(257e) Flexible Heat Integration to Enhance Solar Energy Utilization in a Hybrid CSP Plant

Authors: 
Ellingwood, K., University of Utah
Powell, K. M., University of Utah
Concentrated solar power (CSP) represents a potential means to harness and dispatch solar energy on a large, centralized level. However, like other renewable energy sources, CSP must deal with issues of transiency and flexible operation if it is to be competitive in the modern power grid. Due to its ability to hybridize with fossil fuels through equipment commonality and integrate thermal energy storage (TES), these issues can be mitigated. Further penetration can be achieved by employing holistic automation of CSP systems to leverage TES and hybrid operation. This work proposes a novel operation scheme, referred to here as flexible heat integration (FHI), to enhance solar energy utilization in a solar central receiver—Rankine cycle (SCR-RC) power plant hybridized with natural gas. Typically, in an SRC-RC, concentrated solar energy heats a molten salt and dispatches the salt to a series of heat exchangers in the following order: the steam superheater, steam generator, and feedwater preheater. Under variable conditions (low- and medium-grade solar activity), maintaining the high salt temperatures needed for steam superheating can in result dips in collector thermal efficiency and by extension, diminished solar energy utilization. Instead, FHI explores dispatching solar energy independently to the thermal sinks based upon solar activity rather than in sequence while having hybrid operation via natural gas firing make up for the bypassed energy sinks. For example, during low solar activity, solar energy can be more efficiently collected and stored at a lower temperature and then dispatched to the correspondingly low-temperature feedwater solar preheater while gas firing generates and superheats steam. Likewise, during medium-grade solar activity, a solar salt heated to a medium-level temperature can dispatched to the steam generator and subsequently the feedwater preheater while gas superheats steam. Under such a design, additional TES tanks are needed beyond the conventional two-tank system seen in current SCR-RC power plants. This study is conducted to detail the fundamentals of FHI and how it can be employed to further improve solar energy utilization within a hybrid SCR-RC system thereby increasing CSP penetration and reducing greenhouse emissions relative to the standard SCR-RC power plant.