SolarAdjustTemperature - validation against ASHRAE55-2017

peter.zacharia · June 22, 2020, 11:45pm

I recently got my hands on a copy of ASHRAE55-2017 and decided to do a quick validation check using the C4 validation table. I made some adjustments to the grasshopper script as to adjust inputs (Fbes, Tsol, fsvv etc.). I ended up with the following:

There is consistent errors at each of the validation rows which i isolated to line 1448 in the code (when doing the parallel calc). The formula:

hourERF = ((0.5 * fracEff * skyViewFac * (diffRad + (globHorizRad * groundR))+ (fracEff * ProjAreaFac * fBes * dirNormRad)) * winTrans[HOYS[count]-1]) * (cloA/0.95)

should be (to yield the results in the validation table)

hourERF = ((0.5 * fracEff * skyViewFac * (diffRad + (globHorizRad * groundR))+ ((ProjAreaFac/1.8) * fBes * dirNormRad)) * winTrans[HOYS[count]-1]) * (cloA/0.95)

with 1.8 being the DuBois surface area.

Thoughts? Willing to dive deeper on this as I use this component very frequently.

Pete

peter.zacharia · June 30, 2020, 1:55am

@chris @mostapha any thoughts?

chris · July 1, 2020, 9:42pm

We actually use the ASHRAE55 validation table in the tests of Ladybug[+]:

github.com

ladybug-tools/ladybug-comfort/blob/master/tests/solarcal_test.py#L91-L111


def test_solarcal_validation():
    """Test the SolarCal function against the reference table from ASHRAE-55 2017.
    """
    validation_csv_file_path = './tests/validation_tables/solarcal_validation.csv'
    with open(validation_csv_file_path) as csv_data_file:
        csv_data_file.readline()
        csv_data_file.readline()
        for row in csv_data_file:
            vals = []
            for val in row.split(','):
                try:
                    vals.append(float(val))
                except ValueError:
                    vals.append(str(val))
            i_diff = 0.17 * vals[3] * math.sin(math.radians(vals[0]))
            result = indoor_sky_heat_exch(20, i_diff, vals[3], vals[0], vals[5],
                                          vals[6], 0.6, vals[4], vals[2],
                                          vals[1], vals[7])
            # Values don't match perfectly because we use the original SolarCal splines
            assert result['erf'] == pytest.approx(vals[8], rel=1e-2)
            assert result['dmrt'] == pytest.approx(vals[9], rel=1e-1)

So, if you want something that you can be certain always aligns with ASHRAE55 perfectly, you might want to use Ladybug[+] instead of ladybug legacy.

But I think the fracEff (which seems to be the only thing you changed in your formula there), is equal to 1 / 1.8. So your two formulas there should be equal to one another.

peter.zacharia · July 6, 2020, 6:21am

Thanks for your response @chris. Will use ladybug[+] going forward.

Although 1/1.8 = 0.55 whereas fracEff is = 0.696 or 0.725 depending on posture so that is most likely the source of the error.

Pete