How to Estimate Solar Gain

[Adam2]

Hi, I'm pre-empting the SAP calcs by modelling the expected solar gain - prior to any external shading being installed and would appreciate input re any errors in my approach.

 

I've used PVGIS to estimate the kwh / m2 for my location using a PV panel angle of 90 degrees and azimuth of 45 (SW) to simulate the sun hitting the windows.

Of the two satellite datasets available I used the one with higher values (~5% higher)

 

This gives me the monthly kWh values (1st column) as below.

The 2nd column is the monthly energy x glazing size (after 25% shading reduction).

3rd column is energy after G value reduction (G = 0.4).

I then divided by 29 to get an approx amount of energy per day.

Finally, I figured that the energy would only be coming when the sun was in the right place so estimated over how many hours this daily energy would be delivered to give me an approximation of the energy per hour this would be.

 

Apologies for any inaccurate use of kwh capitalisation! 

 

This excludes energy reflected from external surfaces, not sure how significant this would be or how to estimate.

 

Would appreciate any points to improve this modelling.

 

	Per Month total energy kWh /m2	Energy hitting windows (net of shading)	Net of G and shading		Per Day (avg 29 days)	Approx relevant hours/Day	kw Per hour
Jan	34.5	1,294	518		18	5	3.6
Feb	47.8	1,793	717		25	6	4.1
Mar	81.6	3,060	1,224		42	8	5.3
Apr	93.5	3,506	1,403		48	9	5.4
May	91.6	3,435	1,374		47	10	4.7
Jun	87.5	3,281	1,313		45	10	4.5
Jul	90	3,375	1,350		47	10	4.7
Aug	84.1	3,154	1,262		44	9	4.8
Sep	79.5	2,981	1,193		41	8	5.1
Oct	60.9	2,284	914		32	7	4.5
Nov	41	1,538	615		21	6	3.5
Dec	32	1,200	480		17	5	3.3

 

 

After this step I plan to: 

Model the benefit of different sized shading options to end up with a solar gain in the spring through to autumn period which will be acceptable. Presumably the gain will be greatest when the sun is more aligned to the glazing so I could model this on a per hour basis with different over-hangs etc using the Sustainable by design tools.

Link in with the heat loss calculation spreadsheet to see how the gains will assist with the house heating needs.

Finally, try to work out how an ASHP in cooling mode combined with evening purge/ use of MVHR +/- duct cooling will get us to an overall model of heating and cooling.

 

 

 

 

[Jeremy Harris]

Looks OK to me.  The reflectance of most surfaces would be pretty low, unless they were specifically made reflective (like solar film on windows, or reflective glazing).  Also, the transmittance of glazing can vary quite a lot, so also needs to be taken into account.  Overall, the heat that gets in through the glazing will probably end up being around half, maybe less, of the radiation hitting the outside of it, mainly as a consequence of the reflectivity and transmittance for the specific type of glazing used.

[Ed Davies]

8 hours ago, Adam2 said:

Apologies for any inaccurate use of kwh capitalisation! 

? The “correct” (i.e, standard) capitalization is kWh. The last column in your table should be labelled “kW” (not “kW per hour”) as watts are already a rate (kWh/h = kW). More details.

 

More substantially:

 

8 hours ago, Adam2 said:

Presumably the gain will be greatest when the sun is more aligned to the glazing so I could model this on a per hour basis with different over-hangs…

 

I'm not sure if it's worthwhile but if you want to do more detailed modelling then, yes, the angle of the insolation to the window does make a difference. Of course the simplest reason is the effect of the apparent area of the window being reduced when the sun's off the normal to the glass (cos θ) which is already taken into account by PVGIS. I think it also takes into account the effect of reflection off the outer glass surface using Fresnel's equations. However, if you're being this careful you'll have double or triple glazing so this will have a larger effect. If you want to explore that a bit more I have a calculator which might help.

[Adam2]

Thanks for the info and links. Yes, the PVGIS calc includes the surface angles. The shading model gives the percentage of shade for each hour so will have a look at that next.