To perform the steady-state test satisfactorily, according to ISO 9806-1:1994, certain environmental conditions are required (ISO, 1994):
1. Solar radiation greater than 800 W/m2.
3. Angle of incidence of direct radiation is within ±2% of the normal incident angle.
4. Fluid flow rate should be set at 0.02 kg/s-m2 and the fluid flow must be stable within ± 1% during each test but may vary up to ± 10% between different tests. Other flow rates may be used, if specified by the manufacturer.
5. To minimize measurement errors, a temperature rise of 1.5 K must be produced so that a point is valid.
Data points that satisfy these requirements must be obtained for a minimum of four fluid inlet temperatures, which are evenly spaced over the operating range of the collector. The first must be within ± 3 K of the ambient temperature to accurately obtain the test intercept, and the last should be at the maximum collector operating temperature specified by the manufacturer. If water is the heat transfer fluid, 70°C is usually adequate as a maximum temperature. At least four independent data points should be obtained for each fluid inlet temperature. If no continuous tracking is used, then an equal number of points should be taken before and after local solar noon for each inlet fluid temperature. Additionally, for each data point, a pre-conditioning period of at least 15 min is required, using the stated inlet fluid temperature. The actual measurement period should be four times greater than the fluid transit time through the collector with a minimum test period of 15 min.
To establish that steady-state conditions exist, average values of each parameter should be taken over successive periods of 30 s and compared with the mean value over the test period. A steady-state condition is defined as the period during which the operating conditions are within the values given in Table 4.1.
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