Unglazed Collectors

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Unglazed collectors are more difficult to test, because their operation is influenced by not only the solar radiation and ambient temperature but also the wind speed. The last factor influences the collector performance to a great extent, since there is no glazing. Because it is very difficult to find periods of steady wind conditions (constant wind speed and direction), the ISO 9806-3:1995 for unglazed collector testing recommends that an artificial wind generator is used to control the wind speed parallel to the collector aperture (ISO, 1995b). The performance of unglazed collectors is also a function of the module size

Table 4.1 Tolerance of Measured Parameters for Glazed Collectors

Parameter

Deviation from the mean

Total solar irradiance

±50 W/m2

Ambient air temperature

±1 K

Wind speed

2-4 m/s

Fluid mass flow rate

±1%

Collector inlet fluid temperature

±0.1 K

and may be influenced by the solar absorption properties of the surrounding ground (usually roof material), so to reproduce these effects a minimum module size of 5 m2 is recommended and the collector should be tested in a typical roof section. In addition to the measured parameters listed at the beginning of this chapter, the longwave thermal irradiance in the collector plane needs to be measured. Alternatively, the dew point temperature could be measured, from which the longwave irradiance may be estimated.

Similar requirements for pre-conditioning apply here as in the case of glazed collectors. However, the length of the steady-state test period in this case should be more than four times the ratio of the thermal capacity of the collector to the thermal capacity flow rate mcp of the fluid flowing through the collector. In this case, the collector is considered to operate under steady-state conditions if, over the testing period, the measured parameters deviate from their mean values by less than the limits given in Table 4.2.

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Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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