Ireland is well suited to the deployment of ground source heat pumps (GSHP). These are systems which use the natural warmth of the ground to provide space heating and hot water to homes. Despite this, installations of ground source heat pumps in Ireland have declined in the last 5 years, with the increased market share of air source heat pumps that offer lower installation costs and competitive overall system efficiencies in Irish conditions.

However, GSHP still offers a more competitive energy source, and is independent of climatic conditions. The main obstacle for GSHP is the large capital cost associated with the drilling of many tens of meters of borehole and to increase the overall system efficiently.

The Solar Assisted Ground Source (SAGS) project will combine innovation in the development of new ground source collectors currently not available in Ireland by developing a hybrid ground source and solar thermal system for the Irish market. This innovative project has been funded by the Sustainable Energy Authority of Ireland’s (SEAI) Research and Development Programme

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This project has 3 main goals

1) Carry out field trials of new borehole heat collectors, specifically the GeoKoax technology, recently developed in Germany. It is hoped that this GeoKoax technology will be at least 20% more efficient than existing standard single or double U pipe collectors.

2) Combine a solar thermal collector to the ground side of the heat pump (both the standard U pipe and new GeoKoax) , and use this to raise the ground temperature. It will then be investigated what effect of this on overall system efficiency. It is hoped that this hybrid system will decrease operational costs by 20% through a reduction of heat pump energy usage or through potential shortened borehole depths.

3) Install this new SAGS hybrid ground –solar system in a residential new build and gather performance data as well as baseline data on ground temperatures for one year.

 

Theory

This project seeks to test several different types of borehole heat collectors. These are usually polyethylene pipes, installed in a u loop in a drilled borehole into the bedrock. Fluid flows through this loop, collects the heat of the bedrock and returns it to the heat pump where it is upgraded and used to warm the building. Normally these are configured as single or double U . However a new entrant to the market is the GeoKoax® coaxial heat exchanger, with a pipe in pipe configuration. This will be an innovation in the Irish market.

GeoKoax
The GeoKoax® heat exchanger technology provide passive turbulence to a geothermal collector increasing the efficiency of pumping and operation. It uses 6.5 times as much carrier fluid volume as conventional probe systems allowing for shorter collector installations.

jab09_51Coaxial heat Collector (top) and standard U loop type heat collector (bottom)

The core of the of GeoKoax heat exchanger technology is passive turbulence. The continuous change from laminar flows to turbulent flows increases the heat transfer coefficient by 20% in the case of a 3m probe and by up to 100% in the case of a 10m probe

The geometric concept of the GeoKoax system means that the contact surface area for geothermal heat is twice as large as that for conventional geothermal probes. As result of the homogenous, smooth coating, the space between the probe and the soil is even. This ensures a homogeneous filling of the annular space without any air pockets.

The concentric orientation of the GeoKoax coaxial tube ensures that the space between the coaxial probe and the soil is even, thus leading to significantly lower borehole resistance. This makes for much better absorption of energy, which is relevant for annual performance figures.

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Solar Thermal
Solar thermal panels on rooftops are common site in Ireland today and they prove effective at heating domestic hot water supplies. It is common in Europe to use both a GSHP and a solar thermal panel in a hybrid setup to provide domestic hot water needs. The use  of  these  hybrid  systems  to  improve  the  efficiency  and  long  term  operation  of  ground  source  collectors  has  not  been  trialled  in  Ireland.    The  main  objective  of  this  is  to  reduce  the  effects  of  temperature  decrease  over  a  25  year  operational  profile  reached  mostly  during  the  heating  season  when  GSHPs  are  typically  at  maximum  operational  load.  Several attempts have been documented over the years with limited success.  The  use  of  parallel,  solar  assisted  GSHP  systems  has  the  potential  to  increase  the  overall  efficiency of  the  system  by  up  to  30%  (Seasonal Performance Factor expected  to  be  above  5)

As a heat pump extracts heat from the ground, the ground will be cooled. If the installation  is poorly sized this cooling effect can be extreme and cause freeze damage.  The addition of a solar panel in a hybrid system will aid in maintaining   relatively  stable  ground  temperatures  over  the  long  term  of  the  heat  pump.