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the next generation of CSP plants. Integrating PTES with a CSP plant could have several advantages, as the hybrid plant would be able to generate renewable power as well as providing electricity storage services. In this section, PTES cycles with a supercritical-CO2 working fluid are considered. These systems may either form a stand-alone PTES system, or could be integrated into a CSP plant that uses an sCO2 power cycle. In the latter case, molten salt storage tanks could be charged either by solar heat or electricity that is used to drive an sCO2-based heat pump. Thus, the hot storage tanks are shared between the CSP system and the PTES system. Furthermore, the same sCO2 recompression power cycle is used to convert the stored thermal energy into electricity. Since the CSP and PTES systems share several key components, this hybrid system should require a lower capital investment than two separate systems. is simply the (1) (2) expression indicates that high work ratios are preferable: a low work ratio requires more work to be βprocessedβ to provide the required net work. Thus, high work ratios reduce the sensitivity of a cycle to compression and expansion irreversibilities. charge, such that Supercritical-CO2 PTES cycles and their hybridization with CSP was introduced at the 2019 SolarPACES conference [10], and this paper provided a simplified analysis and considered several key performance indices, such as the round-trip efficiency, the work ratio, and the heat-to-work ratio. The round-trip efficiency, ππ work recovered during discharge as a fraction of the work input during charge: ππππ ππππ ππchg ππ =ππdis The work ratio is the ratio between the compressor worππk input and the expander work output during charge: ππ =ππ π π comp exp The work ratio can be rewritten in terms of the net charging work [12], such that ππnet = (πππ π β 1)ππexp. This The heat-to-work ratio is a similar metric that quantifies the heat that is processed per unit work input during ππ π π = | ππ i n | ππ+ | ππ o u t | ( 3 ) chg Note, that this metric involves all heat exchange processes in and out of the cycle, and also within the cycle (e.g. within recuperators). High heat-to-work ratios imply that large quantities of heat must be exchanged for a given work input, and that the cycle will be more sensitive to heat transfer irreversibilities. The previous study found that sCO2-PTES cycles had relatively high work ratios and high heat-to-work ratios compared to βconventionalβ ideal-gas PTES. That is, sCO2-PTES was less sensitive to turbomachinery inefficiencies, but more sensitive to heat exchanger losses. Furthermore, the study found that sCO2-PTES could potentially achieve very high round-trip efficiencies but that this was contingent on minimizing temperature differences within the heat exchangers. The thermodynamic models have been substantially improved, and updated results are presented in this section. The previous study modelled turbomachinery with isentropic efficiencies and heat exchangers with a fixed temperature difference between the two fluids. This second assumption limited the accuracy of the study since the exchanger performance in terms of an effectiveness ππ and a pressure loss fraction ππ . The variable properties of the ππ properties of sCO2 can vary significantly over a given temperature range. The improved models now define heat working fluid and storage fluids are included and the model calculates the temperature-heat profile in the heat exchanger such that the required effectiveness is obtained. The turbomachinery is modelled using polytropic efficiencies ππ. Other updates to the model include the calculation of parasitic losses that arise due to liquid pumps, air fans, motors and generators. The economic performance is also evaluated. Several capital cost correlations for each component were gathered from a variety of sources, including [13β19]. The levelized cππost of storage (LCOS) is calculated using the fixed charge rate (FCR) method [20], where the LCOS is defined as LCOS = FCR β Ccap + O&M + πΈπΈpriceππin (4) out Where FCR is the fixed charge rate, πΆπΆcap is the capital cost, O&M is the annual operations and maintenance cost, πΈπΈprice is the electricity price, ππin is the annual electricity into the system, and ππout is the annual electricity delivered by the system. The capital cost, LCOS and the uncertainty of these terms are evaluated using a Monte Carlo approach, 3 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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