Go to Forum Home Building Services Open Loop or Closed Loop GSHP?

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    • #31561
      Anonymous

        We're nearing the start for our barn conversion which will be well insulated and as airtight as possible.

        Given the location and surroundings, it has always been our intention to install a GSHP for the hot water and underfloor heating as I want to avoid oil or gas.

        Originally I intended to install loops in the surrounded land but when it turned out that we need a borehole for the water supply, an open loop system was suggested.

        The COP is encouraging but I'm concerned about the potential amount of waste water.

        Any comments and suggestions would be great as we want to use the most efficient system we can.

        Thanks

        Matthew

      • #38109
        Greengauge
        Participant

          An open loop system will normally have two boreholes, one extract and one injection. The HP pumps ground water out from one borehole, extracts some heat from it, then squirts it back into the second borehole – there is no waste as such, you aren't supplying the open system with water from your taps!

          Closed loop systems are much more common, I believe you need quite specific ground conditions for an open loop system. We have recently put in a surface water heat pump and the supply/install co didn't even consider open-loop, so it's basically a GSHP slinky pinned to the bottom of a marina.

          I have no evidence but suspect open loop systems are no less efficient than closed. The cruical parameter is the temperature of your source, and if you have a high enough water table with enough sub-terrainian flow to justify open loop, a borehole closed loop will also recieve these benefits. Technically, I'd have reservations about sucking up god knows what, which 99.9% of the time will be dealt with by filters but… Closed loop avoids this issue.

        • #38110
          Nick Grant
          Participant

            John Cantor is your man, do buy his book!

          • #38111
            Anonymous

              Thank you for your replies, I've only just noticed them today.

              I've recently purchased John Cantor's book thanks! Having had a quick skim through, there's not a great deal about open loop.

              Depending upon who I have spoken to, I receive varying feedback:-

              Open Loop

              Pros

              – Borehole for the water supply is already in, so installation is easy and costs minimised.
              – The temperature is higher and more constant than the 'closed loop slinky' system
              – COE of 5/6 has been suggested

              Cons
              – Filtration of water
              – Electricity required to get the water supply to the open loop heat pump could be excessive

              Closed Loop

              Pros

              – Closed system means no requirement for regular replacement of filters etc
              – Lower electricity usage compared to open loop?

              Cons
              – Lower COE (3/4?)
              – Install cost (although it will just be the excavation for the 'slinky' system
              – Scare stories about permafrost (although with plenty of depth and space between the loops I can't see this being a problem)

              Any further advice would be greatly appreciated as we'll have to make a decision soon in order to benefit from the RHI

            • #38112
              Nick Grant
              Participant

                Been too busy to keep up with Forum but have emailed John.

              • #38113

                John Logan sent me this general info. It might be of interest

                OPEN LOOP GROUND COUPLING – Standing Column Well:
                In situations where the geology is correct, a single drilled well can provide geothermal ground coupling which is 1/2 to 1/3 the cost of equivalent closed loop wells and about 20% more efficient. The two requirements from the geology are (1) a high water table and (2) bedrock which is neither “weathered” nor unstable. One requirement for the water quality is that it is not excessively corrosive due to elevated levels of hydrogen sulphide, sodium, or chlorine.

                A standing column well is constructed similar to a standard water supply well. Water is pumped from the well, through the heat pump, and is returned back into the opposite end of the well. When operating in the heating mode, the heat pump extracts heat energy from the water, reducing its temperature, before returning it to the well. Solar stored heat energy in the ground naturally flows from the bedrock into the water to warm it back up, before it recycles back through the heat pump. Typically the water supplied to the heat pump will lie in the range of 7-10C, which is the reason for the 20% higher efficiency compared to a closed loop well with plastic pipe (an insulator) through which the the heat has to pass into the water/antifreeze mixture.

                Standing column wells for residential homes frequently are used to supply both the domestic water supply and the geothermal water, greatly reducing the cost to install a geothermal system.

                Another difference from a closed loop system is that the standing column well requires a well pump rather than a circulator. However, because of the siphoning effect of the return pipe to the single well (as opposed to an open loop two-well system), the well pump has only to lift the water from the static level of the water in the well to the mechanical room in which the heat pump is located. The pumping power for a correctly designed standing column well system will typically consume less than 5% of the heat energy provided to the building. Taken together with the 20% higher heat pump efficiency of a standing column well, compared to closed loop, there will be significant operating cost savings from the ~15% higher efficiency for the life of the geothermal system.

                A standing column well does not require a high water yield to to work correctly, since the bulk of the circulating water is returned to the well. However, another feature of the standing column well is the possibility to “bleed” off some small percentage of the circulating water through the system, rather than returning 100% to the well. To make up for the bleed, the well will inject warm water from the surrounding bedrock to directly heat up the well water. Typically as little as 10% bleed will allow significantly shorter wells to be drilled and will provide warmer water to the heat pumps, again giving higher heat pump efficiency.

                In the USA, the standing column well is the dominant type of geothermal ground coupling used in all of New England and New York City, where the geology is ideal for this approach.

              • #38114
                Anonymous

                  Thank you Nick for getting John involved and John for asking John Logan's opinion! I'd previously Google'd standing columns and digested as much as I could but it's better to get information from people with more experience.

                  Having read your book I was even contemplating using land drains for the surface water positioned above the closed loops in order to keep the ground wet!

                  The borehole is artesian, so pumping is not excessive and as the water will be filtered to supply the property, my thoughts are to have a branch from the filtered water supply to the heat pump to protect the heat pump as best as possible.

                  As the RHPP deadline is fast approaching, we'll be making a decision soon and I'm leaning (quite heavily!) towards open loop.

                  Next question will be how best to monitor its performance!

                • #38115
                  Nick Grant
                  Participant

                    As you probably realise, John has some answers there. Check out openenergy monitor. Looks interesting and John is trying their system as I hope to but not for heat pumps.

                  • #38116
                    Anonymous

                      Thanks again for your advice John.

                      I'm happy to take any advice on monitoring and logging!

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