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Heat exchange calculation for the foundation:kiva core zone

I'd like some clarifications for the calculation of the heat exchanged through the core zone.

First, how is the area of the core zone determined? I don't see any input where we can define the distance between the perimeter and the core zone.

Second, it says in the documentation that the heat exchange in the core zone is calculated with the deep ground boundary condition. If I understand correctly this can either be set to zeroflux (adiabatic so no exchange) or groundwater (average of outside temp.).

However, it seems that EnergyPlus has a default value of 16°C for the deep ground temperature object. And this value remains no matter if I use the zeroflux or the groundwater deep ground boundary condition. Also I don't see much difference in the inside surface temperature or convection heat transfer rate. Since we can't output outside temperature or heat conduction loss for the ground surface I can't see any difference there neither.

So I'm wondering how is the heat exchange calculated for the core zone of the ground surface if I use the foundation:kiva object? Is there a way to influence these calculations such as defining the area of the core zone or the temperature in the ground?

I'm currently trying to calibrate a small plastic greenhouse with an insulated foundation. Kiva would potentialy represent the heat exchange of the perimeter with more precision. However, if I can't customize the heat exchange through the core zone, I'm thinking I might have more accurate results with other means. Like by letting the outside boundary condition to ground while modifying the Site:GroundTemperature:BuildingSurface for something in between the undisturbed ground temperature at 0.5m of the weather file and the temperature inside the zone.

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For core zones, Kiva uses a 1-D finite difference method to calculate heat exchange between the zone and the deep ground.

The area of the core zone is determined by the area of its floor surface.

Kiva does not use the ground temperature objects. Instead, the deep ground temperature is calculated based on the zero-flux or ground water condition. Unless the deep ground boundary condition is very close to the building, you will likely see very little difference between the two options.

The Site:GroundTemperature:BuildingSurface objects are a much less refined approach to the problem and are not likely to provide a better solution for your example.

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Hi Neal, I stumbled upon this question while searching for some answers regarding using Kiva with EnergyPlus. Is there any way to output the ground temperatures calculated by Kiva? The only output variables that I could find in the .rdd file are related to deep, shallow, and building surface GroundTemperature objects.

( 2020-08-05 13:35:50 -0500 )edit

You cannot output ground temperatures from Kiva at this time. Part of the problem is there can be multiple Kiva instances to represent a single foundation (e.g., a walkout basement), and it is not straightforward to convert 2D temperature maps to an EnergyPlus output.

( 2020-09-09 09:04:54 -0500 )edit