Heat pumps[edit source | editbeta]
A Stirling heat pump is very similar to a Stirling cryocooler, the main difference being that it usually operates at room temperature. At present, its principal application is to pump heat from the outside of a building to the inside, thus heating it at lowered energy costs.
As with any other Stirling device, heat flow is from the expansion space to the compression space. However, in contrast to the Stirling engine, the expansion space is at a lower temperature than the compression space, so instead of producing work, an input of mechanical work is required by the system (in order to satisfy the Second Law of Thermodynamics). The mechanical energy input can be supplied by an electrical motor, or an internal combustion engine, for example. When the mechanical work for the heat pump is provided by a second Stirling engine, then the overall system is called a "heat-driven heatpump".
The expansion side of the heat pump is thermally coupled to the heat source, which is often the external environment. The compression side of the Stirling device is placed in the environment to be heated, for example a building, and heat is "pumped" into it. Typically there will be thermal insulation between the two sides so there will be a temperature rise inside the insulated space.
Heat pumps are by far the most energy-efficient types of heating systems, since they "harvest" additional heat from the environment, rather than turning all their input energy directly into heat. In accordance with the Second Law of Thermodynamics, heat pumps always require the additional input of some external energy to "pump" the collected heat "uphill" against a temperature differential.
Compared to conventional heat pumps, Stirling heat pumps often have a higher coefficient of performance . To date, Stirling systems have seen limited commercial use; however, use is expected to increase along with market demand for energy conservation, and adoption will likely be accelerated by technological refinements.