As more specifiers and building operators recognise the benefits of using CHP to serve district heating networks it is clearly essential that these systems are designed to avoid wasting energy
To that end, the old proverb ‘waste not, want not’ is clearly appropriate – but perhaps modified to ‘waste not energy, want not energy’.
This blog will discuss how energy wastage can be minimised in CHP systems and will also explore the following:
· The benefits of reducing flow/return water temperatures
· The importance of a good Delta T
· Maximising CHP run-times
· Use of thermal storage vessels
· The contribution of reduced flow rates
· Variable speed pumps, smaller pipework and reduced pump energy consumption
· Use of differential pressure control valves (DPCVs)
· Opportunities for integrating flow measurement and energy logging into commissioning modules
· Helping end clients enjoy the benefits of optimum performance with ‘soft landings’
A key consideration in optimising the performance of these systems is to reduce return water temperatures. 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 this is very different to the traditional 82°C/71°C, or even 80°C/60°C flow/return temperatures that are more commonly used nowadays.
As a result, the difference between the flow and return water temperatures (Delta T) becomes the overriding design consideration – and not just for CHP – lower return water temperatures are also beneficial for other heat sources such as heat pumps and condensing boilers.
For instance, the optimum primary circuit Delta T for gas-fired condensing boilers is 55°C/30°C; for heat pumps it is 40°C/35°C.
Consequently, Delta T is just as important for systems that use a combination of heat sources as it is for CHP systems.
Additionally, lower flow water 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.
Maximising run times
The run-time of the CHP plant is critical in achieving maximum energy and carbon benefits – simply because when CHP is running it generates both heat and electricity – you can’t have one without the other.
Clearly, then, the design should try to maximise 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 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 Delta T and utilising a thermal store are both critical elements in maximising the return on investment in CHP.
How to go about it?
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).
Use of 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 Delta T, this will reduce the return water temperature.
In this respect, it is a great help to reduce the flow rate, so that the water spends more time in contact with the air it is heating and a higher level of heat transfer to the space being heated is achieved.
Lower flow and return temperatures will also reduce heat losses from distribution pipework
To gain control over the system Delta T, both the heating and DHW must be variable flow using variable speed pumps to control the flow rate.
Use of smaller, variable speed pumps also results in smaller pipework sizes, reduced capital costs and reduced 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 – such as SAV’s FloCon Watchman. 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 SAV is ideally placed to support the will and help show you the way.