Combined heat and power efficiency

Given the system described in Example 10-11, what is (a) the combined heat and power efficiency assuming that cycle produces 2 tons/hr of 150 psia/400°F steam Assume a feedwater temperature of 60°F. 

Alternatively, this heat duty can be expressed as 1.396 MWt, [4.762 / 3.412 = 1.396 MW]. Thus, the combined heat and power efficiency is calculated as... 

Figure 7.17 Measured electrical and overall (combined heat and power) efficiency for United Technologies Fuel Cell, PC25C 200 kWe PAFC power plant. (Reproduced with permission from Ref. [38].)...

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. 

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),... 

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. 

Goldemberg, J. Johansson, T. B. Reddy, A. K. N. and Williams, R. H. (1985). Basic Needs and Much More in One Kilowatt per Capita. Ainhio 14(4—5) 190-20f). Kaarsberg, T. M., and Roop, J. M. (1999). Combined Heat and Power in Indtisti y How Much Carbon and Energy can Manufacturers. Save IEEE Aerospace and Electronic Systems Magazine, pp. 7-11, January Rosenfeld, A. H., and Bassett, U. (1999). The Dependence of Annual Energy Efficiency Improvement on Price and... 

The preceding section reviewed the application of popular prime movers for the generation of electrical power only. In the conversion of fuel energy to electricity it is shown that heat is rejected, either in the exhaust of a diesel or gas turbine or, alternatively, in the condenser of a thermal power plant. It can be seen that by applying these machines to provide both heat and electricity the total energy recovery can be much greater and efficiency thereby improved. Combined heat and power (CHP) schemes of this nature are well-established methods of producing both heat and power efficiently and economically. 

The cleanliness of the products of combustion is such that the use of heat-recovery equipment is possible without the risk of corrosion. This has led to the development of combined heat and power packages where the overall efficiency is high. 

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. 

T)man (2005) reported on a case study of a CHP (combined heat and power) system installed at a dairy facility in Ireland. The CHP system generates electricity and uses the generated heat to provide steam. The overall efficiency of the process is 58%. Reported energy savings were 1.7M euros/year with reduction of 27,920 tons of CO2. 

The development of district heating systems is linked with the development of other systems such as combined heat and power systems, which generate waste heat, together with power. These increase the fuel use efficiency [35]. Also heat pumps should be introduced in residential heating systems [18]. In some countries like Norway, district heating system s GHG emissions have been compared with those of individual heating systems and it has been found that the first have lower C02 emissions. 

On-site combined heat and power (CHP) which has existed for years, includes turbines, reciprocating engines and steam turbines. Gas turbines in the 500-kW to 250-MW produce electricity and heat using a thermodynamic cycle known as the Brayton cycle. They produce about 40,000-MW of the total CHP in the United States. The electric efficiency for units of less... 

Fluidised bed combustors are now common-place for large-scale operations and are extensively used in large electricity generating stations. More recently, smaller scale units have been developed for use by individual industrial concerns and in operating, as combined heat and power units, can give overall thermal efficiencies of up to 80 per cent. 

A 250 kilowatt atmospheric system is planned for a Toronto, Ontario, Canada site. The system will be operated by Ontario Power Technologies (formerly Ontario Hydro). The unit will supply 145 kilowatts of heat to the site heating system. Electric efficiency is expected to be about 47% (LHV). Operation of the combined heat and power system is expected in late 2001. 

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