Solar thermal collectors and applications

Collector test results and preliminary collector selection

Collector testing is required in order to evaluate the performance of solar collectors and compare different collectors to select the most appropriate one for a specific application. As can be seen from Sections 4.1-4.4 the tests show how a collector absorbs solar energy, how it loses heat, the effects of angle of incidence of solar radiation and the significant heat capacity effects which are determined from the collector time constant.

Finally tests are performed on the solar collectors in order to determine their quality. In particular the ability of a collector to resist extreme operating conditions are exam­ined as specified in International Standard ISO 9806-2 (1995) [109]. The tests are required to be applied in the sequence specified in Table 9 so that possible degradation in one test will be exposed in a later test.

Final selection of a collector should be made only after energy analyses of the complete system, including realistic weather conditions and loads, have been conducted for one year. Also, a preliminary screening of collectors with various performance parameters should be conducted in order to identify those that best match the load. The best way to accomplish this is to identify the expected range of the parameter AT/G for the load and climate on a plot of efficiency n as a function of the heat loss parameter, as indicated in Fig. 23.

Table 9

Sequence of quality tests for solar collectors [109]

Sequence

Test

1

Internal pressure

2

High temperature resistance

3

Exposure

4

External thermal shock

5

Internal thermal shock

6

Rain penetration

7

Freeze resistance

8

Internal pressure (re-test)

9

Thermal performance

10

Impact resistance

11

Final inspection

Collector efficiency curves may be used for preliminary collector selection. However, efficiency curves illustrate only the instantaneous performance of a collector. They do not include incidence angle effects, which vary throughout the year, heat exchanger effects, probabilities of occurrence of Ti, Ta, solar irradiation, system heat loss, or control strategies. Final selection requires determining the long­term energy output of a collector as well as performance cost-effectiveness studies. Estimating the annual perform­ance of a particular collector and system requires the aid of appropriate analysis tools such as F-Chart, Watsun, or TRNSYS. These are analysed briefly in Section 4.6.

Solar thermal collectors and applications

Collector thermal efficiency

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