For locations that do not have steady environmental conditions for long periods of time, the transient or dynamic system test method can be used. This method involves monitoring the transient response of a collector over a number of days, which include both clear and cloudy conditions. The performance data obtained from the dynamic method allow a more detailed characterization of the collector performance in comparison with the steady-state method. The advantages of the dynamic test method are that the test period is much shorter and can be conducted at any time of the year under variable weather conditions. After testing, the data collected over the wide range of operating conditions are fitted to a transient mathematical model of the collector performance. The test data are measured every 5–10 min. For a glazed collector, the following model for the transient useful energy collection could be used (Morrison, 2001):
(4.34)
where ηo, a0, a1, c, and the coefficients Kθ,B and Kθ,D are determined by the correlation of the test measured data.
Equation (4.34) is similar to the second-order equations used for steady-state testing, presented earlier in this chapter, with the addition of a transient term and incident angle modifiers for both beam, Kθ,B, and diffuse, Kθ,D, radiation.
Models that are more complex can be used if the testing program can cover an extended range of operating conditions. In any case, the measured transient data are analyzed using a procedure that compares a set of model coefficients that minimize the deviation between the measured and predicted output. The method should be such that the various parameters should be determined as independently as possible. To be able to satisfy this requirement, sufficient data are needed; therefore, it is required to control the experimental conditions so that all variables independently influence the operation of the collector at various periods during testing. Additionally, a wide range of test conditions are required to determine the incident angle modifiers accurately. An added advantage of the method is that the equipment required is the same as the steady-state testing shown in Figures 4.1 and 4.2, which means that a test center can have the same equipment and perform both steady-state and dynamic testing at different periods of the year, according to the prevailing weather conditions. The primary difference between the two methods is that, in the dynamic method, the data are recorded on a continuous basis over a day and averaged over 5–10 min.
Due to the wider range of collector parameters that can be determined with the dynamic method, it is likely that it may displace the steady-state testing method, even for locations that have clear and stable climatic conditions.
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