11.7 Thermification cycles

In the process of power generation in thermal electric power stations, a great amount of heat is transferred to the low-temperature source, the water cooling the condenser, and is thus lost. The amount of heat transferred to the low-temperature source, q₂, can be decreased by raising the thermal efficiency of the given cycle. However, this loss cannot be fully eliminated since, in accordance with the second law of thermodynamics, the transfer of a certain amount of heat to the low-temperature source is inevitable.

If it is impossible in principle to eliminate the transfer of heat to the low-temperature source, then is this heat unusable? As is known, great amounts of hot water and steam are used for all kinds of technological processes, for heating buildings and hot-water supply.

In ordinary steam power plants with condensing turbines, the condenser pressure is maintained equal to about 4 kPa (0.04 kgf/cm²), i.e. exhaust steam condenses at a temperature of about

29 °C. The heat transferred to the cooling water in such a condenser has a low temperature potential, and it cannot be used for process work or for heating and hot-water supply purposes. Technological processes usually need saturated steam at a pressure from 250 to 2000 kPa (i.e. from about 2.5 to 30 kgf/cm²), and buildings are heated with saturated steam at a pressure from 150 to 260 kPa (1.5 to 2.6 kgf/cm²). Hot water at a temperature reaching 180 °C is also used in some installations.

In order to be able to utilize the heat rejected from the condensing exhaust steam, condenser pressure must be raised, i.e. it is necessary to raise the temperature at which this steam condenses. An increase of the lower cycle temperature will result in a certain decrease in the thermal efficiency and, consequently, in a reduction of the amount of power generated with the same fuel expenditure as before. Therefore, from the point of view of cycle effectiveness, such an operation is disadvantageous. However, the possibility of getting great amounts of heat for industry and heating purposes by some reduction in power generation is very expedient (it makes it unnecessary to construct special heating boiler houses, which as a rule are small and operate with a relatively low efficiency and require therefore a higher fuel consumption; in addition, such boiler houses use heat of a high temperature potential, realized upon fuel combustion in boiler furnaces, to heat a low-temperature working medium, which is irrational due to the inevitable decrease of system availability).

Electric stations engaged in the generation of heat and electric power use back pressure turbines or extraction turbines.

Steam power plants engaged in combined generation of electric power and heat are referred to as heating and power plants, as distinct from power plants fitted with condensing turbines and generating only electric power.

The T-s diagram of a heating and power plant is shown in Fig. 11.33. As usual, the work of the cycle is represented by the area 1-2-3-5-4-6-1, and the area A-3-2-B-A represents the heat q₂ transferred to the external consumer.

Fig. 11.33

Since, as was mentioned above, industry and district heating systems require steam and hot water at a relatively wide range of temperatures and pressures, heating and power plants use back pressure and extraction turbines of different types, depending on the nature of heat consumption.

Figure 11.34 shows the schematic diagram of a heating and power plant with turbines with a deteriorated vacuum. The pressure maintained in the condenser of such a turbine is such that the vapour saturation temperature is sufficiently high to ensure the required heating of the cooling water in the condenser. The cooling water, heated in the condenser to the required temperature, is delivered to district heating systems.

Fig. 11.34

Figure 11.35 shows the schematic diagram of a heating and power plant fitted with back pressure turbines. In these plants there is no condenser and the exhaust steam, leaving the turbine, is directed via a pipeline to be used as industrial steam in apparatuses in which heat is rejected from the steam and the steam condenses; the condensate is returned to the plant as feed water for the boilers. The pressure of the exhaust steam leaving the turbine is determined depending on industrial steam requirements.

Fig. 11.35

The heating and power plant depicted schematically in Fig. 11.36 is fitted with extraction turbines. In this kind of plant steam at quite high conditions is extracted from intermediate turbine stages (from this viewpoint this layout resembles the power plant with regenerative feed-water heaters). The extracted steam can be directed to be used for industrial purposes (so-called industrial steam extraction), and condensate is returned to the plant (Fig. 11.36a), or to special heaters (heat exchangers; HE) in which this steam heats water to be used in district heating systems (so-called district-heating extraction, Fig. 11.36b). It ought to be mentioned that extraction turbines are most widely used in modern heating and power plants.

Fig. 11.36

Economic performance of a heating and power plant is characterized by the heat-utilization factor K, defined as the ratio of the useful work done in the cycle (l_p) plus the heat transferred to the external consumer (q₂) to the amount of heat released upon fuel combustion in boiler furnaces, q₁:

(11.128)

or, which is the same,

(11.129)

where N is the electric capacity of the power plant, B the hourly fuel consumption, Q₁ the lower calorific value of fuel, and Q the amount of heat transferred to the external consumer.

The more perfect the plant, the closer is the heat-utilization factor K to unity, i.e. the least amount of heat losses in the boiler unit and turbine and the mechanical and electrical losses in the generator.