1、Dr. Bo Shen, Dr. Omar Abdelaziz, Dr. Keith Rice and Mr. Van Baxter are Research and Development Staff in Building Technologies Research and Integration Center, ORNL, TN, USA. Mr. Hung Pham is an engineering leader in Emerson Climate Technologies. Cold Climate Heat Pumps Using Tandem Compressors Bo S
2、hen, PhD Omar Abdelaziz, PhD C Keith Rice, PhD Member ASHRAE Member ASHRAE Member ASHRAE Van D. Baxter, P.E. Hung Pham Fellow ASHRAE Member ASHRAE ABSTRACT In cold climate zones, e.g. ASHRAE climate regions IV and V, conventional electric air-source heat pumps (ASHP) do not work well, due to high co
3、mpressor discharge temperatures, large pressure ratios and inadequate heating capacities at low ambient temperatures. Consequently, significant use of auxiliary strip heating is required to meet the building heating load. We introduce innovative ASHP technologies as part of continuing efforts to eli
4、minate auxiliary strip heat use and maximize heating COP with acceptable cost-effectiveness and reliability. These innovative ASHP were developed using tandem compressors, which are capable of augmenting heating capacity at low temperatures and maintain superior part-load operation efficiency at mod
5、erate temperatures. Two options of tandem compressors were studied; the first employs two identical, single-speed compressors, and the second employs two identical, vapor-injection compressors. The investigations were based on system modeling and laboratory evaluation. Both designs have successfully
6、 met the performance criteria. Laboratory evaluation showed that the tandem, single-speed compressor ASHP system is able to achieve heating COP = 4.2 at 47F (8.3C), COP = 2.9 at 17F (-8.3C), and 76% rated capacity and COP = 1.9 at -13F (-25C). This yields a HSPF = 11.0 (per AHRI 210/240). The tandem
7、, vapor-injection ASHP is able to reach heating COP = 4.4 at 47F (8.3C), COP = 3.1 at 17F (-8.3C), and 88% rated capacity and COP = 2.0 at -13F (-25C). This yields a HSPF = 12.0. The system modeling and further laboratory evaluation are presented in the paper. INTRODUCTION As described by Khowailed
8、et al. (2011), in the U. S., the primary target market for cold climate heat pumps is the 2.6 million U.S. homes using electric furnaces and heat pumps in the cold/very cold region, with an annual energy consumption of 0.16 quads (0.17 EJ). A high performance air-source cold climate heat pump (CCHP)
9、 would result in significant savings over current technologies ( 60% compared to strip heating). It can result in an annual primary energy savings of 0.1 Quads (0.1055 EJ) when fully deployed, which is equivalent to 5.9 million tons (5.35 million MT) of annual CO2 emissions reduction. In cold climat
10、e areas with limited access to natural gas, conventional electric ASHPs or electric resistance furnaces can be used to provide heating. During very cold periods, the ASHPs tend to use almost as much energy as the electric furnaces due to their severe capacity loss and efficiency degradation. Present
11、ly, technical and economic barriers limit market penetration of heat pumps in cold climates. R the evaporator exit was assumed to be saturated vapor, i.e. from use of a suction line accumulator. When using a VI compressor with an economizer, the economizer exit superheat degree was set at 10 R (5.6
12、K) and its heat transfer effectiveness was assumed as 70%. The indoor return air temperature was always set at 70F (21.1C). Table 1: Parameters of Indoor and Outdoor Units Parameters (heating mode) Indoor Fin- otherwise, the higher air flow rate and blower power were used. 00.10.20.30.40.50.60.70.80
13、.911.01 . 52.02 . 53.03 . 54.04 . 55.0R ate d C O P at 4 7 F I n te gr ate d C O P w R es Heat at -13 F C ap ac i ty R atioCOP W/WCapacityRatioFigure 3 Ratios of heating capacity relative to 47F, COP at 47F and integrated COP at -13F The heat pump rated capacity at 47F (8.3C), approximately the rate
14、d cooling capacity at 95F (35C) is usually the value used to match a building cooling design load for the sizing selection. Regarding the multi-capacity compressor(s), VS_R4500RPM, 3600RPM, and 2700RPM mean getting the rated heating capacity at 47F (8.3C) by running the VS compressor speed at 4500,
15、3600, and 2700 RPM, respectively. Tandem_RLow means achieving the rated heating capacity at 47F (8.3C), by running a single compressor. The simulation results in Figure 3 compare heating COPs at 47F (8.3C), integrated COPs at -13F (-25C), and ratios of heat pump heating capacity at -13F vs. 47F rati
16、ng point. The ratios of heating capacity were defined as heat pump capacity running at full speed at -13F vs. the rated capacity at 47F. The integrated COPs at -13F (-25C) were calculated by including the supplemental resistance heat needed to match 80% rated heating capacity at 47F (8.3C), i.e. the
17、 building heating load for a well-insulated home at -13F (-25C) in Region V. If no resistance heat was needed, the heat pump COP was used as the integrated COP. It can be seen that over-capacity is the key to match the 75% capacity goal at -13F (-25C) and provide higher integrated COP due to the eli
18、mination of resistance heat use. Four options in Figure 3 are able to reach the DOE capacity goal at -13F (-25C), i.e. 75% relative to the rated capacity at 47F (8.3C) (VS_R4500RPM, 3600RPM, Tandem_RLow and TandemVI_RLow). Tandem_RLow has a higher integrated COP than the VS options, since the VS com
19、pressor has an efficiency drop when running at the top speed of 7200 RPM. The tandem compressors with vapor injection and inter-stage economizing result in the highest integrated COP and the second highest capacity. Based on the analysis, two prototypes were selected for laboratory evaluation. First
20、 was a most cost-effective design, i.e. using equal tandem, single-speed compressors with an electronic expansion valve (EXV) for discharge temperature control. The other is a Premium design, i.e. using equal tandem, VI compressors with inter-stage economizing and discharge temperature control. MOST
21、 COST-EFFECTIVE DESIGN - EQUAL TANDEM, SINGLE-SPEED COMPRESSORS The most cost-effective design using two equal, single-speed compressors is shown in Figure 4. The design considerations are summarized as below: 1. The two equal, single-speed compressors were provided with special “heating application
22、” design features that allow the compressors to operate at higher discharge temperatures than most typical compressors (up to 280F (138C). This enables the heat pump to operate at extremely low ambient temperatures. 2. Current two-speed heat pumps on the market use a single, two-stage compressor hav
23、ing a typical displacement volume split ratio of 100%/67%. In comparison, the tandem compressors have a volume split ratio of 100%/50%, which provides a larger extended-capacity potential, if the heat pump nominal COP and capacity ratings are established for one compressor. That is the reason that t
24、he heat pump using the tandem compressors can reach 75% capacity at -13F (-25C). 3. The CCHP is sized to match a 3-ton/10.6 kW building cooling load using a single compressor. The system uses heat exchangers of a typical 5-ton heat pump. With a single compressor running (cooling mode and moderate te
25、mperatures in heating mode), the heat exchangers are under-loaded, and this provides higher efficiency. That is the key that enabled the CCHP laboratory prototypes to reach a COP 4.0 at 47F (8.3C). 4. The compressor(s) and discharge line are well insulated and placed outside the outdoor air flow str
26、eam, so as to minimize the shell heat loss, as shown in Figure 4. Insulating the compressors impairs the cooling performance; however, its effect is negligible, since the condenser (outdoor heat exchanger) has been oversized for cooling mode. 5. Heating mode discharge temperature control, which uses
27、 an EXV, coupled with a suction line accumulator, is intended to optimize the active charge in the system over an extended operating range. It mitigates the typical charge imbalance problem between cooling and heating modes. A standard thermostatic expansion valve (TXV) is used for cooling mode. Con
28、 de ns erEvap or at orEXVT XVF ilter / Dr ye rF anF an4-WayValveSuctio n Lin e Accum ula to rT an de m Com pr es so r sIn d o o rOu t d o o rOutsi de out door ai r flo w pa thFigure 4 Left: CCHP using tandem, single-speed compressors and an EXV for discharge temperature control in heating mode, righ
29、t: Insulated tandem, single-speed compressors We tested two samples of tandem compressors 1) a pair of typical compressors optimized for cooling mode, and 2) a specially-made pair optimized for heating mode. The latter tandem pair provided better heating performance under all the operating condition
30、s. The comparisons are given in Table 2 and 3 below. Table 2 shows measured performance indices at key ambient temperatures, i.e. 47F (8.3C), 17F (-8.3C) and -13F (-25C), with one or two compressors operating. Table 3 shows HSPFs (as estimated per AHRI Standard 210/240 using the measured performance
31、 results) in Region IV and V, with DHRmin and DHRmax building loads, respectively. Table 2. Performance indices of CCHPs using tandem single-speed compressors. Ambient/Comp(s) 47F, 1 Comp 17F, 2 Comp 17F, 1 Comp -13F, 2 Comp Optimized for cooling mode COP - 4.09 2.76 2.89 1.85 Capacity, kBtu/h (kW)
32、37.96 (11.13) 50.46 (14.79) 25.86 (7.58) 30.04 (8.80) Capacity Ratio to 47F 100% 133% 68% 79% Discharge Temp, F (C) 122 (50.0) 183 (83.9) 131 (55.0) 257 (125) Optimized for heating mode COP - 4.24 2.80 2.97 1.94 Capacity, kBtu/h (kW) 39.72 (11.64) 50.92 (14.92) 25.92 (7.60) 30.25 (8.86) Capacity Rat
33、io to 47F 100% 128% 65% 76% Discharge Temp, F (C) 124 (51.1) 181 (82.8) 124 (51.1) 213 (100.6) %COP Increment 3.7% 1.4% 2.8% 4.9% Table 3. Heating Seasonal Performance Factors of CCHPs using tandem single-speed and VI compressors. Case HSPF/cooling optimized HSPF/heating optimized HSPF/tandem VI com
34、pressors Heating Season Ratings, Region: IV Based on DHRmin 11.04 11.21 11.84 Based on DHRmax 10.90 10.95 11.80 Heating Season Ratings, Region: V Based on DHRmin 9.90 10.03 10.68 Based on DHRmax 9.51 9.59 10.10 PREMIUM DESIGN - EQUAL TANDEM, VAPOR INJECTION COMPRESSORS Use of tandem VI compressors r
35、esulted in increases in both the heating capacity and efficiency. We tested a sample of tandem VI compressors in the same breadboard unit as the most cost-effective configuration. The tandem VI compressors were coupled with an inter-stage flash tank and investigated in three scenarios. The first use
36、d a TXV to control the evaporator exit superheat; the second (Figure 5) used an EXV to control the compressor discharge temperature; the third coupled the discharge temperature control with a suction line heat exchanger (SLHX). It was observed that using an EXV for discharge temperature control led
37、to better performance than using a TXV for compressor suction superheat control. We tested the CCHP using the tandem VI compressors, with and without the SLHX, over extensive ambient temperatures. The SLHX addition didnt show any positive effects on the heat pump COPs and heating capacities. It was
38、observed to increase the compressor suction superheat degree and discharge temperature, which increased the heating capacity per unit refrigerant mass flow rate. However, the increased suction superheat also decreased the suction density, and reduced the compressor mass flow rate. In addition, the c
39、ompressor efficiency was found to decrease due to elevated suction and discharge temperatures. Consequently, neither capacity nor efficiency gain was observed with the SLHX. Therefore, this feature was not selected for the final design. The final system configuration, having the tandem VI compressor
40、s and discharge temperature control, achieved 5% better COPs than the tandem, single-speed compressors (optimized for heating mode) at various ambient conditions. It achieved 88% capacity and 2.0 COP at -13F (-25C), 4.4 COP and 40 kBtu/h (11.7 kW) rated capacity at 47F (8.3C). When delivering 90% ca
41、pacity at 17F (-8.3C), it achieved 3.1 COP. Figure 6 compares the heating capacities of the tandem single-speed compressors and the tandem VI compressors, as a function of the ambient temperature. Figure 7 compares the heating COPs. Table 3 reports the lab-measured HSPFs of the system using the tand
42、em VI compressors and discharge temperature control. CONCLUSION Based on the system modeling and laboratory investigations, two CCHP system configurations are recommended. One is a most cost-effective design using tandem, single-speed compressors (optimized for heating performance), and the other is
43、 a premium configuration using tandem VI compressors. Both configurations achieved the CCHP performance targets. Due to the significant heating capacity reduction of a typical single-speed ASHP, a properly sized ASHP to match a building cooling design load is inadequate for the building heating load
44、 under extremely low ambient temperature conditions. Consequently, multi-capacity compressor(s) are needed to provide proper load matching for both cooling and heating seasons, i.e. using partial capacity to match the building cooling design load, and using the full capacity to match the building he
45、ating load at low ambient temperatures. This facilitates a good balance between reducing the cyclic loss and eliminating the supplemental resistance heat use. Among the multi-capacity compressor(s), using the tandem single-speed compressors is a more cost-effective option than using the VS compresso
46、rs, since the tandem compressors are less expensive and do not need an inverter. In addition, the tandem compressors have a simpler control and no need to be equipped with variable-speed, indoor and outdoor fans, and a specially-made thermostat. A CCHP requires its compressor(s) to work at quite hig
47、h discharge temperatures, necessitating discharge temperature management (e.g., using an EXV for discharge temperature control, or optimizing the charge for heating mode). VI cycles are able to lower the discharge temperature effectively; however, a single VI compressor cant reach the 75% capacity g
48、oal at -13F (-25C). Therefore, the tandem VI compressors were used to facilitate the capacity goal, and achieve the highest COPs, albeit with increased cycle complexity and cost. C o n d e n s e rE v a p o r a t o rF i x e d o r i f i c eT X VF i l t e r / D r y e rC o m p r e s s o r U n i tF a nF
49、a nFlashTankS i g h t G l a s sO i l S e p a r a t o rOilFilter4-WayValveL i q u i d R e c e i v e rH a n d V a l v eS i g h t G l a s sP TP TP TP TE X VIn d o o rOu t d o o rFigure 5 CCHP using tandem VI compressors and an EXV for discharge temperature control in heating mode 01020304050607080- 23 - 13 -3 7 17 27 37 47 57 67HP AirSideHeating Capacity kBtu/hOu td o o r A i r Te m p e r atu r e F2 C o m p s - VI2 C o m p s - 1 Sp e e d1 C o m p - V I1 C o m p - 1 S p e e dLoa d _ DHR m
