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The Building Technologies Office invests in energy efficiency and related technologies to increase the affordability, comfort, and productivity of the nation’s residential and commercial buildings. We perform a wide range of activities including early stage R&D of next generation technologies, integration and validation of technologies in the commercial and residential sectors, and codes and standards work.
I encourage you to take some time to check out our website to learn more about the exciting work coming out of the office and different opportunities to interact with us and support our mission.
A major initiative within our office is the support of grid-interactive efficient buildings (or GEBs). As the primary consumers of electricity, buildings can be utilized as central components in a modern power grid.
There are four key characteristics of a GEB. They are efficient, connected, flexible and smart. Though energy efficiency will still be critical, buildings of the future must go beyond traditional energy efficiency to better support efforts to modernize our power grid. By utilizing increased communications, flexible technologies including variable loads, distributed generation and storage, and analytics to co-optimize efficiency, flexibility, and occupant preferences buildings can play a leading role in supporting grid modernization.
More information on the GEB initiative can be found at the link on the screen, and I encourage you to check out the exciting work in this space.
Flexibility can be achieved through a variety of strategies. One which has gained increasing attention is energy storage. Many projections show energy storage deployments rapidly increasing over the coming decades.
The U.S. Department of Energy launched the Energy Storage Grand Challenge to focus resources from across the department to create a comprehensive program to accelerate the development, commercialization, and utilization of next-generation energy storage technologies. The vision for the Energy Storage Grand Challenge is to create and sustain American global leadership in energy storage utilization and exports, with a secure domestic manufacturing supply chain that does not depend on foreign sources of critical materials. While research and development is the foundation of advancing energy storage technologies, the Department recognizes that the goal of leadership requires addressing associated scale up challenges, including manufacturing, workforce development, valuation, and technology transfer.
The Draft Roadmap outlines a Department-wide strategy to accelerate innovation across a range of storage technologies based on three concepts: Innovate Here, Make Here, Deploy Everywhere.
DOE is requesting feedback to inform the suite of activities proposed in the Draft Roadmap through a Request for Information. More details can be found at the link. Responses to the RFI will be due by August 31.
This is a great way to get involved in helping shape the future of energy storage technologies. BTO will primarily be engaged with the energy storage grand challenge through our work on flexible loads and thermal energy storage technologies.
At times, the conversation around energy storage may appear to be dominated by electrochemical approaches. Thermally based energy storage technologies can provide a number of benefits to applications in the built environment. Significant portions of building loads are already thermally based and thermal loads are major drivers of system peaks. As such it is only natural that thermally based storage options be strongly considered for building applications. Depending on the solution chosen, thermal energy storage can offer numerous advantages over competing technologies. Some examples as shown include lower cost, longer life, and increased efficiency, depending on the conditions which the storage is charged and discharged.
Although there continues to be active research on thermal energy storage materials, many still face challenges to their widespread deployment. Some challenges include cost, storage density, thermal conductivity, decomposition, and phase segregation.
A joint workshop held last year by Lawrence Berkeley national laboratory and the national renewable energy laboratory investigated the needs for dynamic and interactive storage solutions in buildings. The report, which can be found at the link shown, explores how material innovations can play a key role in reducing cost, increasing efficiency, utilization, and the lifetime of thermal energy storage materials.
If you haven’t already, I encourage you to take some time to check out this report and the numerous recommendations it lays out for future research opportunities.
The Building Technologies Office continues to support the development of the next generation of thermal energy storage technologies to make residential and commercial buildings more flexible and resilient.
Some targets we’ve set for future technologies are shown in the table. These include goals for transition temperatures (both for PCMs and TCMs), material cost, energy density, thermal conductivity (keeping in mind dynamic tunability may be desired for future applications), reliability exceeding 20 years, and addressing subcooling limitations.
Meeting these goals will continue to make thermal energy storage technologies competitive with alternative forms of storage.
The rest of this webinar will focus on novel materials and approaches for thermal energy storage technology. Slides from the previous two webinars on ice based and hot water storage can be found at the links shown. And as I said these slides from this webinar will also be posted in the future.
We have a great panel of speakers today. Kyle and Navin from Oak Ridge National Laboratory, Allison and Jason from the National Renewable Energy Laboratory, Patrick from the University of Virginia, Suman from Lawrence Berkeley National Laboratory, and Patrick from Texas A&M university. Each will have an opportunity to talk to us about their work and how they approach the future of thermal energy storage technologies. Again, please use the Q&A function to submit question as we go along, and we will have an open Q&A session at the end.
I will now turn it over to Kyle and Navin. Kyle Glusenkamp here, I am a researcher at Oak Ridge National Laboratory and today with me is Dr. Navin Kumar who will present some of our slide. I’ll talk about salt hydrate materials as part of this webinar today. It’s really a pleasure to be here today. I want to thank BTO and Nelson for the invitation and for putting together the previous ones, which were really informative. So, my goal today is really to establish a common base of understanding for everyone in the audience about what is the landscape of salt hydrates today, and then to identify the R&D challenges and opportunities associated with them.
I will also show you some of the work that we're doing at Oak Ridge, Tennessee. I want to start by acknowledging our partners with University of Tennessee - Knoxville, Georgia Tech, and NEOGRAF. I also want to acknowledge our funding source from BTO as well as the computational resource we’ve taken advantage of during the project.
And specifically at DOE I’d like to thank Sven Mumme and Tony Bouza. I’d also like to thank some other research team members who provided materials for today, Yuzhan Li and Monjay Goswami at ORNL, and Jason Hirschey at Georgia Tech.
So again, I’ll do a review of salt hydrate materials, what are the key challenges, and show you some of the work that we are doing at ORNL.
So we all know that thermal energy storage is really important. There’s a wide range of applications here. I think salt hydrates can be really important in all of these applications. From battery thermal management to building cold storage, HVAC systems. Common PCM materials they basically fit into two categories, inorganic or organic. Today I’ll be focusing on the inorganic option.
So this shows the landscape of phase change materials with melting temperatures between 0 and 65 C. On the y-axis we have energy storage material cost expressed in dollar-per kWh or thermal storage. Note that that’s a log-scale so we’re looking at multiple orders of magnitude in terms of cost. On the x-axis we have energy storage density expressed in kWh/m3. There are a variety of types of materials. The green circles are the salt hydrates, we have water/ice as the “x” and we have organic materials, which are the yellow diamonds which are mostly paraffins. We also have fatty acids, which are another type of organic. We have inorganic mixtures which are basically salt hydrates, which are essentially binary, or ternary mixtures of multiple salts. For reference, the box in the lower right corner are the targets used in the 2019 BENFIT FOA. So obviously the ideal material is going to have a very high energy storage density and a low cost. Looking at this diagram in terms of the landscape of materials out there, salt hydrates are very promising in terms of existing known materials. They have a low cost in terms of dollars per unit energy, they have a high volumetric energy storage capacity, and they also have some other nice properties including that they are nonflammable and the majority of them are nontoxic.
So the way that salt hydrates actually store energy is shown in the schematic in the upper left. It’s basically a hydration and dehydration reaction. On the left hand side you have salt, which is closely associated with hydrated water molecules, and with the addition of heat you can dissociate them and you get the water separating from the salt. The enthalpy of the water on the left-hand side is basically equal to the enthalpy of solid ice-water. The enthalpy of the water on the right-hand side is very similar to the enthalpy of liquid water. So the phase change energy, the energy associated with this reaction, is on the order of the latent heat of ice, of fusion of water. Because of that we have a few properties that arise, we have a very stable crystalline structure, which gives rise to a sharp transition temperature like ice storage. And just looking at the single cations and anions that are available for a singe component salt hydrate, there are over 2000 possible salt hydrate combinations. Once you get into mixing multiple of those to make a binary, you have permutations of that number available. A very common example is CaCl2. And something important to know about salt hydrates, again we’re dealing with inorganic chemistry, every single choice of anion and cation gives rise to a different phase diagram. This is one example of a phase diagram for CaCl2. We unfortunately don’t have time today to go into greater detail on this, but it is important to know that every single salt hydrate material as well as every binary or ternary mixture will have its own unique phase diagram.
So the key challenges of salt hydrates fall into these categories. I believe I have them listed in terms of importance. The very first one is incongruent melting. This goes by a lot of different names. Sometimes they call it phase separation sometimes they call it phase segregation. What this means is that when the salt hydrate melts, it tends to form this anhydrous salt, which will coagulate and precipitate to the bottom of the container. And once it does that it doesn’t like to participate in the subsequent reactions. So obviously this is dropping material out of the active reaction and it is reducing the thermal storage capacity. A second issue is super-cooling. Salt hydrates, and remember when we talk about salt hydrates it is a very heterogeneous space, but there are somethings that we can generalize about them. In general incongruent melting is a significant problem for many salt hydrates, and in general there is a large amount of super cooling for salt hydrates. Moving down the list, we have a limited number of options for phase change temperatures. This arises because we have discrete options since we’re dealing with inorganic chemistry. There are only so many options in the periodic table. And finally corrosion and chemical stability and thermal conductivity can be considered issues. And kinda overarching all of these things are characterization challenges, which can be a subject for another webinar so we won’t get to touching much on them today.
Here’s a cool video, as you can see we have a supercooled substance. The crystal has just dropped, which give rise to a crystallization site, a nucleation site. And this is a real-time video it hasn’t been speed up. You can see how the crystallization propagates once that nucleation starts. Supercooling has long been addressed with additives including borax, which are available today.
So, next I will turn it over to Navin to talk about some of the current state-of-the art. Hi this is Navin. I will be covering more on the current state of art of improving phase change materials thermophysical properties. So one of the biggest problems is phase segregation in salt hydrates. So one of the most commonly used techniques is a thickening agent, which is basically a high viscous polymer or gels, which increase the viscosity of the salt hydrate and reduced the diffusion length. But research and previous literature shows that this only works for a few cycles and will settle out if given enough number of cycles. At ORNL we also ran an MD simulation to see how this viscosity enhancement effects the phase segregation and we’ve seen that eventually the viscosity does not really benefit in reducing segregation. The message is that thickening is not enough to improve the phase segregation in salt hydrates.
Another state of art which is most commonly used to try to improving salt hydrate performance is reducing phase segregation. So there’s two common techniques people have been working on. It’s called a core shell technique and a shape stabilized technique. In the core shell technique the core is basically salt-hydrate, which is encapsulated by polymer, which is not very porous. This prevents the salt from escaping. And the other method is known as a shape stabilizing encapsulation, which uses things like graphite or metal foam to directly integrate salt hydrate into these polymers. And the difference between these two techniques is the porosity of the shell material used. So what is promising in encapsulation. Results have shown this reduced phase segregation and prevents reactivity of hydrates with outside environment.
This is the current results of encapsulation. The table summarizes both techniques. This is summary of some of the encapsulation using sodium sulfate and calcium chloride hexahydrate. One of the message we take away from this is the amount of reduction in latent heat by encapsulation technique. We lose up to 13% in latent heat in the PCM encapsulating. So these processes can become very expensive and also lead to low encapsulation. Also literature reports the encapsulation techniques also tend to break easily after a few cycles due to volume expansion. Next is increase in supercooling without nucleators. So what is the area of research that is necessary for this field is new low cost occurring shell materials. So we want to reduce the cost of encapsulation by coming up with new shell materials and also techniques that are cheap and high encapsulation techniques.
So, what is the other challenge with salt hydrates. It is the phase change temperature. In building applications we are very focused on the temperature range of 15-25C. These two figures highlight the importance. The y-axis is the energy storage material cost ($/kWh) and the x-axis is the volumetric energy storage density. If we look at the top left figure we can see that there is not enough salt hydrate in this temperature range for us to be able to address every application in the building. And also for the 25-35C melting temperature we see the same problem. We have common hydrates, which is CaCl2, NaSO4 and LiNO3. We don’t have any other salt hydrates that fall into this temperature range. There is a huge potential in this area of research to find new hydrates, which fall into this temperature range.
So now I want to focus on ORNL’s way of looking at improving salt hydrates. We look at this Venn diagram. We believe that salt hydrate needs a nucleator to reduce supercooling, some method to improve thermal conductivity, and we believe that physical thickening, which is basically viscosity thickening is not enough to have a long life stability. Therefore we believe that we need to have chemical stability. So in the ideal case we want to find an additive or material that can solve all these four problems in salt hydrates. We most likely need more than one material to fulfill all for targets. This is how we look at our problem and the next slide I’ll show some of our results and how we overcome it.
So, first approach is additive. We have tried various polymers to improve the long-term stability of NaSo4 decahydrate. As we look at the black like with is the pure SSD (sodium sulfate decahydrate) we see that after the second cycle SSD has already decreased. With various polymers we see that we could improve the stability of the SSD. We tried different polymers to see how chemical and physical stability improves the overall performance of salt hydrate. We learned that polyelectrolytes with specific ions exhibited good stabilizing effect. And we were able to get very stable PCM with 150 J of energy over 150 cycles which cost $5.8 per kWh. And reduced the supercooling to less than 3 C. This is one of our ideal composites at the moment and we are still working on it to develop it further.
We are looking into how to modify the phase diagram itself. When we mix various salt hydrates we are going to get different phase diagrams. What we are trying to do is take an incongruent salt hydrate and mix it with a congruent salt hydrate and form anew eutectic hydrate binary system. By this we believe we can prevent phase segregation in the salt hydrate by having a proper melting cycle. So what are the benefits of this technique? First is there a potential of introduction of new salt hydrates based on this eutectic model. We are also developing a model on the side to introduce new salt hydrates in the future. A new eutectic will is incorporated with any high thermal conductivity material, which removes any additives polymers to improve the overall performance of phase segregation in salt hydrates. And one of our success stories, we were able to come up with a new salt hydrate eutectic with a storage capacity of 215 J over 50 cycles. We also reduced the phase change temperature from 32 C to 29 C. This is to highlight what we are working on at ORNL and this is our success story.
Now I want to cover on how we do characterization. I know there’s very common characterization, DSC FTIR techniques. At ORNL we look at the problem a bit differently. We are very interested in looking at the microscale level of the salt hydrate example NaSO4 decahydrate and polymer to see how we’re changing the structures. So what is the benefit of this. The benefit of this we can develop new salt hydrates by understanding the fundamental science on how these salts behave when they melt and react. So this one of our characterization techniques, we use a neutron source technique. We do thermal cycling in-situ in this neutron source. And then we use another technique, too.
We use advanced photon source at Argonne National Lab to do use X-rays to see how the samples behave during their performance and also design new salt hydrate materials. And if you look at the top right corner is a sample we synthesized and send to ANL for testing. I’ll now hand things over to Kyle to complete the presentation.
Thanks, Navin. SO just wrapping up, in this discussion we talked about how salt hydrates are among the most promising PCMs. They are extremely low cost relative to others with high energy densities. Key challenges, phase segregation, which is critical to achieving cycling stability. Limiting supercooling is important and developing new temperature ranges could be important once we start talking about future application. ORNL advanced state of the art using combination of physical and chemical characterization, identifying new eutectics, and gaining insights into fundamental materials using characterization techniques we have such as the spallation neutron source. Thank-you very much.
Thanks, Kyle and Navin. Next we will have Patrick Hopkins. Thanks so much for the introduction and thanks for organizing this. My name is Patrick Hopkins. I’m a professor at the University of Virginia. Today I will be speaking on bio-based materials for thermal energy storage. In particular I’ll be focusing on a novel class of these bio-based materials that are derived from squid ring teeth protein. What we found through previous collaboration was that these squid ring teeth proteins have exceptional thermal properties and the ability to tune the thermal properties, the thermal conductivity, with also novel way to create enhanced energy storage. So as a bit of background my expertise is in thermal conductivity measurements and energy storage measurements. Several years ago we started collaborating with a professor at Penn state who specializes in creating and manufacturing and really genetic engineering of these squid ring teeth protein. After some work a few years ago we discovered the energy storage properties, and in a current effort funded by Sven Mumme and the BENEFIT program I’ll be presenting some of our work with the collaboration between UVA Tandem repeat proteins and Georgia Tech. Investigating these squid ring teeth proteins for novel energy storage.
Just to kind of set the stage for where we’re looking I’m showing here a plot on the right, which is taken from the last year’s BENEFIT call, that was taken from a workshop in Berlin about a decade ago. This is just plotting energy storage capacity kWh/m3 vs thermal capacity as a function of temperature. As you heard from the previous talk, salt hydrates and looking we see here with various PCMs. What typical energy storage densities are if you want to look at the BTO uncharted space where we can look for increased energy storage capacity it’s be down in the regime around room temperature and enhanced energy storage capacity into that target green rectangle. I don't want to talk to you today about is that through a combination of energy storage capacity both latent and sensible heat storage thermal conductivities, these bio-based squid ring teeth materials have the novel approach to achieve this target. And part of the reason behind this not only comes in the energy storage capacity, but also their remarkable thermal conductivities. I’m showing here a derived figure of merit for typical PCMs. One notable work by Patrick Shamberger, who will be talking about this later in the journal of heat transfer work in 2015, is the idea that the figure of merit of a PCM has to do not only with its ability to store the heat but also with its ability to move the heat. Figure of Merit of a of a PCM has to do not only with its ability to store the heat but also its ability to move the heat so really the figure merits related to the thermal diffusivity in this is basically the way that you can consider how rapidly a system can absorb a certain amount of heat flux and then cross over that phase boundary we will hear much more about this later on by Professor Shamberger.
So the other thing is not showing up too well but just as a bit of background we typically think about energy storage capacity or PCM, we typically think about this idea of some latent heat or some energy. Some energy having to get some heat stored having to do with the melt and at a very narrow temperature range. You can store a lot of energy in your device. Here's a pot on the left, just showing them that the typical curb that you get; this is taken from the advanced cooling technologies website and it for a wide range of materials are typically talking about melting in the melting to be anywhere around room temperature like we saw on our previous talk. Too much higher temperatures you can start to define some figure of merit type and some efficiency based on some combination of the thermal conductivity and either the latent heat or the sensible heat or both. So this is just surveying from a previous paper various efficiencies of energy storage based on the latent heat. Really, this is a common metric the people considered PCMs.
I think where we have a lot of a lot of phase space to consider novel materials is also the ability of a material to change thermal conductivity and this brings in the idea of the thermal conductivity switch. So you might consider when you have a PCM that for the figure of merit when you're charging or discharging you would want a high thermal conductivity you want to be able to take that heat flux in and out as rapidly as possible to maximize energy that’s coming in or out. But when you're storing you don't you don't necessarily want that energy to go away; you want a lower thermal conductivity material. So thermal conductivity switching is a concept that has garnered recent attention and it's very common to consider changes in thermal conductivity when you go across the melting transition. This is a regime map that was produced from one of the results of … work a few years ago highlighting many thermal conductivity switches across various solid liquid transitions, you see this very commonly in inorganics.
So really what I'm setting the stage for here is to consider the efficacy of novel, of these novel bio-based squid ring teeth based PCM; we want to consider both their ability to store the heat. Also their ability to dynamically change the thermal conductivity and have a relatively high thermal conductivity, going back to Nelson chart at the beginning looking at thermal conductivity that could be greater that 1 W/m-K and therefore enhancing figure of merit.
What we're going to do is before talking about the thermal properties of these squid ring teeth proteins, I wanted to set the stage about kind of the discovery of these proteins through my colleague at Penn State. Malik Demral and Completely Demerol and tandem repeat the company is manufacturing these protein. The squid ring teeth is a misnomer and in the fact that the protein is not really derived from the squid teeth. You should think of them more like squid ring teeth fingernails but the person that discovered them called them teeth, cuz' it kind of look like teeth. You see pictures of them on the left, those are the suction underneath the squid's tentacles that he's using to claw and grab prey. They have remarkable regenerative properties mechanical, which I'll show in the next couple slides, but what I want to highlight here was the ability to manufacture these into really any shape and sizes and scale up with the anticipation that we can hit metric the target metrics $4 per kilowatt hour based on the manufacturing of these by the base proteins at scale.
Part of the highlights of manufacturing these squid ring teeth protein is the green and carbon neutral process in which they're manufactured. So squid ring teeth proteins are manufactured by fermentation and then purification and drying material processing and not only of a biodegradable but they're recyclable, so you really have a circular carbon economy in the process of creating these nontoxic biodegradable recyclable bio-based PCMs.
If we zoom into the nanoscale level of what one of these squid ring teeth proteins look like, you see a combination of crystalline and amorphous region; that's what shown by the cartoon in the center. They're so what you see the microstructure, really the nanostructures of squid ring teeth to the protein is it complies comprised of two different regions one is a highly crystalline and region called a beta sheet too many protein and then the other are the yellow regions that you're really the amorphous tie chains. So through a combination of both the beta sheet and the amorphous tie-chain you can have some very, very interesting and suitable properties of these teeth proteins so for example these are high strength and very durable due to the semi-crystalline nature due to these beta sheet and the durability in the modulus can be tuned based on the density of the tandem repeat between the beta sheet in amorphous tie-chain.
And I think something that early on when I first started working with Malik Demerol years ago something to got me very excited from a thermal conductivity standpoint was the fact that these bio-based PCM these squid ring teeth protein are self-healable. So here's an example of a of a protein of the protein dog bone that was created at Penn State. The YouTube video on this that actually really interesting stuff here. You can cut it and then he dipped in water added a little bit of heat and then it completely reformed back to its original strength. Behind this is when we saw this ability the self-healing and melding ability of squid ring teeth proteins we started thinking about its ability to change thermal properties with when you're hydrating, and it turns out that this type of protein water can act at the plasticizing agent drastically changing the mechanical properties allowing for this. Also changing thermal properties so when you start thinking about a thermal conductivity switch you're not going to have a larger difference in thermal conductivity switch then when you can do something like this like take something out of contact and then put it back into contact. So we wanted to understand what's happening at the nanoscopic level and understand its thermal properties.
So what I'm summarizing on this slide was the previous collaboration that we had with Penn State on discovering the thermal conductivity of the squid ring teeth proteins is published by my student John Tomko. So as I mentioned before, these SRT proteins are made up of both crystalline and amorphous regions and when hydrated they have quite remarkable properties. When they are hydrated the water acts of the plasticizing agent and changes the vibrational amplitude of the amorphous tie chains so when dry the thermal conductivity and the modulus and thermal conductivity act very similar to what you think from any disorder polymer that's what you're seeing on the right with our measurements of thermal conductivity those are the red dots.
Now when you hydrate, the water interacts with the amorphous tie change increases the vibrational amplitude of the thermal carriers as we confirm neutron scattering and the thermal conductivity of the SRT protein goes out that's what are the blue dots are so not only do you have the ability to change the thermal conductivity when hydration but depending on the tandem repeat sequence and the number of repeats you can increase the thermal conductivity values that are above1 W/m-k. To put this into perspective the thermal conductivity of water 0.6 W/m-K.
So this is not an effective just adding water to this protein it's the fact that the water is interacting and changing the vibrational nature of the protein.
Buy please so that means with this working fluid with this plasticizing agent we are able to create a thermal conductivity switch and that's shown here through the work of really like I mentioned my Ph.D. student John Tomko using our various thermal reflectance measurement system that we have in our lab both time domain thermal reflectance and frequency domain thermoreflectance. Steady-state thermoreflectance where what he's doing is he's creating a switch by adding and removing the plastic sizing agent of the protein you could see a very rapid change and also a large change in thermal conductivity that is cyclable over at least with shown here from our previous paper 20 cycles of on-off going from polymer values up to above 1 W/m-K.
We start thinking about thermal conductivity switches around room temperature so around this target temperature range that we're seeking for energy storage and high figure of merit you now find that these squid ring teeth protein shown the data shown by the red stars are getting thermal conductivity on off ratios that are that are setting the new state-of-the-art for an intrinsic thermal conductivity change of any known material. So not only do these Bio-based PCMs SRT really leave a neutral carbon footprint in manufacturing and provide an interesting way for thermal conductivity but it also provides us a different means to consider energy storage.
The principle of how energy storage and energy storage material would work with a squid ring teeth proteins would be directly related to the plasticizing of the network. So unlike a typical curve that I showed at the beginning where you have some over narrow temperature range you have some heat absorbed melting. What we're talking about in energy storage with these novel bio-based PCM is shifting the glass transition temperature of the PCM.
So consider the SRT in a dry state when you heat it up to at some temperature above room temperature, then you will go across the glass transition temperature and you will increase your heat storage. However, with the plasticizing agent and correct genetic engineering that tandem repeat can be can do then you can shift down the glass transition temperature and thus you now have a dynamic way of storing heat based on the plasticizing agent and the amino acid sequence of the protein, which is all based on the protein manufacture.
This is really what the in addition to the potential for a novel way to consider energy storage and the remarkable thermal conductivity switching metrics that we found our team is creating these SRT proteins that are bio-derived tunable, customizable, self-healing. And really when projected at scale we can hit less than the projected BTO benefit cost of $15 per kilowatt if it's manufactured at scale. So you know really looking at the possibility of these noncorrosive, nontoxic, nonflammable recyclable materials with novel energy storage properties is the real motivation behind this.
This is what our new support from Sven Mumme and his BTO benefit program is providing us the ability to collaborate to really push the limit of bio-based PCMs based on these squid ring teeth protein. So there's a collaboration with my lab at UVA, Ben Allen with tandem repeat and Shannon at Georgia Tech, is to really determine the they're really maximum potential of the squidling teeth protein based on the genetic engineering of the tandem repeat.
I will just stop there with a summary of the previous work today that demonstrates the remarkable thermal conductivity switching, and thank-you so much for your time, and thanks again, Nelson, for organizing.
Remember, you can submit your question to the Q&A and we will get to them at the end. We will now have Suman talking to us about thermochemical energy storage materials. Hello, everyone, and thanks, Nelson, for arranging and thanks, Sven Mumme, for giving me a chance to speak in this webinar. I will be speaking on thermochemical energy storage material, and I'm from Lawrence Berkeley National Lab.
So you know this is just a broad classification of thermal energy storage material. We can store energy in sensible heating latent heating and thermochemical. So the graph here shows the distribution of this different categories of materials we can see in the corner of the sensible heat where the volumetric and gravimetric energy density of those materials are. The last two speakers talked about latent heat in a phase change material was bio-based and inorganic and you can see the energy densities of those materials. Thermochemical storage materials is a class of materials where energy is stored in the reaction. Like latent heat of phase-change where temperature is associated with it, so like for a phase change material you have a transition temperature for thermochemical energy source material you have a reaction temperature.
And the graph shows that it has a wide distribution like you can have the light blue and dark blue siding from dark blue to light blue you can see these are the low temperature thermochemical materials and then you can see it can go all the way to 1200C. The one thing you notice is that the reaction enthalpy, which is the energy stored in this reaction of significantly higher and that is one of the key advantage of thermochemical energy stored material.
So let me walk you through this slide. If you look at the you the energy density you can clearly see that they know that the energy density of the theoretical material is 200-600 kWh/m3. The phase change material organic to inorganic all belong to similar range around 50-150 Kwh/m3. And these reactions can be between solid to gas, solid to liquid, liquid-gas. Broadly when can have two broad classification of this thermochemical material, the reaction can be absorption material or adsorption material. In absorption material there is a reversible reaction. It involved breaking and restoring of strong bonds. It occurs at the molecular level and it alters the composition and morphological composition of the whole solid. Because of this, the material expansion during the reaction can happen. Because it involved breaking and restoring of strong bonds, it can store high energy in comparison to adsorption materials. Adsorption is mostly the surface-based reaction. It involved weaker van der waals and hydrogen bonds compared to the strong bonds in absorption materials. Example of adsoption material would be something like zeolite or silica gel. Kyle and Navin talked about salt hydrate PCM where the phase change of salt hydrate happens that you have solid and it melts and they want a uniform melting. When salt hydrates are used as a TCM material you just want a solid-gas reaction, you don’t want any melting, you don’t want phase change to happen. So you want to hydrate and rehydrate the salt hydrate in a chemical reaction.
OK, so let's just walk into this slide. So how will the thermochemical material act as energy storage for building? If you look at the panel C, say you have a salt hydrate; you can store the energy or you can charge this by using solar or grid and separate salt from the vapor. If you have an open system you can let the vapor escape and just store the anhydrous salt. When you have to discharge it you just provide the moisture and you can release the heat at the desired temperature depending on the end-use like space heating or hot water. This is a reversible solid gas reaction. Can happen in open system format where you can release vapor to the ambient and then use ambient to discharge, or can be a closed loop thermochemical system.
So what do we need for this thermochemical material properties to make it suitable for buildings? We definitely need higher reaction enthalpy. The higher the reaction enthalpy the more energy density to have and you know it is very similar to all the other energy storage material which we have discussed so far. We need high thermal conductivity, nontoxic, noncorrosive, low-cost, nonflammable. What we need here is that we need to have not any side reaction. The reaction should be completely reversible and without any secondary reactions. We need to regenerate it at relatively low temperature. The TCMs have not really been explored for buildings so far because most of them require high temperatures. There's a lot of effort to find new TCM material that are more compatible for buildings.
So I'm going to just cover in my talk what are the main challenges associated with the materials or this thermochemical storage. So as I said if you talk about absorption during the reaction the changes happening at the molecular level. So it is very important that we understand what kind of phases we are dealing with and are they stable during our operating conditions. Because the molecular level defines the theoretical energy density and the stability zones of the various phases. So moving from the molecular level to the next level particle size becomes very important. As I said in absorption where you have molecular changes happening and the morphological structure is changing, the material is undergoing expansion during this reaction. It is very important that you have the right particle size so you can have a good heat and mass transport otherwise your material will pulverize, agglomerate, or melting will happen. All of this is undesirable. for a gas reaction do not want to lose material in melting you do not want face change. I will talk about the composite level in a bit.
So in the structure level if we have in this reaction and I actually said it depends on the crystal structure if you lose a crystal structure during the reaction it changes into amorphous we will lose our energy density. In the molecular stage is important to understand what are the stable phases at the particle level it is very important to correlate your particle size with your reactor level parameters. For example if we have very high heat flux but the vapors cannot escape it will result in the melting of the salt hydrate so that melting in this case is undesirable because we want the reaction to happen between solid and gas. And we do not want make a material inactive.
So to overcome this problem similar to the phase change material people are exploring host matrix. Can we have enough host matrix impregnated with salt so that we can overcome this problem of agglomeration and melting and improve heat and mass transport. So these are the commonly used matrix's or phase change materials and researchers are now using these kind of mattresses for PCM materials so we can have better heat and mass transport. But the challenges here again are that you have to have proper loading of the hydrate of the active material without compromising the energy density too much. For example, zeolite and silica gel, which are also PCM materials, and put in them absorption materials like salt hydrates in this way we aren't adding too much inactive material. All of the approaches have got challenges; there has to be a better heat and mass transport at the composite level so that when you make a system of this material have good energy density the problem with PCM material so far is that they have a very high theoretical energy density 200 to 600 kWh/m3, but when you take it at a system level you don't even have 10% of that. All this is because you lose a lot at the material level you need a lot of optimization at the material level so you can access that high theoretical energy density. if you use composite material with inert matrix you lose a lot of energy density and on top of that if you make a reactor if the design is not optimized you lose further energy density. This PCM class of materials suffers from poor multi cyclic efficiency. Research has been focused on using this material for long-term storage, so what we have shown is up to 10 or 20 cycles you can have at the material level but it drops significantly after. This is due to the points I've mentioned earlier because of the changes going from crystalline to amorphous or pulverization at the particle level or at the composite level there is poor heat and mass transfer. So these are the challenges which have restricted the use of TCM materials in building applications.
So all of these problems would she have mentioned at the material level now you can imagine you have a particle. Its properties are not optimized and when these particles are assembled at a reactor level the reactor, which is a bed of the particles, it further exacerbates these problems. So it becomes you can have localized hot spots, which can create additional mechanical stress and can induce metastable or unstable phases. So it is very important that all the problem of we start with we need to have a bottom-up approach so that the problems are solved at the material Level before we start to solve it at the reactor or system level.
So what are the possible solutions? I think that Kyle and Navin mentioned a bit about how we can come up with new crystal structures and new materials which are more stable and can have better phase change. So similar to that we need to have new material discovery or more effort has to be put into this direction so that we can come up with new salt hydrate solutions or salt mixtures, which are stable, and I'm showing here one study with just looked at 265 hydration reactions. They carried out using high-throughput DFT calculations and they showed some new reactions which have never been explored for TES materials. The stars represent the reaction which have not been explored and we can see that these are at different temperatures. The other approach is that for the existing TCM materials it is very important to have a bottom-up approach and where we not only optimize or just mechanical properties or thermal properties but it needs to be co-optimized. Chemical mechanical and thermal properties of the material needs to be co-optimized such that when we have reactor level or system level we have all the desired properties including energy density high-cyclic efficiency, and thermal conductivity.
Thanks for that, Suman. Now we will have Patrick talking to us on the design of engineered thermal energy storage materials. Great; thank-you. I want to reiterate what our other panelists have said so far; I think there's a lot of great opportunities here or development of thermal energy storage particularly at the basic material Level and I think DOE and BTO are specifically meeting the initiative in this area. The goal of today's talk is really to consider how do we design a material or a particular application. How do we evaluate its performance and how do we develop metrics to help consider different materials solutions. And essentially this really boils down to you what is the right material for the job.
So for any energy storage application you need to think about both energy and power metrics. You can compare this to the analog in the electrochemical storage problem by just thinking about a battery and with batteries we care about how much energy they store but we also care about the storage rate. What is the power for charging or discharging that energy storage, and for many different thermal storage solutions the power can turn out to be the limiting factor.
So we can take a page from the electrochemical energy storage playbook and consider this trade-off space where we're looking at energy density on the horizontal axis and power density on the vertical axis. The question is, how do we do the same thing for thermal energy storage? How do we understand these trade-offs and what are the optimal materials for a particular application? Just like for the electric chemical application, in some cases the energy density metric might be the most important but any of those kind of applications you wouldn't use a capacitor because it's not competitive in that space. Otherwise you can develop specific cost functions to compare the performance of different kind of materials in this energy power density space. So this is an example of what is called an electrochemical Ragone plot and I think we're going to hear more about these in the next talk. We're not considering things in the model level hair; we're really focusing down two materials level. If you want to consider thermal storage materials the rate is basically a function of how quickly you can put heat into some material volume and so if we just think about heat crossing an interface into a semi-infinite medium so it's just sort of one-dimensional heat transfer across the plane, we care about two things. We need to care about connectivity and we need to care about an effective capacitance, how much energy can we store per unit volume. Of course in a PCM the main term that we often focus on is that volumetric latent heat, so this is basically a way to analyze it and we like to evaluate this at the material level.
Be a little bit more quantitative about this. We can look at solutions to heat transfer problems into that semi-infinite medium, and the solutions look different based on the nature of the boundary conditions; constant heat flux or constant temperature. Time and time again we see this term pop up and drop out that really contains all of the material specific parameters that are relevant for this problem. It's been pointed out that this term is very closely related to the thermal effusivity so we can call this essentially an effective thermal effusivity. It includes the square root of the product of the thermal conductivity times the volumetric latent heat.
So this allows us to adopt this figure of merit to start comparing different materials so this would allow you to evaluate the effectiveness of different materials side by side absorbing heat rate. We can you see this as a sort of a material comparison metric but it's important to remember that if you go back to the analytical solutions this figure of merit is proportional to the rate of energy storage or to the temperature rise so you can translate to hear directly back to this rate terms. So we start to see a couple of different classes of materials these are just low temperature and energy storage materials and we can compare them against things that conduct heat very well so copper aluminum graphite and base again on your application you might care more about an energy density so pushing further to the right or maybe the power density so for electronic applications in many cases cooling a chip of some sort. The power is the limiting term so when those cases it might even be best to slap a copper block on the system.
As you know, as material scientists and engineers we like to start to say this is good but how do we go forward, where do we go from here? We start by considering that there are these two key terms that are some things are really strong in the conductivity but don't store a lot of heat where I saw other materials have really large energy storage terms but they are just slower at the conductance level. The conductive things like the aluminum graphite copper are showing up on the upper left things like soap hydrates paraffins are storing heat through their melting so they're effective capacitance showing up on the right. So we can say what happens when we start to mix these two different classes of materials.
By treating these things as homogeneous composites with effective properties we can start to answer that question. So what you're seeing here are curved, which are upper and lower bounds for composites, which are made by combining different connective elements together with a paraffin. So the upper lower bounds are related to the orientation of the different materials within that composite, so if you have your conductive element aligned along the heat flow direction you reach the upper bound if these layers are perpendicular and you reach lower bound. There are a couple important takeaway scare and one is that we can generally outperform a single phase systems but perhaps even more importantly ...
We can use this sort of understanding an approach to interrogate the trade-offs and designs and material space. So this red curve is just highlighting composites that are composed of aluminum and paraffin and the point that I'm sitting at is a function of how much of those two faces I've included in the composite. So again for a specific application you can tune what that composite looks like to develop what is an optimal material.
So this kind of brings up two main questions and the first is related to this general approach of describing things as composites. When can we apply that how well does it work and we've done some work validating this in lamellar systems and I'll talk about that shortly. The next thing is if I want to apply this to a real problem oftentimes I need to go beyond the simple figure of merit, and dive little bit deeper to evaluate it. So we'll talk to an example.
So the answer to when can we apply this composite approach primarily depends on the critical length scales in the system. You can kinda think about if I have this layered material where the if you look at the material on the left the gray is representing layers of aluminum metal and the material in between is a phase change material; as I decrease the spacing between those conductors smaller and smaller and smaller at some point I sent you lie achieve what would be referred to as a one temperature model where temperature is basically only a function of the vertical distance away from that heated surface. So the answer to how thick or how small the spacing need to be. It depends on the time scale, it depends on how far into the material you're introducing your heat pumps, and we kind of go into it in this citation down at the bottom here. I think the important takeaway here is that these links scales are readily manufacturable, so people who work with heat exchangers are making things at these light scales all the time by traditional machining or by brazing approaches with use some added manufacturing techniques to me these.
The following question, how well does it work, these approximation are really closely matched by experimental observation and here I'm just kind of showing basically a top-down look at some components. Some lamellar components that were back filled with phase change materials so as the spacing is large the image on the right see a very large variation in the temperature laterally but asked that spacing get smaller and smaller and smaller we very quickly achieve that one-dimensional model.
A lot of these we can't go too far and validation details here but the actual systems hold up well against our observations. It’s worth really bringing up some porous materials, these have already been mentioned once by another panelist, you can think about either porous or either compressed expanded natural graphite. These are really interesting materials because their length scales, their inherit length scales, tend to be small so they tend to be well-described by the composite approximation. Many cases are controllable, so if you take expanded natural graphite and you compress it to different extents, you are changing the volume fraction of graphite in your composite material. So this allows you then to move somewhere along this curve. The green data points are extracted from someone else's work and kind of plotting them on these composite analysis. So people have been working with these kinds of composites for at least a couple decades but they are very useful systems that have a lot of utility.
So the approach of treating things as composites with effective properties works pretty well for these systems, but any time I make an application that is a little bit more complicated than just putting heat across a planer interface then I need to re-evaluate what an optimal composite looks like. So we need to move beyond this simple figure of merit we talked about and really ask the question of what amount of the two phases and how do I want them distributed within the volume and the answer is that it depends. And it depends on the time scale of the problem it depends on the magnitude of the driving force it depends on the geometry and I can kind of give a brief example here that is based on a cylindrical heat extraction system so taking heat out of or example a heat transfer fluid flowing through a tube.
We can evaluate this at a couple different levels, we can think about just a single degree of freedom in this composite. So what is the volume fraction of the metal a conductor, or the PCM so phi will refer to the volume fraction of the conductive element; and you can have that constant but there's no reason to keep it constant. It turns out that in a lot of cases you want more conductor closer to that pipe because it's helpful to move the heat further the medium. We're only going to talk about this first one because the second topic needs some more complicated approaches to evaluate what is optimal in these cases.
What I'm showing here is basically to plot we have a sort of Cartesian sort of planar surface on the left and a cylindrical surface on the right. The line is basically how much heat per unit area is it being absorbed cumulatively up to a particular time so you'll see this one is 2 to the 0 second so 1 second total. The horizontal axis is showing how much metal did we put in this composite so this was calculated for a combination of aluminum and a paraffin but again we can change material properties to control what the optimal solution is.
So what you see over a very wide range of volume fractions you kind of have a relatively constant amount of heat has been absorbed however that melting front is pushing further and further into the volume because of course it the more metal you put in there the more conductive it is and the less heat it takes to melt a particular volume.
So those circles are the optimal the maximum of heat per unit area or that particular time and the first thing that we see is that if we run the simulation for longer and longer times in a cylindrical case, the optimal volume fraction of metal changes and this is because of the cylinder you’re kind of spreading outwards as you move away from the central tube. So it's advantageous to push that milk front further and further away. Whereas in a planar geometry your optimal volume fraction is fairly constant over time.
We can also ask what happens if I have a higher driving force so all of these were isothermal experiments but if we run to a larger delta T, delta T is the difference between the temperature of the surface at melting temperature, then this curve basically pushes further and further to the right because the sensible heating contributions are stepping in.
All of this was show or heat per area, so we're really saying what is composite in terms of maximizing the total amount of heat absorbed at the time but again the answer changes if we want to optimize some other term so what if we look at heat per unit mass or heat per unit volume, so a lot of the work we do is for aerospace companies and of course mass and volume are very important terms and that not surprisingly pushes the answer way over to the left. So a lot less metal involved because we don't want to push the heat that far we want compact systems and if you evaluate this for different kinds of conductors different kinds of PCMS you're going to get differences.
So you know I think the real takeaway here is that I think it's always important to evaluate what the composite looks like or a particular application different things like geometry, time, magnitude the driving force it all changes with an optimal concept looks like, but at the end of the day these approaches are really driven by this understanding that we can think about a particular volume as a composite inside with effective properties.
I just like to acknowledge: Obviously this is work that a lot of grad students put in a lot of time and I have some collaborators at Texas A&M that have been instrumental in evaluating a lot of this work. So this was supported by ONR and CITMAV along the way but where eager to start applying this rationality to developing composite systems or building base applications. Thank-you very much.
Thanks a lot for that, Patrick. Final presentation will be Allison on designing thermal energy storage devices using the Ragone framework. Thank-you very much, Nelson. My name is Alison Mahvi. I'm a postdoc at the National Renewable Energy Lab and I'm going to be talking more about system-level design and some workout done with Jason Woods.
First, to tie this in with a lot of the other presentations, to kind of go off of what Dr. Shamburger was describing, these materials have some properties that you really want, so generally for a phase change material you want something with a high capacity high latent heat as well as the ability to pack heat in and out of the material easily, so a high thermal conductivity. He's done a lot of work in developing figures of merit for this and specifically for phase change materials which is the focus of this talk. This has led to a lot of work in trying to develop a thermal conductivity phase change material, and generally there's some trade-off between latent heat and thermal conductivity so you have to be in carefully when looking at material properties.
And if my colleagues at NREL has been looking at the material properties side of this looking at organic PCM and salt hydrates trying to make high thermal conductivity form stable materials that can be used in a variety of applications. There's still a lot of open questions on how can you integrate this into buildings envelopes if you heat this from thermal equipment effectively.
We had started looking at NREL specifically for HVAC integrated cold storage, so taking these phase-change composite materials that have high thermal conductivity and integrating them into a charge and a discharge circuit. Where the charge circuit is your typical vapor compression cycle you can then store that cold or cooling inside of this thermal storage device and then when you need to use it you can discharge it to your condition space, but this becomes more complicated problem you're trying to design these devices, there's fluids that are flowing through them how can you do that effectively and what kind of material properties do you need for effective use.
So the way that we're trying to get at the answer to that question is trying to get some inspiration from other fields. Electrochemical storage as mentioned before has a lot of similarities to phase change thermal storage and electrochemical storage there's been a lot of effort in the past looking at different chemistry's also the design of the anode and cathode the design of these systems to make them as efficient as possible or given application. Now we're trying to piggyback on some of that past work to apply that to phase-change Thermal storage devices through trying to optimize them as much as possible.
The reason why we can look at electrochemical storage has to do with similarities between how these two systems operate. So on the left you can see the output of a battery which is the voltage overtime there's some important parts of this battery curve. So ideally your system you get the open-circuit voltage or the battery throughout the entire time that you're discharging and then once your battery is fully depleted your voltage will drop to zero. Generally in an actual battery if you increase the power you'll have some I in an actual battery if you increase the power you'll have some IR losses in your system you'll get further and further away from this ideal output. this allows you to discharge your battery faster but it will also have you get to you you're cut off voltage faster. So the cutoff voltage in a battery is kind of the minimum that whenever design application using can operate under. So at that point you have to turn off your system or your device and any energy left in your battery goes unused. If you look at the phase change storage output of your system, which would be the output lowest temperature, and the device I showed before again you have this ideal output so ideally fluid will come out of this device at the transition temperature however as you increase the power you'll have losses in your system, in this case they are QR losses in a thermal system and again you'll reach this cutoff temperature. So a cutoff temperature in this case for cold storage example is where it's no longer useful for you to continue discharging the system the temperatures are too hot you can't cool your chip or your room effectively.
This is an example of those two plots that I should be forward or call the rate capability curves they just tell you the output of your system versus time or state of charge. So this is an example I have an actual battery lithium ion battery. As you can see this is run at different power rates, so as you increase the power you're getting further and further away from this open circuit voltage. Power in a battery is often specified by the C rate, which is shown in these figures. C rate is the combination of the capacity and the current draw from the battery the amount of power that it's pulling out of it. So a C rate of 1 is defined as a one-hour discharge that's when if you run this battery and you discharging it. It takes an hour to completely get all of the energy out of it. A C rate of 2 is a half-hour discharge. So as you increase C rate you're increasing the amount of current your drawing out of the battery and the amount of power. We can take these rate capability curves and generate Ragone plots. These plots tell you what the specific energy and specific power trade-off is or that specific chemistry and geometry add a different cut-off voltages. So are you can generally as you increase your power or your C rate you can’t get as much energy out of your battery because you're hitting that cut off voltage sooner and so more energy is left in the battery. So generally you go to higher power you lose some of the storage capacity in the battery.
So we wanted to be able to apply this Ragone framework for devices or thermal energy storage to try and get an understanding how to design these things better. The way we did that was generating heat exchanger models so this was a thermal energy storage heat exchanger with a fluid that cooling the thermal load and a phase change material that storing heat or cold. So we discretized the space giving us a lot of information in terms of what's internally happening side the battery, the thermal battery. Where the phase fronts are located, what are the local temperatures inside of the phase change material.
We validated this modeling in experiments. You can see this test section we’re passing fluid through Michael Channel in the center, which is sandwiched between these two phase-change composite slabs and if you look at the fluid outlet temperatures for the output of a thermal energy storage device we find our experiments and our model match quite closely.
So this is showing the discharge process. Inside of these batteries on the top left is the phase of the PCM. So this is looking at both liquid interface between the channel, which is at the bottom, and a phase change material, which is at the top. So as this is discharging you can see this liquid layer building up between the fluid on the bottom and something that is a relatively constant temperature this transition temperature. In the middle that shows the temperature profiles in the system and the temperatures. So you can see that the phase front and how this thermal resistance is building up inside of the system matters a lot for how your fluid at what temperature is changing. So here on the right this is the rate capability curve so as you go from a state of charge 100%, which is when it is cold when the whole thing is at a solid and move to a state of charge of zero when it is completely melted your fluid outlet temperatures will increase because of this increasing and more resistance.
So you can use this information, these rate capability curves or this design, to create a Ragone plot for these types of systems. So the way that you do that is you first run your system at different power rates the same way that you ran a battery. So here you can see as you increase your power or increase your C rate term you're getting further and further away from this ideal base change temperature and you're also crossing the cutoff temperature faster. So the next step is to look at when you cross this cutoff temperature it will tell you information about how long your system has run at a constant power rate and will give you an idea of how much energy you I've been able to discharge out of your system and also was left in it. So then you can calculate the specific power of your device knowing the heat transfer rate that you put into it / the mass or the volume and the specific energy which is related to the amount of time that the system has been on.
You can then take this information and plot it on a log regression a plaque. So here you can see the specific energy on the x-axis and the specific power on the y-axis. Generally at low power you can get all of the capacity out of your system, fully discharged it as you increase the power again in the same way that you find in the electrochemical plot. You tend to have a dropoff in performance this type of information can give us ideas of what types of material properties we need in our system and it also gives us some information of how we want to design what kind of geometry we need.
One thing that has been fairly extensively studied is thermal conductivity. So how do we increase the thermal conductivity of the phase change material to increase your power? Here on the left you can see fluid outlet temperature versus state of charge. So this is the rate capability curve at a C rate of 1, so this is a one-hour discharge or all of these and different thermal conductivities. So generally as you increase thermal conductivity you get closer and closer to this ideal output, you reach your cutoff temperature slower and you'll have better performance. This is because you're reducing your thermal resistance is in your system. This relates to the Ragone lot on the right. So generally as you have higher thermal conductivity you'll move to the upper right side of this plot so he'll have higher specific energy per given specific power but it also tells you a lot about the trade-offs. So there's kind of diminishing returns. Here if we look at the C rate of 1, in here you really after for this particular geometry going above a thermal conductivity of about 10 does it really help you. So if you're trading off material properties of looking at cost of different materials going to very high thermal conductivity materials isn't going to help you much you've already reached near your peak. if you look at building or example which is sometimes you trying to shift and shape load you might be trying to run at lower power something like a four-hour discharge. Going above a thermal conductivity of about 2 is it really helping you again so you need to really understand your application and what kind of power requirements are needed in order to make your material.
Then a second question that you might come across in these types of design questions is what kind of geometry do you need? So when a sample is how much material should you put between each to in order to get your device to work well. So you can put your PCM or more thermal energy source material you can lower your total weight volume and cost. This is usually a trade-off you're interested in looking at. So here you can see as you increase the number of tubes. Again, you're going to the upper right corner of this plot you're getting higher specific energy or given power. At a C rate of 1 you can see you really need a moderate number of tubes in order to get relatively good performance or get most of the capacity out of the battery that you need. As you start to move too much higher C rates at higher powers maybe for something like electronics cooling you need to put things much closer together and that'll increase your cost.
So that was for a high thermal conductivity material, something that has a thermal conductivity of around 1, even if you're looking at buildings or something and has a relatively low power you're going to need a lot of tubes in order to get the full capacity out of it. So there’s certainly a trade-off between things like conductivity geometry of your system, which is related to the cost, and power. So it needs to be something that is carefully balanced between each other from the design stage.
This presentation try to leverage battery research to develop these Ragone plots for thermal energy storage systems to give us more insight into the components design, materials targets for a given application and the storage efficiency and system operation of these devices.
I like to thank BTO for putting this together and giving us the opportunity to talk about at work this is work done by myself Jason and a team of people at NREL, and we're happy to answer any questions in the upcoming panel discussion.
Thanks a lot, Allison. So we're now in the open Q&A portion of our event. You heard from multiple speakers on different materials and approaches for evaluating new thermal energy storage approaches so feel free to use the Q&A box is submit any questions that you may have for the speakers.
Q: With all these technologies what is the target price per kilowatt-hour determined that makes it feasible for production?
A: I can take an initial stab at that. When we look at these thermal energy storage technologies it's not exactly looked at in a vacuum. We also have to compare their cost-effectiveness to other forms of energy storage, so primarily batteries come to mind. So depending on competing technologies and energy conversions that are required to go from electrical to thermal that's essentially what BTO uses to develop our cost targets.
Q: This one is for Patrick Hopkins: Does the switchability of the squid ring teeth provide any benefit to the squid?
A: I appreciate that question and all of the squid concern questions that have been posed. My understanding of this is that the thermal conductivity switching ability is a consequence of the mechanical and self-healing properties of the squid ring teeth. So kind of from a molecular sand point and eons of evolution of the squid, they've created the ability for these regenerative fingernails. So that was a reaction that occurs in the water gives rise to the remarkable mechanical properties and as a consequence the thermal conductivity and energy probably said we measure are from the atomistic mechanical properties. So I think the question is kind of saying does the switching thermal conductivity of the squid help cool its talon OR squid ring teeth, no, I don't think the thermal conductivity is part of the evolution of the squid, but the mechanical properties and self-healing properties that was the evolution of the squid ring teeth and a thermal conductivity is just a byproduct of the mechanical nature. Of course, you know thermal properties and materials are intimately related to their mechanical properties, so it's kind of combined.
Q: This next question was also related to the protein but I think it can expand to the other members of the panel as well: Can you speak to the scalability and manufacturing of thermal energy storage approaches? In Patrick's case proteins for Kyle and Navin salt hydrates, Suman some of the thermochemical approaches and some of the composite work as well.
A: I have a similar question that I tried to answer in the chat earlier on but really what we're trying to look at here is we are projecting at scale will be able to produce fees at under $15 Kilowatt hour we will be able to produce 250,000 L at a volumetric fermentation scale. Currently part of the program is really doing the techno-economic analysis of how we can scale up to that.
This is Kyle. Briefly on the salt hydrates, in terms of the materials and processes we're focusing on, we are very cognizant of what the price might be at scallop and we're trying to avoid processing techniques that would be difficult to scale up. So like Navin showed one of our composites, we project about 5 to $6 per kilowatt-hour. Now, that would be some processing costs on top of that just for the raw materials you need, but I think bottom line is that scalability is very good or the salt hydrate techniques.
This is Suman. I think that for TCM materials we are focusing on the low-cost materials. We are keeping the cost in mind both for the raw material and the scalability so it will be similar to the phase change material using salt hydrate. So cost is not the issue as long as we can scale the properties of the material to the reactor level and system-level.
I’ll put in just a quick word from the composite design side. I think part of the reason of going into this is that the cost of integrating some things like medals and graphite to get heat in and out of the composite can be a significant portion of the overall cost. So one of the things you want to know is how much do I actually need for a particular application. As Allison's talked to got to a lot of this depends on how implementing it into the system so if you're thinking about just creating a larger surface area you can bump up your flux rate that way but there are trade-offs with different morphologies versus putting things like conductive elements in there. Again, in the cases were looking at we're looking at putting this into something that vaguely looks like a heat exchanger so we're trying to make cost kind of comparable and comparable to those components.
This is Kyle again, one more comment regarding scaling. it's really important when you look in the literature there's a lot of information out there of people dealing with milligram quantities especially with people with salt hydrates. Scaling is also important in this respect. If you test something with DSC in a calorimeter where you are using a few milligrams of material you can get very, very different properties than when you scale it up even just a gram scale like we would measure any temperature history device, so scalability also applies in this sense and we're very cognizant of that. I think in general, everyone in the thermal energy storage and phase change material space needs to be really aware and take a critical look at how something was measured not just what number was reported but also what scale it was measured at because the milligram is a completely different story than gram or kilogram scale.
Q: The next question, more of a comment, is: Thermal energy storage it's based on cost per energy or dollars per kilowatt-hour and does not account for the value proposition of improve thermal properties such as thermal conductivity. It would be helpful to add another cost target around a power density such as dollars per kilowatt. I guess I'm just wondering if any palace have boxer comments how does designing these composite systems for improved power density if you have any appreciation of how that impacts the cost of these systems?
A: I expect Patrick would have a longer answer to this, but I just want to mention briefly that we're working on an analysis of how much material you need to use in a heat exchanger depending on the thermal conductivity, and the question is very pertinent and well-posed. And I hope to be able to provide some more quantitative answers to this sometime in the near future.
Sure, I can come in quickly, and I'd love to hear what Allison has to say her side, as well. Ultimately when you think about cost per rate, so much of it depends on the physical implementation, how are you trying to absorb heat out of an airstream or a fluid. And that totally controls what the how you distribute being the material relative to this heat transfer medium. So my answer probably won't be satisfying because it depends, ultimately when it comes down to cost, the governing thing you want to consider is what you're doing with this. If you're trying to displace power from a peak load case. So maybe one portion of the day to another portion of the day. Then ultimately your system needs to store sufficient energy and do it at an adequate discharge rate so both of those things are dictating I would have to be distributed, and then ultimately your economics are based on the cost savings by shifting that electrical load. So I think that we would say that for any particular application you really need to start from the perspective of what do I need to do, how am I trying to displace this heat, and then evaluating things like the cost. For example I'm putting this in some kind of heat exchanger and I know I have a specific rate of discharge that I need to make, then we can kind of change a performance metric into cost per total energy stored by the system whereas we dictated not just by the material were using but by how much graphite or metal or whatever I'm putting in the system to get heat effectively in or out. So again I know that's not super satisfying but maybe Allison can add.
Yeah, I agree with everything Patrick says. I think in general you really have to look at your application and what you need to be able to get out of the system so something that we're looking at right now is for building applications. If you run this in a building and look at different building load profiles and then also look at the rate structures in different places, how much money or what is your economics of moving or shifting your load two different times of the day. That will give you a lot of information about what kind of cost you need for the entire system, which will trickle down to the material cost. So looking at building load profiles it will tell you what kind of rates you need to hit, what kind of C rates you need for your thermal storage system, and then how much can you get out of that from an economic for the building operator. So I think it's a complicated question and something that needs to be looked at in a lot of detail for a given application. For HVAC or hot water heating and building how much does it matter based on your location in the country, as well as kind of the magnitude differences between peak and off-peak.
Q: Does DOE have any plans to enable adoption of PCMS demonstrations and pilots?
A: I can't speak to any concrete plans, but I can mention again that RFI for the Energy Storage Grand Challenge road map. Part of that Grand Challenge is looking at scale up issues, and if you have thoughts or on what would be needed to better commercialize these energy storage technologies I definitely encourage you to submit a response to that request for information, in regards to be always current plans and future activities.
Q: Another question we have is: Can we have a system with a combination of PCM module, sandwich fast discharge characteristics like heating a room from a cold condition, and once the wrapping is done can we get a module with controllable discharge?
A: I can jump in a little bit there. This is already done in the electric chemical batteries space, having high power systems and high energy systems which are generally designed differently. Kind of coupling these two things so that you can get fast discharge but you can also do long duration lower discharge. I think that if this is something that is pretty important for building spaces or could be pretty important if you tried a couple of these two things together. Those could be two different systems that live beside each other or trying to integrate them into a single one which would be a little more complicated, but I think there's some inspiration you can get for doing this type of thing from electrochemical batteries and I agree that I think it is a space that could be explored further.
Q: Another question we have is what is the role of PCMs for seasonal energy storage especially for space heating and cooling. Would this be cheaper in comparison to borehole energy storage?
A: I see a question related to thermochemical energy storage in buildings, so I will pitch in for that so others can jump in. So it's a very good question. Again, the thermochemical materials will you send buildings. In principle yes as long as you can have the charging temperature which are suitable for buildings we need something which can charge at less than 100 degrees C where we can use solar heat to charge it and then we need to have a discharging temperature which is suitable for building applications. Can use it for space Heating, can we discharge at 35 C can we discharged it for the hot water. So I think it principle yes the answer is we can use thermochemical materials for this, the question is always how can we come up with new materials with your stable can we have the psychic ability that we can have a better utilization because if you look at the research which is done in this space it's mostly limited or limited number of cycles because we are looking at TCMS for seasonal storage. For example they want to charge during summer and discharge during winter. So all you need is 10-20 cycles. If you can make the material and system work for 20 cycles, that's good enough. And here if you look at the BTO targets we're talking about 20 years. I want to cycle it every day, so I think that it's with a challenge for the TCM materials will be, and others conviction for PCM.
Yeah, I should comment really quickly. Nelson, can you just remind me with a question asking about seasonal storage? Yes, essentially. So the economics of the seasonal storage problem are pretty challenging and again this is just like the previous peak was talking about. You want to think about what is the payback of the storage unit and if you're using it to charge and discharge every day or even every day for half a year for the hot part of Summer, then that total energy that you're storing adds up pretty quickly and you can get paybacks hopefully in a few years or less. if you're only charging and discharging once per year then basically the cost of the total energy storage would have to be comparable to the cost per Btu to you of that total amount of energy and that's a very challenging number to hit. I think that would be the main limitation for seasonal storage.
I would add to that, we are working on this matrix, what makes sense for thermal energy storage. So I would echo what Patrick is saying that utilization is a big factor. If you can only utilize once a year then the thermal storage doesn’t make sense. But if you can utilize if you increase the utilization factor the cost of the storage comes down dramatically. It is very important that we can cycle and make it more diurnal and we can use it as often as possible you bring the cost of a storage down.
Q: Has anyone been examining the use of a biochar as a matrix given its natural open structure?
A: I can respond really quickly to that, I'm not sure about that particular thing. The ultimate use would largely depend on how to make conductive it is. For something like a biochar it will probably come down to have porethitic is the residual carbon. I don't know the answer to that question but if it's a fairly porethitic carbon then I don't see limit to be used to that. If it's more of a amorphous carbon and it's thermal conductivity is probably not as high as you would like but if it's low enough cost then the PCI material then it could be helpful. The thermal transport properties of that particular substance.
I agree, and I think that one issue when we talked about amorphous carbon or even worth it at carbon or some mixture is that you have thermal conductivity has ranging two orders of magnitude depending on the structure that is a specific problem to address. At ORNL we used compressed expanded graphite and we've also used lignin derived carbon and they both are promising in terms of thermal conductivity as Dr. Hopkins mentioned it really depends on your density you can dial-in however much thermal conductivity you want by choosing how dense of a carbon matrix to use and obviously the density go, the less volume you have left for PCM on material so there is a trade-off there. One of the problems with the ligament the ratchet materials is that they really have inherent structure so you have to basically infiltrate the existing inherit structure with the phase change material and the infiltration can be a challenge.
One more quick comment on the poorest carbon issue, especially on the expanded graphite, one of the reasons that it can work very well as that you can align these graffiti sheets in a particular way and so you can have very anisotropic thermal properties and you can push heat away from a surface that is close to a heat transfer fluid, and that's not always true for all porous carbon materials.
Q: Another question for thermochemical storage: What is the limiting factor for the recycling time?
A: The limiting factor as mention all the challenges with the material properties. You need to have your material active; as long as the material is active and reaction is going reversibly there is no limiting factor. It can go on it can destroy charge and charge. the problem is that especially with absorption when the reaction with the whole structure changes during the hydration and dehydration reaction, you need to have that structure to be stable volume expansion. You need to have materials stay stable during reactions. You don't want it to polarize or disintegrate. You don't want to hear structure to go from crystalline, which can actually incorporate all those water molecules. If it goes from that crystalline structure to amorphous you have lost your energy don't see that as an active material. You need to have very good synergy between heat and mass transport PCM you care about heat transport but you do not know vapor phase from material. In this one you have high heat transport when you have a particle but you have lower mass transport, what's going to happen is instead of that solid gas reaction you're just going to melt your material. Now you'll have phase-change but you won't have a thermochemical material. all of these issues lead to less cycling of the material. To maximize the rate at system level you need to optimize it for 20 cycles, even at material Level it starts to degradation. I see another question on TCM.
Q: I see a question that says, what defines the theoretical density of the thermochemical material?
A: It depends upon the type of material. if you have a salt hydrate, the total enthalpy Delta H is defined by the water capacity. So let's say that it hydrates, let's define that water hydrate by N, so your total theoretical density will be N times Delta H. if you have hydroxide then the […]. is largely determined by the [….] so depends upon what material class you are looking at, that defines the theoretical energy density of that material.
Q: In regards to tunable materials like the squid teeth that Patrick was looking at, it requires water to change its conductivity. How will this water be applied to trigger the switch in an actual application?
A: If you've seen the paper, you'll see the demonstration of the switch on the lab scale is done manually. it was manually pipetting or could be a water pump on a lab scale, but the application that we’re thinking for these would involve it being in some kind of heat exchanger air handler where we'd have a moisturizer plenum that be blowing humid air or saturating the SRT PCM in the device. I will mention that part of this benefit program we have the opportunity to explore that exact design because that would have a lot to do with the efficiency.
Q: Beside material discovery for TCM materials, are there any similar research on searching for other new phase change materials like solid
A: The answer is yes, a resounding yes. I would acknowledge BTO's efforts to look at the fundamental and look and new materials. We are currently LBNL and NREL working on new solid-state PCMS and not just solid to solid phase change, which have high enthalpy change, but we’re also looking for can we have dynamic to tunability. This goes back to the question where people were talking about seasonal, from moving away from seasonal we want to have a PCM which is not only solid to solid phase change with high energy density also has a tunable transition temperature that in summer we have one transition temperature and in winter we have another so that one PCM can solve year long. I hope that answer that question on solid-state PCMS.
Can I jump in on that one as well, just because we're working on this topic under the BTO program as well. There was a slide on the very first presentation showing some of the limitations, The limited salt hydrate compositions that were known within particular temperature ranges of Interest. We're actively engaged in developing eutectic systems, particularly in nitrate and chloride eutectics are targeting some of these absent regions. There’s lock out there in terms of theoretical predictions for eutectic salt hydrates and there is a healthy material space to explore.
I want to thank all of the panelists for speaking today. It was very insightful and we had a very fruitful discussion. So this is the final webinar in our thermal energy storage sequence, in our thermal energy storage webinars. I want to thank you all for attending and I want to encourage you to stay tuned and keep on looking out for future opportunities to get involved with a BTO as we try to further explore and push the boundaries of thermal energy storage capabilities for the built environment and with that thank you all for joining and the slides will be posted to the BTO website in the future.