Introduction
Vapor Compression Cycle
Efficiency
Sizing
Performance
Heat naturally flows "downhill", from higher to lower temperatures. A heat
pump is a machine which causes the heat to flow in a direction opposite of its natural
tendency or "uphill" in terms of temperature. Because work must be done (energy
consumed) to accomplish this, the name heat "pump" is used to describe the
device.
In reality, a heat pump is nothing more than a refrigeration unit. Any refrigeration
device (window air conditioner, refrigerator, freezer, etc.) moves heat from a space (to
keep it cool) and discharges that heat at higher temperatures. The only difference between
a heat pump and a refrigeration unit is the desired effect--cooling for the refrigeration
unit and heating for the heat pump. A second distinguishing factor of many heat pumps is
that they are reversible and can provide either heating or cooling to the space.
One of the most important characteristics of heat pumps, particularly in the context of
home heating/cooling, is that the efficiency of the unit and the energy required to
operate it are directly related to the temperatures between which is operates. In heat
pump terminology, the difference between the temperature where the heat is absorbed (the
"source") and the temperature where the heat is delivered (the "sink")
is called the "lift." The larger the lift, the greater the power input required
by the heat pump. This is important because it forms the basis for the efficiency
advantage of the geothermal heat pumps over air-source heat pumps. An air-source heat
pump, must remove heat from cold outside air in the winter and deliver heat to hot outside
air in the summer. In contrast, the GHP retrieves heat from relatively warm soil (or
groundwater) in the winter and delivers heat to the same relatively cool soil (or
groundwater) in the summer.
As a result, geothermal heat pump, regardless of the season is always pumping the heat
over a shorter temperature distance than the air-source heat pump. This leads to higher
efficiency and lower energy use.
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All heat pumps use a vapor compression cycle to transport heat from one location to
another. In heating mode, the cycle starts as the cold liquid refrigerant within the heat
pump passes through a heat exchanger (evaporator) and absorbs heat from the
low-temperature source (fluid circulated through the earth connection). The refrigerant
evaporates into a gas as heat is absorbed. The gaseous refrigerant then passes through a
compressor where it is pressurized, raising its temperature to over 180 degrees F. The hot
gas then circulates through a refrigerant-to-air heat exchanger where the heat is removed
and sent through the air ducts. When the refrigerant loses the heat, it changes back to a
liquid. The liquid refrigerant cools as it passes through an expansion valve, and the
process begins again.
Although heat pumps are complex internally, they are marvels of compact design for
reliability. Some include features such as additional heat exchangers for water heating,
and microprocessor-based automatic controls and protection devices.
Click
here for an illustrative diagram.
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