Ask the Energy Experts: Desuperheaters can improve heat pump efficiency

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Question:

Can you provide information on heat pump desuperheaters—specifically product and research information about the technology and its application to residential buildings?

Answer:

Let's start with a little background on how heat pumps work. The refrigerant cycle consists of compression of cold refrigerant gas (raising its temperature), cooling and condensing the hot gas to a warm liquid, and expanding the high-pressure warm liquid to a low-pressure gas. A heat pump can reverse the functions of the condenser and evaporator so it can be used to absorb heat from the outdoors to heat the house, or absorb heat from indoors to cool it.

Capture more energy

A desuperheater is a refrigerant-to-water heat exchanger that is located near the compressor discharge, before the condenser. When the refrigerant is compressed, its temperature rises above the saturation temperature for the refrigerant at its discharge pressure—in other words, it is superheated (the energy comes from the heat of compression). Removing the super-heat does two things—it improves the efficiency of the heat pump slightly by extracting the highest-temperature energy from the cycle, and it makes this heat available for other purposes such as water heating (which requires a relatively high temperature of 120 degrees Fahrenheit).

Desuperheaters are most often used on heat pumps in the cooling mode when there is lots of waste heat available. They improve the efficiency of the air conditioner slightly, making this technology useful in warmer climates like the southern United States.

In the heating mode, desuperheaters draw heat from the heat supplied to the house, which reduces the system heating capacity slightly. Most residential desuperheaters deliver only 3,000 to 5,000 British Thermal Units (BTU) per hour, so the reduction in capacity for a three-ton heat pump is minor, except in the coldest weather.

Running a desuperheater in heating mode still provides water heating at the coefficient of performance (COP) of the heat pump—in other words, even though it reduces the output, it is giving you two to three units of heating at the cost of one unit of electricity. Depending on your local climate, it can still be worthwhile to use a desuperheater year-round.

Learn more

Heat Pump Desuperheaters for Supplying Domestic Hot Water—Estimation of Energy Savings and Economic Viability for Residential Applications (1.3MB .pdf) is a 1983 report from the Oak Ridge National Laboratory that provides an excellent technical overview and economic assessment of desuperheaters.

Alabama Power's Web page on water heating systems discusses desuperheaters. While it is aimed at commercial applications, it is also relevant to residential applications.

Nearly every manufacturer of heat pumps offers a desuperheater on one or more models. Energy Experts does not provide product information on the many brands available to consumers. However, a quick Web search on heat pump manufacturers will connect you with the information on specific makes and models.

Resources

Heat Pump Desuperheaters for Supplying Domestic Hot Water—Estimation of Energy Savings and Economic Viability for Residential Applications

Oak Ridge National Laboratory

Alabama Power Water Heating Systems

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