Collector Model according to American Standards (ASHRAE)
Collectors complying with the ASHRAE 96-1980 and 93-1986 standards are modelled analogously to collectors complying with European standards. An overview of the main differences is provided below.
The following applies to the efficiency:
\(\eta(x) = \eta_{0} + a_{1} \cdot \frac{P}{I} + a_{2} \cdot \frac{P^{2}}{I}\)
where
\(P = \ T_{i} – T_{a}\)
\(T_{i}\) = temperature of the fluid flowing through the collector
\(T_{a}\) = ambient temperature
I = irradiance on collector surface
The values measured for η0, a1 and a2 refer to the gross-surface. The relevant standard does not require absorber surface to be measured and so this is equalled to the aperture area. The a1 value is recorded in the absence of wind; in case of wind blowing at 3m/s a1 is to be multiplied by a 1.05 factor for tube collectors, 1.1 for glazed flat-plate collectors and 1.2 for unglazed flat-plate collectors. The influence that wind has on the collector is calculated like provided in the above chapter for European standards. Glazed and unglazed collectors can otherwise be calculated by means of the same model.
Should no indication be available for the mcp heat capacity the relevant field can be left blank; the standard value 10000 J/K will thus be used for the simulation .
The pressure loss can be deduced from three recorded interpolation nodes. Should no indication be available in this respect the relevant fields can be left blank; the following standard values will then be used for the simulation: 72, 180, 288 [l/h] for volume currents, 262, 885, 1784 [Pa] for pressure losses.