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Economic results are also presented in Table 3. These values assume that a CSP plant has already been installed with an RC-sCO2 power cycle and molten salt storage. Therefore, the cost only includes the cost of the medium temperature and cold storage, the charging expander, a heat rejection unit, a motor, and additional pumping during discharge. It is assumed that solar heat is stored at the same time that the charging heat pump operates: if it is economically preferable to store electricity, then it is unlikely that the discharging power cycle would dispatch electricity to the grid. As a result, the solar heat in the molten salt storage is assumed to be โfreeโ and does not incur a cost. Thus, the energy output from the system (used in the LCOS calculation) considers all the electricity generated during discharge. Consequently, the TSRC-sCO2 cycle achieves very low energy storage costs and this system appears to be competitive with other electricity storage systems. A thorough economic assessment should also consider the cost of the CSP system, sCO2 power block, and molten salt thermal storage, as well incorporating the renewable electricity dispatched to the grid. Calculating the value that this combined generation and storage system provides to the grid would provide a more meaningful evaluation of its potential than just considering the cost of the components. TABLE 3: Results comparing the performance of a conventional sCO2 recompression cycle with a time-shifted recompression sCO2 cycle ฬ Charging power input, ๐๐chg net ฬ sCO2 kJ/kg - kJ/kg 100.0 % 45.5 %- % 45.5 $/kWhe - $/kWhe - TSRC 15.8 107.8 49.0 72.9 41.8 100.8ยฑ30 0.032ยฑ0.011 Discharging power output, ๐๐dis Heat engine efficiency, ๐๐HE net Exergetic roundtrip efficiency, ๐๐๐๐๐๐,๐ฅ๐ฅ Net efficiency, ๐๐net Capital cost per unit energy discharged LCOS CONCLUSIONS In this article, Pumped Thermal Energy Storage (PTES) based on supercritical carbon dioxide (sCO2) Brayton cycles are described. Previously reported results have been updated with an improved modelling methodology that provides a techno-economic assessment of these devices. For the nominal cases studies, sCO2-PTES is found to have a lower round-trip efficiency (52.6%) and higher levelized cost of storage (LCOS = 0.27 ยฑ 0.10 $/kWhe) than PTES using an ideal gas which achieves 58.2% and 0.14 ยฑ 0.03 $/kWhe, respectively. The lower efficiencies are predominantly the result of large losses in the heat exchangers due to the variable thermal properties of sCO2. The sCO2-PTES cost may reduce as sCO2 technologies are advanced and commercialized. This study did not conduct an exhaustive design optimization investigation, and further improvements to PTES performance may be obtainable. SCO2-PTES systems have several components in common with next generation Concentrating Solar Power (CSP) plants, namely molten salt thermal storage and an sCO2 power cycle. A hybrid plant that combines PTES with CSP may be able to provide several value streams at lower cost than separate systems. Two methods of hybridizing CSP with PTES are described. The first involves using an sCO2 heat pump to charge the molten salt storage of a CSP plant. The second is known as a โTime-Shifted Recompressionโ sCO2 power cycle. In this concept a heat pump replaces the recompressor in a conventional sCO2 recompression power cycle. This system is found to have a favorable thermodynamic performance with an exergetic round-trip efficiency of 72.9% and a low LCOS of 0.032ยฑ0.011 $/kWhe โ although this value can only be obtained under an optimistic set of assumptions. 8 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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CO2 Organic Rankine Cycle Experimenter Platform The supercritical CO2 phase change system is both a heat pump and organic rankine cycle which can be used for those purposes and as a supercritical extractor for advanced subcritical and supercritical extraction technology. Uses include producing nanoparticles, precious metal CO2 extraction, lithium battery recycling, and other applications... More Info
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