Using A Solar Simulator

In countries with unsuitable weather conditions, the indoor testing of solar collectors with the use of a solar simulator is preferred. Solar simulators are generally of two types: those that use a point source of radiation mounted well away from the collector and those with large area multiple lamps mounted close to the collector. In both cases, special care should be taken to reproduce the spectral properties of the natural solar radiation. The simulator characteristics required are also specified in ISO 9806-1:1994 and the main ones are (ISO, 1994):

1. Mean irradiance over the collector aperture should not vary by more than ±50 W/m2 during the test period.

2. Radiation at any point on the collector aperture must not differ by more than ±15% from the mean radiation over the aperture.

3. The spectral distribution between wavelengths of 0.3 and 3 |im must be equivalent to air mass 1.5, as indicated in ISO 9845-1:1992.

4. Thermal irradiance should be less than 50 W/m2.

5. As in multiple lamp simulators, the spectral characteristics of the lamp array change with time, and as the lamps are replaced, the characteristics of the simulator must be determined on a regular basis.

Table 4.2 Tolerance of Measured Parameters for Unglazed Collectors

Parameter

Deviation from the mean

Total solar irradiance

±50 W/m2

Longwave thermal irradiance

±20 W/m2

Ambient air temperature

±1 K

Wind speed

±0.25 m/s

Fluid mass flow rate

±1%

Collector inlet fluid temperature

±0.1 K

FIGuRE 4.6 Plot of incidence angle modifier against 1/cos(6) — 1 for two types of flat-plate collectors.

The equation for the useful energy collected, Eq. (4.9), is also modified in a similar way.

4.2.2 Concentrating Collectors

Similarly, for concentrating collectors, the performance Eqs. (4.10) and (4.13) described previously are reasonably well defined as long as the direct beam of solar irradiation is normal to the collector aperture. For off-normal incidence angles, the optical efficiency term (ro) is often difficult to be described analytically, because it depends on the actual concentrator geometry, concentrator optics, receiver geometry, and receiver optics, which may differ significantly. As the incident angle of the beam radiation increases, these terms become more complex. Fortunately, the combined effect of these parameters at different incident angles can be accounted for with the incident angle modifier. This is simply a correlation factor to be applied to the efficiency curve and is a function of only the incident angle between the direct solar beam and the outward drawn normal to the aperture plane of the collector. It describes how the optical efficiency of the collector changes as the incident angle changes. With the incident angle modifier, Eq. (4.13) becomes n = FRK,n0 - - C2(T'- T)2 (4.28)

If the inlet fluid temperature is maintained equal to ambient temperature, the incident angle modifier can be determined from

Parabolic Trough Collector Efficiency

Angle (degrees)

FIGURE 4.7 Parabolic trough collector incidence angle modifier test results.

Angle (degrees)

FIGURE 4.7 Parabolic trough collector incidence angle modifier test results.

where \(T = Ta) is the measured efficiency at the desired incident angle and, for an inlet fluid temperature, equal to the ambient temperature. The denominator in Eq. (4.29) is the test intercept taken from the collector efficiency test with Eq. (4.13), with [\o]n being the normal optical efficiency, i.e., at a normal angle of incidence.

As an example, the results obtained from such a test are denoted by the small squares in Figure 4.7. By using a curve-fitting method (second-order polynomial fit), the curve that best fits the points can be obtained (Kalogirou et al., 1994):

For the 1ST collector, the incidence angle modifier Ke of the collector given by the manufacturer is

Ke = cos (9) + 0.0003178(9) - 0.00003985(G)2 (4.31)

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