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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthortony
    • CommentTimeOct 21st 2019
    How larger area should my friend allow for a heat field for a 12kW gshp.

    I said pipes not slinkies all 2m deep and 2m apart
    I'd ask the supplier for a specification relative to required load and soil type.

    some info here https://www.gshp.org.uk/documents/CE82-DomesticGroundSourceHeatPumps.pdf
    found some old spec that suggest 20-30m per 1kW for straight trench depending on gnd type
    just having a look round , this have some detailed design spec. info on sizing relative to ground type
    • CommentAuthortony
    • CommentTimeOct 21st 2019
    So far all I have is 3m apart for the trenches, probably have noisy sandy with hogging stones and clay mixed
    • CommentAuthorSilky
    • CommentTimeOct 22nd 2019 edited
    [edit : I tried to format the values, but the spaces were removed]

    hi Tony, these are the reference values from "Heat Pump Planning Handbook" Jürgen Bonin.

    Depth min-max 1.2 - 2 m

    running 1800 hours/year
    dry sand, gravel 9-12 W/m2
    wet sand, gravel 15-20 W/m2
    dry clay, loam 20-25 W/m2
    wet clay, loam 25-30 W/m2
    water bearing soil 30-40 W/m2

    running 2400 hours/year
    dry sand, gravel 6-10 W/m2
    wet sand, gravel 12-16
    dry clay, loam 15-20
    wet clay, loam 20-24
    water bearing soil 25-32

    I guess these figures are based on Germany where there is more likelihood of a longer, colder winter, so I would treat them as pessimistic for the UK. But he also says that you should tend towards over, rather than under sizing.
    • CommentAuthortony
    • CommentTimeOct 22nd 2019
    Great, thanks
    • CommentAuthortony
    • CommentTimeNov 5th 2019
    No she is talking about boreholes for a 22kW heat pump, how deep should they go?
    make much sense of this lot ? (see above link)

    "4.4.2. Types of vertical GHEs
    There are two basic types of vertical GHEs or borehole heat exchangers (BHE): U-tube and concentric- (coaxial-) tube system configurations (Figure 7). BHEs are widely used when there is a need to install sufficient heat exchanger capacity under a confined surface area, such as when the earth is rocky close to the surface, or where minimum disruption of the landscape is desired. The U-tube vertical GHE may include one, tens, or even hundreds of boreholes, each containing single or double U-tubes through which heat exchange fluid are circulated. Typical U-tubes have a nominal diameter in the range of 20–40 mm and each borehole is normally 20–200 m deep with a diameter ranging from 100 to 200 mm [17]. Concentric pipes, either in a very simple method with two straight pipes of different diameters or in complex configurations, are commonly used in Europe. The borehole annulus is generally backfilled with some special material (grout) that can prevent contamination of ground-water.
    A typical borehole with a single U-tube is illustrated in Figure 8. The required borehole length L can be calculated by steady-state heat transfer equation as follows [13]:

    L=q Rgtg−tf
    where q is the heat transfer rate, in kW; tg is the ground temperature, in K; tf is the heat carrier fluid (antifreeze, refrigerant) temperature, in K; Rg is the effective thermal resistance of ground per unit length, in (mK)/kW.
    The GHE usually are designed for the worst conditions by considering that these needs to handle three consecutive thermal pulses of various magnitude and duration: yearly average ground load qa for 20 years, the highest monthly ground load qm for 1 month, and the peak hourly load qh for 6 h. The required borehole length to exchange heat at these conditions is given by [18]:

    where Rb is the effective borehole thermal resistance; R20a, R1m, R6h are the effective ground thermal resistances for 20 years, 1 month, and 6 h thermal pulses; Δtg is the increase of temperature because of the long-term interference effect between the borehole and the adjacent boreholes. Alternative methods of computing the thermal borehole resistance are presented by Bernier [18] and Hellström [19]."
    Posted By: jamesingrammake much sense of this lot ? (see above link)

    Yes :)

    Basically the more you want to use your GSHP, the deeper the borehole has to be :) There used to be a rule of thumb over here of "one hundred and fifty feet per ton". Google will help you convert feet to metres and tons to kW :) You can do it from 1st principles if you know what the "ton" in ton means :)

    Paul in Montreal
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