Evacuated tube solar collectors (ETC) are increasingly in use worldwide because of their high thermal efficiency and high working temperature compared to the flat plate solar collectors. The efficiency of ETC is substantially enhanced due to the presence of vacuum between the absorber and the cover of evacuated tube solar collector (ETC). This is mainly attributed to the reduction in heat losses by convection ad conduction. The high energy absorption increases the values of solar fraction and instantaneous efficiency. The objective of this paper is to investigate thermal performance of evacuated tube solar water collector in hot and harsh climate like Kuwait climate. An experimental rig facility was first set up to monitor the thermal performance of ETC through one year period. The experimental data along with correlations obtained by linear regression are presented. A detailed developed nonlinear model for evacuated tube solar collectors is presented in the current work with more comprehensive optical and thermal analysis. The variation of the temperature along both the circumferential (fin) and the longitudinal (tube) directions is considered in the present model. The model analyzes separately the optics and the heat transfer in the evacuated tubes allowing the analysis to be extended to different configurations. The predicted numerical values are found to agree well with the experimental values obtained from experimental test facility. The optimum design parameters; collector tube size, mass flow rate and collector tilt angle based on year around ETC thermal performance is determined under the meteorological conditions of Kuwait. The maximum energy generation from the collector corresponds to tilt equal to 25° (i.e. latitude – 5°) and for collector facing south (azimuth angle=0°). The results indicate that the optimum tube length is 1.5 m as at this length a significant enhancement can be achieved in thermal efficiency for other tube diameters studied. The optimal mass flow rate is 30 kg/h.m2 as thermal collector efficiency reaches its highest maximum value of 0.53 at this optimum mass flow rate. Finally, integration of evacuated tube collector in industrial process as soft drink industry can provide 90% of the heat required with life cycle savings of approximately $18,000.
Keywords: Collector Efficiency, Evacuated Tube Collector, Optimum Parameters, Solar Fraction, Solar Heat for Industrial Processes
