A ground source energy (GSE) system can provide a renewable energy solution to a building’s heating and cooling needs if the ground loop is efficiently specified and designed. It is important that the design of the ground loop, and its size, match the profile of the building’s energy use for the system to be successful. ESI can provide the essential skills and knowledge to make sure that the proposed ground loop size meets these needs, taking into account the balance between winter heating and summer cooling.
An initial desk based Site Check can identify typical values of thermo-geological parameters for the site location based on a postcode and site plan. This will give an indication of the closed loop or open loop options available for the site and an indication of the size of ground loop required. For smallest systems this might provide adequate information to feed into simple design tools and enable a suitable conservative design to be chosen.
On most schemes, ESI bases the design on site specific numerical modelling to confirm the long term performance of the design. The layout and depths of the borehole heat exchanges are accounted for explicity and long term ground temperatures are predicted. For larger schemes, FEFLOW modelling allows more detailed and accurate modelling to be undertaken accounting for complex layered geology and the role of groundwater flow. To build confidence in model predictions, it is recommended that site specific thermal properties are obtained. Whichever modelling approach is taken, the circulation pump requirements are assessed using ESI’s custom head loss calculation tools, designed specifically for ground source energy applications. This allows the overall system performance factors to be evaluated as a key part of an integrated system design.
Thermal Response Testing
A Thermal Response Test (TRT) is a standard measurement of the ground thermal conductivity and the borehole resistivity, and will confirm the local geological properties enabling the site specific geological setting to be taken into account.A Thermal Response Test would also quantify the consequences of a unsaturated zone and might also help identify potential benefits from groundwater flow past the array. Tests can take place during drilling of the array and the results delivered in time for a judgement on whether an additional borehole is required when less conservative designs are initially being proposed.
It is highly recommended that ESI work with Installers to design the ground loop to match the requirements of heat pump and the energy demand of the building. ESI’s design services support installations to take best advantage of the available ground area and geological setting.
What is a Thermal Response Test?
A thermal response test involves injecting a known quantity of heat into the ground by circulating a fluid within the ground loop of an installed borehole heat exchanger over a known period of time. Temperature changes in the fluid flowing within the loop are measured over time and the thermal properties of the ground can then be calculated from this data. During the test, it is important that fluid flow within the pipe and energy input (normally electrical energy) to the thermal response test rig are maintained constant. The length of a TRT can vary from 50 to 100 or more hours.
Why Perform a Thermal Response Test?
Ground thermal properties are notoriously difficult to estimate, all too often, rule of thumb approaches or literature values are used to estimate thermal properties. Whilst for very small schemes, it may be appropriate to minimize the overall cost and install a conservative design, on most schemes of 40kW or more, a thermal response test (TRT) should be seen as an essential task that provides site specific data on how the scheme will perform.
Literature values (for example the German VDI standards) tend to give a broad range of thermal properties making it difficult to constrain the design suffciently, particularly on larger sites. Utilising thermal properties gained from literature values may be sufficient for small domestic schemes but are very likely to lead to poor design for larger more complex schemes which will result in increased installation and running costs. It may even risk failure of the system.
Problems arising from schemes where thermal response tests have not been commissioned include:
- Higher initial drilling costs
- Failure of the system to meet the design requirements
- Decreased energy efficiency
- Relying on a top-up system that was not initially costed or included in the design
- Strain on the mechanical system, shorter system life (heat pumps are required to work longer and harder)
- Higher CO2 production due to higher energy consumption
- Non-recovery of ground temperatures (the ground becomes stressed and is not able to replenish the energy it had originally in storage)
- ESI deliver Specialised Thermal Response Testing
ESI offers an adaptable TRT service for geothermal energy installations. The full test can be managed by ESI, arranging the drilling of test bore holes, conducting the tests on site and then interpreting the data. Alternatively the consultants at ESI are able to work purely on the data supplied to them. The whole process is managed and delivered through industry best practice, tests are fully compliant with ASHRAE (2002) and International Ground Source Heat Pump Association (IGSHPA 2009) standards.
ESI’s detailed interpretation of the thermal properties of the ground in relation to the geology and hydrogeology is presented in a concise factual report, which can be delivered within 48 hours of the completion of the test so as to inform the final decision on the drilling requirements on site.
When does a Thermal Response Test become economically viable?
A thermal response test is recommended if the cost of the TRT is less than the amount likely to be saved on the drilling costs. As a guide an effective TRT costs the equivalent of approximately 100 m of closed loop drilling. The larger the scheme the greater the potential saving against drilling costs.
In addition, the system designer and installer has the extra benefit of greater confidence in the appropriate sizing of the scheme and hence the scheme’s lifespan and its long term sustainability. On larger schemes, one or more TRTs should be considered. The cost of undertaking tests has declined with more developers incorporate them as an integral part of the design process.
ESI is a member of the Ground Source Heat Pump Association and follows the Vertical Borehole Standard Guidelines.