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Supercritical CO2 Heat Pumps and Power Cycles for Concentrating Solar Power Joshua D. McTigue,1, a) Pau Farres-Antunez2, Ty Neises1 and Alexander White2 1 National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, CO, USA, 80401 2 Cambridge University Engineering Department, Trumpington Street, Cambridge, UK, CB2 1PZ a) Corresponding author: JoshuaDominic.McTigue@nrel.gov Abstract. Pumped Thermal Energy Storage (PTES) is a promising technology for electricity storage applications. Grid electricity drives a heat pump which moves energy from a cold space to a hot space, thereby creating hot and cold thermal storage. The temperature difference between the storage is later used to drive a heat engine and return electricity to the grid. In this article, supercritical carbon dioxide (sCO2) is chosen as the working fluid for PTES, and results are compared to ‘conventional’ systems that use an ideal gas. Molten salts are used for the hot storage which means that a CSP plant with thermal storage and an sCO2 power cycle could potentially be hybridized with PTES by the addition of a heat pump. This article describes some of the benefits of this combined system which can provide renewable power generation and energy management services. Two methods by which an sCO2 heat pump can be combined with an sCO2 power cycle for CSP are described and techno-economic results are presented. Results indicate that these systems can achieve reasonable technical performance, but that costs are currently high. INTRODUCTION Pumped Thermal Energy Storage (PTES) is a grid-scale energy storage device that stores electricity in a thermal potential between hot and cold media. PTES has been investigated globally (under a variety of names, such as a Carnot Battery) and is receiving widespread commercial interest. PTES has several advantages compared to other electricity storage devices, including no geographical restrictions, long lifetimes, and the ability to use cheap, abundant, non- toxic materials as the storage media. PTES may use a variety of different power cycles, working fluids, and thermal storage systems. Commonly discussed concepts include Brayton cycles with packed beds or concrete storage [1–3], recuperated Brayton cycles with molten salt storage [4], and transcritical carbon dioxide cycles with liquid and ice storage [5,6]. PTES and Concentrating Solar Power (CSP) systems both use similar components such as high temperature thermal storage and power cycles. This work aims to describe systems that hybridize PTES with CSP; components are shared between the two systems to reduce costs, and the combined system can generate both renewable electricity and provide electricity storage services. Previous work has described how PTES can be integrated with conventional CSP plants that use a steam Rankine cycle [7], as well as methods of using a heat pump to enhance the power output of low-temperature solar systems [8]. A growing body of work is exploring whether existing fossil fuel generating plants can be retrofitted with high- temperature heat pumps and thermal storage in order to give a ‘second lease of life’ to existing infrastructure [9]. Thus, the integration of heat pumps and thermal storage with power generation systems is a promising concept for flexible power generation and consumption. At last year’s solarPACES conference, supercritical carbon dioxide (sCO2) PTES cycles for CSP integration were introduced, and these cycles were found to potentially have several advantages compared to PTES using ideal gases, such as large power densities and high round-trip efficiencies [10]. In this article, refinements are made to the computational models that improve the accuracy and detail of results. Technical and economic results of sCO2-PTES cycles that are integrated with CSP plants are presented. 1 This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.PDF Image | Supercritical CO2 Heat Pumps Concentrating Solar Power
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