The efficiencies of thermochemical energy transfer

A general thermodynamic study of thermochemical energy transfer and work production processes is presented. Both gaseous systems in which the effluent of each reactor is not separated into the reactant and product species, and liquid/gas systems in which the effluent separates spontaneously into liquid and gas phases, are treated. the study extends to consideration of non-isothermal reactors, to the individual roles of reactor and heat exchanger in the work production processes and to the significance of the intrinsic work of phase separation. the overall system efficiency is derived as the product of two efficiencies: the energy storage efficiency which defines the fraction of the input energy passed in chemical form to storage and the work recovery efficiency which defines the fraction of this stored energy available as output work. the fundamental thermodynamic processes underlying the derivation of these efficiencies are examined from the point of view of optimization of the design and operation of individual system components. In particular, it is shown that the available work from a thermochemical energy transfer system approaches the maximum value given by the Gibbs' free energy change when the temperature profile of the exothermic reactor is suitably tailored. The work of separation has formed the basis of the analysis of specific system components and has given a useful insight into the understanding of energy storage efficiency. Work recovery efficiencies are calculated for the ammonia/hydrogen-nitrogen system and the paper concludes with a discussion of some practical considerations relating to the recovery of work and the performance that one might ultimately expect from this system.     

Direct work output from thermochemical energy transfer systems

Many important thermochemical transport systems employ hydrogen-based chemical reactions, examples being steam-methane (converting to carbon monoxide and hydrogen) and ammonia (converting to nitrogen and hydrogen). Up to now these have been studied as systems which transport heat in virtual form, i.e. both the input and output energies are heat but in the intervening distance between the terminals energy flows as chemical energy in fluids at ambient temperature. It has been envisaged that if work is required as the output then this can be obtained by providing a conventional heat engine which utilizes the heat generated at the exothermic terminal. In this paper a new concept is presented, whereby the work output is obtained directly from the thermochemical fluid stream. It involves expansion of the fluid through a turbo system after the exothermic reaction. The concept is explored from thermodynamic and economic view points, and the trade-offs between efficiency and cost are discussed. It is shown that the ammonia reaction is a good candidate and consequently a close analysis of this system has been chosen to illustrate the potential of the concept.     

Energy storage efficiency for the ammonia/hydrogen-nitrogen thermochemical energy transfer system

Energy storage efficiency is calculated for the solar thermochemical energy transfer system based on ammonia/hydrogen-nitrogen. the calculation for this system involves generation of thermodynamic data not available in the literature by a method in which use is made of the available phase equilibrium measurements together with application of the criterion that the correct value of separation work for a two-phase mixture must be generated internally by degradation of mixing heat. Energy storage efficiencies for ammonia/hydrogen-nitrogen are derived from the generated thermodynamic data and are shown to increase towards unity as the endothermic reaction approaches completion, with efficiencies greater than 0.90 being obtained for reaction extents exceeding 0.60. the validity of the analysis has been tested successfully by comparison between the thermodynamic predictions and experimental data in the form of measurements of the waste heat rejected from a counterflow heat exchanger operated with liquid ammonia feed and ammonia/hydrogen-nitrogen output.     

Screening of water-splitting thermochemical cycles potentially attractive for hydrogen production by concentrated solar energy

Hydrogen, a promising and clean energy carrier, could potentially replace the use of fossil fuels in the transportation sector. Currently, no environmentally attractive, large-scale, low-cost and high-efficiency hydrogen production process is available for commercialization. Solar-driven water-splitting thermochemical cycles may constitute one of the ultimate options for CO₂-free production of hydrogen. The method is environmentally friendly since it uses only water and solar energy. First, the potentially attractive thermochemical cycles must be identified based on a set of criteria. To reach this goal, a database that contains 280 referenced cycles was established. Then, the selection and evaluation of the promising cycles was performed in the temperature range of 900–2000 °C, suitable to the use of concentrated solar energy. About 30 cycles selected for further investigations are presented in this paper. The principles and basis for a thermodynamic evaluation of the cycles are also given.     

Local storage of solar energy by reversible reactions with sulfates

The problem of the storage of solar energy is of prime importance. Among the possibilities for thermal storage, the use of the heat of a reversible reaction seems to be very attractive because of its high energetic content. In 1961 Goldstein[1] proposed such a method and more recently Wentworth[2] has reviewed many simple stoichiometric decomposition reactions in terms of their reversal temperature. In this paper the authors study the possibilities offered by the decomposition of the sulfates of Mg---Al---Fe---Co---Ni---Cu and Zn in different processes to store solar energy: hot and cold storage with SO₂---O₂ or SO₃ are considered. In addition, an exergy ratio is defined in order to classify the various sulfates. Finally as an example, a determination of the amount of the storage medium is presented for the case of NiSO₄; data from the French Solar power plant (THEMIS) which is under construction were used as a basis and these were compared with the weight of the salt necessary to store a given power.     

Solar-heated rotary kiln for thermochemical energy storage

Thermal energy storage (TES) will improve the efficiency and output of solar power plants. TES based on thermochemical cycles is an interesting option as thermochemical cycles can provide high energy storage densities and allow longer heat storage time. The use of multivalent solid oxide reduction–oxidation (REDOX) reactions for thermochemical heat storage is a promising option. Several concepts are feasible for coupling solar energy to the redox reaction. Among those a directly irradiated rotary kiln is one of the most interesting because it is able to provide high mass flow rates and high amounts of active material. A solar-heated rotary kiln was set-up and operated in the solar furnace of DLR for thermal reduction and oxidation of cobalt oxide. The redox material was fed into the reactor batch wise and reduced on-sun at temperatures of about 900 °C and re-oxidized off-sun in the same rotary kiln. Both steps were carried out in an air atmosphere. Thirty cycles were performed with one batch showing no evident degradation of the material. The results confirm that the rotary kiln is a feasible reactor set-up for the solar reduction of metal oxides and, hence, for thermochemical energy storage.     

Characterization of a thermochemical reactor for thermal solar energy

The main purpose of this work is to elucidate the thermochemical characteristics of a fluidized bed reactor to be used as a solar reactor in thermal energy storage. Zinc sulfate dissociation was studied over the temperature range from 973 to 1123 K. During the reaction problems such as non isothermisity of the bed and pressure drop changes with the reaction, were detected. It was shown that the fluidity increased with temperature and degree of dissociation, but the pressure drop amplitude increased exponentially with gas velocity and particle size when slugging is present in the bed.     

Developing ammonia based thermochemical energy storage for dish power plants

The Solar Thermal Group at the Australian National University has completed an experimental solar-driven ammonia-based closed-loop thermochemical energy storage system. The system uses a cavity receiver containing 20 reactor tubes filled with iron based catalyst material, which collects the radiation from a 20 m² dish solar concentrator. Reliable operation over a range of conditions including cloud transients has been demonstrated. Parallel theoretical investigations have established that maximising the potential for electrical power production from ammonia synthesis reactors, can largely be achieved through appropriate choice of average operating temperature in standard reactors. The possibility of operating the ammonia based system using trough concentrators has also been investigated theoretically, and the preliminary results indicate encouraging energy storage efficiencies in the region of 53%.     

Requirements for designing chemical engines with reversible reactions

Entropy generation during chemical reactions can cause significant irreversibility in chemical engines. These irreversibilities reduce the exergy of the fuel resource preventing potential work production. Understanding the cause of these irreversibilities and developing strategies for reducing them is critical for increasing engine efficiency. All chemical engines can be separated into two categories depending on how they manage the chemical reaction: restrained and unrestrained. A fuel cell with an electric motor is an example of a restrained engine design. Restrained engines have the potential to operate near the reversible limit, with no entropy generation from the chemical reaction. Combustion engines are unrestrained engines, which means the engine design has a built-in irreversibility due to the way it conducts the chemical reaction. This paper defines the requirements necessary to build restrained chemical engines, which helps to identify fundamental strategies for increasing efficiency in both engine designs as well as trade-offs between the two options.     

Research progress of solar thermochemical energy storage

Solar thermal power generation technology has great significance to alleviate global energy shortage and improve the environment. Solar energy must be stored to provide a continuous supply because of the intermittent and instability nature of solar energy. Thermochemical storage (TCS) is very attractive for high‐temperature heat storage in the solar power generation because of its high energy density and negligible heat loss. To further understand and develop TCS systems, comprehensive analyses and studies are very necessary. The basic principle and main components of a solar TCS system are described in this paper. Besides, recent progress and existing problems of several promising reaction systems are introduced. Further research directions are pointed out considering the technical, economic, and environmental issues that existed in the wide application of TCS. Copyright © 2014 John Wiley & Sons, Ltd.     

Thermodynamic analysis of solar thermochemical energy absorption

Thermochemical energy conversion is analysed on a thermodynamic basis with particular interest in obtaining guidance as to solar thermochemical absorber design at an early stage of development when technical and cost information is often unreliable. An earlier-used thermodynamic equivalence technique which is equally applicable to both the endothermic and exothermic reactions has been developed to the point where it gives a clear insight into all sources of heat and work. The method is applied in particular to separating endothermic reactions of which ammonia dissociation is a prime example. A reversibility ratio is defined as the ratio of irreversible to reversible work and it is shown that in a practical solar thermochemical absorber design the reversibility ratio should be minimized, corresponding to reaction temperature minimization and therefore to tower energy losses and to potential for use of lower cost reactor materials and more active catalysts. Values of reversibility ratio are calculated for the ammonia system and are discussed in relation to solar thermochemical absorber design. In a final analysis employing the thermodynamic equivalence technique, it is shown that the apparent paradox between liquid and alternative gas pump work requirements in a liquid/gas thermochemical system is thermodynamically consistent with the internal generation of effective work from the heat source used to drive the endothermic reaction system.     

Corrosion of metals and salt hydrates used for thermochemical energy storage

Solar energy can be efficiently used if thermal energy storage systems are accordingly designed to match availability and demand. Thermal energy storage by thermochemical materials (TCM) is very attractive since these materials present a high storage density. Therefore, compact systems can be designed to provide both heating and cooling in dwellings. One of the main drawbacks of the TCM is corrosion with metals in contact. Hence, the objective of this study is to present the obtained results of an immersion corrosion test following ASTM G1 simulating an open TCM reactor, under humidity and temperature defined conditions. Four common metals: copper, aluminum, stainless steel 316, and carbon steel, and five TCM: CaCl₂, Na₂S, CaO, MgSO₄, and MgCl₂, were studied. Aluminum and copper show severe corrosion when combined with Na₂S, aluminum corrosion is more significant since the specimen was totally destroyed after 3 weeks. Stainless steel 316 is recommended to be used as a metal container material when storing all tested TCM.     

CO2 valorisation based on Fe3O4/FeO thermochemical redox reactions using concentrated solar energy

The solar‐driven dissociation of CO₂ by thermochemical looping via Fe₃O₄/FeO redox reactions is considered. The process recycles and upgrades CO₂ to ultimately produce chemical synthetic fuels from high‐temperature solar heat and abundant feedstock as only inputs. The two‐step process encompasses the endothermic reduction of Fe₃O₄ to FeO and O₂ using concentrated solar energy as the high‐temperature source for reaction enthalpy and the nonsolar exothermic oxidation of FeO with CO₂ to generate CO. The resulting Fe₃O₄ is then recycled to the first step and carbon monoxide can be further processed to syngas and serve as the building block to synthesise various synfuels by catalytic processes. This study examines the thermodynamics and kinetics of the pertinent reactions. The high‐temperature thermal reduction of Fe₃O₄ is realised above the oxide melting point by using concentrated solar thermal power. The reactivity of the synthesised FeO‐rich material with CO₂ at moderate temperature is then investigated by thermogravimetry. FeO conversion higher than 90% can be achieved with reaction rates depending on temperature, particle size and CO₂ concentration. The solar‐produced nonstoichiometric FeO is more reactive with CO₂ than commercial pure FeO. Activation energies of 57 and 68 kJ/mol are derived from a kinetic analysis of the CO₂‐splitting reaction in the range of 600 °C to 800 °C with solar and commercial FeO, respectively. Copyright © 2012 John Wiley & Sons, Ltd.     

Modeling of thermochemical energy storage by salt hydrates

We investigate the capability of salt hydrates to store thermochemical energy as they dissociate into anhydrous salts or lower hydrates and water vapor upon heating using the example of magnesium sulfate heptahydrate as a model salt. An anhydrous salt has a relatively higher energy content than its hydrated counterpart. It can be stably stored over long durations and transported at ambient temperatures. Thermal energy can be released by allowing water vapor to flow across the anhydrous salt, which transforms its chemically stored heat into a sensible form. This has potential as a technology for long-term thermal storage applications, e.g., for storing solar heat during summer months and releasing it in the winter to provide heat for buildings and infrastructure. Water desorption occurs from salt hydrates when they are heated to a critical temperature at which their dehydration is activated. While thermal diffusion governs heat transfer below this temperature, during the dehydration process it is influenced by the desorption kinetics. We model the overall thermochemical process using relations for the conservation of mass and energy, and a relation describing the desorption kinetics, employing a finite difference scheme to solve them. Different cases are considered, which provide guidance about process performance, such as the characteristics of optimally designed salts. The time required for hydrated salts to undergo complete desorption decreases nonlinearly with an increase in the input heat flux.     

Hydrogen production using fusion energy and thermochemical cycles

Thermochemical cycles for the production of synthetic fuels would be especially suited for operation in conjunction with controlled thermonuclear fusion reactors because of very high temperature energy which such reactors could supply. Furthermore, fusion energy when developed is considered to be an inexhaustible supply of energy. Several high-temperature, two step thermochemical cycles for the production of hydrogen are examined. A thermodynamic analysis of the Fe₃O₄-FeO, CrCl₃-CrCl₂, and UCl₄-UCl₃ pairs reveals the feasibility of the processes. A more detailed process analysis is given for the Fe₃O₄-FeO system using steam as the heat transfer medium for decomposing the higher valent metal oxide for oxygen production, and hydrolyzing the lower oxide for hydrogen production. The steam could be heated to high temperatures by refractory materials absorbing the 14 MeV neutrons in the blanket region of a fusion reactor. Process heat transfer and recovery could be accomplished by regenerative reactors. Proposed operating conditions, the energy balance, and the energy efficiency of water decomposition process are presented. With a fusion blanket temperature of 2500 K, thermal efficiencies for hydrogen production (HHV) of 74.4% may be obtained.     

Release of stored thermochemical energy from dehydrated salts

Thermochemical materials, particularly salt hydrates, have significant potential for use in thermal energy storage applications. When a salt hydrate is heated to a threshold temperature, a chemical reaction is initiated to dissociate it into its anhydrous form and water vapor. The anhydrous salt stores the sensible energy that was supplied for dehydration, which can be later extracted by allowing cooler water or water vapor to flow through the salt, transforming the stored energy into sensible heat. We model the heat release that occurs during a thermochemical hydration reaction using relations for mass and energy conservation, and for chemical kinetics and stoichiometry. A set of physically significant dimensionless parameters reduces the number of design variables. Through a robust sensitivity analysis, we identify those parameters from this group that more significantly influence the performance of the heat release process, namely a modified Damköhler number, the thermochemical heat capacity, and the heat flux and flowrate. There is a strong nonlinear relationship between these parameters and the process efficiency. The optimization of the efficiency with respect to the parameters provides guidance for designing engineering solutions in terms of material selection and system properties.     

Doped calcium manganites for advanced high‐temperature thermochemical energy storage

Developing efficient thermal storage for concentrating solar power plants is essential to reducing the cost of generated electricity, extending or shifting the hours of operation, and facilitating renewable penetration into the grid. Perovskite materials of the CaB_xMn_1‐xO_3‐δ family, where B = Al or Ti, promise improvements in cost and energy storage density over other perovskites currently under investigation. Thermogravimetric analysis of the thermal reduction and reoxidation of these materials was used to extract equilibrium thermodynamic parameters. The results demonstrate that these novel thermochemical energy storage media display the highest reaction enthalpy capacity for perovskites reported to date, with a reaction enthalpy of 390 kJ/kg, a 56% increase over previously reported compositions.Copyright © 2015 John Wiley & Sons, Ltd.     

The use of boron for thermochemical storage and distribution of solar energy

Boron has been proposed as a candidate for hydrogen production. In this study a process is described in which boron is used as a means to store and transport solar energy from a production site to motor vehicles, where it is used to generate hydrogen and heat. The proposed multi-step fuel cycle includes no carbon as a reducing agent and, therefore, no release of CO₂ to the atmosphere. This process is safe, mostly involving harmless materials and well-understood technologies. It eliminates the distribution, storage, and pumping of hydrogen at the refueling station, and diminishes the amount of hydrogen stored on the vehicle to a minimum. It is shown that the boron reaction with water, performed on-board of a vehicle, has high hydrogen storage capacity based on both volume and mass, compared with other candidate technologies. An energy balance of the entire process predicts that the overall efficiency of converting solar energy to work by the vehicle engine can be about 11%.     

Thermodynamic simulation of hydrogen based thermochemical energy storage system

The increasing energy demand needs the attention for energy conservation as well as requires the utilisation of renewable sources. In this perspective, hydrogen provides an eco-friendly and regenerative solution toward this matter of concern. Thermochemical energy storage system working on gas-solid interaction is a useful technology for energy storage during the availability of renewable energy sources. It provides the same during unavailability of energy sources. This work presents a performance analysis of metal hydride based thermal energy storage system (MH-TES), which can transform the waste heat into useful high-grade heat output. This system opens new doors to look at renewable energy through better waste heat recovery systems. Experimentally measured PCIs of chosen metal hydride pairs, i.e. LaNi_4.6Al_0.4/La_0.9Ce_0.1Ni₅ (A-1/A-3; pair 1) and LaNi_4.7Al_0.3/La_0.9Ce_0.1Ni₅ (A-2/A-3; pair 2) are employed to estimate the thermodynamic performance of MH-TES at operating temperatures of 298 K, 373 K, 403 K and 423 K as atmospheric temperature (T_atm), waste heat input temperature (T_m), storage temperature (T_s) and upgraded/enhanced heat output temperature (T_h) respectively. It is observed that the system with alloy pair A-1/A-3 shows higher energy storage density of 121.83 kJ/kg with a higher COP of 0.48 as compared to A-2/A-3 pair. This is due to the favourable thermodynamic properties, and the pressure differential between coupled MH beds, which results in higher transferrable hydrogen. Besides, the effect of operating temperatures on COP is studied, which can help to select an optimum temperature range for a particular application.     

Endothermic reactors for an ammonia based thermochemical solar energy storage and transport system

The ammonia dissociation reaction is one of a number of reactions which has been investigated for use in closed loop solar thermochemical energy storage systems, over a period of nearly two decades. A recent series of experiments with an electrically heated high pressure ammonia dissociation reactor has validated a two dimensional pseudo-homogenous theoretical reactor model, established rate parameters for the catalyst used, paved the way for a closed loop demonstration and simulated operation of a receiver/reactor under solar operation. The model has subsequently been used to investigate full sized receiver/reactor options for a 20 m² paraboloidal dish. Technically feasible designs based on: directly irradiated catalyst filled tubes, sodium reflux heat transfer to catalyst tubes and direct absorption of radiation using a windowed pressure vessel, have been identified.