Advanced Concrete Steam Accumulation Tanks for Energy Storage for Solar Thermal Electricity

energies Article Advanced Concrete Steam Accumulation Tanks for Energy Storage for Solar Thermal Electricity Cristina Prieto 1,2,* , David Pérez Osorio 1, Edouard Gonzalez-Roubaud 1, Sonia Fereres 1 and Luisa F. Cabeza 3,*   Citation: Prieto, C.; Pérez Osorio, D.; Gonzalez-Roubaud, E.; Fereres, S.; Cabeza, L.F. Advanced Concrete Steam Accumulation Tanks for Energy Storage for Solar Thermal Electricity. Energies 2021,14, 3896. https://doi.org/10.3390/en14133896 Academic Editor: Dimitrios Katsaprakakis Received: 27 May 2021 Accepted: 24 June 2021 Published: 28 June 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1Abengoa Energía, Calle Energía Solar, 1, 41014 Sevilla, Spain; [email protected] (D.P.O.); [email protected] (E.G.-R.); sonia.ferer[email protected] (S.F.) 2Department of Energy Engineering, University of Seville, Calle San Fernando, 4, 41004 Sevilla, Spain 3GREiA Research Group, University of Lleida, Pere de Cabrera s/n, 25001 Lleida, Spain *Correspondence: [email protected] (C.P.); [email protected] (L.F.C.) Abstract: Steam accumulation is one of the most effective ways of thermal energy storage (TES) for the solar thermal energy (STE) industry. However, the steam accumulator concept is penalized by a bad relationship between the volume and the energy stored; moreover, its discharge process shows a decline in pressure, failing to reach nominal conditions in the turbine. From the economic point of view, between 60% and 70% of the cost of a steam accumulator TES is that of the pressure vessel tanks (defined as US$/kWh th ). Since the current trend is based on increasing hours of storage in order to improve dispatchability levels in solar plants, the possibility of cost reduction is directly related to the cost of the material of pressure vessels, which is a market price. Therefore, in the present paper, a new design for steam accumulation is presented, focusing on innovative materials developed specifically for this purpose: two special concretes that compose the accumulation tank wall. Study of dosages, selection of materials and, finally, scale up on-field tests for their proper integration, fabrication and construction in prototype are the pillars of this new steam accumulation tank. Establishing clear and precise requirements and instructions for successful tank construction is necessary due to the highly sensitive and variable nature of those new concrete formulations. Keywords: storage tank; accumulator; steam; storage; concrete; insulation refractory; high-strength; self-compacting concrete; construction 1. Introduction Large-scale thermal energy storage (TES) is a key component of concentrating solar power plants (CSP), offering energy dispatchability by adapting the electricity power production to the demand curve [ 1 ]. The industry is looking for more economical and efficient TES systems, especially for process heat applications between 150–250 ◦ C. New decarbonization strategies worldwide promote the solarization of the process heat, where direct steam generation is one of the most efficient technologies [ 2 ]. However, the need for effective steam storage penalizes this application. Commercial solar thermal electricity plants today implement only two TES technologies: steam accumulators and molten salts storage [3]. Parabolic trough plants with thermal oil and molten salt towers use two tanks of molten salts as storage system. Molten salt is the most widespread heat transfer fluid for thermal energy storage in CSP commercial applications due to its good thermal properties and reasonable cost [ 2 ]. Direct TES systems, i.e., where the storage fluid is the same as that directly heated by the concentrated solar radiation, have the advantage of reducing system losses and complexity by eliminating heat exchangers between the heated and the storage media; this is used in commercial parabolic trough plants [4]. Direct steam generation plants use steam accumulators, also known as Ruth accumulators [ 5 ]. In these systems, the steam is directly stored at high pressure in accumulator Energies 2021,14, 3896. https://doi.org/10.3390/en14133896 https://www.mdpi.com/journal/energies Energies 2021,14, 3896 2 of 26 tanks. The equipment is charged with the surplus saturated steam produced by the plant. During the discharge, steam is produced by lowering the pressure of the saturated liquid stored within the tank. Steam has the added benefit of being a common working fluid for power generation cycles, also eliminating the need for heat exchangers between the solar field and the power block. First generation STE towers use saturated steam technology. As of January 2016, there were only two commercial CSP plants in operation using steam accumulator TES: PS10 and PS20, both located in Spain. They started commercial operation in 2007 and 2009, respectively, and they were the first two commercial central receiver solar plants in the world and the starting point for the operation of the direct steam generation (DSG) technology. Figure 1represents schematically a flow diagram of the PS10 and PS20 direct steam generation tower plant with a steam accumulator thermal energy storage system. Energies 2021, 14, 3896 2 of 27 Direct steam generation plants use steam accumulators, also known as Ruth accumulators [5]. In these systems, the steam is directly stored at high pressure in accumulator tanks. The equipment is charged with the surplus saturated steam produced by the plant. During the discharge, steam is produced by lowering the pressure of the saturated liquid stored within the tank. Steam has the added benefit of being a common working fluid for power generation cycles, also eliminating the need for heat exchangers between the solar field and the power block. First generation STE towers use saturated steam technology. As of January 2016, there were only two commercial CSP plants in operation using steam accumulator TES: PS10 and PS20, both located in Spain. They started commercial operation in 2007 and 2009, respectively, and they were the first two commercial central receiver solar plants in the world and the starting point for the operation of the direct steam generation (DSG) technology. Figure 1 represents schematically a flow diagram of the PS10 and PS20 direct steam generation tower plant with a steam accumulator thermal energy storage system. Figure 1. Schematic flow diagram of a direct steam generation tower plant (saturated steam) with steam accumulator thermal energy storage system (Source: Abengoa). The PS10 central receiver plant uses a 11 MWe saturated steam Rankine cycle with steam accumulator thermal energy storage. PS10 has 624 heliostats (120 m 2 each) for a total reflective surface of around 75,000 m 2 . It is arranged in circular rows around the tower, concentrating solar radiation on a saturated steam cavity receiver placed on top of a 115 m tower. The storage system provides 20 MWh of storage capacity, giving 50 min effective operational capacity at 50% turbine workload. The system has four tanks that are operated sequentially in relation to their charge status. During full load operation of the plant, the steam accumulators are charged with a proportion of the steam produced at the receiver at 250 °C and 45 bars. When a transient period needs to be covered, energy from the saturated water will be recovered at variable pressure. Figure 2 shows the steam accumulators at PS10. Figure 1. Schematic flow diagram of a direct steam generation tower plant (saturated steam) with steam accumulator thermal energy storage system (Source: Abengoa). The PS10 central receiver plant uses a 11 MWe saturated steam Rankine cycle with steam accumulator thermal energy storage. PS10 has 624 heliostats (120 m 2 each) for a total reflective surface of around 75,000 m 2 . It is arranged in circular rows around the tower, concentrating solar radiation on a saturated steam cavity receiver placed on top of a 115 m tower. The storage system provides 20 MWh of storage capacity, giving 50 min effective operational capacity at 50% turbine workload. The system has four tanks that are operated sequentially in relation to their charge status. During full load operation of the plant, the steam accumulators are charged with a proportion of the steam produced at the receiver at 250 ◦ C and 45 bars. When a transient period needs to be covered, energy from the saturated water will be recovered at variable pressure. Figure 2shows the steam accumulators at PS10. The PS20 central receiver plant has 1255 heliostats (with a surface of 120 m 2 each) and uses the same technology as PS10 but its power cycle reaches 20 MWe with an equivalent storage capacity of 50 min at 50% turbine workload using two steam accumulators that are discharged in sequence. Energies 2021,14, 3896 3 of 26 Energies 2021, 14, 3896 3 of 27 Figure 2. Steam accumulators of PS10 plant (Source: Abengoa). The PS20 central receiver plant has 1255 heliostats (with a surface of 120 m 2 each) and uses the same technology as PS10 but its power cycle reaches 20 MWe with an equivalent storage capacity of 50 min at 50% turbine workload using two steam accumulators that are discharged in sequence. Superheated steam is used in the second generation of STE direct steam towers. This technology uses a second receiver that re-heats the steam produced by the first receiver (evaporator) to reach higher temperatures. The live steam feeding the turbine can reach 540 °C and 130 bars of pressure, increasing by 30% the efficiency of the power cycle compared to first generation PS20. Khi Solar One is a 50 MWe commercial tower plant located in Upington (South Africa) and put into operation in 2016 (Figure 3). It is a second-generation direct steam plant with a steam accumulator TES. It has a tower 205 m high and uses 4120 heliostats with a surface area of 140 m 2 each. The storage capacity of Khi Solar One is 2-h using 19 steam accumulator tanks. Figure 3. Steam accumulators of 50 MWe Khi Solar One plant (Source: Abengoa). Figure 4 shows a schematic flow diagram of the Khi Solar One plant with a superheated direct steam tower and steam accumulator TES. Figure 2. Steam accumulators of PS10 plant (Source: Abengoa). Superheated steam is used in the second generation of STE direct steam towers. This technology uses a second receiver that re-heats the steam produced by the first receiver (evaporator) to reach higher temperatures. The live steam feeding the turbine can reach 540 ◦ C and 130 bars of pressure, increasing by 30% the efficiency of the power cycle compared to first generation PS20. Khi Solar One is a 50 MWe commercial tower plant located in Upington (South Africa) and put into operation in 2016 (Figure 3). It is a second-generation direct steam plant with a steam accumulator TES. It has a tower 205 m high and uses 4120 heliostats with a surface area of 140 m 2 each. The storage capacity of Khi Solar One is 2-h using 19 steam accumulator tanks. Energies 2021, 14, 3896 3 of 27 Figure 2. Steam accumulators of PS10 plant (Source: Abengoa). The PS20 central receiver plant has 1255 heliostats (with a surface of 120 m 2 each) and uses the same technology as PS10 but its power cycle reaches 20 MWe with an equivalent storage capacity of 50 min at 50% turbine workload using two steam accumulators that are discharged in sequence. Superheated steam is used in the second generation of STE direct steam towers. This technology uses a second receiver that re-heats the steam produced by the first receiver (evaporator) to reach higher temperatures. The live steam feeding the turbine can reach 540 °C and 130 bars of pressure, increasing by 30% the efficiency of the power cycle compared to first generation PS20. Khi Solar One is a 50 MWe commercial tower plant located in Upington (South Africa) and put into operation in 2016 (Figure 3). It is a second-generation direct steam plant with a steam accumulator TES. It has a tower 205 m high and uses 4120 heliostats with a surface area of 140 m 2 each. The storage capacity of Khi Solar One is 2-h using 19 steam accumulator tanks. Figure 3. Steam accumulators of 50 MWe Khi Solar One plant (Source: Abengoa). Figure 4 shows a schematic flow diagram of the Khi Solar One plant with a superheated direct steam tower and steam accumulator TES. Figure 3. Steam accumulators of 50 MWe Khi Solar One plant (Source: Abengoa). Figure 4shows a schematic flow diagram of the Khi Solar One plant with a superheated direct steam tower and steam accumulator TES. Energies 2021,14, 3896 4 of 26 Energies 2021, 14, 3896 4 of 27 Figure 4. Scheme of 50 MW Khi Solar One plant (superheated steam) with steam accumulator TES system. The state-of-the-art for steam accumulators uses steel as constructive material to resist high pressure and high temperature water/steam [6]. Steam accumulation tanks are generally cylindrical with elliptical ends and are manufactured from boiler plate. One of the main advantages is that the storage fluid is water, avoiding uncertainty in the price of the storage medium. Steam accumulators are a proven option for compensation of transients and mid-term storage to meet supply/demand curves when there is no radiation, due to their fast reaction times and high discharge rates. Nevertheless, the main disadvantages of steam accumulators are the low volumetric energy density stored as well as the discharge process, which shows a decline in pressure, unable to nominal conditions in the turbine. From the economic point of view, the main cost of a steam accumulator TES is that of the pressure vessel tanks, which can account for 60% to 70% of the TES total thermal costs (in US$/kWh th ) [3]. In general, carbon steel is the most usual material used for the fabrication of steam accumulation tanks. The design presented in this paper seeks to reduce costs by substituting carbon steel with cheaper constructive materials such as concretes. However, the material selected must withstand a combination of pressure (higher than 100 bars), temperature (higher than 300 °C), and long-term thermal cycling while maintaining good durability against chemical attack. The proposed innovative design for the accumulator tank consists of a double-walled cylinder accumulation tank with a post-tensioned concrete layer (a structural layer, made of high-strength self-compacting concrete (HSSCC)) able to withstand high tensile stresses, and an insulating refractory concrete layer (a thermal stability layer) isolating the structural layer from the high temperature steam which can jeopardize the mechanical properties of the outer layer and corrode its steel reinforcements. This multi-layer concept allows higher working temperatures and pressures, at lower cost if it is compared to the current commercial steel steam accumulation tanks. A similar multilayer concept has been used to contain nuclear reactors [7]. These structures tend to use post-tensioned concrete walls, but their working pressure range is 10 times lower than required in solar thermal applications (higher than 100 bars). In case of a loss of coolant accident (LOCA) the heat emitted from the reactor vessel is conducted through the steel shell to the concrete wall which is exposed to atmospheric pressure; if this concrete is heated excessively, cracking phenomena could occur due to large thermal Figure 4. Scheme of 50 MW Khi Solar One plant (superheated steam) with steam accumulator TES system. The state-of-the-art for steam accumulators uses steel as constructive material to resist high pressure and high temperature water/steam [ 6 ]. Steam accumulation tanks are generally cylindrical with elliptical ends and are manufactured from boiler plate. One of the main advantages is that the storage fluid is water, avoiding uncertainty in the price of the storage medium. Steam accumulators are a proven option for compensation of transients and mid-term storage to meet supply/demand curves when there is no radiation, due to their fast reaction times and high discharge rates. Nevertheless, the main disadvantages of steam accumulators are the low volumetric energy density stored as well as the discharge process, which shows a decline in pressure, unable to nominal conditions in the turbine. From the economic point of view, the main cost of a steam accumulator TES is that of the pressure vessel tanks, which can account for 60% to 70% of the TES total thermal costs (in US$/kWh th ) [ 3 ]. In general, carbon steel is the most usual material used for the fabrication of steam accumulation tanks. The design presented in this paper seeks to reduce costs by substituting carbon steel with cheaper constructive materials such as concretes. However, the material selected must withstand a combination of pressure (higher than 100 bars), temperature (higher than 300 ◦ C), and long-term thermal cycling while maintaining good durability against chemical attack. The proposed innovative design for the accumulator tank consists of a double-walled cylinder accumulation tank with a post-tensioned concrete layer (a structural layer, made of high-strength self-compacting concrete (HSSCC)) able to withstand high tensile stresses, and an insulating refractory concrete layer (a thermal stability layer) isolating the structural layer from the high temperature steam which can jeopardize the mechanical properties of the outer layer and corrode its steel reinforcements. This multi-layer concept allows higher working temperatures and pressures, at lower cost if it is compared to the current commercial steel steam accumulation tanks. A similar multilayer concept has been used to contain nuclear reactors [ 7 ]. These structures tend to use post-tensioned concrete walls, but their working pressure range is 10 times lower than required in solar thermal applications (higher than 100 bars). In case of a loss of coolant accident (LOCA) the heat emitted from the reactor vessel is conducted through the steel shell to the concrete wall which is exposed to atmospheric Energies 2021,14, 3896 5 of 26 pressure; if this concrete is heated excessively, cracking phenomena could occur due to large thermal gradients generated inside the wall. In order to solve this problem an intermediate refractory concrete layer is installed. Refractory concretes can resist five–six times higher temperatures than required (higher than 300 ◦ C), but they are not generally working with internal pressures of such magnitudes. Double-walled systems can be found, with smaller thicknesses for less demanding combinations of pressure and temperature, and in general the refractory wall does not have any mechanical or structural properties, which would prevent the use of insulating liners. There are also vessels and tanks that use post- and pre-tensioned concrete walls with different purposes, such as water and oil storage or natural gas containment [ 8 ]. Clinker and/or cement silos, among others, use prestressed concrete walls, but the pressure and temperature parameters are lower than required in solar thermal applications. On the one hand, the development of high-strength self-compacting concrete (HSSCC) is the result of the demand for better performance in present-day structures employing new chemical admixtures. High-strength concretes are those whose characteristic compressive strength exceeds 50 MPa (a typical value [ 9 ], depending on the standards in different countries). Self-compacting concretes are those compacted by the action of their own weight, without any external vibration energy and was introduced in the 1980s to obtain seismic-resistant and durable structures without the need for qualified labor for its compaction [ 10 ]. To achieve elevated flowability and appropriate cohesion, it was essential to combine selected grading distributions with very efficient super-plasticizer admixtures. This was the key in their evolution until current polycarboxylate formulations allowed a satisfactory behavior of fresh concrete [11]. Achieving simultaneously self-compacting and high strength properties requires a high accuracy in establishing a dosage, especially because these are not reached by similar dosing, although combination is possible. Current self-compacting concretes also have elevated compressive strength values, higher than conventional ones, even inside highresistant classification (more than 50 MPa [ 9 ]). The problem emerges when trying to reach values higher than 100 MPa while keeping self-compacting properties, as required for the design of the steam accumulation tanks developed by Abengoa, since there is no product able to cope with both. In a simple search of catalogues and technical data sheets from well-known companies of said industry, it is possible to find similar, but not superior, concretes (Table 1). The only concrete products that could lead these requirements are Ductal from Lafarge and Durcrete. Ductal can reach 180 MPa with self-compacting properties, but the cost is not affordable for steam accumulation tank application; Durcrete assures self-compacting properties with 150 MPa, but its workability and fabrication imply low volumes and unscalable procedures (supply in bags for mixing on site). Table 1. Properties of commercial concretes. Commercial Concrete Compressive Strength Self-Compacting Lafarge Agilia 25–40 MPa Yes UltraSeries >100 MPa No Ductal 180 MPa Yes Portland-Valderrivas High strength 100 MPa No Self-compacting <70 MPa Yes Hanson H-resist <50 MPa Yes Cemex Hormiresist 100 MPa No FYM i.flow <30 MPa Yes Promsa High resistance 120 MPa No Self-compacting <50 MPa Yes Durcrete 150 MPa Yes On the other hand, insulating concretes are widely known in building engineering, raw material manufacturing and power plant equipment, currently developed as standard Energies 2021,14, 3896 6 of 26 commercial products. The objective of these insulating products is to restrain and avoid thermal effects. The majority are focused on conventional pre-manufactured elements such as panels, bricks [ 12 ] or blocks [ 13 ]. Research studies regarding new insulating concretes have been carried out in order to make more sustainable and eco-friendly materials such as rice husk [14–16] or foams [16–18]. Nevertheless, much more research on lightweight concretes has been recently carried out to increase their lightweight characteristics [ 17 , 19 ] and better structural performance [ 20 ] in order to reduce loads on structures. The most commonly used solution to innovate in lightweight concretes is to incorporate new lightweight aggregates (LWA), replacing normal aggregates (NA) in the dosage components [ 21 – 23 ]. One of the most popular additions to this type of concrete is fly ash aggregates [24,25]. After an extensive search of the refractory industry, no commercial product was found to meet both the high thermomechanical values of thermal conductivity and compressive strength required for this design. It is possible to find similar thermal conductivity but insufficient values of compressive strength, and exactly the opposite premise, concrete of similar compressive strength but insufficient thermal conductivity (Table 2). This table shows that the range of commercial refractory concretes is limited. Concretes with high compressive strength have too high thermal conductivities and vice versa. Therefore, these low compressive strength concretes are not used for structural purposes, only for thermal insulation purposes in non-structural constructions, hence the need to develop in this work a new refractory concrete with high structural performance. Table 2. Thermal conductivity of commercial concretes. Commercial Concrete Thermal Conductivity (W/m·K) Compressive Strength (MPa) Alosil A-124 B - 4–6 Promacast MW 1.4 HT 0.34 6–10 Promacast FC 40 0.72 30–40 Horlite 1-2-4 B 2–3 4–5 Horlite 900 LI - 5–7 Horsigel 9362 1.54 5–7 Horlite 1300 SF 0.44 1.8 Thermedia 0.6 0.54 25 Another challenge is that new concrete formulation development in a laboratory setting is only the first step for materials development. It is a challenge to achieve very stringent mechanical and/or thermal properties later, in the field, in an uncontrolled environment and at large scale. Thus, another objective of this research is to demonstrate experimentally that it is possible to elaborate concrete on-field, with the materials, dosage and process determined at small scale in the laboratory and to characterize the variability in its properties. To sum up, it is concluded that thermal energy storage systems in CSP are necessary to offer dispatchability. Currently, molten salts are the most widely used commercial system, while net steam storage still needs to lower costs. The objective of this study is to design a lower-cost accumulator; therefore, a cheaper construction material is selected, i.e., concrete. A new design of the steam accumulation tank is shown, deploying a combination of layers: two concrete walls with an inner metallic shell. The interior wall has a purely thermal purpose; it is an insulating layer to reduce the temperature that reaches the structural concrete. There is no connection between the structural layer and the refractory layer to avoid thermal bridges that would compromise the overall tank system. The structural wall could be damaged at high temperature. The refractory concrete was made once the concrete of the external layer had been built and hardened. Energies 2021,14, 3896 7 of 26 Internally, to protect the refractory concrete layer, a high-performance metallic liner was designed, which avoids contact of steam with the concrete, reducing the risk of contamination. 2. Conceptual Design A cylindrical body and elliptical top are the starting point for the tank design. The inner working pressure (p> 100 bar) and temperature (T > 300 ◦ C) drive the geometrical, structural, and mechanical requirements of the steam accumulation tank. The main design constraint is the need for a concrete wall that can withstand the high tensile stress caused by the inner pressure, while avoiding excessive thermal heating on that wall. This is the reason for a concrete double-wall design. The outer wall is a thick, high-strength posttensioned concrete; the inner wall is made of refractory concrete able to withstand high temperatures with an internal steam-protective shell. Both concrete walls are adjacent but not joined together. The high-strength wall has a double mission: first, to act as a compressive force on the inner layer (refractory concrete) constraining its free expansion due to the inner thermal loads. The second objective of the high-strength wall is to distribute tensile stresses generated by the high internal pressure so that the sum of the compressive stresses generated by the thermal loads, plus the tensile stresses produced by the pressure, is a net compressive stress. A compressive stress state in this wall avoids a cracking phenomenon which could result in a risk for structure durability (fatigue, chemical action for steam by voids, etc.). Hence, the outer wall, stiffer than the inner wall, absorbs most of the traction. Even though a higher thickness produces fewer tensions in the wall by increasing the load distribution area, tensile stresses generated by inner pressure increase in proportion to the radius. So, it is essential to search for an optimum relation between wall thickness, post-tensioning strength, and the characteristic compressive strength of concrete. The inner refractory (insulation) concrete wall prevents the temperature inside the tank from reaching the double-wall interface, avoiding the thermal expansion of the outer wall. Moreover, if this high temperature reaches the high-strength concrete wall, it would alter the mechanical properties of the high-strength concrete and effect the durability of the exterior wall as well as its active (post-tensioning cables) and passive reinforcement (steel bars). It is crucial that this insulating refractory concrete has a low thermal expansion coefficient, so that the that inner compression generated by constraining the thermal expansion is not higher than the tensile stress produced by the inner accumulation pressure, balancing each other. Regarding the concrete quality, both will be compressed continuously during operation. Thus, their compressive strength is a key factor. However, the wall stiffness influences the generation of tensile and compressive stresses, so the elastic modulus of both concretes is also a parameter to optimize. The anchorage zones of the post-tensioning tendons must withstand the concentrated loads that are transmitted by the post-tensioning rings. The number and distribution of these anchorage zones is defined by distribution of the tendons. Due to the cylindrical curvature, friction losses occur between the tendons and the sheaths, reducing the posttensioning force from the middle of the cables (tendons tensioned at both ends). In order to avoid failures by concrete crushing, a minimum height between rows in the anchorage zones must be established. These considerations lead to a design with four anchor points, distributing seven tendon rings in four alternating superposed systems as shown in Figure 5. The design is associated with a complete constructive procedure, which establishes the first development of the external high-strength wall by means of climbing formworks. The next stage is post-tensioning procedure of 85% of the tendons (6 of 7 tendon rows). Next, the inner refractory wall is also built by the means of climbing formworks. Then, the remaining row of tendons is post-tensioned to complete the process with the required pre-compression of the refractory wall. Finally, the semi-spherical dome is built at the top Energies 2021,14, 3896 8 of 26 of the accumulation tank, as shown in Figure 6and Table 3. The tendon system consists of several high rows of seven semi-circular tendons. These rows were arranged in an alternating rotating pattern, so that “compression circles” were completed in the structural wall of the tank as it rose in height. For their construction, stainless steel pipes were used as “inner ducts” in the concrete. Once each height was concreted, the tendons were inserted into these tubes and then a compression grout was injected to prepare the tendons for stressing, which was done once the tank was fully constructed. Energies 2021, 14, 3896 8 of 27 Figure 5. Tendon rings established in the accumulation tank. The design is associated with a complete constructive procedure, which establishes the first development of the external high-strength wall by means of climbing formworks. The next stage is post-tensioning procedure of 85% of the tendons (6 of 7 tendon rows). Next, the inner refractory wall is also built by the means of climbing formworks. Then, the remaining row of tendons is post-tensioned to complete the process with the required pre-compression of the refractory wall. Finally, the semi-spherical dome is built at the top of the accumulation tank, as shown in Figure 6 and Table 3. The tendon system consists of several high rows of seven semi-circular tendons. These rows were arranged in an alternating rotating pattern, so that “compression circles” were completed in the structural wall of the tank as it rose in height. For their construction, stainless steel pipes were used as “inner ducts” in the concrete. Once each height was concreted, the tendons were inserted into these tubes and then a compression grout was injected to prepare the tendons for stressing, which was done once the tank was fully constructed. Figure 6. Accumulation tank conceptual design. Figure 5. Tendon rings established in the accumulation tank. Energies 2021, 14, 3896 8 of 27 Figure 5. Tendon rings established in the accumulation tank. The design is associated with a complete constructive procedure, which establishes the first development of the external high-strength wall by means of climbing formworks. The next stage is post-tensioning procedure of 85% of the tendons (6 of 7 tendon rows). Next, the inner refractory wall is also built by the means of climbing formworks. Then, the remaining row of tendons is post-tensioned to complete the process with the required pre-compression of the refractory wall. Finally, the semi-spherical dome is built at the top of the accumulation tank, as shown in Figure 6 and Table 3. The tendon system consists of several high rows of seven semi-circular tendons. These rows were arranged in an alternating rotating pattern, so that “compression circles” were completed in the structural wall of the tank as it rose in height. For their construction, stainless steel pipes were used as “inner ducts” in the concrete. Once each height was concreted, the tendons were inserted into these tubes and then a compression grout was injected to prepare the tendons for stressing, which was done once the tank was fully constructed. Figure 6. Accumulation tank conceptual design. Figure 6. Accumulation tank conceptual design. Energies 2021,14, 3896 9 of 26 Table 3. Conceptual design parameters. Tank Capacity 3500 m3 Inner refractory concrete wall Height 40 m Inner radius 5.5 m Thickness 0.45 m Outer concrete wall Thickness 3.8 m The new steam accumulation design requires, for its external wall, a post-tensioned high-strength concrete in order to withstand the strong tensile stresses generated by inner steam pressure. Given the large thickness and the strong active reinforcement needed in this external wall, passive reinforcing steel bars are required. These elements make the concrete infiltration and compacting process more difficult and could create voids. For all these reasons, the high-strength concrete must also be self-compacting. Essential properties to be sought in both concretes are the correct distribution and time evolution of temperature through the wall thicknesses. The objective is not to exceed a maximum temperature at the interface of both concretes. This insulating wall must be compressed continuously to assure durability (no cracking) and waterproofing properties. Thermal loads produce an expansion that is constrained, staying internally compressed. Thus, at least a conventional compressive strength (>20–25 MPa for lightweight/ordinary concretes) is required for the new insulating refractory concrete. This “self-compression” phenomenon of the refractory wall, due to thermal loads, is the process that requires the greatest level of control, because it is responsible for strong compression in a wall with a desirable minimum thickness. That is why a low thermal expansion coefficient is required for the refractory concrete, leading to a compromise between the cost of increasing thickness and a decrease in the thermal expansion coefficient. A low porosity and a minimum variation of mechanical properties at high temperatures (>300 ◦ C) are essential. Additionally, the thermal properties of the refractory concrete must be tested under real cycling conditions. Specific technical requirements were defined for the liner: •Yield strength (MPa): 890 MPa •Tensile strength: 940–1100 MPa •High Strength Steel (RQT 901®). 3. Materials and Methods 3.1. Materials As stated above, concrete is the construction material for both tank walls. The outer wall is designed with a reinforced high-performance concrete with self-compacting properties and the inner wall is designed with a special refractory concrete. Both compositions are based on non-commercial concretes because concrete state-of-the-art does not solve the problems of the extreme conditions demanded by the solar thermal industry. The refractory concrete used (Table 4) is developed by Abengoa. based on the performance required for future commercial applications: •Thermal conductivity: λ= 0.2 W/m·K. •Compressive strength (90 days): Rc (90 d) = 20 MPa Energies 2021,14, 3896 16 of 26 Table 7. High-strength concrete formulation [53]. Material Quantity * Aggregate 900–3000 kg/m3 Cement 300–800 kg/m3 Water 90–200 kg/m3 Fly ash 40–150 kg/m3 Filler 15–80 kg/m3 Silica fume 40–80 kg/m3 Metakaolin 40–80 kg/m3 Active dispersing agent 10–20 kg/m3 Polypropylene fibers 0, 1 or 2 kg/m3 * The Dreux-Gorisse method [ 54 ] is one technique among others that allows determining the proportions of the various constituents (sand, cement, gravel, water) of the concrete, starting with a granulometric analysis. The detailed formulation of the high-strength self-compacting concrete developed in the laboratory is presented in Table 8. Results of the characterization of these tests are shown in Table 9, where the slum test shows the good workability achieved; compressive strength results were 112.5 MPa. Table 8. Formulation of high-strength self-compacting concrete for lab tests. Material Test 1 Test 2 Test 3 Cement CEM I 52.5R/SR 500 kg 500 kg 500 kg Water — 160 L 160 L 160 L Silica fume Sika Fume S-92.D 55 kg 55 kg 55 kg Fly ash — 100 kg 100 kg 100 kg Metakaolin — 55 kg 55 kg 55 kg Filler — 30 kg 30 kg 30 kg Aggregate 0/4 — 788 kg 788 kg 788 kg Aggregate 2/8 — 926 kg 926 kg 926 kg Superplasticizer Sika Viscocrete-20 HE 12.5 kg 12.5 kg 13.5 kg Retardant Sika Retarder 50 2.5 kg 5.0 kg 5.0 kg Polypropylene fibers Sika Fiber M-12 0.6 kg 0.6 kg 0.6 kg Table 9. Results of high-strength self-compacting concrete for lab tests. Property Test 1 Test 2 Test 3 Slum-test 670 mm 630 mm 695 mm Compression strength at 7 days 111.5 MPa 105.5 MPa - Compression strength at 28 days 112.5 MPa 112.5 MPa - 4.1.2. Insulating Refractory Concrete Development The new insulating refractory concrete requires stricter thermo-mechanical characteristics. Two key properties are evaluated: thermal resistance (expressed as the inverse of thermal conductivity) and mechanical strength. Concrete thermal resistance is closely linked to density, specifically hardened concrete density, since internal porosity develops the main insulating function due to occluded air in matrix voids [ 13 ]. This porosity, and therefore density, can be reached only by means of two concrete components: aggregates and admixtures. However, to reduce costs, the aggregate type and dosage will limit the insulating nature and thermal resistance reached. The concrete thermal resistance, depends on three main factors: (i) binder type, (ii) aggregate type, closely linked to the insulating function that determines material thermal conductivity, and (iii) total water/concrete (w/c) ratio [ 40 ], along with mechanical strength, which is also closely linked to concrete density. In this case, the influence of the type of binder (cement material) is stronger than for thermal resistance, followed by that of type of aggregate, and effective w/c ratio [55]. Energies 2021,14, 3896 17 of 26 Two parameters influence the achievement of light weight with insulating properties: the effective and the total/theoretical w/c ratio [ 56 ]. When aggregates (dry or wet not saturated) absorb water from the quantity added in the truck mixer, the available water is reduced, and the effective w/c ratio will be lower than the theoretical ratio. In the development of this concrete, moist, non-saturated aggregates were used, therefore water absorption by aggregates caused a reduction in the effective water reacting with the cement. The dosage adjustment was based on this effective ratio. It is very important to determine the water absorbed by the aggregates (by the difference of the instantaneous aggregates moisture and their absorption index). It is also important to determine the difference from the total water of the dosage that will result in the effective level of water to combine with the cement. The decrease of concrete cohesion, due to the use of a higher w/c ratio, causes a decrease of compressive strength. Therefore, the fabrication procedure established in this study considers these aspects, incorporating moisture measurements on field of the aggregates and formulation changes by adjusting the effective w/c ratio. For the selection of cement, two options are evaluated for the temperatures required (>300 ◦ C): ordinary Portland cements or high alumina cement. Portland cements, in their I-52.5R or I-42.5R types, are low cost and allow the use of additives to reduce hydration heat and, in parallel, to increase mechanical strength, taking into advantage the pozzolanic reaction [ 56 ]. On the other hand, their thermal behavior is poor. However, the high alumina cements provide a complete thermal resistance and stability because usual operation exceeds 1000 ◦ C, but unfortunately their high cost would make the TES system unfeasible, due to the increase of at least 500% of the final refractory cost. Besides, high alumina concrete has an elevated hydration heat and its mechanical strength could decrease with time [ 57 , 58 ], which would compromise structural stability if previous drying and thermal treatment is made. High porosity aggregates are selected to provide enough thermal resistance (insulating) capacity. Two kinds of lightweight aggregates, with low density and high porosity, were considered as candidates from most of the current standard and commercial options to achieve a competitive cost: pumice stone (in four grading distributions, PP from Italy and PPP from Puertollano, Spain) and expanded clay (in three grading distributions, AE). The reference used for this pre-selection is taken from the data catalogue of Constructive Elements of CTE (Código Técnico de la Edificación) [ 59 ] which lists concrete density and thermal conductivity values for different types of aggregates, as shown in Table 10. Experimentally, the following grading distributions were analyzed: 0/4 and 6/12 for PP; 0/4, 4/8, and 10/20 for PPP; and 0/4, 3/10 and 8/16 for AE, obtaining density values much lower for the expanded clay (which is selected), directly influenced by its high porosity, inherent to its calcination fabrication procedure [60]. Table 10. Properties of concrete for different aggregates used [59]. Material Density [kg/m3]Thermal Conductivity [W/m·K] Pre-manufacturing concrete 1600 0.69–0.88 Concrete with pumice stone 1300 0.38–0.41 Concrete with expanded clay 1200 0.39–0.41 The rest of the components were selected to ensure the target mechanical strength and concrete workability. The workability was assessed by reaching a soft or fluid concrete by means of the Abrams cone workability test. The achievement of an additional increase of resistance implied the use of fine additions, which helped cement binding power and complements aggregate fine fractions. Factors that determine reaching a fluid or soft consistency were: (i) fines content (additions), (ii) use of admixtures and interaction with cement (plasticizers or super-plasticizers that improve flowability without affecting mechanical parameters), (iii) water content, and (iv) aggregates morphology (use of expanded clays facilitates concrete mass homogeneity). Water content represented the most critical factor Energies 2021,14, 3896 18 of 26 because it leads to a huge variability in concrete property results. Tested additives in this study were fly ashes and silica fume, while super-plasticizer was tested for admixtures. This concrete formulation study began by the combination of the selected components, from mortar dosages until reaching concrete character with coarse aggregates. The main difference in approach compared to the HSSCC development was the combined mechanicalinsulating resistance for the refractory wall, unusual in concrete formulations. Therefore, the work scheme is the following: 1. Target density achievement: fresh state, hardened state after 24 h, hardened state at different ages, hardened state of dry simple (24 h, 45 ◦ C), and hardened state after thermal ageing at 300 ◦C. 2. Thermal resistance assessment. 3. Compressive strength assessment. 4. Behavior assessment after thermal ageing. 5. Conclusions, feedback, and formulation modification. To sum up, the insulating refractory concrete to be developed must exceed conventional values of compressive strength (20–25 MPa in ordinary concretes for structural purposes) and workability (soft or fluid type, more than 6 cm of cone slump), reaching insulating properties by developing a thermal conductivity lower than 0.4 W/m · K. Above all, conventional components materials such as ordinary Portland cement (not the typical high alumina cement in refractories products), lightweight aggregates, simple additions and commercial admixtures were necessary in order to get a competitive cost, much lower than similar commercial concretes at the temperature level indicated. The composition of the refractory concrete with low conductivity is included in the patent WO 2015/055879 A1 [61] and is detailed in Table 11. Table 11. Insulation refractory concrete formulation [61]. Material Quantity * Light aggregate 500–850 kg/m3 Cement 400–700 kg/m3 Water 100–350 kg/m3 Fly ash 50–200 kg/m3 Active dispersing agent 2–10 kg/m3 Ration w/c 0.45–0.75 * The Dreux-Gorisse method [ 54 ] is one technique among others that allows the determination of the proportions of the various constituents (sand, cement, gravel, water) of the concrete, starting with a granulometric analysis. Detailed formulation on the insulating refractory concrete developed in the laboratory is presented in Table 12. Results of the characterization of these tests are shown in Table 13, where adequate consistency was achieved, with a compressive strength higher than the required 25 MPa, and with a thermal conductivity of 0.32–0.33 W/m · K, again lower than that required (0.4 W/m·K). Table 12. Formulation of insulating refractory concrete for lab tests. Material Test 1 Test 2 Test 3 Test 4 Test 5 Test 6 Cement CEM I-52,5R 461 kg 450 kg 450 kg 450 kg 450 kg 450 kg Fly ash — 101 kg 107 kg 107 kg 107 kg 107 kg 107 kg Superplasticizer Glenium C303 SCC 4.66 kg 4.55 kg 4.55 kg 4.55 kg 4.55 kg 4.55 kg Sand AE 0/4 Arlite 506 kg 494 kg 494 kg 494 kg 494 kg 494 kg Gravel AE 3/10 Arlite 218 kg 213 kg 213 kg 213 kg 213 kg 213 kg Water — 196 L 214 L 214 L 214 L 214 L 214 L Relation w/c — 0.65 0.69 0.71 0.71 0.71 0.71 Energies 2021,14, 3896 19 of 26 Table 13. Results of insulating refractory concrete for lab tests. Property Test 1 Test 2 Test 3 Test 4 Test 5 Test 6 Consistency 22 cm 21 cm 21 cm 21 cm 21 cm 21 cm Compression strength at 7 days 24 MPa 25 MPa 21 MPa 20 MPa 20 MPa 19 MPa Compression strength at 28 days 28.5 MPa 28.8 MPa 24.4 MPa 24.0 MPa 23.7 MPa 22.4 MPa Thermal conductivity 0.32–0.33 W/m·K 4.2. Results of Scaling Up Tests 4.2.1. High-Strength Self-Compacting Concrete Fabrication A constructive large-scale sample (50 m 3 ) of the high-strength self-compacting concrete was built to evaluate on-site properties. The ambient conditions during the tests were: min/max temperature 6.1–15.4 ◦ C, average temperature 9.1 ◦ C, min/max relative humidity 45.0–98.8%, average relative humidity 75.5%. Nine different concrete tests were prepared; results are presented below. Due to the high influence of the moisture content in the materials used to produce the concrete mixtures, moisture measurements of sand and gravel were carried out. Table 14 shows that the sand samples had a moisture content of between 3.2% and 5.1%, with a standard deviation of maximum 1.2%, while gravel samples had a moisture content of between 0.95% and 1.7%, with a maximum standard deviation of 0.2%. It should be highlighted that the materials used in tests 8 and 9 were stored outdoors, therefore their moisture content is higher than most of the other samples. Table 14. Moisture content of materials used to produce high-strength self-compacting concrete at pilot plant scale. Test Sand 0/4 Gravel 2/8 Test 1 Test 2 Average St. Dev. Test 1 Test 2 Average St. Dev. 1 3.9 5.3 4.60 0.70 1.4 1.6 1.50 0.10 2 4.4 3.5 3.95 0.45 1.0 1.0 1.00 0.00 3 3.8 3.7 3.75 0.05 1.3 1.1 1.20 0.10 4 2.9 3.7 3.30 0.40 0.8 1.4 1.10 0.30 5 3.2 3.2 3.20 0.00 1.3 1.2 1.25 0.05 6 3.5 3.7 3.60 0.10 1.0 0.9 0.95 0.05 7 3.6 3.7 3.65 0.05 1.2 1.7 1.45 0.25 8 4.3 4.3 4.30 0.00 1.6 1.5 1.55 0.05 9 6.3 3.9 5.10 1.20 1.5 1.9 1.70 0.20 To evaluate these nine samples a slump-flow test was performed (Table 15). Only test 1 and test 3 failed, having bad workability; in all other tests, the compaction was enough to achieve the appropriate mechanical properties. These problems seemed to come from null humidity in the dry additions used (silica fume, fly ash, metakaolin, and filler) as they are stored and prepared inside an industrial facility; this means water absorption is higher than initially expected. Figure 8shows two examples of the slump-flow results in the tests carried out. The relative compression strength results from the successful tests are shown in Figure 9 . The results after 7 days are within 89% and 97% of the target value and after 28 days between 105% and 112% of the target value; therefore, the tests are considered successful. Results show that the scaling up was successful, achieving appropriate mechanical properties when the moisture of the materials to be used is within expected values (too low moisture results in bad workability). Moreover, it was demonstrated that climate conditions (temperature and relative humidity) did not affect the concrete production. Energies 2021,14, 3896 20 of 26 Table 15. Visual inspection and slum test results of the pilot plant scale high-strength self-compacting concrete production. Sample Visual Inspection Slump-Flow Test Test Accepted? T50 (s) Dt(mm) Evaluation 1 A little fluid 11.90 560 Failed No 2A little fluid and disaggregated No segregation 4.16 680 Successful Yes 3 A little fluid 9.00 635 Failed No 4A little fluid and disaggregated No segregation 6.03 610 Successful Need to compaction by vibration Yes 5A little fluid and disaggregated No segregation 5.82 655 Successful Need to compaction by vibration Yes 6No segregation Great aspect and homogeneity 3.07 710 Successful Yes 7No segregation Great aspect and homogeneity 5.88 655 Successful Yes 8 Slight segregation 4.90 610 Successful Need of extra compaction Yes 9Visually disaggregated No segregation 3.72 650 Successful Yes Energies 2021, 14, 3896 21 of 27 (a) (b) Figure 8. Slump-flow tests from (a) test 6 and (b) test 8 high-strength self-compacting concrete production. The relative compression strength results from the successful tests are shown in Figure 9. The results after 7 days are within 89% and 97% of the target value and after 28 days between 105% and 112% of the target value; therefore, the tests are considered successful. Figure 9. Relative compressive strength of the pilot plant scale high-strength self-compacting concrete tests. Results show that the scaling up was successful, achieving appropriate mechanical properties when the moisture of the materials to be used is within expected values (too low moisture results in bad workability). Moreover, it was demonstrated that climate conditions (temperature and relative humidity) did not affect the concrete production. 4.2.2. Insulating Refractory Concrete Fabrication The insulating refractory concrete was developed with the following ambient conditions: min/max temperature 16.2–39.3 °C, average temperature 27.4 °C, min/max relative humidity 16.9–83.5%, average relative humidity 44.1%. Eight different concrete tests were prepared and mixed in two trucks; the results are presented below. The target value of workability is 22 cm for the Abrams cone, set to ensure later transfer from the concrete plant to the construction site, at 30 min distance. In this concrete, the gravel particles were immersed in water for one night to get saturation and ensure the Figure 8. Slump-flow tests from (a) test 6 and (b) test 8 high-strength self-compacting concrete production. 4.2.2. Insulating Refractory Concrete Fabrication The insulating refractory concrete was developed with the following ambient conditions: min/max temperature 16.2–39.3 ◦ C, average temperature 27.4 ◦ C, min/max relative humidity 16.9–83.5%, average relative humidity 44.1%. Eight different concrete tests were prepared and mixed in two trucks; the results are presented below. The target value of workability is 22 cm for the Abrams cone, set to ensure later transfer from the concrete plant to the construction site, at 30 min distance. In this concrete, the gravel particles were immersed in water for one night to get saturation and ensure the absorption grade specified by the supplier (24.10%). The moisture of the sand used in each test was measured, with results from 13% to 22%. With these data and the results from the workability tests, the formulation of each concrete is adjusted to achieve the desired cone (>22 cm). Energies 2021,14, 3896 21 of 26 Once on the construction site, the first mixture seems softer, almost plastic, but with a homogeneous and workable appearance. That is why the workability test is carried out again, obtaining a cone of 17 cm (loss of 5 cm, typical value). Regarding the second mixture, consistency seems fluid and with a homogeneous and workable appearance, reaching cones of 20 cm (again coherent values of loss: 5 cm). Energies 2021, 14, 3896 21 of 27 (a) (b) Figure 8. Slump-flow tests from (a) test 6 and (b) test 8 high-strength self-compacting concrete production. The relative compression strength results from the successful tests are shown in Figure 9. The results after 7 days are within 89% and 97% of the target value and after 28 days between 105% and 112% of the target value; therefore, the tests are considered successful. Figure 9. Relative compressive strength of the pilot plant scale high-strength self-compacting concrete tests. Results show that the scaling up was successful, achieving appropriate mechanical properties when the moisture of the materials to be used is within expected values (too low moisture results in bad workability). Moreover, it was demonstrated that climate conditions (temperature and relative humidity) did not affect the concrete production. 4.2.2. Insulating Refractory Concrete Fabrication The insulating refractory concrete was developed with the following ambient conditions: min/max temperature 16.2–39.3 °C, average temperature 27.4 °C, min/max relative humidity 16.9–83.5%, average relative humidity 44.1%. Eight different concrete tests were prepared and mixed in two trucks; the results are presented below. The target value of workability is 22 cm for the Abrams cone, set to ensure later transfer from the concrete plant to the construction site, at 30 min distance. In this concrete, the gravel particles were immersed in water for one night to get saturation and ensure the Figure 9. Relative compressive strength of the pilot plant scale high-strength self-compacting concrete tests. The relative compression strength results after 1, 7, 28, and 90 days from the two truck concretes are shown in Figure 10. Results from the two truck tests are very similar, reaching the stated target after 90 days, but being at 92%–97% of the target already after 28 days. Energies 2021, 14, 3896 22 of 27 absorption grade specified by the supplier (24.10%). The moisture of the sand used in each test was measured, with results from 13% to 22%. With these data and the results from the workability tests, the formulation of each concrete is adjusted to achieve the desired cone (>22 cm). Once on the construction site, the first mixture seems softer, almost plastic, but with a homogeneous and workable appearance. That is why the workability test is carried out again, obtaining a cone of 17 cm (loss of 5 cm, typical value). Regarding the second mixture, consistency seems fluid and with a homogeneous and workable appearance, reaching cones of 20 cm (again coherent values of loss: 5 cm). The relative compression strength results after 1, 7, 28, and 90 days from the two truck concretes are shown in Figure 10. Results from the two truck tests are very similar, reaching the stated target after 90 days, but being at 92%–97% of the target already after 28 days. Figure 10. Relative compressive strength of the pilot plant scale insulating refractory concrete tests. Results show that controlling the moisture of the materials is needed to obtain good workability and, for this concrete type, ambient temperature affects this workability. Good control of water content in the concrete mixture when performing cone tests ensures that the concrete produced achieves the desired mechanical properties. 4.2.3. Thermal Cycling of the Insulating Refractory Concrete The results of compression strength and Young modulus of the insulating refractory concrete after 100 thermal cycles are shown in Table 16. The compression strength decreased by 41% and the Young modulus by 60%, making the validation of the results difficult. Figure 10. Relative compressive strength of the pilot plant scale insulating refractory concrete tests. Results show that controlling the moisture of the materials is needed to obtain good workability and, for this concrete type, ambient temperature affects this workability. Good control of water content in the concrete mixture when performing cone tests ensures that the concrete produced achieves the desired mechanical properties. Energies 2021,14, 3896 22 of 26 4.2.3. Thermal Cycling of the Insulating Refractory Concrete The results of compression strength and Young modulus of the insulating refractory concrete after 100 thermal cycles are shown in Table 16. The compression strength decreased by 41% and the Young modulus by 60%, making the validation of the results difficult. Table 16. Compression strength test results and elastic modulus (Young modulus) of insulating refractory concrete cylindrical samples after 100 thermal cycles. Sample Compression Strength (MPa) Variation from Non-Cycled Concrete Young Modulus E (GPa) Variation from Non-Cycled Concrete Cylindrical samples 100 ×200 cm 16.06 +/−0.48 −41% 5.4 −60% Table 17 shows that, even though the thermal conductivity of the insulating refractory concrete is lower than the commercial refractory concrete (Table 2), it remains almost constant after 100 thermal cycles. Table 17. Thermal conductivity results of insulating refractory concrete cylindrical samples after 100 thermal cycles. Number of Cycles Thermal Conductivity (W/m·K) Thermal Resistance (mK/W) 0 0.330 — 100 0.276 0.0195 5. Conclusions A complete process of new materials development facing high thermo-mechanical requirements is carried out within a new technology developed by Abengoa, consisting in a cylindrical tank for thermal energy steam accumulation, composed of a double concrete wall made of an external wall of self-compacting high-strength concrete and an internal wall of insulating refractory concrete. Components and dosages analysis are carried out from materials selection to concrete formulations, counting on essential tests for this application (workability, compressive strength, and thermal conductivity) until the proposal of definitive dosages, both patented by Abengoa Solar NT. On-field tests are performed to verify and validate the process of concrete fabrication, implementing the established formulation and fabrication procedures developed during previous research activities. During all the fabrication tests as well as the concreting processes, it is determined and confirmed that, in special concretes like these, there are several external and internal factors that affect the material in a critical way and could modify the dosages initially defined. The main conclusion from the development of the high-strength self-compacting concrete (HSSCC) is: • The moisture of aggregates and additions is the main factor found during this concrete fabrication. This point is directly related to the storage conditions of the raw materials. It is shown that external storage could create an excess of moisture in the aggregates and additions that could cause changes in the hydration process of concrete fabrication. This fact triggers a dosage correction that might be inaccurate if, in following concreting processes, the storing conditions are different, when water absorption increases and, therefore, causes a drier concrete mix that does not meet the expected workability requirements. The main conclusions from the development of this insulating refractory concrete are: • Effective w/c ratio is the key control parameter for the success of insulating refractory concrete fabrication, supported by workability tests with the Abrams cone method. Energies 2021,14, 3896 23 of 26 A suitable and accurate previous determination, as well as a constant control during concrete fabrication, permits the achievement of the target resistance and thermal requirements. The success of this insulating refractory concrete is based on a referent effective w/c ratio establishing limits for the fabrication process. Besides, workability verification presents value of 20 cm in the Abrams cone for acceptance of the concrete mixture transfer. • Aggregates and their physical properties represent the second most important factor to rigorously control the properties of insulating refractory concrete. The selection of such special aggregate, expanded clay, determines that porosity and absorption grade are critical points to match specified requirements in all the mixtures fabricated. It is essential to previously control aggregate porosity and absorption in every batch, since grains absorb a fraction of the dosage water during fabrication, reducing the planned amount of water to be combined with the cementitious products. This fact has a negative effect if the dosage is not previously corrected, leading to structurally weak and non-uniform concrete. • The instantaneous moisture of aggregates is identified as a key parameter. Dosage correction by the absorbed water is directly related to that parameter at the time the compounds are mixed. Due to very high absorption of the selected gravel, presaturation of gravel grains is prescribed so that dosage water is not absorbed by aggregates. Regarding sand, the formulation correction is applied by means of the previous measurement of moisture just before the charge of the other component materials. This factor is also linked to the first (effective w/c ratio), creating a control parameter triangle which must be the axis around which concrete is fabricated: w/c, absorption grade and aggregates moisture. • Finally, ambient temperature is confirmed as the external factor per excellence in concrete fabrication. Moreover, with such high temperatures during this research, this factor becomes critical. It is necessary to minimize temperature affection on concrete with measures such as: (i) previous saturation of aggregates, (ii) protection of mixture fabrication hoppers from direct radiation of sun and (iii) a quick and efficient transfer to construction site to maintain required workability. The effect of thermal cycling on refractory insulating concrete is a decrease in the thermal conductivity (and therefore an increase in insulating effect), but also a reduction in the mechanical strength. After 100 thermal cycles, samples showed a 40% reduction in compressive strength and a 60% in Young Modulus’ values when compared to reference concrete samples. In both types of concrete developed, strict control and monitoring of the concreting process are essential. As shown in this paper, workability problems could appear even though control is meticulous; the absence of this kind of control probably drives failure in the concreting process. For different reasons in each type of concrete developed, aggregates moisture, ambient temperature and w/c ratio have arisen as the key factors for the success of these special materials in the newly designed steam accumulation tank. Although very encouraging results were obtained in this study, further research is needed to corroborate the formulations, to carry out their scalability, and to perform tests at pilot plant scale. Author Contributions: Conceptualization, C.P. and D.P.O.; methodology, C.P., E.G.-R. and L.F.C.; formal analysis, C.P., E.G.-R. and S.F.; investigation, E.G.-R. and D.P.O.; resources, C.P. and L.F.C.; data curation, D.P.O.; writing—original draft preparation, C.P., S.F. and L.F.C.; writing—review and editing, D.P.O. and E.G.-R.; visualization, C.P. and L.F.C.; supervision, C.P. and L.F.C.; project administration, C.P.; funding acquisition, C.P. and L.F.C. All authors have read and agreed to the published version of the manuscript. Funding: The research leading to these results has received funding from CDTI in the project Innterconecta Thesto (ITC-20111050). The work is partially funded by the Spanish government (ENE2015-64117-C5-1-R (MINECO/FEDER)). This work was partially funded by the Ministerio de Ciencia, Innovación y Universidades de España (RTI2018-093849-B-C31—MCIU/AEI/FEDER, UE) and by the Ministerio de Ciencia, Innovación y Universidades—Agencia Estatal de Investi- Energies 2021,14, 3896 24 of 26 gación (AEI) (RED2018-102431-T). This work is partially supported by ICREA under the ICREA Academia programme. Data Availability Statement: Data is available upon request to the corresponding authors. Acknowledgments: The authors would like to thank all the Chirivo Construcciones, Hormigones Aznalcóllar, Elabora, Azcatec, Next Force Engineering, Universidad de Granada, Aidico, Centro Andaluz de Metrología, AICIA and Abengoa teams involved in Solúcar Platform during prototype construction and start-up. Cabeza would like to thank the Catalan Government for the quality accreditation given to her research group GREiA (2017 SGR 1537). GREiA is a certified agent TECNIO in the category of technology developers from the Government of Catalonia. Conflicts of Interest: The authors declare no conflict of interest. References 1. Liu, M.; Steven Tay, N.H.; Bell, S.; Belusko, M.; Jacob, R.; Will, G.; Saman, W.; Bruno, F. Review on concentrating solar power plants and new developments in high temperature thermal energy storage technologies. Renew. Sustain. Energy Rev. 2016 , 53, 1411–1432. [CrossRef] 2. International Renewable Energy Agency (IRENA). Innovation Outlook: Thermal Energy Storage; International Renewable Energy Agency: Abu Dhabi, United Arab Emirates, 2020. 3. González-Roubaud, E.; Pérez-Osorio, D.; Prieto, C. Review of commercial thermal energy storage in concentrated solar power plants: Steam vs. molten salts. Renew. Sustain. Energy Rev. 2017,80, 133–148. [CrossRef] 4. 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