Cycling Evaporator Fans to Reduce Energy Costs in Controlled Atmosphere Storage

Transcription

1 ~~ Cycling Evaporator Fans to Reduce Energy Costs in Controlled Atmosphere Storage C ontrolled atmosphere refrigeration systems are sized to rapidly remove field heat from fruit after harvest. Afterfield heat is removed, typically less than 10 percent of that cooling capacity is needed to maintain long-term storage temperatures. This Technology Update explains how you can save significant amounts of energy and money by cycling evaporator fans in controlled atmosphere storage after field heat has been removed. The quality of the fruit remains unchnnged. Four Central Washington Case Studies Many businesses are taking advantage of the energy savings gained from cycling evaporator fans. A demonstration project* conducted at a central Washington commercial controlled ahnosphere (CAI facility found that cycling evaporator fans 1 hour on and 7 hours off would not affect fruit quality and would maintain temperature control (Waelti and Cavalieri 1990). The demonstration project was conducted in two identical CA rooms storing Golden Delicious apples over two storage seasons, from December through April. This facility reduced total refrigeration-associated electrical energy use during the storage period by an estimated 69 percent. Fan cycling saves this facility about $20,000 per year. Three other fruit packing and CA storage facilities in central Washington are described in Table 1. These facilities adopted a more conservative fan cycling schedule of 2 hours on, 6 hours off. After installing computerized control systems to cycle evaporator fans and reduce refrigeration head pressure, these plants have a combined savings on their electric bills of $52,000 per year. This is a documented average annual saving of 23 percent. In addition to the electricity saving, moisture loss was reduced (which increased marketable weight) and fruit quality improved compared to previous storage seasons. Advantages of Cycl ing Evaporator Fans Background The goal of CA storage is to keep fruit fresh (harvest quality) longer by reducing the fruit s respiration rate. This is done by decreasing the amount of oxygen to 1 to 2 percent, increasing the amount of nitrogen to 96 to 98 percent, and maintaining relatively high (1.5 to 2.0 percent) carbon dioxide levels in the storage environment. The respiration rate is further reduced by storing the fruit at approximately 31 F (depending on fruit variety and atmospheric gas concentrations). Toll-free Hot1 line FAX Electronic Bulletin Board Bonneville POW E R A DM I N I ST RAT ION

2 Table 1. Three Case Studies Annual Plant Verified Annual Computer Energy Energy $Savings Control Facility Use Savings at System Additional. Description (kw h/yr ) (kwh/yr) $0.03/kWh cost* Benefits Pl"Ll 3,000, ,000 $23,100 $100,000 The computer can: Fresh packing line 3 common stvrage moms Sequence refrigeration 12 CA rooms compressors and 475 h refrigeration compressors control head pressure hp evaporator fans 2 catalytic oxygen burners 2 molecular sieve adsorbers Monitor and control oxygen, carbon dioxide,and nitrogen concentrations w 3,000, ,000 $16,500 $50,000 Defrost evaporator coils Fresh packing line when needed 4 commun storage rooms 5 CA rooms Control fruit temperature 455 h refrigeration compressors by refrigerating only when required hp evaporator fans 18 3-hp evapraator fans 1 catalytic oxygen burner 3 molecular sieve adsorbers I!k"Q2 1,700, ,000 $12,600 $45,000 6 CA rooms 325 hp refrigeration compressors hp evapcrratw fans 16 5-hp evaporator fans 1 catalytic oxygen burner 1 molecular sieve adsorber Monitor relative humidity within the CA rooms *This type of computer control, system is often eligible for financial incentives to assist in purchase. Check with your local utility regarding incentives that may be available in your area. Note: This table reflects the verified energy savings and cost of adding computerized control in CA storage for three central Washington facilities. In each of the facilities the computer cycles the evaporator fans and reduces head pressure. The fan cycle is 2 hours on, 6 hours off. Because the respiration rate is approximately 7.5 times greater at a field temperature of 86 F than at a storage temperature of 31 F, field heat must be removed as rapidly as possible. For example, consider fruit that is harvested at 70 F and allowed to remain at this temperature for 24 hours. After that period, it is cooled normally to 32 F. This delay will cause the fruit to lose 7 to 10 days from its potential storage life. For this reason, CA cooling systems are sized to rapidly remove field heat from the fruit immediately after harvest. A typical CA storage facility (Figure 1) has a central refrigeration system for the buildinp. The building is composed of several airtight rooms that usually hold 1 to 2 million pounds of fruit each. Each CA room has at least one evaporator with electric fans that circulate the atmosphere across the evaporator coils and throughout the room. Common practice in CA facilities has been to continuously operate evaporator fans during the entire storage season. Continuous Operation of Evaporator Fans Continuous operation of evaporator fans creates three problems: As the fans operate, they introduce energy (heat) into the CA room that must then be removed by the cooling system. All the energy consumed by the fans' motors stays in the room and must be removed by the refrigera- 2 tion system. In a typical installation, about 70 percent of the load on the refrigeration system after fruit cc~1- down is due to the continuous operation of the fans. The velocity of the circulating atmosphere causes the fruit to lose moisture more rapidly than if the fans were off. The loss of moisture from living tissue is called transpiration. The velocity of air moving over the fruit affects the rate of moisture loss due to transpiration. Some weight loss is due to the loss of carbohydrates during respiration; however, the largest percentage of weight loss is due to transpiration. Ap-

3 ples stored at 37 F lose weight through transpiration at about 0.5 percent/week, whereas they lose weight through respiration at only 0.05 percent/week. This moisture loss not only reduces marketable weight but may further decrease value by reducing the product grade. The moisture removed from the fruit by air movement condenses and /or freezes on the evaporator coils. This significantly reduces air flow and decreases efficiency. Additionally, the coils must be defrosted more frequently. While the coils are being defrosted, more energy is introduced into the room. Also, the water collected from the defrosted coils is disposed of, completing the transfer of moisture from the fruit to the drain. Evaporator Fan Cycling The three problems associated with continuous fan operation can be alleviated by means of cycling the evaporator fans. Another advantage to fan cycling is reduced energy use. Suppose, for example, you have a 1,000-bin CA room with four evaporator fans with a total fan capacity of 10 hp.2 Let's compare three fan operation scenarios for this example: (1) continuous fan operation, (2) 1 -hour-on, 1 -hour-off cycling, and (3) 1 -hour on, 7-hoursoff cycling (Figure 2). The first bar represents estimated energy use for continuous fan operation (23 hours/day with 1 hour for defrost). The estimated daily power consumption is 275 kwh, of which 170 kwh is required to Figure 1. Typical CA storage facility, plan view I r-- C'A Room I t t I t Figure 2. Daily electrical energy usage P n Refrigeratioil Sys1em c'ompressor t t t t I t t t t 1 1 t t t LF Bin Storage Room 3

4 V The Electric Ideas Clearinghouse is a conzprehensive information source for conzrnercial and industrial ener J users. It is operated by the $shington State Energ Office and is part of the Electric Ideas technology transfer pro am sponsored by partici atirzg uti Borznmil P T ities and the e Power Adnzirzistration, V Technology Update CH-42 V Electric Ideas Clearinghouse 925 Plum St. S.E. P.O. Box Olympia, Washington V Toll-free Hotline: Fax: Electronic Bulletin Board: V To obtain this factsheet in an alternate ormat, contact the Clearing f iouse. V TDD (206) DOE/BP-2125 June C Reprinted from Washington State Energy Office. Industrial Division. Cycling Evaporator Fans to Reduce Energy Costs in Controlled Atmosphere Storage. Energy Tips for Industry. WAOENG December a GPO

5 power the evaporator fans and 45 kwh is required to remove the evaporator fan energy from the room. Running the fans only half of the time reduces energy use, as illustrated by the second bar. Heat gain by conduction and respiration is not affected by reducing fan operation; however, energy use is reduced by approximately 39 percent. As illustrated by the third bar of the graph, cycling the evaporator fans on for 1 hour and off for 7 hours (12.5 percent cycle) causes daily energy usage to drop to 87 kwh. This is a reduction of nearly 70 percent of the energy used in continuous fan opera tion. Is it possible to operate the evaporator fans only 12.5 percent of the time and still maintain desired fruit storage temperatures? The answer is yes, unce fruit has been cooled to 31 F or the desired storage temperature. This was proven in the demonstration project at the central Washington commercial CA storage facility (Waelti and Cavalieri 1990). In fact, in the demonstration the refrigeration system and evaporator fans operated as seldom as 1 hour on and 11 hours off before fruit temperature increased. Fruit temperature rose only 0.36 F, even after cycling 1 hour on and 11 hours off for 6 days in April. Additionally, no stratification of the gas atmosphere ever occurred. How to Cycle Evaporator Fans There are four ways to cycle fans: (1) manual fan cycling, (2) time clock fan cycling, (3) computerized fan cycling, and (4) fan operation on demand. Manual Fan Cycling Manual fan cycling is the least expensive method, but requires a hu- man operator and can be time consuming. Because of these factors, fans are typically not cycled off as often as is optimal. The procedure is as follows: 1. The operator manually turns off the fans at the disconnect or at the motor starter. 2. The operator turns off all power to the refrigeration controls for the room being fan cycled. This includes the liquid line solenoid valve and the back pressure regulator. The control power to the defrost time clock should also be turned off because defrosting is not desirable or necessary during the off period of fan cycling. 3. When the off period is over, the operator restarts the equipment in an orderly sequence. Startup mutines are normally dictated by management operating procedures Le., start fan 1, start fan 2, energize refrigera tion control circuit). Time Clack Fan Cycling Time clock fan cycling is a low-cost method ($1 00 to $200 per room) that is simple to operate. The time clock performs the same procedures as in manual fan cycling on a time schedule determined by the operator. The operator still needs to monitor CA conditions in order to adjust the cycling schedule. Computerized Fan Cycling Computerized fan cycling is the most flexible way to guarantee reduced energy costs. This method uses a computer to cycle fans and to monitor and operate the overall refrigeration and CA systems. However, it is the most expensive method to install ($10,000 to $1 00,000). The computer s hard-wired relays cycle fans and control refrigeration valves. Probes located at carefully chosen places in open air space in the mom or within bins of fruit sup- ply temperature information to the computer. The computer uses this information to determine if the current fan cycle is sufficient, should be reduced, or can be lengthened. In addition to controlling fan cycling, some computerized control systems provide benefits such as sequencing refrigeration compressors; controlling head pressure; monitoring and controlling oxygen, carbon dioxide, and nitrogen concentrations; defrosting evaporator coils when needed; and monitoring relative humidity within the CA room. Fan Operatian on Demand Fan operation on demand is a system that has been used in New York and adjacent states for several years. It is similar to the time clock fan cycling, but differs because fan operation is controlled by the room thermostat that controls the refrigeration. The evaporator fans run only when there is demand for refrigeration. Implementation costs are minimal ($ per room), requiring only that the fan starter relay be connected to temperature probes either in the return air stream or, optimally, in the bins of fruit. The computerized fan cycling system could also be configured to operate the refrigeration system evaporator and fans on demand. Caution: During extended periods of very low outdoor temperatures, a time clock system or the cumputer should be used to turn on the evaporator fans for a brief period every 8 hours or so regardless of the cycling or demand schedule. This should be done to ensure that a sufficiently uniform temperature is maintained within the storage room. Otherwise, the fruit stored next to outside walls may become too cold. 4

6 Summing It All Up As much as 70 percent of evaporator fan energy can be saved after harvest by cycling the evaporator fans. Based on experience, a ratio of 1 hour on and 7 hours off is adequate for most well-insulated storage facilities in central Washington during the winter months. Cycling evaporator fans can reduce moisture loss and improve fruit quality. Evaporator fans can be cycled manually, mechanically, or by computerized controls. H Strategically locating temperature probes in bins of fruit (not inside the fruit) or at appropriate locations in open air spaces can trigger the refrigeration system to operate only when the fruit actually needs cooling. Bibliography Andre, N., and M.L. Hellickson. Computer Simulation of Transient Refrigeration Load in Cold Storage for Apples and Pears. ASAE Paper No St. Joseph, Michigan: American Society of Agricultural Engineers. June Bartsch, J.A. "Reducing Energy Costs in CA Storage.'' Post Harvest Pomology Newsletter. Washington State University and U.S. Department of Agriculture. 4(2):9 (July-August 1986). Bartsch, J.A. Summation of Slide Presentation on Methods of Cycling Fans. 90th. pp North Amherst, Massachusetts: Massachusetts Fruit Growers Association Bartsch, J.A., and G.D. Blanpied. Cooling Rates of Apples in Tight and Spaced Stacking Patterns. Staff Report, Agricultural Engineering. No. 84-3, pp Department of Agricultural Engineering, New York State College of Agriculture and Life Sciences Hardenburg, R.E., A.E. Watada, and C.Y. Wang. The Commercial Storage of Fruits, Vegetables, and Florist and Nursery Stocks. Agriculture Handbook No. 66. U.S. Department of Agriculture Waelti, H., and R. Cavalieri. Refrigeration Energy Efiiciency Demonstration Project Final Report. No Olympia, Washington: Washington State Energy Office Zaugg, K.R. Controlled Atmosphere Storage Refrigeration System Operation for Golden Delicious Apples. Unpublished M.S. thesis. Pullman, Washington: Washington State University Waelti, H. and R. Cavalieri Refrigeration Ener<gy Efficiency Dcmonstation Project Final Keport. No Washington State Energy Office, Olympia, Washington 'Estimated heat conduction into the storage was based on insulation resistance of R-3 for the floor, R-10 for the walls, and for the ceiling. Respiration heat was assumed to be 80,000 kcal per day. A refrigeration system coefficient performance of 3.7 was used for this analysis. Assumed average outside temperature was 45 F and storage room temperature was 32 F. 5