Energy storage in a central heating system: Spa School field trial

Cranfield Institute of Technology and British Gas plc have undertaken a collaborative programme of tests of a system which interposes a heat store between a boiler and a distributive central heating network. This has culminated in a field trial at Spa School in London. It was predicted to lead to significant energy savings, to a lower installed boiler capacity requirement, and to reductions in installation costs for new and for some retro-fitted systems. Specifically, a thermally insulated 3235 1 hot water store was installed in the circuit between the existing modular boiler set and the hydronic ‘radiator’ network. The pipework was arranged so that the heating system could be operated either as configured originally or in the storage mode, in which case two of the five modular boilers were valved off. The two configurations were operated alternately (for four week periods), and their performances were carefully monitored over the 1985/86 heating season. The storage system performed successfully with a 40% lower installed boiler capacity. Boiler cycling was reduced by a factor of 15 and the mean boiler load factor rose from 37 to 70%. Analyses of results show that a 6% saving in annual fuel consumption was achieved by the storage system as compared with that of the highly efficient conventional system employing direct external compensation of the boilers. However, all this saving is attributable to the faster building preheat achieved with the store as, surprisingly, the annual efficiencies of the two systems were the same (in contradiction to the findings of other trials). The reasons for this are examined. Where direct compensation is not possible, the reduction in boiler cycling demonstrated will lead to a further substantial efficiency advantage of at least 10%.