Unlike closed loop geothermal installations, open loop systems, in particular, standing column well (SCW), require more diligence from the designer than just the well field.  In the former case, the HDPE supply and return pipes are handed through the foundation wall to the HVAC contractor to connect to the extended range geothermal heat pump or heat pump loop circulator.  In the case of SCW, however, one needs to consider the operation of the submersible pumps and additional hardware and controls inside the mechanical room to deal with well water, heat exchange and system control optimization under building part load.

Stepping back, the SCW designer needs to do some investigative research into the expected hydrogeology of the site identified for drilling.  Regions of known or suspected karst formation or limestone caverns, H-C rich areas, and water chemistries with high Fe-Mn and TDS, should be avoided.  But what about other drilling challenges, such as dealing with large blown yields and/or unconsolidated depths…how do these considerations affect the practicality of SCW geothermal installations?

All of these design considerations are delineated here in a step by step procedural set of best practices to improve the chances for a successful SCW installation. Here are my thirteen steps to success:

1. Site hydro-geological research

There are a number of resources available to support an investigation of the hydrogeology of a given site and an assessment of the suitability for SCW deployment.  For well field designers, a starting point is to check with local (water well) drillers to see what they know or, at least, suspect about the site.  Note that drillers will likely only have knowledge about the first 300’ of drilling. The next stop is your local USGS office www.USGS.gov for maps and pertinent geological information.  In PA, the Dept of Conservation & Natural Resources (DCNR) offers access to site records through its Webdriller website.  Most States offer similar access to drilling information through DNR.  Finally, a national undertaking to provide state by state information for geothermal assessment is underway, which can be accessed from www.stategeothermaldata.org

2. Thermal testing

If your project is greater than 50 tons, it is cost effective to initiate a thermal test of a representative well.  This implies that a level of commitment has already been afforded by the owner to proceed with drilling a SCW test well.  Remember that the test well will either become part of the overall geothermal well field, or it stands as a useable (potable) water well, should the findings not support further investment in the SCW design.

The importance of the thermal measurement cannot be overemphasized.  This measurement, which includes thermal conductivity, thermal diffusivity and relaxation characteristic of the well) is critical to the proper design basis for the geothermal well field.  For fortuitous situations in which there is significant ground water flow, the designer can take advantage of the higher effective conductivity and substantially reduce the amount of drilled feet to meet a given load, thus reducing the capital expense of the project and increasing the ROI.

3. Preliminary design (# and placement of wells)

Armed with the thermal measurements from the test well, the next step is a preliminary design of the well field.  Here the physical constraints of the site limit the number and placement of wells required to serve the load.  The design process includes modeling & simulation to translate the thermal test results into a workable field design that establishes the number and placement of wells.  Note that the spacing requirement between SCW at 40’-50’ is a factor of two greater than closed loop geometries, but each SCW handles typically 6X the load of a closed loop well.

It is at this point that SCW shows an advantage.  The ability to place fewer wells in closer proximity around the perimeter of the building, rather than having to reserve a dedicated field (usually more remote) for closed loop arrays, now is recognized as a benefit.  Remember though, that one needs to leave room for any future maintenance of the well, such as replacement of the submersible pump.

At this stage, a preliminary site plan can be developed in anticipation for bidding out the drilling and installation of the SCW pump strings.

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