The Federal Energy Management Program (FEMP) provides acquisition guidance for geothermal heat pumps, which are an ENERGY STAR-qualified product category. Federal laws and requirements mandate that agencies meet these efficiency requirements in all procurement and acquisition actions that are not specifically exempted by law.
Most manufacturers display the ENERGY STAR label on complying models. For a model not displaying this label, check the manufacturer's literature to determine if it meets the efficiency requirements outlined by ENERGY STAR.
Meeting Efficiency Requirements for Federal Purchases
Visit the ENERGY STAR website for the most up-do-date geothermal heat pump efficiency levels and product specification information, and a list of qualified geothermal heat pumps.
Buying Energy-Efficient Geothermal Heat Pumps
When contracting or buying from a commercial source, specify or select ground-source heat pumps with the ENERGY STAR label. Visit the ENERGY STAR website for a list of complying models.
These requirements apply to all forms of procurement, including guide and project specifications; construction, renovation, repair, energy service, operation and maintenance (O&M) contracts; lease agreements; and solicitations for offers. Energy performance requirements should be included in all evaluations of solicitation responses. Buyers shall insert the standard clause from FAR section 52.223-15 into contracts and solicitations that deliver, acquire, furnish, or specify energy-consuming products for use in Federal facilities. Agencies can claim an exception to these requirements through a written finding that no ENERGY STAR-qualified or FEMP-designated product is life-cycle cost effective for a specific application.
The technical feasibility of geothermal heat pumps depends on the availability of geothermal resources and the specifics of the application. Given an ample supply of groundwater (and an acceptable means of disposing of it), an open-loop system may be a viable option. Such systems usually include a plate heat exchanger to transfer heat between the groundwater and a common water loop inside the building. Zone heat pumps exchange heat with the common loop. Surface water from lakes and streams can also be used in an open loop system, but applications are usually limited to warmer climates or to cooling-only applications in colder climates.
In closed-loop systems, the earth itself can be used as the heat source and sink by way of vertical or horizontal ground-coupled heat exchangers. Most large systems use vertical heat exchangers, which consist of polyethylene u-tube pipes in deep (typically 150-250 feet) boreholes. Horizontal loops require more land area, but are usually less costly to install, depending on the types of soil and rock formations encountered at the site. Closed loops can also be located in lakes, ponds and other bodies of surface water.
There are various types of geothermal heat pump systems. Hybrid systems using several different geothermal resources, or a combination of a geothermal resource with outdoor air (e.g., a cooling tower), are other technology options. Hybrid approaches are particularly effective where cooling needs are significantly larger than heating needs. Where local geology permits, the "standing column well" is another option. In this variation of an open-loop system, one or more deep vertical wells are drilled. Water is drawn from the bottom of a standing column and returned to the top. During periods of peak heating and cooling, the system can bleed a portion of the return water rather than reinjecting it all, causing water inflow to the column from the surrounding aquifer. The bleed cycle cools the column during heat rejection, heats it during heat extraction, and reduces the required bore depth.
The installed cost of geothermal heat pump systems can be somewhat higher than that of conventional space conditioning equipment, but this depends on a number of factors, including the particular geothermal resource to be used, and whether the project involves new construction or renovation of an existing facility. For new commercial applications, the installation cost of a well-designed geothermal heat pump system is competitive with the cost of most conventional alternatives. Even in applications where geothermal heat pumps have higher first costs, however, their life cycle cost is usually lower than other alternatives, given their substantially lower energy and maintenance costs.
Because geothermal heat pumps may be more costly to purchase than more conventional systems, direct procurement may be problematic. FEMP has a variety of programs that allow Federal facilities to leverage available resources with private financing to fund energy conservation measures, including geothermal heat pumps.
When selecting ground-source heat pumps, choose models that qualify for the ENERGY STAR label, all of which meet FEMP recommendations. Alternatively, specify a coefficient of performance (COP) and energy efficiency ratio (EER) COP and EER that meet the recommended levels. Since GSHPs are an inherently efficient technology, the FEMP and efficiency thresholds include the great majority of models for sale. Models with efficiencies that substantially exceed these levels, however, are widely available. The most efficient models, though, generally involve dual compressor systems and increased heat exchange area, and thus cost significantly more.
A proper assessment of the building's peak heating and cooling loads is critical to the design of a GSHP system. As with all heating and cooling equipment, oversizing of geothermal heat pumps, besides raising purchase cost, will result in decreased energy efficiency, poorer humidity control, and shorter product life, all due to excessive on-off cycling.
Accurate knowledge of the properties of the geothermal resource is also crucial in the design of a geothermal heat pump system. For ground-coupled systems, important parameters include the thermal conductivity and temperature stability of the soil formation. In larger installations, these properties are often measured directly in short-term tests at one or more locations on the site. Because ground heat exchangers represent a significant portion of the cost of these types of systems, it is important to size ground loops accurately. Software tools for ground loop sizing are available from a number of vendors.
The design of groundwater systems depends on several properties of the water source, including temperature, well flow rates, and water quality. Surface water systems, whether open or closed loop, depend on the temperature profile of the surface water body (through all seasons, as this may vary significantly).
Finding More Information
Federal supply sources are the U.S. General Services Administration (GSA) and Defense Logistics Agency (DLA). GSA sells products through its Multiple Awards Schedules and GSA Advantage! DLA offers products through the Defense Supply Center Philadelphia and DOD EMALL.
Lawrence Berkeley National Laboratory provided supporting analysis for this acquisition guidance.