1、Tania Ullah is a mechanical engineer in the Energy and Environment Division, National Institute of Standards and Technology, Gaithersburg, MD. William Healy is the group leader of the Heat Transfer and Alternative Energy Systems Group, National Institute of Standards and Technology, Gaithersburg, MD
2、. The Performance of an Auxiliary Heat Pump Water Heater Installed in a Dual-Tank System in a Net-Zero Energy Residence Tania Ullah William M. Healy, Ph.D. Member ASHRAE Member ASHRAE ABSTRACT In the effort to achieve low-energy operation of residential buildings, advanced water heating technologies
3、 are vitally important. This paper explores the year-long performance of a 189 L (50 gal) heat pump water heater (HPWH) serving as an auxiliary unit to an active, indirect solar thermal water heater with a 303 L (80 gal) storage tank in a net-zero energy test home located in Gaithersburg, MD, USA. T
4、he systems were subjected to a representative water use schedule for a virtual family of four between July 2013 and June 2014. We investigate the effect of inlet water temperature on the overall system Coefficient of Performance (COPsys) of the HPWH and the units space conditioning impact, as these
5、factors can vary substantially depending on the extent to which hot water demand is met by the solar thermal water heater. Field testing showed that the installed HPWH used 1104 kWh in the year and had a COPsys of 1.41, not reaching the manufacturers reported Energy Factor (EF) of 2.33 over the cour
6、se of the 12-month testing period. The difference was largely due to the fact that the hot water load delivered by the unit was much less than if it were the sole water heater. The study of a HPWH in this unique configuration is valuable considering regulatory trends away from electric resistance st
7、orage water heaters, such as current standards in the United States that require EFs greater than 1.9 for electric water heaters with storage volumes greater than 208 L (55 gal). INTRODUCTION Water heating is the second largest energy consumer in homes, amounting to 18 % of the total energy use in r
8、esidences (DOE 2012). For a high performance home, particular attention needs to be paid to minimizing all loads such that renewable technologies can provide the energy required to operate space heating and cooling equipment, water heaters, appliances, lighting, and plug loads. The Net-Zero Energy R
9、esidential Test Facility (NZERTF), a detached single-family test home built in Gaithersburg, Maryland, used the most energy efficient commercially-available water heating technologies. The primary means of water heating is accomplished with a solar thermal water heater. During times when solar irrad
10、iance is low or when hot water demand is high, this system would normally engage electric resistance elements in its storage tank for auxiliary heating. However, in the case of the NZERTF, auxiliary heating is instead provided by a heat pump water heater (HPWH) located downstream of the solar storag
11、e tank, making this a dual-tank water heating system. A two-tank configuration with an electric resistance water heater is not unusual, but the purpose of this paper is to provide data on how a HPWH performs in this scenario. HPWHs use a vapor compression cycle to draw heat from the ambient air to h
12、eat water. Their recent popularity is highlighted by a U.S. Department of Energy (DOE) report stating that shipments of Energy Star qualified integrated HPWHs increased 630 % between 2006 and 2009 (DOE 2010b). The presence of HPWH technology will increase furthermore in upcoming years due to DOE eff
13、iciency standards that require electric storage water heaters above 208 L (55 gal) to have a minimum Energy Factor (EF) of at least 1.9, depending upon storage volume (DOE 2010a). In this paper, data from a field-tested HPWH in a dual-tank solar water heating system are provided to show how the incr
14、eased inlet temperature affects its overall performance, and estimates are provided for comparison to an electric resistance unit that could be installed for auxiliary water heating in its place. NZERTF DOMESTIC WATER HEATING The NZERTF uses an active, closed-loop solar thermal system as its primary
15、 method for water heating. The system utilizes two solar collectors (1.1 m (3.8 ft) by 2.0 m (6.6 ft) aperture dimensions, facing true south at an 18.4 tilt) and a 303 L (80 gal) storage tank with its auxiliary heating element disabled. In its stead, a HPWH provides hot water in the event that the s
16、olar thermal water heating system cannot meet the demand. The unit consists of a 189 L (50 gal) storage tank with an integrated air source heat pump and two 3800 W electric elements. The HPWH was operated in the “Hybrid” mode with a temperature set-point of 48.9 C (120.0 F). The control logic of the
17、 HPWH in Hybrid mode is as follows: When the differential between the set-point temperature and the reading of a temperature sensor located in the top portion of the tank is 16.7 C (30.0 F) or more, the heat pump will turn off and the 3800 W top element will be energized. Once the top temperature se
18、nsor reading reaches the set-point, the element turns off and the heat pump comes on to heat the remainder of the tank (i.e., until the reading of the sensor at the bottom portion of the tank also reaches the set-point). While the HPWH has a second 3800 W electric element, it is not energized in thi
19、s mode. Hybrid mode ensures that the heat pump provides a majority of the hot water load while electric resistance is enlisted only when the heat pump cannot provide enough hot water. In the Hybrid mode, under test conditions of 57.2 C (135.0 F) set-point temperature and 19.7 C (67.5 F) ambient temp
20、erature, the manufacturer-reported EF, Coefficient of Performance (COP), and standby loss are 2.33, 2.36, and 0.20 C/h (0.36 F/h), respectively. WATER USE CONTROL AND MONITORING The NZERTF was used to demonstrate that a home similar in size and amenities to those in the surrounding community could g
21、enerate as much energy through onsite renewable sources as used by a typical family of four (Fanney et al. 2015). The family was in fact a virtual family whose water-use and electricity-use behaviors were automated according to a weekly schedule derived from the Building America Research Benchmark D
22、efinition (Hendron and Engebrecht 2008). Over the course of each day, 44 water draws were initiated at the sinks, showers, and baths in the house by a real-time event controller according to a water draw schedule described by Omar and Bushby (2013). The clothes washer was initiated for two cycles ea
23、ch on three days of the week, and the dishwasher was initiated for a single cycle five days a week. Approximately 2570 L (680 gal) of mixed hot and cold water were utilized in the house per week. The water temperature at the inlet and outlet of the HPWH storage tank and the ambient temperature were
24、measured with immersed Type-T thermocouples with a calibrated uncertainty (k=2) of 0.1 C ( 0.2 F). The water flow through the solar thermal storage tank and the heat pump water heater was measured by pulse-output paddle-type flow meters with a resolution of 0.013 gal/pulse (0.049 L/pulse) and a cali
25、brated uncertainty (k=2) within 1.7 % of reading. HPWH power was measured at the circuit breaker using current transformers with an uncertainty (k=2) that did not exceed 2 % of reading, and electrical energy use was determined from a time integration of power. Solar irradiance was measured with a py
26、ronometer in the plane of the thermal collector array. Temperature and flow data were collected by the house data acquisition system and thermal energy calculations were made at 3-s intervals during water draw events, while the electrical energy data and ambient conditions were recorded every minute
27、. RESULTS AND DISCUSSION Heat Pump Water Heater Efficiency Table 1 shows monthly HPWH performance data for the year of testing. As a result of solar insolation and, thus, the water heating contribution of the solar thermal system varying monthly, the average HPWH inlet water temperature, THPWH,in, d
28、uring times of draws ranged from a minimum of 23.3 C (74.0 F) in December to a maximum of 46.1 C (114.9 F) in June. This inlet temperature impacted the amount of time the heat pump and the heating elements operated according to the control logic explained above. The heat pump monthly total runtime r
29、anged from a minimum of 57 h in June to a maximum of 178 h in January. The heating elements were inactive for all of June and active most often in November (partly on account of a defect with the heat pump unit). Likewise, the total electrical energy used by the HPWH, EHPWH, ranged from 45 kWh in Ju
30、ly to 156 kWh in December. The result was that the thermal energy contributed by the HPWH, Qdel,HPWH, reached its low in the summer (35 kWh in June) and peaked in the winter (244 kWh in December), as the solar thermal water heaters capacity to meet the virtual familys hot water demand changed season
31、ally. Qload is the total energy in hot water delivered to fixtures and water-utilizing appliances. The overall system Coefficient of Performance, COPsys, is an efficiency metric that is the ratio of thermal energy delivered by the HPWH, Qdel,HPWH, to the electrical energy used to produce it, EHPWH,
32、computed as follows: = (,)(1) where m is the mass of hot water delivered to the fixtures, cp is its specific heat, THPWH,out is the outlet water temperature of the HPWH, and THPWH,in is the inlet water temperature. The COPsys is akin to the EF, although the rated EF is measured under specific test c
33、onditions outlined below from which the present HPWH operation deviates. HPWHs generally have EFs above 2.0 since the work done by the heat pump extracts heat from the surrounding air for water heating and the manufacturer of the NZERTF unit reports an EF of 2.33. Monthly COPsys indicate that this l
34、evel of efficiency is never reached; the COPsys did not surpass 1.68 (January). According to the DOE test method for rating residential water heaters in place at the time of the manufacturers rating (DOE 2010a), HPWHs were subjected to a 24-h simulated use test where 243 L (64.3 gal) of hot water wa
35、s drawn, maintaining the inlet temperature at 14.4 C (58.0 F) and the set-point at 57.2 C (135.0 F), for a target temperature rise of 42.8 C (77.0 F). As shown in Table 1, the average temperature rise (difference between the inlet and outlet water temperatures) was as low as 4.6 C (8.3 F) and as hig
36、h as 25.4 C (51.2 F). As the mass of water drawn on a daily basis also changed depending on the day of the week, the daily thermal output of the HPWH, Qdel,HPWH, ranged from -2.0 kWh to 12.7 kWh, rather than being fixed at Qdel,sim use = 11.9 kWh as it is during the 24-hour simulated use test. Figur
37、e 1 shows the daily COPsys between July 2013 and June 2014 as a function of Qdel,HPWH. It should be noted that these data do not account for any changes in stored energy within the tank from the start to the end of the day. The hollow diamond symbols serve to differentiate the days in which electric
38、 resistance was used from the days in which only the heat pump operated (solid diamonds). The manufacturer-reported EF at Qdel,sim use is placed on the plot (solid circle) as a reference to the HPWH performance under rating conditions. Table 1. Monthly Heat Pump Water Heater Performance, July 2013 J
39、une 2014 Month Solar Insolation kWh/m2 Tbasement C (F) RH % THPWH,in C (F) THPWH,out C (F) HP Run Time h Elmnt. Run Time h Qload kWh Qdel,HPWH kWh EHPWH kWh COPsys Jula 152 21.6 (70.8) 52.2 43.2 (109.8) 51.6 (124.9) 56 0 252 42 45 0.93 Auga,b 123 21.7 (71.0) 51.4 35.7 (96.3) 51.8 (125.3) 86 2 218 10
40、8 71 1.52 Sep 158 22.1 (71.9) 51.9 41.8 (107.2) 51.5 (124.7) 69 1 238 68 57 1.20 Oct 114 21.2 (70.2) 51.6 37.6 (99.6) 51.6 (124.9) 105 1 269 119 82 1.44 Novc 102 20.4 (68.8) 41.2 30.5 (86.9) 52.0 (125.6) 113 11 283 172 130 1.32 Decc 73 20.1 (68.1) 38.0 23.3 (74.0) 51.8 (125.2) 160 10 326 244 156 1.5
41、6 Jan 101 19.8 (67.6) 31.4 23.7 (74.6) 51.3 (124.4) 178 4 343 240 143 1.68 Feb 98 19.5 (67.2) 30.7 25.6 (78.0) 51.3 (124.3) 153 4 330 208 125 1.66 Mar 117 19.5 (67.1) 30.6 28.8 (83.9) 51.3 (124.3) 149 4 341 187 121 1.55 Apr 153 19.6 (67.3) 40.0 38.9 (102.1) 49.9 (121.9) 92 1 300 84 73 1.16 May 161 2
42、0.8 (69.4) 51.6 44.2 (111.5) 50.9 (123.6) 69 0d 277 55 55 0.99 June 164 21.3 (70.3) 53.3 46.1 (114.9) 50.7 (123.2) 57 0 251 35 46 0.77 Year Total 1518 1287 38 3428 1563 1104 Year Avg 20.6 (69.1) 43.6 34.9 (94.9) 51.3 (124.4) 1.41 a Data loss on 7/1/2013 and 8/2/2013 8/6/2013; therefore, monthly valu
43、es in table exclude these days. b Between 8/24/2013 and 9/3/2013, the pumps of the solar thermal water heater heat exchanger were not operational due to failure of electrical connection to glycol circulating pump. c Between 11/25/2013 and 12/5/2013, the heat pump of the heat pump water heater was no
44、t operational due to a control wire being disconnected. d Resistance element run time in May was not “0” but a very small value rounded to 0. Figure 1 Daily averaged overall system Coefficient of Performance (COPsys) of the heat pump water heater as a function of its thermal output, July 2013 June 2
45、014. The overall COPsys of any storage-type water heater will decline as the thermal output of the water heater goes to zero, i.e., as the temperature entering the unit nears the set-point temperature. This condition happens because of two factors: (1) the numerator in Equation 1 goes to zero, and (
46、2) the water heater heater must have a minimum amount of electrical energy input on a daily basis to make up for thermal standby losses. For HPWHs, an added effect is that the refrigerant-to-water heat exchange efficiency decreases as the temperature of the water entering the heat pump compressor in
47、creases. While the installed unit is capable of reaching its rated efficiency, it does not operate under the conditions that would allow it to do so for most days of the year. In addition to the daily COPsys shown in Figure 1, the data are compared to a COPsys curve (solid black line) that has been
48、calculated for a typical electric storage water heater using equations from the Water Heater Analysis Model (WHAM) (Lutz et al. 1998). This theoretical unit has a rated EF of 0.95 and recovery efficiency, rec, of 0.98, but it operates with a tank temperature set-point of 48.9 C (120.0 F) as is the c
49、ase for the HPWH under test. At Qdel,sim use, the COPsys of the NZERTF HPWH is 2.5 times greater than the COPsys of an electric storage water heater determined using WHAM to adjust for the different stored water temperature. However, that factor diminishes as the HPWH delivers less thermal energy; at approximately Qdel,HPWH 1 kWh and below, the COPsys data for the HPWH (diamond symbols) and the electric storage curve (black line) converge. In
