Ground temperatures

rankoromane · Posts: 66

I have looked through the DB help and Eplus documentation on ground heat transfer and am still not sure how to specify temperatures.

To have a constant temp throughout the year for 0.5m ground depth seems unrealistic though I guess this comes from the idea that the ground temps should be 2 deg less than the average monthly indoor temp. This would imply there was always a net heat transfer to the ground. For a conditioned building in a hot climate there would surely be a net heat transfer to the building from the ground - certainly in summer.

Also the energy plus weather file has a set of ground temperatures and the location tab also has a set. Which ones are used?

We assume that ground heat transfer is a significant effect on buildings in this part of the world and therefore we are concerned to get this right.

Andy Tindale · Posts: 2432 Location: Stroud

Yes, I agree that it is probably unrealistic to assume constant ground temperatures at 0.5m depth, but this data is used in the absence of any other reliable data.

The ground temperature data used in the simulations is that defined at the site level on the Location tab. The ground temperature data in the epw weather files represents temperatures for an undisturbed site without any buildings and is highly unrealistic for sites which have buildings, especially large ones.

We plan to improve the ground modelling capability of DesignBuilder in future versions, possibly using some weighted average of the undisturbed ground temperatures and assumed building temperatures - 2degC, the weighting depending on the floor area of the building. If anyone knows of any algorithms which work in this way I would be very glad to know of them.

In the meantime it is up to you to define the boundary conditions for your model including ground temperatures.

Regards,

Andy

rankoromane · Posts: 66

I am now using the Eplus ground modeler and what I find is that you would be much better using the average monthly outdoor temp as your starting point than the current default of 16 degrees throughout the year. This would of course be at the underside of slab level now and not at 0.5 m depth. The other problem I have come across though is that for unconditioned buildings the temperature under the slab can exceed the limit you have set of 25 degrees for ground temperatures. Would it be possible to raise this limit?

rankoromane · Posts: 66

I also got the following response from the energy plus newsgroup regarding ground temperatures which suggests that the 2degC assumption should be reviewed.

Andy Tindale · Posts: 2432 Location: Stroud

Steve Potter · Posts: 572 Location: newcastle

Ground temperature calculation module
Would anyone out there be interested in researching and developing a 'module' for calculating appropriate ground temperatures to use in EnergyPlus (and other simulation programs)? The module could use the EnergyPlus slab calculation program or whatever other code/algorithms there might be out there. Input to the module might be building geometry, ground floor area, perimeter length etc.

Andy

rankoromane · Posts: 66

The ground modelling preprocessor in Eplus is not that difficult to use. It has it's own idf file and weather file input but as you say the only things that change in there are slab area and perimeter, ground and slab properties and the indoor average monthly temperatures. The outputs are the heat flux through the slab and the underslab temperatures.

I am not a computer programmer but the dos batch files are fairly easy to use.

rankoromane · Posts: 66

Andy Tindale · Posts: 2432 Location: Stroud

OK, sorry, this change will filter through in versions after 1.2.0.041.

pkalnitz · Posts: 1

Shortcuts

Login