The fundamental operating principle of air-conditioning systems is to absorb heat from a space at the cooling coils, transport the heat outside by way of the refrigerant fluid, then discharge the heat to the atmosphere via a fluid-to-air or fluid-to-water heat exchanger. The former is an air-cooled chiller for large loads or a packaged DDX unit for small independent systems.
The refrigerant circulates through a finned heat exchanger, through which a fan draws air. The fluid can also circulate through a fluid-to-fluid heat exchanger to heat up water passing through the coils.
Instead of rejecting the waste heat to the atmosphere it is recovered and reused, to heat domestic hot water.
For commercial air-cooled chillers, this accessory can provide a steady volume of hot water since the compressors operate much of the time. An auxiliary storage tank enhances the ability to handle high demand or peak loads. It can also provide—via an integral thermal element—a gas or electric backup if the desuperheater is unable to satisfy the demand, such as during the winter (unless the building has a large interior zone that needs cooling all year).
The savings calculated are more than just substituting a heat source for the existing water heater. Also, the air-cooled chiller fans do not have to run as much of the time since the circulating water absorbs much of the heat from the refrigerant, so the heat does not have to be rejected at the fan coils. The only energy cost for the system is a small in-line circulating pump to move fluid through the desuperheater from the storage tank when the water temperature drops below setpoint.
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Excerpt from Energy Conservation Projects.