9.3   Combined heat and power (CHP)

 

The inherently poor efficiency of the turbine-generator in power generation applications has always been regarded with concern by turbine designers. The reasons for this are discussed in Section 3 of this chapter, where it will be seen that the major part of the heat produced by the burning of fuel in the boiler is rejected as low grade energy in the condenser cooling water.

It has long been appreciated that there is a major thermal advantage to be obtained if such low grade energy can be employed to provide heating for domestic and industrial use. It is the purpose of CHP schemes to exploit this potential.

The condenser pressure in a typical turbine for power generation is in the range of 50-120 mbar. The function of the cooling water in the condenser is to extract the latent heat of evaporation from the steam exhausted by the turbine and this is done at the saturation temperature corresponding to the steam pressure, i.e., at 33-45°C to 49.5°C for pressures of 50-120 mbar. Unfortunately, temperatures as low as these would be unacceptable for transmission to domestic heating systems, so it is necessary to consider higher temperatures in the range 80-120°C which give corresponding pressures in the range 0.5-2.4 bar. Industrial processes may need even higher temperatures; so a turbine used in a CHP scheme will either incorporate steam extraction at a pressure higher than condensing pressure, or will exhaust directly at the higher pressure. The former is known as an extraction or pass-out turbine whereas the latter is called a back pressure turbine.

Many plant configurations exist to suit the particular requirements of each specific application. Two examples are given here to illustrate the range of plant and some of the possibilities. The first example is of a condensing turbine designed at the outset for a domestic heating installation with electrical power generation as well. The second example illustrates how an existing large power station might be converted for a similar heat load.

In both cases, steam is extracted from the turbine at two different pressures, as shown in Figs 1.130 and 1.131. More than two pressures could be used with the advantage of higher thermal efficiencies but at the expense of greater complication. The extraction steam is passed to two district water heaters, so that an equal temperature rise is obtained across each of them.

District heating supply scheme with special turbine design

The condensing turbine shown in Fig 1.130 meets the usual requirement of a domestic heating application where, depending on the ambient temperature, a variable quantity of hot water at the required temperature must be supplied. It comprises a conventional single-flow HP cylinder with an asymmetric double-flow IP cylinder supplying steam to two series-connected district water heaters. The steam from each IP exhaust is also supplied to two separate different design double-flow LP cylinders, being throttled before expansion. This enables the steam pressure at the IP exhausts, and hence the heated water temperature, to be held constant over a wide load range.

Electrical power is also generated by means of a conventional shaft-driven generator. The asymmetric IP cylinder provides a neat solution to the problem of achieving controlled extraction pressures, since con¬trol valves can readily be supplied in the inter-cylinder crossover piping downstream of the IP exhausts. The asymmetry is provided by designing for a different number of stages in each flow of the turbine.

CHP convertion for a district heating supply scheme

Figure 1.131 depicts three large existing turbines for power which have been modified to incorporate steam extractions to a bled-steam range. This sup¬plies an auxiliary back pressure turbine providing further electrical power generation and feeding two district water heaters from its asymmetric double-flow cylinder. For clarity, the feedheater systems of the three main turbine-generators have not been shown. Because steam is bled from all three main units if they are running, there is a loss in the water inven¬tory of each unit which must be made good by the provision of a condensate return system, suitably con¬trolled to return the correct water quantity to each unit.

All the plant shown in Fig 1.131 would normally be installed at the existing power station. The only modification to the existing turbine-generators would be the provision of bleed and condensate return tap¬pings, with control valves as shown. The district water system would include the heaters, pumps and pro¬vision for make-up and heat storage so that the operating periods of the auxiliary turbine generator and its loading schedule can be controlled with a degree of independence from the heat demand.

Such a scheme could provide heat as well as power to a large city located many kilometres away from the power station.

The total 'efficiency' or utilisation of heat in the fuel in district heating schemes may be over 80%, which makes it attractive for countries with cold winters. One difficulty which has prevented a more widespread use of such schemes is the cost and com¬plication of distributing the heat thus provided to hundreds of domestic premises. The most successful schemes have been applied to new developments of large apartment blocks. With ever increasing fuel costs and the need to conserve valuable fossil fuels, the economics of such schemes are likely to find increasing favour.

 

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