IES-VE User Forum

Hi all,

I did a test comparison study and noticed something interesting between the solar gain and external conduction loss of two models:
- Model 1: No adjacent buildings modeled
- Model 2: adjacent buildings modeled

In the end, the total energy of both buildings are relatively the same, however it was found that the cooling in model 1 was less than model 2? Given the climate is San Francisco (very mild, typically 60-65 degrees year round) The following solar results were:
- Model 1: Solar gain = 1807469 kBtu
- Model 2: Solar gain = 1600648 kBtu

This is what we expected to see, with the adjacent buildings shading, model 2 would have a lower solar gain. However, looking at the external conduction loss:
- Model 1: External conduction loss = -3418165 kBtu
- Model 2: External conduction loss = -3273866 kBtu

We expected to see external conduction loss as opposed to gain because of SF's mild climate, but we couldn't understand why the external conduction loss for the higher solar gain case is higher (during hours of operation and non-operation). Given that the system is operating with zero unmet load hours (which it is reported to be doing), the delta T between the interior and exterior is relatively the same in both models. On a given cooling day, model 2 and model 1 rarely differ in room temperatures.

If the system is maintaining set point in the space for both models, how is it that the model with larger solar gain is losing more heat through the facade? Instead, shouldn't the increased solar gain be reflected in the cooling energy in model 1 vs the external conduction loss?

Any input would be appreciated,
Thanks!

I think what you are seeing there is thermal absorption by the building fabric. The external conduction gain/loss is actually driven by the difference between room air temperature and the internal surface temperature of that construction. This means that it can be hotter outside than in and you still see a conduction loss at certain times due to thermal lag.

In your case the room air temperature is being held constant in both scenarios but model 1 has more energy entering the space, some of which will start to heat the walls (ie you are losing energy to the wall) and thus will show as a conduction loss in the results.

You could try a test by switching the constructions to something thin and lightweight to see the difference.

CP

Complex Potential,

Ahhh... you're absolutely right, we didn't think about that.

Thank you!