Heat Rejection Equipment
Engines produce more heat than power. All of the heat liberated by the engine must be dissipated or the engine will overheat and fail. Ideally, this heat will be utilised by the site heat load. In practice, this cannot be realised.
There are three options:
- Modulate the engine output to reduce electricity and heat generation when heat energy is not being fully utilised.
- Dump excess heat into radiators or some other heat sink to enable full electrical output to continue.
- A combination of 1 and 2 above.
If there are periods where the full electrical output is required from the CHP plant, but the heat load will be absent, then the heat rejection equipment must be capable of dumping the full heat output of the CHP plant.
This will happen if, for example:
- The CHP plant must provide a stand-by or auxiliary supply and the heat load must be assumed to be the worst case.
- There are periods of zero heat load, but full electricity output is required (e.g. in summer).
In most situations, for instance in hotels, leisure centres and hospitals, a background heat load is always present (usually the hot water supply), and the heat rejection equipment can be sized to be less than the full heat load, with modulation of output as a secondary method of balancing the heat load. At present, turbines have less flexibility in this respect.
The normal means of rejecting unwanted heat will be by fan-cooled heat rejection units. These units must be sited so that they:
- Ensure a supply of cool air at all times.
- Avoid discharging warm air into other cooling equipment, windows, air-conditioning ducting and spaces where people may stand or walk.
- Comply with planning rules.
- Avoid noise. Fans are inherently noisy. Reducing the air velocities involved can reduce noise levels but the units required are usually larger and more expensive. Another option is to fit silencers. Variable speed fans should also be considered as these alter noise levels and power consumption in line with cooling demands.
The design of heat rejection units should incorporate design margins that allow for:
- Fouling (normal = 10%).
- Variability in engine heat output (normal = 5-10%).
- An appropriate ambient design temperature (25-30°C) - noting that the units will probably be in greatest use in the hottest weather.
Other Topics
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