HEAT NETWORK | SWAFFHAM PRIOR The data was used as a proxy to get an annual demand profile for a typical property type. We had to use a real blend of data sources to get to an approximation of heat use in the village, Troup explains. Hybrid heat pumps While work to establish the villages heat demand was undertaken, the team also had to confirm that the field owned by the council had the ability to supply sufficient heat. Originally, the plan was for 267 boreholes, but thermal testing revealed that this density would have reduced the productivity of the GSHPs significantly because of the amount of heat that would have been extracted by that many ground-loop arrays. To sustain the temperature in the ground and thereby allow the heat pumps to work more efficiently Bouygues E&S looked at installing additional air source heat pumps (ASHPs). Iterative modelling established that a 3:1 ratio of GSHP to ASHP was optimal (two 0.75MW GSHPs and one 0.5MW ASHP). The strategy is harnessing the benefits of both technologies, says Miles Messenger, head of energy performance contracting at Bouygues E&S. Theres a benefit to using ASHP in the summer, as the warmer air gives rise to a higher coefficient of performance. This also gives the GSHPs ground loops a chance to replenish, ready for the next heating season. The seasonal replenishment results in a lower thermal degradation rate of the ground loop array, so fewer ground loops are needed theyre reduced by more than 50% to meet the long-term demand, Messenger adds. One of the key challenges of the project was ensuring compatibility with a diverse range of dwellings. The heat network is designed to supply heat at around 72C, in line with that generated by oil-fired central heating. The reason this is important is that many older homes, unless retrofitted to high energy efficiency which can be costly, cannot take individual low temperature ASHPs and get the level of warmth and comfort they would like, says Councillor Lorna Dupr, Cambridgeshire To gauge heat demand, meters were installed in properties representative of the range of building types in Swaffham Prior Originally, the plan was for 267 boreholes, but thermal testing revealed that this density would have reduced the productivity of the ground source heat pumps significantly OPTIMISING THE HEAT PUMPS The ASHPs will act as lead from mid-April to early October. They will be used all year round, but make a comparatively small contribution in winter. Switchover to GSHP is based on the real time COP, driven by the air temperature (ASHP COP) and glycol temperature (GSHP COP). The mode of operation will shift depending on these variables, though it will also be influenced by the RHI tariff and solar PV availability. In general: Winter: The GSHP will act as lead and the ASHP will act to meet peak loads only. Shoulder: Either will act as lead, depending on outside air temperature and ground loop temperature, as well as tariff optimisation. Summer: ASHP will act as lead and GSHP will act to meet peak loads only (unlikely to be required). County Councils environment and green investment committee chair. The high temperature will enable residents to dispense with conventional oil-fired boilers and connect to the network through a heat interface unit, without having to upgrade radiators or insulation. Minimal alteration to the tertiary systems is required, says Messenger. We have options for all common types of domestic system; however, we do need to convert heating systems from gravity-fed to mains pressure. The main and unavoidable disruption for occupants is associated with the heat network connections, which require the installation of trenched heat pipework from the property boundary to the building. As conventional heat pump technology is unable to achieve 72C efficiently, Bouygues E&S selected heat pumps with ammonia as the refrigerant. This is able to generate higher temperatures while maintaining a reasonably good COP but there is a downside. The corrosive and hazardous nature of this refrigerant led to significant mitigations in design, says Messenger. These include compartmentalisation of the energy centre spaces, emergency extraction ventilation, and leak-detection systems, which had to comply with the Dangerous Substances and Explosive Atmospheres Regulations and the EU ATEX directive. The scheme will be linked to a private electricity network connected to a 28MW 22 February 2022 www.cibsejournal.com CIBSE Feb22 pp20-22, 24 Swaffham Prior.indd 22 28/01/2022 16:19