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This report describes how solar system tanks fail to stratify, a new solar system control strategy that allows stratification, a one-dimensional analytical model of thermally stratified tanks, experimental measurement of thermal stratification in tanks, correlation of experimental measurements with empirical constants in the analytical model, and a procedure for designing thermally stratified tanks. Failure to stratify is explained in terms of the critical Richardson number. The key to the new control strategy is to avoid a Richardson number that decreases during solar collection. The analytical model is an approximate solution based on assumptions that (1) the solution is a function of elevation and time, only, (2) plug flow exists, (3) flow rate is constant, (4) the cross-sectional areas of the tank and tank wall are constant, (5) there is a step change of inlet temperature, (6) there is heat transfer between the tank wall and the water, and (7) thermal losses from the tank are negligible. Empirical constants in the theory are determined by adjusting them until the best least-squares fit with experimental data is obtained and correlating the constants with the Fourier and Richardson numbers. The new control strategy allows tanks to stratify and reduces the average collector operating temperature by as much as 20/sup 0/C. The result is up to 20% improvement in system performance. The analytical model and correlation of its empirical constants allow system designers to select from several alternatives the tank configuration that provides the best stratification. |
