Energy Conservation in Apple Storages
Refrigerated and CA storage of tree fruits in the Pacific Northwest requires large amounts of electrical energy. The highest electrical demands for fruit storage occur in the fall during cool down. This coincides with a period when the hydropower generating capacity is low. Rapidly increasing cost of electrical energy has sparked an interest in reducing energy consumption in fruit storage.
Air System Design
Most modern apple storage rooms operate with approximately 30 air changes per hour under continuous operation. These high air flow rates provide fast and uniform cool down rates throughout a room. However, once the field heat is removed from the fruit, much lower air rates would be sufficient to maintain uniform temperatures throughout a room.
Fan Cycling Tests
Gilbert E. Yost, Agricultural Engineer, USDA-ARS, Tree Fruit Research Laboratory, Wenatchee, has investigated different evaporator coil fan cycling schemes, such as 12 hours on - 12 hours off; 6 hours on - 6 hours off; 8 hours on - 16 hours off; and 16 hours on - 8 hours off. This work was conducted over a five-year period in newly-constructed rooms and also in older rooms. Yost has concluded that fan and refrigeration cycling will work, proper fruit temperatures are maintained, and a significant amount of energy can be saved, as illustrated in Figure 1. For a more detailed report, the reader is referred to an article by Mr. Yost in the January 1983 issue of The Goodfruit Grower titled "Cycling fans in Apple Cold Storage Rooms Can Be a Good Way to Conserve Energy". A summary of his findings includes:
The air flow pattern in a loaded room does not change throughout the storage.
In the rooms tested, the average fruit temperatures were maintained between 31° and 32°F. with the fans run continuously, one-third of the time, one-half of the time, or two-thirds of the time.
There was a maximum fruit temperature gradient from the coldest to the warmest fruit of 3-4°F. Fruit temperature differences are affected by other factors than fan cycling schemes, such as room layout, total fan capacity, stacking pattern, stacking accuracy, and others.
In "typical" rooms with coils located above the bins blowing air out over the bins, with air returning between the stacks as well as filtering back through the bins, the apples in the top bins were the coldest. (Figure 1.)
In well-insulated rooms, the average fruit temperature rose 1°F. when the fans were shut off for a 36 hour period. Theoretically, using a heat of respiration of 300 BTU/bin/day, the fruit temperature would rise 1°F. per day.
Fruit quality evaluation indicated that there was no significant difference between the fruit held in rooms with continuous fan operation and that in rooms with cycled fan operation.
New York Study
A fan cycling experiment was conducted and reported by Bartsch (1983) in Peru, New York during the coldest part of the winter from mid-January to mid-February, a 27-day period. The technique used in that test was to operate the evaporator coil with the refrigeration thermostat, thus the fans were operating only when refrigerant was flowing through the coils. The average outside ambient temperature over the period was approximately 20°F. with a high of 45°F. and a low of -15°F.
During the 27-day period, the fans operated an average of 5½ hours per day, with a total energy consumption of 1162 KWH recorded. With continuous fan operation, the fans would have used 4860 kWh during the same period, thus the savings were 3698 kWh. These savings amount to 76% of what normally would have been used. At 3¢/kWh, the savings were $111.00 for an 800-bin room for the 27 day period. It should be noted that the savings calculation includes only the fan operation. Additional savings were obtained from reduced compressor operation. It needs to be pointed out that the study was done during very cold weather; in warmer weather the savings would be significantly less. No information is available on the variation of fruit and room air temperatures during this experiment.
The researchers who have studied fan cycling in storage rooms are warning people to use caution when experimenting with fan cycling schemes. It is important to know the air temperature and fruit temperature at several locations in the room. When the fans are not operating during a long period of time, there is danger of fruit freezing, especially during extremely cold weather. Also, the heat of respiration is most likely to raise the temperature of the top bins in the center of the room. A monitoring system with continuous recording should include temperature readings in these critical areas.
Bartsch and Blanpied (1982) describe in detail how to make and install a thermocouple air temperature monitoring system in the article titled "Temperature Monitoring Systems for Cold Storage", reprinted in this issue of the newsletter. Additional thermocouples could be used to measure fruit temperature in appropriate places in the room. Yost (1983) recommends the following: "A system to do this monitoring could be to:
Use nine bins with an apple probed for temperature monitoring placed about 6 inches deep in each bin, and put these bins in the top layer of bins, three across the back, three across the middle, and three across the front of the room
Use at least two air temperature probes, one in the front of the coils and one behind the coils or in the return air flow of the room
Connect all air and fruit probes to a recording device that automatically prints out temperatures at least four times every 24 hours
Use several regular thermometers for "back-up" temperature readings in case of electronic failure".
Other recommendations include: (1) when the evaporator fans are operated on an intermittent basis, the time period between defrosts should also be extended; and (2) during extended periods of low temperature the fans should be operated for a 20-minute period at least every 6-8 hours.
Air Circulation and Fruit Temperature
One major concern is the possibility that with reduced fan operation there may not be enough air movement through the stacks to remove all the heat of respiration that is being generated by the fruit. Air circulation systems for apple storage are designed to provide large air volumes that are required during cool down of the fruit. As a general rule, the air system is designed to provide about 1000 cfm/ton of refrigeration with a 10°F. or less temperature difference between the return air and the air leaving the coils. After the fruit has been cooled down this temperature difference will be only 2°F. or less. Thus, during the storage period the capacity of the air system is much larger than would be required to remove only the respiration heat and heat gain from other sources.
It is essential to have not only a sufficient volume of air, but also to have equal air movement in all parts of the storage. The importance of proper stacking of the pallet bins cannot be overemphasized. The general practice is to start stacking near the wall opposite the evaporator fans and bring the stack rows back toward the fans. About a one-foot air space is maintained along the end wall and less along the side walls. A 6-inch space between the stack rows is common practice. However, in some rooms variations from 3 inches or less in some rows to 8 inches or more in other rows can be observed. Air velocity measurements that I have taken in storages show that air velocities vary proportionally with the air gap width between the stacks. Average velocities recently measured are 50 ft/min for 2-inch gaps, 125 ft/min for 4-inch gaps, 200 ft/min for 6-inch gaps, and 300 ft/min for larger gaps. Since the air flow volume is the product of the velocity times the cross-sectional area, the actual air flow differences between various gap sizes become very large.
For example, using the above figures, the air flow rate in a 4-inch gap would be about five times larger than that in a 2-inch gap; in a 6-inch gap it would be twelve times larger than in a 2-inch gap, and in an 8-inch gap it would be 24 times larger. Air flow rates through the space between the bins stacked on top of each other were found to be much more uniform with velocities between 100-200 ft/min. Measured air velocities around cross-stacked bins near the door were very low, from almost zero to 50 ft/min.
It has been demonstrated that fan and refrigeration cycling can work. It will save energy, lower energy costs, and still maintain optimum apple temperatures during storage.
Expanded room temperature monitoring is essential to assure that proper temperatures are maintained throughout the whole storage room. This type of cold room instrumentation also provides management with better data on which to base their storage operation decisions for each type of cold room.
With reduced fan operation, uniform air circulation is needed to maintain acceptable temperature levels throughout the storage. Accurate stacking of bins with uniform spacing between bins is essential to maintain uniform air flow to all parts of the storage room.
Bartsch, J. A. 1983. Reducing Electrical Costs in Cold Storages. Proceedings of the 1983 Apple Storage and Fruit Quality Workshop. Agricultural Experiment Station and the Extension Service, University of Vermont, Miscellaneous Publication 106.
Bartsch, J. A. and G. D. Blanpied. 1982. Temperature Monitoring Systems for Cold Storage. Extension Bulletin 430, Department of Agricultural Engineering, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca, New York 14853.
Yost, G. E. 1983. Cycling Fans in Apple Cold Storage Rooms Can Be A Good Way to Conserve Energy. The Goodfruit Grower, Vol. 34, No. 2, January 15, 1983.
Extension Agricultural Engineer, Cooperative Extension, Washington State University, Pullman, WA
Post Harvest Pomology Newsletter 1(4):13-17