Solar Water Heating Total Area Equation Units

Hello, 

I am trying to calculate the total collector area required for a solar water heating system using a surrogate model I created with PySAM.

The documentation has:

• Nameplate Capacity = Total collector area * (FRta  - FRUL * 0.03)

With units of:

• Nameplate Capacity [kW]
• Total collector area [m2]
• FRta [dimensionless]
• FRUL [W/m2]
• 0.03 [????]

I am trying to calculate total collector area. My question is regarding the units. If you carry the units through the equation above, you end up with bogus/nonsensical units, especially since the FRta factor is dimensionless.

Can you explain how you get the proper units from this equation? What are the units for FRta and/or what are the units for the 0.03 factor? On my side, I can get my model to match SAM output if I convert capacity to kW, assign kW/m2 to FRta and units of K to the 0.03 factor, and convert FRUL to kW/m2, but I would prefer to know the details behind this calculation.

Thanks

Hi Kurban,

I agree that the dimensions don't seem to make sense. The equation in the SAM user interface for the solar water heating model system capacity is:

system_capacity = area_coll * ncoll * ( FRta - FRUL * ( 30 / 1000 ) )

For what it's worth, this system_capacity value is not used in the solar water heating performance model simulation except to calculate the capacity factor. It is used by the financial model to calculate any costs or incentives specified in $/kW.

We don't have a developer working on the solar water heating model for the moment, but I will see if I can get an answer to this and follow up.

Best regards,
Paul.

Hi Kurban,

There is an implied multiplication of FRta by 1000 W/m2:

system_capacity [W] = total_collector_area [m2] * ( FRta [] * 1000 W/m2 - FRUL [W/m2-C] * 30 [C] )

Converting to kW:

system_capacity [kW] = total_collector_area [m2] * ( FRta [] * 1 kW/m2 - FRUL [W/m2-C] * 30 [C] / 1000 W/kW )

The rated system size  equation is derived from Equation 2.1.4 of the draft technical manual and assumes an irradiance of 1000 W/m² and temperature difference of 30°C.

DiOrio, N.; Christensen, C.; Burch, J.; Dobos, A. (DRAFT 2014). Technical Manual for the SAM Solar Water Heating Model. ( PDF 150 KB )

We will revise the description in Help to make that clearer.

For more information about SAM's solar water heating model, see:

sam.nrel.gov/solar-water-heating.html

samrepo.nrelcloud.org/help/solar_hot_water.htm

Best regards,
Paul.