Combined heat and power

A more complex utility is combined heat and power (or cogeneration). Here, the heat rejected hy a heat engine such as a steam turbine, gas turbine, or diesel engine is used as the hot utility. 

Now let us take a closer look at the two most commonly used heat engines (steam and gas turbines) to see whether they achieve this efficiency in practice. To make a quantitative assessment of any combined heat and power scheme, the grand composite curve should be used and the heat engine exhaust treated like any other utility. 

Example 6.5 The stream data for a heat recovery problem are given in Table 6.7. A problem table analysis for AT , = 20°C results in the heat cascade given in Table 6.8. The process also has a requirement for 7 MW of power. Two alternative combined heat and power schemes are to be compared economically. 

Figure 6.35 Alternative combined heat and power schemes for Example 6.4.

This is particularly important when considering the effect that combined heat and power generation (cogeneration) has on utility waste. 

Combined heat and power cogeneration). Combined heat and power generation can have a very significant effect on the generation of utility waste. However, great care must be taken to assess the effects on the correct basis. 

For most chemical plants, process steam is used at pressures of 1.825 MN/m" (250 psig), saturated or lower. When combined heat and power generation is economically justified, the steam may be generated at about 5.96 MN/m" (850 psig) appropriately superheated and used to drive back-pressure steam turbines passing out process steam at the required pressure level. 

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-... 

Fig. 1.6. Cogeneration plant (combined heat and power plant).

For a gas turbine operating as a combined heat and power plant, the energy utilisation factor (EUF) is a better criterion of performance than the thermal efficiency. It is defined as the ratio of work output (W) plus useful heat output (Qu) to the fuel energy supplied (F),... 

THE GAS TURBINE AS A COGENERATION (COMBINED HEAT AND POWER) PLANT... 

This criteria of performance has less relevance to a combined heat and power plant which provides heat and generates electrical power. For an open circuit gas turbine plant, a more logical criterion is the energy utilisation factor (EUF) which can be calculated as... 

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. 

Chapter 9. The as turbine as a cogeneration (combined heat and power) plant... 

Horlock, J.H. (1997). Cogeneration—Combined Heat and Power Plants. 2nd edn, Krieger, Malabar. Florida. 12 Porter. R.W. and Mastanaiah, K. (1982), Thermal-economics analysis of heat-matched industrial cogeneration systems. Energy 7(2). 171-187. 

In recent years there has been a rapid growth in the number of combined heat and power (CHP) and combined cycle gas turbine (CCGT) plants, driven mainly by gas turbines using natural gas, sometimes with liquid fuel available as stand-by. Governments have encouraged the construction of these plants, as their efficiency is high and they produce less carbon dioxide than conventional coal and oil-burning power stations. However, they present some hazards, as gas turbines are noisy and are therefore usually enclosed. 

Horlock, J. H. (1997). Cogeneration—Combined Heat and Power Thermodynamics and Economics. Malabar, FL KiTcgcr Publishing Company. 

The oldest form of combined heat and power is the smokejack, developed in Tibet to turn prayer wheels during religious ceremonies. Captured Tartar slaves introduced this device into Europe by the early fourteenth centuiy and Leonardo da Vinci sketched one around 1480. Commentators as diverse as Montaigne (1580), John Evelyn (1675), and Benjamin Franklin... 

The introduction of the internal combustion engine in the late nineteenth century opened up an entirely new approach to combined heat and power. Rather than using the same fluid for the heat and power process as was the case with hot air and steam processes, the tremendous waste heat generated by the internal combustion process can easily be transformed into useful heat. Cogeneration applications using stationary engines were common in Europe prior to World War I and remain quite popular because the heat is relatively... 

The United States obtained about 9 percent of its electricity from combined heat and power (cogeneration) systems as of 1997. Cogeneration is more prevalent in some European nations than in the... 

About 10 percent of the district energy systems are part of combined heat and power systems where both electric energy and useful thermal energy are produced. The electric generation capacity of these systems totals about 3,500 MW, about 0.5 percent of... 

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