Case Study: Oxford Brookes
All three phases of Oxford Brookes University made use of our Heat Bank (TM) Pandora Thermal Stores to provide distributed storage from a central CHP plant, with each store delivering domestic hot water to eight accommodation units.
The Pandora was selected as it provided an almost fit and forget solution, with no discharge pipes, and instantaneous mains hot water overcoming Legionella issues.
The CHP plant required return temperatures of 60C or below, while the client required DHW secondary return circuits fed from each store to remain at 60C almost continuously, with alarms fitted to circuits that trigger if DHW temp drops for too long. These two requirements, hand in hand, meant that when stores were sitting idle, they need to cool the primary return to 60C or below, using DHW water at 60C - practically impossible with a steady state of affairs.
The solution was to setup controls so that the DHW circuit starts at 60C, and once cooled a few degrees, reheat is initiated. The design uses very close approach heat exchangers so that the return to the boiler can be kept at or below 60C. The DHW temperatures stay above 55C at all, rising to 60C every few minutes.
The current systems using mains logic, and standard electro-mechanical controls. No electronics.
Also, in our view, the 60C requirement for Legionella is are too strict, given this is an instantaneous system with no stored DHW. There is a reasonable volume of water in each DHW loop, however the circuit need only be periodically sterilised. This would allow the normal setpoint to be dropped to 50C, providing an improvement in CHP efficiency.
We also feel that the system can benefit from been actively managed, allowing heat network temperatures to be weather compensated to heating requirements, rather than be kept hot all the time for DHW maintenance. With stores communicating to time reheat sessions to coincide, the heat network need only ramp up in temperature for short spurts. The Pandora stores can be heated up to 90C if available, and are capable of driving secondary return losses for a significant time before calling for more.