COMPARISON OF FIXED ASYMMETRICAL AND SYMMETRICAL REFLECTORS FOR EVACUATED TUBE SOLAR RECEIVERS

A computer simulation of the relative performance of certain truncated symmetrical and asymmetrical fixed reflector designs for solar energy collection was performed. The comparison was on the basis of annual energy delivered to a circular cylindrical evacuated tube receiver per unit of mirror area, but seasonal load information was also included to determine possible seasonal load matching advantages of one type of collector over another when storage is employed. Circumsolar radiation models based on recent correlations in the literature were included. Major conclusions were that: (1) Annual solar fractions of between 80% and 95% seem to be feasible with a load matching collector used with moderate energy storage, with 80-90% being a likely optimum; (2) CPC reflectors always gave the best annual output performance per unit of mirror area, and the lowest receiver area for situations of constant annual load, regardless of whether the storage is included in the system; (3) Asymmetrical concentrators are more cost-effective for strongly seasonally asymmetrical load patterns; (4) Fixed parabolic systems required much more receiver area than the symmetrical CPC and asymmetrical systems investigated; (5) Concentration levels utilizable in fixed systems are higher than previously supposed, with 3.5:1 in an asymmetrical reflector being optimal or nearly optimal for the domestic load pattern used in the study: (6) Using a load matched reflector, the amount of storage required to achieve solar fractions of total thermal energy (space heating, water heating, clothes drying) above 90% in a residence appears to be much lower than previously thought.