1、Hot Gas Bypass Defrosting Strategy for Residential Heat Pump Song Li Dennis Nasuta William Hoffman Associate Member ASHRAE Associate Member ASHRAE Ron Domitrovic, PhD. John Bush Cara Martin Member ASHRAE Member ASHRAE Associate Member ASHRAE ABSTRACT This paper describes a single-circuit hot gas byp
2、ass defrost strategy, similar to those used in commercial refrigeration applications, for a residential heat pump. Conventional heat pump systems defrost the outdoor coil by temporarily reversing the direction of the cycle such that the indoor unit acts as an evaporator and the outdoor unit acts as
3、a condenser, which dissipates heat to melt frost from the surface of the coil. This requires that the heat pump temporarily ceases to heat the home and actually uses the indoor coil to extract heat during defrosting. The hot gas bypass strategy does not require a reversal of the cycle, but instead b
4、ypasses some hot gas from the compressor discharge line through some or all of the circuits of the evaporator (outdoor unit) coil to remove frost. Similar defrosting strategies are successfully utilized in large-scale refrigeration systems such as those used in supermarkets, but these methods are no
5、t commonly implemented in smaller systems for the residential or light-commercial markets. A prototype system was developed for investigation consisting of a manifold system that allows hot gas to be bypassed from the compressor discharge line through any or all of the five circuits on the outdoor u
6、nit coil. Following the construction of the prototype system, proof-of-concept laboratory testing of the heat pump was carried out. Experimentation confirmed the capability of the hot gas defrosting strategy and provided initial quantitative results of the impact on system performance and energy con
7、sumption. The prototype has the capability to maintain partial heating capacity in the conditioned space while simultaneously defrosting the outdoor unit; this can be a substantial advantage over conventional systems. Hot gas bypass defrosting configurations of this type could make heat pumps a more
8、 attractive option to many users deterred by the limitations of conventional reverse-cycle defrosting. INTRODUCTION The concept of hot gas defrosting is common and successfully utilized in larger systems, including commercial refrigeration systems, but this approach has not been fully evaluated for
9、residential and light commercial heat pumps. A commercially-available 2.5-Ton (30,000 Btu/hr or 8.8 kW), 13-SEER residential heat pump system was modified and instrumented for experimental evaluation of a hot gas bypass (HGB) defrosting technique. The prototype system Dennis Nasuta and Song Li are T
10、hermal Engineers at Optimized Thermal Systems (OTS), Inc. in College Park, MD. William Hoffman is an Experimental Technician and Cara Martin is the Engineering Manager at OTS. Dr. Ronald Domitrovic is Program Manager and John Bush is a Senior Engineer at the Electric Power Research Institute (EPRI)
11、in Knoxville, TN. was modified such that each of the five circuits of the outdoor heat exchanger can be bypassed with hot gas coming from the compressor discharge line. Valves in each circuit allow for partial restriction of hot gas flow; this allows the remaining refrigerant (not used for hot gas b
12、ypass) to travel through the condenser and provide some heating capacity during defrosting. Numerous strategies of defrosting the outdoor unit by controlling defrosting order, flow rates, and refrigerant pressure drop were evaluated to understand the potential of the technology for residential and l
13、ight-commercial applications. This exploratory study found that the technique is effective, but not yet as efficient as conventional reverse-cycle defrosting. Several system improvements have been identified that may make this technology more efficient than conventional defrosting strategies. PROTOT
14、YPE SPECIFICATION A prototype system was constructed consisting of a small, commercially available residential heat pump system with several key modifications. The unit included a matching pair of indoor and outdoor units rated at 2.5 tons and a custom manifold system for hot gas bypass testing. Fig
15、ure 1 shows the prototype refrigerant flow diagram. As can be seen, each of the five circuits in the outdoor coil can allow hot gas to flow through from the discharge line. Metering valves before and after the outdoor coil allow for flow adjustments in each circuit. Instrumentation locations are den
16、oted in the drawing for all relevant measurements: Temperature (T), Pressure (P), refrigerant mass flow rate (), Power (W), and Relative Humidity (RH). Figure 1 Hot Gas Bypass Heat Pump Prototype Refrigerant Schematic Experimental Setup The prototype unit was installed in the laboratory with the out
17、door unit in an environmental chamber and the indoor unit in a closed-loop wind tunnel. The environmental chamber was used to maintain constant temperature (1 K / 1.8F) and relative humidity (1.5% RH) for the outdoor unit during testing. In order to provide repeatable and realistic conditions for th
18、e indoor unit, the unit was installed in a horizontal configuration in a closed-loop wind tunnel to provide a controlled air volume at constant temperature at the coil inlet. Testing procedures were designed to approximate the conditions prescribed by AHRI Standard 210/240 (AHRI 2008). Temperature a
19、nd humidity conditions were maintained as described in the H2 test condition from the standard. Air entering the indoor unit was maintained at 21.1C (70F) dry bulb and 10C (50F) wet bulb (20% RH). Humidity in the indoor loop was not strictly controlled since the indoor coil rejects sensible heat onl
20、y and humidity levels have a negligible impact. Conditions in the environmental chamber were controlled to assure that an ambient dry bulb temperature of 1.7C (35F) was maintained, and wet bulb temperature was kept as close to 0.6C (33F) (82% RH) as possible. Initial experiments were carried out by
21、operating the heat pump in heating mode under the conditions described above. The unit was first run under factory default defrost to establish a baseline. The amount of frost that accumulated on the outdoor coil face right before the reverse cycle defrost (RCD) was initiated was used as the signal
22、for when defrosting was needed for subsequent HGB tests. Each defrost strategy was evaluated until frost was visibly removed from the coil face. Frost was “measured” using visual inspection only, through the use of video cameras installed for the purposes of experimentation. Following this prelimina
23、ry investigation, several tests were conducted under repeatable conditions such that energy input and heating energy output could be compared over a fixed time period. These results provide insight into the overall energy-efficiency of the different defrosting strategies. Experimental Findings Table
24、 1 summarizes the results of the first eight tests. In these experiments, the factory defrosting strategy was compared to several different HGB defrosting strategies. The duration of defrosting time is recorded along with the approximate heating capacity (if any) provided during defrosting. The foll
25、owing findings are evident from the tests summarized in the table: The HGB method is capable of defrosting the entire coil, even under conditions of severe frostaccumulation where the coil is fully-blocked. All tested hot gas configurations require more time than the factory (reverse-cycle) defrosti
26、ng option todefrost the entire coil. The order of the circuits receiving hot gas has an impact on the performance of the HGB method. Ingeneral, testing showed that defrosting from the bottom circuit upwards requires less time than defrostingfrom the top circuit down. Top down defrosting methods caus
27、e water to drip onto lower circuits, increasingthe frost thickness and requiring additional defrost time. By restricting the flow in the defrost circuit, heating capacity in the indoor unit can be maintained during adefrost period, unlike the conventional method, which cools the space during defrost
28、ing. Restricting the flow at the outlet of the defrost circuit results in a faster defrost time than restricting flow atthe inlet because high pressure hot gas is more effective at defrosting the coil.Additional testing was performed after initial conclusions were established from the exploratory te
29、stingdescribed above. The addition of calibrated nozzles in the indoor unit test loop allowed for an energy balance calculation and confirmation of the indoor unit heating capacity during defrosting. Tests of this type were performed over a period of 225 minutes (about four hours) in order to examin
30、e the energy consumption and heating capacity of the unit; the results are summarized in the following sections. Table 1. Initial Test Results Summary # Defrost Mode Severity of FrostAccumulation Defrost Order (1 is top, 5 is bottom) HGB Restriction Indoor Air Flow Rate (m3/hr / CFM ) Approx. Defros
31、t Time (minutes) *Approx.HeatingCapacityDuringDefrost (kW / MBtu/hr) 1 Factory Severe - - Not measured 58 -6 / 20.5(cooling)2 HGB Severe 5,4,3,2,1 Fully-open Not measured 32 Not measured 3 HGB Severe 1,2,3,4,5 Fully-open Not measured 35 Not measured 4 HGB Severe 3,5,4 Fully-open Not measured 33 Not
32、measured 5 Factory Less-severe - - Not measured 2.5 -6 / 20.5(cooling)6 HGB Less-severe 3,4,5,2,1 Inlet-restriction 2,260 /1,330 48 3.75 / 12.87 HGB Less-severe 3,4,5,2,1 Inlet-restriction 1,759 / 1,035 41 3.4 / 11.6 8 HGB Less-severe 3,4,5,2,1 Outlet-restriction 1,687 / 993 12 1.7 / 5.8 *These resu
33、lts are estimated and not directly measured on the air-sideFactory Defrost For the factory default condition, the unit was set to a 90 minute default defrost. The frost was allowed to accumulate for a full 90 minutes, achieving severe frost conditions. The cycle was then reversed and the unit underw
34、ent default defrosting. Data recording started after the system reached steady state, and the entire analysis includes three defrost cycles. As seen from Figure 2, during factory default defrosting, the average heating-interrupted time due to defrosting was 4.5 minutes. Prior to defrosting during no
35、rmal operation, the unit provided around 7 to 8 kW (2427 kBtu/hr) of heating capacity, which slowly degraded as frost accumulated on the outdoor coil. A similar trend is also observed for the system coefficient of performance (COP). It should be noted that in conventional systems, depending on the p
36、articular unit, the indoor coil fan may either be on or off during the defrost period. For the particular unit used for testing in this experimental effort, the indoor fan does not turn off during defrosting. A significant drop in indoor unit air outlet temperature occurs during defrosting, thus the
37、 cooling effect was observed as a negative heating capacity in Figure 2. This can have the effect of reducing the thermal comfort of the interior space. Figure 2 Heating Capacity (Q indoor), Power Consumption (W compressor) and COP for the Factory Default Test Factory Defrost with Reduced Indoor Flo
38、w Rate Because some heat pumps reduce the speed or disable the indoor fan during defrosting, another test was conducted with reduced flow rate for the indoor unit. The air flow rate was reduced from 1,767 m3/hr (1,040 CFM) to 1,164 m3/hr (685 CFM). This resulted in a reduced system efficiency during
39、 the RCD and thus increased defrosting time by approximately 22% to a total of 5.5 minutes. It can be assumed that a system in which the indoor blower turns off completely during defrosting would require even more time to complete defrosting. HGB with Restricted Inlet For the subsequent HGB test, da
40、ta recording was also started after the unit had reached steady state, then the factory default defrost was disabled. After a similar amount of frost accumulated as compared to the baseline, the HGB method was employed using a circuit order of 3, 4, 5, 2 and 1. In this test, the inlet metering valve
41、 was restricted to maintain indoor unit heating capacity and the outlet valve was fully open. The hot gas refrigerant was only able to achieve a pressure of 1,200 kPa (174 PSI), and a temperature of 13C (55.4F). After frost was completely removed, the system was switched back to normal operation unt
42、il there was a need for the next defrost. Figure 3 summarizes the systems performance. A total of three HGB defrost periods were necessary before the end of the data analysis. During frost accumulation, indoor heating capacity and COP both showed a decreasing trend. However, unlike the baseline case
43、 (Figure 2), heating capacity was maintained to a certain extent (3-4 kW / 1014 kBtu/hr) and heating was not interrupted during the average defrost time of 25 minutes. Figure 3 Heating Capacity (Q indoor), Power Consumption (W compressor) and COP for the HGB Inlet Restricted Test HGB with Restricted
44、 Outlet In the restricted outlet case, the outlet metering valve was restricted to attain higher pressure and temperature hot gas, which in turn, shortened the defrost time. Since the inlet of each circuit was fully open, hot gas reached temperatures up to 30C (86F) and pressure between 1,500 and 2,
45、000 kPa (218290 PSI), which are significantly higher than those achieved by restricting the inlet (13C / 55.4 F and 1200 kPa / 174 PSI). The indoor unit heating capacity and COP for this test condition are plotted in Figure 4. A total of three HGB periods were required before the end of the data ana
46、lysis. The average defrost time was 7.5 minutes, which is significantly shorter than the inlet restricted case. However, the deviation in heating capacity and COP is larger as compared to restricting the inlet. Nevertheless, by utilizing the HGB method, some small amount of heating capacity is maint
47、ained during defrosting. One significant finding from this configuration was a substantial drop in suction temperature and pressure. Investigation of the refrigerant states through HGB defrosting circuits revealed the cause. In an example instant, refrigerant intended for HGB defrosting enters the o
48、utdoor circuit at 1.8 MPa (261 PSI) and 29C (84.2F) as vapor, and exits at 1.7 MPa (247 PSI) and 0C (32F) as liquid after having exchanged heat for defrosting. The refrigerant must then merge with the low-side refrigerant passing through the other four outdoor unit circuits at 0.5 MPa (73 PSI) and -
49、10.8C (12.6F) in vapor phase. This mixing process causes the hot gas circuit to undergo a significant pressure drop and evaporation process; it absorbs over 2 kW (7 kBtu/hr) of heat, which causes the temperature at the suction port to drop to nearly -25C (-13F). This process has a negative impact on the system performance. The implementation of a suction line heat exchanger may help systems of this type to heat this refrigerant stream, increase suction temperature and thus improve efficiency. Figure 4 Heating Capacity (Q indoor), Power Consumption (W compr
