CHP plants

The term "cogeneration is sometimes used to describe a combined power plant, but it is better used for a combined heat and power (CHP) plant such as the one shown in Fig. 1.6 (see Ref. [2] for a detailed discussion on CHP plants). Now the fuel energy is converted partly into (electrical) work (W) and partly into useful heat (Qu) at a low temperature, but higher than ambient. The non-useful heat rejected is (2nu-... 

In Fig. 9.2a, the work output from the unfired plant is shown to be equal to unity and the heat supply F q = 4.0. Further, it is assumed that the useful heat supplied is (2u)cg = 2.25 and the unu.sed non-useful heat is (2nu)cg = 0-75. An important parameter of this CHP plant is the ratio of useful heat supplied to the work output, = (2u)cg cg = 2.25. 

A second criterion of performance sometimes used is an artificial thermal efficiency (tja) in which the energy in the fuel supply to the CHP plant is supposed to be reduced by that which would be required to produce the heat load (Qu) in a separate heat only boiler of efficiency (tjb). i e- by (Qu/ b)- The artificial efficiency (tja) is then given by... 

A third performance criterion developed for combined heat and power plant involves compari.son between the fuel required to meet the given loads of electricity and heat in the CHP plant with that required in a reference system . The latter involves conventional plants that meet the same load demands (indicated by subscript D), for example, in a conventional electric power station and in a heat only boiler. 

The above simple analysis has to be modified for a supplementary fired CHP plant such as that shown in Fig. 9.3c, meeting a unit electrical demand and an increased heat load A. The reference. system fuel energy supplied is now... 

Thus the FESR is less attraetive when there is a large heat load and a WHB with supplementary firing is used. In general, the FESR is probably the most useful of the CHP plant performanee eriteria as it ean be used direetly in the eeonomie assessment of the plant [1. ... 

In general, a gas turbine CHP plant may not exaetly mateh the eleetrieity and heat demands. A plant with a recuperator may meet the heat load (Qu)cg = but not the power load (Wgg < Wd= 1) so extra power from the grid is required (Wc) as illustrated in Fig. 9.4. Following a procedure similar to that given in Section 9.2.3 it may be shown [ 1 ] that the performance parameters for the total plant are then... 

Fig. 9.4. Unmatched CHP plant taking power from the grid.

For an unmatched gas turbine CHP plant, meeting a power load (Wcg = = 1) bul... 

We now illustrate numerically the full range of operation of a gas turbine CHP plant,... 

A gas turbine plant with an overall efficiency t]cq = 0.25 matching a heat load Acc, = 2.25 is again considered as the basic CHP plant also implied is a non-useful heat rejection ratio (Cnu)cg( cg = [1 ( cg)( g + 1)1 =. 3/16. For FESR calculations, we again take the conventional plant efficiency as 0.4 and the conventional boiler efficiency as 0.9. At the fully matched condition the.se assumptions previously led to EUF = 0.8125 and FESR = 0.2. 

There are many gas turbine CHP plants in operation for a range of purposes and applications. Here we describe the salient features of two such plants, each operating with a WHR but also with supplementary firing which can be introduced to meet increased heat demands. 

The CHP plant which replaced these two separate energy supplies is based on a Ruston TB gas turbine (rated at 3.65 MW) which can meet the electrical demand of 3.2 MW and is connected to the grid so that excess electrical power can be sold. By providing full gas power, up to 12 t/h of saturated steam can be produced at 191°C and 13 bar. Five supplementary gas burners can be engaged to increase the steam... 

This fuel cell has shown promise for combined heat and power systems (CHP systems). In such systems, the waste heat is used to heat buildings or to do work. Efficiency in a CHP plant can reach 80%. These plants could replace heating plants and power sources in colleges and universities, hotels, and apartment buildings. 

The capital cost of the CHP plant is estimated to be 3 million pounds (5 million dollars). Combined heat and power is expected to give net savings of 700,000 ( 1,150,000) per year. The plant is expected to operate for 10 years after the completion of construction. 

Today, large amounts of biomass are already used to generate heat and electricity (mainly wood) and are predicted to increase further (e.g., wood-pellet-fuelled boilers, wood-chip-fuelled CHP plants, electricity generation from biogas). 

In contrast to the operation of vehicles, electricity and heat for stationary applications can be generated by the combustion of solid biomass without upstream biomass conversion to pure hydrogen (or methanol, BTL or DME). The efficiency of the direct use of solid biomass is generally higher. The overall efficiency of a solid-biomass-fuelled heat and power (CHP) plant is typically about 70% to 80% direct combustion of solid biomass (e.g., wood chips, wood pellets) in suitable boilers for heat generation only can reach an efficiency of more than 90%. 

CHP plant condensing heat exchanger, assuming an overall efficiency of about 100% (heat and electricity). 

For natural-gas-fuelled CHP plants, the same line of argumentation holds as for the stationary use of hydrogen from biomass. It is more reasonable to use natural gas directly than to convert it to hydrogen first and then to heat and electricity. High electrical efficiencies can be reached in the stationary sector by feeding natural gas to molten-carbonate fuel cells (MCFC) and solid-oxide fuel cells (SOFC). Molten-carbonate fuel cells have the added advantage of using C02 for the electrolyte (see also Chapter 13). 

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