Getting the best from CHP
With the growing use of CHP serving district-heating networks it is essential that these systems are designed to avoid wasting energy. Beata Blachut of SAV Systems explains how this can be achieved
The old proverb ‘waste not, want not’ will be familiar to many MBS readers. And its meaning is clear — the less we waste the more we have. This principle can clearly be applied to our approach to energy and the systems that use it, perhaps with a modification to ‘waste not energy, want not energy’.
One of the key criteria for designing efficient CHP systems is to recognise that this technology operates more efficiently at lower return-water temperatures than are traditionally used in UK heating systems. The guidance from the Chartered Institution of Building Services Engineers (CIBSE) 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.’
Clearly these temperatures are very different to the traditional 82/71°C or, even, 80/60°C flow/return temperatures that are more commonly used nowadays.
This means that the difference between the flow and return water temperatures (ΔT) becomes the over-riding design consideration; lower return-water temperatures are also beneficial for other heat sources, such as heat pumps and condensing boilers.
For instance, the optimum primary circuit ΔT for gas-fired condensing boilers is 55/30°C; for heat pumps it is 40/35°C.
Consequently, ΔT is just as important for systems that use a combination of heat sources as it is for CHP systems.
Additionally, lower flow temperatures are better suited to the relatively mild UK climate, where heating systems are often over-sized for the few very cold days we may experience each year.
At the risk of stating the obvious, one of the key 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.
As a result, the design should seek to ensure maximum run-times for the CHP plant by achieving a good ΔT 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 to the grid. Lower return temperatures also avoid the problem of having to stop the CHP plant from running to allow it to cool.
This means that achieving a good ΔT and utilising a thermal store are both critical elements in maximising the return on investment in CHP.
But what is the best way to go about this?
The first step in increasing system efficiency is to reduce the flow temperature (e.g. to the 70°C described above, rather than the traditional 82°C). Using weather compensation allows flow temperatures to be reduced even further on mild days, thereby reducing heat losses.
Then, as long as sufficient heat is removed from the system via the radiators, fan coils, heat-interface units or other terminal units to achieve a good ΔT, 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.
Lower flow and return temperatures will also reduce heat losses from distribution pipework
To gain control over the system ΔT, both the heating and DHW must be variable flow using variable-speed pumps to control the flow rate. Using smaller variable-speed pumps also results in smaller pipework sizes, reducing capital costs and pump energy consumption.
Delivering these efficiencies in practice requires a whole-system approach that makes use of differential-pressure control valves (DPCVs), variable-speed pumps, ultra-low flow commissioning modules and the use of flexible pipework to facilitate re-configuration of the system through its life.
There are also significant benefits to making use of commissioning modules with integral electronic flow measurement and energy data logging capabilities. These enable continuous monitoring of flow rates and the real-time energy performance of each zone of terminal units so that any areas of energy waste can be identified and rectified.
There is already increasing awareness in the UK around the problem of unnecessary energy waste in buildings, embodied in the forthcoming publication ‘Heat networks: code of practice for the UK’ by CIBSE and the Association for Decentralised Energy (formerly the CHPA), due for publication in the first half of 2015.
The key challenge will be to ensure that these principles are embodied in the design of LTHW systems with CHP and other technologies. To quote another proverb, ‘Where there’s a will, there’s a way,’ and that will needs to come through a culture change that treats the prevention of energy waste as a priority. I would suggest that building-services engineers are ideally placed to apply their will and lead the way.
Beata Blachut is product manager for CHP with SAV Systems.