ASHRAE Journal - February 2009 - 69

Heat Pumps for Cold Climates By Kurt Roth, Ph.D., Associate Member ASHRAE; John Dieckmann, Member ASHRAE; and James Brodrick, Ph.D., Member ASHRAE he majority of air-source heat pumps (ASHPs) are installed in moderate to warm areas of the United States, mostly south of the Mason-Dixon Line. Two major issues make conventional ASHPs unattractive as a heat source in cold climates. First, buildings in colder climates often have appreciably smaller design cooling loads than design heating loads. Second, ASHP heating capacity and coefficient of performance (COP) decrease as the outdoor temperature, To, decreases because the temperature lift across the compressor increases. For these reasons, conventional ASHPs sized to meet cooling loads cannot meet the full heating loads at lower To, necessitating extensive use of costly, and energy-inefficient, electric resistance heating. Conceptually, a heat pump designed for a cold climate would have sufficient capacity to meet heating loads around To~0°F at a reasonable COP, and would require limited (ideally, no) electric-resistance heat on an annual basis. Such a design would enable effective use of heat pumps in much colder climates than current designs. For example, it could extend the region where heat pumps could reduce heating primary energy consumption or cost over much of the northern U.S. Several design modifications and technologies have been proposed or introduced (alone and in combination) for coldclimate heat pumps. • Sizing the ASHP for heating instead of cooling can increase capacity. It does not, however, address the problem of reduced heating cycle efficiency and capacity as To decreases. It also can lead to excessive cycling—and the resulting efficiency decrease—at moderate heating loads and during cooling season (since the system is oversized for those conditions).1 Excessive cycling can, in turn, significantly decrease dehumidification effectiveness.2 • Multiple or modulating compressors address mismatched loads by sizing compressor capacity to meet heating design loads at full capacity, while part-load operation efficiently satisfies cooling loads and dehumidification.2,3 The problem of reduced heating cycle efficiency as To decreases still remains.4 • Geothermal heat pumps, also known as ground- or watercoupled or ground- or water-source heat pumps (GSHP), overcome the problem of reduced cycle efficiency in cold ambient air by extracting heat from the soil or groundwater at approximately constant underground temperatures. In theory, they can achieve near-constant heating February 2009 T and cooling efficiencies year-round. In practice, in colder regions the quantity of heat extracted from the ground (during the heating season) is larger than that rejected to the ground, which depresses the ground temperature around the loop and GSHP efficiency. Still, effectively designed GSHPs in the northern U.S. can achieve average heating COPs on the order of 3 (including pumping power).5 • Increased outdoor coil capacity enables the ASHP extract more heat at a given To. • Carbon dioxide (CO2 ) refrigerant cycles that exploit the thermodynamic characteristics of CO2 to provide about 35% greater capacity at To = 17°F. This decreases the use of electric resistance heating and also significantly reduces system oversizing relative to the design cooling load. CO2 also rejects heat over a wider temperature range, enabling higher air delivery temperatures without thermodynamic penalty.6 • Mechanical liquid subcooling increases capacity (~10%) and efficiency (~5%). • Optimization of the indoor and outdoor coil circuiting for heating mode also could enhance capacity and efficiency. Within the past five years, at least two U.S. companies have introduced ASHPs designed for cold climates. Units developed by both companies have two compressors that operate in series. At moderate To, a single compressor (with multiple capacities) operates to meet the heating loads. When To falls below the point where the single compressor can effectively meet the heating load, a second “boost” compressor with a large volumetric capacity also runs. In one unit, the refrigerant exiting the first compressor enters the second compressor.7 The other unit has a heat exchanger that transfers heat between two vapor-compression cycles, i.e., the condenser of the first cycle serves as the evaporator of the second cycle, but each cycle uses separate refrigerant.8 Both configurations decrease the lift of both compressors, increasing their efficiencies and capacities. In addition, two-stage units can incorporate a refrigerant economizer that expands a portion of the liquid refrigerant leaving the condenser to a pressure between that of the evaporator and condenser (i.e., the inlet pressure of the second compression stage). This expanded refrigerant accepts heat from the liquid refrigerant, subcooling the liquid prior to expansion, and further increasing cycle capacity and efficiency. ASHRAE Journal 69

ASHRAE Journal - February 2009

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