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Heat Pump Heat Resources
Air
Outdoor air is a universal heat-source and heat-sink medium for
heat pumps and is widely used in residential and light commercial
systems. Extended-surface, forced-convection heat transfer coils
transfer heat between the air and the refrigerant. Typically,
the surface area of outdoor coils is 50 to 100% larger than that
of indoor coils. The volume of outdoor air handled is also greater
than the volume of indoor air handled by about the same percentage.
During heating, the temperature of the evaporating refrigerant
is generally 10 to 20°F less than the outdoor air temperature.
When selecting or designing an air-source heat pump, two factors
in particular must be considered: (1) the outdoor air temperature
in the given locality and (2) frost formation.
As the outdoor temperature decreases, the heating capacity of
an air-source heat pump decreases. This makes equipment selection
for a given outdoor heating design temperature more critical for
an air- source heat pump than for a fuel-fired system. The equipment
must be sized for as low a balance point as is practical for heating
without having excessive and unnecessary cooling capacity during
the summer. A procedure for finding this balance point, which
is defined as the outdoor temperature at which heat pump capacity
matches heating requirements, is discussed in Chapter 45.
When the surface temperature of an outdoor air coil is 32°F
or less, with a corresponding outside air dry-bulb temperature
4 to 10°F higher, frost may form on the coil surface. If allowed
to accumulate, the frost inhibits heat transfer; therefore, the
outdoor coil must be defrosted periodically. The number of defrosting
operations is influenced by the climate, air-coil design, and
the hours of operation. Experience shows that, generally, little
defrosting is required when outdoor air conditions are below 17°F
and 60% rh.. However, under very humid conditions, when small
suspended water droplets are present in the air, the rate of frost
deposit may be about three times as great as predicted from psychrometric
theory. Then, the heat pump may require defrosting after as little
as 20 min of operation. The loss of available heating capacity
due to frosting should be taken into account when sizing an air
source heat pump.
Following commercial refrigeration practice, early designs of
air-source heat pumps had relatively wide fin spacing of 4 to
5 fins/in., based on the theory that this would minimize the frequency
of defrosting. However, experience has proven that effective hot
gas defrosting permits much closer fin spacing and reduced size
and bulk of the system. In current practice, fin spacings of 10
to 20 fins/in. are widely used.
WATER
Water can be a satisfactory heat source,. City water is seldom
used because of cost and municipal restrictions. Groundwater (well
water) is particularly attractive as a heat source because of
its relatively high and nearly constant temperature. The water
temperature is a function of source
depth and climate, but, in the United States, generally ranges
from 40°F in northern areas to 70°F in southern areas.
Frequently, sufficient water is available from wells for which
the water can be reinjected into the aquifer. The use is nonconsumptive
and, with proper design, only the water temperature changes. The
water quality should be analyzed, and the possibility of scale
formation and corrosion should be considered. In some instances,
it may be necessary to separate the well fluid from the equipment
with an additional heat exchanger. Special consideration must
also be given to filtering and settling ponds for specific fluids.
Other considerations are the costs of drilling, piping, pumping,
and a means for disposal of used water. Information on well water
availability, temperature, and chemical and physical analysis
is available from U.S. Geological Survey offices in many major
cities.
Heat exchangers may also be submerged in open ponds, lakes, or
streams. When surface or stream water is used as a source, the
temperature drop across the evaporator in winter may need to be
limited to prevent freeze-up.
In industrial applications, waste process water (e.g., spent
warm water in laundries, plant effluent, and warm condenser water)
may be a heat source for heat pump operation.
Sewage, which often has temperatures higher than that of surface
or groundwater, may be an acceptable heat source. Secondary effluent
(treated sewage) is usually preferred, but untreated sewage may
used successfully with proper heat exchanger design.
Use of water during cooling follows the conventional practice
for water-cooled condensers.
Water-to-refrigerant heat exchangers are generally direct-expansion
or flooded water coolers, usually of the shell-and-coil or shell
and-tube type. Brazed plate heat exchangers may also be used.
In large applied heat pumps, the water is usually reversed instead
of the refrigerant.
GROUND
The ground is used extensively as a heat source and sink, with
heat transfer through buried coils. Soil composition, which varies
widely from wet clay to sandy soil, has a predominant effect on
thermal properties and expected overall performance. The heat
transfer process in soil depends on transient heat flow. Thermal
diffusivity is a dominant factor and is difficult to determine
without local soil data. Thermal diffusivity is the ratio of thermal
conductivity to the product of density and specific heat. The
soil moisture content influences its thermal conductivity.
There are three primary types of ground-source heat pumps: (1)
groundwater, which is discussed in the previous section; (2) direct
expansion, in which the ground-to-refrigerant heat exchanger is
buried underground; and (3) ground-coupled (also called closed
loop ground-source), in which a secondary loop with a brine con
nects the ground-to-water and water-to-refrigerant heat exchangers
Ground loops are can be placed either horizontally or vertically.
A horizontal system consists of single, or multiple, serpentine
heat exchanger pipes buried 3 to 6 ft apart in a horizontal plane
at a depth 3 to 6 ft below grade. Pipes may be buried deeper,
but excavation costs and temperature must be considered. Horizontal
systems that use coiled loops referred to as slinky coils are
also used. A vertical system uses a concentric tube or U-tube
heat exchanger.
SOLAR ENERGY
Solar energy may be used either as the primary heat source or
in
combination with other sources. Air, surface water, shallow groundwater,
and shallow ground-source systems all use solar energy indirectly.
The principal advantage of using solar energy directly as a
heat source for heat pumps is that, when available, it provides
heat
at a higher temperature than the indirect sources, resulting in
an
increase in the heating coefficient of performance. Compared to
solar heating without a heat pump, the collector efficiency and
capacity are increased because a lower collector temperature is
required.
Research and development of solar-source heat pumps has been
concerned with two basic types of systems—direct and indirect.
The direct system places refrigerant evaporator tubes in a solar
collector, usually a flat-plate type. Research shows that a collector
without glass cover plates can also extract heat from the outdoor
air.
The same surface may then serve as a condenser using outdoor air
as a heat sink for cooling.
An indirect system circulates either water or air through the
solar collector. When air is used, the collector may be controlled
in such a way that
(1) the collector can serve as an outdoor air preheater,
(2) the outdoor air loop can be closed so that all source heat
is derived from the sun, or (3) the collector can be disconnected
from the outdoor air serving as the source or sink.
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