Antoni Gil

Review of Solar Thermal Storage Techniques and Associated Heat Transfer Technologies

Thermal energy storage is a key component of solar power plants if dispatchability is required. On the other hand, although different systems and many materials are available, only a few plants in the world have tested thermal energy storage systems. Here, all materials considered in literature and/or used in real plants are listed, the different systems are described and analyzed, and real experiences are compiled. The associated heat transfer technologies to support and improve these systems are described and analyzed.

New Thermal Energy Storage Materials From Industrial Wastes: Compatibility of Steel Slag With the Most Common Heat Transfer Fluids

Slag is one of the main waste materials of the iron and steel manufacturing. Every year about 20 × 106 tons of slag are generated in the U.S. and 43.5 × 106 tons in Europe. The valorization of this by-product as heat storage material in thermal energy storage (TES) systems has numerous advantages which include the possibility to extend the working temperature range up to 1000 °C, the reduction of the system cost, and at the same time, the decrease of the quantity of waste in the iron and steel industry. In this paper, two different electric arc furnace (EAF) slags from two companies located in the Basque Country (Spain) are studied. Their thermal stability and compatibility in direct contact with the most common heat transfer fluids (HTFs) used in the concentrated solar power (CSP) plants are analyzed. The experiments have been designed in order to cover a wide range of temperature up to the maximum operation temperature of 1000 °C corresponding to the future generation of CSP plants. In particular, three different fluids have been studied: synthetic oil (Syltherm 800®) at 400 °C, molten salt (Solar Salt) at 500 °C, and air at 1000 °C. In addition, a complete characterization of the studied slags and fluids used in the experiments is presented showing the behavior of these materials after 500 hr laboratory-tests.

Design of a 100 kW Concentrated Solar Power on Demand Volumetric Receiver With Integral Thermal Energy Storage Prototype

A new concept of Thermal Energy Storage (TES) system based on current available technologies is being developed under the framework of the Masdar Institute (MI) and Massachusetts Institute of Technology (MIT) collaborative Flagship Program. The key feature of this concept lies on concentrating sun light directly on the molten salt storage tank, avoiding the necessity of pumping the salts to the top of a tower thereby avoiding thermal losses and pumping and electric tracing needs inherent in most conventional CSP plants.This Concentrated Solar Power on Demand (CSPonD) volumetric receiver/TES unit prototype will be tested in the existing MI heliostat field and beam down tower in Abu Dhabi (UAE) which will collect and redirect solar energy to an upwards-facing final optical element (FOE). These energy will be concentrated on the aperture of the prototype designed to store 400 kWh of energy allowing 16 hours of continuous production after sunset using Solar Salt (60%NaNO3 + 40%KNO3) as storage material.The tank is divided in two volumes: one cold in the bottom region, where Solar Salt is at 250 °C and another hot on the upper region, at 550 °C. A moving divider plate with active control separates both volumes. The plate includes mixing enhancement features to help with convection on the hot volume of salts.It’s expected that results will demonstrate the technical feasibility and economic viability of this concept allowing its scale up at commercial size.Copyright

New Thermal Energy Storage Materials From Industrial Wastes: Compatibility of Steel Slags With the Most Common Heat Transfer Fluids

Slag is one of the main waste materials of the iron and steel manufacturing. Every year about 20 million tons of slag are generated in the United States and 43.5 million tons in Europe. The revalorization of this by-product as heat storage material in thermal energy storage systems would have numerous advantages which include: the possibility to extend the working temperature range up to 1000 °C, the reduction of the system cost and, at the same time, the decrease of the quantity of waste in the iron and steel industry.In this paper, two different electric arc furnace slags from two companies located in the Basque Country (Spain) are studied. Their thermal stability and compatibility in direct contact with the most common heat transfer fluids used in the concentrated solar power plants are analyzed. The experiments have been designed in order to cover a wide temperature range up to the maximum operation temperature of the future generation of concentrated solar power plants (1000 °C). In particular, three different fluids have been studied: synthetic oil (Syltherm 800®) at 400 °C, molten salt (Solar Salt) at 500 °C and air at 1000 °C. In addition, a complete characterization of the studied slags and fluids used in the experiments is presented showing the behavior of these materials after 500 hour laboratory-tests.Copyright

Concentrated solar power on demand demonstration: Construction and operation of a 25 kW prototype

Currently, the majority of concentrated solar power (CSP) plants built worldwide integrate thermal energy storage (TES) systems which enable dispatchable output and higher global plant efficiencies. TES systems are typically based on two tank molten salt technology which involves inherent drawbacks such as parasitic pumping losses and electric tracing of pipes, risk of solidification and high capital costs. The concept presented in this paper is based on a single tank where the concentrated sunlight is directly focused on the molten salt. Hot and cold volumes of salt (at 565 °C and 280 °C, respectively) are axially separated by an insulated divider plate which helps maintain the thermal gradient. The concept, based on existing technologies, seeks to avoid the listed drawbacks as well as reducing the final cost of the TES system. In order to demonstrate its feasibility, Masdar Institute (MI) and Massachusetts Institute of Technology are developing a 25 kW prototype to be tested in the Masdar Solar Platform...

CSPonD demonstrative project: Start-up process of a 25 kW prototype

The current concept of commercial concentrated solar power (CSP) plants, based on the concept of a solar field, receiver, storage and power block, experienced significant growth in the past decades. The power block is the most well know part of the plant, while solar field depends on the receiver technology. The dominant receiver technologies are parabolic troughs and central towers. Most thermal energy storage (TES) relies on two tanks of molten salts, one hot and one cold serviced by pumps and piping systems. In spite of the technical development level achieved by these systems, efficiency is limited, mainly caused by thermal losses in piping, parasitic losses due to electric tracing and pumping and receiver limitations. In order to mitigate the these issues, a new concept called Concentrated Solar Power on Demand (CSPonD), was developed, consisting of a direct absorption Solar Salt CSP receiver which simultaneously acts as TES tank. Currently, in the frame of the flagship collaborative project between the Masdar Institute (UAE) and the Massachusetts Institute of Technology (USA) a 25 kW demonstrative prototype is in its final building phase at the Masdar Institute Solar Platform. The present paper, explains the demonstration prototype based on the CSPonD concept, with emphasis on the planned start-up process for the facility.The current concept of commercial concentrated solar power (CSP) plants, based on the concept of a solar field, receiver, storage and power block, experienced significant growth in the past decades. The power block is the most well know part of the plant, while solar field depends on the receiver technology. The dominant receiver technologies are parabolic troughs and central towers. Most thermal energy storage (TES) relies on two tanks of molten salts, one hot and one cold serviced by pumps and piping systems. In spite of the technical development level achieved by these systems, efficiency is limited, mainly caused by thermal losses in piping, parasitic losses due to electric tracing and pumping and receiver limitations. In order to mitigate the these issues, a new concept called Concentrated Solar Power on Demand (CSPonD), was developed, consisting of a direct absorption Solar Salt CSP receiver which simultaneously acts as TES tank. Currently, in the frame of the flagship collaborative project between ...

Optical property characterization of molten salt mixtures for thermal modeling of volumetrically absorbing solar receiver applications

A method for experimentally determining the attenuation coefficient of high temperature semi-transparent liquids for volumetrically absorbing solar receiver applications was developed. The method was used to measure the attenuation coefficient over a broad spectral range in a 40 wt. % KNO3:60 wt. % NaNO3 binary nitrate molten salt mixture (solar salt). The measured absorption bands extend over 98% of the re-emission spectrum of the salt, indicating that thermal redistribution within the salt itself via radiative participating media effects is negligible. In addition, the effects of the salt’s purity and thermal decomposition on the optical properties were also investigated and the light penetration depth is shown to vary significantly in the presence of impurities. The implications of these results for solar receiver design and modeling are discussed.A method for experimentally determining the attenuation coefficient of high temperature semi-transparent liquids for volumetrically absorbing solar receiver applications was developed. The method was used to measure the attenuation coefficient over a broad spectral range in a 40 wt. % KNO3:60 wt. % NaNO3 binary nitrate molten salt mixture (solar salt). The measured absorption bands extend over 98% of the re-emission spectrum of the salt, indicating that thermal redistribution within the salt itself via radiative participating media effects is negligible. In addition, the effects of the salt’s purity and thermal decomposition on the optical properties were also investigated and the light penetration depth is shown to vary significantly in the presence of impurities. The implications of these results for solar receiver design and modeling are discussed.

Industrial waste materials and by-products as thermal energy storage (TES) materials: A review

A wide variety of potential materials for thermal energy storage (TES) have been identify depending on the implemented TES method, Sensible, latent or thermochemical. In order to improve the efficiency of TES systems more alternatives are continuously being sought. In this regard, this paper presents the review of low cost heat storage materials focused mainly in two objectives: on the one hand, the implementation of improved heat storage devices based on new appropriate materials and, on the other hand, the valorisation of waste industrial materials will have strong environmental, economic and societal benefits such as reducing the landfilled waste amounts, reducing the greenhouse emissions and others. Different industrial and municipal waste materials and by products have been considered as potential TES materials and have been characterized as such. Asbestos containing wastes, fly ashes, by-products from the salt industry and from the metal industry, wastes from recycling steel process and from copper refining process and dross from the aluminium industry, and municipal wastes (glass and nylon) have been considered. This work shows a great revalorization of wastes and by-product opportunity as TES materials, although more studies are needed to achieve industrial deployment of the idea.A wide variety of potential materials for thermal energy storage (TES) have been identify depending on the implemented TES method, Sensible, latent or thermochemical. In order to improve the efficiency of TES systems more alternatives are continuously being sought. In this regard, this paper presents the review of low cost heat storage materials focused mainly in two objectives: on the one hand, the implementation of improved heat storage devices based on new appropriate materials and, on the other hand, the valorisation of waste industrial materials will have strong environmental, economic and societal benefits such as reducing the landfilled waste amounts, reducing the greenhouse emissions and others. Different industrial and municipal waste materials and by products have been considered as potential TES materials and have been characterized as such. Asbestos containing wastes, fly ashes, by-products from the salt industry and from the metal industry, wastes from recycling steel process and from copper ...

Parametric and Thermal Management Optimization of a Steel Slag Based Packed Bed Heat Storage

In this work steel slag, one of the main by-products of the steelmaking industry, is proposed as a competitive and effective heat storage material. The implemented storage design suggested for this material is a solid packed bed arrangement based in the temperature stratification (thermocline) phenomena. In particular, two different solutions based on different storage tank geometries, cylindrical and conical, have been modeled by using computational fluid dynamic (CFD) methods. In addition, both geometries have been simulated under two different operation modes as a function of the used heat transfer fluid: solar salt and air. This selection permitted to investigate the operation of the proposed storage for current CSP technologies which make use of molten salt as storage/heat transfer fluid and also the analysis of the system when the operation parameters are potentially associated to new generation CSP plants at higher temperatures, above 600 °C. The comparison between the simulated systems has allowed to determine the influence of the driving parameters on the proposed storage solution, such as the operation temperature range, nature of the heat transfer fluid or geometrical implications. The thermal management of the storage unit has also been shown during a transient operation up to a reproducible behavior.Overall, the selected parameters for the presented modeling analysis have revealed the high potential of steel slag as heat storage material and the suitability and flexibility of the implemented packed bed solution.Copyright