The most valuable form of energy is electricity. Unfortunately, large scale storage is difficult and electric power must be produced when used. A base load in the electric system from wind power, hydro power or other renewable resources delivers electrical energy whereas thermal plants (nuclear or combustion based) supply electricity at a specified power and thus ‘keep the balance’ in the system.
In a thermal plant a fuel (natural gas, coal, oil, biomass, waste, nuclear material) is used to heat up and evaporate a media (steam, combustion air, or other) at a given pressure and subsequent make the gas expand either through a turbine or in a cylinder to generate electric power. Fundamental laws states that such a conversion has an upper limit on the efficiency. Almost half of the energy is waste heat.
The proper use of waste heat from thermal plants is to locate the plants close to cities and supply DH systems for heating buildings. In case the plant is an engine or a simple cycle gas turbine, waste heat is available at sufficiently high temperatures to be used directly. For a steam turbine plant waste heat temperature is often close to the ambient temperature. The process is modified to obtain waste heat at DH system temperatures without losing too much of the electrical efficiency. The lost electricity is typically 10-20% of the production of useful heat.
In a steam turbine the low end pressure is a vital parameter. With sea water cooling it could be 0.02 Bar whereas the use of DH water may give a back pressure of 0.5-0.8 Bar.
A plant to cover heat requirements is designed as a back pressure turbine, which has a linear relation between electricity and heat production.
In a plant to produce electricity mainly, steam can be extracted from the turbine and condensed with DH water. The ratio between lost power and produced heat is in the range 0.1 to 0.2. Heat production can be dropped within seconds and release the lost power as a spinning reserve.
Near large cities, the power plants will produce large amounts of heat during winter and most electricity during the summer. The CHP-plant is built to operate in back pressure mode during winter and with extraction the rest of the year.
Normally it will be very profitable to connect a heat accumulator corresponding to at least 8 max. load hours. It is inexpensive and has many benefits:
- Maintain the pressure in the DH system,
- Decouple the production of electricity and heat,
- Peak load unit in the DH system.
In electricity only mode efficiency is in the range 40-60%; In CHP mode energy utilization is of the order 90-95%.
Replacing individual boilers with low temperature DH, based on CHP and accumulators is a cost effective and efficient solution. Ramboll is a world leading consultant in all three.
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