1、AN-04-5-3 Heat Pump System Performance in Northern Climates Paul W. Francisco Member ASHRAE Bob Davis David Baylon Larry Palmiter ABSTRACT Single-value ratings for heat pumps and air conditioners have become industry standards and are widely used by consumers, designers, and program managers for equ
2、ipment selection and prediction ofperformance. On the heating side, the standard rating is the heating seasonal performance factor (HSPF), and on the cooling side the standard rating is the seasonal equipment eficiency ratio (SEER). These values are determined under set conditions. However, the actu
3、alperfor- mance of the equipment depends on the climate in which it is being used. Furthermore, the overall energy use can begreatly impacted by factors such as control strategy and duct losses. This paperpresents the results of computer modeling using the bin method to estimate the impact of climat
4、e, certain common control strategies, sizing approaches, and duct losses on the HSPF of heat pumps in two climates in the northwest United States. Comparisons with jeld data and observations show impacts on heat pump performance consistent with modeling results. INTRODUCTION As energy prices increas
5、e, people are paying more atten- tion to the seasonal efficiency of heat pumps and air condi- tioners. This efficiency is often characterized by a single number, which is determined at a specific set of test condi- tions. For heat pumps, this number is the HSPF (heating season performance factor), a
6、nd for air conditioners the number is the SEER (seasonal energy efficiency ratio). These values are used by consumers to compare conditioning systems, by program managers and regulators to predict The SEER rating uses the results of operating the air conditioner at three different conditions (AN 199
7、4). All of them are done at 82F outdoor temperature and 80F indoor temperature at the indoor coil, for a sensible load temperature difference of only 2F. Two of the tests are done with low indoor humidity (dry coil)+ne of these is done at steady- state conditions (typically about 10 to 15 minutes af
8、ter the unit has begun operation); the other is done with the compressor operating for 6 minutes and then off for 24 minutes. These two results are used to determine the part-load factor, which accounts for losses due to the cycling of the compressor. The third test is done with higher indoor humidi
9、ty such that the coil is wet and is also done under steady-state conditions. The SEER rating is the EER (energy efficiency ratio, defined as the ratio of the compressor output in kBtuh to the power input in kW) for this third test multiplied by the part-load factor. The fact that the SEER rating is
10、higher than the commonly published EER rating is because the commonly published EER rating test is performed at 95F outdoor dry-bulb. The HSPF uses a similar technique to get the part-load factor as that used in determining the SEER (AM 1994). The HSPF test, however, uses a much broader range of con
11、ditions to get the rating. Compressor efficiencies at both 17F and 47F are used, as well as the defrost penalty at 35“F, using a 90-minute defrost cycle. The data are applied to multiple design loads and in multiple climates, using a bin calculation technique. This results in many different HSPF rat
12、ings for each piece of equipment. The rating that is published is based on the U.S. Department of Energy Climate Region IV using the minimum design load. savings and establish incentives, and by designers to specify equipment. Despite the broad use of these single-value ratings, the actual seasonal
13、performance of a specific piece of equipment P.W. Francisco is a research specialist at the Building Research Council, School of Architecture, University of Illinois, Champaign. D. Baylon is president, B. Davis is a research scientist, and L. Palmiter is senior scientist at Ecotope, Inc., Seattle, W
14、ash. 442 02004 ASHRAE. can be very different from the ratings provided. One of the primary causes of this is that the conditions under which the rating tests are performed may not represent the location of interest. For example, if the equipment is located in a climate that differs significantly fro
15、m the climate selected for publi- cation of HSPF values, the seasonal efficiency of the heat pump may be very different from the stated rating. The differ- ences between a specific climate and the rating conditions can result in either an improvement or a reduction in seasonal performance. This fact
16、 is well known among experts, but the way in which these ratings are used suggests that this infor- mation is not common knowledge among users, practitioners, and policy-makers (for example see EPA 1998 and ICC 1998). Another factor that can affect the actual performance of a heat pump is the contro
17、l strategy, especially on the heating side. The point at which the backup heating comes on can have a major impact on the actual energy use of a heat pump, since the efficiencies of electric resistance (100%) and natural gas combustion (typically about 80% for standard gas furnaces, 95% for condensi
18、ng types) are both much less than the effi- ciency of a compressor (capacity divided by power input, usually 200% to 400% at the standard rating points). Averag- ing the backup contribution into the overall performance will reduce the apparent equipment efficiency. Backup heat is considered in HSPF
19、ratings, with the assumption that the backup heat only comes on when necessary. Experience with the manner in which systems get installed in the Pacific North- west shows that the desire to ensure customer comfort or prevent customer complaints results in significantly greater use of backup heat. Du
20、ct losses can also have a major effect on the system performance. Duct losses cause the system to run longer than it would otherwise and on the heating side can cause the backup to be required at warmer temperatures than if there were no duct losses. This effect was evaluated for air condi- tioners
21、under hot conditions by Walker et al. (1998), who found that, even with good sizing and installation, the deliv- ered cooling under the conditions evaluated could be as much as 50% lower than the rated output. Palmiter and Francisco (1997) showed that duct losses in the heating mode had a larger imp
22、act on the system efficiency for heat pumps than for furnaces, with the efficiency loss often double that for furnaces for the same duct leakage. This is because delivery tempera- tures are much lower for compressors than for furnaces and extra load is often made up by backup heating elements. This
23、paper describes the results of computer simulations to investigate the performance of heat pump systems for given ratings, installation practices, and duct systems in two loca- tions in the Pacific Northwest-Seattle and Spokane, Wash- ington. These computer simulations use a bin method calculation,
24、similar to that used in the determination of the HSPF ratings, to estimate the efficiency of the heating system at each bin and then combine the results to provide a seasonal efficiency calculation. No cooling season results are reported in this paper, though a similar method can be applied. Results
25、 of both heating and cooling simulations are included in a previous report (Francisco et al. 2002). The results are expressed as overall seasonal efficiencies. There are three pertinent efficiencies. The first is duct effi- ciency (the ratio ofthe heat output required with no ducts to the heat outpu
26、t with the ducts). The second is the equipment effi- ciency (ratio of the heat energy output to the operating energy input). The equipment efficiency is also affected by the duct losses because the duct losses change the amount of time that the equipment runs in each bin. The last efficiency is the
27、heat- ing seasonal system efficiency (referred to as the system effi- ciency), which is the product of the duct efficiency and the equipment efficiency. System efficiency is also the ratio of the house heating load (seasonal) to the actual operating energy input, Le., the amount of energy that was a
28、ctually required to keep the house warm throughout the year. It is this last effi- ciency that actually describes how well the system as a whole is performing and is an index for how much it will cost to heat the house as well as a baseline for potential savings calcula- tions. Note that the duct ef
29、ficiency as defined here is the same as the distribution efficiency in ASHRAE Standard 152 (ASHRAE 2003), with the exception that in Standard 152 the distribution efficiency includes an “equipment efficiency factor” that here is included in the equipment efficiency. SIMULATION PARAMETERS The followi
30、ng sections describe the model and the combi- nation of factors affecting heat pump performance that are considered in this paper. These include climate descriptions, building descriptions, duct loss characteristics, heat pump characteristics, and different control strategy options. Simulation Model
31、 The analysis in this paper is based on computer simula- tions. These computer simulations use a bin method calcula- tion to calculate the system efficiency at each bin. The model includes a specification of the building load at each tempera- ture bin, explicit performance characteristics from the m
32、anu- facturers literature, the effect of any backup resistance heat required to meet the building load at each bin, and the impact of duct losses. The duct calculation includes the regain of useful heat that is initially lost to unconditioned spaces but recovered to the heated zones and the interact
33、ion of unbal- anced leakage with natural infiltration. In each bin, the compressor supplies as much heat as possible under the spec- ified control strategy, with enough resistance heat added to supply any shortfall. The results from each bin are multiplied by the number of hours in the bin and then
34、summed to get annual heating season results. These annual results are used to get the efficiencies described previously. The bin method is a generally recommended method for evaluating seasonal energy performance. This is the method recommended in ACCA Manual H (ACCA 1984) and described in McQuiston
35、 and Parker (1 988). The method is also recognized by ASHRAE (Knebel 1984, which also outlines ASHRAE Transactions: Symposia 443 - Boston Salt Lake City - Philadelphia -18 -13 -8 3 2 7 12 17 22 27 32 37 42 47 52 57 Temperature Bin Center, F Figure 1 Comparison of Spokane climate to other locations.
36、the general technique). The duct model adapts the ASHRAE Standard 152 equations to the bin model format to calculate overall distribution efficiency. Climate Descriptions Seattle has a fairly mild climate, with a small number of hours below freezing. The average temperature during the months of Octo
37、ber to April is about 46F. Due to high humid- ity levels during the heating season, there is often a significant need for defrost. On the other hand, there are relatively few hours in temperature bins below 32F. In most ways the Seattle climate is favorable for the use of heat pumps. Spokane, which
38、is located in eastern Washington, has a large number of hours below freezing, where heat pump performance drops off considerably and the need for backup heating increases. The average temperature during the months of October to April is about 36F. As seen in Figure 1, the heat- ing season in Spokane
39、 is comparable to eastern locations such as Boston and Philadelphia (which is typical of the climate used for the standard HSPF rating) and Rocky Mountain loca- tions such as Salt Lake City. This graph shows the hours in each bin below the balance point of the prototype houses (55F) as a fraction of
40、 the total number of hours below the balance point. This method of describing the climate is the way in which climates are assigned climatic regions for purposes of determining HSPF. To reinforce the extent to which Spokane looks like these locations, Minneapolis is also shown. Seattle is also shown
41、 to emphasize how favorable Seat- tle is for heat pumps. Building Descriptions Two prototype buildings were evaluated. The first is a 1350 ft2 single-story house built over a vented crawlspace. The floor and the wall are insulated to R-19, the attic to R-38. The overall house UA is 377 BtuN“F. In th
42、e model, the crawlspace is thermally connected to the ground temperature, which is set to the annual average temperature. The return ducts are all in the attic, and the supply ducts are all in the crawlspace. Both sides were assumed to have 10% duct leakage to outside as a percentage of air-handler
43、flow. The supply ducts were assumed to have R-4 insulation, with the return ducts uninsu- lated. This is common practice in the Pacific Northwest and would also apply to many cases where a heat pump is being added to an existing building. There are codes that require greater insulation than that use
44、d in the modeling; in these cases, the duct losses will be lower if the insulation is installed well. The supply duct surface area was 365 ft2. Because of the single return system, the return ducts were assumed to have a surface area of about 80 ft2. For a 3.5 ton heat pump with a system flow rate o
45、f 1400 cfm, this results in a return duct conduction efficiency of 96.5% if the ducts are uninsulated, using the equations in ASHRAE Standard 152P (ASHRAE 2003). Reducing the airflow rate to 1000 cfm would provide a conduction efficiency of 95.2%, and increasing the surface area to 100 ft2 (with 140
46、0 cfm) gives a conduction efficiency of 95.7%. Because the return ducts have a conduction loss of less than 5%, so the effect of insulating these ducts on the over- all efficiency is small. The second building is a 2 184 ft2 split-level house, with one-half of the house being built over a conditione
47、d basement and the other over a crawlspace. The house UA is 565 Btuh/“F. The return ducts are entirely within the conditioned space, and the supply ducts are half in the attic and half between the floors. The supply ducts are assumed to have 5% duct leakage to outside and are insulated to R-4, and t
48、he return ducts are assumed to have no leakage or conduction losses. The supply duct surface area outside the conditioned space is 205 ft2. Heat Pump Characteristics and Control Strategy Options Two 3.5 ton heat pumps were considered. The first has an 8.2 HSPF rating, while the second has a 7.2 HSPF
49、 rating. Performance characteristics were taken from manufacturers data and include the defrost penalty. The system airflow rate for both units is 1400 cfm, which is the standard recommen- dation of 400 cfdton. The 3.5 ton heat pump is oversized for the buildings described. This is an artifact from a preliminary study that led to the project from which these results originated (Francisco et al. 2002). In that project, a variety ofbuilding insulation levels were considered, and the heat pump was sized for the building with the largest load. In order to make comparisons across build
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