Delta T controllers and thermal stores
One of the key features of CHP is that it generates both heat and electricity – you can’t have one without the other. So the cost and environmental benefits of using CHP are linked to the time the CHP plant runs. Therefore, where CHP is included in the mix of heat sources, and a good Delta T is achieved, a thermal storage vessel should also be included in the system. When the demand for hot water is low, any excess heat can then be stored in the storage vessel, so that the CHP can continue to operate and generate electrical power for use in the building
Consequently, achieving a good Delta T and utilising a thermal store are both critical elements in maximising the return on investment in CHP.
The first thing to do in ensuring there is effective Delta T control in the system is to reduce flow temperatures to as low a temperature as possible while ensuring sufficient heat is delivered to the terminal units.
Reducing the flow temperature then has the potential to reduce the return temperature, as long as sufficient heat is removed from the water at the terminal unit (e.g. radiator, fan coil). This can be achieved by reducing the flow rate, so that the flow water spends more time in contact with the heat exchanger and loses more heat to the space. The same principle applies when plate heat exchangers are used for the domestic hot water (DHW) rather than hot water cylinders.
Achieving a good Delta T therefore involves the use of a variable speed pump so that the flow rate of the water can be reduced to maximise heat transfer at the terminal unit. An added benefit of this is that it enables use of a smaller pump that costs less to buy and operate.
In turn, variable speed pumping necessitates the use of two-port valves such as thermostatic radiator valves (TRVs). However, two-port valves are not designed to function with variable pressures, so differential pressure control valves (DPCVs) should also be included to enable the two-port control valves to operate as designed.
When all of these factors are taken into account it is clear that a few relatively simple changes to the way we think about heating system design can help to realise the full potential of low carbon energy sources. Furthermore, adopting the principles of 70/40 flow/return temperatures will deliver those benefits to all sizes of application, from schools and nursing homes to the largest office blocks and factories.