Geological Survey / TRT / Design
Geological surveys and/or TRT's are strongly recommended for larger GSHP installations
A geological survey will give a strong indication of the thermal characteristics at the site where the boreholes are to be drilled. This will enable the MCS look up tables to be used to estimate more accurately the heat recovery per metre of borehole. In the absence of any geological data, a responsible driller will estimate the borehole requirement using conservative values for heat recovery. This can result in 5-10% more borehole metres being drilled than absolutely necessary.
On larger installations, this 5-10% can easily mean a difference of one borehole or more. As each borehole is likely to add £4-5K to the cost of the total installation it soon begins to make sense to invest in a little more research to achieve a more accurate estimation of total borehole requirements.
- As a rule of thumb, installations of up to around 15kW Heat pump capacity can be estimated solely using the MCS tables
- Installations of 15 to 30 kW should be dimensioned with the help of a geological survey
- Installations of above 30 kW definitely require a geological survey and could well benefit from a TRT.
Thermal Response Tests evolved largely in Scandinavia and Germany over the last 15 or 20 years. They measure the thermal characteristics of one or more boreholes at a specific site. Test data is used in a separate planning process to extrapolated and predict the performance of more boreholes, i.e. a borehole array.
For multiple boreholes, it is essential to correctly dimension the borehole number, depth and spacing.
Two major inputs are required.
1) The dimensions and seasonality, of the heating and cooling load.
2) The thermal characteristics of the geology.
The first is the responsibility of the architect and/or heating engineer. Independently of who is responsible for the second, be it the architect, heating designer or heat pump installer; Synergy will recommend a Geological Survey. We will gladly arrange this. This is a desk exercise, utilising geological maps and archived records to provide a description of site geology.
Although this will describe the thermal qualities of the underlying strata, it is an estimate, not based on site measurements.
To provide better guidance for the planning of larger installations, a site specific test is recommended to provide site specific values which can be input to the planning process.
TRT is such a procedure.
A measuring module is used, usually trailer mounted. Few drilling companies have this equipment. More often, specialist companies provide the equipment and personnel for the test, which can take some days.
The cost is not insignificant and can range between £3000 and £6000, depending on company and scope. Therefore, TRT's are usually carried out for larger projects, i.e. arrays of 10 boreholes or more, (Or, GSHP's in excess of 50kW)
A TRT produces data which will determine the optimum borehole array. A 100m to 120m borehole, with installed and tested collector loop, can cost between £3000 to £5000. A TRT resulting in the saving of one or more boreholes can therefore be self financing. Conversely, TRT data can ensure that sufficient boreholes are provided to meet the heating and cooling load.
A TRT gives the end client and heat pump designer/installer, the assurance that correct array dimensions will avoid performance deficiencies in the longer term.
A TRT test is usually made on the first borehole completed, i.e. equipped with loop and grouted. The measurement period is typically 3-5 days including setup. If the test is separate to the main drilling operation, extra costs will arise for additional drilling equipment transport and setup.
Fluid is pumped around the loop. Heat energy is injected into the fluid and ingoing and outgoing temperatures are recorded, as is the flow rate, typically for 50 or 72 hours. It follows that for this time, a circulation pump and heater must run and the equipment be monitored.
A report is issued which will aid the design of the borehole array on the basis of actual heat transfer figures from the site. This is the most accurate method of array design.
Synergy can arrange a TRT. We will not normally recommend a TRT for smaller arrays, i.e. six boreholes or less, (Heat pumps of 30 kW or less). For these, if the client wishes to have a safety margin, it is simpler to specify an additional borehole. There remains a middle band of performance i.e. 30 - 50 kW, where TRTs may be specified at the discretion of the planners
Borehole Array Design
For larger installations it is advisable to have the boreholearray designed by a professional hydro-geologist. Dedicated software will be used to specify the depth, spacing of the boreholes and the orientation of the array in order to maximise the effect of any ground water movement. The designer should be indemnified so that his design quality is backed up by a financial guarantee.