LOW CARBON CASE STUDY THERMAL ENERGY
Heating mode, with heat pump
pump to supply ground water to a heat pump situated in the basement of the new wing. In summer, when cooling is needed,
ground water is pumped from the cool borehole at a rate of up to 46 cu m/hr. Depending on the cooling demand, the ground water is either used to cool the building directly or it is passed through the heat pump, to lower its temperature further, before being used to cool the building. The system will operate in direct mode at
the beginning of the cooling season, when the cold well temperature is at its lowest (having been charged over winter). In this mode, system efficiencies are very high because there is no electrical demand from the heat pump’s compressor. However, if the cooling demand is too high the heat pump will kick in, to bring the chilled water temperature down to the required level (see diagrams). Once it has left the heat pump, the warmed water is returned to the ground at 20C through the warm borehole, charging the aquifer with heat ready for winter. In winter the ground water flow is
of 10m to 15m is probably the limit but this depends greatly on the depth and construction of the borehole and also the rest water level’. To provide heating and cooling to
the museum’s new wing, two 350mm diameter, 80m deep boreholes are needed. After the scheme’s feasibility had been established, a second borehole was drilled 70m away from the first, just outside at the western boundary of the new wing. A smaller-diameter borehole, not part of the ATES system, was also drilled into the aquifer to serve the wing with water for flushing toilets. From readings and measurements taken from all three boreholes, the groundwater flow direction and velocity were established and fed into the computational thermal model of the system. The aquifer is at a standing temperature of around 12C. The aquifer surrounding the main
eastern borehole is used to store heat energy, while the aquifer surrounding the western borehole stores cooling energy. Each borehole contains a submersible
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reversed so that warmed ground water passes through the heat pump. Once its heat has been extracted, the cooled ground water is returned to the aquifer at approximately 8C, via the cold borehole, where it is used to recharge the ground ready for summer. The control logic is very simple. The
system includes a 350kW output heat pump, twin heat exchangers and two 1,000-litre buffer tanks, one for the heating circuit and one for cooling. ‘The demand is led by the two buffer vessels,’ explains Boid. If the building is asking for heat, it will get it from the warm vessel at 45 C, while cooling will be from the cold vessel at 6C.
Factfile Principles of aquifer thermal energy storage (ATES) While the groundwater
During the warm season, water from the cold store at around 7C to10C is passed through a heat exchanger, providing direct cooling water to the building. The heat pump is available automatically as support in periods of peak demand. The store circuit water
will pick up energy from the building and thus be raised in temperature to around 18C to 20C (or higher for fresh air load). This water, the temperature
of which is higher than the natural groundwater temperature, will be run to an underground ‘warm energy’ store. The heat stored in the
warm energy store is used for heating during the winter. Water from the store at around 20C is passed through a heat exchanger and connected into a heat pump, which in turn provides water at around 40C to 50C for use in building heating.
passes through the heat pump it cools to around 7C. The cooled water is run to the underground ‘cold energy’ store. The cold stored in the
‘cold energy’ store is used for cooling, completing the annual cycle. Any excess heat or cold
in the system over a year is balanced using an external heat exchanger. Source: Iftech (
www.iftech.co.uk)
October 2011 CIBSE Journal 25
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