HEAT NETWORKS | QUEENS QUAY HEAT PUMP A NEW ERA FOR HEAT A large-scale ammonia water source heat pump at the Queens Quay heat network in Glasgow promises to decarbonise heat for new and existing buildings. Vital Energis Lee Moran explains the challenges of maximising efficiency for varied building stock in this innovative scheme W hile there is considerable enthusiasm for large-scale water source heat pumps for new-build projects, they arent always considered as suitable for retrofit buildings with traditional secondary side heating systems. With correct design, however, they are an intelligent solution for most projects or as in the case of the Queens Quay project, commissioned by West Dunbartonshire Council a mixture of new-build and retrofit connections. The Queens Quay energy solution has two 2.65MW ammonia water source heat pumps, built by Star Refrigeration, and a 130m3 thermal store at the heart of the low carbon system. This provides around 80% of the 51,000MWh annual heat demand, with the remainder supplied by backup, gas-fired boilers. There is also scope for two heat pumps to be added as the build out progresses and the heat demand increases. The colourfully lit chimney contains the heat pump ventilation system and includes the emergency ammonia purge system, which ensures there is adequate dispersion and no impact on locals in the event of a leak. Three boiler flues also terminate in the chimney. A riverwater abstraction system has been installed at the Queens Quay Basin, which takes water from the river Clyde and circulates it through the heat pumps, before returning it to the river, with a stipulation that it cannot be returned more than 3K cooler than its original abstraction temperature. The heat pump converts the latent heat from the river into low-temperature hot water, which is distributed via a 1.5km district heat network serving the 23-hectare development. It will eventually serve 1,200 homes, and the associated infrastructure needed to support them, such as health centres and commercial facilities. Delivering a hybrid district heating system While the heat pumps were capable of supplying heat at 80C+, there was strong motivation to lower the lowtemperature hot water (LTHW) flow temperature as much as possible, because every 1K reduction resulted in an increased heat-pump efficiency of 1.5%. This created a dilemma for the designers, who would need to keep temperatures relatively high to meet the needs of the existing buildings, but low enough to get maximum efficiency from the pump. Historical heating systems served by gas boilers operate on 82C flow and 71C return temperatures, meaning primary flow temperatures of up to 90C are commonly used in district heating schemes to satisfy this requirement across a hydraulic break, such as a plate heat exchanger. A detailed review of the existing systems concluded that these buildings can operate at 75C flow and 60C return, while new buildings have been designed to operate at 70C and 45C return. Weather compensation can reduce summertime temperatures for further efficiency benefits. The flow temperature has to be high enough to meet the needs of the four retrofit buildings but, as these constitute 10% of the demand, they dont influence the overall network distribution temperature too much and we can still prioritise overall system efficiency. If the balance of loads tended towards a higher retrofit percentage, then the reciprocal would be true. However, reduction in temperature via weather compensation is important to ensure the system 38 August 2021 www.cibsejournal.com CIBSE Aug 21 pp38-40 Queen s Quay.indd 38 23/07/2021 12:13