Dynamic optimization of a hybrid solar thermal and fossil fuel system

by Kody M. Powell, John D. Hedengren and Thomas F. Edgar

Solar Energy 2014 108, 210-218

This work illustrates the synergy that exists between solar thermal and fossil fuel energy systems. By adding degrees of freedom and optimizing the system, more solar energy can be harvested by operating in a “hybrid” mode, where a portion of the demand is met by solar energy, with the remainder provided by a supplemental fuel, such as natural gas. This requires allowing temperatures in the solar field and storage tanks to vary, permitting the system to meet the demand by a combination of solar and fossil energy, rather than one or the other, and by allowing the heat transfer fluid to bypass storage. The addition of thermal energy storage provides the opportunity for dynamic optimization, where the degrees of freedom can be exploited over the entire time horizon to yield optimal results: maximizing the total amount of solar energy harvested. The problem is solved using a simultaneous solution method that concurrently minimizes the objective function and solves the system’s constraints. This methodology is demonstrated on a parabolic trough solar thermal plant with a two-tank-direct thermal energy storage system. Results show that 9% more solar energy can be harvested on a sunny day by using this methodology. On a day with intermittent sunlight, 49% more solar energy can be harvested with the same system. Dynamic optimization enables more cost effective solar integration in areas with lower or intermittent sources of solar incidence.

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