Abstract
This paper concerns the ongoing studies on a Concentrated Solar Power (CSP) plant in operation in Ottana (Italy), comprising a 629 kW organic Rankine cycle (ORC) unit fed by a linear Fresnel solar field. Hexamethyldisiloxane (MM) and “Therminol SP-I” are used respectively as ORC working fluid and heat transfer fluid in the solar receivers. A two-tank direct Thermal Energy Storage (TES) system is currently integrated in the CSP plant, serving as a direct interface between solar field and ORC. With the view of improving the solar facility, two alternative TES configurations were proposed in this study: a one-tank packed-bed TES system using silica as solid storage media and another similar one including encapsulated phase-change material (molten salt). Comprehensive mathematical models were developed for simulating daily behaviour as well as for assessing yearly performance of the various TES technologies. Furthermore, a preliminary economic analysis was carried out. Results showed poorer response of the one-tank TES system to large fluctuations in the ORC inlet fluid temperature, leading to reduction in the mean ORC efficiency (18.2% as against 19.7% obtained with the two-tank TES). Conversely, higher energy storage density and lower thermal losses were obtained adopting the one-tank TES, resulting in about 5% more annual solar energy yield. Invariably, equivalent annual ORC energy production of 0.92 GWh/year was obtained for the three TES configurations. Additionally, adopting a one-tank TES system meant that the purchase costs of a second tank and its storage medium (thermal oil) could be saved, resulting in investment costs about 45% lower and, ultimately, levelized cost of storage about 48% lower than what obtains in the two-tank TES system.
Highlights
Some controversies still exist regarding the capabilities of renewable energy systems to fully and sustainably satisfy global energy needs someday, the critical roles the renewables will play in the future energy mix are never in doubt
It would be imperative to shut down a concentrated solar power (CSP) plant when solar irradiation is unavailable or insufficient to operate the system, thereby increasing the system down-time, whereas surplus solar energy is often wasted at other times when it exceeds the nominal value
Research activities are in progress towards optimal design/selection of thermal energy storage (TES) systems for real CSP plants, and this generally informed the relevance of the study being reported in this research article
Summary
Some controversies still exist regarding the capabilities of renewable energy systems to fully and sustainably satisfy global energy needs someday, the critical roles the renewables will play in the future energy mix are never in doubt. If CSP systems would be operated optimally to play their intended roles in the sustainable energy infrastructure, the limiting challenges should be minimized For this reason, CSP designs usually integrate thermal energy storage (TES) systems,[11,12,13] which serve to accumulate excess solar thermal energy during surplus times for deferred usage in times of insufficiency.[14] In addition, to mitigate the fluctuating effects of solar irradiation on CSP-based systems, TES systems serve the purpose of increasing the proportion of solar energy being exploited, thereby maximising the overall investment.[15,16] the practical deployments of TES in real CSP plants often attract higher degrees of freedom for designers, with attendant reductions in economic efficiencies of the integrated systems. Research activities are in progress towards optimal design/selection of TES systems for real CSP plants, and this generally informed the relevance of the study being reported in this research article
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