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So I need to calculate how big geothermal heat pumps I need for a building. Heat pump is used for heating and to heat used water.

If I know how many kW's are needed for heating the building and the calculated flow rate of used water, what is the simplest and quickest way to calculate how much power is required from the pump(s)?

Let's say AC heating requires 20 kW, underfloor heating requires 5 kW and flow rate of used warm water 0,3 l/s. And I know dT for AC heating is for example 35 and underfloor heating is 10 and used warm water leaves the pump at 58 degrees celsius.

How should I calculate it? And how do I convert this all to annual kWh used?

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  • $\begingroup$ Basic idea is to work out the total heat demand then divide that by the Coefficient of Performance. But the detail is not simple - buildings require more or less heat depending on the season, one resource used is temperature in degree-days to calculate the heat demand necessary. $\endgroup$ – Solar Mike Jan 14 '19 at 17:34
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Usually supply is the limiting factor not the pump. However say you have a pump that has an output of q liters of water per second, with a usable Delta temperature of 10 Celsius degrees.

So this pump can deliver q*10 kcalories energy per second, which is q*11.622 Watt hour or q*41.80 kilojoules.

So you can either calculate how much available geothermal energy your system has by calculating liters per second times dt available. Or you calculate your demand and concert it to flow of the pump.

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In order to calculate geothermal pumps, the best place to start is from ASHRAE Technical Committee 6.8.

Having gone through the ropes myself, and since you already know the heating loads, I'll provide you with what is -in IMHO- a very useful shortcut. (Keep in mind that this should not be a substitute for a proper geothermal study following the guidelines from ASHRAE TC6.8).

The maximum energy you can get by a meter of geothermal tube is about 50$\left[\frac{W}{m}\right]$. This is:

  • (Almost) Independent of soil type,
  • (almost) independent of tube diameters (between 10mm and 50mm just to set some limits)
  • (almost) independent of rate of flow (when flow is below a few liters/second)
  • (almost) independent of deep or shallow geothermal arrangement
  • assuming you use water as a cooling medium in the tube

So, if you need 25[kW] of heating power you'd need about 500[m] of tubing (either a long one or 5x100[m]).

Regarding the temperature differences you are stating, they don't really matter. This is because in any case you'd have a heat pump with a primary (input) circuit connected to the geothermal tubing, and a second (output) circuit with the hot water to the temperature you want. Something like the following:

enter image description here

One tradeoff you need to be aware of is the following:

   The higher the temperature difference on the output circuit the smaller the flow
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