1、OR-05-1 6-1 Humidity Effects on Supermarket Ref rig e rated Case En erg y Pe rfo rrna n ce : A Database Review Douglas Kosar Associate Member ASHRAE ABSTRACT Many studies have highlighted that American supermar- kets are energy-intensive commercial users of energy with approximately 54.5 billion kWh
2、 of electricity consumed annu- ally. Refrigeration and HVAC systems account for about 50% and 10% of the electrical load, respectively, and refrigeration compressors alone constitute about 30%. There is a direct and strong interaction between the refrigerated display cases and space conditioning sys
3、tems and the store air conditions. The conditioned store air exchanges heat and moisture with the refrigerated cases. Most refrigerated rasa are designed to operate in an environment of 55% RH and 75F (24C). Howevel; removing additional moisture with HVAC systems to lower store RHlevels at 75F (24C)
4、 couldyield overall HVAC and refrigeration energy cost savings. Drier store air will reduce the latent load on the refrigeration compressors by reducing the moisture entering the display cases. This will lead to less condensation and frost formation, reductions in defrost cycles, decreases in anti-s
5、weat heater energy requirements, and improvements in temperature stability ofproducts. It has been over a decade since Howell and Adams (I 991) surveyed the “Efects of Indoor Space Conditions on Refrig- erated Display Case Performance I under ASHRAE Research Project 596. At that time they cited the
6、“limited amount of experimental or measured data available.” Since then, labo- ratory andjeld work in this area has generated signijkant additionaljndings, although not always extensive or consis- tent in their results. This paper willprovide an updated review of currently available databases, from
7、computer simulations, laboratory tests, and$eld evaluations, that address the e 15.1 % for the open, five-shelf dairy/deli case; 13 3% for the open coffin frozen food case; and no savings for the closed, glass door, six-shelf reach-in frozen food case. If the antisweat heaters are turned off for the
8、 closed, glass door, six-shelf reach-in frozen food case, then the total case saving is 1.3 1% for reductions in humidity from 50% to 35% RH. The results for the Faramarzi et al (2000) database are summarized in Table 3. ASHRAE Transactions: Symposia 1055 Howell and Adams (1991) Database This databa
9、se was generated by a computerized model that emphasized simulation of the heat and mass transfer at the display case opening and in turn calculated eight components of the refngerated case load. In their final report, the authors pointed out that only three display case load components are affected
10、 by the store relative humidity: the infiltration (latent) load, the anti-sweat heater load, the defrost energy load. It is further stated that these load components will also be affected by other factors, including the type of display case, length and width of opening, case temperature, door openin
11、gs per hour (for closed-door reach-in types of cases), and store traffic patterns and operating hours. Eleven case types were analyzed, but in order to have a common basis for comparison in this paper, only five types are discussed here. Those five cases include two low-temperature The energy requir
12、ements for the anti-sweat heaters were stated to be proportional to the difference in the store air dew- point temperature and the display case temperature. There- fore, the energy savings from anti-sweat heaters, when lower- ing the store relative humidity, were calculated with a ratio (AP) between
13、 the anti-sweat heater load (lower dew-point differential) at the lowered store humidity level and the anti- sweat heater load (higher dew-point differential) at 55% RH store condition. With this ratio, the savings were: 20.1% for glass door reach-in frozen food (regardless of door opening time) and
14、 15.1% for the single-shelf tub or coffin ice cream case. Anti-sweat heaters are not typical of medium-tempera- ture cases, so that Howell and Adams data are not reported here. The results for the Howell and Adams (1 99 1) database are also summarized in Table 3. cases: glass door reach-in frozen fo
15、od case and single-shelf tub or coffin ice cream case. The other three cases to be discussed are medium-temperature cases: multi-shelf meat case, multi-shelf dairy, and multi-shelf produce case. The authors equated the simulated reductions in loads on the display case at lower store humidity levels
16、directly to decreases in compressor energy requirements to meet that load. The authors final report for ASHRAE Research Project 596 makes no explicit reference to condensing temperatures at which those percentage reductions in case load and, in turn, compressor energy are calculated. However, a read
17、er might infer from the text that those calculations are at the refrigera- tion equipment manufacturers standard 95F (35C) outdoor rating condition. Compressor load reductions, or compressor energy savings, were calculated using a ratio (TP) between total (sensible and latent) case load at a selecte
18、d relative humidity and total case load at 55% store relative humidity (and fixed dry-bulb temperature of 75“F/24“C). Lowering the relative humidity from 55% to 35%, was projected to result in compressor load and energy savings of: 14.9% fora glass door reach-in frozen food case (when the doors are
19、opened 100% of time and only 3.7% when the doors are opened only 10% of time), 11.4% for single-shelf tub or coffin ice cream case, 28.5% for multi-shelf produce case, 28.0% for multi-shelf dairy case, and 21 .O% for multi-shelf meat case. Savings in defrost energy were calculated using a ratio (DP)
20、 for water vapor transfer relating moisture exchange across the air curtain (or through the door openings) at a selected value of store relative humidity and the moisture exchange across the air curtain (or through door opening) at 55% store relative humidity. Electric defrost was analyzed for all d
21、isplay cases. The savings in defrost energy were: 38.6% for glass door reach-in frozen food, assuming that the doors are open 100% of the time (3.9% for glass door reach-in frozen food but with a more realistic assumption that the doors are opened only 10% of the time), 36.8% for single-shelf tub or
22、 coffin ice cream case, 57.0% for multi-shelf produce case, 59.4% for multi-shelf dairy case, and 46.8% for multi-shelf meat case. Henderson (1999 and 2001) Database This database contains field-monitored data on compres- sor, defrost, and anti-sweat heater energy use that was collected at four supe
23、rmarkets. At each of the four supermarkets, the energy consumption of the low- and medium-temperature compressor racks was recorded separately along with outdoor and indoor conditions at each store. The compressor energy use data collected were over a wide range of outdoor temperature and indoor hum
24、idity. Since the energy use of the refrigeration compressors depends primarily on outdoor temperature, the author attempted to separate out the effect of indoor humidity from outdoor temperature with multiple linear regression analyses. The compressor energy savings were measured on both the low- an
25、d medium-temperature compressor racks in these four stores, where the low-temperature compressor rack consumption energy usage ranged from 600 to 1500 kWhlday and the medium-temperature compressor rack from 400 to 1200 kWhiday, with very strong sensitivities to ambient temperature as expected. The a
26、uthor reported no impact of store humidity on the energy use of the low-temperature rack. The savings reported for the medium-temperature rack ranged from 10.1% to 16.1% for reductions from 55% to 35% RH (a range of average savings from 3.69 to 5.95 kWh/day per gr/lb humidity reduction). The medium-
27、temperature cases in these four stores each had modest 10 kWhiday or less electric defrost energy usage that showed no sensitivity to store humidity. The electric defrost was temperature terminated. The low-temperature cases in these four stores typically had 60 to 90 kWhday elec- tric defrost energ
28、y usage and showed savings of 4.2% to 7.4% for reductions from 55% to 35% RH (a range of average savings from O. 13 to 0.24 kWhlday per grlb humidity reduc- tion) with temperature terminated control. At three of the four supermarkets, anti-sweat heaters were not controlled and ran continuously (400
29、to 540 kWhlday), so no savings were achieved with store humidity reductions. At the fourth store, however, a “Sweat-Miser control system was 1056 ASHRAE Transactions: Symposia implemented to cycle the anti-sweat heaters (based on store humidity). The slope of the anti-sweat heater savings trend line
30、 was 5.9 kWWday for each %RH drop (or 5.8 kWh/day per grlb) and the savings were 25.0% for a decrease in RH from 55% to 35%. Again, the results for the Henderson (1999, 2001) data- base are summarized in Table 3. Henderson and Khattar (1999) Database This database presents field-monitored data from
31、two supermarkets (designated A and B), which were monitored for the impact of the indoor relative humidity on certain refriger- ation system energy uses. As in the previous database, the compressor energy use data collected were over a range of outdoor temperature and indoor humidity. However, in re
32、porting the results from these two stores, the energy consumptions of the low- and medium- temperature compressor racks were totaled together. As previ- ously, the authors attempted to separate out the impact of indoor humidity from the dominant outdoor temperature effect with multiple linear regres
33、sion analysis. At store B, a summation of all refrigeration compressor energy use (plus direct defrost electric energy use) yielded a projected 7.0% reduction for a decrease in store humidity from 55% to 35% RN (savings of 9.9 kWhday per %RH drop). Also at Store B, mechanical controls were used to c
34、ycle anti- sweat heaters based on store and case dew-point differentials. The slope of the anti-sweat heater savings trend line was 4.6 kWWday per each %RH drop and the savings were 18.1 % for a decrease in RH from 55% to 35%. The author further stated that, in theory, this savings could be 9.1 kWh/
35、day for each %RH drop with the savings 35.8% for a decrease in RH from 55% to 35% and still prevent condensation and fogging. At Store A, the load imposed on the refrigeration system by the hot gas defrost was measured under both a time termi- nated and temperature terminated control scheme. The red
36、uc- tion in refrigeration load (and compressor energy savings) attributable to the temperature terminated defrost was 1 .O% for a decrease in RH from 55% to 35% (savings of 4.0 kWh/ day per %RH drop). However, the author noted that these savings versus time terminated defrost may not be realized at
37、a given humidity condition if time terminated defrost is too short or temperature terminated defrost is set too high. Also in Store A, an advanced DDC control replaced the mechanical control for the anti-sweat heaters. With the DDC control, the anti-sweat heaters were full on at 55% RH and full off
38、at 25% RH for a savings of 66.7% from 55% to 35% RH (7.8 kWh/day per %RH drop). The data for the Henderson and Khattar (1999) database are listed in Table 3, as well. Tyler (1972, 1973) and Laverrenz (1 984, 1987) Database A refngerated case manufacturer has released several technical notes since th
39、e 1970s addressing the issue of refrig- erated case performance and store humidity levels. According to these notes, some of their work is the basis for the figures noted earlier in the ASHRAE Handbooks. The database high- lighted in this paper is the most recent by Laverrenz (1 987). In that databa
40、se there is an Exhibit A that shows a variety of store air temperature and relative humidity conditions. An enthalpy condition is noted next to each condition as the determinant of the refrigeration load. A change in refngeration load for medium- and low-temperature cases is listed separately for ea
41、ch condition, too. The memo states that “to break this down into further details such as style of case would not gain signif- icant change in numbers; one exception to this is the Glass Door Merchandiser.” It further states that “these load changes can be interpreted as direct increases or decreases
42、 in electric consumption of the refrigeration compressors.” And finally, the “information as indicated was very general and conserva- tive in nature.” For a decrease in FH from 55% to 35%, the compressor load (energy) savings were shown to be 5.9% for the low-temperature case and 19.7% for the mediu
43、m-temper- ature case. No explicit reference is made to the source of this database or to the condensing temperature at which those percentage reductions in case load and, in turn, compressor energy are determined. However, a reader might infer from the text that those calculations are based on labor
44、atory tests and at the refrigeration equipment standard 95F (35C) outdoor rating condition. The data from this final database-Tyler (1 972,1973) and Laverrenz (1984, 1987) database-is listed in Table 3. Comparison The refrigerated display case load (compressor energy) savings from these different da
45、tabases are plotted together for comparison with low-temperature cases in Figure 4 and medium-temperature cases in Figure 5. Note that no closed door style cases are plotted-only open style cases are plot- ted-for low-temperature cases. For the low-temperature cases, a sizeable spread in data- base
46、results can seen, with the “conservative” Laverrenz (1 987) database showing the least savings and the laboratory- based Faramarzi et al. (2000) database showing the most savings, while the ASHRAE Handbook (2003) and analyti- cally based Howell and Adams (1 99 1) database predict very similar saving
47、s in between. Case load (compressor energy) savings ranged from 5.9% to 17.3% for a store humidity reduc- tion from 55% to 35% RH. As shown in Table 2, low temperature case compressors constitute approximately 40% to 60% of the total case energy consumption, which is on the order of 2.5 to 3.0 kWh/d
48、ay per linear foot of display case at design (75OF24”C and 55% RH indoors, 85F outdoors). Closed door cases saving 3% at 35% RH would yield compressor energy reductions on the order of 0.08 kWWday-ft. Tub or coffin cases saving 12% at 35% RH would yield compressor energy reductions on the order of 0
49、.33 kWWday-ft. ASHRAE Transactions: Symposia 1057 1000 - - -FmARLIME4T -FARAMMI DAIRY 1 -CMOVYEUMEA, 35% 40% 45% 50% 55% Relative Humidity at 75 F 35% 40% 45% 50% 55% Relatlve Humidlty at 75 OF Figure 4 Low-temperature refrigerated display case load Figure 5 Medium-temperature refrigerated display case and compressor energy consumption versus load and compressor energy versus relative relative humidity ( 75“F/24“C). humidity ( 75“F/24“C). For the medium-temperature cases, a somewhat tighter bunching of databases centers around the ASHRAE Hand- book (2003), except for sig
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