Delta T and CHP
One of the fundamental characteristics of CHP is that it generates both heat and electricity. You can’t have one without the other, so the overall energy and carbon benefits are inextricably linked to the run-time of the CHP plant. Consequently, where CHP is included in the system the design should seek to ensure maximum run-times for the CHP plant by achieving a good Delta T and using a thermal storage vessel to store hot water when demand is low. In this way the CHP continues to operate and generate electrical power for use in the building or export the grid.
Consequently, achieving a good Delta T and utilising a thermal store are both critical elements in maximising the return on investment in CHP.
In ensuring that there is effective control of Delta T in the system, the first step is to reduce flow temperatures (e.g. to the 70° described above, rather than the traditional 80°). Then, as long as sufficient heat is removed from the system via the radiators, fan coils or other terminal units, this will reduce the return water temperature. This higher level of heat transfer to the space being heated will be greatly facilitated by reducing the flow rate, so that the water spends more time in contact with the air it is heating. The same principle applies when plate heat exchangers are used for the domestic hot water
(DHW) rather than hot water cylinders.
A further benefit of this is that variable speed pumps can be used to control this flow rate, as slower flow rates can be achieved with smaller, variable speed pumps, resulting in lower capital costs and reduced pump energy consumption.
To a non-engineer these measures may seem complex but are in fact relatively simple to incorporate into the design of the system. Teaming up with specialists that understand the benefits of designing for 70/40 flow/return temperatures will help to deliver maximum energy, cost, and environmental benefits.