Modeling of Solar Storage Tanks

Abstract

In solar system simulations, the user wants fast yet reasonably accurate calculations. A new tank model for TRNSYS has been written to include features such as: conduction along the tank wall, internal heat exchangers, internal time steps, static heat at inlet and outlet pipes, tanks of non-cylindrical cross section, components entered as heights, and automatic tank mass balances. The energy balance equations in the model are solved numerically using the Crank-Nicolson solution technique. A side-by-side comparison of the Crank-Nicolson, Euler, analytical, and other solution techniques is presented. The comparison shows both the solution method's accuracy and relative speed. The new model uses internal time steps, which can vary in size. The model takes the largest time step possible without exceeding a fraction of the critical Euler time step. The fraction of critical Euler time step is entered by the user. Internal time steps also allow the model to cycle auxiliary heaters on and off at full power within the TRNSYS time step in a manner that represents reality. The new model includes the option for internal heat exchangers. Heat exchangers may be modeled from curve fits of experimental data, or a generalized heat transfer correlation may be used. Although the geometries of internal heat exchangers can vary, a reasonable result is still obtained using a generalized correlation. Model results are compared against experimental results obtained from literature. A good match occurs in most situations. The FORTRAN computer code is listed in the appendix, along with other supporting data.