Operation & Maintenance of Power Plants
Re-using waste heat
Jeroen van Ruitenbeek and Johan van der Kamp look at the efficient use of waste heat in combined heat and power systems.
G
enerating electricity, a (fossil) fuel – gas, coal, or oil – is converted into heat and subsequently is being used to generate high pressure, high temperature steam. Ten, in a steam turbine connected
to a generator, a significant part of the energy in this steam is converted into electrical energy to be supplied to the power grid. In large modern power stations, approximately 40 per cent of the energy in fossil fuel is converted into electrical energy, the remainder being discharged in (cooling) water as waste heat, and/or into the air. Tat waste energy does nothing other than heating the planet. It is physically and technically almost impossible to
convert a higher per centage than the above mentioned 40 per cent of the energy from fossil fuel into electrical energy. It is, however, possible to make efficient use of the waste heat - for example, for district heating. However, this solution also has negative consequences, because it usually means that an even smaller proportion of the energy in the fuel is converted into electrical energy. In most cases, just one third (33 per cent) of the potential energy in fossil fuel than is converted into electrical energy. But in these applications, a larger proportion of the
remaining two-thirds of the waste heat can be used for heating greenhouses, buildings, or processes. Tis waste heat can also be stored underground as a seasonal energy supply. Processes making efficient use of part of the waste heat are called CHP (combined heat and power).
Location of the CHP Most electricity is still generated in large power stations with an efficiency of 40 per cent. Where the power station is built is determined by the supply and/or availability of fuels because the electricity can easily be transported over long distances. In CHP plants, however, the waste heat cannot be
transported over long distances. Tese power stations must therefore be built close to the waste heat users. For that same reason, CHP plants are often smaller, as there is often a limited local capacity to use large amounts of waste heat. Waste incineration plants are much smaller, for example, than the larger power stations. But these waste incineration plants are very suitable for CHP. Te electricity supplied by these waste incineration plants is supplied to the power grid and the waste heat is used nearby.
Te electrical efficiency of a power station is mainly 18
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determined by two criteria that influence the efficiency of the turbine:
● Te higher the pressure and temperature of the steam before the turbine, the higher the efficiency of the turbine, but also:
● Te lower the pressure and temperature of the steam after the turbine, the higher the efficiency of the turbine.
High pressure, high temperature Condensers are used to achieve the lowest possible pressure and temperature after the turbine. Because the steam boiler and the steam turbine in the first case (high pressure, high temperature steam) are relatively very expensive for smaller systems, its operators of smaller systems opt to work with lower pressures and temperatures. Also, since very low pressure after the turbine can only be achieved when the condenser is very large and very expensive, the operators of small systems often opt for a higher pressure after the turbine. Together, this means that the electrical efficiency of a
combined heat and power plant is usually significantly lower than that of a large (expensive) power station.
Fig. 1. A-frame at existing plant.
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