High Temperature Indirect Solar Thermal Hydrogen Processes

Cpc For Solar Furnace

More recently, higher temperature processes have been considered at T gt 2000 K , such as two-step thermal chemical cycles using metal oxide reactions.2 The first step is solar the endothermic dissociation of the metal oxide to the metal or the lower-valence metal oxide. The second step is non-solar, and is the exothermic hydrolysis of the metal to form H2 and the corresponding metal oxide. The net reaction is H2O H2 0.5 O2, but since H2 and O2 are formed in different steps, the need for...

References

Backus, A Historical Perspective on Concentrator Photovoltaics, Proceedings of the International Solar Concentrator Conference for the Generation of Electricity or Hydrogen, Alice Springs, Australia, November 2003. 3. R. M. Swanson, Straight Talk About Concentrators, Future Generation Photovoltaic Technologies First NREL Conference, Denver, Colorado, American Institute of Physics Conference Proceedings 404, October 1997, p. 277-284. 4. R. M. Swanson, The promise of...

Direct Solar Thermal Hydrogen Processes

Thermochemical Reactor

The problems with materials and separations at such a high temperature make direct decomposition not attractive at this time. The production of hydrogen by direct thermal splitting of water generated a considerable amount of research during the period 1975-1985. Fletcher and co-workers stressed the thermodynamic advantages of a one-step process with heat input at as high a temperature as possible.25-27 The theoretical and practical aspects were examined by Olalde,28 Lede,29-31 Ounalli,32...

Cpv Hydrogen Hybrid

Renewable Energy and the Hydrogen Krishnan Rajeshwar, Robert McConnell, Kevin Harrison, and Stuart Licht 1 Renewable Energy and the Terawatt 2 Hydrogen as a Fuel of the 3 Solar Energy and the Hydrogen 4 Water Splitting and 5 Completing the Loop Fuel 6 Concluding 2. The Solar 1 Introduction Basic Properties of the 2 The Spectral Distribution of the Sun as a Radiation Source 20 3 The Earth's Atmosphere as a 4 Utilization of Solar Spectral Regions Spectral Response of 5 Reference Spectral 3....

Water Splitting and Photosynthesis

Pourbaix Diagramm H2o

The decomposition of liquid water to form gaseous hydrogen and oxygen is a highly endothermic and endergonic process with AH 285.9 kJ mol and AG 237.2 kJ mol. This reaction may be driven either electrochemically or thermally via the use of solar energy. The standard potential AE for Reaction 1 corresponding to the transfer of two electrons is given by In Eq. 2, F is the Faraday constant 96485 C mol-1 and the negative sign denotes the thermodynamically non-spontaneous nature of the water...

Photochemical Diodes and Twin Photosystem Configurations for Water Splitting

As mentioned earlier, photochemical diodes489 can be either of the Schottky type, involving a metal and a semiconductor, or a p-n junction type, involving two semiconductors which can be the same, i.e., a homojunction or different, a heterojunc-tion . Only the latter type is considered in this Section involving two irradiated semi-conductor electrolyte interfaces. Thus n-TiO2 and p-GaP crystal wafers were bonded together through the rear Ohmic contacts with conductive Ag epoxy cement.489 The...

The Hydrogen Evolving Reaction HER Hydrogen Evolution Catalysts

Iron Iron Hydrogenase Structure

The vast majority of hydrogen evolving catalysts are still mostly limited to noble-metal colloids or solids.257-260 Early studies on solar or photo-driven hydrogen production used these colloids along with a sensitizer such as Rubpy and electron relays e.g., MV2 with modest success.15,16,261-268 The development of good molecular catalysts for HER remains a surprisingly elusive goal excepting the biologically produced iron-only and iron-nickel hydrogenases.269 These enzymes are able to catalyze...

Hydrogen Vision Using Hybrid Solar Concentrators

National Research Council and National Academy of Engineering believes that one of the four most fundamental technological and economic challenges for the hydrogen economy is To reduce sharply the costs of hydrogen production from renewable energy sources over a time frame of decades 23 Table 4. Cost comparison for the hybrid CPV production of electrolytic hydrogen. Note that 1 kg of hydrogen has the energy equivalent of one U.S. gallon of gasoline. Table 4. Cost comparison for the...

Types of Approaches

Photoelectrolysis Cell Configuration

Figure 1 illustrates the interfacial energetics involved in the photoelectrochemical evolution of H2. Thus, the electronic energy levels in the semiconductor and in the contacting solution are shown on a common diagram. In a semiconductor, the filled electronic levels valence band or VB and the empty levels conduction band or CB are separated by a forbidden gap, namely, the band gap energy, Eg.98-100 Photoexcitation of the semiconductor with light of energy equal to or exceeding Eg i.e., with...

Electrolysis of Water

Alkaline Electrolysis Stack

Personally, I think that 400 years hence the power question in England may be solved somewhat as follows. The country will be covered with rows of metallic windmills working electric motors, which in their turn supply current at a very high voltage to great electric mains. At suitable distances there will be great power stations where during windy weather the surplus power will be used for the electrolytic decomposition of water into hydrogen and oxygen In times of calm, the gases will be...

Natures Photosynthetic Machinery

Photosynthetic Machinery

Natural photosystems have been extensively studied and the machinery of photosynthesis found to be highly modular, organized on multiple scale levels and compart-mentalized.32 Importantly, all natural photosystems are molecule-based and their function can be understood both at the schematic level and, in many cases, at the molecular-level. Highly specialized components interact in controlled manner to ultimately deliver a product reduction equivalents and to effectively deal with waste...

Theory of Hybrid Solar Hydrogen Generation

Solar Power Water Electrolysis

Thermally assisted solar electrolysis consists of i light harvesting, ii spectral resolution of thermal sub-bandgap and electronic super-bandgap radiation, the latter of which iiia drives photovoltaic or photoelectrochemical charge transfer V i 2O , while the former iiib elevates water to temperature T, and pressure, p finally iv V i O driven electrolysis of H2O T,p . A schematic representation for this solar thermal water electrolysis photothermal electrochemical water splitting is presented...

Germanium and Silicon

Given the very high level of technological infrastructure that already exists for these elemental semiconductors because of microelectronics applications, it is not surprising that both these materials were examined early on in the evolution of the field of photoelectrolysis of water. As mentioned in an introductory paragraph, cathodic reduction of the Ge surface is accompanied by H2 evolution.58,559 However, we are not aware of studies under irradiation of Ge electrodes from a HER or OER...

Energy Transfer Quenching Antenna Complexes

Osmium Antenna Complex

Energy transfer quenching is the mechanism that natural photosystems use to harvest light over large areas using an 'antenna system' and subsequently funnel this energy to a specific site. In green plants, the antenna is composed of thousands of chlorophyll molecules which, when photoexcited to the triplet state 3Chl , rapidly pass their energy to neighboring molecules eventually resulting in the electronic excitation of the special pair of chlorophyll molecules in the photochemical reaction...

Contributors

National Renewable Energy Laboratory 1617 Cole Blvd., Golden, CO 80401 marie_ghiradi nrel.gov National Renewable Energy Laboratory NREL MS3911 Ph 303-384-7091, F 303-384-7055, Kevin_Harrison nrel.gov National Renewable Energy Laboratory 1617 Cole Blvd., Golden, CO 80401 johanna_levene nrel.gov 4201 Wilson Blvd., Arlington, VA 022230 Chemistry Department 100 Morrissey Boulevard University of Massachusetts, Boston, MA 02135-3395 Ph 617-287-6156, stuart.licht umb.edu Department of Chemistry and...

Reductive and Oxidative Quenching Dyads and Triads with Donors and Acceptors

Phenolate Oxidation

Once light energy has been harvested in the form of a localized excited state, spatial separation of the electron-hole pair becomes important to prevent wasteful recombination reactions and to spatially direct the oxidizing and reducing equivalents to the appropriate co-catalysts. Dyads are usually composed of a sensitizer S and a donor D or acceptor A moiety whereas and triads are typically composed of S with both a D and A covalently attached. As opposed to antenna assemblies, the mode of...

Opportunities for Renewable Energy

Integrating electrolyzers with renewable energy system can present challenges as well as unique benefits. Currently most renewable energy systems produce power and interconnect with thte electrical grid via some form of power electronics PE . To use electrical grid power, today's commercial electrolyzers also have some type of power electronics interface that can represent a significant portion of the overall system cost.43 The power electronics convert alternating current AC from the grid to...

The Earths Atmosphere as a Filter

Sun Angle Absorption Atmosphere Position

Above, we described the solar resource at the top of the atmosphere. The atmosphere acts as a continuously variable filter for the ETR radiation. The atmosphere has stable components of 78 nitrogen, 21 oxygen, and 1 argon and other noble gases. There are also variable components in the atmosphere, such as water vapor 0 to 2 of the total composition , and gases dumped into the atmosphere by man-made and natural process, such as carbon dioxide, 0.035 , methane, and nitrous oxides.6 At a...

Energy Efficiency

Solar Powered Electrolysis Water

Energy efficiency is defined as the higher heating value HHV of hydrogen divided by the energy consumed by the electrolysis system per kilogram of hydrogen pro- Table 3. Efficiency of selected electrolyzers in the market. Table 3. Efficiency of selected electrolyzers in the market. PIEL division of ILT Technology s.r.l. duced. See Eqs. 5 and 6 for details on this calculation. HHV is used as opposed to the lower heating value LHV because in commercial electrolyzers in 2005, the water...

Stability

Water Oxidation Ruthenium

The stability of Rubpy complexes is a result of a number of factors including 1. the large electronic stabilization found in these octahedral low spin d6 second row transition metal complexes while the symmetry of the complexes in Fig. 7 is not Oh, to a first approximation an octahedral coordination environment adequately describes many of the electronic properties 2. the considerable covalent character of the bond as reflected in the similar electronegativities for Ru 2.2 and N 3.0 , and 3....

Efficiency of Photoelectrolysis

In a regenerative solar energy conversion system, the device efficiency n is simply given by the ratio of the power delivered by the photovoltaic converter and the incident solar power Ps in W m2 or mW cm2 . However, we are concerned here with devices producing a fuel H2 and several expressions exist for the device efficiency. Thus, this efficiency can be expressed in kinetic terms 70,192 In Eq. 1, AG is the standard Gibbs energy for the water splitting reaction generating H2, Rh2 is the rate...

Comparison of Solar Electrochemical Thermal Hybrid Water Splitting

Solar Water Splitting

Solar electrochemical, solar thermal,1,2 and solar thermal electrochemical hybrid3 hydrogen generation are introduced in this Section. Water electrolysis and electrolysis using solar concentrator technology were discussed in Chapters 3 and 4. The thermal and the hybrid processes will be discussed in depth in subsequent Sections of this chapter. At high temperatures gt 2000 C , water chemically disproportionates to hydrogen and oxygen. Hence, in principle, by using solar energy to directly heat...

Single versus Multi Electron Processes

Charge Transfer Complexes

Photoexcitation is almost always a one-photon, one-electron process.202 As seen in reactions 5 and 6, water splitting is a multi-electron process. One significant challenge in the development of artificial photosystems has been addressing this mismatch. As seen in reactions 19 and 20, one-electron routes towards water splitting involve the formation of highly reactive intermediates and require considerably higher potentials202,203 than needed for the corresponding multi-electron reactions 5 at...

Nitrides Oxynitrides and Oxysulfides

We have seen that introduction of nitrogen into the TiO2 lattice has a favorable effect in terms of sensitizing it to the visible range of the electromagnetic spectrum Table 6 . The line between doping and new phase formation is one of degree and the studies on nitridation of a given parent oxide exemplify this point. Thus the band gap of Ta2O5 shrinks from 4.0 eV to 2.1 eV by nitriding it in a NH3 atmosphere to yield Ta3N5.476 This material evolves H2 and O2 under visible irradiation k lt 600...

Conclusions

There exists an opportunity to change the face of our energy consumption from one of polluting our air, water, and land to one more in harmony with the environment. The environment and the economy are often at odds for resources, but it does not have to be that way. Renewable hydrogen seems to possess the ability to transition the world's carbon-based economy into a near-carbon-free economy. Hydrogen can be extracted from all fossil fuels as well as split from water using the electricity from...

Utilization of Solar Spectral Regions Spectral Response of Materials

Spectral Response Reflectance

Why this extended discussion of the solar spectral distribution The primary reason is provided by the example discussing the sensitivity of our eyes, in the previous Fig. 7. Temperature and color relationships for various natural and artificial sources of optical Fig. 7. Temperature and color relationships for various natural and artificial sources of optical Fig. 8. Decreasing direct beam spectral irradiance as aerosol optical depth at 500 nm increases from 0.05 top curve to 0.04 bottom curve...

Metal Chalcogenide Semiconductors Cadmium Sulfide

Like TiO2 in the case of oxide semiconductors, CdS is the prototype of compound semiconductors containing a chalcogen S, Se or Te . Next to TiO2, CdS is perhaps the most extensively studied from a photocatalytic water splitting perspective, particularly in powdered form in aqueous dispersions. Table 13 contains a chronological listing of representative studies that have appeared on CdS dating back to 1960--well before when the first studies on TiO2 had begun to appear. The energy band gap of...

Photophysics and Photochemistry

All three Ru II complexes in Fig. 7 have strong absorption bands e 15,000 - 20,000 M-1cm-1 in the visible region from 450 to 480 nm which are assigned to Ru dn bpy n MLCT transitions. As described previously, Ru bpy 3 2 refers to the thermally equilibrated 3MLCT excited-state which is the state responsible for luminescence. This 3MLCT state is converted from the initial 1MLCT state often denoted Ru bpy 3 2 with near unit efficiency in less than 300 fs.99,100 The 3MLCT state decays by both...

Recent Work on TiO on Photosplitting of Water or on the Oxygen Evolution Reaction

Table 7 contains a compilation of studies that have appeared since 1985. Several points are worthy of note here. The vast majority of the entries feature studies on TiO2 powders rather than on electrodes in a photoelectrochemical cell configuration. In this light, the new studies can be regarded as offshoots inspired by the earlier pre-1985 studies on co-functional photocatalysts and the cyclic cleavage of water.4,30 Second, many of the new studies address two key issues with the earlier...

Reference Spectral Distributions

Renewable Resource Data Center Spectral

From the discussion in the preceding Sections, even without addressing the influence of clouds, it is clear that the terrestrial solar spectrum is highly variable. So, how can we relate the spectral responses and spectral optical properties of materials to each other when this variability is present The answer is to establish a standard spectral distribution with which to compute performance. Then comparisons can be made based on standard set of conditions. Furthermore, if measurements are made...

Hydrogen as a Fuel of the Future

Para Hydrogen And Ortho Hydrogen Pie

Jules Verne appears to be one of the earliest people to recognize, or at least articulate, the idea of splitting water to produce hydrogen H2 and oxygen O2 in order to satisfy the energy requirements of society. As early as 1874 in The Mysterious Island, Jules Verne alluded to clean hydrogen fuels, writing Fig. 2. The terawatt renewable energy challenge the energy mix has to switch from the panel on the left to the panel on the right to cap CO2 levels at safe limits. Data from the International...

Other Binary Oxides

Table 8 contains a compilation of studies on other binary oxides that have been examined for their applicability to drive the photoelectrolysis of water. As cited earlier, general reviews are available on many of the oxides listed in Table 8.32'62'65 Other than TiO2, Fe2O3 and WO3 are two of the most widely studied among the binary oxide semiconductors, and studies on these oxides have continued to appear right up to the time of the writing of this Chapter. Tungsten oxide shares many of the...

Miscellaneous Multinary Oxides

In this catch-all Section, we mainly discuss the spinel structures with the generic formulas, AB2O4 and A2MO4. The unit cell of the spinel structure is a large cube, eight times 2 x 2 x 2 the size of a typical face-centered cube.385 We also discuss the delafossite-type structure ABO2 in which the A cation is in linear coordination Table 11. Studies on tantalate and niobate photocatalysts for the splitting of water. Table 11. Studies on tantalate and niobate photocatalysts for the splitting of...

Electrolysis System

Hydrogen Generator System Diagram

The system used to produce hydrogen via electrolysis consists of more than just an electrolyzer stack. A typical electrolysis process diagram is shown in Fig. 6.45 The primary feedstock for electrolysis is water. Water provided to the system may be stored before or after the water purification unit to ensure that the process has adequate feedstock in storage in case the water system is interrupted. Water quality requirements differ between electrolyzers. Some units include water purification...

Commercial Electrolyzer Technologies

Electrolyzers produced in 2006 range in sizes from less than 0.1 kg day-1 to over 1000 kg day-1. The smallest systems, in the under 0.5-kg day-1 range, are used for the production of hydrogen at the lab scale, and are designed as hazard-free alternatives to high pressure gas cylinders.26 Electrolyzers are sized to meet the system requirements of the existing high purity hydrogen markets. However, only systems above the 0.55-kg day-1 production rate are viable for producing hydrogen as a...

Titanium Dioxide Early Work

Historically, this is the material which really sparked interest in the solar photoelec-trolysis of water. Early papers on TiO2 mainly stemmed from the applicability of TiO2 in the paint pigment industry255 although fundamental aspects such as current rectification in the dark in the reverse bias regime shown by anodically formed valve metal oxide film electrolyte interfaces was also of interest e.g., Ref. 52 . Another driver was possible applications of UV-irradiated semiconductor electrolyte...

[Rubpy

Rubpy Latimer Diagram

Latimer diagram of the excited- and ground-state redox processes for Ru bpy 3 2 . Potentials given in V vs. NHE reference electrode. Ru bpy 3 - Ru bpy 3 - Ru bpy 3 Fig. 5. Latimer diagram of the excited- and ground-state redox processes for Ru bpy 3 2 . Potentials given in V vs. NHE reference electrode. given step in the cycle. If we analyze the energetics of the water-splitting reaction within these two manifolds, as shown in reactions 9 to 17, we can see that by designing a system...

Describing the Hybrid CPV System

Solar Hydrogen Civilization

This approach first proposed by Solar Systems in Australia employs a dish concentrator that reflects sunlight onto a focal point see Fig. 6 . At the focal point is a spectral splitter heat mirror that reflects infrared solar radiation and transmits the visible sunlight to high-efficiency solar cells behind the spectral splitter. Figure 7 schematically shows the transmission across the solar spectrum wavelengths. The reflected infrared radiation is gathered by a fiber-optics light pipe and...

Mimicking the Oxygen Evolving Center Water Oxidation Catalysts

Mn2 Bipy Oxygen Evolving

Developing good molecular catalysts for water oxidation remains one of the most challenging aspects of artificial photosynthesis. The oxidation process requires a number of difficult steps including the binding of at least two waters in close proximity, the removal of 4 electrons from these two waters, the formation of an O-O bond, the release of the product O2, and the regeneration of the catalyst starting state. Several comprehensive reviews on molecular water oxidation catalysts have...

Bichromophores Increasing Excited State Lifetimes

Azapentacene

Reversible or equilibrated energy-transfer between two chromophores with similar excited-state energies has also been used as a strategy for extending excited-state lifetimes of both Rubpy and Ruterpy analogues and has been the topic of several excellent recent reviews.141-143 Conjugates of a ruthenium-polypyridyl chromophore with an organic chromophore that has a triplet excited state similar in energy to the 3MLCT state can show reversible triplet-triplet energy transfer between the two...

The Spectral Distribution of the Sun as a Radiation Source

Global Solar Radiation Extraterrestrial

In this chapter we briefly describe the solar spectral distribution, or distribution of energy with respect to wavelength, over the region of the electromagnetic spectrum of use to renewable energy systems. The sun radiates energy at wavelengths ranging from the X-ray and gamma ray spectral region out into the very long wavelength radio spectral region. We will restrict our discussion, for the most part, to solar energy in the wavelength region between the ultraviolet UV of wavelength 250...

Perovskite Titanates and Related Oxides

Methanol Oxide

Perovskites have the general formula, ABX3, with SrTiO3 being a prototype. They contain a framework structure containing corner-sharing TiO6 octahedra with the A cation in twelve-coordinate interstices.384,385 Several hundred oxides have this struc Fig. 6. Twin-compartment photoelectrochemical cell for the photocatalytic generation of H2 from water using electrodeposited p-Cu2O from Ref. 382 . TCO is the transparent conducting oxide substrate and A is an electron donor in the anode compartment....

Low Cost Hydrogen from Hybrid CPV Systems

Wind Hybrid System Schema

We are now in a position to understand why this new high-efficiency solar electric technology provokes a fresh look at the challenge of generating hydrogen from water using sunlight. In addition to generating solar electricity at low cost, CPV systems have the potential to produce hydrogen through an electrolysis process. The generation of electrolytic hydrogen from solar energy is critically important to the world's long-term energy needs for several reasons. The feedstock water and supplied...

Introduction and Scope

This Chapter explores the possibility of using sunlight in conjunction with semiconductor electrolyte interfaces for the production of hydrogen from water and other suitable solvents. The underlying principles of solar energy conversion using semiconductor electrolyte interfaces have been discussed in several review articles, book chapters and books,1-46 and will not be repeated here. This field of photoelectroche-mistry had its early origins in attempts to use inorganic semiconductor...

More on Nomenclature and the Water Splitting Reaction Requirements

Flatband Potential Semiconductor

A bewildering array of terms have been deployed in this field thus, a few clarifying remarks appear to be in order. The term photoelectrochemical refers to any scenario wherein light is used to augment an electrochemical process. This process could be either uphill Gibbs free energy charge being positive or downhill negative AG in a thermodynamic sense. In the former case, the process is called photosynthetic the reaction H2O H2 O2 being an example while the latter would be a photocata-lytic...

Future Outlook and Concluding Remarks

A hybrid solar thermal electrochemical process combines efficient photovoltaic devices and concentrated excess sub-bandgap heat into highly efficient elevated temperature solar electrolysis of water to generate H2 fuel. Efficiency is further enhanced by excess super-bandgap and non-solar sources of heat but diminished by losses in polarization and photo-electrolysis power matching. As also shown earlier in Chapter 4 and elaborated further here, solar concentration can provide the high...

Amouyal E. Homsi A. Chambron J.-c. Sauvage J.-p. J. Chem.

Rev. 1992, 92, 435-461. 2. Cole-Hamilton, D. J. Bruce, D. W. In Comp. Coord. Chem. V. 6 Wilkinson, G., Gillard, R. D., McCleverty, J. A., Eds. Pergamon Books Ltd Maxwell House, NY, 1987 Vol. 6. 3. Holten, D. Bocian, D. F. Lindsey, J. S. Acc. Chem. Res. 2002, 35, 57-69. 4. Kodis, G. Liddell, P. A. de la Garza, L. Clausen, P. C. Lindsey, J. S. Moore, A. L. Moore, T. A. Gust, D. J. Phys. Chem. A. 2002, 106, 2036 - 2048. 5. Fukuzumi, S. Imahori, H. In Electron Transfer...