1、W. Vance Payne is a Mechanical Engineer with the HVAC the rated SEER of the heat pump is 15.8 Btu W-1h-1(4.63 W W-1). Annual heating efficiency was 7.10 Btu W-1h-1(2.09 W W-1); the units rated HSPF is 9.05 Btu W-1h-1(2.65 W W-1). These field measured efficiency numbers include dedicated dehumidifica
2、tion operation and standby energy use for the year. Annual sensible heat ratio was approximately 70%. Standby energy consumption was 5.2% and 3.5% of the total electrical energy used for cooling and heating, respectively. INTRODUCTION Buildings consumed 41% of all energy used in the United States in
3、 2010, with residential buildings and commercial buildings accounting for 22% and 19%, respectively. In addition to consuming more energy than the transportation or industrial sectors, buildings represent the fastest growing sector of energy usage (DOE 2010). Thus, many buildings have been designed,
4、 constructed and monitored throughout the world to demonstrate the feasibility of achieving net-zero energy. Parker (2009) presents a history of low energy homes and presents annual performance data from a dozen very low energy homes in North America. In Washington D.C. in 2001, a 2885 ft2(268 m2) m
5、odular ZEH called the “Solar Patriot” or Hathaway home was built to demonstrate a ZEH in a mixed humid climate. An advanced geothermal heat pump was used for space conditioning. A 6 kW photovoltaic (PV) system was installed with the objective of reaching zero energy on an annual basis. This is in th
6、e same weather region as the NIST NZEH and offers some electric energy use comparisons. Musall et al. (2010) summarizes the research of the International Energy Agencys Annex 52 “Towards Net Zero Energy Buildings” and states that “during the last 20 years more than 200 reputable projects with the cl
7、aim of a netzero energy budget have been realized all over the world.” Sherwin et al. (2010) present the performance of four near net-zero energy homes in Florida instrumented to provide data on electrical consumption and generation, indoor conditions, and outdoor weather. NZEH#2 used a 19 SEER air
8、conditioner (AC) for cooling and averaged 216 kWh month-1for cooling alone while maintaining an average dry-bulb temperature of 78.3F (25.7C) and 52.2% RH: standby power was 55 W. NZEH#3 used a ground-source heat pump (GSHP) and average 453 kWh in September of 2008 cooling in a northern Florida clim
9、ate. NZEH#4 used a 18.4 SEER two-speed air source heat pump with 9.1 HSPF while using 3444 kWh, 24% of the total electric use, for cooling and heating while operating at low speed 85% of the time with indoor conditions averaging 75.7F (23.7 C) and 54.5% RH. Appliance loads were not separately measur
10、ed for any of these NZEH sites. TEST HOUSE The net-zero energy residential test facility, NZERTF, is a unique facility that resembles a residence yet is truly a laboratory (Figure 1). The house is two stories having 2715 ft2(252 m2) of living area, a 1453 ft2(135 m2) full basement, and a 1162 ft2(10
11、8 m2) passively conditioned attic. The water, lights, and appliance usage utilized by a family of four were simulated in the NZERTF according to occupancy schedules. Details of these control schedules can be found in Omar and Bushby (2013) and Kneifel (2012). Sensible heat energy generated by occupa
12、nts was simulated in various rooms, but internal latent load simulation was concentrated in the kitchen. Though natural gas could be supplied to the house, during the first year of operation all of the equipment and appliances were powered by electricity either from the sites 10.2 kW (DC) solar phot
13、ovoltaic (PV) system or the main power grid. The building envelope was constructed using a continuous air barrier system to minimize infiltration, and ventilation was provided by a heat recovery ventilator, HRV. Five blower door tests were conducted at various stages of construction; the final test
14、after the house was complete yielded a leakage rate of 472 cfm (802 m3/h) at 0.20 in H2O (50 Pa) corresponding to 0.63 AC h-1. HVAC operations consumed 51% of all electrical energy used on the site. Figure 1 Net-zero energy residential test facility. AIR-TO-AIR HEAT PUMP SYSTEM The heating and air c
15、onditioning system used for the first year of operation in the NZERTF consisted of a HP system that incorporates a dedicated dehumidification cycle (Figure 2). The air distribution duct system was designed for less than 0.5 in H2O (124.5 Pa) external static pressure drop at the air handler with supp
16、ly and return duct airflow rates of 1200 cfm (2039 m3h-1)with all registers fully open. The outdoor unit incorporates a two-speed scroll compressor with two modulated hot gas valves on the compressor discharge that send hot refrigerant gas through a third pipe to the indoor reheat heat exchanger dur
17、ing active dehumidification. A supply air temperature sensor provides the control signal used to proportionally modulate the flow of hot refrigerant gas to maintain a preset supply temperature during dedicated dehumidification. The indoor air handler unit contains a variable speed indoor fan. At the
18、 Air-Conditioning, Heating, and Refrigeration Institute (AHRI) rating condition (AHRI 2008), the A-Test cooling capacity is 26 kBtu h-1(7.60 kW) and the EER (COP) is 13.05 Btu W-1h-1(3.82 W W-1). In the heating mode at AHRI rating condition, the unit has a heating capacity of 26.6 kBtu h-1(7.80 kW).
19、 The unit has a seasonal energy efficiency ratio (SEER) of 15.8 Btu W-1h-1(4.63 W W-1) and a heating seasonal performance factor (HSPF Region IV) of 9.05 Btu W-1h-1(2.65 W W-1). Figure 2 Heat pump refrigerant circuiting and instrumentation. INSTRUMENTATION AND UNCERTAINTY The heat pump system was in
20、strumented to determine important operational parameters and efficiency. The refrigerant circuit is shown in Figure 2 with temperature, pressure, and refrigerant mass flow sensors. The HP has its own dedicated data acquisition system that continuously monitors both refrigerant and air side condition
21、s. Air side capacity (sensible and latent) and component power demand are continuously measured to give instantaneous values of efficiency (COP). The instrumentation, data acquisition system, and measurement uncertainty associated with the heat pump system, as well as all other electrical/mechanical
22、 subsystems within the NZERTF are described in Davis et al. (2014). COOLING AND HEATING SEASON PERFORMANCE The heat pump was operated as a single zone system with a thermostat located in the living room area on the first floor. During heating mode or defrost mode operations, the indoor unit controll
23、er could energize up to 10 kW of electric resistance heat. The thermostat setpoints in the cooling and heating modes were 75.0F (23.8C) and 70.0F (21.1C), respectively, without setback. Table 1: Instrumentation Uncertainty for the Air-Source Heat Pump Instrument RangeTotal Uncertainty at the 95 % Co
24、nfidence Level Transducer voltage measurement 0 to 10 VDC 5 mVDCT-type thermocouples 16F to 131F (-10C to 55C) 1.0F (0.6C)Barometric pressure 20 to 30 in Hg (0.667 to 1.001 bar) 1 % of readingHigh pressure transducer 1000 psig (6895 kPa) 0.25 % of readingLow pressure transducer 500 psig (3447 kPa) 0
25、.25 % of readingAir pressure differential (ESP1) 0 to 0.75 in H2O (0 to 187 Pa) 0.8 % of readingIndoor blower and controls power meter 0 to 300 VAC, 5 Amps, 1000 W 5 WIndoor total power meter 0 to 300 VAC, 100 Amps, 20 000 W 100 WOutdoor unit power meter 0 to 300 VAC, 20 Amps,4000 W 20 WSupply air d
26、ry-bulb temperature sensor -20F to 120F (-28.8C to 49C) 0.9F (0.5C)Supply air dew-point temperature sensor -20F to 120F (-28.8C to 49C) 1.8F (1.0C)Return air dry-bulb temperature sensor -40F to 140F (-40C to 60C) 0.4F (0.2C)Return air dew-point temperature sensor -4F to 212F (-20C to 100C) 1.5 % of
27、readingOutdoor air dry-bulb temperature sensor -40F to 140F (-40C to 60C) 0.4F (0.2C)Outdoor air dew-point temperature sensor -4F to 212F (-20C to 100C) 1.5 % of readingCoriolis refrigerant mass flow meter 0 to 80 lbm min-1(0 to 2180 kg h-1)0.15 % of reading1-External Static Pressure Cooling and hea
28、ting thermal energy for the heat pump can be seen in Table 2. For the entire cooling season the heat pump produced a seasonal COP of 10.07 Btu W-1h-1(2.95 W W-1) (Figure 3a). Although not directly comparable, this seasonal COP was 36% lower than the rated SEER of the system. The measured performance
29、 was lower than the rated SEER value due in part to different indoor setpoint conditions than the standardized tests, differences in the cumulative outdoor temperature conditions, ventilation thermal loads, and standby power demand. Heating thermal loads are also shown in Table 2 with their associat
30、ed monthly COP in Figure 3b. For the entire heating season COP of the system was 6.96 Btu W-1h-1(2.04 W W-1), a value that was 23% lower than the rated HSPF. The resistance heat operated more frequently than anticipated in the heating season due to the inherent control logic of the thermostat. The t
31、hermostat heating configuration allows the user to prescribe differential temperatures relative to the current setpoint temperature and delay times or the maximum amount of time a given stage is allowed to operate before energizing the next higher stage. The delay time control logic appears to be ef
32、fective in the cooling mode, but produced unnecessary usage of electric resistance heat in the heating mode. By limiting the 2ndstage delay time to a maximum of 40 min, the thermostat required 3rdstage electric heat even though 2ndstage heating was increasing the temperature in the house. DEDICATED
33、DEHUMIDIFICATION PERFORMANCE In the cooling mode, the thermostat was set such that the active dehumidification mode of the HP unit was engaged if the relative humidity (RH) reached 50%. Studies of high performance homes with mechanical ventilation showed that they were generally able to maintain the
34、 conditioned space below 60% RH (Rudd et al. 2014). An RH setpoint of 50% was thus selected for the NZERTF to be more restrictive and more comfortable. Active Table 2: Cooling and Heating Thermal Loads Cooling, kWh Jul-2013 Aug-2013 Sep-2013 Oct-2013 Apr-2014 May-2014 Jun-2014 2123 1621 937 306 67 6
35、03 1560Heating, kWh Oct-2013 Nov-2013 Dec-2013 Jan-2014 Feb-2014 Mar-2014 Apr-2014 56 832 1351 2157 1635 1423 253a) b)Figure 3 Cooling (a) and heating (b) season COP. dehumidification control consists of two distinct stages: stage 1 lowers the indoor blower speed for approximately 15 min or until th
36、e RH setpoint is reached; stage 2 begins after 15 min of stage 1 operation by modulation of the two outdoor unit hot gas bypass valves to control indoor supply air temperatures to a pre-selected “room neutral” temperature. Table 3 shows the operating times and electrical energy use during active deh
37、umidification. Active dehumidification consumed approximately 892 kWh of electrical energy, or 6.8 % of all the house electrical energy used during the first year of operation. Figure 4a shows the operational dehumidification efficiency (or energy factor) in units of liters per kilowatt hour. The U.
38、S. Environmental Protection Agency issues Energy Star ratings for dehumidifiers ( 75 pints day-1or 35.5 liter day-1) with efficiencies of 1.85 L kWh-1or greater (EPA 2015). While the HP dedicated dehumidification efficiency was lower than Energy Star, this mode of dehumidification does not add heat
39、to the conditioned space as do most portable dehumidifiers. Table 3: Active Dehumidification Mode Runtimes Active Dehumidification with Reheat Runtime (min) Jul-2013 Aug-2013 Sep-2013 Oct-2013 Apr-2014 May-2014 Jun-20149891 9510 4198 2280 0 829 7989Active Dehumidification Electrical Energy (kWh) 246
40、 246 114 60 0 22 204a) b)Figure 4 Dedicated dehumidification mode efficiency (a) and sensible heat ratio while maintaining 50% RH (b). The key feature of the NIST net-zero home that most contributes to its low energy consumption is the tight and highly insulated envelope. This tight envelope require
41、s the introduction of outdoor air for ventilation which, in the summer months, introduces large amounts of moisture into the space. This moisture was removed by the HP to hold humidity levels at the 50% RH setpoint. The heat pump operated normally and in the dedicated dehumidification mode to produc
42、e a sensible heat ratio (SHR) of approximately 70% (Figure 4b). This was the SHR needed to maintain a 50% indoor RH and to keep the occupants more comfortable. Human comfort and maximum heat pump SEER are at odds with each other. The main air-conditioning system should be designed to efficiently pro
43、vide comfort in all climate zones; therefore, maximum possible SEER will vary with climate zone just as average daily humidity varies with climate zone. Heat pumps and air-conditioners, with their controls, should be rated for dehumidification performance by a non-burdensome laboratory test method.
44、EFFECT OF STANDBY ENERGY ON PERFORMANCE Cooling and heating annual energy efficiency were affected by the HPs standby power demand. Table 4 shows the electrical energy consumed by the heat pump and the percentage of standby time during the cooling and heating modes. Overall the standby energy consum
45、ption was 5.2% and 3.5% of the total electrical enery used for cooling and heating, respectively. Table 4: Standby Energy Use Cooling Standby Energy (kWh) and Percentage of Time in Standby Mode Jul2013 Aug2013 Sep2013 Oct2013 Apr2014 May2014 Jun201412.6, 34% 17.2, 46% 23.4, 64% 25.6, 87% 3.1, 99% 30
46、.5, 82% 16.2, 45%128.6 kWh standby cooling, 5.2% of total cooling electrical energy Heating Standby Energy (kWh) and Percentage of Time in Standby Mode Oct2013 Nov2013 Dec2013 Jan2014 Feb2014 Mar2014 Apr20146.8, 98% 25.2, 72% 21.3, 59% 14.3, 38% 14.9, 45% 21.5, 59% 29.4, 93%133.2 kWh, 3.5% of total
47、heating electrical energyHEAT RECOVERY VENTILATOR IMPACT Ventilating the house using an HRV resulted in a 7% savings in heat pump energy use on average over the year compared with ventilating without heat recovery. The impact on the heat pump energy use ranged from a 5% increase in cooling months to
48、 a 36% savings in heating months (Figure 5). However, in this climate, the annual savings in heat pump energy were offset by the increased power consumption of the HRV compared to a supply fan without heat recovery. These findings are consistent with the literature studies, which were conducted usin
49、g simulations (Rudd et al. 2013). Figure 5 Heat pump electrical energy use due to mechanical ventilation. HOUSE AND HEAT PUMP PERFORMANCE SUMMARY During the first year of operation, the homes annual energy consumption was 13039 kWh (4.8 kWh ft-2or 51.7 kWh m-2), and the 10.2 kW solar PV generated an excess of 484 kWh. The HP consumed a total of 6225 kWh (2442 kWh cooling, 3783 kWh heating) of electrical energy while transferring a total of 14924 kWh of thermal energy (7217 kWh cooling, 7707 kWh heating) between the i
