Calculating solar gain - what data to use and how much detail

[Joey]

Hi all, looking for some advice regarding the calculation of solar gain/overheating risk! I've seen a couple of threads by @Lilly_Pines and have taken a similar approach as they did with their solar gain calculator, using solar irradiance data from PVGIS and then calculating the amount of energy entering the house based on glazing area and G value etc. 

 

Our current design has a substantial amount of south (slightly south east) facing glazing, including a proposed 6m set of sliding doors spanning most of the length of the kitchen/diner. I want to get a grasp of the data and make sure sufficient shading is provided and any required alterations can be made whilst we're early in the project! Ideally I want to optimise the shading to limit overheating, but take advantage of as much solar gain during the colder months as possible. It would also be nice to incorporate things now to give us a good chance of passing the part O dynamic modelling.

 

My questions are:

 

• Which data from PVGIS would be best to use for solar irradiance? The options are clear-sky irradiance, global, direct, and diffuse. Clear-sky is much higher than the others and, as this is apparently a calculated theoretical value based on, surprisingly, a completely clear sky, this is a large overestimation of what would be seen in real life. I've assumed global irradiance is the most suitable value so far, which seems to be the sum of direct and diffuse and then a little something extra for confusion.

 

• What level of detail would you say is sufficient to calculate overheating risk? Daily averages? Monthly? Hourly? When modelling heat loss and solar gain based on monthly/daily average irradiance and temperature data, I can apply shading solutions calculated with the tools from the sustainable by design website (https://www.susdesign.com/) and end up with figures showing the monthly/daily average solar gain is x% of the heating requirements for the same period. Where gains are less than the required heat input, this would suggest, on average, overheating risk is limited. However, when I download actual historical hourly data points from PVGIS and model how shading would have performed in say 2020 on a per hour basis, although the total daily gains might be less than the required heat input, at certain points during a given day the solar gain might exceed the heating requirement at that time point, sometimes significantly. I'm assuming if 4000 W of solar energy enters the house during an hour, but the heat requirement for the same period is only 2000 W, some of the additional energy will be absorbed by the mass of the building and released over time, not just all be dumped straight into the air and massively increasing the internal temperature? Basically, if I calculate the daily average solar gain to be lower than the average daily heat demand, is that enough? Or is the finer detail required?

 

Might be overthinking things!

 

Thanks for your help

   

[SteamyTea]

2 hours ago, Joey said:

if I calculate the daily average solar gain to be lower than the average daily heat demand, is that enough

Also look at the peaks because

2 hours ago, Joey said:

When modelling heat loss and solar gain based on monthly/daily average irradiance and temperature data, I can apply shading solutions calculated with the tools from the sustainable by design website

which possibly (I don't know the software) takes night time into account.

 

It is also worth adding in the reflectivity of the glazing, the steeper the angle, the more gets bounced off.  I am not sure of the G-Value covers that.

 

Solar film is another option to get you though the regs, cheap and removable.