12 - Heat Pumps, Heat Recovery, Gas Cooling and Cogeneration.pdf - Heat pump & solar energy - andisow

CHAPTER 12

HEAT PUMPS, HEAT

RECOVERY, GAS COOLING,

AND COGENERATION SYSTEMS

12.1 BASICS OF HEAT PUMP AND HEAT

RECOVERY 12.1

Heat Pumps 12.1

Heat Pump Cycle 12.2

Classiﬁcation of Heat Pumps 12.3

HVAC&R Heat Recovery Systems 12.3

Heat Balance and Building Load

Analysis 12.4

12.2 AIR-SOURCE HEAT PUMP

SYSTEMS 12.5

System Components 12.6

Suction Line Accumulator 12.8

Operating Modes 12.9

System Performance 12.9

Cycling Loss and Degradation Factor

12.11

Minimum Performance 12.12

Defrosting 12.12

Controls 12.13

Capacity and Selection 12.13

12.3 GROUNDWATER HEAT PUMP

SYSTEMS 12.13

Groundwater Systems 12.14

Groundwater Heat Pump System for a

Hospital 12.14

Groundwater Heat Pump Systems for

Residences 12.15

12.4 GROUND-COUPLED AND SURFACE

WATER HEAT PUMP SYSTEMS 12.17

12.5 AIR-TO-AIR HEAT RECOVERY 12.19

Types of Air-to-Air Heat Recovery 12.19

Effectiveness 12.19

Fixed-Plate Heat Exchangers 12.20

Runaround Coil Loops 12.21

Rotary Heat Exchangers 12.21

Heat Pipe Heat Exchangers 12.22

Comparison between Various Air-to-Air

Heat Exchangers 12.25

12.6 GAS COOLING AND

COGENERATION 12.25

Gas Cooling 12.25

Cogeneration 12.25

Gas-Engine Chiller 12.27

Gas Engines 12.27

Exhaust Gas Heat Recovery 12.28

Engine Jacket Heat Recovery 12.28

Cogeneration Using a Gas Turbine 12.29

REFERENCES 12.29

12.1 BASICS OF HEAT PUMP AND HEAT RECOVERY

Heat Pumps

A heat pump extracts heat from a heat source and rejects heat to air or water at a higher tempera-

ture. During summer, the heat extraction, or refrigeration effect, is the useful effect for cooling,

whereas in winter the rejected heat alone, or rejected heat plus the supplementary heating from a

heater, forms the useful effect for heating.

A heat pump is a packaged air conditioner or a packaged unit with a reversing valve or other

changeover setup. A heat pump has all the main components of an air conditioner or packaged unit:

fan, ﬁlters, compressor, evaporator, condenser, short capillary tube, and controls. The apparatus for

changing from cooling to heating or vice versa is often a reversing valve, in which the refrigerant

ﬂow to the condenser is changed to the evaporator. Alternatively, air passage through the evaporator

may be changed over to passage through the condenser. A supplementary heater is often provided

when the heat pump capacity does not meet the required output during low outdoor temperatures.

12.1

12.2

CHAPTER TWELVE

A heat pump system consists of heat pumps and piping work; system components include heat

exchangers, heat source, heat sink, and controls to provide effective and energy-efﬁcient heating

and cooling operations. HCFC-22, HFC-134a, and HFC-407C are the most widely used refrigerants

in new heat pumps. According to the data in the EIA’s Commercial Buildings Characteristics, for

the 57 billion ft 2 (5.3 m 2 ) of air conditioned commercial building ﬂoor area in the United States in

1992, the use of heat pumps for heating and cooling was about 15 percent (by ﬂoor space).

Heat Pump Cycle

A heat pump cycle comprises the same processes and sequencing order as a refrigeration cycle ex-

cept that the refrigeration effect q 1 4 or q rf , and the heat pump effect q 2 3 , both in Btu / lb (J / kg), are

the useful effects, as shown in Fig. 12.1. As deﬁned in Eqs. (9.7) and (9.9), the coefﬁcient of perfor-

mance of a refrigeration system COP ref is

COP ref

h 1 h 4

q 1 4

W in

(12.1)

where h 4 , h 1 enthalpy of refrigerant entering and leaving evaporator, respectively, Btu / lb (J / kg)

W in work input, Btu / lb (J / kg)

The coefﬁcient of performance of the heating effect in a heat pump system COP hp is

COP hp

q 2 3

W in

(12.2)

and the useful heating effect q 2 3 can be calculated as

q 2 3 h 2 h 3 h 1 h 4 h 2 h 1

(12.3)

FIGURE 12.1 Heat pump cycle: ( a ) schematic diagram; ( b ) cycle on p - h diagram.

HEAT PUMPS, HEAT RECOVERY, GAS COOLING, AND COGENERATION SYSTEMS

12.3

where h 2 enthalpy of hot gas discharged from compressor, Btu / lb (J / kg)

h 3 enthalpy of subcooled liquid leaving condenser, Btu / lb (J / kg)

Here polytropic compression is a real and irreversible process. Both the subcooling of the liquid

refrigerant in the condenser and the superheating of the vapor refrigerant after the evaporator

increase the useful heating effect q 2 3 . Excessive superheating, which must be avoided, leads to a

too-high hot-gas discharge temperature and to a lower refrigeration capacity in the evaporator.

Classiﬁcation of Heat Pumps

According to the types of heat sources from which heat is absorbed by the refrigerant, currently

used heat pump systems can be mainly classiﬁed into two categories: air-source and water-source

heat pump systems. Water-source heat pumps can again be subdivided into water-source, ground-

water, ground-coupled, and surface water heat pump systems. Water-source heat pump systems are

discussed in Chap. 29.

Heat pump systems are often energy-efﬁcient cooling / heating systems. Many new technologies

currently being developed, such as engine-driven heat pumps, may signiﬁcantly increase the system

performance factor of the heat pump system. Ground-coupled heat pumps with direct-expansion

ground coils provide another opportunity to increase the COP of the heat pump system.

HVAC&R Heat Recovery Systems

An HVAC&R heat recovery system converts waste heat or cooling from any HVAC&R process to

useful heat or cooling. Here heat recovery is meant in a broad sense. It includes both waste heat and

cooling recovery. An HVAC&R heat recovery system includes the following:

The recovery of internal heat loads — such as heat energy from lights, occupants, appliances, and

equipment inside the buildings — by reclaiming the heat rejected at the condenser and absorber of

the refrigeration systems

The recovery of heat from the ﬂue gas of the boiler

The recovery of heat from the exhaust gas and water jacket of the engine that drives the

HVAC&R equipment, especially engine-driven reciprocating vapor compression systems

The recovery of heat or cooling from the exhaust air from air conditioning systems

Although heat pump systems sometimes are used to recover waste heat and convert it into a

useful effect, a heat recovery system is different from a heat pump system in two ways:

In a heat pump system, there is only one useful effect at a time, such as the cooling effect in sum-

mer or the heating effect during winter. In a heat recovery system, both its cooling and heating

effects may be used simultaneously.

From Eq. (12.2), the coefﬁcient of performance of the useful heating effect in a heat pump is

COP hp q 2 3 / W in , whereas for a heat recovery system, the coefﬁcient of performance COP hr is

always higher if both cooling and heating are simultaneously used and can be calculated as

COP hr

q 1 4 q 2 3

W in

(12.4)

A heat pump is an independent unit. It can operate on its own schedule, whereas a heat recovery

system in HVAC&R is usually subordinate to a refrigeration system or to some other system that

produces the waste heat or waste cooling.

12.4

CHAPTER TWELVE

Heating or cooling produced by a heat recovery system is a by-product. It depends on the opera-

tion of the primary system. A heat recovery system can use waste heat from condenser water for

winter heating only if the centrifugal chiller is operating.

A centrifugal chiller that extracts heat from a surface water source (e.g., a lake) through its evap-

orator and uses condensing heat for winter heating is a heat pump, not a heat recovery system. Heat

recovery systems that are subordinate to a centrifugal or absorption refrigeration system are dis-

cussed in Chaps. 13 and 14, respectively. Recovery of waste heat from industrial manufacturing

processes to provide heating shows great potential for saving energy. These heat recovery systems

must be closely related to the speciﬁc requirements of corresponding manufacturing processes and

are not discussed here.

Heat Balance and Building Load Analysis

The building load consists of transmission gain or loss, solar radiation, ventilation load and infil-

tration load, people, electric lights, appliances (or equipment), and heat gains from fans and

pumps. Building load is actually the load of the cooling or heating coil in an air-handling unit, or

DX coil in a packaged unit or air conditioner. On hot summer days, solar radiation and the latent

ventilation load must be included in the building load. However, both sunny and cloudy days may

occur in cold weather; therefore, building load is calculated and analyzed with and without solar

radiation in cold weather, especially for the control zones facing south in a building in northern

latitudes, or facing north in southern latitudes, where solar radiation is often a primary cooling

load on sunny days.

Figure 12.2 shows the building load analysis of a typical ﬂoor of a multistory building without

solar radiation in winter. In this ﬁgure, line ABCDE represents the building load curve at various

outdoor temperatures T o , in °F (°C). Point A on this curve represents the summer design refrigera-

tion load Q rl , in MBtu / h (kW). During summer design load, all the space cooling loads are offset

by the cold supply air, and the condensing heat of the refrigeration system is rejected to the cooling

tower.

When the outdoor temperature T o drops below 75°F (23.9°C) on the left side of point B on the

building load curve, then the following things occur:

The perimeter zone may suffer a transmission loss.

Solar radiation is excluded.

The latent load of the outdoor ventilation air is no longer included in the building load because

the outdoor air is often drier than the space air.

T F , hot condenser water is supplied to the heating coils in the

perimeter zone to satisfy the heating load. Here T F indicates the outdoor temperature at point F .

When the outdoor temperature drops to T D , the heat recovered from the interior zone plus the

power input to the compressor is exactly equal to the heating load of the perimeter zone. No supple-

mentary heating is needed. Point D is called the break-even point of the building. When T o

T D ,

supplementary heating is necessary to maintain a desirable space temperature.

As the outdoor temperature falls to T H , the cooling coil load in the interior zone becomes

zero. The refrigeration compressors are turned off. No recovery of condensing heat is possible.

When T o

heating.

The area of triangle FGH indicates the heat energy recovered from the internal loads in the

interior zone, which is used to offset the heat losses in the perimeter zone by means of hot

condenser water. Similar building load curves with solar radiation for the entire building and for

the south-facing zones in the building should be calculated and analyzed in winter. For building

load with solar radiation, break-even point D will move to a lower T o . Recovered heat will be

greater, and supplementary heating will be less. For south-facing zones in buildings in northern

T H , all the heat needed for the perimeter zone will be provided by the supplementary

If the outdoor temperature T o

HEAT PUMPS, HEAT RECOVERY, GAS COOLING, AND COGENERATION SYSTEMS

12.5

FIGURE 12.2 Building load analysis of a typical ﬂoor in a multistory building without solar radiation.

climates, cold supply air may be required during sunny days in the perimeter zone even in

winter.

12.2 AIR-SOURCE HEAT PUMP SYSTEMS

In an air-source heat pump system, outdoor air acts as a heat source from which heat is extracted

during heating, and as a heat sink to which heat is rejected during cooling. Since air is readily avail-

able everywhere, air-source heat pumps are the most widely used heat pumps in residential and

many commercial buildings. The cooling capacity of most air-source heat pumps is between 1 and

30 tons (3.5 and 105 kW).

Air-source heat pumps can be classiﬁed as individual room heat pumps and packaged heat

pumps. Individual room heat pumps serve only one room without ductwork. Packaged heat pumps

can be subdivided into rooftop heat pumps and split heat pumps.

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