Refrigeration Heat Recovery

Water heating can be one of the larger energy uses on a dairy farm according to a study of 32 New York State dairy farms. In the study, water heating was the first or second largest energy use on farms that did not have a refrigeration heat recovery unit. Dairy farms use large amounts of water to clean and sanitize milking equipment. Based on the New York State dairy farm energy study, if a refrigeration heat recovery unit was used, the average energy use for water heating was 140 kWh/cow-yr equivalent (propane and heating oil were converted to the equivalent kWh, 3413 Btu/kWh). If no heat recovery unit was used, water heating was 338 kWh/cow-yr equivalent, a 58% increase in energy use.

How does a refrigeration heat recovery unit work?
A refrigeration heat recovery unit captures waste heat from the refrigeration system and transfers some of the heat into water, pre-heating water before it enters the water heater. The most popular units are comprised of a water tank with a heat exchanger wrapped around the outside of the tank. The hot refrigerant flows through the heat exchanger on its way to the condenser unit. The heat from the refrigerant is transferred through the tank wall into the water. Thermal buoyancy causes the warmest water to rise to the top of the tank. When hot water is used, water flows from the refrigeration heat recovery tank into the water heater, while well water flows into the heat recovery tank. These units can typically reduce the water heating by 50%.

Refrigeration heat recovery units and well water precoolers that cool milk using well water as a coolant, are competing technologies. A precooler will reduce the heat available for the refrigeration heat recovery unit to capture. On dairies with less than 100 cows it may not be economical to use both a precooler and refrigeration heat recovery unit. Typically, a refrigeration heat recovery unit will provide greater energy and cost savings than a precooler. Dairies with more than 150 cows will normally benefit from using both technologies.

Why should a refrigeration heat recovery unit provide a larger savings than a precooler?
The overall efficiency of a gas or oil fired water heater is typically only 50 to 55% while the efficiency of an electric water heater is typically about 80 to 90%. Electricity is a more expensive energy source however. A refrigeration system is 200% efficient removing two units of energy from the milk for each unit of electrical energy used. Adding a Refrigeration Heat Recovery unit will increase the refrigeration system efficiency because of the increased capacity to dissipate heat while at the same time capturing heat in water to reduce water heating costs. Adding a precooler will reduce refrigeration needs but also reduce the amount of heat available for preheating water in a refrigeration heat recovery unit. Because of the lower efficiency of water heaters (50 to 85%), it is usually more economical to displace water heating before reducing refrigeration needs with a precooler. On dairies with less than 100 cows, it is usually not economical to use both a precooler and refrigeration heat recovery unit.