Industrial and commercial cooling Boreholes
Digging deep for a solution
A supermarket has adopted an unusual process for providing energy for instore cooling. Kevin Stickney of Greenfield Energy explains how the technology works
T
he Sainsbury’s store in Crayford, Kent, became the supermarket group’s largest after the outlet underwent a major refurbishment programme last year that included a range of measures to
increase energy efficiency and cut carbon. Following the extension work that was done on the store, it more than doubled in size to 100,000 sq ft. As part of its remit to grow its business but not its carbon footprint, Sainsbury’s decided to adopt borehole technology at the enlarged site, which has been fully operational since September. The ‘geo exchange’ system uses a series of closed
loop boreholes drilled to depths of around 200 metres to access the site’s natural geothermal potential. The boreholes provide cooling for the stores CO2 refrigeration systems – which are also the largest in the Sainsbury’s estate – then capture and store this otherwise waste heat for space heating and domestic hot water. This is believed to be the first time that a closed loop borehole system has been integrated with a carbon dioxide refrigeration system. A control system ensures efficient heat rejection
(and savings) for the energy-intensive refrigeration processes that run the chillers and freezers. This ‘rejected’ thermal energy is carried by a vegetable- based glycol and water fluid in the pipework manifold to the closed loop boreholes to transfer this heat to the underground rock strata, which are used as a massive natural thermal storage buffer. The thermal energy is then later released to provide heating and hot water – via high efficiency heat pumps – throughout the building. The thermal management system comes as
a packaged element designed to provide a fully integrated system throughout its design, installation, commissioning and operation. The retail area ‘cooling’ process involves no separate comfort cooling or air conditioning. The frozen-food, chilled-food and
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fresh-produce cabinets represent a 24/7 load on the refrigeration packs. Cooling to the store is a ‘by- product’ of these refrigeration systems. The process provides a stable and energy-efficient heat rejection ‘sink’ (condensing medium) for the CO2-based refrigeration packs, instead of a traditional air-based gas cooler, which would work to reject this energy into the air. Liquid returned from the boreholes is sent firstly to the CO2 refrigeration packs, where it provides the necessary cooling (condensing the refrigerant from a gas to a liquid). This additional energy is added to the borehole liquid – which generally adds up to 10K to its temperature. This pre-heated fluid then travels to the heat pump circuit to supply an enhanced energy source for the heat pump evaporators. This is ‘recycled’ energy that would otherwise be rejected to atmosphere. Once the heat pumps have extracted the energy they
require – which varies generally from 10% to 100% of the waste heat from refrigeration throughout the day – the residual energy is sent to the boreholes. This could mean energy extraction from the boreholes if the heating load is greater than the refrigeration load, or energy rejection to the boreholes if the heating load is lower. If the energy to and from these circuits is ‘balanced’, the fluid can be re-circulated at the surface, bypassing the boreholes: in other words, the system has the ability to recycle thermal energy at the surface before using the borehole array (where appropriate). The specialist design and install team’s predictions –
Circulation pumps manage the thermal energy from refrigeration packs
> February 2011 CIBSE Journal 47
Greenfield Energy
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