Resumen de Almacenamiento de calor sensible en materiales de base cemento para infraestructuras de centrales termosolares | sensible heat storage in cement-based materials for solar thermal power plants infrastructures
The continuous increase in the global energy demand has intensified the negative effects on climate change. World energy production, mainly based on fossil fuels, is the principal source of CO2 emissions. The fossil fuel depletion has been considered a future challenge and some researchers highlight the need for a holistic solution. For that reason, society needs for accelerating the transition into renewable energy sources and regions such as European Union (EU) are struggling to cope and shape its future and to attain a net-zero emissions continent for 2050. Having this context in mind, the seek for an economy based on a climate neutral needs being met from renewable energies and optimized technologies of energy harvesting, storage and transport to the final consumer at a low price. The present thesis is developed within this context and aims at storing thermal energy from the solar source.
Solar energy is an alternative energy source whose main advantages are sustainability and the no-generation of Greenhouse Gas (GHG) emissions. The limitation of the Solar Thermal Power Plants (STPP) is the intermittence of the source because of the lack of solar radiation at nights or in daily periods with clouds. Thermal Energy Storage (TES) systems are the solution to the problem since they ensure the viability of the STPP by satisfying the energy demand when the solar input is insufficient. Moreover, the development of thermal energy storage technology achieves a higher level of sustainability of the STPP.
Because of the increasing construction of STPP around the World, the interest in TES systems able to store energy at high temperatures has also been increased. In recent years, many experimental studies have demonstrated the usefulness of concrete as solid material for thermal energy storage. Besides, some studies have probed its appropriateness in Concentrated Solar Power (CSP) Plants. Concrete presents several advantages such as its worldwide availability at a relatively low price, it is easy to handle, and it can be worked onsite.
The architecture of 2-tanks is presented in mostly CSP with storage systems, but recent studies have demonstrated the use of a single-one thermocline tank gives better results in terms of cost reduction. In a thermocline tank, both cold and hot Heat Transfer Fluid (HTF) are placed inside the tank and separated due to stratification. When the tank is charging energy, the heated molten salts come into the tank from the upper part and displace the thermocline region to the bottom. At the same time, the cold HTF is being gone out of the thermocline from the bottom part. In the discharging process, hot molten salts get out of the tank from the top and the thermocline zone is moved to the top.
However, as the research goes on, the limits of operation at high temperatures are changing and the medium of thermal storage needs other properties. For example, brand-new studies on molten salts from CSP Plants have demonstrated the suitability for working even at higher temperatures up to 600°C. Therefore, the thermal energy storage material has to be able to store energy at a higher temperature without compromise its performance when it is being operated.
Not only concrete can be used for the role of thermal energy storage. Energy infrastructures also need insulating materials able to withstand high temperatures with the aim of reducing the heat transfer losses to the environment. Those plants also need materials to support the structures and concrete is widely used in structures. The desired thermo-physical properties of concrete depends on the application as an insulating material or as a thermal energy storage material would require a balance between low or high thermal conductivity and low thermal losses. The analysis of the heat transfer at high temperatures for applications of thermal energy storage is of interest to predict the appropriateness of the application analysed in working conditions.
Determining properties at high temperatures gets even more complicated if the working temperature arrives at 600°C. The lack of devices and test protocols for measuring thermal and electrical properties makes this issue a hard task. Also, as the experimental tests are too complex, the lack of data makes that the results of computational models do not fit with the real operation, affecting the expectations. The real operation of the facility will be far from the designed and optimal solution simulated before the construction of the installation.
The behaviour of concrete exposed to high temperatures has been studied for a long time from the point of view of the material in the presence of fire. However, in recent years, new applications in energy infrastructures have consolidated their own line of research. When a concrete structure is exposed to fire, it has to resist high temperatures (1000°C). Nevertheless, the boundary conditions differ to the application of energy storage owing to the fact that in a fire the material is designed for resisting under Ultimate Limit State (ULS) and the second for Serviceability Limit State (SLS). The novel use of concrete as a TES in CSP Plants requires a cyclic operation where the temperature ranges during the performance. Nevertheless, the commissioning of those concrete infrastructures for working at high temperatures implies exposure at high temperatures and this area still remains unexplored.
Not only the operation of the concrete under charging and discharging processes is important, but also the commissioning and preconditioning of this infrastructure for achieving better results in operation.
To prove that the previous lab-scale results could be applicable to the industry, and with the aim to search the different options of concrete in a CTES infrastructure in CSP Plants, this thesis goes deeper into both the commissioning and operating of concrete under high temperatures.
Given the above motivation, the main aim of this thesis is:
To demonstrate the capacity of thermal energy storage of concrete at temperatures up to 550°C and to validate the performance in the commissioning and operating conditions of concrete infrastructures at high temperatures.
Some partial objectives are proposed in order to achieve the global aim:
1) Analyses of the current knowledge on concrete exposed at high temperatures, including the evolution of thermal parameters at high temperatures and possible risks due to the operation at high temperatures. Identification of gaps. This study also includes the literature review of materials and configurations of thermal energy storage in CSP Plants.
2) Design of High Thermal Performance Concrete and experimental characterisation. Identify different concrete dosages and evaluation of the performance at high temperatures and to thermal cycles, following temperature regimes of molten salts (limits of stability between 290-550°C). This study allows evaluating whether the concrete as thermal energy storage is a suitable material to withstand long-term high temperature regimes. The effect of the heat is evaluated through changes in mechanical and physicochemical properties as well as the risk of spalling.
3) Characterisation of thermal properties of different concrete compositions exposed to high temperatures and thermal cycles. Evaluation of the effect of the heat in concrete at high temperatures.
4) Design and propose the commissioning of concrete infrastructures exposed to high temperatures through the evolution of the moisture content and dehydration of concrete by monitoring the evolution of the electrical resistance during the heating process. The demonstration of the viability of this challenging idea implies proposing methodologies for the commissioning, which are validated in different concrete compositions, to evaluate the drying process by monitoring the structure in real time.
5) Up-scaling and thermal operation of concrete specimens at high temperatures simulating the operating conditions of a CSP Plant. Commissioning of the test protocols at a higher scale and monitoring of thermal and electrical properties within the up-scaled concrete, simulating the operation of a thermocline tank. Lessons learned for the suitability of this application.
6) Simulations of the up-scaled concrete exposed to high temperatures assuming the evolution of thermal conductivity with the temperature obtained experimentally.
7) Analysis of the results.
To achieve the objectives mentioned above, the study has been developed in the following steps:
1) Review of the state of the art in thermal energy storage materials and operating conditions of CSP Plants as well as literature related to concrete exposed to high temperatures. This study is analysed in order to find the requirements of the materials for the operation in CSP Plants and the risks and transformations within the concrete when it is exposed to temperatures up to 600°C.
2) Proposal of thermal tests for both the commissioning and testing of concrete structures for being used in CSP infrastructures. These experimental studies start from a Technology Readiness Level (TRL) 3 (experimental proof of concept) to reach TRL 4 (technology validated in the laboratory).
3) Evaluation of physicochemical, mechanical and thermal properties before, during and after the thermal performance. These properties are evaluated within TRL 3 to TRL 4.
4) Developing a non-destructive monitoring system to assess the preconditioning of the concrete through a dying process before operation. This study starts in TRL 2 (technology concept formulated) and the performance is demonstrated in a relevant environment, achieving a TRL 5 and allowing to apply for a patent of invention.
5) Validation of the above-developed experimental methodologies in up-scaled concrete elements for a relevant environment and improving the technology level from TRL 4 to TRL 5.
6) Simulations of heat transfer in concrete infrastructures for CSP Plants. Analysis of different case studies using input data obtained in the experimental tests.
7) Continuous analysis of the results of the work and proposal of future lines of work related to the research.
The research presented in this thesis aimed at improving the knowledge on the use of concrete for infrastructures of thermal energy storage in Solar Thermal Power Plants. The research was carried out at lab scale and then it was validated at an up-scaled section of a thermocline tank made of concrete. The TRL was improved up to TRL 5 because the technology was simulated in a real environment.
The main discoveries of this research figure as follows:
- Regarding the use of concrete for TES infrastructures:
o The design and testing of concretes under regimes of temperatures of a CSP Plant were validated for being used in a TES infrastructure.
o It has been demonstrated that with a proper design and selection of and components, the concrete can withstand thermal cycles up to 550°C. The concrete composition should include Polypropylene fibres for avoiding the risk of spalling during the first heating at high temperatures.
o Regarding the thermal exposure, the most significant degradation was produced after the first thermal cycle. On the contrary, the stabilization of the mechanical and physicochemical properties in long-term performance is a positive aspect for TES applications. It was validated through compressive strength, mass variation, ultrasonic pulse velocity, porosity, X-ray diffraction, thermogravimetric analysis and microcracking analysis at residual conditions.
- Regarding the evolution of the thermal parameters with repetitive heating and cooling cycles:
o The thermal conductivity, volumetric heat capacity and thermal diffusivity experienced changes after cycles but measured at residual conditions.
o The volumetric heat capacity experienced a slower decrease in its value when the concrete was exposed to thermal cycles, achieving between 1.5-1.6·10^6 J/(m3·K) after being exposed to 5 thermal cycles.
o The thermal diffusivity experienced significant changes in the binary composition containing basalt and CAT aggregates (B-C) and the one containing 100% CAT (C). The thermal parameter experienced decreases of around 35-40% of its dried value after being exposed to thermal cycles. The ternary mix B-C-S15% experienced a more stable response, with a variation of around 20% after being exposed to thermal cycles. The values of diffusivity ranged between 0.4-0.5·10^(-6) m2/s.
- Regarding the evolution of thermal conductivity measured at high temperature:
o A protocol for measuring thermal conductivity at temperatures up to 600°C was developed, being the most complete work up to date in the evolution of thermal conductivity with temperatures up to 600°C and for repetitive heating and cooling cycles. This development has allowed the validation of the protocol at lab scale, with an improvement in the TRL up to TRL 4.
o The thermal conductivity of concrete at high temperatures depends on the concrete mix and the aggregates. At room temperature, the values of thermal conductivity range for most of the concrete studied between 1.2-2 W/(m·K), range obtained in concretes with calcareous, basalt, CAT and their combinations of aggregates. The use of siliceous aggregates improved thermal conductivity.
o In all the mixes, the biggest loss in the thermal conductivity takes place during the drying stage due to the loss of free water. When heating up to 600°C, the siliceous mix experienced the biggest drop (↓50%) in the thermal conductivity compared to its initial value, while CAT and ternary mixes displayed more stable behaviour.
o In the first cooling down to 300°C, the thermal conductivity of most concrete mixes does not vary significantly, but siliceous and calcareous mixes recovered between 20-40% of the value at 600°C. In the second heating, the thermal conductivity achieves again the previous values at the equivalent temperatures. The last cooling produced a slight recovery in the thermal conductivity when samples were cooled at 200°C in mixes using siliceous, calcareous and basalt aggregates whereas the other mixes maintained the same value of the thermal parameter.
o The concrete mixes analyzed, excluding the siliceous mix, are recommended for thermal energy storage applications with temperatures between 290-550°C (as expected in TES systems with Solar Salt as HTF), because of the stabilization of their thermophysical properties.
o The assumption of constant thermal conductivity in simulations of concrete for TES systems is not appropriate for the first operating cycles and has to be decided depending on the concrete compounds.
- Regarding the commissioning of concrete infrastructures at high temperatures:
o A protocol for monitoring continuously the evolution of the drying for the heating process in real time based on the measurement of the electrical resistance has been proposed and validated in a TRL 5.
o The level of drying detectable with the measure of the electrical resistance was above 90% of the total mass lost and it was verified for all the concretes tested.
o The methodology developed regarding the measure of the electrical resistance for following the drying process continuously has been validated at lab-scale and up-scale and can be applied for preconditioning large-scale infrastructures of concrete before the operation at high temperatures.
o The continuous measure of the electrical resistance permits following the drying process for a heating process and, hence, can avoid the spalling phenomenon.
o The procedure for detecting the drying process of a concrete structure continuously and non-destructive could be applied to industrial applications such as infrastructures of energy at high temperatures.
o The industrial application of this protocol was proved in the mock-up section of a thermocline tank.
o A total of 20 patents were found and compared to the invention. The requirements for patentability (novelty and inventive activity) were analysed and the invention fulfilled those requirements compared to the prior art found.
o A National Patent was applied with the title “Procedimiento y equipo de medida para detector, de forma continua y no destructiva, el secado de una estructura de hormigón”. The assigned application number is P202031076 and it was registered at OEPM Madrid Office.
- Regarding the thermal operation and simulations of the up-scaled mock-up section of a thermocline tank made of concrete for TES:
o A mock-up of a section of a thermocline tank was built. The section of the thermocline tank included different layers from the heat source to the external surface, as follows: 1) heating mats simulating the heat coming from the molten salts, 2) a steel liner working as a container for the molten salts, 3) an air chamber for allowing the thermal expansion of the steel liner during the thermal operation, 4) a concrete layer (up-scaled up to 230 litres) as a structural element of the infrastructure able to withstand high temperatures and 5) an insulating layer to minimize the heat transfer losses to the environment.
o A recommendation of pouring the concrete by phases in a real tank is given to reduce the peak of temperature reached during the setting period.
o A pre-heating process of the thermocline during the commissioning is needed to reduce risks such as high thermal gradients in the layers that could induce high internal stresses and affect the mechanical performance.
o An initial heating process of the tank is needed before the entrance of the melted salts in the tank with a temperature higher than the melting point in order not to freeze the salts when they are incoming into the tank.
o The concrete element withstood accurately the operation under regimes of high temperatures. For that reason, its use as an insulating element in thermal energy storage tanks is highly recommended.
o The convection heat transfer mechanism in the air chamber highly influences the evacuation of heat from the bottom to the top, which was forced when the heat source was operating at full power (500°C) and free when it was at the minimum cyclic temperature (200°C).
o The convection heat flow accelerates the heat transfer losses and for a thermocline tank concept, having an air gap is not the most efficient because the ratio cost vs energy is reduced. Nonetheless, the gap is needed from the mechanical point of view because the tank filled with molten salts will expand during the operation.
o A balance between the geometry, construction phase and the thermal operation of the infrastructure must be found.
o The simulations showed that the majority of the heat supplied was lost to the environment from the air chamber of the configuration tested.
The results reported and the main conclusions related to the discoveries have led to achieving all the partial and main objectives of the present thesis.
