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The University of South Florida, under the Baseload CSP FOA, developed a thermal energy storage system based on encapsulated phase change materials (PCM) that meets the utility-scale baseload CSP plant requirements at significantly lower system costs.

Approach

Previous thermal energy storage (TES) concepts cost about $27 per kilowatt hour thermal (kWht). The University of South Florida proposed a TES system concept that could potentially reduce the cost to as low as $3.54/kWht to make it competitive with fossil fuels and allow for a capacity factor increase to 75% or greater. Specific objectives included:

  • Developing an encapsulated PCM for thermal energy storage in the temperature range of 300°–450°C with a cyclic performance (charge/discharge heat transfer) capability of at least 1,000 cycles
  • Designing a one-tank storage system using spherical (or other shape) PCM capsules immersed in synthetic oil with an integrated heat exchanger for charge/discharge testing.
Proposed TES system with packed-bed configuration.

Innovation

The University of South Florida's project employed a latent heat storage system that used a phase change material (salt) encapsulated by a shell (metal) to overcome the barrier of low thermal conductivity. Capsules were placed in direct contact with the heat transfer fluid to achieve the highest heat transfer rates. There were two major challenges to the success of this concept:

  • Forming porous macrospheres of the PCM material at optimum size and pore volume to account for the volume change from solid to liquid phase
  • Encapsulating the macrospheres of PCM in a higher melting temperature material, preferably a metal, at low cost.

The research team also developed a two-dimensional numerical model to look at diffusion-natural, convection controlled heat transfer in an encapsulated sphere.

Conclusion

This project successfully developed encapsulated PCM with provision for expansion/contraction during melting/freezing: 

  • Characterized the PCMs of interest 
  • Tested capsules for thermal cycling 
  • Exceeded the 1st Phase milestone of 50 cycles 
  • Successfully completed 200 cycles (continuing) 
  • Developed a numerical model for melting/solidification 
  • Developed a manufacturing plan for capsules 
  • Cost estimate of a TES system based on the developed PCM capsules – $14/kWhth 
  • Preliminary discussions with companies on further testing and development for commercialization 

Final Report

Goswami, D. Yogi. Development and Demonstration of an Innovative Thermal Energy Storage System for Baseload Power Generation. No DE-EE0003590, 2012. doi:10.2172/1049940. 

Publications, Patents, and Awards

Learn about other DOE competitive awards for concentrating solar power research that are in progress.