Environmental Engineering Reference
In-Depth Information
Heat flow:
Movement of heat from within the Earth to the surface, where it is dis-
sipated into the atmosphere, surface water, and space by radiation.
Heat pump:
A year-round heating and air-conditioning system employing a refriger-
ation cycle. In a refrigeration cycle, a refrigerant is compressed (as a liquid)
and expanded (as a vapor) to absorb and reject heat. The heat pump trans-
fers heat to a space to be heated during the winter period and by reversing
the operation extracts (absorbs) heat from the same space to be cooled dur-
ing the summer period. The refrigerant within the heat pump in the heating
mode absorbs the heat to be supplied to the space to be heated from an out-
side medium (air, ground, or ground water) and in the cooling mode absorbs
heat from the space to be cooled to be rejected to the outside medium.
Heat pump, air source:
The most common type of heat pump. The heat pump absorbs
heat from the outside air and transfers the heat to the space to be heated in
the heating mode. In the cooling mode, the heat pump absorbs heat from the
space to be cooled and rejects the heat to the outside air. In the heating mode,
when the outside air approaches 32°F or less, air-source heat pumps lose effi-
ciency and generally require a back-up (resistance) heating system.
Heat pump efficiency:
The efficiency of a heat pump (that is, the electrical energy
to operate it) is directly related to temperatures between which it operates.
Geothermal heat pumps are more efficient than conventional heat pumps or
air conditioners that use the outdoor air because the ground or groundwater
a few feet below the Earth's surface remains relatively constant throughout
the year. It is more efficient in the winter to draw heat from the relatively
warm ground than from the atmosphere where the air temperature is much
colder, and in summer transfer waste heat to the relatively cool ground than
to the hotter air. Geothermal heat pumps are generally more expensive to
install than outside air heat pumps. However, depending on the location,
geothermal heat pumps can reduce energy consumption (operating cost)
and, correspondingly, emissions by more than 20% compared to high-effi-
ciency outside air heat pumps. Geothermal heat pumps also use the waste
heat from air-conditioning to provide free hot water heating in the summer.
Heat pump, geothermal:
A heat pump in which the refrigeration exchanges heat
(in a heat exchanger) with a fluid circulating through an earth connection
medium (ground or groundwater). The fluid is contained in a variety of loop
(pipe) configurations depending on the temperature of the ground and the
ground area available. Loops may be installed horizontally or vertically in
the ground or submersed in a body of water.
Heat rate:
A measure of generating station thermal efficiency commonly stated as
Btu per kilowatt-hour.
Note:
Heat rates can be expressed as either gross or
net heat rates, depending on whether the electricity output is gross or net
generation. Heat rates are typically expressed as net heat rates.
Heating value:
The maximum amount of energy available from burning a substance.
Heliostat:
A mirror that reflects solar rays onto a central receiver. A heliostat auto-
matically adjusts its position to track daily or seasonal changes in the sun's
position. The arrangement of heliostats around a central receiver is also
called a
solar collector field.
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