• Energy Research

Abstract This work discusses a preliminary thermodynamic assessment of three different supercritical CO 2 (sCO 2 ) power cycles applied to a high temperature solar tower system, with maximum temperatures up to 800 °C. The thermal power is transferred from the solar receiver to the power block through KCl-MgCl 2 molten salts as heat transfer fluid, therefore an indirect cycle configuration is considered assuming a surrounded field as the one of Gemasolar plant. The most promising cycle configuration in terms of solar-to-electric efficiency is selected, optimizing the cycle turbine inlet temperature to achieve the best compromise between cycle and receiver performance: the highest efficiency at design conditions is achieved by the Recompression with Main Compression Intercooling (RMCI) configuration with a solar to electric efficiency of 24.5% and a maximum temperature of 750 °C. The yearly energy yield of the proposed power plant is estimated with a simplified approach and results in the range of 18.4%: the performance decay from design to average yearly conditions is mostly due to the optical and thermal efficiencies reduction (−10.8% and −16.4%, respectively).

Abstract This work discusses a preliminary thermodynamic assessment of three different supercritical CO 2 (sCO 2 ) power cycles applied to a high temperature solar tower system, with maximum temperatures up to 800 °C. The thermal power is transferred from the solar receiver to the power block through KCl-MgCl 2 molten salts as heat transfer fluid, therefore an indirect cycle configuration is considered assuming a surrounded field as the one of Gemasolar plant. The most promising cycle configuration in terms of solar-to-electric efficiency is selected, optimizing the cycle turbine inlet temperature to achieve the best compromise between cycle and receiver performance: the highest efficiency at design conditions is achieved by the Recompression with Main Compression Intercooling (RMCI) configuration with a solar to electric efficiency of 24.5% and a maximum temperature of 750 °C. The yearly energy yield of the proposed power plant is estimated with a simplified approach and results in the range of 18.4%: the performance decay from design to average yearly conditions is mostly due to the optical and thermal efficiencies reduction (−10.8% and −16.4%, respectively).

In the near future, a significant increase of the fraction of electricity produced by variable renewable energy sources (wind and PV) is expected. In this scenario, concentrating solar power (CSP) plants, thanks to the implementation of cost-competitive thermal energy storage, can provide zero-emission back-up power improving system stability. Advanced sCO2 cycles are considered a promising option for next generation CSP plants, due to their higher efficiency, compactness of turbomachinery, simpler plant arrangement, no water consumption, high performance at part-load and fast transients. In the present work, a solar tower plant adopting sodium as heat transfer fluid (HTF) and a recompressed sCO2 cycle with a maximum temperature of 700°C as power conversion system is studied. The net cycle efficiency (46%) is maximized and a preliminary design of the heat exchangers is performed. The off-design operation of the cycle is then investigated evaluating the system performance using different operating strategies for two cases: (i) load variation and (ii) ambient temperature variation. The obtained results are compared in terms of cycle efficiency and in terms of compressor operating points, providing useful information on the compressors design.

In the near future, a significant increase of the fraction of electricity produced by variable renewable energy sources (wind and PV) is expected. In this scenario, concentrating solar power (CSP) plants, thanks to the implementation of cost-competitive thermal energy storage, can provide zero-emission back-up power improving system stability. Advanced sCO2 cycles are considered a promising option for next generation CSP plants, due to their higher efficiency, compactness of turbomachinery, simpler plant arrangement, no water consumption, high performance at part-load and fast transients. In the present work, a solar tower plant adopting sodium as heat transfer fluid (HTF) and a recompressed sCO2 cycle with a maximum temperature of 700°C as power conversion system is studied. The net cycle efficiency (46%) is maximized and a preliminary design of the heat exchangers is performed. The off-design operation of the cycle is then investigated evaluating the system performance using different operating strategies for two cases: (i) load variation and (ii) ambient temperature variation. The obtained results are compared in terms of cycle efficiency and in terms of compressor operating points, providing useful information on the compressors design.

Long duration energy storage systems with 8-12 hours of capacity are one of the best options to reduce the increasing curtailment of renewable energy, being able to provide intra-day storage. Among those systems, Carnot batteries operating with CO2 can be a promising solution due to the relatively high round trip efficiencies (up to 60%), being site-independent, including the possibility to store and sell both cold and hot thermal power in addition to electricity. In this work, the detailed sizing of the main components of a CO2 Carnot battery is proposed: in particular, an interesting and promising feature of the system is represented by in the adoption of the same heat exchangers during the charging and the discharging phase, while the cold and hot storage systems are inspired by the commercial solution proposed by Echogen Power Systems. Specifically, the hot storage consists of two heat transfer fluid loops: a pressurized water loop and a diathermic oil loop, requiring two different insulated tanks, whereas the cold storage is based on an ice slurry tank. A routine in MATLAB has been developed to properly design the system and optimize its main variables to maximize the round-trip efficiency and minimize the storage costs, but also including the calculation of the levelized cost of storage. The battery is simulated with a cold storage at 0°C and hot storage between 71.6°C and 293.7°C, with the cycle maximum pressure of 250 bar and a round-trip efficiency of 54.6%. The specific capital cost of the system is 2033 €/kWel,ch, with the largest share being the storage systems. The levelized cost of storage is estimated around 0.1 and 0.3 €/kWh, depending on the electricity selling price, and a relatively large internal temperature difference in the hot storage (20°C) is suggested for these systems. Conference Proceedings of the European sCO2 Conference6th Edition of the European Conference on Supercritical CO2 (sCO2) for Energy Systems: April 09–11, 2025, Delft, The Netherlands, p. 387

Long duration energy storage systems with 8-12 hours of capacity are one of the best options to reduce the increasing curtailment of renewable energy, being able to provide intra-day storage. Among those systems, Carnot batteries operating with CO2 can be a promising solution due to the relatively high round trip efficiencies (up to 60%), being site-independent, including the possibility to store and sell both cold and hot thermal power in addition to electricity. In this work, the detailed sizing of the main components of a CO2 Carnot battery is proposed: in particular, an interesting and promising feature of the system is represented by in the adoption of the same heat exchangers during the charging and the discharging phase, while the cold and hot storage systems are inspired by the commercial solution proposed by Echogen Power Systems. Specifically, the hot storage consists of two heat transfer fluid loops: a pressurized water loop and a diathermic oil loop, requiring two different insulated tanks, whereas the cold storage is based on an ice slurry tank. A routine in MATLAB has been developed to properly design the system and optimize its main variables to maximize the round-trip efficiency and minimize the storage costs, but also including the calculation of the levelized cost of storage. The battery is simulated with a cold storage at 0°C and hot storage between 71.6°C and 293.7°C, with the cycle maximum pressure of 250 bar and a round-trip efficiency of 54.6%. The specific capital cost of the system is 2033 €/kWel,ch, with the largest share being the storage systems. The levelized cost of storage is estimated around 0.1 and 0.3 €/kWh, depending on the electricity selling price, and a relatively large internal temperature difference in the hot storage (20°C) is suggested for these systems. Conference Proceedings of the European sCO2 Conference6th Edition of the European Conference on Supercritical CO2 (sCO2) for Energy Systems: April 09–11, 2025, Delft, The Netherlands, p. 387

Abstract This paper presents the definition of a detailed optimization model for planning the operation of a standalone hybrid microgrid. The mathematical model is expressed in a general way in order to handle a large number of microgrid configurations in which the demand of different goods (storable or not) has to be fulfilled, as AC and DC electricity, heat, cooling, drinkable water and ice production. The operation of the microgrid is defined by a rolling-horizon strategy that includes solving of a mixed integer linear problem. The goal of the optimization problem is to define the programmable units schedule that leads to the minimum operating cost, ensuring that the demand of each good is fulfilled and exploiting the forecast of production and consumption of non-programmable units over a certain time horizon. The model handles multi-input and multi-output programmable units, considering part-load performance curves, start-up penalizations and several other operation constraints. To demonstrate the model ability, the paper addresses two examples of microgrid configuration: the first test case shows the operation cost reduction attainable with this strategy in comparison with commonly adopted heuristic dispatch strategies; the second one demonstrates the capability to manage a complex system, provided with an Organic Rankine Cycle power system powered by both a biomass boiler and a solar Fresnel collector.