Comparative study of single and multi-layered packed-bed thermal energy storage systems for CSP plants

Abstract: In order to overcome the increasing demand–supply energy gap due to the rapid urbanization, labor productivity, consumerism and depletion of fossil fuel resources, there is a need for the development of technologies with renewable energy sources. Phase change materials are one of the most appropriate materials for effective utilization of thermal energy from the renewable energy resources. As evident from the literature, development of phase change materials is one of the most active research fields for thermal energy storage with higher efficiency. This review focuses on the application of various phase change materials based on their thermophysical properties. In particular, the melting point, thermal energy storage density and thermal conductivity of the organic, inorganic and eutectic phase change materials are the major selection criteria for various thermal energy storage applications with a wider operating temperature range. The strategy adopted in improving the thermal energy storage characteristics of the phase change materials through encapsulation as well as nanomaterials additives, are discussed in detail. Specifically, the future research trends in the encapsulation and nanomaterials are also highlighted.

Abstract: Concentrated solar power (CSP) is today recognized as a unique renewable energy for electricity generation due to its capability to provide dispatchable electricity incorporating thermal energy storage (TES). Molten salts TES is the most widespread technology in commercial CSP but the industry is looking for cheaper and more efficient TES systems and phase change materials (PCM) have been highlighted as potential low cost and high energy TES systems. This paper presents a completely new concept of PCM energy storage systems to be used in solar thermal electricity plants with its technical assessment. A cascade type PCM storage system is evaluated, using four buckets with the PCM organized based on melting temperature and the latent energy of the materials. Daily, monthly, and annual transient simulations of the plant performance are carried out. The main conclusion is the similarity between this new concept and the commercial two-tank indirect molten salt system. The cumulative power production over the year is similar and the net production of both systems is well matched.

Abstract: Conventional jaggery making process utilizes the bagasse for boiling of sugar cane juice which releases pollutants into the atmosphere and high particulate matter from these emissions causes air pollution. In this article, solar powered jaggery industry with freeze pre-concentration is proposed with conventional and modified heating pans. The system performance, environmental impacts and economic feasibility were assessed by carrying out 4E (Energy-Exergy-Environment-Economic) analyses using the developed mathematical model. These systems were designed to produce 300 kg of jaggery per day when operated for 7.5 h in 3 batches with average solar direct normal irradation of 662 W/m2 and 343 °C. These systems are integrated with auxiliary heating for uninterrupted production in the absence of sunlight. These systems can mitigate nearly 2015.95 to 3062.15 tons of CO2 emission during its 25 years of lifespan under 300 clear days of operation each year. Jaggery produced by this technique is rich in its colour and completely safe for human consumption as no artificial clarificants are used. Amount invested in these systems can be recovered in a span of 12.03 to 13.45 years for jaggery selling price of USD.0.514/kg or INR.36/kg.

Abstract: The thermal energy storage system is a pivotal system for solar thermal plants for improving reliability. The stability in the thermocline is more significant to clarify and improve the performance of thermal energy storage tank which legitimately shows the quality of the thermocline. In this stability analysis investigation, the modern engineering energy storage material concrete was used as a filler material for high-temperature thermal energy storage applications as a result of the intrinsic properties. A comprehensive laminar and k-e turbulent flow energy transport model accounts for the heat transfer fluid and filler material with adiabatic and non-adiabatic conditions using LTNE (Local Thermal Non-Equilibrium model). The axial, radial, and diagonal temperature differences were identified which was used to calculate the stability of the thermocline. A thermal energy storage tank size of 1 m height and 0.250 m diameter with a 0.030 m size of filler material packed with an average porosity of 0.3 for the storage capacity of 150 kWh/m3 is used for solar process heating applications considered for the present study. The thermocline stabilities are performed with Reynolds numbers, Re varied from 1 to 3000. It is found that the Re = 1 provides better stability in the axial and radial direction as well as diagonal than other Reynolds number. It is observed that Re = 1 provides superior discharging efficiency for nearly 5.84 hrs which is highly suitable for solar process heating applications and the discharging efficiency consistently drops, when Re increases from 1 to 3000. The wall condition of the tank and velocity of the heat transfer fluid is highly disturbing the thermocline in the radial direction and it creates a ‘spike’ profile in the axial direction. Based on the newly introduced stability scale, the effective length of packing and timing to achieve stability is identified for H/D = 4. From that result, the top and bottom layer of the thermocline tank porosity is also found which is used to decide the porosity of the packed bed distributors. The identified porosity for the top and bottom distributors in the ɛ = 0.3 thermal energy storage tank is less than 0.3 is more suitable for provide the uniform flow in the tank.

Abstract: Convective motions are a multi-physics phenomenon, in which flow and transport processes interact in a two-way coupling. The density of the fluid depends on the value of transport variable and this back-coupling leads to non-linear behaviour. For the classical constellation of a closed fluid container heated from below convective motions appear, when a critical threshold for the Rayleigh number is exceeded. The heat transfer due to convection is much higher than in the case of pure conduction. Here systems of three layers are examined in detail. Using numerical CFD modelling it is shown that in layered systems different convective flow patterns appear than in the single layer case. The number and constellation of convection cells characterize steady flow patterns. Using a parametric sweep over the relevant parameter range of layer Rayleigh numbers and layer thicknesses we determine diagrams that show the excess heat or mass transfer of the dominant convection patterns, measured by the Nusselt- or Sherwood numbers.

Abstract: Concentrated solar thermal power generation is becoming a very attractive renewable energy production system among all the different renewable options, as it has have a better potential for dispatchability. This dispatchability is inevitably linked with an efficient and cost-effective thermal storage system. Thus, of all components, thermal storage is a key one. However, it is also one of the less developed. Only a few plants in the world have tested high temperature thermal energy storage systems. In this paper, the different storage concepts are reviewed and classified. All materials considered in literature or plants are listed. And finally, modellization of such systems is reviewed.

Abstract: The published heat transfer data obtained from steady and nonsteady measurements are corrected for the axial fluid thermal dispersion coefficient values proposed by Wakao[1]. The corrected data in the range of Reynolds number from 15 to 8500 are correlated by the analogous form of the mass correlation proposed by Wakao and Funazkri[2]: Nu= 2 + 1.1 Pr13 Re0.6

Abstract: Thermal storage improves the dispatchability and marketability of parabolic trough power plants allowing them to produce electricity on demand independent of solar collection. One such thermal storage system, a thermocline, uses a single tank containing a fluid with a thermal gradient running vertically through the tank, where hotter fluid (lower density) is at the top of the tank and colder fluid is at the base of the tank. The thermal gradient separates the two temperature potentials. A low-cost filler material provides the bulk of the thermal capacitance of the thermal storage, prevents convective mixing, and reduces the amount of fluid required. In this paper, development of a thermocline system that uses molten-nitrate salt as the heat transfer fluid is described and compared to a two-tank molten salt system. Results of isothermal and thermal cycling tests on candidate materials and salt safety tests are presented as well as results from a small pilot-scale (2.3 MWh) thermocline.

Abstract: A literature review was carried out to critically evaluate the state of the art of thermal energy storage applied to parabolic trough power plants. This survey briefly describes the work done before 1990 followed by a more detailed discussion of later efforts. The most advanced system is a 2-tank-storage system where the heat transfer fluid (HTF) also serves as storage medium. This concept was successfully demonstrated in a commercial trough plant (13.8 MW e SEGS I plant; 120 MWh t storage capacity) and a demonstration tower plant (10 MW e Solar Two; 105 MWh t storage capacity). However, the HTF used in state-of-the-art parabolic trough power plants (30-80 MW e ) is expensive, dramatically increasing the cost of larger HTF storage systems. Other promising storage concepts are under development, such as concrete storage, phase change material storage, and chemical storage. These concepts promise a considerable cost reduction compared to the direct 2-tank system, but some additional R&D is required before those systems can be used in commercial solar power plants. An interesting and likely cost-effective near-term option for thermal energy storage for parabolic trough power plants is the use of an indirect 2-tank-storage, where another (less expensive) liquid medium such as molten salt is utilized rather than the HTF itself.