It is very clear that one of the most important factors in securing future energy supplies is to prevent, or at least minimise, the energy that is wasted in building services systems such as CHP and district heating networks. The less we waste, the more we have and the longer it lasts.
The philosophy at the heart of our company embodies this principle – namely ‘waste not energy, want not energy’. And this is a principle that can be applied in practice through smarter design.
Indeed, this is a concern that has come to the attention of the Chartered Institution of Building Services Engineers (CIBSE), and has been addressed through its guidance on the design of such systems. The guidance states:
“It is recommended that, for new systems, radiator circuit temperatures of 70ºC (flow) and 40°C (return) are used, with a maximum return temperature of 25°C from instantaneous domestic hot water heat exchangers.”
When following this guidance the difference between the flow and return water temperatures (ΔT) becomes the overriding design consideration.
And what makes this approach even more important is the fact that other energy-efficient heat sources that are now commonly being used – such as condensing boilers and heat pumps – also benefit from lower return water temperatures.
For instance, the optimum primary circuit ΔT for gas-fired condensing boilers is 55°C/30°C; for heat pumps it is 40°C/35°C.
The challenge of run times
When using CHP one of the key design challenges is to ensure that the run times of the CHP plants are optimised by making maximum use of both the heat and the electricity generated.
This is best achieved by designing the system to achieve a good Delta T to ensure efficient heat production and using the thermal storage vessel to store heat when demand for it is low.
In this way, the CHP plant will continue to run for longer periods of time and, in doing so, continue to generate electrical power for the building.
A further benefit of the lower return water temperatures is that the CHP plant does not overheat, so as such it does not have to be stopped to prevent the unit being damaged by overheating.
The following steps should be considered in designing such a system:
- Reduce flow temperature to 70°C.
- Use weather compensation to allow flow temperatures to be reduced even further on mild days, thus reducing heat losses.
- Use load tracking CHP units to enable the plant to respond to variable loads.
- Reduce flow rates to achieve a higher level of heat transfer, as the hot water spends more time in the terminal units.
- Use variable speed pumps for heating and DHW to control flow rates.
- Reduce pipework sizes in line with use of variable speed pumps.
- Include differential pressure control valves and pressure-independent ultra-low flow commissioning modules.
- Specify flexible pipework to facilitate reconfiguration of the system if required in the future.
- Include flow measurement and energy data logging in the commissioning modules to enable continuous monitoring of flow rates and real-time energy performance by zone.
By embodying all of these principles in the design, energy performance will be optimised and energy waste will be minimised, if not eliminated.
To address these issues CIBSE and the Association for Decentralised Energy (ADE) are in the process of publishing a ‘Heat Networks: Code of Practice for the UK’, details of which are summarised here .