ATES system in direct cooling mode – directly from the cold well with no heat pump


Cooling mode, with heat pump


HP: Heat Pump HEX: Heat Exchanger DCS: Dry-Cooler Supply DCR: Dry-Cooler Return


CS: Cold Supply CR: Cold Return WS: Warm Suppky WR: Warm Return


The reversible energy store drives up the coefficient of performance of the heat pump because it helps drive up the temperature difference between the heat pump’s on- and off- temperatures


an ATES solution. ‘Because heat rejection in an ATES system is into the aquifer, the scheme does not need rooftop refrigeration machines,’ says Richard Shennan, a director at Mott MacDonald Fulcrum. He says the system’s other big advantage is that it has low running costs, which is important for the museum. ATES systems are common in the


Netherlands, but before it could be used in London the engineers had to ensure ground conditions were suitable. Greenwich is situated in the London basin. Before construction could commence, the engineers had to establish conditions in the chalk aquifer, buried over 60m below the surface beneath layers of gravel, impermeable clay and thin layers of sedimentary deposits. Rainwater water enters the aquifer mainly from the Chiltern Hills to the north and the Downs to the south, before making its way slowly towards the axis of the syncline and eventually to the coast following the hydraulic gradient. The speed at which water moves through the aquifer and the rate at which water


24 CIBSE Journal October 2011


could be extracted were key factors in determining whether the ATES system would be suitable for the new wing. If the ground water moves too fast it will carry away the heating or cooling energy stored underground before it can be utilised. ‘You have to test to confirm the system’s feasibility because of variations in the chalk,’ explains Shennan. A trial borehole was drilled outside of


the wing’s eastern boundary to establish the amount of water that could be extracted from the aquifer. ‘In London’s chalk aquifer, we say that 40 cubic metres per hour is a good flow rate to achieve, per well, if you have a sustainable drawdown associated with that rate,’ says Nicholas Boid, a director of IFTech, the AETS system suppliers. The borehole was tested at flow rates


of up to 50 cu m/hr. At this rate the water level in the borehole dropped from its rest level of 12.5 metres below ground level to 17.5m below ground level, a drawdown of 5m. ‘This was a very positive performance,’ Boid says. According to him, ‘a drawdown


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