Dinc Ibrahim. Refrigeration systems and applications 2th edition

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Advanced Refrigeration Cycles and Systems 251

ﬂowing through the nozzle 1. The expansion causes a drop in pressure and an enormous increase in

velocity. Owing to the high velocity, ﬂash vapor from the tank 2 is drawn into the swiftly moving

steam and the mixture enters the diffuser 3. The velocity is gradually reduced in the diffuser but

the pressure of the steam at the condenser 4 is increased 5–10 times more than that at the entrance

of the diffuser (e.g., from 0.01 to 0.07 bar).

This pressure value corresponds to the condensing temperature of 40

◦

C. This means that the

mixture of high-pressure steam and the ﬂash vapor may be liqueﬁed in the condenser. The latent

heat of condensation is transferred to the condenser water, which may be at 25

◦

C. The condensate

5 is pumped back to the boiler, from which it may again be vaporized at a high pressure. The

evaporation of a relatively small amount of water in the ﬂash tank (or ﬂash cooler) reduces the

temperature of the main body of water. The cooled water is then pumped as the refrigeration carrier

to the cooling-load heat exchanger.

An ejector was invented by Sir Charles Parsons around 1901 for removing air from steam engine

condensers. In about 1910, the ejector was used by Maurice Leblanc in the steam ejector refriger-

ation system. It experienced a wave of popularity during the early 1930s for air conditioning large

buildings. Steam ejector refrigeration cycles were later supplanted by systems using mechanical

compressors. Since that time, development and reﬁnement of ejector refrigeration systems have

been almost at a standstill as most efforts have been concentrated on improving vapor compression

cycles (Aphornratana et al., 2001).

Furthermore, another typical gas-driven ejector is shown schematically in Figure 5.27a. High-

pressure primary ﬂuid (P) enters the primary nozzle, through which it expands to produce a low-

pressure region at the exit plane (1). The high-velocity primary stream draws and entrains the

secondary ﬂuid (S) into the mixing chamber. The combined streams are assumed to be completely

mixed at the end of the mixing chamber (2) and the ﬂow speed is supersonic. A normal shock

wave is then produced within the mixing chamber’s throat (3), creating a compression effect, and

the ﬂow speed is reduced to a subsonic value. Further compression of the ﬂuid is achieved as the

mixed stream ﬂows through the subsonic diffuser section (b).

Figure 5.27b shows a schematic diagram of an ejector refrigeration cycle. It can be seen that a

boiler, an ejector, and a pump are used to replace the mechanical compressor of a conventional

system. High-pressure and high-temperature refrigerant vapor is evolved in a boiler to produce

the primary ﬂuid for the ejector. The ejector draws vapor refrigerant from the evaporator as its

secondary ﬂuid. This causes the refrigerant to evaporate at low pressure and to produce useful

refrigeration. The ejector exhausts the refrigerant vapor to the condenser where it is liqueﬁed. The

liquid refrigerant accumulated in the condenser is returned to the boiler via a pump while the

Mixing chamber

Primary nozzle

Pressure

Boiler

Ejector

Condenser

Evaporator

Subsonic diffuser

(a)

(b)

Figure 5.27 Schematic of (a) a jet ejector and (b) a simple jet ejector refrigeration system (Aphornratana

et al., 2001).

252 Refrigeration Systems and Applications

remainder is expanded through a throttling valve to the evaporator, thus completing the cycle. As

the working input required to circulate the ﬂuid is typically less than 1% of the heat supplied to

the boiler, the COP may be deﬁned as the ratio of evaporator refrigeration load to heat input to the

boiler as follows:

COP =

(5.66)

where

is evaporator refrigeration load in kW and

is heat input to the boiler in kW.

In the past, Aphornratana et al. (2001) have developed a new jet ejector refrigeration system

using R-11 as the refrigerant as shown in Figure 5.28. All vessels in the systems were constructed

from galvanized steel. The boiler was designed to be electrically heated, with two 4 kW electric

heaters being located at the lower end. At its upper end, three bafﬂe plates were welded to the vessel

to prevent liquid droplets being carried over with the refrigerant vapor. The evaporator design was

similar to that of the boiler. A single 3 kW electric heater was used to simulate a cooling load.

A water-cooled plate type heat exchanger was used as a condenser. Cooling water was supplied at

◦

C. The boiler was covered with 40 mm thickness of glass wool with aluminum foil backing.

The evaporator was covered with 30 mm thickness of neoprene foam rubber. A diaphragm pump

was used to circulate liquid refrigerant from the receiver tank to the boiler and the evaporator.

The pump was driven by a variable-speed 1/4 hp motor. One drawback of using the diaphragm

pump is cavitation of the liquid refrigerant in the suction line due to pressure drop through an

inlet check valve. Therefore, a small chiller was used to subcool the liquid R-11 before entering

the pump. Figure 5.28c shows a detailed schematic diagram of the experimental ejector. The

nozzle was mounted on a threaded shaft, which allowed the position of the nozzle to be adjusted.

A mixing chamber with throat diameter of 8 mm was used: in the inlet section of the mixing

chamber, the mixing section is a constant are a duct while in the exit section, the mixing section

is a convergent duct.

Aphornratana et al. experiments showed that an ejector refrigeration system using R-11 proved

to be practical and could provide reasonably acceptable performance. It can provide a cooling

temperature as low as −5

◦

C. The cooling capacity ranged from 500 to 1700 W with COP ranging

from 0.1 to 0.25.

5.6 Thermoelectric Refrigeration

This type of system is used to move heat from one area to another by the use of electrical energy. The

electrical energy, rather than the refrigerant, serves as a “carrier.” The essential use of thermoelectric

systems has been in portable refrigerators, water coolers, cooling of scientiﬁc apparatus used in

space exploration, and in aircraft. The main advantage of this system is that there are no moving

parts. Therefore, the system is compact, quiet, and needs little service.

Thermoelectric coolers are solid state equipment used in applications where temperature sta-

bilization, temperature cycling, or cooling below ambient temperature are required. There are

many products using thermoelectric coolers, including charge-coupled device (CCD) cameras, laser

diodes, microprocessors, blood analyzers, and portable picnic coolers.

Thermoelectrics are based on the Peltier Effect, discovered in 1834, by which DC current applied

across two dissimilar materials causes a temperature differential. The Peltier effect is one of the three

thermoelectric effects, the other two being known as the Seebeck effect and Thomson effect . Whereas

the last two effects act on a single conductor, the Peltier effect is a typical junction phenomenon.

The three effects are connected to each other by a simple relationship (Godfrey, 1996).

The typical thermoelectric module is manufactured using two thin ceramic wafers with a series of

P- and N-doped bismuth telluride semiconductor materials sandwiched between them. The ceramic

material on both sides of the thermoelectric adds rigidity and the necessary electrical insulation.

Advanced Refrigeration Cycles and Systems 253

(a)

(b)

(c)

Ejector

Boiler

Cooling water

Condenser

Evaporator

Receiver tank

To R12 chiller

Pressure transducer

Type K thermocouple

Liquid level sensor

Electric heater

Mixing chamber

To the condenser

From the evaporator

Primary nozzle

Nozzle position adjusting device

From the boiler

Figure 5.28 (a) Schematic of the experimental ejector refrigerator. (b) Photograph of the experimental refrig-

erator. (c) The ejector used in the experimental set-up (Aphornratana et al., 2001).

The N type material has an excess of electrons, while the P type material has a deﬁcit of electrons.

One P and one N make up a couple, as shown in Figure 5.29. The thermoelectric couples are

electrically in series and thermally in parallel. A thermoelectric module can contain one to several

hundred couples. As the electrons move from the P type material to the N type material through

an electrical connector, the electrons jump to a higher energy state absorbing thermal energy (cold

side). Continuing through the lattice of material, the electrons ﬂow from the N type material to

the P type material through an electrical connector, dropping to a lower energy state and releasing

energy as heat to the heat sink (hot side).

Thermoelectrics can be used to heat and to cool, depending on the direction of the current. In an

application requiring both heating and cooling, the design should focus on the cooling mode. Using

a thermoelectric in the heating mode is very efﬁcient because all the internal heating (Joulian heat)

and the load from the cold side is pumped to the hot side. This reduces the power needed to achieve

the desired heating.

254 Refrigeration Systems and Applications

−

−−

−

+−

DC source

(a) (b)

Body to be cooled

(heat source)

Heat sink

“N” type

semiconductor

“P” type

semiconductor

Electrical insulation

(good heat conductor)

Electronic carriers

moving heat to

the heat sink

Figure 5.29 (a) Cross-sectional view of a typical thermoelectric cooler. (b) Practical thermoelectric coolers

(Courtesy of Melcor Corporation).

5.6.1 Signiﬁcant Thermal Parameters

The appropriate thermoelectric for an application depends on at least three parameters. These

parameters are the hot surface temperature (T

), the cold surface temperature (T

), and the heat

load to be absorbed at the cold surface (

The hot side of the thermoelectric is the side where heat is released when DC power is applied.

This side is attached to the heat sink. When using an air-cooled heat sink (natural or forced

convection), the hot side temperature can be found using the following heat-transfer equation:

= T

+ R

(5.67)

where T

is the ambient temperature in

◦

C, R is the thermal resistance of the heat exchanger (

◦

C/W),

and

is the heat released to the hot side of the thermoelectric in W.

The heat-transfer balance equation for the thermoelectric cooler becomes

W (5.68)

where

is the heat absorbed from the cold side in W and

W is the electrical input power to the

thermoelectric cooler in W. The COP of a thermoelectric refrigerator is deﬁned as

COP =

(5.69)

Note that the thermal resistance of the heat sink causes a temperature rise above ambient temper-

ature. If the thermal resistance of the heat sink is unknown, then estimates of acceptable temperature

rise above ambient temperature are as follows (Godfrey, 1996):

• Natural convection: 20 to 40

◦

• Forced convection: 10 to 15

◦

• Liquid cooling: 2 to 5

◦

C (rise above the liquid coolant temperature).

The heat sink is a key component in the assembly. A heat sink that is too small means that

the desired cold side temperature may not be obtained. The cold side of the thermoelectric is the

side that gets cold when DC power is applied. This side may need to be colder than the desired

temperature of the cooled object. This is especially true when the cold side is not in direct contact

with the object, such as when cooling an enclosure.

Advanced Refrigeration Cycles and Systems 255

The temperature difference across the thermoelectric relates to T

and T

as follows:

T = T

− T

(5.70)

In a recent paper, Godfrey (1996) studied the thermoelectric performance curves and the rela-

tionship between the temperatures and the other parameters, as well as other parameters that are

required to calculate the thermal loads for the design. The thermal loads can be classiﬁed as follows:

• Active loads. I

R heat load from the electronic devices and any load generated by a chemical

reaction

• Passive loads. Radiation (heat loss between two close objects with different temperatures), con-

vection (heat loss through the air where the air has a different temperature than the object),

insulation losses, conduction losses (heat loss through leads, screws, etc.), and transient load

(time required to change the temperature of an object).

It is also important that all thermoelectrics are rated for I

max

, V

max

, Q

max

,andT

max

, at a speciﬁc

value of T

. Operating at or near the maximum power is relatively inefﬁcient because of internal

heating (Joulian heat) at high power. Therefore, thermoelectrics generally operate within 25 to 80%

of the maximum current. The input power to the thermoelectric determines the hot side temperature

and cooling capability at a given load. As the thermoelectric operates, the current ﬂowing through

it has two effects: (i) the Peltier effect (cooling) and (ii) the Joulian effect (heating). The Joulian

effect is proportional to the square of the current. Therefore, as the current increases, the Joulian

heating dominates the Peltier cooling and causes a loss in net cooling. This cut-off deﬁnes I

max

for

the thermoelectric. In fact, for each device, Q

max

is the maximum heat load that can be absorbed by

the cold side of the thermoelectric. This maximum occurs at I

max

and V

max

, and with T = 0

◦

The T

max

value is the maximum temperature difference across the thermoelectric. This maximum

occurs at I

max

and V

max

and with no load (Q

= 0 W). These values of Q

max

and T

max

are well

treated by Godfrey (1996).

Example 5.6

Suppose we have a thermoelectric application with a forced convection type heat sink with a

thermal resistance of 0.15

◦

C/W, an ambient temperature of 25

◦

C, and an object that needs to be

cooled to 5

◦

C. The cold side of the thermoelectric will be in direct contact with the object. The

hot side temperature is 35

◦

C and the electric current and voltage are 3.6 A and 10 V, respectively.

Determine the temperature difference across the thermoelectric T and the heat absorbed from the

cold side

. Also, determine the COP of the system.

Solution

The temperature difference across the thermoelectric is

T = T

− T

= 35 − 5 = 30

◦

The heat released to the hot side of the thermoelectric is

= T

+ R

⇒

− T

(35 − 25)

◦

0.15

◦

C/W

= 66.7W

Then the heat absorbed from the cold side becomes

−

W ⇒

− IV = 66.7W− (3.6A)(10 V)



1AV



= 30.7W

256 Refrigeration Systems and Applications

TheCOPofthesystemis

COP =

30.7 W

36 W

= 0.853

5.7 Thermoacoustic Refrigeration

Garrett and Hoﬂer (1992) pointed out that two recent events are responsible for the new era in

refrigeration before the beginning of the twenty-ﬁrst century. The most signiﬁcant of these is the

international agreement (signing of the Montreal Protocol) on the production and consumption of

chloroﬂuorocarbons (CFCs), which were found to be causing the depletion of the stratospheric

ozone layer. The second event was the discovery of “high-temperature” superconductors and the

development of high-speed and high-density electronic circuits, which require active cooling and

hence a new approach to refrigeration, or thermoacoustic refrigeration, which was ﬁrst discovered

by Wheatley et al. (1993) in August 1983. The simplicity of the hardware involved in thermoacous-

tic machines is best appreciated by examining a concrete example. In the mid-1990s, S.L. Garrett

and his colleagues at the Naval Postgraduate School in Monterey, California, developed two ther-

moacoustic refrigerators for the Space Shuttle. The ﬁrst was designed to cool electronic components

and the second was intended to replace the refrigerator-freezer unit used to preserve blood and urine

samples from astronauts engaged in biomedical experiments (Garrett and Backhaus, 2000).

Thermoacoustic refrigeration is considered a new technology, attaining cooling without the need

for refrigerants. The basic mechanism is very simple and efﬁcient. A loudspeaker creates sound

in a hollow tube which is ﬁlled with an ordinary gas. In fact, thermoacoustic refrigeration utilizes

high-density sound waves to transfer heat due to the thermoacoustic effect (i.e., acoustic energy).

Therefore, the working ﬂuid in this system is acoustically driven gas. The process itself utilizes

standing acoustic waves in an enclosed cavity to generate the mechanical compression and expansion

of the working ﬂuid (gas in this case) needed for the cooling cycle. The technique has the potential

for high-efﬁciency operation without the need for cooling liquids or mechanical moving parts.

These factors make the concept amenable to miniaturization to chip-scale dimensions for thermal

management of electronic components.

The interaction between acoustics and thermodynamics has been known ever since the dispute

between Newton and Laplace over whether the speed of sound was determined by the adiabatic or

isothermal compressibility of air. At the present time, the efﬁciency of thermoacoustic refrigerators

is 20−30% lower than their vapor-compression refrigerators. Part of that lower efﬁciency is due

to the intrinsic irreversibilities of the thermoacoustic heat transport process. These intrinsic irre-

versibilities are also the favorable aspects of the cycle, since they make for mechanical simplicity,

with few or no moving parts. A greater part of the inefﬁciency of current thermoacoustic refrigera-

tors is simply due to technical immaturity. With time, improvements in heat exchangers and other

subsystems should narrow the gap. It is also likely that the efﬁciency in many applications will

improve only because of the fact that thermoacoustic refrigerators are well suited to proportional

control. One can easily and continuously control the cooling capacity of a thermoacoustic refrig-

erator so that its output can be adjusted accurately for varying load conditions. This could lead to

higher efﬁciencies than for conventional vapor-compression chillers which have constant displace-

ment compressors and are therefore only capable of binary (on/off) control. Proportional control

avoids losses due to the start-up surges in conventional compressors and reduces the inefﬁciencies

in the heat exchangers, since such systems can operate over smaller temperature gaps between the

coolant ﬂuid and the heat load.

The research focus of the Thermoacoustics Laboratory in ARL at Pennsylvania State Univer-

sity in cooperation with Los Alamos Research Laboratory is the study of acoustically driven heat

transport. Their goals include an improved understanding of fundamental thermoacoustic processes

and the development of new thermoacoustic refrigerators and heat engines with increased power

Advanced Refrigeration Cycles and Systems 257

(a)

(b)

Figure 5.30 (a) A thermoacoustic refrigerator and (b) its application to a refrigerator (Courtesy of Pennsylvania

State University Applied Research Laboratory).

density, temperature span, and efﬁciency, and the commercialization of those devices. The labora-

tory provides the infrastructure to support research on the basic processes required to understand

this emerging, environmentally friendly refrigeration technology. This facility also supports the

fabrication and testing required to produce complete, full-scale operational prototype refrigeration

systems for military and commercial applications such as food refrigerators/freezers and air condi-

tioners. Their prototypes have been ﬂown on the Space Shuttle and have been used to cool radar

electronics onboard a US Navy warship. Thermoacoustic refrigerators with cooling powers ranging

from a few watts to chillers with cooling capacities in excess of 10 kW are currently in operation or

under construction. Figure 5.30a shows a thermoacoustic refrigerator developed by this laboratory

and it is operational for running a small refrigerator such as in Figure 5.30b.

Although thermoacoustic refrigerators have not been commercialized yet and are considered

to be still at a developmental stage, it is known that they can be used for any kind of cooling.

Conventional, single-stage, electrically operated thermoacoustic refrigerators can reach cold side

temperatures that are two-thirds to three-quarters of ambient temperature, so they are not well

suited to cryogenic applications below −40

◦

C. However, thermoacoustically driven pulse-tube style

refrigerators can reach the cryogenic temperatures required to liquefy air or natural gas. In their early

commercial stages, they will probably be limited to niche applications such as in military systems

which are required to operate in closed environments and food merchandizing where toxicity

is an important issue. As global environmental mandates and legislations/amendments become

essential, one can expect the scope of thermoacoustic applications to expand both domestically and

in emerging markets.

5.8 Metal Hydride Refrigeration Systems

For the ﬁrst time, a group of Japanese companies (JNT, 1996) have succeeded in making operational

an innovative, CFC-free, metal hydride (MH) refrigeration system using hydrogen absorbing alloys

(MH alloys) for cold storage at low temperatures. Their state-of-the-art MH refrigeration system can

keep the temperature in a cold storage area below −30

◦

C. This was a real landmark in the ﬁeld. The

258 Refrigeration Systems and Applications

joint R&D group in 1995 demonstrated an MH refrigeration system under test conditions by cooling

a 100 m

cold storehouse. They succeeded in continuously operating the system with a store room

temperature below −30

◦

C. The MH system can be made as compact in size as a conventional vapor-

compression refrigeration system. The system can be incorporated easily, therefore, into automatic

vending machines and display cabinets for frozen foods. At the end of the year, the group also

completed a trial model of an automatic vending machine equipped with an MH refrigeration system

which can be used for commercial operation once the system size has been reduced. In addition,

the system is safe since hydrogen is absorbed and stored in MH alloys. Ammonia absorption

refrigerating machines have also been proposed as an alternative CFC-free system for cold storage.

However, it is not possible to make ammonia systems as small in size as MH systems, and the

strong toxicity and highly irritating odor of ammonia are serious obstacles to their widespread use.

JNT (1996) stated that the MH refrigeration system is a very safe as well as clean and environ-

mentally friendly, CFC-free refrigeration system. Hydrogen is sealed in gas-tight cylinders, and,

being far lighter than air, rapidly diffuses into the atmosphere in the accidental event of its leakage.

Thus, the danger of explosion caused by hydrogen is slight. In addition, their system has additional

advantages as follows:

• By not using CFCs or ammonia, the system is applicable to wide-ranging uses.

• The system needs a heating source to generate energy for refrigeration, but can save energy

consumption for this purpose by the use of waste heat or by operating in combination with a

cogeneration system.

• It has no moving elements except for pumps circulating water and brine. In particular, its hydrogen

system, driven solely by heat input, does not need the manipulation of valves. Thus, rarely

suffering breakdowns, the MH system is easy and simple to operate and maintain.

• Driven by heat, the system consumes 20% less electric power than conventional, electrically

powered compression refrigeration machines.

• Having no sliding and vibrating components such as compressors, the system operates with low

noise levels.

• Because the MH refrigeration system is safe and simple to control, it is possible to design

refrigerating units using this technology with wide varieties of cooling capacities ranging from

10−10,000 kW.

5.8.1 Operational Principles

When MH alloys (e.g., TiZrCrFe series) come into contact with hydrogen, the alloys absorb hydro-

gen by an exothermic reaction and store it as MHs. In reverse, the alloys easily dissociate and

discharge hydrogen by an endothermic reaction. Employing this endothermic reaction when MH

alloys discharge hydrogen, MH refrigeration systems implement a refrigeration cycle by a combi-

nation of two types of alloys. One type works at a higher temperature and the other at a lower

temperature, both under their own equilibrium hydrogen pressure. The working principles of MH

refrigeration systems are illustrated in Figure 5.31.

MH alloys absorb or discharge hydrogen at certain constant equilibrium hydrogen pressure levels,

determined by temperature. MH refrigeration systems use an MH alloy (MH-A) driving hydrogen

to carry out the regeneration process on the high-temperature side of the system and another MH

alloy (MH-B) refrigerating brine on the low-temperature side. Each of these alloys has the relation

between temperatures and equilibrium hydrogen pressures as shown in Figure 5.31a.

Regeneration Process. (1) To raise the hydrogen pressure of the MH-A by raising its temperature,

the alloy is heated (Q2-1). Hydrogen is discharged from the MH-A and moves to the MH-B

with lower hydrogen pressure. (2) The MH-B absorbs hydrogen, thus generating heat. However,

Advanced Refrigeration Cycles and Systems 259

MH-A MH-B

(Hydrogen driving alloy) (Refrigerating alloy)

Flow of hydrogen

MH-A MH-B

(Hydrogen driving alloy) (Refrigerating alloy)

Flow of hydrogen

High temperature

heat source

(steam or hot water)

Cooling water

Regenerative process : MH-A discharges hydrogen; MH-B absorbs/stores hydrogen

Refrigerating process : MH-B discharges hydrogen and refrigerates brine; MH-A absorbs/stores hydrogen

(Discharge) (Absorption/storage)

Brine for cooling of allo

Brine for refrigeration

3 (Absorption/storage) 4 (Discharge)

2-1

1-1

2-2

1-2

(b)

MH-A

MH-B

2-1

1-1

2-2

1-2

Temperature (1/T )

Pressure (log P )

(a)

Figure 5.31 (a) Working principle of MH refrigeration system and (b) illustration of MH refrigeration system

principle (JNT, 1996).

the circulation of cooling brine (Q1-1) suppresses the rise in the MH-B temperature, thereby

preventing the pressure of the MH-B from increasing. The MH-B continues to absorb and store

hydrogen in this way.

Refrigeration Process. (3) When all the hydrogen of the MH-A is transferred to the MH-B, the

former alloy is cooled by cooling water (Q2-2), thereby lowering its hydrogen pressure. (4)

Hydrogen is discharged from the MH-B. This has a lower hydrogen pressure than that of the

MH-A and moves to the latter alloy. Reducing its temperature by the hydrogen discharge, the

MH-B refrigerates brine (Q1-2).

To continuously cool brine for refrigeration, an MH refrigeration system has two sets of high-

and low-temperature MH alloy pairs. While one set of the alloys operates in the regeneration

process, the other set works in the refrigeration process. Each of these MH alloys is packed in

a heat exchanger cylinder. This enables the exchange of thermal energy with a heating medium

(steam, hot water, etc.), cooling water, or brine. Shell and tube type heat exchangers are used.

260 Refrigeration Systems and Applications

Figure 5.32 Flow chart of the MH refrigeration system for low-temperature cold storage (JNT, 1996).

The joint R&D group (JNT, 1996) applied this MH refrigeration system for a low-temperature

cold storage, using methanol as the heat-transfer medium for the low-temperature side and

hot/cooling water as the heat-transfer medium for the high-temperature side (Figure 5.32). MH

alloy cylinders (heat exchangers) are shown as forming part of the refrigerating process.

5.9 Solar Refrigeration

The developing worldwide shortage of petroleum emphasizes the need for alternative energy sources

which are both inexpensive and clean. There has been high interest in, and high potential use of,

renewable energy sources since the energy crisis faced during the 1970s. During the last few

decades, an increasing effort based on research and development has been concentrated on the

utilization of renewable energy sources, for example, solar energy, wind energy, tidal waves, biogas,

geothermal energy, hydropower, and hydrogen energy. Among these sources, solar energy for

refrigeration applications is very popular because it is direct and easy to use, renewable, and

continuous, maintains the same quality, is safe and free, and is environmentally friendly.

The continuous supply of solar energy to the earth’s surface is equal to a power of about 100,000

TW. Approximately one-third of the radiation impinging on land area and accumulated over less

than 2 hours should sufﬁce to satisfy the entire primary energy demand by humans for the period of

1 year (Dincer, 1997; 2003). More than 25% of the total energy in the world is consumed for heating

and cooling of buildings and providing hot water. Therefore, the diversion of this particular energy

demand to an alternative source would result in a substantial reduction in the world’s dependence on

fossil fuels. The annual incidence of solar energy on buildings in the United States is several times

the amount required to heat these buildings; approximately 10

kW·h of solar energy is received

on earth annually. It has been projected that by the year 2020 from 25 to 50% of the thermal energy

for buildings could be provided from the sun (Dincer et al., 1996). Consequently, solar energy is an

available energy source for many applications ranging from electricity generation to food cooling.

5.9.1 Solar Refrigeration Systems

Many food products (e.g., fruits, vegetables, meats, dairy products) are stored in cooling units

for periods of the order of weeks at temperatures between 0 and 4

◦

C in order to prevent