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 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.
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.
The PS20 central receiver plant has 1255 heliostats (with a surface of 120 m2 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 4 shows a schematic flow diagram of the Khi Solar One plant with a superheated direct steam tower and steam accumulator TES.
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 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 high-resistant 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).
On the other hand, insulating concretes are widely known in building engineering, raw material manufacturing and power plant equipment, currently developed as standard 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,
15,
16] or foams [
16,
17,
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,
22,
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.
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.
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 post-tensioned 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 post-tensioning 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 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.
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®).
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. 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, pre-saturation 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.