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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.
4. Pool of local drillers capable of 800’-1500’
Drilling constitutes greater than 50% of the SCW geothermal project expense. The balance is roughly: 25% in the sleeve and pump string installation, and the remaining 25% in the piping, HVAC, electrical and controls subcontracts. If the drilling expense is out of line, then this can blow the entire project. To ameliorate this situation, the bid specification should not be overly restrictive, but must insist on the total amount of drilled feet required to handle the load. To be successful, bids must be received from as many qualified drillers as practical. If there is little or no competition, the drilling expense can rise significantly (a factor of 2-3X) over expectations, which would kill the project.
5. Planned in-house or contracted services to perform maintenance
Work with the owner or owner’s representative to make them aware that a SCW geothermal system will require some (periodic) maintenance. This includes filtration of well water, monitoring pressure drops, temperatures, and flows in the system, and gathering measures to insure optimum performance of the system. Monitoring the health of the submersible pumps (impedance) will help support a decision for replacement scheduling. Who is going to do this? If the building already has dedicated engineering staff, this activity can be added after appropriate training. Otherwise, the owner will have to contract for these services. The ability to monitor the system remotely will allow a service engineer to be dispatched as required to address any maintenance issues as they arise.
6. Building identified use of bleed water
Once again, the value of sitting down with the owner or owner’s representative early-on to discuss potential use of ground water can offer mutual benefit. Toward the end of the cooling season, for example, when water in the bore has risen 30º-35º from the start of the season, it is advantageous to include a bleed operation to limit the temperature rise. This involves diverting a portion (e.g. 10%) of the water prior to returning to the well field, which reduces the heat flux on the well bore and at the same time causes ambient ground water to make-up the static level balance. The big question here is whether the owner sees a value in the use of that water by using bleed in the building as a grey water source (flushing, etc.), or externally for landscape maintenance. This can be a win-win, which can result in fewer wells (less drilled feet) and lower capital cost.
7. Development of installed cost & benefit
From knowledge gained from all of the previous steps, the designer is now in a position to bring all of this information together to present a proposal to the owner which includes the geothermal system installed cost estimate and the net benefit based on simulation analysis. The owner will then take this study result and consider his/her ROI.
8. Building owner decision
Many projects have gotten to this point and that’s as far as they have gotten. Clearly, the ability to sell the project is key to going forward. Many universities, for example, see their commitment to geothermal as part of their educational mission. As non-profits, they don’t qualify for federal tax credits, or state incentives, but they can justify their investment in the future since their facilities and location are fixed and determined. This is not the case for residential or commercial building owners. At current HVAC expense of $0.76/sf, with a 48% annual savings benefit offering from geothermal, the 12-15 yr payback without incentives cannot be expected to drive a commitment. But, with accelerated depreciation (5 yr property), utility incentives, and reduction in water use, the owner can see a pretty good ROI
9. Detailed well field design (pumps, distribution piping, power wiring, etc.)
Assuming that the owner has committed to the investment in the SCW geothermal system, the next phase is the detailed design of the well field, which includes the placement of wells, design depth, configured as a SCW, connection piping & wiring, burial specification, access ports, and building penetration(s). This is a set of engineering drawings that will either go-out on bid to driller/installers, or developed as part of a design-build project.
10. HVAC design interface: piping manifold, filtration, heat exchanger, flow control & balance, bleed option
But, it doesn’t end with the geothermal field design. Rather, the design extends into the mechanical room, the domain of the HVAC contractor, who may or may not have any experience with geothermal systems. Since the general contractor may not have selected his mechanical contractor based on having any prior knowledge & experience with geothermal, the onus for geo-portion of the HVAC design specification then falls on the geothermal system designer. For this reason, the designer must be capable of generating engineering drawings & specifications of exactly how the exterior geothermal piping will interface with the building extended range heat pump loop. We’re dealing with well water here. The use of plastic pipe for conveyance of well water is recommended as long as it complies with local codes. Elements of the geo-HVAC design include: the geothermal pipe supply and return manifold, which includes isolation valves, etc., duplex strainer or spin-down (centrifugal) filters, well water to HP loop heat exchanger (typically SS, but base this on the results of a water assay), flow control and balance valves, and optional bleed hardware. The HVAC contractor will be the installer, so that the price for this piece of the HVAC installation will come from him.
11. Part load controller and its interface to the building HP loop
The operation of the submersible pumps is addressed by the PLC master controller, which might interface with AquaVAR pump controllers for VFD operation to service part load. In another configuration, a custom controller is used to activate specific rows of wells to meet the part load condition without the need for VFDs. Geothermal operation is triggered by loop temperature set point, e.g. 40ºF (heating season) and 85ºF (cooling season), temperature extremes. Additional control complexity enters when traditional HVAC backup units are employed in a hybrid design to supplement for geothermal at the end of the season. So, wiring and controls are a separate subcontract that includes power wiring out to the field.
12. DAQ & reporting requirements (display option)
Since the geothermal design has already penetrated the building, one should seriously consider the inclusion of monitoring points and a data acquisition system to measure and report performance. This could be integrated into the existing building management system (BMS). Like solar installations, where the amount of energy collected from the sun is displayed for all to see as they enter the building, geothermal systems should include a visual display of energy savings as appropriate feedback to its occupants. A complete system could be installed for under $25K (<1% of a 100 ton geothermal installation).
13. Documentation and training
No one does this, but how short sighted is that! It is likely that a separate engineering services contract will be solicited by the owner to provide the monitoring and maintenance of the geothermal system operation. There is a need for communication here! What was installed; how does it operate; how do you monitor operation; what maintenance is anticipated; where do I go for help? Documentation and training is the appropriate hand-off.
I hope that this information is helpful to those engaged in SCW project development. I entertain any feedback that you are willing to share with our readership.
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