Risk managing open loop geothermal systems

Property developers reviewing ways of meeting the 10% renewables planning criteria (the Merton Rule) frequently consider ground source heat pump (aka geothermal) systems. Benefits include load capacity, long-term cost savings, space savings, noise reduction and planning advantages (eg compared with wind turbines).

If the proposed development has a significant cooling requirement, these systems may be the only viable renewable option. There are risks, however, which require careful management.

Geothermal systems fall broadly into open loop and closed loop categories. Open loop systems abstract groundwater from one or more boreholes, passing it through a heat exchanger before discharging it, generally back to the underlying aquifer. Closed systems use a series of pipe loops in the ground through which a fluid is circulated via a heat exchanger unit. Open loop systems are often used in large commercial developments with load requirements of 250-350kW upwards, because they can be more cost-effective than closed systems on this scale. Open loop is inherently more risky and is therefore the focus of this article.

The key risks are to do with borehole yields, licensing and thermal interference. Open loop systems use groundwater as an energy source or sink, so success depends on obtaining a sufficient flow rate. Borehole yields are difficult to predict, but the likely range can be investigated at feasibility stage by a hydrogeologist. Factors to consider include geology, aquifer properties and topography. Test boreholes can be drilled, but the fluid properties of aquifers are notoriously variable, so it is always possible to be surprised – both pleasantly and unpleasantly. Contractors will not guarantee borehole yields and so the client must bear the risk. By commissioning a detailed hydrogeological feasibility report, the engineer or project manager can communicate the risk to the client and ensure a diligent assessment has been undertaken before expensive drilling operations commence.

The Environment Agency (EA) regulates abstraction and discharge of water from and to aquifers in England and Wales. The key factor it considers is whether the scheme is likely to have an adverse impact on the environment and existing water abstractions. Such impacts include significant changes in water level and temperature. Because this issue could potentially limit the use of the final scheme, it is important to investigate it at an early stage. The feasibility assessment should include evaluation of this risk.

Where the initial review indicates significant potential for impact on existing water users, and where sufficient data are available, groundwater models can be used to investigate the off-site transport of thermal energy and water level changes in the area in more detail. The level of risk associated with licensing the scheme can thereby be understood. If the risk is unacceptable to the client, alternative renewables or building temperature control system options can be considered before significant investment in geothermal has been made. (Closed loop systems do not require abstraction licences and discharge consents.)

Thermal interference

Thermal interference occurs where the temperature of water drawn from the boreholes changes as a result of short-circuiting of warmed or cooled water discharged via the injection boreholes. This can reduce the efficiency of the system and even make it inoperable until groundwater temperatures return to normal.

Numerical modelling studies by Zenith International have indicated such temperature recovery could take several decades. Thermal interference is most likely to occur when the separation distance between abstraction and injection boreholes is limited and when there is a significant imbalance in the thermal energy exchange with the groundwater system over an annual cycle.

Preferential flow paths in the aquifer (fractures and fissures) increase the risk of thermal interference. Tools for assessing the risk range from professional judgement to numerical models. The best approach depends on the size of the scheme, the extent of the load imbalance, the separation distance between the boreholes and the data available. This judgement should be made in consultation with a hydrogeologist.

If there is a significant risk and data are available, a numerical model of the groundwater system and proposed geothermal scheme should be developed at feasibility stage. The model will evaluate the potential thermal interference effects so any design alterations can be made early on.

The risk of thermal interference due to unexpected direct fracture connections should be borne by the client. However, the contractor/consultant must consider the critical parameters at design stage, because thermal interference effects can occur without significant direct fracture connections (eg where large, unbalanced loads are applied to boreholes with limited separations). These factors can be evaluated at design stage, so a blanket exclusion of responsibility for thermal interference by the contractor/consultant should not be accepted.

Where the risk of direct fracture connections is high (eg in fractured aquifers and where borehole separations are limited), the geothermal contractor/consultant could reasonably exclude this risk from its warranty, but should make recommendations for quantification of the risk. Once the system is operational, it is essential to monitor the energy exchange with the ground in order to verify that the operational building loads are similar to the design loads.

Fundamentally, the client or consulting engineer must ensure that sufficient data are supplied by the contractor/consultant during the design and development process to allow apportionment of responsibility in the event of a failing geothermal system.

Two main procurement options are available for geothermal systems: design and build contracts, and consultancy/project management packages. There are pros and cons with both options (Table 1).

Case study

Park House occupies an entire city block of 0.42ha on London’s Oxford Street. The scheme provides 100,000ft2 of retail and 165,000ft2 of office space as well as 39 apartments. The heating and cooling will incorporate an open loop ground source heat pump system predominantly for cooling the office and retail units and for heating and cooling the flats.

Developer Land Securities chose a consultancy and project management package, using geothermal consulting team Zenith International and Loopmaster Europe. The team set out the risks of the proposed system; how these could be managed; and the steps to be taken to ensure the system will deliver the maximum possible energy savings (and thus CO2 emissions) in the long term.

Recommendations (which would also have been applicable for a D&B contract) included a hydrogeological feasibility study, drilling of two test boreholes to prove yields, running a tracer test between the boreholes to evaluate the magnitude of direct fracture connections between the boreholes and quantification of the risk of thermal interference through a numerical modelling study.

Feasibility assessment

Development of the geothermal system began with a feasibility assessment. Two boreholes were then drilled (with a horizontal separation of 50m) to determine the capacity likely to be achieved from the completed system, which will comprise six boreholes.

Drilling works were closely managed to ensure the boreholes were drilled to specification, to minimise the risk of delays

and maximise the chances of achieving sufficient flow rates. The boreholes were constructed to a high standard, and substantial yields (more than 30 litres per second) achieved.

The tracer test comprised injection of a fluorescing dye into one borehole as part of a recirculation pumping test and monitoring the arrival time and concentration at the second borehole. The results indicated that 30% of the water injected through the recharge borehole was circulating directly back to the abstraction borehole.

This high level of direct recirculation was caused by extensive fracturing in the upmost section of the saturated aquifer. A numerical model was developed of the groundwater system beneath the site and the proposed geothermal scheme to evaluate the magnitude of thermal interference.

The model incorporates three-dimensional fracture flow equations, which allowed the local fracture system to be represented within a broader equivalent homogeneous mass model. It was therefore possible to calibrate the model against both the pumping test and the tracer test data.

On completion of the calibration phase, the model was used to simulate long-term operation of the geothermal system. Two building load scenarios were run to accommodate uncertainty in the long-term heating and cooling requirements of the development: a best estimate (based on TAS modelling) and a worst-case scenario (best estimate loads + 20%). The simulations indicated that significant thermal interference effects were likely to occur, with abstraction water temperatures reaching 18°C within the 25-year simulation. The project team concluded that this represented an unacceptable risk.

Numerical modelling

A contingency plan, which involved increasing the separation between injection and abstraction boreholes, was made in the planning stages. This was not the first choice because of programme and cost issues, but it was concluded that the level of thermal interference risk outweighed these.

The decision was made to drill the abstraction boreholes for the ground source system servicing the retail units at the increased separation. The numerical model was used to determine whether the original boreholes could service the residential loads alone without thermal interference.

Results indicated the risk of interference associated with these lower, more balanced loads was small. The original boreholes will therefore be used in this way. Model results also indicated the scheme was unlikely to have a significant impact on existing groundwater abstractions and hence the licensing risk was low. The remaining four boreholes will be drilled this year and the numerical model updated to confirm that the increased separation has reduced the risk of thermal interference to a satisfactory level.

In summary, geothermal systems are an effective option for achieving the renewables planning rule and can deliver long-term energy and cost savings for heating and cooling, but careful risk management is required. Risk registers can be used to ensure all risks have been defined and allocated.

The client or project manager must ensure sufficient data are gathered to allow apportionment of responsibility if necessary. Careful prequalification of geothermal contractors and consultants is important because a fundamental design flaw may not be exposed until the system has been operational for several years.