2 Basic Concepts

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CHAPTER 2

TYPES & ASSESSMENT PARAMETERS

Combined heat and power (CHP, also known as cogeneration) can provide thermal

energy for buildings or processes, while simultaneously generating part of the

electricity needed at the site.

2.1 CLASSIFICATION OF COGENERATION SYSTEMS

Cogeneration systems are normally classified according to the sequence of energy use

and the operating schemes adopted.

A cogeneration system can be classified as either a topping or a bottoming cycle on

the basis of the sequence of energy use. In a topping cycle, the fuel supplied is used to

first produce power and then thermal energy, which is the by-product of the cycle and

is used to satisfy process heat or other thermal requirements. Topping cycle

cogeneration is widely used and is the most popular method of cogeneration.

2.1.1 BOTTOMING CYCLE

In a bottoming cycle, the primary fuel produces high temperature thermal energy and

the heat rejected from the process is used to generate power through a recovery boiler

and a turbine generator. Bottoming cycles are suitable for manufacturing processes

that require heat at high temperature in furnaces and kilns, and reject heat at

significantly high temperatures. Typical areas of application include cement, steel,

ceramic, gas and petrochemical industries.


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FIG 2.1 Bottoming Cycle

2.1.2 TOPPING CYCLE

The four types of topping cycle cogeneration systems are briefly explained infollowing table

Table 2.1 Types of topping Cycles

A gas turbine or diesel

engine producing electrical

or mechanical power

followed by a heat recovery

boiler to create steam to

drive a secondary steam

turbine. This is called acombined-cycle topping

system.

The second type of system

burns fuel (any type) to

produce high-pressure

steam that then passes

through a steam turbine to

produce power with the

exhaust provides lowpressure process steam.

This is a steam-turbine

topping system.

A third type employs heat

recovery from an engine

exhaust and/or jacket

cooling system flowing to a

heat recovery boiler, where

it is converted to process

steam / hot water for furtheruse.

The fourth type is a gas

turbine topping system. A

natural gas turbine drives a

generator. The exhaust gas

goes to a heat recovery

boiler that makes process

steam and process heat

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2.2 ASSESSMENT PARAMETERS OF COGENERATION

SYSTEMS

Before proceeding with the description of cogeneration technologies, it is necessary to

define certain indices, which reveal the thermodynamic performance of a

cogeneration system and facilitate the comparison of alternative solutions (systems).

Numerous indices have appeared in the literature. The most important of those are

defined in this chapter:

Efficiency of the prime mover (e.g. of gas turbine, Diesel engine, steam turbine):

where

shaft power of the prime mover,

fuel power (flux of the fuel energy) consumed by the system.

fuel mass flow rate,

lower heating value of fuel.

Electrical efficiency:

Where is the net electric power output of the system, i.e. the electric power

consumed by auxiliary equipment has been subtracted from the electric power of the

generator.

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Thermal efficiency:

where

is the useful thermal power output of the cogeneration system.

Total energy efficiency of the cogeneration system:

The quality of heat is lower than the quality of electricity and it is decreasing with the

temperature at which it is available. For example the quality of heat in the form of hot

water is lower than the quality of heat in the form of steam. Consequently, one may

say that it is not very proper to add electricity and heat, as it appears in Eq. (2.5).

Therefore following relations are also used:

Power to heat ratio:

Table Heat-to-power ratios and other parameters of cogeneration systems

CogenerationSystem Heat-to-powerratio (kWth/

kWe)

Power output(asper cent of fuelinput)

Overallefficiency(percent)

Back-pressure steamturbine

4.0-14.3 14-28 84-92

Extraction-condensingsteam turbine

2.0-10.0 22-40 60-80

Gas turbine 1.3-2.0 24-35 70-85

Combined cycle 1.0-1.7 34-40 69-83

Reciprocating engine 1.1-2.5 33-53 75-85

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Fuel energy savings ratio:

where

total fuel power for separate production of and ,

fuel power of the cogeneration system producing the same amounts of and

In order for a cogeneration system to be a rational choice from the point of view of

energy savings, it must be FESR > 0.

Equations (2.3)-(2.6) lead to the following relations:

It should be mentioned that the power to heat ratio is one of the main characteristics

for selecting a cogeneration system for a particular application.

2.4 COGENERATION IS MOST ATTRACTIVE WHEN

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The demand for both steam and power is balanced i.e. consistent with the

range of steam: power output ratios that can be obtained from a suitable

cogeneration plant.

A single plant or group of plants has sufficient demand for steam and power to

permit economies of scale to be achieved.

Peaks and troughs in demand can be managed or, in the case of electricity,

adequate backup supplies can be obtained from the utility company.

Table 7.3: Typical Heat: Power Ratios for Certain Energy Intensive Industries

Industry Minimum Maximum Average

Breweries 1.1 4.5 3.1

Pharmaceuticals 1.5 2.5 2.0

Fertilizer 0.8 3.0 2.0

Food 0.8 2.5 1.2

Paper 1.5 2.5 1.9

The ratio of heat to power required by a site may vary during different times of theday and seasons of the year. Importing power from the grid can make up a shortfall in

electrical output from the cogeneration unit and firing standby boilers can satisfy

additional heat demand. Many large cogeneration units utilize supplementary or boost

firing of the exhaust gases in order to modify the heat: power ratio of the system to

match site loads.

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2 Basic Concepts