Dynamic modeling of a solar receiver/thermal energy storage system based on a compartmented dense gas fluidized bed

Fluidized beds may be considered a promising option to collection and storage of thermal energy of solar radiation in Concentrated Solar Power (CSP) systems thanks to their excellent thermal properties in terms of bed-to-wall heat transfer coefficient and thermal diffusivity and to the possibility to operate at much higher temperature. A novel concept of solar receiver for combined heat and power (CHP) generation consisting of a compartmented dense gas fluidized bed has been proposed to effectively accomplish three complementary tasks: collection of incident solar radiation, heat transfer to the working fluid of the thermodynamic cycle and thermal energy storage. A dynamical model of the system laid the basis for optimizing collection of incident radiative power, heat transfer to the steam cycle, storage of energy as sensible heat of bed solids providing the ground for the basic design of a 700kW th demonstration CSP plant

High Temperature Sensible Storage-Molten Salts

The technological limitation in the use of solar energy is intermittent generation due to the lack of storage capacity. The impediment that plants do not generate at night and cloudy days makes their massive participation in the energy matrix complex. For this reason, storage systems allow solar thermal power plants a more stable generation of electrical energy independent of the variability of the solar resource. These systems increase the performance and competitiveness of CSP technologies in terms of LCOE (Levelized Cost of Energy) (Stepper, 2014). The main types of thermal energy storage are classified into thermochemical storage, latent heat storage, and sensible heat storage. In the first case, this type of storage involves a reversible chemical reaction where the used medium must have the ability to completely dissociate in the temperature range of the solar field heat. The amount of heat stored will depend on the heat of reaction and the degree of conversion that is achieved in the exothermic and endothermic process. In the case of latent heat storage, the materials used undergo a phase change at a temperature within the upper and lower range of the solar field. The typical phase used is solid-liquid. These systems are governed by the specific heat of the material and by the enthalpy of the phase change, which allows a large amount of energy to be stored in a smaller volume, and thus at a lower cost if compared to sensible heat storage systems (González-Roubaud et al., 2017)

Latent-Heat Augmentation of Thermocline Energy Storage for Concentrating Solar Power – A System-Level Assessment

Molten-salt thermocline tanks are a low-cost energy storage option for concentrating solar power plants. Despite the potential economic advantage, the capacity of thermocline tanks to store sufficient amounts of high-temperature heat is limited by the low energy density of the constituent sensible-heat storage media. A promising design modification replaces conventional rock filler inside the tank with an encapsulated phase-change material (PCM), contributing a latent heat storage mechanism to increase the overall energy density. The current study presents a new finite-volume approach to simulate mass and energy transport inside a latent heat thermocline tank at low computational cost. This storage model is then integrated into a system-level model of a molten-salt power tower plant to inform tank operation with respect to realistic solar collection and power production. With this system model, PCMs with different melting temperatures and heats of fusion are evaluated for their viability in latent heat storage for solar plants. Thermocline tanks filled with a single PCM do not yield a substantial increase in annual storage or plant output over a conventional rock-filled tank of equal size. As the melting temperature and heat of fusion are increased, the ability of the PCM to support steam generation improves but the corresponding ability of the thermocline tank to utilize this available latent heat decreases. This trend results from an inherent deconstruction of the heat-exchange region inside the tank between sensible and latent heat transfer, preventing effective use of the added phase change for daily plant operations. This problem can be circumvented with a cascaded filler structure composed of multiple PCMs with their melting temperatures tuned along the tank height. However, storage benefits with these cascaded tank structures are shown to be highly sensitive to the proper selection of the PCM melting points relative to the thermocline tank operating temperatures

Assessment of the potential of solar thermal small power systems in small utilities

The potential economic benefit of small solar thermal electric power systems to small municipal and rural electric utilities is assessed. Five different solar thermal small power system configurations were considered in three different solar thermal technologies. The configurations included: (1) 1 MW, 2 MW, and 10 MW parabolic dish concentrators with a 15 kW heat engine mounted at the focal point of each dish, these systems utilized advanced battery energy storage; (2) a 10 MW system with variable slat concentrators and central steam Rankine energy conversion, this system utilized sensible thermal energy storage; and (3) a 50 MW central receiver system consisting of a field of heliostats concentrating energy on a tower-mounted receiver and a central steam Rankine conversion system, this system also utilized sensible thermal storage. The results are summarized in terms of break-even capital costs. The break-even capital cost was defined as the solar thermal plant capital cost which would have to be achieved in order for the solar thermal plants to penetrate 10 percent of the reference small utility generation mix by the year 2000. The calculated break-even capital costs are presented

High temperature packed bed thermal storage for solar gas turbines

A thesis submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy, 2016Solar powered gas micro-turbines present opportunities for off-grid power generation. Due to the intermittent nature of the solar energy supply, existing Solar Gas Turbine (SGT) plants employ hybridisation with fossil fuels to generate dispatchable power. In this work sensible heat and latent heat storage solutions are investigated as a means of increasing the solar share of a SGT cycle, thus reducing the consumption of diesel fuel. The sensible heat storage concept was based on a pressurised packed bed of spherical ceramic particles, using air as the heat transfer fluid. An axisymmetric, two-phase heat transfer model of the system was developed, based on the continuous solid phase approach. The model was successfully validated against experimental data from a packed bed of alumino-silicate particles over the temperature ranges of gas turbine cycles (350-900 °C and 600-900 °C). The validated numerical model was utilised to conduct a parametric design study of a six hour (1.55 MWhth) storage system for a gas micro-turbine. The results show that a high storage efficiency and high utilisation factor can be achieved when combining sensible heat storage in alumina with fossil fuel hybridisation, with somewhat lower values without hybridisation. An analysis of different inventory geometries showed that a packed bed of spherical particles is best suited to pressurised sensible heat storage. The latent heat storage concept was based on a pressurised packed bed of Encapsulated Phase Change Material (EPCM) particles. Sodium sulphate was identified as a suitable phase change material for the gas turbine cycle. The sensible heat storage model was extended to account for intra-particle temperature gradients and phase change within the particles. The intra-particle phase change model was validated against published experimental data for a single EPCM sphere heated and cooled by convection. The full EPCM storage model was further successfully validated against experimental data from a packed bed of macro- encapsulated sodium sulphate particles with alumina shells, up to a temperature of 950 °C. A comparison of the two storage concepts for a 7 m3 bed shows that a packed bed of en- capsulated sodium sulphate particles would have a 36% lower energy storage capacity than a bed of solid alumina particles. This is due to the limited melt fraction in the EPCM bed when a temperature limit is placed on the base. Increasing the packed bed volume to 10.5 m3 would provide a comparable thermal performance to the 7 m3 solid alumina bed, at a 12% lower storage mass. A hybrid three-layer packed bed is proposed to increase the volumetric energy storage density. Modelling shows that this concept could provide a small increase of 5.3% in the amount of energy discharged above 850 °C, compared to the solid alumina particles only

Techno-economic heat transfer optimization of large scale latent heat energy storage systems in solar thermal power plants

Concentrated solar power plants with integrated storage systems are key technologies for sustainable energy supply systems and reduced anthropogenic CO2-emissions. Developing technologies include direct steam generation in parabolic trough systems, which offer benefits due to higher steam temperatures and, thus, higher electrical efficiencies. However, no large scale energy storage technology is available yet. A promising option is a combined system consisting of a state-of-the art sensible molten salt storage system and a high temperature latent heat thermal energy storage system (LHTESS). This paper discusses the systematic development and optimization of heat transfer structures in LHTESS from a technological and economic point of view. Two evaluation parameters are developed in order to minimize the specific investment costs. First, the specific product costs determine the optimum equipment of the latent heat storage module, i.e. the finned tube. The second parameter reflects the interacting behavior of the LHTESS and the steam turbine during discharge. This behavior is described with a simplified power block model that couples both components

Energy and exergy analysis of the Kalina cycle for use in concentrated solar power plants with direct steam generation

AbstractIn concentrated solar power plants using direct steam generation, the usage of a thermal storage unit based only on sensible heat may lead to large exergetic losses during charging and discharging, due to a poor matching of the temperature profiles. By the use of the Kalina cycle, in which evaporation and condensation takes place over a temperature range, the efficiency of the heat exchange processes can be improved, possibly resulting also in improved overall performance of the system. This paper is aimed at evaluating the prospect of using the Kalina cycle for concentrated solar power plants with direct steam generation. The following two scenarios were addressed using energy and exergy analysis: generating power using heat from only the receiver and using only stored heat. For each of these scenarios comparisons were made for mixture concentrations ranging from 0.1 mole fraction of ammonia to 0.9, and compared to the conventional Rankine cycle. This comparison was then also carried out for various turbine inlet pressures (100bar to critical pressures). The results suggest that there would be no benefit from using a Kalina cycle instead of a Rankine cycle when generating power from heat taken directly from the solar receiver. Compared to a baseline Rankine cycle, the efficiency of the Kalina cycle was about around 5% lower for this scenario. When using heat from the storage unit, however, the Kalina cycle achieved efficiencies up to 20% higher than what was achieved using the Rankine cycle. Overall, when based on an average assumed 18hours cycle, consisting of 12hours using heat from the receiver and 6hours using heat from the storage, the Kalina cycle and Rankine cycle achieved almost equal efficiencies. A Kalina cycle operating with an ammonia mole fraction of about 0.7 returned an averaged efficiency of about 30.7% compared to 30.3% for the Rankine cycle

PHASE CHANGE MATERIALS (PCM) FOR THERMAL CONTROL DURING SPACECRAFT TRANSPORTATION

Phase Change materials (PCMs) absorb and release latent heat during their phase transition nearly at constant temperature. The latent heat storage phenomena using PCMs provides much higher storage density, with a smaller or zero temperature difference while storing and releasing of heat. PCMs have 5-14 times more heat capacity per unit volume than sensible storage materials that merits their usage as passive thermal control systems. They are effectively complemented with active thermal control systems in order to minimize their duty cycles and optimize the capacity. This paper discusses a passive thermal control system using PCMs to maintain the temperature within the limits inside the enclosures used for transportation of spacecrafts. Further, various applications of PCMs in the thermal control architecture as applied to spacecrafts are also discussed. The paper also discusses about the technologies such as Onboard power generation, Universal Spacecraft thermal control architecture and other significant spacecraft applications

Exergy recovery from solar heated particles to supercritical CO2

In this work, the technical feasibility of a fluidized and a fixed bed heat exchanger in a concentrating solar power (CSP) tower for heat recovery applications is analysed using Two-Fluid Model simulations. The heat recovery process analysed in this work corresponds to the discharge of sensible heat from solid particles. In the cases studied, the fluidizing agent of the bed is carbon dioxide (CO2) in supercritical conditions and the particles, which constitute the bed material, are sensible heat storage material. CO2 is gaining attention in its application as a working fluid in thermodynamic cycles for power generation, especially in transcritical and supercritical conditions due to its high density and excellent heat transfer characteristics. Currently, research is focused on exploring the CO2 capabilities when used in combination with CSP technologies, together with systems that allow the storage and recovery of the solar thermal energy. Fixed or fluidized beds work as a direct contact heat exchanger between the particles and the working fluid that percolates through the bed material. Several bed configurations are presented to derive the optimal configuration of the bed that enhances the efficiency from both the energetic and the exergetic points of view. The results indicate that a fixed bed heat exchanger produces a maximum increase of availability in the CO2 flow during longer times than a fluidized bed heat exchanger. Therefore, to maximise the exergy recovery from solar heated particles to supercritical CO2 a fixed bed heat exchanger is more suitable than a fluidized bed heat exchanger

Optimized synthesis/design of the carbonator side for direct integration of thermochemical energy storage in small size Concentrated Solar Power

Two of the most attractive characteristics of Concentrated Solar Power are the high-quality heat exploitable and its capacity for thermal energy storage, which enhance the energy dispatchability in comparison with other renewable sources such as photovoltaics or wind. Consistent efforts are therefore direct to the research of suitable thermodynamic cycles and energy storage systems with low thermal losses and high operating temperatures. However, in the most developed technologies, based on sensible and latent heat storage, high thermal losses are the direct consequence of high operating temperatures. As alternative, Thermochemical Energy Storage systems are gaining attention in the last years.The present work investigates the adoption of a novel Calcium-Looping system for Thermochemical Energy Storage, focusing on the integration on carbonator side. This key integration is directly linked to the energy delivery from the energy storage system and therefore power generation capacity of the plant. An optimization of the carbonator side plant is performed for a direct integration layout, where carbon dioxide from the carbonator evolves through the power block. This analysis aims to maximize the system efficiency acting both on the process components operation and on the thermal transfer between the involved streams. The optimization relies on a novel method based on a genetic algorithm. The pinch analysis is adopted for this study and proper constraints are provided to obtain a configuration exploiting only the renewable energy source. A multi-objective optimization is performed to find out the heat exchanger network topology changes that occur for different operating conditions and derived from this analysis suggestion for systems integration are provided. Keywords: Concentrated Solar Power, Calcium-Looping, HEATSEP, Brayton cycle, Long term energy storag