Concentrated solar power for renewable electricity and hydrogen production from water—a review

Abstract

Today's world suffers from an increasing dependence on fossil fuels, either for electricity production, transportation or reagent for the chemical industry. A technological revolution in hydrogen and electricity production is important to support the future needs and lead the world towards a better future. For that, technological and economical barriers have to be broken. Concentrated solar power (CSP) has been proving to be a valid means to start this revolution and produce electricity and hydrogen from completely renewable sources—water and the sun. Although solid steps should be taken to solve the current limitations and increase the technical and economical viability of these projects, there are conditions to begin this revolution using factual bridges from the current fossil technologies to renewable technologies.

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B. Coelho

Bruno Coelho is a mechanical engineering PhD student at the Faculty of Engineering of University of Porto–FEUP. He graduated at FEUP with a masters degree in chemical engineering in 2008 with special emphasis on environment and energy. He has done several presentations at international conferences and is co-author of a few papers under publication. His main research interests are on solar energy and concentrated solar power, especially the production of renewable electricity and chemicals.

A. C. Oliveira

Professor Armando C. Oliveira is an Aggregate Professor at the Department of Mechanical Engineering of FEUP. He is Head of the New Energy Technologies Research Unit of the Institute of Mechanical Engineering–FEUP. He has coordinated and participated in 13 European research and development projects related to the development of new and sustainable energy systems. He is Secretary General of the World Society of Sustainable Energy Technologies, Executive Editor of the International Journal of Low Carbon Technologies (Oxford University Press, UK) and a member of the Editorial Board of the International Journal of Ambient Energy (Ambient Press Ltd, UK).

A. Mendes

Professor Adélio Mendes received his PhD degree from the University of Porto in 1993. He is Aggregate Professor at the Department of Chemical Engineering of FEUP. He coordinates a large research team with research interests in a variety of topics, mainly in fuel cells, dye sensitised solar cells, photoelectrochemical cells for hydrogen production, photocatalysis, catalytic membrane reactors, membrane and adsorbent-based gas separations and numerical methods. Prof. Mendes authored and co-authored more than 100 papers and filled 18 patents.

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Broader context

This review paper looks at the different routes for producing electricity and chemicals from concentrated solar power (CSP). Specifically, the production of hydrogen from water, biomass, fossil fuels, electricity and steam based on CSP power is analysed. This is one of the most up-to-date topics of investigation on energy and environmental science and has a major impact in the research community. In today's fossil fuel dependent world, a technological revolution in fuels and electricity production is important to support the future needs and lead the world towards a better future. For that, technological and economical barriers have to be broken. Concentrated solar power (CSP) has been proven to be a valid means to start this revolution and produce electricity and hydrogen from completely renewable sources—water and the sun. This review addresses the current limitations, technical and economical viability of these technologies, and indicates some possible factual bridges connecting the current fossil technologies to renewable technologies.

Introduction

Hydrogen is being indicated as the fuel of the future. In addition, the so-called “hydrogen economy” has been receiving great governmental support, public and media attention. However, as there are no naturally abundant supplies of hydrogen on the surface of the earth, hydrogen has to be obtained from another primary source and should be referred as an “energy carrier”. A projected increase in global energy demand, especially due to the progressive industrialization of developing countries in Asia and South America, will culminate in a world net electricity generation increase of 77 percent, from 18.0 trillion kilowatt-hours in 2006 to 23.2 trillion kilowatt-hours in 2015 and 31.8 trillion kilowatt-hours in 2030¹—the economic and geopolitical implications of future limitations in oil supply, and consequent concerns about energy supply security, put the discussion about renewable electricity production and the use of hydrogen as an energy carrier on the agenda.

The transport sector is particularly vulnerable to oil supply scarcity, since it is still 95% dependent on oil worldwide.² It represents 18% of primary energy use and about 17% of global CO₂ emissions, with the vast majority of emissions coming from road transport.² Transport is also responsible for 20% of the projected increase in both the global energy demand and greenhouse gas emissions (GHG) until 2030.³ For example, using solar hydrogen in fuel cell (FC) cars can reduce life cycle GHG emissions by 70%, compared to advanced fossil fuels.⁴ Solar hydrogen production allows the reduction of fossil energy requirements by a factor of 10 compared to conventional technologies.⁴ The only environmental impacts are associated with the construction of infrastructures for collecting solar energy, as well as with the hydrogen transport and storage. The major impacts are due to large steel needs for building a CSP plant, since today's steel production technology is responsible for fossil and mineral resource consumption as well as particulate, GHG, and other harmful emissions.⁴

Regarding these scenarios, hydrogen appears attractive as a fuel because it can solve the major fossil fuel environmental, political and economical down points. Hydrogen is advantageous because it can be produced from water and renewable energy, and can be used in highly efficient fuel cells producing only water as a by-product. This may represent a technological step forward to provide a sustainable energy source that can reduce negative environmental effects on the climate, such as greenhouse gas emissions, and satisfy the world’s future fuel demand.

1 Concentrated solar power (CSP)

1.1 CSP state of the art technologies

CSP technologies are presented as the technologies with the greatest economical and technical potential for energy production in the European Union, Middle East and North Africa (EU/MENA), with a total potential of 632.1 PWh/year.⁵

Large-scale commercial implementation of CSP has been done in California, USA and Spain and is now a successful model for many CSP promoting countries. The bulk of new capacity is seen in the Mediterranean bay, which may lead this region to be the heart of renewable CSP electricity generation. This region has abundant solar radiation, cheap land and high electrical demand. These factors have already propelled the growth of the CSP market in the region, with countries such as Spain, Algeria, Morocco, Israel and UAE investing in the promotion and development of CSP plants.⁵

The basic operation principle is to concentrate solar radiation into a receiver, specially designed for high absorption and to reduce heat loss. A fluid flowing through the receiver absorbs the heat, and high temperature–high pressure steam is generated to drive a turbine. Air, water, oil and molten salts are used as heat transfer fluids. Electricity can be generated with traditional Rankine power cycles using steam turbines, or with the Brayton cycle using gas turbines, or using Stirling engines. These CSP systems have been proven to be technically and economically viable in large scale megawatt (MW) units focused on the production of electricity on demand. Several authors have described the currently operating and under construction/project CSP plants.^6–9 The major plants are based on four different optical configurations: tower, parabolic trough, dish and linear Fresnel.

Parabolic trough collectors (PTC), linear Fresnel systems (LF) and power towers or central receiver systems (CRS) can be coupled to cycles of 10 to 200 MW of electric capacity, with thermal cycle efficiencies of 30 to 40%. On the other hand, Dish-Stirling engines (DSE) are normally used for decentralized generation in the range of 10 kW.⁴

1.2 CSP complementary characteristics for thermochemical applications

CSP systems have an advantage for thermochemical applications because they can provide electricity and high temperature process heat to drive endothermic chemical reactions.

Within different CSP technologies each has specific and different operating conditions. PTC and LF are 2D concentrating systems able to concentrate the radiation flux 30 to 80 times, heating the thermal fluid up to 393 °C.⁹ On the other hand, CRS and DSE are 3D concentrating systems that can achieve higher concentration ratios, between 200 and 1000 for CRS and 1000 to 4000 for DSE, with working temperatures above 1000 °C.⁹

Due to a lower concentration capacity of PTC and LF and the size limitations of DSE, the technology with higher potential for hydrogen production is CRS. With a secondary concentration, CRS systems can achieve concentrations of over 1500 suns and temperatures above 1500 °C. The collected thermal energy can be used to provide heat so that hydrogen is produced by chemical reactions that split the water molecule. Many such reaction sets have theoretical efficiencies around 40%. Combined with annual efficiencies for solar thermal systems of about 45%, the solar-to-hydrogen efficiency of this method could go above 20%, with much lower installed collector costs than photovoltaic systems (PV).¹⁰

World CSP implementation is in exponential progress. Currently there are 560 MW of CSP plants in operation, and adding to that, currently there are 984 MW under construction around the world and more than 7463 MW announced to be in development.⁸ In a moderate technological growth scenario, CSP is expected to achieve above 68 000 MW of installed power by 2020 and beyond 230 000 MW by 2030.⁸

Solar thermochemical applications are not as developed as solar thermal electricity generation, but they employ the same solar concentrating technologies. In order to scale-up solar reactors, parameters such as the reactor volume and the loading with the required redox/catalyst coating have to be optimized, taking into account the solar flux and the resulting temperature distribution, the heat transfer characteristics, the reaction rates and transient phenomena due to reactor operation at alternating solar flux conditions.¹¹

The combination of these systems presents interesting synergies that can be exploited.

2 Renewable hydrogen generation from CSP

Primary resource overview

The different primary energy resources could play important roles in connecting bridges from the current technologies to future hydrogen economy technologies. As presented in Fig. 1 there are several routes based on CSP technology to produce hydrogen, which can work considering different configurations of the CRS receptor: external, indirect or direct.

Fig. 1 CSP routes for renewable hydrogen production.

2.2 Hydrogen: supply overview

A “hydrogen economy”, even if based in hydrogen production from renewable sources, faces difficulties to its implementation. Hydrogen production, delivery and safety are major concerns to a full dissemination of hydrogen as a fuel.

Hydrogen transportation and storage is a major logistics problem. Compressed hydrogen storage in lightweight compound bottles or tanks has an energy density of 4.4 MJ L⁻¹ (700 bar) and represents a cost increase of 0.2–1.0 €/kg ³ in hydrogen production price. These tanks currently have operating pressures of up to 700 bar. As tank pressure increases, the energy needed for hydrogen compression also increases. To compress hydrogen to 200 bar, 18% of the total energy stored in the tank is spent.¹²

Depending on the delivery distance, hydrogen liquefaction could also be considered, as the total capital costs could be reduced. For a 100 km delivery distance, the energy consumed percentage to the delivery is 6.27% for compressed hydrogen, while it is 0.87% for liquid hydrogen. These values are still above gasoline (0.19%), propane (0. 27%) and methanol (0.42%).¹³

Operating pressures for liquid storage of hydrogen range from 1 to 3.5 bar and can provide energy densities of up to 8.4 MJ L⁻¹. The main problems related to liquid hydrogen are the energy required for liquefaction, insulation and boil-off losses. About 30% of the total energy can be consumed in the liquefaction process. Besides, in the current pressure vessels for passenger cars there is a hydrogen loss of 2–3% per day.¹² In novel double-walled vacuum insulated tanks these losses could be reduced to 0.2–0.4%.¹²

Alternatively, hydrogen may be stored in hydride compounds. Metal hydrides could have very good hydrogen densities, up to 151 kg m⁻³ (aluminium–AlH₃), but with a large system mass. Therefore, the gravimetric densities do not exceed 2 wt.% at normal pressure and temperature conditions.¹³ With auxiliary heating the gravimetric densities can be increased to 7 wt.%.¹⁴ Higher densities can be reached with the use of chemical hydrides. Such hydrides are usually formed with elements such as B, Al, Mg, and Li. Compounds such as LiBH₄ can reach gravimetric densities of 18 wt.% and Al(BH₄)₃ can carry 17 wt.% of hydrogen. But compounds that achieve these high efficiencies tend to have slow hydrogen releasing mechanisms. Also, boron hydrides, which provide the highest storage capacities, produce volatile borates, which have high hysteresis and can potentially damage fuel cell systems.¹²

The vehicle autonomy is another problem to the use of hydrogen as a fuel. Hydrogen storage solutions are not yet able to provide a range greater than 500 km³ while meeting all the performance parameters; also, it is necessary to build a new distribution infrastructure. The costs of transportation and building a distribution infrastructure accounts for 20–40% of total hydrogen costs; the remaining 60–80% share is attributed to hydrogen production.³

Despite the presented difficulty for hydrogen use as a fuel, the hydrogen market for other applications is still a growing market. The hydrogen production technologies can be switched towards sustainability, basing hydrogen production in renewable resources.

2.3 Electricity and steam: electrolysis

CRS plants for electricity generation are currently in the commercialization phase. These plants are considered one of the most promising CSP technologies for electricity generation and future production of chemicals. A commercial 50 MW solar-only CRS plant with molten salts is expected to produce electricity at 0.155 €/kW h_e (where kW h_e is kilowatt hour of electricity).⁶ Within other CSP technologies this is a promising value, below the predicted electricity cost for mass produced dish engine systems (0.193 €/kW h_e) and current parabolic trough (0.172 €/kW h_e) even when compared to future parabolic trough with direct steam generation (0.162 €/kW h_e).⁶

There are other CRS technologies currently in the commercialization phase that present interesting electricity costs. That is the case for CRS saturated steam system with an estimated electricity cost of 0.169 €/kW h_e and the atmospheric air central system with an electricity cost of 0.179 €/kW h_e.⁶ One of the most promising systems, though still in the experimental phase, is the pressurized air receiver system which works with a combined cycle, allowing better efficiencies and lower electricity production costs of 0.139 €/kW h_e.⁶

These promising prices of electricity can be competitive with other renewable technologies such as photovoltaic (PV). Electrolysis electricity needs could therefore be supported by a CRS plant and hydrogen produced with current commercial electrolysis equipments. Electrolysis could be performed in an acid or alkaline medium. Despite the fact that the discovery of electrolytic water decomposition was first observed in acidic water, in industrial plants the alkaline medium is preferred, because corrosion is more easily controlled and cheaper construction materials can be used. Considering an alkaline electrolyte, hydrogen is generated at the cathode while oxygen is produced at the anode, according to eqn (1) and (2). The charge equalization proceeds by ionic conduction.

2H₂O + 2e⁻ → H₂ + 2OH⁻ (cathode) (1)

2OH⁻ + ½O₂ → H₂O + 2e⁻ (anode) (2)

The major problem of conventional electrolysis is high electricity consumption. It reduces the overall solar-to-H₂ efficiency (14% ¹⁵) and increases the cost of hydrogen (2.1–6.8 €/kg ¹⁶) when compared to other approaches. Other technologies for hydrogen production, such as proton exchange membrane electrolysis and steam electrolysis have been developed in more recent years and could get good results, especially with the use of CSP. Steam electrolysis is a technology that can reach higher energy efficiency when compared to alkaline and proton exchange membrane electrolysis. That occurs because a substantial part of the energy needed for the electrolysis process is added as heat, which is much cheaper and efficient than electrical energy. In this system, hydrogen is produced with a solar-to-H₂ efficiency of 20%¹⁵ and a cost of 5.5–6.7 €/kg.¹⁷

2.4 Fossil fuels and biomass

Fossil fuels and biomass already contain a great deal of chemical energy, contrarily to water, and therefore the required solar plant for the same hydrogen production is significantly smaller, leading to lower overall costs and a significantly lower selling price. The current costs of biomass and fossil fuels can be, in the near term, more competitive for hydrogen production than for water technologies, and provide a good bridge for the future technologies. However, the evolution of prices and availability of biomass and fossil fuels should be considered in the case of market-wide implementation. Basic research was done from 1975, with several earlier projects,¹⁸ and with larger projects in later years. Distributed in different branches of fossil plus CSP technologies, some of these assignments are: solar steam gasification of petroleum coke;^19–21 solar steam reforming of methane, based on the thermal decomposition of methane and steam at about 900 °C²² (project SOLASYS²³ and SOLREF²⁴); and high temperature solar chemical natural gas cracking for co-production of hydrogen and carbon black (project SOLHYCARB²⁴), based on the thermal decomposition of CH₄ into C and H₂ at temperatures in a range of 1225 °C to 2025 °C,²⁵ in various designs of solar reactors,²⁶ with a solar-to-chemical experimental efficiency of 16% and a predicted efficiency of 31% to be achieved.²⁷ Besides current research, there is still a need for new developments, to achieve better performances, overtake the current technological barriers and be competitive. The mid-term strategy should be the upgrading of fossil fuels and biomass, using CSP as a renewable heat source. After establishment of fossil fuels and biomass plus CSP technologies, the long-term strategy should naturally tend to the substitution of fossil fuels and biomass by the use of water as primary energy carrier and raw material for hydrogen production.

2.5 Water: solar thermolysis

Thermolysis is known as the most direct method to obtain hydrogen. High temperature heat, provided for example by CSP, induces a one-step direct thermal water molecule decomposition into H₂ and ½O₂. The technically tolerable reaction temperature limit is around 2225 °C, which theoretically allows dissociation levels of around 4% at atmospheric pressure.²⁸ Generally the thermolysis reactor is constructed using special refractory materials, capable of enduring chemically active environments and very high temperatures, usually above 1225 °C.²⁸ The reactor has to tolerate significant temperature gradients and temperature swings during its operation and lifetime, without degradation, so that it is possible to use concentrated solar power to provide heat for its operation. To achieve the requested temperatures with CSP systems, high concentration ratios are required. The solar field should have a secondary concentrator or be over-designed to support several working hours per day at the required temperature, and that, with addition of the special materials required, leads to unsupportable expenses and reduced near-term feasibility and competitiveness. Additionally, direct thermolysis produces a mixture of H₂ and O₂ that requires high-temperature separation. If allowed to cool, the gases would form an explosive mixture that would be hazardous to personnel and plant.⁹ Several solutions are proposed for this separation, such as a very rapid cooling (quenching) of the mixture, with a reduction of more than 1225 °C within milliseconds, followed by traditional hydrogen separation methods, such as diffusion membranes or hydrogen separation techniques at reaction temperature.²⁸ Near 2225 °C experiments proved that even at such high temperatures, only 25% of water dissociation was observed.²⁹ Nevertheless, thermolysis is still a major area for research, but still needs a breakthrough in material, product separation techniques and process design to allow its commercial implementation.

2.6 Water: solar thermochemical cycles

The search for water thermochemical cycles started worldwide in the late 1960s. During the first 10 years, about 20 publications dealing with thermochemical cycles appeared.³⁰ After this, activity rapidly increased, especially between 1974 and 1986, indicating a very strong increase in interest.³¹ After that period, interest dropped until now, when the need for investigation of new fuels has increased, as well as the need of new innovations to solve hydrogen production problems from water thermochemical cycles to facilitate the shift of hydrogen production from fossil fuels to renewable sources.

The water-splitting thermochemical cycles solve the H₂/O₂ separation problem and allow operation at relatively moderate upper temperatures. Previous studies performed on H₂O-splitting thermochemical cycles were mostly characterized by the use of process heat at temperatures below 925 °C.³² These cycles required multiple steps (one endothermic high temperature step supported by the CSP heat and then followed by exothermic reaction steps) and suffer from inherent inefficiencies associated with heat transfer and product separation at each step. Currently the CSP technology has improved and new, more efficient, cycles have been tested at higher temperatures.

There are, however, as in the direct water thermolysis process, solar peculiarities in comparison to conventional thermochemical processes: high thermal flux density and frequent thermal transitions. Because of the fluctuating solar radiation and weather conditions, variations in the regeneration temperature of the cycles of about ± 50 °C on the receiver have been observed.³³ The two most prominent receiver concepts are the volumetric-air receiver and the solid particle receiver.³⁴ Therefore, CRS and thermochemical commercial processes need to be adapted and, to reduce the solar transients, it is ideal to store solar high-temperature heat in a thermal storage system, and use the stored thermal energy continuously. However, thermal storage at temperatures above 450 °C is very difficult to achieve in an economically competitive way.¹⁶

From all possibilities, several authors³⁵ compiled a database of 280 thermochemical water cycles. These cycles can be divided into “high-temperature” (Table 1) and “low-temperature” categories (Table 2), based on their number of reaction steps and on the operating temperature, above and below 1400 °C, respectively.

Table 1 Thermochemical “high temperature” water splitting cycles adapted from ref. 10

Cycle	Reaction steps	T/°C
FeO/Fe3O4	Fe3O4 → 3FeO + ½O2	2000–2300
	3FeO + H2O → Fe3O4 + H2	400
Zn/ZnO	ZnO → Zn + ½O2	1600–1800
	Zn + H2O → ZnO + H2	400
Sodium manganese	Mn2O3 → 2MnO + ½O2	1400–1600
	2MnO + 2NaOH → 2NaMnO2 + H2	625
	2NaMnO2 + H2O → Mn2O3 + 2NaOH	25
Cadmium carbonate	CdO → Cd + ½O2	1450–1500
	Cd + H2O + CO2 → CdCO3 + H2	350
	CdCO3 → CdO + H2O	500
Hybrid cadmium	CdO → Cd + ½O2	1450–1500
	Cd + 2H2O + CO2 → Cd(OH)2 + H2 (electrochemical)	25 (ambient)
	Cd(OH)2 → CdO + CO2	375

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Table 2 Thermochemical “low-temperature” water splitting cycles adapted from ref. 36

Cycle	Reaction steps	T/°C
Westinghouse cycle (or hybrid sulfur cycle)	H2SO4 → H2SO4 + H2O	875
	H2SO4 → H2O + SO2 + ½O2	875–1275
	2H2O + SO2 → H2SO4 + H2 (electrolysis)	80
General Atomics process (or sulfur–iodine process)	H2SO4 →H2SO4 + H2O	875
	H2SO4 → H2O + SO2 + ½O2	875–1275
	2H2O + I2 + SO2 → H2SO4 + 2HI (Bunsen reaction)	100
	2HI → I2 + H2	300–500
UT-3 cycle	CaBr2 + H2O → CaO + 2HBr	700–760
	CaO + Br2 → CaBr2 + ½O2	500–600
	Fe3O4 + 8HBr → 3FeBr2 + Br2 + 4H2O	200–300
	3FeBr2 + 4H2O → Fe3O4+ 6HBr2 + H2	550–650
Hybrid copper chloride cycle	2Cu2 + 2HCl → 2CuCl + H2	425
	4CuCl → 2Cu + 2CuCl2 (electrochemical)	25 (ambient)
	2CuCl2 + H2O → Cu2OCl2 + 2HCl	325
	Cu2OCl2 → 2CuCl + ½O2	550

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All of the “high temperature” cycles could use CSP heat for thermal reduction of the metal oxide. In the simplest version of the cycle, the oxide is completely reduced to a lower valence state. In the following exothermic reaction, the reduced oxide is put in contact with steam to produce hydrogen and regenerate the original oxide. This is the model for the cycles most closely examined in the literature, Zn/ZnO and FeO/Fe₃O₄.¹⁰ These cycles have only two steps, leading to simple process separations and a low potential for energy losses between cycle steps and during separations. Of all the oxides that are capable of performing the subsequent hydrolysis, ZnO has the lowest decomposition temperature, and would be predicted to have a cycle efficiency of 45% and a solar-to-hydrogen efficiency of 17%.¹⁶

The cycle based on the FeO/Fe₃O₄ pair has also received large amounts of attention in the literature, but its operating temperatures are much higher than in the Zn/ZnO cycle, causing it to lose efficiency and causes material problems. To avoid these problems, some moderated temperature oxide cycles have been proposed: the case of cadmium carbonate (CdO) and sodium manganese (Mn₂O₃). These oxides, when reduced, do not directly split water, so the process must be achieved through more than one additional step, as shown in Table 1. These steps introduce the possibility of side reactions and the consequent need of complicated separations steps. Sodium manganese exhibits thermal reduction at a lower temperature than ZnO and FeO without recombination problems. However, the subsequent steps of the cycle form a sodium–manganese compound that makes the sodium recovery very difficult.¹⁰

Those difficulties are reduced when using a water-splitting cycle, presented in Table 2, which can work at lower temperature. Nonetheless, the cycles with more than three reaction steps suffer from a significant reduction in efficiency, with the use of solar high temperature heat.²⁸ From this perspective, the Westinghouse cycle, with two reaction steps (Table 2), and the General Atomics process with three reaction steps (Table 2), are good solutions. The UT-3 cycle is a clear example of this assessment; the cycle when powered by nuclear energy has an estimated 15% overall efficiency and when operated in solar mode the efficiency is reduced to 8%.³⁶ For the hybrid cycles, an important aspect to determine is the equilibrium of the acid concentration and the cell voltage in order to find the minimum voltage for the best electrochemical efficiency. The hybrid low temperature cycles, which is the case of the hybrid copper chloride cycle, use an electrochemical step to reduce cycle operating temperatures, and can lead to more efficient solar operation and may eliminate some of the complex steps of similar cycles. However, they require the use of electricity that may drive up the final hydrogen cost; on the other hand, these hybrid cycles require less capital.³⁷ Some of the presented water-splitting thermochemical cycles were and are being tested in several projects such as SOLZINC,²³ HYDROSOL and HYDROSOL II,²⁴ HYTHEC,³⁸ and STCH.³⁹ The CRS technological experience and knowledge is used to integrate new components and produce a new, high value and high potential product such as hydrogen.

2.7 CSP thermal applications and other solar technologies to produce hydrogen

There are other technologies that allow the production of hydrogen by a combined solution with solar energy. This is the case with water electrolysis using electricity from PV systems⁴⁰ that can be competitive with CSP for remote small applications. In cases of large scale applications, the electricity produced by CSP is expected to have lower costs than the electricity produced by PV and, therefore, the hydrogen electrolysis plant is more favourable for interaction with CSP systems. Another technology used is the photo-electrochemical system for hydrogen production.^41,42 However, the photo-electrode used in the method of photo-electrochemical decomposition is not yet at the level required for commercialization, and, therefore, should be further studied and developed.⁴³ Still, the use of nanostructured materials for photocatalytic H₂ production is presenting very promising results and could boost this technology.⁴⁴

CSP heat can also be used in a great number of different applications other than hydrogen production, such as process heat, handling of hazardous wastes and testing and synthesis of different materials, e.g. carbon nanotubes and refractory oxides.³⁰ In low temperature applications, CSP can be used for seawater desalinization, supplying heat for multi-stage flash evaporation (MSF), multi-effect distillation (MED), thermal vapour compression (TVC) or supplying clean electricity for mechanical vapour compression (MVC) or reverse osmosis (RO) processes.⁴⁵ CSP can therefore be a means to start solving the alarming water crisis in developing countries with water scarcity, and contribute to the delay of a catastrophic depletion of groundwater resources that would have major effects on economic development and social peace.⁴⁶ CSP solar heat can also be used to provide solar cooling in separated⁴⁷ or integrated solutions, with production of power, cooling and water support.⁴⁸

3 Economic perspective

Table 3 presents a perspective of the expected hydrogen producing prices for different technologies. The majority of these technologies are still at project/prototype scale and it is still difficult to obtain results in comparable conditions.

Table 3 Hydrogen production cost and efficiencies per cycle

Cycle	Production costs (€/kg)	Operating temperature of cycle/°C	Cycle efficiencies (%)	Solar to hydrogen efficiencies (%)
a Adjusted prices based on a 50 MW CSP plant.^16 b Based on a 46 MW CPC Si-G reactor receiver on a CRS.^15 c Based on a solid particle 700 MWth (thermal megawatt) receiver on a CRS.^15 d Adjusted prices based on current process designs and small scale pilot plants.^17 e Based on a CRS-CSP with day and night operation for a projected output of 20000 N m^3 h^−1.^36 f Hybrid copper chloride cycle with a desalinization plant using nuclear energy and adjusting the capital costs to solar energy.^37 g Based on a molten salt 700 MWth (thermal megawatt) receiver on a CRS-CSP plant configuration. The cycle efficiencies consider the efficiency of electricity production by CSP.^15 h CRS-CSP with a heliostat field from 2188–8750 m^2 with elemental carbon production based on ref. 50. i Based on a 5 kW particle flow reactor in a solar furnace.^27 j Based on the SOLASYS reformer with 50 MW power.^22 k Based on a 5 kW reactor in a solar furnace.^20,21 l Based on ref. 51 and considering a cost increase of 25% for the provision and implementation of the carbon capture costs.^16 m Process energy efficiency–energy value of produced hydrogen divided by the energy input—based on ref. 51.
Metal oxide cycles	3.5–13^a	1400–2300	45–60 ^b	17–22 ^b
Westinghouse cycle	3.9–5.6^a	875–1275	51^c	22^c
General Atomics process	2.4–7.9^d	875–1275	45^c	19^c
UT-3 cycle	3.9–4.2^e	700–760	47^e	8^e
Hybrid copper chloride	4.0–5.5^f	550	49^g	23^g
Water electrolysis with CSP electricity	2.1–6.8^a	—	30^g	14^g
High temperature solar steam electrolysis^a	5.5–6.7^d	750–950	45^g	20^g
Solar methane cracking	3.0–3.9^h	1600–1900	70^h	9.1–31^i
Solar steam reforming of methane	1.8–1.9^j	900	86^j	63^j
Solar petroleum coke gasification	—	1600–2100	48–87^k	9–20^k
Commercial coal gasification (with/without carbon sequestration)	0.8/0.64^l	600–1000	63^m	—
Commercial natural gas steam reforming (with/without carbon sequestration)	0.66/0.53^l	900	83^m	—
Biomass gasification	0.85–1.7^l	1100	40–50^m	—
Photocatalytic water splitting	3.5^l	—	10–14^m	—

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Hydrogen produced by the General Atomics process has the lowest production cost when compared to Westinghouse, UT-3 and hybrid copper chloride cycles. The hydrogen produced from the metal oxide based cycle has the greatest variation in production costs and is well above all low temperature cycles in the worst price scenario (Table 3). These high costs are mainly due to the high consumption of metal oxide that is not currently produced on a large scale. The production cost differences among all the water thermochemical cycles are not very expressive and, due to the early stage of market implementation, it is still undefined what cycle would be the cheapest one. In terms of thermochemical cycle efficiency, metal oxide cycles can achieve the highest efficiencies. Depending on the metal selected, the cycle efficiencies vary from 45 to 60%. Afterwards, depending on the receiver configuration, the optical and receiver efficiencies vary from 50 to 55% and 67 to 78%, respectively, and determine the solar-to-hydrogen efficiency of 17 to 22%.

Comparing renewable technologies with the current methods, there are significant differences in hydrogen production costs. Today's hydrogen is mainly produced from steam reforming of natural gas without carbon sequestration, which has significantly lower costs than hydrogen generated from water. However, the currently produced hydrogen from natural gas releases 7.3 kg of carbon dioxide per kilogram of hydrogen; if the hydrogen production is from coal gasification, the emissions would increase to 29 kg CO₂/kg H₂.⁴⁹ Even if carbon sequestration is considered, the prices of hydrogen production would still be unmatchable to the remaining technologies.

Solar steam reforming of methane can be a first step for the integration of CSP in hydrogen production. It presents a lower hydrogen price than methane solar cracking and is the method with lower hydrogen equivalent cost after current hydrogen production technologies. This method could be the earlier described technological bridge from current technologies to completely renewable hydrogen production from water. The early stage petroleum coke gasification method, based in CSP could be another interesting method. It uses an unexplored and rich fraction of the refineries products to produce a valued product such as hydrogen, with interesting efficiencies.

Another technology that should be considered is biomass gasification. Although there are still some major difficulties in cleaning the syngas, the best case hydrogen price scenario could be competitive, as presented in Table 3.

Electricity generated from CSP can also be used to perform water electrolysis. This system could function as a module of the CSP power plant, conferring flexibility and a means to easily convert electricity excesses into a market added-value product. Electrolysis presents, nevertheless, lower overall efficiencies when compared to water thermochemical cycles. This is due to significant larger losses in the electricity generation power block. However, the high temperature solar steam electrolysis is expected to achieve solar-to-hydrogen efficiencies of about 20% and could therefore be considered a competitive solution.

4 Conclusions

The use of concentrated solar power for hydrogen production could be a near term reality. Due to environmental emission restrictions and oil shortage, hydrogen could be a future solution as a renewable energy carrier to substitute fossil fuels. There are, however, several problems, such as hydrogen storage and distribution, that should be solved before the implementation of hydrogen as a fuel. On the other hand, hydrogen that is currently consumed can progressively start being produced from renewable resources.

Large scale production of hydrogen from water could be a long-term solution after the possible implantation of hydrogen as a fuel. Water direct thermolysis and thermochemical cycles have been extensively studied and tested and could be future processes for renewable hydrogen production. Nevertheless, there is still a need for some breakthroughs that can solve the current technological problems. However, the costs and efficiency estimates are promising for water thermochemical cycles such as metal oxide, sulfur iodine and hybrid sulfur cycles. Hydrogen produced from electrolysis could also be a viable renewable solution to hydrogen production if based on CSP electricity, but would have predicted efficiencies below water thermochemical cycles. To boost efficiencies, high temperature steam electrolysis may be used but the current associated costs are slightly above the best cost scenario for other technologies.

Meanwhile, biomass gasification and concentrated solar power cracking/reforming/gasification of fossil fuels with carbon sequestration, particularly methane solar steam reforming, present themselves as bridges that can provide several competitive solutions to assure the current hydrogen demand of the world is met and encourage the use of hydrogen as a fuel, shifting the economy from fossil to renewable energy.

Acknowledgements

The authors are grateful to the Calouste Gulbenkian Foundation for grant ref.: 104299.

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Footnote

† This article results from the Hyceltec 2009 meeting.

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