Sunday, March 17, 2019
Orchards, coolers and geo-thermal
Refrigeration is a large consumer of electricity in an orchard operation. One of the trends that is pushing that consumption is the move to be the first on the market with any particular variety. Breeders are selecting earlier ripening or fruit that acquires color sooner in the season.
What ends up happening is that the orchard reduces the amount of fruit it is picking in the middle of October when the average of the night and day temperature is fifty Fahrenheit and increasing the amount of fruit it picks in the last half of August when the average N/D temperature is sixty-eight degrees.
One strategy for saving energy exploits the difference between the night day temperature. Parking the 800 pound bins of apples in a shed where massive amounts of cool, night air blows over them is a cheap way to move the heat out of the apples.
If you assume that the daily swing is 20 degrees and you can only pull 10 degrees out of the apples, then you saved having to pull 8000 BTU out of each bin. A modest, family orchard can easily pick 40,000 pounds of apples a day during the peak season. That equates to a 400,000 BTU savings a day.
The downside of this approach is that apple varieties that ripen early in the season rarely store well. Even waiting twelve hours to start the cool down process might result in a significant loss of shelf life.
Another strategy is to sink the heat to ground-water. Yup, geo-thermal.
Most serious orchards have irrigation capability. Ground water is about 55 degrees and water, due to its compactness, is amenable to reverse-flow (low entropy) heat exchangers. That means that you can get your compressed refrigerant to within a few degrees of your ground water.
Refrigerant-to-air heat exchangers are basically car radiators. The air flows perpendicular to the flow of the refrigerant and there is always a significant temperature penalty. The other issue is that it is more expensive, energy wise, to blow air than to pump water.
In round numbers, a cooler running at 35 degrees and is dumping heat to 55 degree water uses 42% of the energy that a system that cools the compressed refrigerant to 85 degrees uses.
Furthermore, the trends just get worse as the temperature rises AND the grid is already straining for capacity in August.
Every little bit helps!
Another benefit to geothermal is that plumbing and heat exchangers are fragile. Orchards have tractors. Orchards have fork trucks. Orchards hire teenagers.
Refrigerant-to-air heat exchangers are fragile. They really are just like a car radiator. They are big and must be mounted above ground level.
Refrigerant-to-ground water heat exchangers are much physically more robust and because they are compact, they can be further armored by putting them in dedicated sheds and protecting them with rails or Jersey barriers.