By Martin Crane, owner and managing director, Carbon Alternatives
Substation performance: As per F103-7, the test explores the performance for DHW, space heat and standby separately. DH design consultants’ specifications rarely mention substation performance during standby; the UK HIU Testing Procedure now highlights this key area that many UK designers were unaware of. Some of the tests are done at typical DHW and space heat hot water demands, which test the substation under conditions that reflect real world operation. These typical demands were assessed from a large set of customer heat meter records. Unlike the Swedish test which is pass or fail, the UK HIU Testing Procedure takes measurements from which a volume weighted annual return temperature (VWART) can be calculated for the substation. This calculation reflects the substation performance during supply of typical space heating and DHW loads and while on standby (at all other times). VWART was chosen as the performance metric as return temperature is the dominant factor affecting the efficiency of DH systems, and also as this concept is relatively easy to understand. Assessing impacts of other DH variables: This element of the UK HIU Testing Procedure demonstrates the impact on the DH return temperatures of variables such as DHW temperature and the temperature of the space heating system flow and return. From the results produced by the Testing Procedure, designers should improve their understanding of the impacts of modifying variables such as primary and secondary flow temperatures. Highlight impacts of poor commissioning: In the UK it is rare to see requirements for pre-settable (flow controlled) valves on radiators supplied from DH networks. Many UK designers have little exposure to operating schemes and they design in the belief that if the radiators are sized for 70/40°C operation, that is what they will work at. Of course there are few complaints when the radiators actually work at, say, 70/62°C as the radiator pushes out lots of heat and the residents are happy. The householder is unaware of how high return temperatures increase the cost of DH.
From a low base of only a few percent of households being on district heating (DH), the UK is now developing more DH. In some areas, such as London, it is a requirement for new developments, but at times this can result in the DH being just another box to tick as part of getting permission to build. At these new built housing developments, it has taken some time to realise the importance of the long term operating costs of the DH. DH has often been poorly implemented, the designer’s focus is meeting a theoretical peak demand and the building contractors feel that if domestic hot water (DHW) flows and the radiators are hot, their job is done. There has been insufficient attention throughout the process to deliver lowest lifecycle cost in operation. Also, consideration of the environmental performance tends to stop once the high level design choices are made. The UK Government made available some research funding to improve DH systems. Using this funding, I developed a testing procedure for individual house / apartment substations with indirect connection for the space heat. This is known as the UK HIU Testing Procedure, and comprises a series of tests which are conducted in a testing facility. In the UK, individual apartment substations are called hydraulic (or heat) interface units, abbreviated to HIUs. The Swedish District Heating Association test standard F103- 7 was used as a starting point for the UK test. The substation configuration and capacity was chosen to best represent those currently being installed in the UK. The research project included testing 5 substations to gather data on their performance. The UK HIU Testing Procedure is much broader than just the performance of the plate heat exchangers at peak loads, which is the usual performance information a substation supplier will provide. The Test Procedure: 1) determines the performance of individual substations for heat loads seen in operation, 2) demonstrates the impact of other DH design variables on substation performance and, 3) highlights issues arising from poor commissioning.
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