Dinc Ibrahim. Refrigeration systems and applications 2th edition
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Heat Pumps 341
6.25.5 Fan Efficiency and Power Requirements
A major source of energy loss in a heat pump is outdoor and indoor fan power consumption for air
source and air sink units. For example, a slower, larger diameter fan uses less power than a normal
fan at the same capacity; many units now incorporate these larger fans.
6.25.6 Compressor Modification
Compressor developments are directed toward improved reliability, better performance under all
load conditions, and improved low-temperature heating capacity. The basic mismatch between the
variation with ambient temperature of typical building losses (or gains) and the heating (or cooling)
capacity of the heat pump remains. This mismatch arises, as indicated earlier, because of the
necessity for supplementing the compressor at low outdoor temperatures with electric resistance or
other heating devices as well as to on–off cycling under part-load conditions. Capacity modulation
tries to overcome these difficulties by achieving a better match between capacity and load.
6.25.7 Capacity Modulation
Capacity modulation can be brought about in a number of ways. A number of two-speed compressor
systems are now available involving speed-halving, which is achieved by switching from a two-pole
to a four-pole motor configuration. Modulation is also achieved by using multiple compressors and
cylinder off-loading as the load varies. Variable speed systems use an electronic “black-box” static
inverter unit to achieve power conversion. The high first cost of some of those high-performance
systems is a barrier to greater penetration of the market, which could be overcome, in the case
of the variable speed systems, by significant cost reductions in the electronic inverter technol-
ogy. Capacity and speed controls have been applied cost-effectively for large compressors. For
smaller units, capacity control can be expensive and part-load operation is usually less efficient than
full-load operation.
6.25.8 Heat Exchangers
Heat exchanger effectiveness can be improved by increasing the heat-transfer coefficient across
the heat exchanger or by increasing the coil area. The geometry of the heat exchangers greatly
influences the performance of heat pumps. To achieve this, many of the systems commercially
available have larger coil areas than the older models.
6.25.9 Refrigerants
Refrigerants that may be useful on the basis of suitable boiling points, modest pressures, reasonable
performance, and so on, can be classified as HCFCs, HFCs, blends, and natural working fluids
(Table 6.12). The gases are much cheaper than the fluorocarbons and are preferred in some industrial/
commercial applications. They may, however, be toxic, noxious, poisonous, and flammable
and many pose explosion hazards. The most widely used fluorocarbon refrigerants were R-11,
R-12, R-22, and R-502, accounting for nearly 92% of the consumption in the world. Concern about
the potential impact of fluorocarbons on the atmosphere has been widespread for some time. The
342 Refrigeration Systems and Applications
Table 6. 12 Some common heat pump refrigerants.
Common Groups Examples
HCFCs Phased out as stated above.
HFCs R-134a, R-152a, R-32, R-125, R-143a
Blends R-407C, R-410A
Natural working fluids Ammonia, Water, Carbondioxide, HCs
sources of fluorocarbons vary from aerosols to refrigerators, manufacturing processes, and heat
pumps. Normally, alternative working fluids for heat pumps are expected to fulfill the requirements
(e.g., reliability, cost effectiveness, environmental friendliness, commercial availability, etc.) as
given in Chapter 2. Moreover, the COP of the systems should be maintained or be even higher, in
order to make heat pumps a potential energy-saving alternative.
6.26 Operational Aspects of Heat Pumps
As mentioned earlier, the task of the heat pump is to transport heat for either cooling or heating.
The heat carrier of the system is the refrigerant. The term heat pump includes the refrigeration part
of the total plant, that is, generally the heat exchanger on the cold side, the temperature-raising
device with the introduction of drive energy, the heat exchanger on the warm side, and, in most
cases, an expansion device for completing the refrigeration cycle. All refrigeration machines are
suitable for use as heat pumps. The following have been used (Dincer, 2003)
• the cold-air machine, using air as the working fluid.
• the cold vapor machine, using evaporation and condensation of the working fluid which can be
water vapor or a coolant; the energy can be supplied using compression, absorption, or the steam
jet principle.
• the thermoelectric principle (Peltier effect). As in refrigeration, the cold vapor heat pump with
mechanical compression of the coolant vapor is by far the most important. The highly developed
technology of this design can also be fully utilized for heat pumps.
In all the designs mentioned, it is possible to use simultaneously both the cooling and the heating
effects to good purpose. Varying utilizations are also possible at different times, for example, cooling
in summer and heating in winter, as required, in particular, for air conditioning. There are two
methods for this type of operation:
• Changing the flow medium in the heat exchangers; for example, in the heat exchanger on the
cold side, using cold water during summer for air conditioning (cooling) and groundwater as the
heat source during winter or in the heat exchanger on the warm side, using water for the cooling
system in summer and warm water for heating the building in winter.
• Changing the coolant circuit so that, during summer operation, the heat exchanger on the warm
side, that is, that which transfers heat to groundwater, becomes the heat exchanger on the cold side
in winter by extracting heat from the groundwater. In the same way, the other heat exchanger,
which in summer cools the cold water circuit on the cold side for the air conditioning, in
Heat Pumps 343
winter becomes the heat exchanger on the warm side which heats the heating water for the
air conditioning. The coolant circuit changeover is possible only in thermoelectric systems and
in cold vapor systems with piston compressors and appropriate reversing valves, because the
difficulty of changeover in other systems is too great.
The following are some guidelines that should be followed to provide efficient, comfortable
operation of air source heat pumps (Carrier, 2000).
• Do not set the temperature back at night or when you are at work unless a programmable heat
pump thermostat is used. Since heat pumps operate differently than fossil fuel heating systems,
setback of a standard heat pump thermostat can actually increase energy consumption. This is due
to the use of supplemental heaters to bring the house temperature back to the desired setpoint.
Use of supplemental heaters will reduce the efficiency of the heat pump system and result in
higher energy costs.
• Keep the temperature setpoint consistent. A standard heat pump thermostat has two controls, one
for the heat pump and one for the supplemental heat. If the temperature difference between the
room and thermostat setpoint is more than 2 or 3
◦
C, the supplemental heat will be activated.
Manually adjusting the thermostat will result in greater reliance on the supplemental heaters and
will reduce the efficiency of the heat pump system and increase operating costs.
• Replace filters regularly. Vacuum dirt and dust from the indoor coil once a year to prevent
restricted air flow.
• Adequate air flow through a heat pump system is critical to ensure efficient and comfortable
operation.
• Keep supply vents open and free from obstruction. Closing off supply vents will restrict airflow,
and reduce system efficiency and the life of the compressor.
A heat pump is one of the most energy-efficient heating and cooling systems available today.
Unlike other types of heating systems which convert fuel or electricity directly to heat, a heat pump
is designed to move heat from one place to another. Even at temperatures as cold as −18
◦
Cor
below, the heat pump is able to extract heat from outside air to use in heating a house.
6.27 Performance Evaluation Aspects of Heat Pumps
The heat delivered by a heat pump is theoretically the sum of the heat extracted from the heat source
and the energy needed to drive the cycle. The steady-state performance of an electric compression
heat pump at a given set of temperature conditions is referred to as the COP which is defined as
the ratio of heat delivered by the heat pump and the electricity supplied to the compressor.
For engine and thermally driven heat pumps the performance is indicated by the PER as given
earlier. The energy supplied is then the higher heating value of the fuel supplied. For electrically
driven heat pumps, a PER can also be defined by multiplying the COP with the power generation
efficiency. The COP or PER of a heat pump is closely related to the temperature lift, that is, the
difference between the temperature of the heat source and the output temperature of the heat pump.
The COP of an ideal heat pump is determined solely by the condensation temperature and the
temperature lift (condensation–evaporation temperature).
Figure 6.32 shows the COP for an ideal heat pump as a function of temperature lift, where
the temperature of the heat source is 0
◦
C. Also shown is the range of actual COPs for various
types and sizes of real heat pumps at different temperature lifts. The ratio of the actual COP of
a heat pump and the ideal COP is defined as the Carnot efficiency. The Carnot efficiency varies
344 Refrigeration Systems and Applications
20 30 40 50 60
0
5
10
15
Coefficient of performance, COP
h
c
= 0.5 – high-efficiency residential and commercial units
h
c
= 0.3 – conventional domestic space conditioning units
h
c
= 0.6 – large, advanced electric heat pumps
Figure 6.32 Coefficient of performance profiles (Courtesy of IEA-HPC ).
Table 6. 13 Typical COP/PER range for heat pumps with different drive energies.
Heat Pump Type COP PER
Electric (compression) 2.5–5.0 –
Engine (compression) – 0.8–2.0
Thermal (absorption) – 1.0–1.8
Source: IEA-HPC (2001).
from 0.30 to 0.5 for small electric heat pumps and 0.5 to 0.7 for large, very efficient electric heat
pump systems. The COP/PERs for different heat pump types at evaporation 0
◦
C and condensing
temperature 50
◦
C are given in Table 6.13.
As stated earlier, the operating performance of heat pumps over the season is rated by the seasonal
factors, for example, SEER and HSPF. In fact, it takes into account the variable heating and/or
cooling demands, the variable heat source and sink temperatures over the year, and includes the
energy demand, for example, for defrosting. These seasonal factors can also be used for comparing
heat pumps with conventional heating systems (e.g., boilers), with regard to primary energy saving
and reduced CO
2
emissions. For evaluating electric heat pumps the power generation mix and the
efficiencies of the power stations must be considered.
Heat Pumps 345
Table 6. 14 Typical COP/PER for heat pumps with different drive energies.
Heat Pump Type COP PER
MVR 10.0–30.0 –
Closed cycle, electric 3.0–8.0 –
Closed cycle, engine – 1.0–2.0
Absorption (Type I) – 1.1–1.8
Heat transformer (Type II) – 0.45–0.48
Source: IEA-HPC (2001).
6.27.1 Factors Affecting Heat Pump Performance
The performance of heat pumps is affected by a large number of factors. For heat pumps in buildings
these include
• the climate (annual heating and cooling demand and maximum peak loads),
• the temperatures of the heat source and heat distribution system,
• the auxiliary energy consumption (pumps, fans, supplementary heat for bivalent system, etc.),
• the technical standard of the heat pump,
• the sizing of the heat pump in relation to the heat demand and the operating characteristics of
the heat pump, and
• the heat pump control system.
Industrial heat pumps often have a higher COP/PER than the heat pumps for buildings. This is
mainly due to small temperature lifts and stable operating conditions. Typical COP/PER ranges for
industrial heat pumps are given in Table 6.14.
As mentioned earlier, a heat pump may use only one-third as much energy as electric resistance
heat (e.g., electric furnace and baseboards) during mild winter weather (outdoor temperature about
7
◦
C). In the heat pump industry, this is described as a COP of 3. COP is the ratio of heat output to
electrical energy input. A number of factors prevent air source heat pumps from maintaining COPs
of 3 throughout the heating season:
• Air temperature. Heat pumps operate at temperatures colder than 7
◦
C much of the winter.
When the temperature is −6.6
◦
C, the COP of the heat pump will be closer to 2 than 3.
• Defrost. If there is very cold refrigerant flowing through the outdoor heat exchanger, ice can
form on the coils, just as it does in freezers. When outdoor temperature is below 6.4
◦
C, the heat
pump may need to defrost periodically. To melt the ice, the heat pump takes heat from the house
to heat the outdoor coils, which reduces average heat pump efficiency.
• Supplemental heat. As the air gets colder outside, the heat pump fails to provide the necessary
heat to keep the house comfortable. At some outdoor temperature it will be too cold for the
heat pump to provide all the heat the house needs. To make up the difference, heat pumps have
a supplemental heating system, usually electric resistance coils (basically an electric furnace
inside the heat pump indoor cabinet). This part of the system is sometimes called “back-up” or
“emergency” heat because the same coils can be used to provide some or all of the heat in the
event of heat pump failure. Since the supplemental electric heating system does not operate with
the same efficiency as the heat pump (the COP of electric resistance heat is 1), the total heat
346 Refrigeration Systems and Applications
pump COP will be much lower when the supplemental heat is on. In this regard, gas and oil
furnaces provide supplemental heat in some new homes with heat pumps. Existing gas and oil
furnaces can also be used as supplemental heat with “add-on” heat pumps that allow a heat pump
to be added to an existing system. Controls for these systems are different since the combustion
system and the heat pump do not operate at the same time. Special care is required to ensure that
there are proper air flows for both the heat pump and the furnace. The economics of purchasing
and operating this type of system will depend on local energy costs.
• Cycling losses. When heating systems first start up, they need to operate for a while just to get
warm enough to heat the house. When they are shut off, there is still heat in the system that
does not get into the house. The losses associated with stopping and starting the heat pump are
referred to as cycling losses.
• Heating season performance factor (HSPF). The industry standard test for overall heating
efficiency provides this rating known as HSPF. This laboratory test attempts to take into account
the reductions in efficiency caused by defrosting, temperature fluctuations, supplemental heat,
fans, and on/off cycling. The higher the HSPF, the more efficient the heat pump. A heat pump
with an HSPF of 6.8 has an average COP of 2 for the heating season. To estimate the average
COP, we divide the HSPF by 3.6. In fact, HSPF is a rough predictor of the actual installed
performance. HSPF assumes specific conditions that are unlikely to coincide with the climate.
Most utility-sponsored heat pump programs require that heat pumps have an HSPF of at least
6.8. In practice, many heat pumps meet this requirement. Some heat pumps have HSPF ratings
above 9. In general, more efficient heat pumps are more expensive.
• Seasonal energy efficiency ratio (SEER). Cooling performance is rated using the SEER. The
higher the SEER the more efficiently the heat pump cools. The SEER is the ratio of heat energy
removed from the house compared to the energy used to operate the heat pump, including fans.
The SEER is usually noticeably higher than the HSPF since defrosting is not needed and there is
no need for expensive supplemental heat during air-conditioning weather. Except in an area where
cooling is more important than heating, the HSPF is a more important measure of efficiency than
the SEER.
6.28 Ground-Source Heat Pumps (GSHPs)
Efficient heating performance makes GSHP a good choice for the heating and cooling of commercial
and institutional buildings, such as offices, stores, hospitals, hotels, apartment buildings, schools,
restaurants, and so on. The three main parts are the heat pump itself which includes the compressor,
blower, air, and water coils; the liquid heat exchange medium which is either well water, pond,
lake, or river water, or a buried earth loop filled with water and glycol; and the air delivery system
(ductwork).
GSHP systems can heat water or heat/cool the interior space by transferring heat from the
ground outside, but they can also transfer heat within buildings with a heat-producing central core.
GSHP technology can move heat from the core to perimeter zones where it is required, thereby
simultaneously cooling the core and heating the perimeter.
GSHP systems are also used as heat recovery devices to recover heat from building exhaust air
or from the waste water of an industrial process. The recovered heat is then supplied at a higher
temperature at which it can be more readily used for heating air or water.
As with air-to-air heat extraction technology, geothermal (groundwater/ground source) technol-
ogy utilizes a type of heat pump known as a GSHP. This type of geothermal heat pump device
extracts its heat from water rather than from air. While the principles are fundamentally similar,
the methodology varies in that water is pumped through a special type of heat exchanger and is
either “chilled” by the evaporating refrigerant (in the heating mode) or heated by the condensing
refrigerant (in the cooling mode).
Heat Pumps 347
A GSHP is essentially an air conditioner that also runs in reverse: in winter, it extracts heat from
the ground for heating. These systems have been used more efficiently than most other systems.
For example, when you put one unit of energy into a furnace, you get somewhat less than one
unit back out as heat. But when we put the same unit of energy into a heat pump, we get more
than triple as the return. That is because heat pumps do not use energy to create heat; instead, they
move heat that already exists.
1. GSHPs start with a closed loop of buried pipes containing a fluid that can carry heat. The pipes
may lie in a shallow long curved trench, or they may jut deep into the ground.
2. For heating (Figure 6.33), the pipe fluid absorbs heat from the earth. The fluid passes through
a heat exchanger (acting as an evaporator), where it transfers heat to a refrigerant.
3. The refrigerant, which flows through another closed loop in the heat pump, then boils. The
vaporized refrigerant travels to the compressor, where its temperature and pressure are increased.
4. The hot gas continues to two heat exchangers (acting as condensers), one to heat the house’s
water and the other for space heating. At each, the refrigerant gives up some heat. A fan blows
across the space-heat condenser to move the warmed air through the house. The refrigerant,
again a liquid, repeats the process.
5. In summer, the cycle reverses to remove heat from the house. Some of the heat is used for hot
water; the remainder is dumped into the earth via the ground loop.
GSHP systems use the earth’s stored energy and simply transfer it into a home or business for
heating, cooling, and generating hot water. These efficient and economical heating and cooling
systems draw in well water, extract energy from it, and transfer heat into the home or business.
1
2
3
4
5
Ground loop
Hot water
condenser
Space-heat
condenser
Compressor
Receiver
Reversing
valve
Evaporator
Expansion
valve
Check
valve
Fan
Figure 6.33 Schematic of a ground-source heat pump.
348 Refrigeration Systems and Applications
Ground
direct
expansion
Groundwater
(well water)
Ground
coupled
Below
ground
Figure 6.34 Three GSHP categories.
There are several reasons to consider a GSHP for applications, including the following:
• Efficient heating and cooling. When measured against other existing systems, the heat pump
provides higher COP.
• Durability and low maintenance. They require 30–50% less maintenance than a fossil-fuel-
based heating and cooling system. Most geothermal systems can run without any repair main-
tenance for over 20 years. The only replacement required is for the air filter (e.g., every 3
months).
• Low domestic water heating cost on demand. All residential and many commercial heat pumps
use an optional hot water generator circuit which can save 50–65% on water heating costs.
Three GSHP categories are summarized in Figure 6.34, namely groundwater, ground coupled,
and direct expansion. The first two use water, or a water solution with an antifreeze (brine or glycol),
for intermediate transport of heat from (to for cooling) the ground. In groundwater systems, water
is removed from underground, though often reinjected later. In ground-coupled systems, a closed
loop or loops of recirculated fluid is (are) used to couple the heat pump to the ground. For direct
expansion systems, refrigerant is actually circulated in a buried heat exchanger; the name derives
from direct expansion (evaporation) of the refrigerant in the ground without an intermediate heat
transport fluid. The terms ground and earth are usually synonymous, depending on the context.
6.28.1 Factors Influencing the Impact of GSHPs
The impact of heat pumps is determined by a number of factors which are common to many
countries and regions but which vary considerably, depending on the circumstances under which a
heat pump is being deployed. The main factors which influence the impact of heat pumps include
the following:
• climatical conditions,
• energy policy considerations,
• building factors,
• utility considerations,
• economic criteria, and
• competing systems.
Heat Pumps 349
6.28.2 Benefits of GSHPs
A GSHP is one of the most efficient means available to provide space heating/cooling for homes
and offices. It transfers the heat located immediately under the earth’s surface (or in a body of
water) into a building in winter, using the same principle as a refrigerator that extracts heat from
food and rejects into a kitchen. A heat pump takes heat from its source at low temperature and
discharges it at a higher temperature, allowing the unit to supply more heat than the equivalent
energy supplied to the heat pump. Many people are familiar with air-to-air heat pumps, which use
outdoor air as the source of heat. These units are well suited for moderate climates, but they do
not operate efficiently when the outdoor temperature drops below −10
◦
C and there is little heat
left in the air to extract. Here we list some of the key benefits of GSHP systems as follows:
In terms of cost savings,
• lower annual heating and cooling bills – 30–60% on average,
• option to use excess heat for water heating,
• high reliability, low maintenance, and
• less space requirements for heating and cooling equipment-frees space for manufacturing or work
facilities.
In terms of enhanced comfort and appearance,
• individual zone control,
• outdoor equipment not necessary,
• safe, noncombustion process,
• quiet operation,
• greater convenience,
• simultaneous heating and cooling of different zones, and
• no more fuel deliveries.
In terms of improved environment,
• renewable, environmentally friendly energy source,
• reduced harmful emissions (no on-site combustion of fossil fuels), and
• self-contained system – less potential for refrigerant leaks.
In addition to heating and cooling the building interior, GSHP units can also provide domestic
water. Some major features and technical details of GSHPs are as follows.
6.28.2.1 Efficiency and COP
The major advantage of a GSHP system is that the heat obtained from the ground (via the condenser)
is much greater than the electrical energy that is required to drive the various components of the
system. The efficiency of a unit is the ratio of heat energy provided versus the electrical energy
consumed to obtain that heat, referring to the COP. In some countries, there is a minimum limit
for the COP of the GSHP units. For example, in Canada it must exceed 3.0 (i.e., for every kilowatt
of electricity needed to operate the system, the GSHP provides 3 kW of heat energy).
6.28.2.2 Cost
With a COP of 3.0, the cost of heating would be one-third (i.e., two-thirds less) of the cost to operate
an electric resistance heating system, such as baseboards or electric furnace. With a COP of 6.0,
the savings can be as much as three-quarters of the price of electric heating and cooling. As earth
energy technology improves and the COP increases above 6.0, the operating savings also increase.
350 Refrigeration Systems and Applications
6.28.2.3 Comfort
A GSHP system warms air in smaller increases over a longer period of time, compared to the burst
of a combustion oil or gas furnace. As a result, homeowners notice a stable level of heat with no
peaks or troughs, less drafts, and so on.
6.28.2.4 Environment
GSHP is preferred by many people and institutions because it is an environmentally benign tech-
nology, with no emissions or harmful exhaust. For example, the GSHP industry was the first in
Canada to move away from damaging CFCs. Although most GSHP units require electricity to
operate the components, a high COP means that GSHP systems provide a significant reduction in
the level of CO
2
,SO
2
,andNO
x
emissions (all linked with the issue of greenhouse gas emissions
and global warming).
6.28.2.5 Suitability for Public and Commercial Utilization
GSHPs are ideally suited for public and commercial places, due to the following advantages:
• Sports fields and parking lots provide an excellent heat sink for the thermoplastic loops, and are
easy and inexpensive to install.
• Decentralized control allows simultaneous heating or cooling of different zones, allowing each
teacher to set a different temperature for any room in the school.
• The heating/cooling loads of a school are well-matched to the performance output of a heat
pump.
• Locating equipment inside the building reduces possible damage from adverse weather condi-
tions and vandalism, and eliminates the need for roof penetrations (further securing the building
envelope and allowing enhanced design with nonhorizontal rooftops).
• The smaller mechanical space requirements for heat pumps and the elimination of large air ducts
result in more available space for classrooms and lower costs (the physical dimensions of a
school building can drop by 5%).
• Student health and safety is increased by the elimination of combustion heat and the need for
fuel storage, as well as the absence of possible carbon monoxide leaks and superior levels of
indoor air quality.
• Modularity allows reduced inventory for boilers and chillers.
• GSHP installations are known for quiet operation, high reliability, greater ventilation and dehu-
midification, and less drafts.
6.28.2.6 Other Possible Applications
GSHP units can be used for the dehumidification of indoor swimming pool areas, where the unit
can dehumidify the air and provide condensation control with a minimum of ventilation air. The
heat recovered from the condensed moisture is then used for heating domestic/pool water or for
space heating.
6.28.3 Types of GSHP Systems
Heat pumps tap the natural geothermal energy underground to heat and cool houses. A fluid is
circulated through an underground loop of polyethylene pipe. The fluid absorbs heat from the earth