Optical Characterization of Solar Collectors form Outdoor Measurements

Sammanfattning: Due to high installation costs and relatively low output, the active use of solar energy is limited. The output from a collector can be improved e.g. by using a selective absorber, AR-treated cover glass or transparent insulation. The output/cost ratio can also be improved by the use of reflectors. The performance of a collector depends highly on the incidence angle dependent optical efficiency. In this report, detailed methods for measuring the optical efficiency is developed. In order to evaluate the performance of a collector, measurements are needed. For the evaluations in this work, a dynamic test method has been used. The collector output is modelled as:  = h0bKta(Q)Ib + h0dId – kDT - (mC)e(dTf/dtð) where h0 is the optical efficiency, I the irradiance, k the heat loss factor, DT the temperature difference between collector and ambient air, and (mC)e the effective collector thermal capacitance. Kta(Q) is a modifier accounting for the dependence of varying incidence angles during the day. It is often modelled as: Kta(Q) = 1 - b0(1/cos(Q) - 1) where b0 is the “incidence angle modifier coefficient”. In order to evaluate the energy output from different collector types, measurements were made on a number of collector prototypes. In the analysis, the collector parameters were identified by MLR on the measured data. In order to verify the determined parameters, the modelled output was compared with the measured output. The annual energy output was then estimated by using the collector parameters in the simulation program MINSUN. In one study in the work, the incidence angle dependence of the absorptance was investigated by outdoor testing. The tested absorbers had coatings of nickel-pigmented aluminum oxide (Ni-Al2O3), and sputtered nickel/nickel oxide (Ni-NiOx). The results showed that the Ni-Al2O3 absorber has a slightly better performance than the Ni-NiOx absorber at high incidence angles. In another study, detailed comparative tests were made on different glazings in order to study the influence of AR treatment on the collector output. The tests indicated that the AR treatment can increase the annual output by 9% (at Top = 50°C). Usually a structured glass is installed with the structures facing the absorber. The evaluation indicates, however, that facing the structure outwards can increase the annual performance by 4%. A detailed study showed that the b0 factor generally depends on the incidence angle. MaReCo collectors are studied in the work. This is a reflector collector, specially designed for northern latitudes. The MaReCo principally consists of an asymmetric reflector trough with a single, double-sided selective, absorber that runs along the trough. The purpose of the MaReCo is to replace the collector box, insulation, and some of the absorber material by a reflector. The standard MaReCo has an acceptance angle interval of 20° - 65°, outside which the reflector is not active and the absorber only works with radiation direct from the sun. The MaReCo concept is flexible and can be used for stand-alone as well as building integrated applications. Several MaReCo prototypes have been tested in the work. The estimated yearly energy output at 50°C from a stand-alone MaReCo with Teflon and from a Roof-MaReCo, both at a tilt of 30°, were 282 and 336 kWh/m² respectively. The Spring/Fall-MaReCo is a special version that has a low optical efficiency during the summer. In this way, a larger collector area can be installed for increasing the solar fraction of the system without increasing the risk of overheating. The test results estimate a yearly energy output of 222 kWh/m² from this collector. For an asymmetric collector (e.g. the MaReCo), the incidence angle dependence will be different in different directions. The angular analysis then has to be made in two perpendicular planes (longitudinal and transverse) of the collector. In the transverse plane, not only the properties of glass and absorber affect the output, but also the reflectivity and shape of the reflector. In order to handle this, a biaxial incidence angle modifier should be used. One example is the common “product model”: Kta(Q) = KL(QL,0)KT(0,QT). Shortcomings of this model are that it is not correct for plane collectors and that it is not defined for concentrators where normal incidence is outside the acceptance interval. In this work, a new expression for a biaxial incidence angle modifier is suggested: Kta = fL(Q)gTL(QT). The factor fL(Q) gives information about the influence of the glazing and gTL(QT) accounts for the influence of the reflector. This expression differs in principle from the product model, since QL is not used. In order to study the suggested model, measurements were made on MaReCo collectors. The “no-loss efficiency” was determined by eliminating the effect of heat losses from the measured output. The factor fL(Q) was decided from measurements in the L direction made around the equinox (when QT is constant). The factor FT(QT) was determined from measurements in the T direction for constant QL. In order to keep QL constant, the collector was rotated to a north/south direction. The results were then used to calculate the reflector factor, gTL(QT), as the ratio FT(QT)/fL(QT). The parameters were then used to model the energy output. The analyses indicate that the new suggested biaxial expression can be used to model the collector output for asymmetric collectors where the standard model does not work. One drawback of the method is, however, that it requires measurements to be made around either spring or autumn equinox. The new suggested model has also been tested for modelling the angular performance of PV modules with concentrators.