HEAT NETWORK | CITIGEN had embedded carbon within the CHPs, which are still within their payback period, so this solution allows us to make best use of the technologies we already have, Oluwole says. Three Carrier AquaForce 61XWHZE heat pumps have been installed. These use a twin-rotor screw compressor and operate on ultra-low global warming potential (GWP) refrigerant R1234ze. In total, the heat pumps provide 4MW of additional heating capacity and an additional 2.8MW of cooling. Along with heat pumps, three 450mm-diameter boreholes were drilled down into the aquifer, 200m below the street. The borehole installation was key to the commercial viability of the decarbonisation works. One of the challenges we face in decarbonisation is achieving a commercial rate of return; the boreholes give us the ability to tap into the Non-Domestic Renewable Heat Incentive to help bring the payback period for this project into an acceptable range for the company, explains Oluwole. Chatzidiakos agrees: The upgrade would still work without the boreholes, but we would lose a lot of financial benefits, he says. Space constraints meant the boreholes had to be drilled in the road outside the energy centre. The road is supported by a series of subterranean arched vaults THREE SOURCES OF HEAT USED BY HEAT PUMPS The ground boreholes will be used as a heat source for most of the year. The low-grade heat that had previously been wasted by Citigen, such as heat rejected from the chillers, can now be stored in the ground boreholes. This is then upgraded by the heat pumps into useful heat when required, saving cost and carbon. The heat in the ground could diminish over time, so, to avoid that, we store the recovered process waste heat in the ground to keep the temperature in balance, says Chatzidiakos. The final source of heat is the cooling network itself. We designed the heat pump installation in a way that allows them to be used in combined heating and cooling mode, explains Chatzidiakos. In this mode, the chilled water is not routed to the chillers; instead it is piped direct to the heat pumps. Because the chilled water has effectively absorbed heat from buildings, if we take the chilled water return to the heat pump, we can use it as a heat source to provide heat back to the network, says Chatzidiakos. He adds that this is the lowest carbon mode of operation because the chillers do not need to run, but heat is still being removed from the cooling network. How it works Mains gas supply CHP Electricity supply with access to the basement. To reach the aquifer from the road, the boreholes had to be sleeved through the vaulted space to enable the auger to drill down 200m to the aquifer. They also need to be threaded between three rail lines under and next to the site. Electricity generated by the CHP engines is used to power the heat pumps. Chatzidiakos says the scheme has been designed to enable three sources of heat to be used by the heat pumps: ground boreholes; low-grade from chillers and other plant; and the cooling network (see panel, Sources of heat used by heat pumps). Whatever the source of heat, the heat pumps upgrade the energy harvested to produce hot water at 80C. That heat can be supplied to the network, saved for future use in the thermal store, or pumped down into the boreholes for longer-term storage. The high flow temperature of the district heating system is because the system serves legacy heating systems in buildings such as the Guildhall. This is not a typical heat pump solution because were supplying older buildings on the network; the Guildhall, for example, requires heat at 85C, while newer buildings on the network with underfloor heating, for example, only require heat at 45C, says Oluwole. To provide for both, the system is designed for the higher temperature demand of older buildings on the basis How the heat pumps are being integrated in the Citigen energy centre Electric chillers Gas boilers Credit: Paul Weston www.geniusandme.com Heat pumps Thameslink tunnels Power export to electricity Grid Electricity supply to customers Thermal store Heat supply to customers Heat exchangers Aquifer Cooling supply to customers Boreholes Storing and recovering heat in the ground via 200m-deep boreholes London Underground 48 May 2022 www.cibsejournal.com CIBSE May 22 pp46-48, 50-51 Citigen Supp.indd 48 22/04/2022 18:11