Characteristic heat balance equations
In a solar thermal collector the heat balance can be written according to the following equation:
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T represents the collector temperature and C its effective thermal capacity. qabs is the power absorved by the collector from incident solar radiation, qioss, is the power lost to the environment. The power delivered by the collector for use is given by:
quse = m cp (te - ti) (2)
With m being the flow rate, te and ti , the outlet and inlet collector temperature respectively and Cp the fluid heat capacity. In the SS test method the term on the right, in equation (1), is considered equal to zero and it is possible to define a steady-state efficiency:
Л= Л0 - a1(tm - ta)/G - a2(tm - ta)2/G (3)
where tm is the mean fluid temperature and ta the ambient temperature. Parameters q0, ai and a2 are determined based on test data that respects the conditions listed in Table 1 and using a least square fit method. Since one of the conditions is that the incidence angle on collector aperture is lower then 20°, dependence of optical efficiency, q0, on incidence angle has to be determined in a separate test. In this test, the Incidence Angle Modifier (IAM) is determined based on global irradiance incident on collector aperture.
Also the thermal capacity of the collector is only determined if and optional test is performed (see section 6.1.6 of EN 12975-2:2006)
In the case of QD test method, equation (1) can be written, for the case of glazed collectors, as:
Q/A = F(xa)enКдь (0)Gb + F(ia)enKMGd - c1(tm - tj-c2(tm - tj - c5dtm /dt
where A is the collector reference area, Gb and Gd are beam and diffuse irradiance incident on the collector, respectively.
In QD test method, the parameters that characterize the collector are:
F(xa)en;Keb (0);K9d ;c1;c2;c5 (5)
i. e., optical efficiency (equivalent to ^0 in SS), IAM for beam radiation, IAM for diffuse radiation, thermal loss coefficients (equivalent to a1 and a2 in SS) and effective thermal capacity. The fact that the IAM is decoupled in its component for beam radiation, K0b(0), and the component for diffuse radiation, K0d, is an additional advantage of QD test method, specially in the characterisation of collectors with more complex optical characteristics than flat plate collectors [3,4], e. g, evacuated tubular collectors or CPC type collectors.
