The system being upgraded at E.ON’s Citigen energy centre in the heart of London’s square mile will draw from the natural warmth of the earth as well as recycling waste heat from power generation that would otherwise vent into the atmosphere.
From Citigen, which is housed in Charterhouse Street in the old Port of London Authority building and another building nearby, an 11km network of insulated pipes weave their way underground, buried beneath public roads and hidden in basements and tunnels. These deliver heating and cooling to the City of London’s buildings including the Guildhall, Barbican Centre and the Museum of London.
Citigen’s pumps push heat through 6km of pipes at a temperature of 95°C flow, 72°C return, to 27 substations, for distribution to the buildings served by the plant. The buildings are hydraulically separated via individual plate heat exchangers.
Seven of the substations also provide cooling; chilled water leaves the energy centre at a temperature of 6°C and returns at approximately 11°C - 12°C, depending on demand.
In the latest refit, three Carrier AquaForce 61XWHZE heat pumps have been installed. These utilise a twin-rotor screw compressor and operate on ultra-low global warming potential refrigerant R1234ze. In total, the heat pumps provide 4MW of additional heating capacity and an additional 2.8MW of cooling. The Carrier units upgrade the energy harvested to produce hot water at 80°C.
They are supplemented by three boreholes that extend into the aquifer 200m below London streets and provide zero-carbon heating and cooling for buildings served by its network.
Three 450mm diameter boreholes each house two 150mm-diameter pipes, one each for abstraction and recharge. Each works as a standing column well and can be used as a heat source (providing water at perhaps 9°C in winter) or sink (returning water at perhaps 25°C in summer). The system is estimated to extract about 2.8MW of heat capacity.
Space constraints meant the boreholes had to be drilled in the road immediately outside the energy centre. The road is supported by a series of subterranean arched vaults. To reach the aquifer from the road, the boreholes had to be sleeved through the vaulted space so enable the auger to drill down 200m to the aquifer.
Citigen also draws on a thermal store – a 30m-high vertical vessel, volume 320m3, said to be able to hold up to 8MW of heat. It was installed as part of a £27m upgrade in 2013 that also saw two 16MW dual-fuel (gas/diesel) combined heat and power (CHP) engines removed and replaced with two MWM TCG 2032 V16 gas-powered generators. They provide a total of 9MW heat capacity and 8.6MWe electrical generation capacity.
In a normal year, 92% of the heat is supplied from the CHP, with an 8% contribution from three 3MW gas boilers.
The heat pumps are powered from the CHP, making best use of the power generated on-site. They take renewable heat from the boreholes, combined with recycled heat from the cooling towers. That heat can either be saved for future use in the thermal store within the building or pumped down into the boreholes for longer term storage.
Waste heat is recovered from engine exhausts, turbochargers, jacket cooling, and lubricating oil and transferred via heat exchangers to the network, increasing efficiency. Cooling is provided by three 1.1MW low-voltage screw chillers and a 3MW high-voltage screw chiller. Eight cooling towers on the roof provide waste heat removal from the CHP intercooler and electric chillers.
Gas is used to run the CHPs and boilers. Electricity import can come from the CHPs or from the grid to run the pumps and electric chillers. The heat pump can use electricity from the CHP or imported from the grid (when the CHP is not running).
The system also exports electricity back to the grid; in 2019 before COVID, that amounted to up to 33GWh.
Citigen is also home to E.ON’s UK City Energy control room. From here over 70 district heating schemes across the UK covering 43,000 customers. Two operators working 12-hour shifts join a team of six there, which include three maintenance engineers, two project managers and one general manager.
STRATEGIC ROLE
The project forms part of the city’s wider efforts to meet net zero targets and contributes to London’s climate emergency plans.
Michael Lewis, E.ON UK Chief Executive, says: “Tackling the environmental impact of heating, especially in densely populated areas, will be key to meeting the UK’s 2050 net zero targets. Part of that challenge means re-imagining how energy is provided to homes, businesses and cities. In taking the next step and installing heat pump and geothermal technology at Citigen we’re making a powerful statement of what can be done to reduce carbon usage on a large scale.
“The Citigen building itself has a long and storied history, and our £4m investment in a zero carbon heating and cooling system gives it an exciting new chapter and makes sure it’s fit to support the capital’s cleaner future.“
Energy Minister Lord Callanan said: “Heat networks offer an effective way of reducing carbon emissions while bringing down costs to consumers. E.ON’s project is a commercial vote of confidence in heat networks and heat pumps, meaning homes and businesses across the City of London will benefit from clean heat and is another great example of how the pace of rolling out cutting-edge low-carbon technologies is being accelerated across the UK.”
Catherine McGuinness, the City of London Corporation’s Policy Chair, said: “This scheme offers a really creative solution to helping meet our energy needs and tackle climate change, by tapping into the earth as a sustainable energy source. Innovative projects like this will play a hugely important role in helping us meet the ambitious goals set out in our Climate Action Strategy of achieving net zero carbon status for our operations by 2027 and for the Square Mile as a whole by 2040.”