Dinc Ibrahim. Refrigeration systems and applications 2th edition

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Heat Pumps 311

(d) Repeating calculations for T

= 18

◦

CandT

=−6

◦

C, we obtain

COP =

163.2kJ/kg

34.0kJ/kg

= 4.80

min

= ex

= 3.30 kJ/kg

3.30

34.0

= 0.097 = 9.7%

dest,total

= w − ex

= 34.0 − 3.30 = 30.7kJ/kg

The COP in the ground-source heat pump case is 32.6% higher than that in the air source cycle.

The second-law efﬁciency of the cycle decreases and the total exergy destruction decreases.

6.13 Mechanical Vapor-Recompression (MVR) Heat Pump Systems

The open-cycle vapor recompression evaporator provides a very efﬁcient means of concentrating

dilute solutions using the solvent removed as the operating ﬂuid. The latent heat of vaporization is

recovered when the evaporated vapor is condensed following compression and the excess solvent

is then available for recovery if required. Alternatively, the process may be used to obtain a purer

solvent, as, for example, in desalination of sea water. The ejection of gas from a nozzle into an

expander can be used to increase the pressure in a secondary circuit in which the same gas is

used as a refrigerant. This method has been applied using steam as the working ﬂuid, primarily

to obtain cooling using a conventional steam boiler, but the efﬁciency of all such systems is low.

Heat pumping applications may exist where there is spare steam, possibly in large-scale total

energy schemes.

Although the heat pump’s operational method of transferring heat from a colder to a hotter reser-

voir remains the same under various conditions, the means of realizing this method differ according

to temperature level, range of temperature rise, variety of heat sources, applicable processes, and so

on. These differences characterize the system’s design and the choice of hardware for an industrial

heat pump. Compared to various other heat pump systems, the MVR heat pump type for latent heat

recovery with a high thermal efﬁciency was thought to be better. It completely replaces conventional

systems by means of newly developed component technology, improved performance, operation and

service characteristics and reduced installation costs. This tendency has been particularly important

since the ﬁrst oil crisis. This system, which recovers and reuses latent heat, is remarkably effective

for processes such as concentration, distillation, and rectiﬁcation, where the COP is often higher

than 20. The compressor used for this MVR system is a specially designed steam compressor.

The goal of this system is to achieve a high COP (3−6) by restricting the temperature rise to the

minimum required for a refrigerant vapor which has a high latent heat (539 kcal/kg, 1 atmospheric

pressure, 100

◦

C in the case of water) thus also minimizing the power required. The essentials

of this technology are as follows (Kuroda, 1986): to efﬁciently compress water vapor (which has

a high speciﬁc volume and a comparatively low compression ratio) and raise its temperature; to

efﬁciently exchange this latent heat with a smaller difference in temperature; and to combine both

technologies. The combination of these technologies was successful (Figure 6.12). The application

ﬁelds of these technologies, however, were limited with regard to the availability of process streams

containing liquid water and similar materials. On the other hand, the extension of application ﬁelds

of such a highly efﬁcient system was considered to be important for various industrial processes.

If the compression ratio of water vapor could be increased, resulting in higher temperatures, the

application ﬁelds could be further extended.

312 Refrigeration Systems and Applications

Steam

compressor

Steam at

90° C

Steam at 80° C

Steam for

starting heat

pump

(Preheating

heat exchanger)

80° C

33° C

30° C

90° C

Liquid

at 90° C

Raw liquid

Distilled liquid

Concentrated liquid

Heat

Latent heat

0.72at

Figure 6.12 A MVR heat pump system (Kuroda, 1986).

The recently developed high-pressure, two-stage, centrifugal compressor with a high compression

ratio has permitted improvement of the technology and extension of the application ﬁelds. As a

basis for the development of heat pump technology, the above-mentioned technology is available

not only for working with water vapor but also for working with ﬂuids such as ammonia, Freon,

hydrocarbons, and so on. These technologies have been developed to provide higher temperatures

and higher efﬁciencies. Although water is considered to be most suitable for heat pumps used for

high-temperature ranges, it is desirable to produce optimum combinations for various conditions

and systems.

6.14 Cascaded Heat Pump Systems

In the case of cascaded systems, it is possible to use different refrigerants in the two cycles,

each refrigerant being selected as the optimum for the particular temperature range of operation.

Figure 6.13 illustrates a schematic of a cascaded system and its log P −h diagram. Another way

of increasing the COP is to use cascaded heat pumps. The combination of heat pumps will have

a higher COP

than the COP

(heat delivered/power absorbed) of a single heat pump performing

the same duty. A cascade is probably applicable only to very large systems and will mostly be

applied for process heating in industry. However, the use of cascade systems in geothermal and

district heating schemes looks extremely promising.

6.15 Rankine-Powered Heat Pump Systems

The Rankine-cycle heat pump converts low-temperature heat (100−200

◦

C) to usable process steam.

In this system, the resource temperature, normally industrial waste heat, is raised through the addi-

tion of mechanical work. An organic Rankine cycle turbine extracts energy for this mechanical

work from the heat source to drive the heat pump compressor. Figure 6.14 illustrates the operation

of a Rankine-cycle heat pump system. The heat stream ﬁrst passes through the water evapora-

tor to generate low-pressure steam, which is then compressed by a centrifugal compressor to the

desired pressure. Steam is discharged at superheated conditions and may be desuperheated at the

Heat Pumps 313

High-pressure

cycle

Low-pressure

cycle

Heat exchanger

Condenser

Compressor

log P

Figure 6.13 Horizontal cascade heat pump cycle and its log P −h diagram.

Residual

waste heat

Refrigerant

evaporator/

preheater

Feed

pump

Condenser

Cooling

water

Turbine

Compr

-essor

Saturated

steam

Steam

evaporator

Waste heat

Condensing

load provided

by process

Rankine bottoming

cycle

Open-cycle

heat pump

Superheated steam

out

Figure 6.14 A Rankine-powered heat pump system (Adapted from Koebbeman, 1982).

user’s option. The highest temperature is utilized in the water evaporators, minimizing the steam

compressor pressure ratio and also the power required by the Rankine drive. After the water evap-

orator, the waste heat passes through the refrigerant evaporator and into the refrigerant preheater.

Therefore, the Rankine cycle working ﬂuid (R-113) is heated from condenser conditions to saturated

liquid at evaporator conditions. The refrigerant evaporator vaporizes the working ﬂuid, which is

then expanded through the turbine to produce the required compressor power. The turbine exhausts

into the condenser, where the refrigerant vapors are condensed to liquid and returned to the hot

well to repeat the cycle.

314 Refrigeration Systems and Applications

As shown in Figure 6.14, the Rankine-powered heat pump system consists of two separate

modules: the heat exchanger module and the power module. The heat exchanger module consists

of three shell and tube heat exchangers: the water evaporator, the refrigerant evaporator and the

refrigerant preheater. All three units are connected in series on the tube side, which contains the

waste heat stream. Waste heat enters the tube side of the water evaporator and evaporates fresh

water on the shell side. Then, vapor separation is achieved by gravity in the open volume above

the tube bundle. Vapor generated in the water evaporator is piped to the compressor inlet.

6.16 Quasi-Open-Cycle Heat Pump Systems

District heating systems provide thermal energy to their customers in the form of hot water or steam.

These systems can use one or more types of heat sources to meet the thermal load, including boilers,

cogeneration systems, or low-grade heat sources in conjunction with a heat pump.

Most large-scale heat pumps operate using the closed-cycle concept, and usually use a chlorinated

ﬂuorocarbon as the working ﬂuid. An alternative to this approach is the quasi-open-cycle heat pump.

The quasi-open-cycle heat pump concept deals with the use of low-grade local energy resources

in district heating systems that use hot water as the transport medium. The particular low-grade

energy resources of interest in this concept are water resources, such as ﬂooded mines and sewage

ponds, and waste heat resources, such as low-pressure waste steam, hot water, and exhaust gases

from industrial processes.

Figure 6.15 illustrates the quasi-open-cycle heat pump concept. A fraction of the water returning

from the distribution network is bled off, throttled in an expansion valve, and injected into the

Post office

BMC/GMF

Temperature

control loop

Prime mover

waste-heat

recovery equipment

Direct-contact

condenser

System

pump

Hot

well

Screw

compressor

Natural

gas

engine

Injection

water

(Saturated

vapor)

Evaporator

Monsanto

chemical

plant

(Waste water)

rec

system

source

fuel

Figure 6.15 A quasi-open-cycle heat pump system (Adapted from Kunjeer, 1987).

Heat Pumps 315

evaporative heat exchanger. The heat exchanger is connected to the low-grade energy resource

which transfers heat to the low-pressure water and produces saturated steam. The steam produced

in the heat exchanger is then compressed in the compressor. The superheated steam that exits

the compressor then enters a direct-contact condenser where the remaining fraction of the district

heating system return water is added. The water that exits the direct-contact condenser is then

pressurized by the district heating system pump where it enters the prime mover’s waste heat

recovery equipment and, if applicable, is distributed throughout the district heating system where

it transfers its thermal energy to various users (Kunjeer, 1987).

The quasi-open-cycle heat pump is “open” in the sense that the compressor working ﬂuid is the

same as the district heating hot water transport media. The system, however, resembles the closed

cycle in that two heat exchangers, a direct-contact condenser, and a surface-area type evaporator,

are used. The quasi-open-cycle heat pump has several advantages over the closed cycle, including

the following:

• The working ﬂuid is water, which is nontoxic and has excellent thermal properties.

• Since the high-temperature heat exchanger is a direct-contact condenser as opposed to a primary

surface heat exchanger, the capital cost of this heat exchanger is much lower.

The quasi-open-cycle heat pump is found to be best suited for the higher temperature heat

resources such as those found in the waste streams of industrial processes. This is due mainly to

the thermodynamic properties of steam. At low temperatures, the vapor-speciﬁc volume is quite

large and, because of the pressure–temperature relationship, a reasonable temperature rise is obtained

when the compressor operates with a large pressure ratio.

6.17 Vapor Jet Heat Pump Systems

In the vapor jet heat pump, the kinetic energy of a vapor jet, produced by heat input, is utilized for

compressing the refrigerant vapor. In principle, this is a compression process which is, however,

operated without input of mechanical energy. The operation of a vapor jet compressor was explained

earlier in Chapter 5. In the injection nozzle, the drive vapor at pressure P

is expanded, and a

vapor jet with a velocity several times the velocity of sound is produced. This carries forward the

expansion vapor at pressure P

and accelerates it. Because of the decreased pressure on the suction

side, evaporation takes place and the vapor is cooled by extracting the evaporation enthalpy. The

pressure of the vapor mixture is increased in the diffuser to the condensing pressure P at which

condensing can take place in the condenser. The deﬁnition of a COP for vapor jet heat pumps leads

to difﬁculties. For industrial purposes, the so-called speciﬁc vapor consumption, that is, the ratio

of drive vapor quantity to suction vapor quantity, is given; relevant tables are available from the

manufacturers. In thermodynamic terms, the deﬁnition of a heat ratio analogous to the AHP would

be logical using the enthalpy of the vapor mixture and the enthalpy of the drive vapor. Because

the evaporation enthalpy is very high for water at about 2000 kJ/kg, the process is mostly carried

out with water vapor. But, for technical process reasons other media are also used.

6.18 Chemical Heat Pump Systems

A number of other methods of heat pumping have been proposed, which have not, to date, been

tested in practical devices. These include vortex tubes (so-called chemical heat pumps). The vortex

tube heat pump makes use of an effect known as the Ranque effect. If a high-pressure gas is

injected tangentially into a tube, a vortex is formed and the gas at the center of the tube is at a

lower temperature and pressure than the gas near the tube wall. The gas can be extracted separately

316 Refrigeration Systems and Applications

from these two regions, yielding heated or cooled gas as required. Although, in principle, air can

be used to provide heat by this means, a sufﬁciently efﬁcient device worth developing has never

been demonstrated.

Chemical heat pump systems have been proposed based on the mixing and dissolution of two

components. Thermodynamic analysis and consideration of material properties have been carried out

theoretically, but no practical machine has emerged, the most common problem being irreversibility.

A sorption and desorption system for hydrogen on and from lanthanum penta-nickel has been

proposed. Such systems are unlikely to be used other than in extremely specialized applications.

The basis of a chemical heat pump can provide temperature upgrading at high temperatures. This

type of temperature upgrading has been difﬁcult to accomplish with conventional technologies.

Basically, the chemical heat pump uses a high- and low-temperature heat source, needs only a

small amount of mechanical energy input, and, depending on which component element reactions

are selected, can deliver useful heat at a desired temperature. A wide variety of combinations of

working reactants are conceivable for chemical heat pumps. The most common type is the reversible

thermochemical reaction of CaO/Ca(OH)

that is used along with the evaporation and condensation

of water to complete a heat pump cycle (see Hasatani et al., 1988).

The operation of this chemical heat pump consists of the reactions given in the following

equations:

CaO(s) + H

O(g) ⇒ Ca(OH)

(s) + 1.858 × 10

kJ/kg (6.33)

and

O(g) ⇒ H

O(l) + 2.316 × 10

kJ/kg (average value of 293−641 K) (6.34)

Figure 6.16 shows the relationship between the reaction equilibrium pressure P

and temperature

for the reaction in Equation 6.33 which is given by line 2–4 in the ﬁgure. Also shown is the

relationship between the saturated steam pressure P

and temperature for Equation 6.34, which is

given by line 1–3.

Reactor

Condenser/

evaporator

ln P

1/T

: water vapor pressure at

reaction equilibrium

: saturated water vapor

pressure

Figure 6.16 ln P −1/T diagram for a chemical heat pump (Adapted from Hasatani et al., 1988).

Heat Pumps 317

For the heat release mode, consider a hermetically sealed reaction system having a reactor and

an evaporator ﬁlled with CaO and water, respectively. If heat (Q

) is added to the evaporator

from a medium temperature (T

) heat source, the water in the evaporator becomes pressurized

steam (path 1–3 in Figure 6.16). Owing to the pressure difference between P

and P

, this steam

enters the reactor to undergo an exothermic hydration reaction with CaO. This causes the temper-

ature in the reactor to rise (2–4) to temperature T

at which point high-temperature heat (Q

)

becomes available.

In the heat storage (regeneration mode), heat Q

from a medium-temperature (T

) source

is added to the reactor which contains Ca(OH)

formed as described above. At the same time,

the condenser is cooled to a temperature T

. Under these conditions, the Ca(OH)

undergoes

an endothermic dehydration reaction to release steam. The steam shifts from the reactor to the

condenser (path 2−1 in Figure 6.16) due to the pressure difference between the two chambers.

There it condenses by releasing its latent heat of condensation to the low-temperature heat sink (T

An experimental unit developed by Hasatani et al. (1988) is shown schematically in Figure 6.17.

The evaporator/condenser (1) and the reactor (2) are made of stainless steel and are cylindrical in

shape, having both an inside diameter and a height of 150 mm. Both containers are equipped with

a cooling coil (4), thermocouple insertion tube (5), and an electric heater (6). Each of these items

is arranged symmetrically about the center of the container. Both containers are also equipped with

a pressure gauge (7). In addition, the evaporator/condenser has a water level gauge (8) and the

reactor has an auxiliary external heater (9). The auxiliary heater consists of nichrome wire wound

around the reactor’s outer surface. The two containers are connected to each other by stainless

steel piping via valve V2. A vacuum pump (3) is used to obtain the proper pressure level in

the reactor. The equipment is insulated with an adiabatic material to reduce heat loss. For the

reaction system employed, the temperature T

shown in Figure 6.16 is in principle about 640 K.

At this temperature, the pressure of the saturated steam in the evaporator is 27.5 MPa. The present

experimental apparatus was not designed for such high pressures. For this reason the evaporator

temperature was kept below about 430 K for this experiment.

Evaporator/Condenser

Vacuum pump

Thermocouple

Pressure gauge

–V

Valve

Reactor

Coil heat exchanger

Heater

Level gauge

Auxiliary heater

CaO or Ca(OH)

powder

Figure 6.17 Schematic diagram of the experimental unit (Hasatani et al., 1988).

318 Refrigeration Systems and Applications

6.19 Metal Hydride Heat Pump Systems

A large amount of thermal energy is involved in the dehydriding and hydriding reactions of hydrogen

with hydride-forming materials. Heat involved in each reaction can be used to extract and supply

heat for thermal energy storage and conversion, air conditioning, heating, drying, and humidity

control devices. Such hydride-forming material is called a metal hydride and has been considered

as the material extensively applicable to energy storage and conversion. A heat pump is a typical

application for the dehydriding and hydriding characteristics of metal hydrides, particularly as a

cooling and temperature upgrading device. In the principle of these heat pumps, the reaction between

hydrogen and hydride-forming materials is reversible. It is characterized by rapid kinetics and large

amounts of heat in an endothermic desorption reaction and exothermic absorption reaction in the

following equation:

x+y

= MH

+ x/2H

± H (6.35)

The amount of heat (H ) in each reaction has an average value of 6.4–9.2 kcal/mole of H

160–230 kJ/kg of alloy. In the case of Mg-based alloys, much higher values are obtained. In the

former case, approximately 1000 kg of the alloy are necessary for a heat pump with a capacity of

100 kW.

To run a heat pump cycle, a set of two paired metal hydride heat exchangers are necessary.

With this system, a continuous countercurrent ﬂow of hydrogen can be maintained even though

the hydriding and dehydriding reactions are executed in a batch-wise manner. Figure 6.18 shows

a metal hydride heat pump cycle providing a low temperature (T

) sink which can be used for air

conditioning and refrigeration purposes. The points on 1–4 indicate the ﬁnal states that the metal

hydride and metal achieve during execution of the cycle. Lines labeled MH

and MH

are lines

of constant concentration for the low vapor pressure (low concentration of H

) and high vapor

pressure (high concentration of H

) metal hydrides. These equilibrium temperature and pressure

relations are known as van Hoff plots.

The cycle is executed as follows (for clarity, a single pair of high and low pressure hydride heat

exchangers is used to describe the cycle). Let metal M

be at temperature T

and pressure P

,and

let metal hydride MH

be at temperature T

and pressure P

. With these conditions, metal hydride

log P

Temperature (1/T)

= highest temperature

= lowest temperature

= intermediate temperature

Some examples metal hydrides:

LaNi, LaNi-Al, MmNi, LaNi, La-Ni

, LaNi

6.7

0.14

, TiMn, etc.

Figure 6.18 Log P and 1/T diagram of the heat hydride heat pump (Adapted from Suda, 1987).

Heat Pumps 319

is heated at point 1 to raise its temperature from T

to T

. At the same time, it is used to supply

hydrogen to metal M

. Metal M

, reacting with hydrogen from MH

, releases energy causing its

temperature to rise from T

to T

. At point 2, metal M

has absorbed all the hydrogen to become

at T

and P

. Metal M

is now cooled to T

by rejecting heat to the atmosphere. MH

with

its vapor pressure P

is then used to supply H

to metal M

. Since the release of H

is an

endothermic reaction, the temperature of MH

drops. Once it reaches temperature T

, heat is added to

the metal hydride, MH

, to maintain this temperature until all the hydrogen has been driven off. The

hydrogen is absorbed by the metal M

until the metal hydride MH

is formed. The energy released

during this exothermic absorption of H

is rejected at point 4, thereby maintaining the low-pressure

metal hydride at T

n−m

and P

. The cycle can now be repeated. As summarized in Figure 6.18, heat

is rejected to the atmosphere (T

) at points 2 and 6. At point 1, a high temperature source (T

)

adds heat to the cycle; and at point 3, heat is absorbed from a low-temperature source (T

6.20 Thermoelectric Heat Pump Systems

Thermoelectrics are based on the Peltier effect, discovered by Peltier in 1834, that when a direct

electric current passes round a circuit incorporating two different metals, one contact area is heated

and the other cooled, depending on the direction of current ﬂow. The Peltier effect provides a

means for pumping heat without using moving parts. In a circuit containing two junctions between

dissimilar conductors, heat may be transferred from one junction to the other by applying a DC

voltage (Figure 6.19). To be effective, the conductors must provide high thermoelectric power and

low thermal conductivity, combined with adequate electrical conductivity. Such a combination of

properties is not to be found in metallic conductors, and this principle could not be applied to heat

pumping until the advent of semiconductor materials, typically bismuth, antimony, selenium, and

tellurium alloys. The effectiveness of materials for such applications is measured by the “ﬁgure of

merit,” Z, deﬁned by Heap (1979):

Z =

kρ

(6.36)

where a is the Seebeck coefﬁcient, k is the thermal conductivity, and ρ is the electrical resistivity

of the material. Using presently available materials for which Z = 0.003 K

−1

, thermoelectric heat

pump performances about half as good as those of typical vapor-compression machines can be

predicted. Practical devices only achieve half these predicted values.

Material

Junction bar

Current

Figure 6.19 Schematic representation of the Peltier effect.

320 Refrigeration Systems and Applications

The limitations of known materials and the scope for possible future developments of new mate-

rials have been considered by various researchers. If semiconductors were to be developed with

Z = 0.006 K

−l

or greater there could be a considerable widening of thermoelectric heat pumping

applications, and a COP comparable with those obtainable with vapor-compression machines might

be achieved. At present, thermoelectrical devices are not competitive with vapor-compression heat

pumps, but they may ﬁnd increasing use in specialized cooling applications where power require-

ments are low or where silent operation is necessary. Close temperature control of electronic

components and cold stores in nuclear submarines are examples of these.

To operate semiconductor Peltier cells, a high direct current at a low voltage is required and

consequently a large number of cells are connected together in series. Heat exchangers are also

required at hot and/or cold junctions to transfer heat as needed. During the past two decades, custom

Peltier (thermoelectric) modules and Peltier coolers have been designed, with heat pump capacities

ranging from a few watts to thousands of watts. Currently, some companies are conducting research

and development to optimize all Peltier performance parameters independently for maximum heat

pump performance.

By using IsoFilm heat spreaders (Figure 6.20a) to effectively transfer heat to and from the Peltier

module (Figure 6.20b), higher power density Peltier coolers without sacriﬁcing their maximum

temperature differentials are designed and manufactured. Since the manufacturing cost of a Peltier

module is highly dependent on its size and not its heat pumping capacity, higher power density

designs have a higher performance versus cost ratio. A very high power density (50 W/cm

) Peltier

technology using deposited thin ﬁlm bismuth–telluride (Bi

) with LIGA-processed copper junc-

tions and a proprietary insulation scheme is under development. This entire device is manufactured

using wafer fabrication techniques. This twenty-ﬁrst century wafer-scale processing is a paradigm

shift from the mechanical processing currently employed by today’s Peltier module manufacturers.

(a)

(b)

(c)

Figure 6.20 (a) Translucent view of an IsoDie heat spreader. (b) The view of a custom Peltier module.

Concepts, Inc.).