Dinc Ibrahim. Refrigeration systems and applications 2th edition

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Heat Pumps 291

Customarily in the United States heat pumps are classiﬁed for the heating of buildings according

to the type of heat source (ﬁrst place) and type of heat carrier (second place). A distinction can be

made between the terms:

• heat pump, covering only the refrigeration machine aspect, and

• heat pump plant, which, besides the heat pump itself, also contains the heat source.

This differentiation is due to heat from the heat source being transferred to the cold side of the

heat pump by an intermediate circuit, the cold carrier.

Another usual classiﬁcation differentiates between

• primary heat pumps which utilize a natural heat source present in the environment, such as

external air, soil, groundwater, and surface water,

• secondary heat pumps which reuse waste heat as heat source, that is, already used heat, such as

extract air, waste water, waste heat from rooms to be cooled, and

• tertiary heat pumps which are in series with a primary or secondary heat pump in order to raise

the achieved, but still relatively low, temperature further, for example, for hot water preparation.

Furthermore, heat pumps are generally classiﬁed by their respective heat sources and sinks.

Depending on cooling requirements, various heat source and heat sink arrangements are possible

in practical applications. The six basic types of heat pump are as follows:

• water-to-water,

• water-to-air,

• air-to-air,

• air-to-water,

• ground-to-water, and

• ground-to-air.

In each of these types the ﬁrst term represents a heat source for heating or a heat sink for cooling

applications. Schematics of the common types of heat pumps are also shown in Figure 6.2.

6.6.1 Water-to-Water Heat Pumps

In these heat pump systems, the heat source and the heat sink are water. The heat pump system

takes heat from a water source (by coil A) while simultaneously rejecting it to a water heat sink

(by coil B) and either heats or cools a space or a process. In practice, there are many sources of

water, for example, waste water, single or double well, lake, pool, and cooling tower. These heat

pumps use less electricity than other heat pumps when they are properly maintained. However,

without proper maintenance the operating costs increase dramatically.

Table 6.5 shows typical COPs for a water-to-water heat pump operating in various heat distri-

bution systems. The temperature of the heat source is 5

◦

C, and the heat pump’s Carnot efﬁciency

is 50%.

6.6.2 Water-to-Air Heat Pumps

Some heat pumps have been designed to operate utilizing a water source instead of an air source

simply by designing the outdoor heat exchanger to operate between the heat pump working ﬂuid

292 Refrigeration Systems and Applications

Heat pump

Heat pumpHeat pump

Heat pump

Water

Air

Ground

(a) Water-to-water (b) Water-to-air

(

)

Ground-to-water

(

)

Ground-to-air

Figure 6.2 Types of heat pumps (here, the ﬁrst part refers to the heat source for the outdoor coil during the

heating process and the second part indicates the medium treated by the refrigerant in the indoor coil).

Table 6. 5 Example of how the COP of a water-to-water heat

pump varies with the distribution/return temperature.

Heat distribution system (supply/return temperature) COP

Conventional radiators (60/50

◦

C) 2.5

Floor heating (35/30

◦

C) 6.0

Modern radiators (45/35

◦

C) 3.5

Source: IEA-HPC (2001).

and water instead of between the working ﬂuid and air. These so-called water-to-air heat pumps

have advantages over the air-to-air type if a relatively warm source of water is available which

does not require an excessively large amount of pumping power. In particular, industrial waste heat

might be used.

In this case, the difference from the water-to-water heat pump is in the method of treating

the air. This system provides heating and cooling of air with water as the heat sink or source.

The same sources of water can be used in these systems. These systems are less efﬁcient than the

water-to-water systems because of the much lower heat-transfer coefﬁcient of air. These systems

are commonly used in large buildings and sometimes in industrial applications to provide hot or

cold water.

The water-to-air heat pump removes heat from water and converts it to hot air in exactly the

same way that a cold water drinking fountain removes the heat from the water and discharges

the heat from the side or back of the drinking fountain.

Heat Pumps 293

6.6.3 Air-to-Air Heat Pumps

These systems use air on both sides (on coils) and provide heating or cooling. In the cooling mode,

heat is removed from the air in the space and discharged to the outside air. In the heating mode, heat

is removed from the outside air and discharged to air in the space. In these units, it is necessary to

provide defrost controls and periods to maintain maximum efﬁciency. These are the most popular

systems for residential and commercial applications because of easy economical installation and

lower maintenance cost.

Depending on climate, air source heat pumps (including their supplementary resistance heat) are

about 1.5–3 times more efﬁcient than resistance heating alone. Operating efﬁciency has improved

since the 1970s, making their operating cost generally competitive with combustion-based systems,

depending on local fuel prices. With their outdoor unit subject to weathering, some maintenance

should be expected.

The most popular heat pump is the air source type (air-to-air) which operates in two basic modes:

• As an air conditioner, a heat pump’s indoor coil (heat exchanger) extracts heat from the interior

of a structure and pumps it to the coil in the unit outside where it is discharged to the air outside

(hence the term air-to-air heat pump).

• As a heating device, the heat pump’s outdoor coil (heat exchanger) extracts heat from the air

outside and pumps it indoors where it is discharged to the air inside.

6.6.4 Air-to-Water Heat Pumps

In Figure 6.2, these systems work in reverse of the water-to-air heat pumps: they extract heat from

ambient or exhaust air to heat or preheat water used for space or process heating. The system is

simply reversed. Heat is extracted from the air inside the home and transferred to water and put

back in the ground. All the householders select the temperature that makes their homes as cool as

they wish.

6.6.5 Ground-to-Water and Ground-to-Air Heat Pumps

In these systems, coil A in Figure 6.2 is buried underground and heat is extracted from the ground.

These heat pump systems have limited use. Practical applications are limited to space heating where

the total heating or cooling effect is small, and the ground coil size is equally small. This system

requires the burial of several meters of pipe per ton of refrigeration, thus requiring a large amount

of land.

6.6.6 Basic Heat Pump Designs

The four basic heat pump designs for space heating and cooling employ

• air as the heat source/sink and air as the heating and cooling medium,

• air as the heat source/sink and water as the heating and cooling medium,

• water as the heat source/sink and air as the heating and cooling medium, or

• water as heat source/sink and water as the heating and cooling medium.

Each of these basic designs can supply the required heating and cooling effect by changing

the direction of the refrigerant ﬂow, or by maintaining a ﬁxed refrigerant circuit and changing

the direction of the heat source/sink media. A third alternative is to incorporate an intermediate

transfer ﬂuid in the design. In this case the direction of the ﬂuid is changed to obtain heating or

294 Refrigeration Systems and Applications

Table 6. 6 Typical delivery temperatures for various heat and cold

distribution systems.

Application Supply Temperature Range (

◦

Air distribution

• Air heating

• Floor heating (low-temperature)

30−50

30−45

Hydronic systems

• Radiators

• High-temperature radiators

• District heating (hot water)

45−55

60−90

70−100

District heating

• District heating (hot water/steam)

• Cooled air

100−180

10−15

Space cooling

• Chilled water

• District cooling

5−15

5−8

Source: IEA-HPC (2001).

cooling and both the refrigerant and heat source/sink circuits are ﬁxed. The ﬁxed refrigerant circuit

designs, generally referred to as the indirect type of application, are becoming increasingly popular,

particularly in the larger capacities.

6.6.7 Heat and Cold-Air Distribution Systems

Air is one of the most widely used distribution media in the mature heat pump markets, especially

in the United States. The air is either delivered directly to a room by the space-conditioning unit or

distributed through a forced-air ducted system. The output temperature of an air distribution system

is usually in the range of 30−50

◦

C in heating applications.

Water distribution systems (so-called hydronic systems) are predominantly used in many

countries, particularly in Europe, Canada, and the northeastern part of the United States.

Conventional radiator systems require high distribution temperatures, typically 60−90

◦

C. Today’s

low-temperature radiators and convectors are designed for a maximum operating temperature of

45−55

◦

C, while 30−45

◦

C is typical for ﬂoor-heating systems. Table 6.6 summarizes typical

supply temperature ranges for various heat and cold distribution systems.

Because a heat pump operates most effectively when the temperature difference between the

heat source and heat sink (distribution system) is small, the heat distribution temperature for space

heating heat pumps should be kept as low as possible during the heating season.

6.7 Solar Heat Pumps

During the past two decades, there has been increasing interest in solar-assisted heat pumps. In this

system, solar energy is used as the heat source for a heat pump. The main advantage of this system

Heat Pumps 295

Saturated steam

Condenser

Compressor

Electricity

Evaporator

Circulation pump

Solar pond

Solar radiation

Expansion

valve

Figure 6.3 A solar heat pump (Tu, 1987).

is that solar energy supplies heat at a higher temperature level than other sources and, therefore,

provides an increase in the COP. As compared to a solar heating system without a heat pump,

collector efﬁciency and capacity are materially increased because of the lower collector temperature

required. Direct utilization of solar energy depends on sufﬁcient solar radiation. However, there

is still an extremely large amount of solar energy stored in the atmosphere and the ground at

temperatures between 0 and 20

◦

C, and this source is also available during the winter months,

which can be used by everyone. Research and development efforts for using solar energy for heat

pump operation have been focused on direct and indirect systems. In a direct system, refrigerant

evaporator tubes are incorporated with a solar collector system, usually ﬂat-plate collectors, and

their surface extracts heat from the outdoor air. The same surface may be employed as a condenser

using outdoor air as a heat sink for cooling. An indirect system utilizes either water or air circulated

through the solar collector.

Figure 6.3 shows a schematic diagram of a simple solar-assisted heat pump system that was

employed by Tu (1987). In the operation, heated water (∼80

◦

C) from the solar pond (1) and

the heat pump working ﬂuid (water) exchange heat in the evaporator (2). Water vapor from the

evaporator (2) is compressed by a compressor (3) and then condensed in the condenser (4), giving

up its heat to the user to produce steam at 120

◦

C from saturated water at 120

◦

Thermal storage coupled heat pumps are primarily associated with heat reclaim units for domestic

water heating. Sometimes they are used for heat storage as part of large commercial cool storage

installations. Thermal energy storage works well in such applications. Use of thermal storage has

recently become common in residential applications.

6.8 Ice Source Heat Pumps

Ice-making systems can be conﬁgured to provide heating alone, or heating and cooling, for a

building or process. The conversion of water to ice occurs at a relatively high evaporator temperature

and COP compared to air source heat pumps operating at low ambient temperatures. These systems

can provide the necessary heating, with the resulting ice disposed of by melting with low-grade heat,

for example, solar energy. In addition, they can be used for cooling through diurnal and seasonal

296 Refrigeration Systems and Applications

storage applications. The energy consumption savings resulting from the COP of a conventional

heat pump system are achieved. Using off-peak night and weekend rates reduces power costs, and

the ice produced is used for building cooling requirements. As a result, a system can be developed

that consumes less energy. Ice source heat pumps follow two basic approaches. The ﬁrst involves

using the ice builder principle, with coils in a large tank as the evaporator components of a heat

pump system. The second one utilizes a fragmentary-type ice maker as the evaporator of a heat

pump, with ice stored in a tank (as a mixture of ice and water). Many variations and combinations

may be developed from these basic systems.

6.9 Main Heat Pump Systems

Numerous heat pump cycles are available in practice, including

• vapor-compression cycle,

• Stirling cycle,

• Brayton cycle,

• Rankine cycle,

• absorption cycle,

• compression cycle,

• open cycle,

• recompression cycle, and

• compression/resorption cycle.

In conjunction with the above classiﬁcation, in the following sections, we will focus on the following

heat pump systems:

• vapor-compression heat pump systems,

• MVR heat pump systems,

• cascaded heat pump systems,

• Rankine powered heat pump systems,

• AHP systems (including heat transformers, resorption heat pumps, diffusion heat pumps),

• thermoelectric heat pump systems,

• vapor jet heat pump systems,

• quasi-open-cycle heat pump systems,

• chemical heat pump systems, and

• metal hydride heat pump systems.

6.10 Vapor-Compression Heat Pump Systems

A great majority of heat pumps on the market are simple four-component vapor-compression cycle

systems. The term vapor compression refers to the use of a mechanical compressor. During the

last three decades, many forms of these heat pump systems have been developed. Figure 6.4

shows the most common four-component vapor-compression heat pump system, consisting of four

main components:

• an evaporator (where heat is absorbed into a boiling refrigerant),

• a compressor (which raises the pressure, and hence temperature, of the refrigerant),

• a condenser (where the absorbed energy and compressor power are released), and

• an expansion device (where the refrigerant liquid changes from a high temperature liquid to a

low-pressure and low-temperature mixture of liquid and vapor).

Heat Pumps 297

Expansion valve

Evaporator

Condenser

Compressor

Source Sink

Figure 6.4 A single-stage vapor-compression heat pump.

It is important to recognize that any vapor-compression heat pump system has three distinct ﬂuid

circuits:

• A source from which heat is removed.

• A sink to which heat is delivered.

• A refrigerant circuit through which the energy is transferred.

Furthermore, the components of a vapor-compression heat pump are connected to form a closed

circuit, as shown in Figure 6.5. A volatile liquid, known as the working ﬂuid or refrigerant, circulates

through the four components. In the evaporator the temperature of the liquid working ﬂuid is kept

lower than the temperature of the heat source, causing heat to ﬂow from the heat source to the

Heat in

Heat out

Fuel

Engine

Exhaust− +

cooling

water heat

Compression

Evaporation

Evaporator

Expansion

Expansion valve

Condensation

Condenser

Compressor

Figure 6.5 Closed cycle, engine-driven vapor-compression heat pump (Courtesy of IEA-HPC ).

298 Refrigeration Systems and Applications

Heat in

Heat out

Compression

Evaporation

Evaporator

Expansion

Expansion valve

Condensation

Condenser

Electric motor

Electricity

Compressor

Figure 6.6 Closed-cycle, electric-motor-driven vapor-compression heat pump (Courtesy of IEA-HPC ).

liquid, and the working ﬂuid evaporates. Vapor from the evaporator is compressed to a higher

pressure and temperature. The hot vapor then enters the condenser, where it condenses and gives

off useful heat. Finally, the high-pressure working ﬂuid is expanded to the evaporator pressure and

temperature in the expansion valve. The working ﬂuid is returned to its original state and once

again enters the evaporator. The compressor is usually driven by an electric motor and sometimes

by a combustion engine.

An electric motor drives the compressor (see Figure 6.6) with minimal energy losses. The overall

energy efﬁciency of the heat pump strongly depends on the efﬁciency by which the electricity is

generated. When the compressor is driven by a gas or diesel engine (see Figure 6.5), heat from

the cooling water and exhaust gas is used in addition to the condenser heat. Industrial vapor-

compression type heat pumps often use the process ﬂuid itself as working ﬂuid in an open cycle.

These heat pumps are generally referred to as mechanical vapor recompressors,orMVRs.

In practice, several types of these four components are available. For example, evaporators and

condensers may be plate, shell-and-tube, or ﬁnned coil heat exchangers, and compressors may be

reciprocating, screw, rotary, centrifugal, and so on. Expansion devices may be ﬂoat and thermostatic

expansion valves or capillaries. A further discussion of these components is given in Chapter 3.

First, it should be pointed out that in principle the vapor-compression heat pumps utilized for

heating purposes also can be used for cooling purposes by incorporation of a four-way valve.

The latter allows for the inversion of the role of the heat exchangers (evaporator–condenser). In

practice, proper sizing of the heat exchangers is necessary, taking into account heating and cooling

loads. It is evident that the heat exchangers have to be of the same type (air or water). Hereafter,

special attention is given to the heating function of domestic heat pump systems while the above

remark concerning cooling operation should be kept in mind. Depending upon the capacity of the

heat pump compared to the peak heating load of the building, distinction can be made between

monovalent and bivalent heat pump systems:

• Monovalent systems. In a monovalent system the heat pump is designed to cover the peak

heating losses of the building. The heat pump is the only device delivering heat during the

whole heating season and therefore has to be sized at the maximum heating load of the building.

This leads to considerable overcapacity of the heat pump, which leads to more intermittent

Heat Pumps 299

use and therefore less efﬁcient heat pump operation and lower average COP values. Therefore,

monovalent systems are applied only when heat sources like groundwater and ground heat are

available. Such systems are characterized by large investments. On the other hand, the seasonal

COP values usually are higher than those which can be obtained with air systems.

• Bivalent systems. Bivalent systems are used whenever the performance of the heat pump dete-

riorates because of low source temperatures. Air heat pumps are of this type. In these systems

heat pumps of smaller capacity are utilized. They can provide the heat to the building down to

the outside temperature (e.g., design temperature). If it goes below this temperature, an auxiliary

system has to be called into operation. Depending upon the operation of the system one may

distinguish between parallel bivalent systems and alternative bivalent systems.

As a measure of the energetic performance of space heating heat pumps, one may take the

average COP over a heating season. The value of this COP will depend upon the characteristics of

the heating season. Table 6.7 lists typical seasonal COP for a number of systems (lowest average

temperature being −10

◦

C). Table 6.7 also lists the type and capacity of the auxiliary system and

the percentage of the heat load provided by the heat pump. Of course only bivalent heat pump

systems require auxiliary heat, the parallel systems requiring less of this heat than the alternative

ones. The seasonal COP values listed in Table 6.7 take into account the electricity consumed by

fans, pumps, and so on. The water–water monovalent heat pump shows the highest COP values

because of the high temperature of the heat source available throughout the whole heating season.

The lower temperatures of ground sources considerably reduce the COP values. For the bivalent

systems the heat pump COP loses a lot of its meaning since it does not take into account auxiliary

heat. Finally, Table 6.7 also lists the seasonal primary energy efﬁciency which is the ratio of heat

produced to the primary energy consumed to produce this heat. Here one has taken into account the

fact that all the heat pumps are electrically driven and that the efﬁciency of electricity production

Table 6. 7 Heat pump systems and their performance comparisons.

Heat Pump

System

Heat Source Source

Temperature

(

◦

Auxiliary

Heating

Type

Auxiliary

Heating

Capacity

(%)

Heat

Pump

Delivered

Heat (%)

Seasonal

COP

Primary

Energy

Efﬁciency

Monovalent

soil/water

Soil –2 to 10 – – 100 2.0 to 3.0 66 to 100

Monovalent

water/water

Ground or

surface water

10 – – 100 3.0 to 4.0 100 to 130

Bivalent

parallel

air/water

Outside air –10 to 15 Oil or gas

boiler

40 90 2.5 to 3.0 70

Bivalent

alternative

air/water

Outside air –10 to 15 Oil or gas

boiler

100 60 to 70 1.6 to 1.8 75

Bivalent

extracted

air/air or

water

Extracted air 18 to 20 Direct

electric

heating

75 to 85 50 to 60 1.6 50

Source: Berghmans (1983b).

300 Refrigeration Systems and Applications

is about 33%. The last column shows that only the monovalent heat pump systems using the soil

or ground may give rise to primary energy savings loads compared with conventional oil or natural

gas heaters.

The heat pump runs on an evaporation–condensation cycle, just like traditional air conditioners

and refrigerators. A typical vapor-compression heat pump has several vital components as mentioned

earlier: a refrigerant that circulates continuously in a closed-cycle, a motor-driven compressor, a

pair of heat exchangers which can alternate in the roles or condenser and evaporator, and some

form of expansion valve that can be used to control a pressure drop and hence the temperature

change of the working ﬂuid. A heat pump is essentially an air conditioner with a few additions.

A heat pump has a reversing valve, two metering devices and two bypass valves. This allows the

unit to provide both cooling and heating. Here, we discuss the vapor-compression heat pump for

both cooling and heating.

6.10.1 The Cooling Mode

In the cooling mode (e.g., cooling operation in summer), the outdoor coil becomes the condenser

and the indoor coil the evaporator. By condensing water vapor out of the circulated interior air,

the heat pump can also dehumidify like a traditional air conditioner. Figure 6.7 shows a vapor-

compression heat pump in cool mode. The cycle operates as follows: the compressor (1) pumps

the refrigerant to the reversing valve (2). The reversing valve directs the ﬂow to the outside coil

(condenser) where the fan (3) cools and condenses the refrigerant to liquid. The air ﬂowing across

the coil removes heat (4) from the refrigerant. The liquid refrigerant bypasses the ﬁrst metering

device and ﬂows to the second metering device (6) at the inside coil (evaporator) where it is

metered. Here it picks up heat energy from the air blowing (3) across the inside coil (evaporator)

and the air comes out cooler (7). This is the air that blows into the home. The refrigerant vapor

(8) then travels back to the reversing valve (9) to be directed to the compressor to start the cycle

all over again (1).

6.10.2 The Heating Mode

To provide heat from this same unit the evaporator and condenser must essentially switch places.

That is, heat must be moved from the outside air to the indoor coil for discharge. This is

Inside coil

Vapor

Compressor

Outside coil

Compressed vapor

Liquid refrigerant

Figure 6.7 A vapor-compression heat pump running for cooling (Courtesy of DHClimate Control).