Calculating a Solar Wall in PHPP
SolarWall is the brand name of a transpired air collector system that was developed in 1989 and has since been manufactured by a Canadian company. A south-facing wall (in the northern hemisphere) is covered by a dark, perforated sheet metal which forms a solar air collector. Ambient air is drawn through the holes in the metal cover into the gap behind the cladding, resulting in a temperature increase, and is then supplied to the building’s ventilation system. The approach is also combined with building-integrated photovoltaics. In the following, we refer to all of these systems as Solar Walls. Similarly to a subsoil heat exchanger, the effect of a Solar Wall is small when combined with a high-efficiency ventilation heat recovery. Only the small remaining ventilation heat losses can be reduced by the Solar Wall.
For calculating the temperature increase by a Solar Wall, an hourly simulation is required, e.g. in RETscreen. For the use in PHPP, this temperature increase is averaged over each month.
The Solar Wall should only be considered in the Heating worksheet, not in the Heating Load (no guarantee that there will be enough solar radiation during the design period) or the Summer/Cooling calculations (assuming that there is a heat recovery bypass for the summer, which will usually be required).
The PHPP ventilation calculation in the Heating worksheet is connected with the outgoing temperature of the Solar Wall, month by month. This requires some modifications in the hidden cells. We take advantage of the fact that the PHPP already accounts for a separate air temperature coming from a subsoil heat exchanger (SHX). Assuming that there is no SHX in the building, the corresponding cells can be used, and the following changes are made in PHPP 9:
- Set the SHX efficiency to 100% in the Heating worksheet: Heating!I35 = 100%
- Calculate the monthly average air temperatures leaving the Solar Wall, e.g. in a separate SolarWall Calcs worksheet.
- Adjust the formula for the calculation of the heating degree hours for the SHX in Heating!T100:AE100, which no longer relates to the annual average ground temperature from the Ground worksheet but to the air temperature leaving the Solar Wall.
- Similarly adjust the formula for the equilibrium temperature in Heating!T121:AE121.
This results in a sufficiently precise calculation of the effects of the Solar Wall, including the monthly variations.
For PHPP 10, the procedure is slightly different and a bit more complicated:
- Set the SHX efficiency to 100% in the Heating worksheet: Heating!I35 = 100%
- Calculate the monthly average air temperatures leaving the Solar Wall, e.g. in a separate SolarWall Calcs worksheet.
- Adjust the formula for the calculation of the heating degree hours for the SHX in Heating!T100:AE100, which no longer relates to the annual average ground temperature from the Ground worksheet but to the air temperature leaving the Solar Wall.
- This last step is the most complicated, and for practical applications its impact turned out to be on the order of 0.1 kWh/(m²a), so that it may also be neglected.
In Ground!E125:P200, in all formulae for the calculation of the equilibrium temperature in winter ('GG-Temp Winter'), where the conductance of the SHX is multiplied with the annual average ground surface temperature (Heating!\$R\$103*\$P\$11), replace the reference to the annual average ground surface temperature by the respective air temperature leaving the Solar Wall. The equilibrium temperature in summer need not be changed because we assume that the Solar Wall does not take effect in summer.
NOTES:
- If the Solar Wall produces temperatures above the indoor temperature setpoint, a possible heat recovery bypass is not taken into account. The calculation is conservative in this respect.
- The wall that is covered by the Solar Wall system has reduced heat losses due to the higher temperature at its outside. This effect can be accounted for separately.
- One might consider the alternative option to just determine an effective SHX efficiency η* SHX instead that represents the effects of the Solar Wall. The major difficulty is that this effective SHX efficiency varies strongly between months. In practical examples, its values were between 55% and 140% during the heating period, and negative values of -300% occured outside the heating period.This may result in considerable inaccuracies, which is why this approach is discouraged.