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Electricity storage is becoming one of the main challenges for the massive deployment of renewables. Thermal energy storage is gaining momentum, with molten salts-based systems being the state-of-the-art technology. As an alternative, Thermochemical Energy Storage (TCES) is a promising system that can increase the system performance in terms of energy storage density, maximum heat discharge temperature and long-term storage capacity. This work proposes and preliminary evaluates the performance of a novel modular Thermochemical Energy Storage (TCES) system based on the Calcium-Looping (CaL) process. The modularity allows its integration with multiple renewable energy sources into different high-temperature applications, including hard-to-decarbonise applications. Heat for the charging stage is provided by electrical heaters connected either to a Photovoltaic (PV) facility or directly to the grid. Limestone particles are heated, and once they reach their decomposition temperature (∼ 930 °C at 1 bar), the released CO 2 is removed from the reactor, cooled, compressed, and stored. When the stored energy discharge is required, CO 2 is supplied to the reactor to produce calcium carbonate and release the stored heat (∼ 400–875 °C). In the proposed concept, the solids remain in the reactor, and CO 2 is added or removed depending on the stage. As cases of study of the concept, energy storage systems integrated with PV plants of 0.5 and 20 MW e and grid-based storage are considered. An hourly simulation throughout the year is performed. Results show an energy density above 1 GJ/m3, higher than current molten-salt-based systems. The levelized energy storage cost is below 100 €/MWh for the grid-based storage, highlighting its potential to contribute significantly to the global energy transition. • A novel modular thermochemical electricity storage system is proposed. • The levelized storage cost ranges from approximately 85–190 €/MWh. • All analysed cases exhibit a high energy storage density (∼ 1 GJ/m3). • The implementation of an energy arbitrage strategy proves to be cost-effective. [ABSTRACT FROM AUTHOR]

This paper provides a review of negative carbon capture technologies. Based on these technologies, here it is proposed an innovative negative emissions power plant combining the generation and storage of energy from biomass, photovoltaic, and concentrated solar power, capturing and recovering CO 2 by producing H 2 or CH 4 as green energy carriers. The main features of the system are i) large-scale energy production system with negative CO 2 emissions; ii) 100% renewable system based on biomass and solar energy with the possibility of integrating other renewables; iii) synergistic integration of processes and systems; iv) recovery of O 2 generated by photovoltaic-driven electrolysis within the process of partial biomass oxycombustion and v) solar-driven limestone calcination. A detailed model of the entire plant is developed to evaluate the integration of the process. The model performance is assessed on an hourly basis throughout the whole year. The base case results show an energy consumption from 1 to 2.1 MJ/kg CO 2 to capture 60–77% of CO 2 emitted from the biomass plant and green methane production of more than 7500 tons/year. The negative emissions associated with the process are -612 kg CO 2 /MWh. It justifies the interest in the proposed negative emissions power plant. [Display omitted] • A synergetic integration of CO2 capture, energy storage and methane production is proposed. • A biomass power plant is considered, resulting in -612 kg CO2/MWh negative emissions. • Solar PV and CSP are integrated into the process to reduce the SPECCA to 1–2.1 kg CO2/MWh. • A large amount of solids storage is required to achieve 70–90% CO2 capture capacity. [ABSTRACT FROM AUTHOR]

• Solar photovoltaics stored by using cheap and abundant materials. • The Calcium-Looping process is integrated as electricity stored system (40% efficiency) • Electric-to-electric performance of the whole system reaches 57%. • A notable cost reduction could be achieved as alternative to batteries. Solar photovoltaic plants are today a competitive alternative to power plants based on fossil fuels. Cost reduction in photovoltaics modules, scalability and ease of installation of these plants are enabling a rapid worldwide expansion of the technology. Nevertheless, dispatchability still remains as the major challenge to overcome due the intrinsic variability of solar energy. Most of the current solar photovoltaic facilities at large scale lack energy storage while those with storage systems rely on expensive batteries. Batteries are based on elements such as nickel, lithium or cadmium whose scarcity hinder their sustainability for storing energy in the large scale. This manuscript presents a novel concept to integrate thermochemical energy storage in photovoltaic plants. Furthermore, the concept is also directly adaptable to wind power plants to store surplus energy. The paper analyses the suitability of the Calcium-Looping process as thermochemical energy storage system in solar photovoltaics plants. The system works as follows: part of the power produced in the solar plant provides electricity to the grid while the rest is used to supply heat for calcination of calcium carbonate. After calcination, the products of the reaction – calcium oxide and carbon dioxide- are stored separately. When power production is required, the stored products are brought together in a carbonation reactor wherein the exothermic reaction releases energy for power production. The overall system is simulated to estimate the process behaviour and results show that storage efficiencies of ∼40% can be achieved. Moreover, an economic analysis is developed to compare the proposed system with batteries. Due to the low price of natural calcium oxide precursors, such as limestone, and the expected longer lifetime of equipment as compared to batteries, the Calcium-Looping process can be considered as a potential alternative for improving dispatchability in solar photovoltaic plants. [ABSTRACT FROM AUTHOR]

The Calcium-Looping process is a promising thermochemical energy storage method based on the multicycle calcination-carbonation of CaCO 3 -CaO to be used in concentrated solar power plants. When solar energy is available, the CaCO 3 solids are calcined at high temperature to produce CaO and CO 2 , which are stored for subsequent utilization. When power is needed, these reaction by-products are fed into a carbonator reactor where energy is released from the exothermic carbonation reaction. In comparison with currently commercial energy storage systems, such as solar salts, the Calcium-Looping process presents several benefits such as the feasibility to work at significantly higher power cycle temperatures, a higher energy storage density and the possibility to store energy in the medium-long term. The present manuscript analyzes a number of novel Calcium-Looping configurations for energy storage combined with CO 2 cycles in a solar tower plant. The high overall efficiencies achieved (32–44%, defined as the ratio of net electric power production to net solar thermal power entering the calciner) indicate a potential interest for the integration of the Calcium-Looping process in Concentrating Solar Power Plants, although major technological challenges related to the design of the solar receiver and of the high temperature solids handling devices remain to be faced. [ABSTRACT FROM AUTHOR]

Efficient, low-cost and environmentally friendly storage of thermal energy stands as a main challenge for large scale deployment of solar energy. This work explores the integration into concentrated solar power plants of the calcium looping process based upon the reversible carbonation/calcination of calcium oxide for thermochemical energy storage. An efficient concentrated solar power-calcium looping integration would allow storing energy in the long term by calcination of calcium carbonate thus overcoming the hurdle of variable power generation from solar. After calcination, the stored products of the reaction (calcium oxide and carbon dioxide) are brought together in a carbonator reactor whereby the high temperature exothermic reaction releases the stored energy for efficient power production when needed. This work analyses several power cycle configurations with the main goal of optimizing the performance of the overall system integration. Possible integration schemes are proposed in which power production is carried out directly (using a closed carbon dioxide Brayton power cycle) or indirectly (by means of a steam reheat Rankine cycle or a supercritical carbon dioxide Brayton cycle). The results obtained show that the highest plant efficiencies (up to 45–46%) are achievable using a closed carbon dioxide Brayton power cycle. [ABSTRACT FROM AUTHOR]

The Calcium-Looping (CaL) process, based on the multicyclic calcination-carbonation of CaCO 3 /CaO, is considered a promising Thermochemical Energy Storage (TCES) technology to be integrated into Concentrating Solar Power (CSP) plants. This work proposes a novel CaL integration that operates at low-pressure calcination under pure CO 2 and a moderated temperature. Low-pressure calcination (0.01 bar) provides a suitable solution to mitigate CaO sintering and its consequent loss of reactivity in the carbonation stage. Since the temperature for quick calcination in a pure CO 2 atmosphere is decreased (from around 950 °C at 1 bar to 765 °C at 0.01 bar), the energy losses at the receiver are minimised. In addition, a reduced calcination temperature allows for the use of metallic receivers already tested at the MW-scale, which significantly increases the CSP-CaL integration reliability. Moreover, multicycle CaO reactivity is promoted in short residence times, allowing the use of a simpler reactor design. Furthermore, there is an increase of 85% in the energy storage density of the system. The proposed plant proposes a smooth integration of the CaL process in CSP plants, with a moderate storage level and supported by a natural gas backup system (solar share higher than 50%). The results show that the solar thermal-to electric efficiency is above 30%. [ABSTRACT FROM AUTHOR]

• The proposed SCC-TCES allows boost the solar share in combined cycles above 70%. • Receiver thermal-to-electric efficiency are in the range 45–50%. • Annual performance is evaluated through four solar radiation clusters from real data. • A 360° heliostats solar field with three cavity receivers (200 m tower) is designed. The present work proposes integrating a high-temperature thermochemical energy storage cycle to boost the solar contribution in solar combined cycles. The main feature of the plant is the possibility of storing solar energy at a very high temperature and releasing it on demand to drive the combined cycle in the absence of solar radiation. Based on the reversible calcination-carbonation of CaCO 3 /CaO, the Calcium-looping process is proposed since it allows power production above 900 °C by using cheap, non-toxic and widely available raw materials (i.e. limestone or dolomite). Based on an air-open and a CO 2 -closed combined cycle, two potential configurations are modelled and analysed, including designing a 360° solar field with a 200-meter tower. The novel solar combined cycle analyzed in the present work enhances the annual solar share above 50%, whilst the current state-of-the-art technology is below 15%. From actual solar irradiation data and clustering analysis, results show overall plant efficiencies over 45% (considering off-design performance) with a very high dispatchability, which justifies the interest in further developing this novel cycle. [ABSTRACT FROM AUTHOR]

• A fully novel High Temperature Storage Solar Combined Cycle (HTSSCC) is proposed. • Thermochemical energy storage allows a 24 h operation without fuel input. • The solar share in the solar combined cycle is highly enhanced (ideally up to 100%). • The base case analysed leads to a net solar-to-electric efficiency of 44.5%. The integration of Concentrating Solar Power (CSP) in combined cycles is a subjects of increasing attention. Combined cycles require high temperature at the gas turbine inlet (typically over 1000 °C), which hinders plant operation in the absence of direct solar radiation using currently commercial storage technologies based on molten salts (with a temperature limit around 600 °C). Thus, solar power share in current Integrated Solar Combined Cycles (ISCC) is typically lower than 20%, while most of the thermal power required is provided by natural gas. The present manuscript proposes the integration in combined cycles of a Thermochemical Energy Storage (TCES) system based on the Calcium-Looping process, which can release the stored energy at temperatures above 1000 °C. The storage charging step uses the heat provided by a CO 2 stream previously heated in a high-temperature solar receiver. The configuration of the solar receiver-calciner is fundamental to determine the amount of storable energy. Results from the conceptual model simulation predict overall plant efficiencies above 45% (excluding solar side losses), suggesting a high potential for the development of this novel integration that would allow enhancing the solar share in combined cycles. [ABSTRACT FROM AUTHOR]

• Validated linear optimisation model for calcium-looping process integrated with CSP. • Multi-objective operational optimisation for hybrid solar power plants with TCES. • Multi-objective optimisation is key to balance competitiveness and dispatchability. • Configurations described as case studies can achieve capacity factors around 70%. • Affordable full-dispatchability possible in locations with excellent solar resource. Energy storage is key to decarbonising the energy sector by reducing intermittency and increasing the integration of renewable energy. Thermochemical energy storage (TCES) integrated with concentrated solar and photovoltaic power plants, has the potential to provide dispatchable and competitive energy. Here we develop a multi-objective optimisation framework to find the best operational strategy of a hybrid solar power plant with a TCES system. The model uses a typical meteorological year to optimise one-year hourly operation. The results demonstrate that the integration of a calcium-looping process as TCES in a concentrated solar power plant provides dispatchability and, when hybridised with photovoltaic, enhances its competitiveness with current electricity prices. The low mismatch between supply and demand, even when a fixed commitment is required throughout the year, together with a high overall efficiency, indicates that the integration of calcium-looping in hybrid solar power plants is an opportunity to increase the penetration of solar energy in the power sector. Through the optimisation framework presented, a seasonal energy storage analysis can be developed, although a second optimisation stage is required to improve the sizing of the main components of the system in order to further reduce the energy costs. [ABSTRACT FROM AUTHOR]

Energy storage based on thermochemical systems is gaining momentum as a potential alternative to molten salts in Concentrating Solar Power (CSP) plants. This work is a detailed review about the promising integration of a CaCO 3 /CaO based system, the so-called Calcium-Looping (CaL) process, in CSP plants with tower technology. The CaL process relies on low cost, widely available and non-toxic natural materials (such as limestone or dolomite), which are necessary conditions for the commercial expansion of any energy storage technology at large scale. A comprehensive analysis of the advantages and challenges to be faced for the process to reach a commercial scale is carried out. The review includes a deep overview of reaction mechanisms and process integration schemes proposed in the recent literature. Enhancing the multicycle CaO conversion is a major challenge of the CaL process. Many lab-scale analyses carried out show that residual effective CaO conversion is highly dependent on the process conditions and the CaO precursors used, reaching values in a wide range (0.07–0.82). The selection of the optimal operating conditions must be based on materials performance, process integration, technology and economics aspects. Global plant efficiencies over 45% (without considering solar-side losses) show the interest of the technology. Furthermore, the technological maturity and potential of the process is assessed. The direction towards which future works should be headed is discussed. • Energy storage based on limestone, one of the most abundant materials on Earth. • Process integration schemes with efficiencies up to 45–46%. • Multicycle CaO conversion depends on process conditions and CaO precursor. • Process equipment well-known in the cement industry, excepting solar calciners. [ABSTRACT FROM AUTHOR]

Energy storage is a key factor in the development of renewables-based electrical power systems. In recent years, the thermochemical energy storage system based on calcium-looping has emerged as an alternative to molten salts for energy storage in high-temperature concentrated solar power plants. This technology still presents some challenges that could be solved by integrating the thermochemical energy storage system based on calcium hydroxide. This work studies a novel concentrated solar power system integrating calcium-looping and calcium hydroxide thermochemical energy storage systems. The results show that the combined use of hydration-dehydration cycles in the calcination-carbonation processes of the calcium looping for energy storage could partially solve the issue related to the multicyclic deactivation of calcium oxide. The improvement in the conversion of calcium oxide during carbonation is demonstrated experimentally when hydration-dehydration cycles are combined. Numerical simulations demonstrate the technical feasibility of the integrated process, with efficiencies ranging between 38-46%, improved with the increase in calcium oxide conversion in the carbonator, showing the potential of the proposed integration. • Novel integration of thermo-chemical energy storage systems based on CaO. • High temperature calcium looping as main high-efficiency energy storage system. • Medium temperature calcium hydroxide integrated as secondary energy storage system. • Dehydration-hydration processes enhance CaO multicyclic activity in carbonation. • Combined joint operation with roundtrip efficiencies exceed 45%. [ABSTRACT FROM AUTHOR]

Thermochemical energy storage is attracting interest as a relevant alternative energy storage system in concentrating solar power plants. Efficient, low-cost, and environmentally friendly thermal energy storage is one of the main challenges for the large-scale deployment of solar energy. The reversible hydration/dehydration process of calcium oxide is one of the most promising concepts for energy storage integration at intermediate temperatures in solar plants. The efficient integration of concentrated solar power with a thermochemical energy storage system based on the calcium hydroxide concept, individually or integrated into a hybrid system with sensible heat storage, can be a feasible solution for long-term energy storage. Efficient energy recovery and subsequent power production are crucial. This work presents a novel analysis of the indirect integration of different power cycle configurations to optimise the roundtrip efficiency of the system. Steam Rankine, closed CO 2 Brayton, and organic Rankine cycles are considered. The analyses show power block efficiencies in the range of 38–50%, with a global roundtrip efficiency of 37.1% in the case of the CO 2 supercritical cycle. [ABSTRACT FROM AUTHOR]

Thermochemical energy storage (TCES) is considered a promising technology to overcome the issues of intermittent energy generation in Concentrated Solar Power (CSP) plants and couple them with yearly electricity demand. The development of this technology could favor the commercial deployment of CSP, which is considered as a key factor for new challenges in reducing GHG emissions. Among other possibilities, using the Calcium Looping (CaL) process for TCES is an interesting choice mainly due to the low cost of natural CaO precursors such as limestone (below $10/ton) and the high energy density that can be achieved (around 3.2 GJ/m 3 ). This manuscript explores several configurations in order to maximize the performance of the CSP-CaL integration with the focus on power cycle integration in the carbonator zone. For this purpose, firstly, a discussion about the possibility of using open and closed power cycles is carried out, which leads to the conclusion that a CO 2 closed cycle is more appropriate. Then, a closed regenerative CO 2 Brayton cycle is analyzed in further detail and optimized by means of the pinch-analysis methodology. A main output is that high plant efficiencies (of about 45%) can be achieved using a simple closed CO 2 Brayton power cycle. The optimized integration layout shows good performances at carbonator to turbine outlet pressure ratios around 3, thus allowing for a feasible integration of the power cycle in the CSP-CaL system. [ABSTRACT FROM AUTHOR]

The development of novel energy storage technologies is crucial for the massive deployment of large-scale renewable energy systems. This paper presents the conceptual study of an integrated system for the large-scale storage of solar thermal energy in the form of thermochemical energy based on calcium hydroxide. Calcium oxide-based storage media are very promising because they are an abundant and inexpensive resource with high energy density and the possibility of storage under ambient conditions. The equilibrium temperature of the reaction is around 500 °C, so the integrated system operates in a temperature range similar to that of concentrating solar power plants and molten salt storage, being able to reach higher temperatures in power production depending on the partial pressure of the steam generated in the reaction. The study discusses the technological challenges of the system, highlighting the importance of recovering the latent heat from condensation of the steam generated in the dehydration reaction, which represents 38% of the solar thermal energy entering the reactor during the charging phase, and proposes recovery mechanisms. The analysis shows the potential competitiveness of the technology as a large-scale energy storage system. By considering the full recovery of the latent heat of the steam, the system achieves a round-trip thermal efficiency of more than 80%, an overall thermal-to-electrical efficiency of 40% and LCOE values of 100 USD/MWhe. [ABSTRACT FROM AUTHOR]

Energy storage is the main challenge for a deep penetration of renewable energies into the grid to overcome their intrinsic variability. Thus, the commercial expansion of renewable energy, particularly wind and solar, at large scale depends crucially on the development of cheap, efficient and non-toxic energy storage systems enabling to supply more flexibility to the grid. The Ca-Looping (CaL) process, based upon the reversible carbonation/calcination of CaO, is one of the most promising technologies for thermochemical energy storage (TCES), which offers a high potential for the long-term storage of energy with relatively small storage volume. This manuscript explores the use of the CaL process to store Concentrated Solar Power (CSP). A CSP–CaL integration scheme is proposed mainly characterized by the use of a CO 2 closed loop for the CaL cycle and power production, which provides heat decoupled from the solar source and temperatures well above the ∼550 °C limit that poses the use of molten salts currently used to store energy as sensible heat. The proposed CSP–CaL integration leads to high values of plant global efficiency (of around 45–46%) with a storage capacity that allows for long time gaps between load and discharge. Moreover, the use of environmentally benign, abundantly available and cheap raw materials such as natural limestone would mark a milestone on the road towards the industrial competitiveness of CSP. [ABSTRACT FROM AUTHOR]

Energy storage systems are crucial for the massive deployment of renewable energy at a large scale. This paper presents a conceptual large-scale thermoelectrical energy storage system based on a transcritical CO 2 cycle. The concept is developed through the analysis of three high-efficiency systems: renewable energy storage using a thermoelectric energy storage system based on a reversible heat pump; a CO 2 storage system; and novel integration of energy storage using a reversible heat pump and geological injection of CO 2. The latter system efficiently integrates energy and CO 2 storage, taking advantage of the synergies between the operational requirements of both systems. The system uses CO 2 captured in stationary sources as a working fluid to store energy from renewables. The energy is stored and recovered in geological formation and heat/cold tanks, with energy storage based on sensible or latent heat of ice and water. A fraction of the CO 2 is expected to be permanently sequestered in the geological formation. The analysis of the time evolution of the system, under different operation profiles, shows the interest of the concept as a feasible integration for energy storage and CO 2 capture based on renewable energy, with an electric-to-electric efficiency varying between 40 and 50 %. • Reversible heat pump for electrothermal energy storage. • Transcritical CO 2 as working fluid and water for storage. • Inclusion of captured CO 2 and geological storage. • Round-trip efficiencies varying between 40 and 50 %. [ABSTRACT FROM AUTHOR]

This study introduces an innovative solution to address the challenges arising from the volatile natural gas market and the growing integration of renewable energy sources within the industrial sector. The research strives to confront this challenge by including renewable methanol (CH 3 OH) and converting it into methane (CH 4), with an intermediate step involving synthesis gas (CO/H 2) by using concentrating solar power. This approach provides a sustainable and adaptable solution to reduce dependence on natural gas. The process entails a methanol decomposition reaction at moderate temperatures (<350 °C). Subsequently, the synthesis gas is compressed to 40 bar, stored, and discharged through a methanation process that can be conducted at high temperatures (>500 °C). The resulting methane is used as fuel for gas turbines and can also serve as feedstock in the chemical industry. The simulations were conducted in ASPEN HYSYS and yielded overall system efficiencies exceeding 29% and roundtrip efficiencies of 44%. Through techno-economic optimisation of the reaction conditions, competitive levelized fuel costs (LCOF) of €172/MWh and future LCOE values of €145/MWh were achieved. These findings present an innovative strategy for integrating gas turbine cycles and additional conversion pathways for green methanol. • A novel flexible storage system for conventional power generation is proposed. • Green methanol to methane conversion process from intermediate step to synthesis gas. • Overall system efficiencies above 29% and roundtrip efficiencies of 44% were achieved. • Competitive levelized fuel cost of €172/MWh and future LCOE values <€265/MWh are reached. [ABSTRACT FROM AUTHOR]