The particles of any physical system above absolute zero temperature are always moving. The higher the temperature—that is, the hotter the system—the more energetic the particles’ motion. As the particles interact with each other, their random motions tend to redistribute the energy more and more equally among themselves all the time, since there are more ways to distribute energy more evenly than there are to distribute it less evenly. This means that the system’s temperature becomes more and more uniform throughout—warmer regions tend to cool, and cooler regions tend to warm, as their particles’ random motion flows from where the temperature is higher to where it is lower. Heat flow in the opposite direction, from lower to higher temperature, is so unlikely on a large scale that in practice it never happens on its own. But it is possible to make heat flow out of lower-temperature regions into higher-temperature regions, if some additional energy is put into a machine that forces the heat to flow that way, using a fluid or other medium that is made to absorb heat from the cooler region and circulate to emit it into the warmer one.
This kind of warming, using machines that reverse the spontaneous flow of heat in this way (heat pumps), contrasts with such heating methods as burning fuel or running electric current through a resistor. Without a heat pump, all the heat added to the warm region comes directly from the combustion or the electric current, while the energy of random particle motion in the cool region stays pretty much where it is. With a heat pump, only some of the heat comes from the energy input while the rest is energy transferred from the cool region. If a given heat pump’s performance is close enough to the theoretical maximum, its energy input for the same heat output will be less than the heat generated by more direct methods.
Two common uses of heat pumps are in water heaters and in heating and air-conditioning systems. In a heat-pump water heater, heat is made to flow from the surrounding air into the water tank. In a heat-pump heating and air-conditioning system for a building, heat is either pumped from the cold outside into the building to heat it, or else from the inside to the outside to cool the building. One problem in designing a heat pump for either a building system or a water heater is the accurate estimation of how the air- or water-temperature requirements will differ at different times, since they vary in irregular ways that depend on their users’ activities. Also, since heat pumps require an energy input to make the energy of particles’ random motions flow from cooler to warmer regions, they cost something to run, which may be more or less than what other methods of heating and cooling cost. Recent research sponsored by the Department of Energy addresses what conditions make heat pumps superior or inferior to other heating and cooling devices, and which designs cost least and use less energy.
The report “Measure Guideline: Heat Pump Water Heaters in New and Existing Homes”[Information Bridge] from Steven Winter Associates, Inc. of the Consortium for Advanced Residential Buildings describes some experiments conducted with heat pump water heaters in homes. According to the results, heat-pump water heaters that use electrical resistors to back up the heat pump do offer energy and cost advantages in warmer regions of the US. In other parts of the country with different climate, the advantages are with purely electrical-resistance heaters or oil-burning and gas-burning heaters. A heat-pump water heater also requires certain operating conditions, including an ambient temperature above 50 °F, a floor able to support the heater’s weight, a condensate drain in the floor, and room volume with minimum clearances around the heater. At present, heat-pump water heaters initially cost more than heaters of other types, so to offer a cost advantage their operating costs have to be low enough to more than offset the initial cost.
The way local climate affects heat-pump water heaters is dealt with in further detail in the National Renewable Energy Laboratory (NREL) report “Heat Pump Water Heater Technology Assessment Based on Laboratory Research and Energy Simulation Models”[Information Bridge]. Whereas the findings described in the previous report were from water heaters tested in homes, the tests described in this report were conducted in a laboratory to represent conditions in different US climate regions. The five integrated heat-pump water heaters tested were found to be efficient under most of these conditions. The water heaters’ storage tank sizes and control schemes were found to have significant effects on the heaters’ energy efficiency and hot-water delivery in high-demand situations. A heater’s recovery time and operating range also need to be appropriately balanced. The report’s authors find that in general, heat-pump water heaters are very promising as an energy-saving technology.
Field tests and laboratory experiments can tell us whether heat pumps are more efficient than other old or new methods of heating water under given conditions, but experimenting to determine how heaters’ performances depend on the many variables that affect them requires much time and equipment. If the significant parameters are accurately represented in a mathematical model based on the laws of thermodynamics and fluid mechanics, calculations can be used instead of experiments to show which type of heater will use the least energy or cost least to run.
The advent of heat-pump water heaters affects the potential market for other types of water heaters, particularly solar water heaters. Mathematical comparison of these two types is made in the NREL report “Low-Cost Solar Water Heating Research and Development Roadmap”[Information Bridge] to determine which segment of the market is best suited for heat-pump water heaters, so that development of innovative, low-cost solar water heaters can be focused where the largest market opportunities exist for them. The mathematical model used for this report indicates that electrical-resistance water heaters would use significantly more energy than both heat-pump and solar water heaters anywhere in the US, so either heat-pump or solar heaters would offer an advantage over electrical-resistance heaters; on the other hand, natural-gas water heaters would use more energy than heat-pump water heaters only in southern regions of the US, and more energy than solar water heaters anywhere in the US. The US region of solar heaters’ superiority to natural gas heaters corresponds closely to where freeze conditions are likely at some point during the year. The report’s authors thus conclude that “the focus of the [solar water heater] activity should be on innovative low-cost solutions with adequate freeze protection and applicability to cold climates. Such systems could be used in all U.S. locations; however, they need to be optimized for cold climates such that they yield sufficient savings to be worthwhile, and such that freezing conditions are not a concern for long-term durability in the target market.” They also note that the Energy Department and the National Renewable Energy Laboratory have already developed simple and effective solar water heaters for regions that don’t experience freeze conditions.
A more detailed mathematical model of heat-pump water heaters, designed to account more accurately for several factors (e.g., how the water temperature varies with height inside the heater, the discontinuous way water is drawn from the heater, the variation in heating output with inlet water temperature, and how the heater behaves when it affects and is affected by its particular environment and that environment’s heating and cooling system), is described in the NREL slide presentation “Heat Pump Water Heater Modeling in EnergyPlus”[Information Bridge] and fact sheet “NREL Develops Heat Pump Water Heater Simulation Model”[Information Bridge]. The new model’s simulation of heat-pump heaters’ energy use turns out to yield figures within 2% of what lab experiments indicate. Like the earlier mathematical model used for the aforementioned report on solar water heaters, the new model predicts that heat-pump water heaters would use less energy than electrical-resistance water heaters anywhere in the US, but less energy than natural-gas water heaters only in southern regions of the US.
There are also mathematical models for air conditioning that are used to determine the most cost-effective air conditioning system for a given building. For input data, these models use rated values of air-conditioner parameters such as capacity and efficiency together with performance curves that show how these values vary with environmental conditions. An investigation into improving these models, reported in the NREL slide presentation “Improving Air-Conditioner and Heat Pump Modeling”[Information Bridge], shows that the simulation of an air conditioning system’s annual performance is affected little by what performance curve is chosen for input. On the other hand, the Seasonal Energy Efficiency Ratio (SEER)[Wikipedia] of high-efficiency equipment is generally lower for higher-tonnage units. These considerations led to a new mathematical model of air conditioning systems in which their Seasonal Energy Efficiency Ratio and tonnage are combined with specific products’ performance ratings to estimate how a given system will perform.
How to determine what size a house’s heating, ventilation, and air conditioning system needs to be is also addressed in the report “Strategy Guideline: HVAC Equipment Sizing”[Information Bridge] from IBACOS, Inc. This report emphasizes that the calculation of a house’s heating and cooling load is just the first step in an iterative process to design the house’s HVAC system; the remaining steps are to select cooling and heating equipment and to design the ducts. Once a duct system configuration and preliminary design are completed, achieving the airflow for the room-by-room heating and cooling loads may require selecting different equipment. The report describes the selection process for two example houses, one a split-system air conditioner and furnace in Chicago, Illinois, the other a heat-pump system in Orlando, Florida.
Variable-capacity heat pumps offer the ability to vary compressor speed and output capacity to match the load on a house and thus reduce cycling losses, and also offer the ability to provide higher heating capacities at lower temperatures by running the compressor at higher speeds. The Oak Ridge National Laboratory report “Performance of Variable Capacity Heat Pumps in a Mixed Humid Climate” [Information Bridge] describes a recent experiment with variable-capacity heat pumps installed in two research houses in Knoxville, Tennessee, that ran into problems which precluded conclusive results. On the other hand, an innovative laboratory approach to testing one type of variable-capacity pump (mini-split heat pumps) is described in the NREL report “Laboratory Test Report for Fujitsu 12RLS and Mitsubishi FE12NA Mini-Split Heat Pumps”[Information Bridge]. As summarized in the fact sheet “NREL Documents Efficiency of Mini-Split Heat Pumps”[Information Bridge], National Renewable Energy Laboratory researchers worked with colleagues at Purdue University's Herrick Labs and Ecotope, Inc. to refine and apply this new approach to a suite of mini-split heat pump products. From performance tests of two mini-split heat pumps across a variety of operating conditions, researchers found that both heat pumps achieved manufacturer-reported performance at rating conditions, but at other temperature and humidity conditions their capacity ranged from 40% above to 54% below the manufacturer-reported values. The fact sheet notes the importance of the mini-split heat pump’s coefficient of performance significantly exceeding the rated efficiency at low compressor speeds.
Investigations related to ground-source heat pumps, which pump heat into or out of the ground instead of the outside air, are described in three recent reports. “In-Depth Look at Ground Source Heat Pumps and Other Electric Loads in Two GreenMax Homes”[Information Bridge], from Steven Winter Associates, Inc. of the Consortium for Advanced Residential Buildings describes the performance of ground-source heat pumps (among other things) in two GreenMax demonstration homes in Wisconsin belonging to WPPI Energy. The ground-source heat pumps for both homes proved to have lower coefficients of performance for heating and lower energy efficiencies for cooling than the heat pumps’ ratings.
Ground-source heat pumps use less energy than other systems in the northern and central US where heating needs are either dominant or are comparable to cooling needs. For climates in which cooling needs dominate heating needs, hybrid ground-source heat pump systems have been developed that use additional heat sinks to reject excess heat. However, comprehensive analysis of the benefits of hybrid ground-source heat pumps has been limited and often non-conclusive. Results of a project by investigators at the University of North Texas and Florida International University with others to gather and analyze cost/benefit data on ground-source and hybrid ground-source heat pumps in southern states are presented in “Analysis of Energy, Environmental and Life Cycle Cost Reduction Potential of Ground Source Heat Pump (GSHP) in Hot and Humid Climate”[Information Bridge]. The report notes among other things that despite ground-source heat pumps’ often having higher initial costs than conventional systems, significant potential exists for their wide application in hot/humid climates, especially where they can be used in combination with solar energy. The report concludes that “the application of [ground-source heat pumps] in Florida (and hot and humid climate in general) shows a good potential”.
A recent report (“Recovery Act - Geothermal Technologies Program: Ground Source Heat Pumps Final Scientific/Technical Report”[Information Bridge]) from the TRAK International division of Harris Companies provides information on the design and construction of a geothermal ground-source heat pump system that was used for both water heating and space heating/cooling—and also ice making on a very large scale—at the Civic Arena of Eagen, Minnesota. Whereas most geothermal heat pumps heat and cool water and air to moderate temperature ranges, the Eagen Civic Arena system’s staged heat pumps heat and cool water/glycol streams to temperatures ranging below 15 °F (~ -9 °C, or 264 K) and above 150 °F (~ 66 °C, or 339 K). As of May 2012, the system had been operating successfully for over a year without significant technical problems.
Heat pumps are considered as one possible component of a heating and cooling system in “A Feasibility Study: Ductless Hydronic Distribution Systems with Fan Coil Delivery”[Information Bridge] by the Alliance for Residential Building Innovation. The study finds that hydronic systems, which use water in pipes instead of air in ducts to transfer heat[Wikipedia], can reduce annual heating and cooling energy use by up to 22% when substituting pipes, pump, small distributed fan coils, and a water-to-air heat pump for the ducts, air handler, indoor coil, and conventional air-to-air heat pump unit of similar rating as the air-to-water unit. From use of the same mathematical modeling software that was used for “Low-Cost Solar Water Heating Research and Development Roadmap”[Information Bridge], hydronic distribution appears to be economically viable in three of the four climates evaluated. The report notes that the lack of low-cost, small, ceiling-mounted fan coils, and the limited availability and higher cost of air-to-water heat pumps, are major barriers to hydronic systems’ widespread application, and that the cost and limited availability of air-to-water heat pumps could be overcome by producing an add-on refrigerant-to-water conversion kit to adapt any heat pump to serve as a water chiller-heater.
Steven Winter Associates, Inc.
Consortium for Advanced Residential Buildings
National Renewable Energy Laboratory
University of Colorado, Boulder
Oak Ridge National Laboratory
University of North Texas
Florida International University
ClimateMaster Geothermal Heat Pump Systems
Florida Power & Light Company
Oak Ridge National Laboratory
Gulf Power Company
Egg Geothermal Air Conditioning and Pool Heating
Alliance for Residential Building Innovation
City of Eagen, Minnesota
Prepared by Dr. William N. Watson, Physicist, DOE Office of Scientific and Technical Information