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EDN Europe's Editor Graham Prophet posts a selection of comments and insights prompted by the many items of industry news and rumour that cross the editorial desk or are gathered on his frequent travels to interviews, press conferences and events around Europe - and further afield - and somehow never find their way to the magazine or the web site, recovering some of the information otherwise lost in the noise level...

Wednesday, July 01, 2009

Getting into hot water

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A random thought on energy efficiency. We are now being preached the message on a daily basis that if our electric motors/lighting systems/etc etc had more sophisticated controllers, that the energy savings available could be very large. When you say ‘sophisticated’, you generally imply precision of regulation to a set-point, minimal hysteresis and so on. I was reminded that hysteresis is a parameter that also has an optimum value – this came to mind when I was looking at my domestic heating system, which hadn’t been operating correctly. It heats domestic hot water via an indirect loop from a gas boiler and the controls are simple. A bimetallic thermostat on the hot water tank signals the boiler to run, and the boiler has its own temperature sensor that shuts it down if the return flow from the tank heating loop is at too high a temperature – if, that is, it has failed to dump heat into the tank because the contents are already hot. I noticed that the system would run intermittently, over extended periods, when no hot water was being drawn. Simple; the tank thermostat was sticking, calling for heat after the tank contents were already hot; the boiler would run for a short time, get hot water back in the return flow, then shut down until the pipework cooled off and the cycle would repeat. So I was burning fuel to heat up a few metres of pipework, over and over again, to no good effect. Not environmentally desirable and at today’s prices, not a good idea.
As I installed the new thermostat, I realised that this is one case where a significant amount of hysteresis is a desirable characteristic. The optimum behaviour is that the heat source runs for a time, switches off, and doesn’t use any more fuel ‘hunting’ to maintain a set-point; then, when more heat is required, it runs under efficient conditions until its control point is reached. This particular loop can tolerate a high level of hysteresis – because it is always used in a mixed flow, I can accept quite a wide variation in the hot water temperature. So in this case – and assuming it’s working correctly! – the simple, old-fashioned bimetallic switch is actually the ideal product for the job. I could instrument the tank with a silicon temperature sensor, measure to 0.1C and probably control to within a couple of degrees, but it wouldn’t increase efficiency at all. If you follow that thought, there must be an optimum value for that hysteresis: could an adaptive controller find out what it is?

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