Massive grid-scale energy storage for next-generation concentrated solar power

The world’s energy system is currently in the transformation process towards a fully renewable and sustainable future. Since very recently, this transformation process is not only driven by the objective of mitigating climate change, but also by geopolitical and economic reasons. The cost of renewable electricity generation, such as photovoltaic (PV) and wind, has fallen dramatically, being now lower than the marginal power generation cost of traditional fossil-fired or nuclear power plants [1]. However, the way towards a new energy system with highest shares of variable renewable sources is not exempt from complications. We are moving towards a completely renewable energy system where power is produced in a highly decentralized and uncontrolled manner. Thus, the key requirements for the efficient operation of the future energy system are both massive energy storage and highly flexible power generation. The future’s ideal power plant needs to provide “adaptive” power generation, being able to generate power during hours of high demand (high price periods, morning and evening), and to store energy efficiently, when electricity demand is low, but renewable energy is available in excess (low price periods, midday).

The solar resource available on Earth exceeds the current world’s energy demand several hundred times, thus, in areas with a high solar resource, Concentrated Solar Power (CSP) aims to play a crucial role [2]. This technology concentrates the direct solar radiation to obtain high-temperature thermal energy that is converted into electricity by means of a thermodynamic cycle with an electric generator. State-of-the-art CSP together with the well-known Thermal Energy Storage (TES) systems already provides a proper dispatchable electricity generation that can stabilize the electrical grid. Furthermore, CSP can be conveniently integrated into the electrical grid and the energy system as a whole (e.g., industrial process heat [3], desalination plants [4] and production of solar fuels [5].

Additionally, CSP holds great potential as a viable solution for hybridization with various thermal energy technologies since it shares technology with conventional power generation and can be readily integrated with other energy sources into a synergistic system. This integration offers numerous potential benefits, such as enhanced dispatchability and reliability, improved efficiency, cost reduction through equipment sharing, and the possibility of flexible operation by interchanging between energy sources [6,7]. Over the past few decades, the majority of research efforts have primarily focused on exploring thermal hybridization possibilities with coal, natural gas, biofuel [8] and geothermal [9]. However, coal and natural gas are not completely sustainable, biofuels may suffer from limited cost-effective availability, necessitating alternative fuel sources, and geothermal energy exhibits location constraints and operates with inefficient power cycles at low temperatures [6,7]. As a result, new possibilities for hybridization are arising, expanding beyond thermal-based technologies. This include the integration of PV and wind energy sources, which underscores the need to investigate novel approaches for integrating CSP into hybrid systems [7,10,11].

Within the CSP sector, several distinct types of technologies can be distinguished depending on the way of concentrating the solar radiation onto the receiver [12]: Parabolic Trough Collectors (PTC), Linear Fresnel Reflectors, Parabolic Dish Collectors and Solar Power Towers (SPT). Currently, overall installed capacity of CSP worldwide has reached around 6–6.5 GW in 2020 [1,13] and the most installed technology is PTC, which constitutes an 80% of the operational plants [14]. Nevertheless, over the last few years, the sector is witnessing a profound change where SPT is drawing more attention given that it is able to achieve high solar concentration factors (above 1000 suns) and operate at higher temperatures than PTC, which results in the use of power blocks with higher efficiencies. Therefore, there are more SPT plants under construction and development than for the rest of CSP technologies [15] and it is considered that SPT presents a higher room for improvement and a bigger potential for cost decrease due to a higher solar-to-electric efficiency, higher energy densities, a low-cost solar field, less maintenance and oil-free plants with lower environmental impact.

Nevertheless, the recent decline in the cost of PV power generation (Levelized Cost Of Electricity, LCOE = 3–4 $c/kWhe [1]) has significantly lowered the competitiveness of SPT technology and there is no point in producing electricity during the day (LCOE = 10–15 $c/kWhe [1]), when PV is running at full capacity and electricity prices are low (the so-called “duck curve” [16,17]). In this context, the unique and very valuable advantage of CSP with conventional TES – thus providing dispatchable operation – is no longer competitive enough, and a CSP concept breakthrough is needed. Hence, in addition to the advances in tower receivers and solar field components [18,19], two recent strategies aims to increase the competitiveness of next-generation CSP through massive grid-scale energy storage (see Fig. 1): (A) advanced TES systems [[20], [21], [22]] and (B) integration of Excess Electricity Storage (EES) systems [11,22,23]. Both strategies can be suitably combined with each other and, at the same time, integrated with novel Advanced Power Blocks (APB) to increase the efficiency of converting the stored energy back into electricity [[24], [25], [26]].

Firstly, integrating advanced TES systems, which work in conjunction with concentrated solar energy, will overcome the limitations and disadvantages of state-of-the-art TES technology (sTES), mainly based on conventional molten salts that present significant challenges to commercial CSP plants: limited operation temperature, high freezing temperature point, alternating thermal stress of tanks and serious corrosion issues [27,28]. These limitations can result in storage tank ruptures, as exemplified by recent leakage incidents at the “Crescent Dunes” CSP plant in the USA, “Gemasolar” CSP plant in Spain and “Noor-III” CSP plant in Morocco, which have caused substantial economic losses [27]. The most promising advanced storage alternatives are classified into the following: (i) Sensible Heat Storage (SHS): novel molten salts, particles, sensible packed-bed thermocline and liquid metals; (ii) Latent Heat Storage (LHS): phase change materials packed-bed thermocline and (iii) Thermochemical Energy Storage (TCES): hydrides, hydroxides, carbonates and redox reactions.

Secondly, in order to accomplish an authentic revolution in the sector, a complete paradigm shift based on a new CSP business model is necessary. Therefore, it has been recently proposed to employ CSP technology not only for producing electricity based on solar energy (e.g., CSP3 from the SunShot [20]), but also for integrating massive Excess Electricity Storage (EES) by directly using low-value excess energy from other renewable energy technologies that lack their own feasible energy storage systems [11,22,23]. In comparison with the popular electrical energy storage technologies, CSP is able to overcome its major limitations, e.g., battery electric storage presents high environmental impact [29], recyclability issues, limited lifetime [30] and higher cost [31] and pumped hydro storage brings about strong geographical restrictions due to the necessity of geographical height and water availability [32]. As function of electricity prices in the grid (which could even be negative at certain times, e.g. during the midday), the CSP plant is even being paid for receiving electricity that can be later on used in the power block when power is demanded by the grid. Thus, this solution is able to increase the capacity factor of the CSP plant while sharing the power block [33,34]. As it is defined in the update of the European Strategic Energy Technology Plan of 2023 [22], the utilization of CSP must facilitate the variable renewable energy sources penetration in the future electrical system, mainly through hybrid plants such as PV-CSP systems. Examples of recent PV-CSP projects include the “Cerro Dominador” plant in Chile and the “Noor Energy 1/DEWA IV” in the United Arab Emirates [15]. Notably, the “Noor Midelt” CSP project in Morocco, with a capacity of 800 MW, stands as the pioneering hybrid PV-CSP plant that incorporates an electric heater to directly store the power produced by the PV system in the TES system, thus paving the way for the next-generation CSP technology [23,35]. The most encouraging massive electricity storage that can be integrated with CSP technology are classified in two subgroups (see Fig. 1): (i) CSP with Power-To-Heat-To-Power (P2H2P) [33,34,[36], [37], [38]] and (ii) CSP with Compressed Gas Energy Storage (CGES) or Liquefied Gas Energy Storage (CLES) [[39], [40], [41], [42], [43]].

In order to convert this stored energy into electricity in a more efficient way, the integration of Advanced Power Blocks (APB) with CSP is essential. The emerging storage alternatives (TES and EES) can operate at higher temperatures, hence, the conventional sub-critical Rankine cycle can be conveniently replaced by most innovative APB, e.g., (i) an Air-Brayton Combined Cycle (ABCC) [[44], [45], [46]] or by (ii) a supercritical Carbon Dioxide (sCO2) cycle [20,47,48]. The integration of power blocks is mainly dependent on the selected energy storage systems, operating conditions and working fluids of the CSP plant.

Main results will be obtained thanks to these novel strategies: power grid stabilization with higher shares of renewable energy sources and efficient, economic and, in short, more competitive CSP. Thus, it is expected that this disruptive change in the CSP paradigm will be able to increase the efficiency of the plants, increase the Energy Storage Density (ESD), increase the reliability and lifetime, maintain a minimum environmental impact, improve the flexibility of operation and decrease the cost.