Types of Heat Source and Heat Sink Media

Numerous heat sources and distribution fluids can be used with heat pumps and there are many variations on the basic system design. Following are brief descriptions of commonly used heat source and heat sink media.

Outdoor air represents a virtually unlimited heat source and heat sink medium for heat pumps and is widely used in small-capacity systems. Other sources of air for heat pumps include air from internal spaces and air from process or HVAC exhaust systems. Since air is a relatively inefficient heat transfer medium, a large surface area is required for heat exchange coils.

Since outside air temperature fluctuates so widely, set point design is a critical task. As the outdoor temperature decreases, heating capacity and efficiency of an air source heat pump decreases rapidly. Supplemental heating sources are generally required in colder climate regions. As outdoor air temperature increases, capacity and efficiency in cooling mode decreases. Air source heat pumps also require defrosting capability and are subject to degraded performance due to frost build-up. Exhaust air is usually a better heat source than outdoor air, but is usually an inferior heat sink due to relatively higher temperatures. It also has a low variation in temperature, which is advantageous. Disadvantages are limited load capability, compared with outside air, and, in cases where the exhaust stream contains dirt, grease, or other impurities, the potential for coil degradation.


There are many types of water sources that can serve as heat source and heat sink for heat pumps, including various types of ground (well) water and surface water from lakes, ponds, oceans, and rivers. Other potentially usable sources include municipal water supply, cooling tower water, and various types of waste water or fluid, such as sanitary sewage or discarded process water.

Temperature will be a function of climate and source depth. Ground water is especially effective because of its relatively high and constant temperature. Heat exchangers can also be submerged in surface water sources, though the temperature drop across the evaporator in winter may need to be limited to prevent freeze-up. Water quality should be analyzed for the potential of scale formation and corrosion. In some cases, it may be necessary to separate the water fluid from the equipment with an additional heat exchanger. Water-to-refrigerant heat exchangers are usually DX or flooded-water cooler types.

Ground (Earth)

The ground is commonly used as a heat source and heat sink medium, with heat transfer occurring through buried coils. Heat can be transferred directly with the refrigerant in the buried coil or indirectly through a secondary loop in which heat is transferred between a circulating brine and the ground. While suitability as a heat source and heat sink varies depending on soil composition, the ground exhibits relatively low temperature variation.

Solar Energy

Solar energy can be an effective heat pump heat source. When available, it provides heat at a higher temperature than other conventional sources, which produces a greater heating COP. The advantage of using solar as a heat pump heat source instead of directly for heating is that collector efficiency and capacity is increased due to a lower temperature requirement.

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