Unlike steam turbines, gas turbines burn fuel directly. Large industrial gas turbines operate with energy conversion efficiencies of up to 39% but smaller gas turbines, often derived from aeroengines, can operate at up to 42% efficiency. Gas turbine generating capacities range from 3 MW up to over 250 MW. Units of any size can be used in CHP systems and gas turbines are probably the cheapest prime movers available today. However they are best suited for continuous base-load operation. Regular output change can increase wear and maintenance significantly.
Most modern gas turbine installations burn natural gas, though some burn distillate. The heat output from a gas turbine is all found in its exhaust. This is a high-temperature source and it can be used to generate high-temperature, high-pressure steam. Hence a gas turbine will normally only be used in a CHP application where there is a need for high-quality steam. This steam will be generated in a waste-heat boiler attached directly to the turbine exhaust.
Two features provide the gas turbine with additional flexibility in CHP applications. Firstly the turbine is capable of generating steam of sufficient quality to power a steam turbine. This means that steam demand can be
Figure 5.3 Block diagram of gas-turbine-based CHP system balanced with electricity output by exploiting unwanted steam in a steam turbine to generate additional power. Secondly the exhaust from a gas turbine contains a considerable quantity of oxygen because the gas turbine combustion system employs an excess of air. This means that if necessary a waste-heat boiler can be fitted with a supplementary firing system to generate additional steam. This allows a gas turbine CHP system to be matched accurately to heat and electricity demand, allowing efficiencies of up to 90%.
Principal emissions from a gas turbine are nitrogen oxides. These can be controlled by using a special combustion system. Additional exhaust gas treatment may be necessary to meet more stringent environmental regulations. Like the steam turbine, a gas turbine CHP system is only likely to be considered in an industrial situation.
Micro turbines are tiny gas turbines with capacities of from 25 to 250 kW. Many of these units are still in the development phase but some are now being deployed. These are often designed for CHP applications.
Micro turbines operate at extremely high speeds, often up to 120,000 rpm. Typical designs incorporate the turbine components and the generator on a single shaft. Bearings are air lubricated minimising wear. They can burn natural gas, gasoline, diesel and alcohol.
Micro turbine efficiency is low, in the 20-30% range. The exhaust heat can be used to generate low-pressure steam or hot water. The units are quiet relative to most engines so they can be run close to dwellings or in commercial environments. Their size range makes them suitable for commercial or light industrial applications. Multiple units operating in parallel can be used to increase capacity. Micro turbines are likely to become widely available by the end of the first decade of the twenty-first century.
Fuel cells are electrochemical devices, like batteries, that convert a fuel directly into electricity. All fuel cells operate at an elevated temperature but some require very high temperatures while others work at only moderate temperatures.
Fuel cells are among the most efficient ways of converting fuel into electricity. Efficiencies range from 36% in operating units available today to a predicted 55-60% in high-temperature units under development. In CHP applications they can deliver up to 85% efficiency. Fuel cells are extremely good at load following, where their part-load loss of efficiency is minimal.
Low-temperature fuel cells such as the phosphoric acid fuel cell are well suited to distributed generation applications. The units are virtually noiseless so they can be positioned close to homes or offices without undue problem. Emissions are negligible too. However the cells are relatively expensive. These cells require hydrogen (normally derived from natural gas) and are easily poisoned, so the fuel must be very clean. Heat output is suitable for producing hot water but not steam.
Higher-temperature cells can burn natural gas directly without need for pre-treatment. These cells can produce both high-quality steam and hot water. Large units are likely to be deployed in a combined cycle configuration rather than for CHP but some companies are designing small high-temperature solid oxide fuel cells specifically for small commercial and even domestic environments.
While low-temperature fuel cells are commercially available, high-temperature fuel cells are still in their development phase, with the earliest models beginning to appear in commercial situations. Costs are high but these are expected to come down as economies of scale are realised. This technology is considered by many to be the best for future power generation, particularly in a hydrogen economy. It is particularly well suited to distributed generation CHP applications.
A nuclear reactor is used in a power station as a source of heat energy, the heat being used to raise steam to drive a steam turbine. Thus in principle nuclear power can be used for CHP in exactly the same way as any other source of heat. While nuclear power is normally seen as best suited to base-load power generation in large-capacity plants, some attempts have been made to design and build nuclear plants to provide heat and power. In Russia and Eastern Europe some nuclear plants supply district heat and nuclear units have also been used to provide both electricity and heat for seawater desalination. The environmental concerns attached to nuclear generation have limited this type of use.
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