Dinc Ibrahim. Refrigeration systems and applications 2th edition
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Heat Pumps 281
Table 6. 2 Output capacities in heat pump applications.
Application Output Capacity (kW)
Residential hot water heater 1−3
Residential single room 1−4
Residential single-family residence 4−30
Residential multifamily residence
• (noncentralized units)
• (centralized units)
1−20
20−400
District heating 400−10,000
Commercial 20−1,000
Industry 100−30,000
Source: Berghmans (1983a).
Table 6. 3 Delivery temperatures in various applications.
Application Delivery Temperature (
◦
C)
Space cooling (chilled water) 5−8
Space cooling (cooled air) 10−15
District heating (cool water) 10−30
Warm air heating 30−50
Warm water floor heating 30−50
Warm water radiators (low temperature) 45−55
Warm water radiators (forced convection) 55−70
Warm water radiators (free convection) 60−90
District heating (warm water) 80−100
District heating (hot water/steam) 100−180
Industrial process, water 60−110
Industrial process, steam 100−200
Source: Berghmans (1983a).
shops and small offices to 1 MW for large commercial centers. A greater range, from 100 kW to
30 MW, is found in the industrial sector. The delivery temperature also varies with the requirements
of a particular application. Table 6.3 summarizes the temperature requirements for a number of uses.
Heat pumps for residential heating and cooling can be classified into four main categories depend-
ing on their operational function:
• Heating-only heat pumps for space heating and/or water heating applications.
• Heating and cooling heat pumps for both space heating and cooling applications.
282 Refrigeration Systems and Applications
• Integrated heat pump systems for space heating, cooling, water heating, and sometimes exhaust
air heat recovery.
• Heat pump water heaters for water heating.
In residential applications, room heat pumps can be reversible air-to-air heat pumps (ductless
packaged or split type units). The heat pump can also be integrated in a forced-air duct system or
a hydronic heat distribution system with floor heating or radiators (central system).
They often use air from the immediate surroundings as heat source, but can also be exhaust-
air heat pumps, or desuperheaters on air-to-air and water-to-air heat pumps. Heat pumps can be
both monovalent and bivalent, where monovalent heat pumps meet the annual heating and cooling
demand alone, while bivalent heat pumps are sized for 20–60% of the maximum heat load and
meet around 50−95% of the annual heating demand. The peak load is met by an auxiliary heating
system, often a gas or oil boiler. In larger buildings the heat pump may be used in tandem with a
cogeneration system.
In commercial/institutional buildings the heat pump system can be a central installation connected
to an air duct or hydronic system, or a multizone system where multiple heat pump units are
placed in different zones of the building to provide individual space conditioning. Efficient in large
buildings is the water-loop heat pump system, which involves a closed water loop with multiple
heat pumps linked to the loop to provide heating and cooling, with a cooling tower and auxiliary
heat source as backup.
In residential, commercial, and institutional buildings, recently there is an increasing interest in
room-type controlled heat pumps (Figure 6.1). In addition to some benefits such as greater comfort,
reduced noise, and reduced energy use, some features of this type of system are
• preventing operation when connection is made to the wrong supply voltage or if the wiring is
incorrect,
• preventing overheating of the compressor, fan motor, and power transistor,
• detecting refrigerant undercharge and evaporator freeze-up, and
• maintaining the pressure balance by controlling the on/off switching cycle of the compressor.
6.3.1 Large Heat Pumps for District Heating and Cooling
Many large electrically driven heat pumps are in operation worldwide today and even more are
planned for the future. Large heat pumps are defined as equipment with an output of about 500 kW
or more. These are particularly used for district heating and cooling applications.
The point has often been made that the heat pump is competitive and is well established in
markets where cooling is required, too. These markets are
• simultaneous cooling and heating (double utilization) such as in more recent HVAC applications
or in the classical commercial cases of skating rink plus swimming pool, or refrigeration plus
hot tap water production and
• consecutive production of cold and heat in HVAC plants with the same equipment, known as
the heating/cooling heat pump (air cooling and dehumidifying in the summer season and heating
and possibly humidifying during the winter season).
The large heat pump for district heating and cooling use proves to be well-suited:
• for base load coverage in systems without combined heat and power (CHP) generation;
• for low load, low-temperature summer operation for domestic hot water production;
• where supply and/or return temperatures are low;
Heat Pumps 283
Indoor unit
Outdoor unit
Multijointed split-fin
heat exchanger
Compound
curved
propeller
fan
L-shaped, two-row
heat exchanger
Two-cylinder rotary
compressor with DC
brushless motor
Digitally controlled inverter
RAC controller
DC fan motor
Cross-flow fan
Figure 6.1 A controlled room heat pump (Itoh, 1995).
• where water is available as a heat source, for instance, cleared sewage water, industrial waste
water, lake, or sea water. (There are also plants with a heat capacity up to 2.5 MW using ambient
air as a heat source.)
6.4 Heat Pump Applications in Industry
The potential for industrial heat pumps has become promising. It is estimated that in highly indus-
trialized countries, up to 40% of industrial primary energy demand can be saved by the application
of heat pumps. In several countries strong efforts are being made to assist the introduction of
industrial heat pumps.
In terms of the economy, effectiveness, savings, recovery of waste, and the environment, the
industrial heat pumps compete with several alternative technologies, for example, boilers, heat
pipes, and regenerators. Lehmann (1983) pointed out that the following requirements must be met
for the industrial heat pump to prevail against these alternatives:
• manufacture of industrial heat pumps with lower initial cost,
• development of heat pumps with output temperatures between 150 and 300
◦
C,
• intensive and detailed analyses of different industrial heat pumps for specific industries,
• better adaptation of process technologies to heat pump applications,
• increased cooperation among heat pump users, practitioners, and engineers, and
• increased information collection about existing plants and operating experience.
284 Refrigeration Systems and Applications
Process heat pump opportunities in the food industry include both closed and open cycle designs.
For closed-cycle heat pumps, this industry offers a unique combination of using large quantities of
moderately warm (60
◦
C) water for processing and cleanup while having a ready source of waste
heat that is currently being thrown away by the refrigeration plant condensers. Reclamation of some
of this heat makes increasing economic sense to plant operators. For open-cycle designs, a number
of food processes involve evaporation.
When heat pumps are used in drying, evaporation, and distillation processes, heat is recycled
within the process. For space heating, heating of process streams and steam production, heat pumps
utilize waste heat sources between 20 and 100
◦
C.
Industrial applications show a great variation in the type of drive energy, heat pump size, oper-
ating conditions, heat sources, and the type of application. The heat pump units are generally
designed for a specific application, and are therefore unique. The following are some major types
of industrial heat pumps (IEA-HPC, 2001).
• Mechanical vapor recompression (MVR) systems, classified as open or semi-open heat pumps.
In open systems, vapor from an industrial process is compressed to a higher pressure and thus a
higher temperature, and condensed in the same process, giving off heat. In semi-open systems,
heat from the recompressed vapor is transferred to the process via a heat exchanger. Because
one or two heat exchangers are eliminated (evaporator and/or condenser) and the temperature
lift is generally small, the performance of MVR systems is high, with typical COPs of 3–9.
Present MVR systems work with heat-source temperatures from 70 to 80
◦
C and deliver heat
between 110 and 150
◦
C, in some cases up to 200
◦
C. Water is the most common working fluid
(i.e., recompressed process vapor), although other process vapors are also used, notably in the
petrochemical industry.
• Closed-cycle compression heat pumps are described in detail in Section 6.10. Currently applied
working fluids limit the maximum output temperature to 120
◦
C.
• AHPs are not widely used in industrial applications. Present systems with water–lithium bromide
as the working pair achieve an output temperature of 100
◦
C and a temperature lift of 65
◦
C.
The COP typically ranges from 1.2 to 1.6. The new generation of advanced AHP systems are
expected to have higher output temperatures up to 260
◦
C and higher temperature lifts.
• Heat transformers have the same main components and working principle as AHPs. With a heat
transformer waste heat can be upgraded, virtually without the use of external drive energy. Waste
heat of a medium temperature (i.e., between the demand level and the environmental level) is
supplied to the evaporator and generator. Useful heat of a higher temperature is given off in the
absorber. All present systems use water and lithium bromide as the working pair. These heat
transformers can achieve delivery temperatures up to 150
◦
C, typically with a lift of 50
◦
C. COPs
under these conditions range from 0.45 to 0.48.
• Reverse Brayton cycle heat pumps recover solvents from gases in many processes. Solvent
loaded air is compressed, and then expanded. The air cools through the expansion, and the
solvents condense and are recovered. Further expansion (with the associated additional cooling,
condensation, and solvent recovery) takes place in a turbine, which drives the compressor.
Heat pumps are available for many industrial processes ranging from the pulp and paper industry
to the food industry for a large number of applications, including:
• space heating,
• process water heating and cooling,
• steam production,
• drying and dehumidification processes, and
• evaporation and distillation and concentration processes.
Heat Pumps 285
The most common waste heat streams in industry are cooling water, effluent, condensate, mois-
ture, and condenser heat from refrigeration plants. Because of the fluctuation in waste heat supply,
it may be necessary to use large storage tanks for accumulation to ensure stable operation of the
heat pump. Some common applications can be summarized as follows (IEA-HPC, 2001):
• Space heating. Heat pumps can utilize conventional heat sources for heating of greenhouses
and industrial buildings, or they can recover industrial waste heat that could not be used directly,
and provide a low to medium temperature heat that can be utilized internally or externally for
space heating. Mainly electric closed-cycle compression heat pumps are used.
• Process water heating and cooling. In many industries, warm process water in the temperature
range from 40 to 90
◦
C is needed, particularly for washing, sanitation, and cleaning purposes.
Heat pumps offer good potential for such applications and may be a part of an integrated system
that provides both cooling and heating. Although electric closed-cycle compression heat pumps
are mainly installed, some AHPs and heat transformers may find application.
• Steam production. In industrial processes, vast amounts of low-, medium-, and high-pressure
steam in the temperature range from 100 to 200
◦
C are consumed. In the market, at present, the
high-temperature heat pumps can produce steam up to 300
◦
C. In this regard, open and semi-
open MVR systems, closed-cycle compression heat pumps, cascade systems, and some heat
transformers are employed.
• Drying and dehumidification process. Heat pumps are used extensively in industrial dehumid-
ification and drying processes at low and moderate temperatures (maximum 100
◦
C). The main
applications are drying of pulp and paper, various food products, wood, and lumber. Drying of
temperature-sensitive products is also interesting. Heat pump dryers generally have high COP
(with COP of 5–7), and often improve the quality of the dried products as compared with tradi-
tional drying methods. Because the drying is executed in a closed system, odors from the drying
of food products, and so on, are reduced. Both closed-cycle compression heat pumps and MVR
systems are used.
• Evaporation, distillation, and concentration processes. Evaporation, distillation, and concen-
tration are energy-intensive processes, and most heat pumps are installed in these processes in
the chemical and food industries. In evaporation processes the residue is the main product, while
the vapor (distillate) is the main product in distillation processes. Most systems are open or
semi-open MVRs, but closed-cycle compression heat pumps are also applied. Small temperature
lifts result in high performance, with COPs ranging from 6 to 30.
In addition to the above-mentioned processes, there are some significant applications of heat
pumps, covering a very wide range, from connected loads of a few watts for the thermoelectric
heating/cooling units in the food industry to loads of several megawatts for large vapor-compression
plants in industry, including
• Small, mass-produced, hot water heaters, sometimes combined with refrigerators and with con-
nected loads between 200 and 800 W.
• Heating heat pumps for individual rooms, single-family houses, smaller office buildings, restau-
rants, and similar projects. Package heat pumps (in closed casings) are also available as split units
with an indoor and an outdoor section for installation in the open. Mass-produced, sometimes
on a large scale, these have a heat output often with supplementary heating (electric, liquefied
gas, warm water) up to about 120 kW, and connected loads from 2 to 30 kW.
• Heat pumps for heating and heat recovery for large air-conditioning plants in office blocks,
department stores, and similar projects. Appropriately adapted mass-produced chilled water units
as well as systems individually assembled from the usual components for large refrigeration
plants are used. Heat output is more than 1200 kW and the connected load is between 20 and
286 Refrigeration Systems and Applications
400 kW. If the heat pump is also used for cooling in summer, it is often better to use it to recover
heat from the extract air in winter than to use an additional recuperative heat exchanger.
• Heating–cooling heat pumps for cooling and heating of rooms, objects of mass flows. The main
task of these plants, also determining the control, is usually for either cooling or heating, not
both, since the other effect is an additional gain which is not available during nonoperational
periods of the system and can only be supplied by a store, for example, a hot water boiler.
• Waste heat utilization heat pumps for utilizing or reusing discharged heat which cannot be reused
immediately because of its low temperature. This is so, for example, in drying processes in which
the waste heat contained in the extracted water vapor is used for heating the drying air, or in
laundries where practically all the applied heat energy is discharged with the waste water and
can be recovered by a heat pump. The plants are controlled by heat demand, often combined
with the storing of waste heat.
In the case of the industrial heat pump usually, a payback period of less than 4 years, sometimes
even of 2 years, is required. Applications are therefore limited to cases where temperature levels
and therefore COPs are favorable and utilization factors are high. Four basic heat pump systems
are in use, available, or being developed for industrial low- and high-temperature applications:
• the closed-cycle compression heat pump, up to about 115
◦
C;
• the open-cycle heat pump (steam compressor) for much higher temperatures;
• the AHP (Type I), up to about 110
◦
C; and
• the “Type II” AHP (heat transformer), up to about 180
◦
C.
Various types of heat pumps are widely used in industry. However, as environmental regulations
become stricter, it will become more important to use industrial heat pumps that reduce emissions,
improve efficiency, and limit the use of groundwater for cooling.
To ensure the sound application of heat pumps in industry, processes should be optimized and
integrated. Through process integration improved energy efficiency is achieved by thermodynam-
ically optimizing total industrial processes. An important instrument for process integration is
pinch analysis, a technology to characterize process heat streams and identify possibilities for heat
recovery. Such possibilities may include improved heat exchanger networks, cogeneration, and heat
pumps. Pinch analysis is especially powerful for large, complex processes with multiple operations,
and is an excellent instrument to identify sound heat pump opportunities.
6.5 Heat Sources
To understand the basic principle of the heat pump, one must realize that heat is a form of energy,
the quantity of which is quite independent of the temperature which happens to exist at the time.
In air, soil, and water, in air extracted from buildings, and in waste water of any kind, there are
enormous quantities of heat which are useless only because the temperature is too low. From all
these sources, heat can be extracted, and with a small expenditure of additional, high-grade energy
a heat pump can upgrade the waste heat to a temperature suitable for room heating.
The primary heat sources include air, water, and soil. In practice, air is the most common source
for heat pumps while water- and soil-source systems are less commonly applicable. In general, air,
soil, and groundwater are considered practicable as heat sources for small heat pump systems while
surface water, sea water, and geothermal systems are more suited to larger heat pump systems.
As far as low-temperature sources are concerned, ground or surface water, air, and soil are most
commonly used.
The technical and economic performance of a heat pump is closely related to the characteris-
tics of the heat source. An ideal heat source for heat pumps in buildings has a high and stable
Heat Pumps 287
Table 6. 4 Commonly used heat sources.
Heat Source Temperature Range (
◦
C)
Ambient air –10–15
Exhaust air 15–25
Groundwater 4–10
Lake water 0–10
River water 0–10
Sea water 3–8
Rock 0–5
Ground 0–10
Waste water and effluent >10
Source: IEA-HPC (2001).
temperature during the heating season, is abundantly available, is not corrosive or polluted, has
favorable thermophysical properties, and requires low investment and operational costs. In most
cases, however, the availability of the heat source is the key factor determining its use. Table 6.4
presents commonly used heat sources. Ambient and exhaust air, soil, and groundwater are practical
heat sources for small heat pump systems, while sea/lake/river water, rock (geothermal), and waste
water are used for large heat pump systems.
Several heat pump configurations can be visualized utilizing a seemingly inexhaustible number
of energy sources. Some of these energy sources are outside air, sensible heat from stream or well
water, latent heat diffusion from water (ice formation), warm discharge effluents from industry,
fireplace waste heat, and heat generation in sewage. Most of these energy sources are not widely
available to the general public. Four types of heat pump systems are in common use in practice:
• single-package heat pumps using an air source,
• split-system heat pumps using an air source,
• single-package heat pumps using a water source, and
• split-system heat pumps using a water source.
Single-package heat pumps have all the essential components contained within a single unit
while split-system heat pumps house the essential components in two separate units (i.e., one unit
outdoors and one unit indoors).
Here, we present the most common heat sources for heat pumps.
6.5.1 Air
While ambient air is free and widely available, there are a number of problems associated with its use
as a heat source. In the cooler and more humid climates, some residual frost tends to accumulate on
the outdoor heat-transfer coil as the temperature falls below the 2–5
◦
C range, leading to a reduction
in the capacity of the heat pump. Coil defrosting can be achieved by reversing the heat pump cycle
or by other less energy-efficient means. This results in a small energy penalty because during the
defrost cycles cool air is circulated in the building. Provided the defrost cycle is of short duration,
288 Refrigeration Systems and Applications
this is not significant. In addition, for thermodynamic reasons the capacity and performance of the
heat pump fall in any case with decreasing temperature. As the heating load is greatest at this
time, a supplementary heating source is required. This device could be an existing oil, gas, or
electric furnace or electric resistance heating; the latter is usually part of the heat pump system.
The alternative to the provision of a supplementary heating device is to ensure that the capacity
of the heat pump is adequate to cope with the most extreme weather conditions. This can result in
over-sizing of the unit at a high additional capital cost and is not cost-effective compared with the
cost of supplementary heating devices.
Exhaust (ventilation) air is a common heat source for heat pumps in residential and commercial
buildings. The heat pump recovers heat from the ventilation air and provides water and/or space
heating. Continuous operation of the ventilation system is required during the heating season or
throughout the year. Some units are also designed to utilize both exhaust air and ambient air.
For large buildings exhaust air heat pumps are often used in combination with air-to-air heat
recovery units.
Outside ambient air is the most interesting heat source as far as availability is concerned. Unfor-
tunately when the space heating load is the highest, the air temperature is the lowest. However, tem-
peratures are not stable. The COP of vapor-compression heat pumps decreases with decreasing cold
source temperature. In addition at evaporator temperatures below 5
◦
C air humidity is deposited on
the evaporator surface in the form of ice. This does not improve the heat transfer and leads to lower
working fluid temperatures and therefore lower COP values, depending upon the temperature of the
air flowing over the evaporator. If ice formation occurs periodic de-icing of the evaporator surface
has to be applied. This invariably leads to decreased values of the overall system COP (5−10%).
6.5.2 Water
Water-source units are common in applied or built-up installations where internal heat sources or
heat or cold reclamation is possible. In addition, solar or off-peak thermal storage systems can be
used. These sources have a more stable temperature, compared to air. The combination of a high
first cost solar device with a heat pump is not generally an attractive economic proposition on either
a first cost or a life-cycle cost basis.
Groundwater is available with stable temperatures between 4 and 10
◦
C in many regions. Open
or closed systems are used to tap into this heat source. In open systems the groundwater is pumped
up, cooled, and then reinjected in a separate well or returned to surface water. Open systems
should be carefully designed to avoid problems such as freezing, corrosion, and fouling. Closed
systems can be either direct expansion systems, with the working fluid evaporating in underground
heat exchanger pipes or brine loop systems. Owing to the extra internal temperature difference,
heat pump brine systems generally have a lower performance, but are easier to maintain. A major
disadvantage of ground-water heat pumps is the cost of installing the heat source. Additionally,
local regulations may impose severe constraints regarding interference with the water table and the
possibility of soil pollution.
Most groundwater at depths more than 10 m is available throughout the year at temperatures
high enough (e.g., 10
◦
C) to be used as low temperature source for heat pumps. Its temperature
remains practically constant over the year and makes it possible to achieve high seasonal heating
COPs (3 and more). The pump energy necessary to pump up this water has a considerable effect
upon COP (10% reduction per 20 m pumping height). It is necessary to pump the evaporator water
back into the ground to avoid depletion of groundwater layers.
The groundwater has to be of a purity almost up to the level of drinking water to be usable
directly in the evaporator. The rather large consumption of water of high purity limits the number
of heat pump systems which can make use of this source. Also, surface waters constitute a heat
source which can be used only for a limited number of applications.
Heat Pumps 289
Groundwater at considerable depth (aquifers) may offer interesting possibilities for direct heating
or for heating with heat pump systems. The drilling and operating costs involved require large-scale
applications of this heat source. The quality of these waters often presents serious limitations to
their use (corrosive salt content).
Groundwater (i.e., water at depths of up to 80 m) is available in most areas with temperatures
generally in the 5–18
◦
C range. One of the main difficulties with these sources is that often the water
has a high dissolved solids content, producing fouling or corrosion problems with heat exchangers.
In addition, the flow rate required for a single-family house is high, and ground-water systems
are difficult to be used widely in densely populated areas. Inclusion of the cost of providing the
heat source has a significant impact on the economic attractiveness of these systems. A rule of
thumb seems to be that such systems are economic if both the supply and the reinjection sources
are available, marginally economic if one is available, and not cost-competitive if neither source is
available. In addition, if a well has to be sunk, the necessity for drilling teams to act in coordination
with heating and ventilation contractors can pose problems. Also, many local legislatures impose
severe constraints when it comes to interfering with the water table and this can pose difficulties
for reinjection wells.
Surface water such as rivers and lakes is in principle a very good heat source, but suffers from the
major disadvantage that either the source freezes in winter or the temperature can be very close to
0
◦
C (typically 2–4
◦
C). As a result, great care is needed to avoid freezing on the evaporator. Where
the water is thermally polluted by industry or by power stations, the situation is somewhat improved.
Sea water appears to be an excellent heat source under certain conditions and is mainly used
for medium-sized and large heat pump installations. At a depth of 25–50 m, the sea temperature is
constant (5−8
◦
C), and ice formation is generally not a problem (freezing point −1to−2
◦
C). Both
direct expansion systems and brine systems can be used. It is important to use corrosion-resistant
heat exchangers and pumps and to minimize organic fouling in sea water pipelines, heat exchangers,
evaporators, and so on. Where salinity is low, however, the freezing point may be near 0
◦
C, and
the situation can be similar to that for rivers and lakes in regard to freezing.
Waste water and effluent are characterized by a relatively high and constant temperature through-
out the year. Examples of possible heat sources in this category are effluent from public sewers
(treated and untreated sewage water) in a temperature range of 10–20
◦
C throughout the year, indus-
trial effluent, cooling water from industrial processes or electricity generation, and condenser heat
from refrigeration plants. Condenser cooling water for electricity generation or industrial effluent
could also be used as heat sources. The major constraints for use in residential and commercial
buildings are, in general, the distance to the user and the variable availability of the waste heat
flow. However, waste water and effluent serve as an ideal heat source for industrial heat pumps to
achieve energy savings in industry.
Apart from surface water systems which may be prone to freezing, water source systems generally
do not suffer from the low-temperature problems of air source heat pumps because of the higher
year-round average temperature. This ensures that the temperature difference between the source
and sink is smaller and results in an improvement of the performance of the heat pump. The
evaporator must, however, be cleaned regularly. The heat transfer at the evaporator can drop by as
much as 75% within approximately 5 months if it is not kept properly clean. The costs of cleaning
become relatively low for larger projects so that the use of this source may become economic.
6.5.3 Soil and Geothermal
Soil or sub-soil (ground source) systems are used for residential and commercial applications and
have similar advantages to water source systems, because of the relatively high and constant annual
temperatures resulting in high performance. Generally the heat can be extracted from pipes laid
horizontally or sunk vertically in the soil. The latter system appears to be suitable for larger heat
290 Refrigeration Systems and Applications
pump systems. In the former case, adequate spacing between the coils is necessary, and the avail-
ability of suitably large areas (about double the area to be heated) may restrict the number of
applications. For the vertical systems, variable or unknown geological structures and soil thermal
properties can cause considerable difficulties. Owing to the removal of heat from the soil, the soil
temperature may fall during the heating season. Depending on the depth of the coils, recharging
may be necessary during the warm months to raise the ground temperature to its normal lev-
els. This can be achieved by passive (e.g., solar irradiation) or active means. In the later case,
this can increase the overall cost of the system. Leakage from the coils may also pose problems.
Both the horizontal and vertical systems tend to be expensive to design and install and, more-
over, involve different types of experts (one for heating and cooling and the other for laying the
pipe work).
Rock (geothermal heat) can be used in regions with no or negligible occurrence of groundwater.
Typical bore hole depth ranges from 100 to 200 m. When large thermal capacity is needed the
drilled holes are inclined to reach a large rock volume. This type of heat pump is always connected
to a brine system with welded plastic pipes extracting heat from the rock. Some rock-coupled
systems in commercial buildings use the rock for heat and cold storage. Because of the relatively
high cost of the drilling operation, rock is seldom economically attractive for domestic use.
The ground constitutes a suitable heat source for a heat pump in many countries. At small depths,
temperatures remain above freezing. Furthermore, the seasonal temperature fluctuations are much
smaller than those of the air. Heat is extracted from the soil by means of a glycol solution flowing
through tubing embedded in the ground. If a horizontal grid of tubing is utilized, several hundred
square meters of surface area are needed to heat a single family building. In urban areas such space
is rarely available. In addition, considerable costs are involved. For these reasons vertical ground
heat exchangers are more preferred presently.
Geothermal heat sources for heat pumps are currently utilized in various countries, particularly
in the United States, Canada, and France. These resources are generally localized and do not
usually coincide with areas of high-density population. In addition, the water often has a high salt
component which leads to difficulties with the heat exchangers. Owing to the high and constant
temperatures of these resources, the performance is generally high.
6.5.4 Solar
Solar energy, as either direct or diffuse radiation, is similar to air in its characteristics. A solar-source
heat pump or a combined solar/heat pump heating system has all the disadvantages of the air source
heat pump, such as low performance and extreme variability, with the additional disadvantage of
high capital cost, particularly because in all cases a heat-store or back-up system is required. In
areas with high daily irradiation, this may not be the case.
Each of the above-mentioned heat sources for heat pumps presents some drawbacks. Presently,
considerable research is devoted to the technical problems involved and alternative heat sources.
Also, solar energy may provide a suitable heat source. Unfortunately, solar systems presently are
very costly. Furthermore, the intermittent character of solar energy requires the use of large and
costly storage volumes.
6.6 Classification of Heat Pumps
A systematic classification of the different types of heat pumps is difficult because the classification
can be made from numerous points of view, for example, purpose of application, output, type of
heat source, and type of heat pump process. If the heat is distributed via a mass flow, for example,
warm air or warm water, this mass flow is called the heat carrier .