Page 181 - Design of Solar Thermal Power Plants
P. 181

166         3. GENERAL DESIGN OF A SOLAR THERMAL POWER PLANT

                       0.44
                       0.43
                       0.42
                       0.41                    η=η 0 -U  (t f,i -t a )/G bp
                      Instantaneous efficiency  0.39
                                                   L
                       0.40
                       0.38
                       0.37
                       0.36
                       0.35    η 0 =0.43
                                          2
                                 U =1.92 W/(m K)
                                  L
                       0.34
                       0.33
                       0.32
                        0.00   0.01   0.02   0.03   0.04   0.05   0.06
                                                 2
                                         (t f,i -t a )/G  (m K/W)
                                              bp
                          FIGURE 3.25  Collector transient efficiency curve.
            value, which is not completely consistent with its strict physical meaning,
            this method does not have any influence on the prediction of long-term
            thermal performance of collectors. It is because influences of various
            factors in the actual process on U L are insignificant, which have been
            widely verified. In the ASHRAE 93 standard, a calculation example has
            offered the testing model based on parameters determined through
            regression to calculate the hourly output energy of collector and the
            whole-day output energy of collector after accumulation, the calculation
            conditions and results of which are shown in Table 3.6 [23].
               As described in the section of “significance and application,” this test
            method is intended to provide test data essential to the prediction of the
            thermal performance of a collector in a specific system application in a
            specific location. In addition, to the collector test data, such prediction
            requires validated collector and system performance simulation models
            that are not provided by this test method. The results of this test method
            therefore do not by themselves constitute a rating of the collector under
            test. Furthermore, it is not the intent of this test method to determine
            collector efficiency for comparison purposes since efficiency should be
            determined for particular applications.” Therefore, such solar collector
            test method is not offered to conduct quality rating or search for certain
            quality parameters or indices of a particular component of a collector
            under specific conditions; the fundamental target is to design a collector
            application system. As thermal performance of collector is changeable,
            when comparing two solar collectors, daily output energy and annual
            cumulative output energy of collector must be considered, instead of
   176   177   178   179   180   181   182   183   184   185   186