Large differences between sunLoadConstant and sunLoadFairWeather models

B_R_Khan · January 3, 2022, 04:22

Hi!

I was using solar load model to calculate irradiance on a solar panel. With fair weather model, the qr (irradiance) comes out to be 880 W/m2 on front surface. Keeping all other parameters same i-e sun direction, start time and start day, when I use constant sun load with values:

Code:

   sunLoadModel          sunLoadConstant;

	directSolarRad		835;
	diffuseSolarRad	134;

It gives me qr value of 410 W/m2 on front surface.

I do not understand such large difference in irradiance even though in constant model, the direct solar radiation is 835 W/m2.

Please guide.

Thanks!

B_R_Khan · January 6, 2022, 03:17

Update:
So I found out that the direct solar radiation calculated by fair weather with default values of A, B and C in simple Solar Car comes out to be 1930 W/m2 which is very unrealistic and hence the high value of 880 W/m2 I was getting on panel surface. When I used values from table 7 from Chapter 30 of the 2001 ASHRAE Handbook of Fundamentals, the irradiance on solar panel surface through fair weather model came out to be 460 W/m2 with (2 1 ) spectral distribution and (0.3 0.7) panel absorptivity and emissitivity.

Weird that OpenFoam tutorial was not using values from table 7 of Ashrae handbook even though the guide says that fair weather model is taken from ASHRAE

January 3, 2022, 04:22	Large differences between sunLoadConstant and sunLoadFairWeather models	#1
B_R_Khan Member Bushra Rasheed Join Date: Dec 2020 Posts: 97 Rep Power: 6	Hi! I was using solar load model to calculate irradiance on a solar panel. With fair weather model, the qr (irradiance) comes out to be 880 W/m2 on front surface. Keeping all other parameters same i-e sun direction, start time and start day, when I use constant sun load with values: Code: sunLoadModel sunLoadConstant; directSolarRad 835; diffuseSolarRad 134; It gives me qr value of 410 W/m2 on front surface. I do not understand such large difference in irradiance even though in constant model, the direct solar radiation is 835 W/m2. Please guide. Thanks!

January 6, 2022, 03:17		#2
B_R_Khan Member Bushra Rasheed Join Date: Dec 2020 Posts: 97 Rep Power: 6	Update: So I found out that the direct solar radiation calculated by fair weather with default values of A, B and C in simple Solar Car comes out to be 1930 W/m2 which is very unrealistic and hence the high value of 880 W/m2 I was getting on panel surface. When I used values from table 7 from Chapter 30 of the 2001 ASHRAE Handbook of Fundamentals, the irradiance on solar panel surface through fair weather model came out to be 460 W/m2 with (2 1 ) spectral distribution and (0.3 0.7) panel absorptivity and emissitivity. Weird that OpenFoam tutorial was not using values from table 7 of Ashrae handbook even though the guide says that fair weather model is taken from ASHRAE francescomarra likes this.