Key Issues for a consultant to Consider when Utilising Ground Source Technology - Page 2
Underfloor heating should be considered if the floor construction features a screed layer since the flow temperature could generally be reduced to around 30-35OC. Where suspended timber floors are present, any underfloor heating pipework, typically installed within the joist void, would need to be embraced by a steel heat transfer plate, which not only increases cost but also slows the build programme. Since the flow temperature into these underfloor circuits would need to be increased to around 45oC (to drive the heat through the overlying chipboard and final floor finish), there is no greater operating efficiency than if the heat pump was serving radiators.
As well as heating, heat pumps can be reverse cycle, i.e. designed to provide cooling or heating. Heat pumps can either be set for cooling or heating. For systems which require heating in some areas and cooling at the same time in another area, multiple units can be configured to switch between heating and cooling as the loads change.
For distributing cooling within a building, the ideal situation is to use a blown air system (such as air handling units) as this will deliver the cooling where it is needed. Blown air systems can also provide heating as well as cooling, however they do generally need higher temperatures to produce heat, which can reduce the efficiency of the heat pump.
Underfloor is not an ideal means of delivering cooling as the absorption rates are low due to the high thermal mass, the cooling is not delivered in the right place and there is a possibility of condensation occurring on the floor.
For cooling systems a buffer vessel is required to avoid the heat pump short cycling.
View our Domestic Schematics or Plantroom Schematics Page to view typical schematics for applications utilising Underfloor Heating.
4. Energy Sources
With any development one of the first decisions is which energy source is the most suitable to provide the low grade energy input for the heat pump. This energy source can be air, water or the ground.
With air systems the installation is easier, however there can be noise issues and air source heat pumps are generally less efficient than ground or water.
Using water source is ideal as water has a high heat transfer rate and installation again is reduced, however suitable water sources are not always adjacent to the development and even if present issues such as extraction rates, boat traffic, etc might need to be taken into account.
The most common source of energy for any development is from the ground. The ground provides a good heat transfer rate and because the temperature is generally constant below one metre efficiency rates are high. The downside is there is additional ground works to complete, however as this is generally done at the early stages of the project, costs and disruption are usually minimal.
If the energy source selected is the ground this now presents a further question; horizontal arrays or vertical boreholes?
Horizontal arrays for small developments are generally the lower cost option, but do require the largest amount of available ground, as the trenches can be up to 50m and need to be spaced 5m apart. A rough rule of thumb is that the amount of ground available needs to be twice the area to be heated.
Boreholes are more expensive to drill for smaller developments due to the need of specialist drillers and equipment; however the area of ground required is less. The amount of energy available from a borehole will depend on the material that the hole is being drilled into and typically you can expect 3 to 5kW from a 70-100m deep borehole. Each borehole would need to be approximately 5 to 6m apart.
For larger developments (typically above 100kW) due to the economies of scale, boreholes might be a lower cost solution then trenching and in developments of this size it is advised that a trial borehole is drilled and a thermal response test carried out. This will provide accurate data as to the amount of energy that can be extracted from a borehole and the results can be used by a borehole consultant to provide an accurate borehole field design.
Kensa heat pumps can be used with any energy source and can provide guidance on the best energy source for the application, including helping with any horizontal ground array design. If vertical boreholes are required, Kensa can recommend a number of drilling contractors who, generally, would also install the pipework and grout.
Kensa can also provide a Thermal Response Test and can recommend a borehole specialist to design the borehole field if required.
Some companies have expressed an initial desire to have all the work handled by a single contractor, primarily because of some ‘nervousness’ linked to the performance of the technology. It should be noted that the groundwork contractor is merely providing a connection to the ‘heat source’ in the same way that the utility companies are responsible for providing connections to the gas, electricity or water grids. In a typical project featuring a gas boiler, the plumber would only be responsible for installing the boiler and would not handle the provision of the gas supply to the property. Kensa is merely suggesting a mirroring of this approach.
Click on the below links to find out more about the different energy sources available to heat pumps:-
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