Ground Source Heating and Cooling
(Contributed by Ric Horobin, Zenith International)
Sources of renewable energy are increasingly required at a range of scales as we aim to reduce man-made greenhouse gas emissions and thus the potential effect on the world’s climate. The expressions of this need are diverse and include: national government and EU targets on the use of renewables; local planning criteria that mean that developers must consider incorporating onsite renewable energy sources into new developments; and pressure from the public and non-governmental organisations to abandon our current reliance on fossil fuels. There are also potential energy security, cost and price stability benefits that will accrue from using renewable technologies and it may be that these become the main drivers in the future.
Open Loop or Closed Loop?
Ground source heat pumps (GSHP’s) are amongst an array of technologies that can greatly reduce our reliance on fossil fuels. A GSHP taps the heat energy contained within the ground at shallow depths so providing heating and cooling for buildings in a renewable way. GSHP’s can be divided into two types, closed loop and open loop. At the heart of both types is a heat pump (identical in operation to a refrigerator) that is designed to remove or supply heat as required via a fluid which flows around the system. A closed loop system comprises a sealed pipe network buried within the ground through which the fluid is continuously circulated. Closed loop systems may use either shallow (less than 2m) horizontal pipes or deep vertical pipes depending on the ground conditions. In contrast an open loop system uses an abstraction borehole to supply groundwater as the fluid for the heat pump and then re-injects the water, often back to the aquifer using a re-injection borehole but sometimes to a near by watercourse. Both systems make use of the fact that the temperature of the ground (below a depth of around 10 – 20 m) is relatively stable over the year and has a strong correlation with the local annual average air temperature. A GSHP scheme uses the ground as a source or sink for heat energy (the heat pump is simply a way of moving the heat from one place to another) and such schemes can directly substitute for carbon fuel-based heating and cooling plant.
A closed loop ground source system does not impact the groundwater resource itself, as it does not change natural sub-surface water flow and as such is not directly regulated by the environment agencies. However pollution caused directly or indirectly by a closed loop scheme would still come within the remit of the environment regulators e.g pollution caused by a leaking system or the cross-connection of two aquifer units. An open loop ground source system uses groundwater and can impact on the groundwater resource and groundwater temperature and has the potential to impact surface water where there is connection with groundwater. Open loop schemes are regulated, as direct abstraction of groundwater and discharge of wastewater (either hotter or colder) is involved; both of these activities are governed by national legislation.
Successful System Design
The design and implementation of a thermally sustainable (and therefore successful) open loop GSHP system is largely a hydrogeological problem, as it involves: the assessment of available groundwater resources; the potential impact on other water abstractions; the potential impact on the environment; and the regulatory aspects of water abstraction.
The added dimension in assessing the hydrogeology is the impact of short-term and long-term groundwater temperature changes on the successful operation of a given GSHP systems and potentially the operation of adjacent systems and the wider environment. Unwanted changes to the abstraction water temperature, caused by flow of higher or lower temperature injected water towards the abstraction borehole, have the potential to render a system inoperable in a short period of time. This effect is termed thermal interference. If present, not only is thermal interference likely to result in failed GSHP’s, but there is also a greater risk to the wider environment and could indirectly cause thermal/heat pollution. A GSHP system that has a balanced annual thermal load, i.e. it is used for heating in the winter and cooling in the summer, is less likely to effect groundwater temperatures in the long-term.
Assessing the risks at the design stage requires a good knowledge of the hydrogeology, the thermal properties of the aquifer and adjacent rocks and how thermal energy is transported by water flow and heat conduction through rocks. This is often done by the use of numerical groundwater models that incorporate heat energy abstracted or re-injected to the ground by the building. Such modelling requires an in depth understanding of planned building heating and cooling loads and how they impact water re-injection temperatures. The models can be also used to validate the thermal sustainability of proposed alterations to the building design and how this will impact the available groundwater resource and are thus an invaluable tool for hydrogeologists and building service engineers alike.
Challenges Lie Ahead
There is growing use of open loop GSHP’s that use multiple pairs of abstraction and injection boreholes and involve significant groundwater abstraction and re-injection rates. However, as these schemes become more common and potentially closer together, there are both technical and regulatory challenges that must be met if the successful and sustainable use of this technology is to both continue and increase, and thus make a real contribution to reducing greenhouse gas emissions.
Further Information about GSHPs
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