Dynamic optimization of a hybrid solar thermal and fossil fuel system Kody M. Powell a,⇑ , John D. Hedengren b , Thomas F. Edgar c a The University of Texas at Austin, McKetta Department of Chemical Engineering, 200 E. Dean Keeton St., Stop C0400, Austin, TX 78712-1589, United States b Brigham Young University, Department of Chemical Engineering, United States c The University of Texas at Austin, McKetta Department of Chemical Engineering, United States Received 24 August 2013; received in revised form 10 May 2014; accepted 8 July 2014 Communicated by: Associate Editor Halime Paksoy Abstract 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. Ó 2014 Elsevier Ltd. All rights reserved. Keywords: Dynamic optimization; Hybrid solar thermal systems; Thermal energy storage 1. Introduction Solar energy has tremendous potential to produce emis- sion-free electricity (Zhang et al., 2013). With similarities to conventional power generation methods, solar thermal, or concentrated solar power (CSP), can be a low-cost alterna- tive to fossil-fuel-based systems (Barlev et al., 2011). In order to provide reliable base load power, however, CSP systems must be equipped with large-scale thermal energy storage (TES) and/or a backup energy source, such as natural gas or diesel fuel (Kuravi et al., 2013; Zhang et al., 2013). Because of the intermittency of solar energy, it generally must rely on other energy technologies to ensure that con- sumer demand for power is always met. Hybrid systems, which combine solar thermal and other energy technolo- gies, have been proposed as an alternative to solar-only power generation (Peterseim et al., 2013). For instance, solar thermal power can be combined with conventional power generation technology so that solar energy is aided by proven power generation technology, such as gas and steam turbines (Jamel et al., 2013). When gas and steam turbines are used in tandem, solar energy can be used to http://dx.doi.org/10.1016/j.solener.2014.07.004 0038-092X/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +1 512 471 5150; fax: +1 512 471 7060. E-mail address: kody.powell@utexas.edu (K.M. Powell). www.elsevier.com/locate/solener Available online at www.sciencedirect.com ScienceDirect Solar Energy 108 (2014) 210–218