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Zdeslav Hrepic Fort Hays State University Physics Department Zdeslav Hrepic Fort Hays State University Physics Department

Two top reasons for Energy Alternatives 1. Global Warming n Greenhouse gas changes since Two top reasons for Energy Alternatives 1. Global Warming n Greenhouse gas changes since the industrial revolution are believed to be closely related to the change in climate that has been observed (Intergovernmental Panel on Climate Change (IPCC), 2007) 2. Energy Independence – National Security

Global Warming The NOAA Annual Greenhouse Gas Index (AGGI) w Global averages of the Global Warming The NOAA Annual Greenhouse Gas Index (AGGI) w Global averages of the concentrations of the major, well-mixed, long-lived greenhouse gases - carbon dioxide, methane, nitrous oxide, CFC-12 and CFC-11 from the NOAA global flask sampling network since 1978. These gases account for about 97% of the direct radiative forcing by long-lived greenhouse gases since 1750. The remaining 3% is contributed by an assortment of 10 minor halogen gases w (NOAA Earth System Research Laboratory, 2007)

World Carbon Dioxide Emissions by Region, 2001 -2025 (Million Metric Tons of Carbon Equivalent) World Carbon Dioxide Emissions by Region, 2001 -2025 (Million Metric Tons of Carbon Equivalent) w World carbon dioxide emissions expected to increase by 1. 9 percent annually between 2001 and 2025. w Much of the increase in these emissions is expected to occur in the developing world where emerging economies, such as China and India, fuel economic development with fossil energy. Developing countries’ emissions are expected to grow above the world average at 2. 7 percent annually between 2001 and 2025; and surpass emissions of industrialized countries near 2018. w (US DOE Energy Information Administration, 2004)

Solar Potential w The Sun energy drives the planet – powers oceanic and atmospheric Solar Potential w The Sun energy drives the planet – powers oceanic and atmospheric currents, the water cycle, typhoons, hurricanes, and tornadoes… w Sun continuously delivers to Earth 1. 2 × 105 terawatts of power which dwarfs every other energy source, renewable or nonrenewable. w Human civilization produces and uses energy currently at rate of about 13 TW w The San Francisco earthquake of 1906, with magnitude 7. 8, released an estimated 1017 joules of energy, the amount the Sun delivers to Earth in one second. w Earth's ultimate recoverable resource of oil, estimated at 3 trillion barrels, contains 1. 7 × 1022 joules of energy, which the Sun supplies to Earth in 1. 5 days. w The amount of energy humans use annually, about 4. 6 × 1020 joules, is delivered to Earth by the Sun in one hour. Source: (Crabtree & Lewis, 2007)

USA Concentrated Solar Power Resource Potential Map w (US DOE Energy Information Administration, 2007 USA Concentrated Solar Power Resource Potential Map w (US DOE Energy Information Administration, 2007 a)

Solar Versatility Sunlight can be converted into: w electricity by exciting electrons in a Solar Versatility Sunlight can be converted into: w electricity by exciting electrons in a solar cell. w chemical fuel via natural photosynthesis in green plants or artificial photosynthesis in human-engineered systems. w heat for direct use or further conversion to electricity (concentrated or unconcentrated) (Lewis & Crabtree, 2005) w All three link seamlessly with existing energy systems w Image source: (Crabtree & Lewis, 2007)

Solar Versatility Added bonus … w Stardust spacecraft (NASA image) in flight. Photovoltaic systems Solar Versatility Added bonus … w Stardust spacecraft (NASA image) in flight. Photovoltaic systems were an important power source for that mission. Solar cells have not only enabled exploration of space, the solar system, and the Earth in great detail, they also have enabled the emergence of the telecommunications industry. (US Department of Energy, 2006)

Solar vs. Fossil fuels w Account for 80% - 85% of our energy consumption Solar vs. Fossil fuels w Account for 80% - 85% of our energy consumption w are finite w distributed unevenly w When combusted produce greenhouse gases and harmful environmental pollutants Solar energy w effectively inexhaustible w Widely available and unrestricted by geopolitical boundaries w Environmentally friendly, especially hen directly used – no threat to health or climate w Theoretically enormous supply However fossil fuels still much cheaper than solar w fossil-fuel are concentrated sources of energy, whereas the Sun distributes photons uniformly over Earth at a more modest energy density. w The use of biomass as fuel is limited by the production capacity of the available land water. (Crabtree & Lewis, 2007)

But… w Despite the enormous energy flux supplied by the Sun, the three conversion But… w Despite the enormous energy flux supplied by the Sun, the three conversion routes supply only a tiny fraction of our current and future energy needs. w Solar electricity, at between 5 and 10 times the cost of electricity from fossil fuels, supplies just 0. 015% of the world's electricity demand. w Solar fuel, in the form of biomass, accounts for approximately 11% of world fuel use, but the majority of that is harvested unsustainably. w Solar heat provides 0. 3% of the energy used for heating space and water. w Source: (Crabtree & Lewis, 2007)

USA: Solar Thermal and Photovoltaic Collector Manufacturing Activities 2006 Data For: 2006; Report Released: USA: Solar Thermal and Photovoltaic Collector Manufacturing Activities 2006 Data For: 2006; Report Released: October 2007 w (Energy Information Administration (EIA), 2007 a)

The Role of Renewable Energy Consumption in the Nation's Energy Supply, 2005 Data for: The Role of Renewable Energy Consumption in the Nation's Energy Supply, 2005 Data for: 2005 Release Date: July 2007 w (US DOE Energy Information Administration, 2007 a)

Projections: World Marketed Energy Use by Fuel Type, 1980 -2030 w International Energy Outlook Projections: World Marketed Energy Use by Fuel Type, 1980 -2030 w International Energy Outlook 2007 Report (US DOE Energy Information Administration, 2007 c)

Global energy in perspective w World Energy consumption by fuel source in quads (1 Global energy in perspective w World Energy consumption by fuel source in quads (1 quad = 1. 06 × 1018 joules) for the years 1999 and 2020 (estimate) for various fuel sources. w Source: (Moniz & Kenderdine, 2002)

Missing Link – Conversion Efficiency w The best commercial solar cells based on single-crystal Missing Link – Conversion Efficiency w The best commercial solar cells based on single-crystal silicon are about 18% efficient. w Laboratory solar cells based on cheaper dye sensitization of oxide semiconductors are typically less than 10% efficient w Laboratory solar cells based on even cheaper organic materials are 2– 5% efficient. w The cheapest solar electricity comes not from photovoltaics but from conventional induction generators powered by steam engines driven by solar heat, with efficiencies of 20% on average and 30% for the best systems. w (Crabtree & Lewis, 2007)

Missing Link – Conversion Efficiency w The utilization gap between solar energy's potential and Missing Link – Conversion Efficiency w The utilization gap between solar energy's potential and our use of it can be overcome by raising the efficiency of the conversion processes, which are all well below their theoretical limits. (Crabtree & Lewis, 2007)

Conversion to Electricity Solar Cells w (Wikipedia, 2007 a) Conversion to Electricity Solar Cells w (Wikipedia, 2007 a)

P-N Junction w A p-n junction in thermal equilibrium with zero bias voltage applied. P-N Junction w A p-n junction in thermal equilibrium with zero bias voltage applied. Electrons and holes concentration are reported respectively with blue and red lines. Gray regions are charge neutral. Light red zone is positively charged. Light blue zone is negatively charged. The electric field is shown on the bottom, the electrostatic force on electrons and holes and the direction in which the diffusion tends to move electrons and holes. (Wikipedia, 2007 d)

Bandgap Energies of Semiconductors and Light w When light shines on crystalline silicon, photons Bandgap Energies of Semiconductors and Light w When light shines on crystalline silicon, photons with a certain level of energy can free electrons in the semiconductor material from their atomic bonds to produce an electric current. w This level of energy, known as the "bandgap energy, " is the amount of energy required to dislodge an electron from its covalent bond allow it to become part of an electrical circuit. To free an electron, the energy of a photon must be at least as great as the bandgap energy. w However, photons with energy > the bandgap energy will expend that extra amount as heat when freeing electrons. (US Department of Energy, 2006) w Image from: (US Department of Energy, 2006)

Solar Cell Efficiency w William Shockley and Hans Queisser established a theorethical efficiency limit Solar Cell Efficiency w William Shockley and Hans Queisser established a theorethical efficiency limit of 31% for the performance of solar cells (Shockley & Queisser, 1961). w The analysis was based on four assumptions: n n illumination with unconcentrated sunlight, a single p–n junction, one electron–hole pair excited per incoming photon, and thermal relaxation of the electron–hole pair energy in excess of the bandgap. w The efficiency limit of 31% for those conditions still a research goal. n The best single-crystal Si cells have achieved 25% efficiency in the laboratory and about 18% in commercial practice. (Crabtree & Lewis, 2007) w Cheaper solar cells made from other materials operate at significantly lower efficiency (Crabtree & Lewis, 2007)

The three generations of solar cells. w w w First-generation - based on expensive The three generations of solar cells. w w w First-generation - based on expensive silicon wafers; 85% of the current commercial market. Second-generation - based on thin films of materials such as amorphous silicon, nanocrystalline silicon, cadmium telluride, or copper indium selenide. The materials are less expensive, but research is needed to raise the cells' efficiency. Third-generation - the research goal: a dramatic increase in efficiency that maintains the cost advantage of second-generation materials. Their design may make use of carrier multiplication, hot electron extraction, multiple junctions, sunlight concentration, or new materials. Goal: Achieving high efficiency from inexpensive materials with so-called third-generation cells The horizontal axis represents the cost of the solar module only; it must be approximately doubled to include the costs of packaging and mounting. Dotted lines indicate the cost per watt of peak power.

The Shockley–Queisser limit can be exceeded by violating one or more of its premises The Shockley–Queisser limit can be exceeded by violating one or more of its premises Violating Premise 1: illumination with unconcentrated sunlight, w Concentrating sunlight allows for a greater contribution from multi-photon processes; that contribution increases theoretical efficiency limit to 41% for a single-junction cell with thermal relaxation. (Crabtree & Lewis, 2007) w Image source: (Wikipedia, 2007 b)

The Shockley–Queisser limit can be exceeded by violating one or more of its premises The Shockley–Queisser limit can be exceeded by violating one or more of its premises Violating Premise 2: a single p–n junction, w Multi junction cells: A cell with a single p–n junction captures only a fraction of the solar spectrum: photons with energies less than the bandgap are not captured, and photons with energies greater than the bandgap have their excess energy lost to thermal relaxation. Stacked cells with different bandgaps capture a greater fraction of the solar spectrum; the efficiency limit is 43% for two junctions illuminated with unconcentrated sunlight, 49% for three junctions, and 66% for infinitely many junctions. (Crabtree & Lewis, 2007) A multijunction device is a stack of individual single-junction cells in descending order of bandgap (Eg). The top cell captures the high-energy photons and passes the rest of the photons on to be absorbed by lower-bandgap cells. (US Department of Energy, 2006)

The Shockley–Queisser limit can be exceeded by violating one or more of its premises The Shockley–Queisser limit can be exceeded by violating one or more of its premises Violating Premise 3: one electron–hole pair excited per incoming photon w Carrier multiplication is a quantum-dot phenomenon that results in multiple electron– hole pairs for a single incident photon. w Carrier multiplication was discussed by Arthur Nozik in 2002 and observed by Richard Schaller and Victor Klimov two years later. w In bulk-semiconductor solar cells, when an incident photon excites a single electron– hole pair, the electron–hole pair energy in excess of the bandgap is likely to be lost to thermal relaxation, whereas in some nanocrystals most of the excess energy can appear as additional electron–hole pairs. If the nanocrystals can be incorporated into a solar cell, the extra pairs could be tapped off as enhanced photocurrent, which would increase the efficiency of the cell. w Nanocrystals of lead selenide, lead sulfide, or cadmium selenide generate as many as seven electrons per incoming photon, which suggests that efficient solar cells might be made with such nanocrystals. w Significant obstacles impede implementation of this method. We cannot attach wires to nanocrystals the way we do to bulk semiconductors; collecting the electrons from billions of tiny dots and putting them all into one current lead is a problem in nanoscale engineering that no one has solved yet. A second challenge is separating the electrons from the holes, the job normally done by the space charge at the p–n junction in bulk solar cells. Those obstacles must be overcome before practical quantum-dot cells can be constructed. w Cited: (Crabtree & Lewis, 2007)

The Shockley–Queisser limit can be exceeded by violating one or more of its premises The Shockley–Queisser limit can be exceeded by violating one or more of its premises Violating Premise 4: thermal relaxation of the electron–hole pair energy in excess of the bandgap. w Hot-electron extraction provides way to increase the efficiency of nanocrystal-based solar cells by tapping off energetic electrons and holes before they have time to thermally relax. (Nozik, 2005) w Femtosecond laser and x-ray techniques can provide the necessary understanding of the ultrafast decay processes in bulk semiconductors and their modification in nanoscale geometries that will enable the use of hot-electron phenomena in nextgeneration solar cells. (Crabtree & Lewis, 2007)

Other cell technologies Thin film technologies w w w The various thin-film technologies currently Other cell technologies Thin film technologies w w w The various thin-film technologies currently being developed reduce the amount (or mass) of light absorbing material required in creating a solar cell. This can lead to reduced processing costs from that of bulk materials (in the case of silicon thin films) but also tends to reduce energy conversion efficiency, although many multi-layer thin films have efficiencies above those of bulk silicon wafers. (Wikipedia, 2007 c) Thin-film cells offer advantages beyond cost, including pliability, and potential integration with preexisting buildings and infrastructure. (Crabtree & Lewis, 2007) Novel conducting polymers enable solar cells that are flexible, inexpensive, and versatile. The new materials can be coated or printed onto flexible or rigid surfaces. (Image courtesy of Konarka Technologies. ) (Crabtree & Lewis, 2007)

Triple Junction Thin Film Cell w (United Solar Systems Corp. , 2004) Triple Junction Thin Film Cell w (United Solar Systems Corp. , 2004)

Other cell technologies Dye-sensitized and Organic solar cells w Dye-sensitized solar cells were introduced Other cell technologies Dye-sensitized and Organic solar cells w Dye-sensitized solar cells were introduced by Michael Grätzel and coworkers in 1991(O'Regan & Grätzel, 1991) w Dye-sensitized solar cells effectively separate the two functions provided by silicon in a traditional cell design. Normally the silicon acts as both the source of photoelectrons, as well as providing the potential barrier to separate the charges and create a current. In the DSSc, the semiconductor is used solely for charge separation, the photoelectrons are provided from a separate photosensitive dye. Additionally the charge separation is not provided solely by the semiconductor, but works in concert with a third element of the cell, an electrolyte in contact with both. (Wikipedia, 2007 a) w Organic/polymer solar cells are built from thin films (typically 100 nm) of organic semiconductors such as polymers and small-molecule compounds like polyphenylene vinylene, copper phthalocyanine (a blue or green organic pigment) and carbon fullerenes. Energy conversion efficiencies achieved to date using conductive polymers are low at 6% efficiency[18] for the best cells to date. However, these cells could be beneficial for some applications where mechanical flexibility and disposability are important. (Wikipedia, 2007 c)

Conversion to Electricity Solar Cells – Types and Efficiencies w (Kazmerski et al. , Conversion to Electricity Solar Cells – Types and Efficiencies w (Kazmerski et al. , 2007)

Conversion to Fuels w Photosynthesis – Nature’s way w An estimated 100 TW of Conversion to Fuels w Photosynthesis – Nature’s way w An estimated 100 TW of solar energy go into photosynthesis, the production of sugars and starches from water and carbon dioxide. w Green plants convert sunlight into biomass with a typical yearly averaged efficiency of less than 0. 3% - Enough for plants to cover the Earth but too low to readily satisfy the human demand for energy. w The early stages of photosynthesis are efficient: n Two molecules of water are split to provide four protons and electrons for subsequent reactions, and an oxygen molecule is released into the atmosphere. w The inefficiency lies in the later stages, in which carbon dioxide is reduced to form the carbohydrates that plants use to grow roots, leaves, and stalks. w The research challenge is to make the overall conversion process between 10 and 100 times more efficient by improving or replacing the inefficient stages of photosynthesis. w Cited: (Crabtree & Lewis, 2007)

Conversion to Fuels w w w The metabolic pathways of plants have evolved for Conversion to Fuels w w w The metabolic pathways of plants have evolved for organisms' survival and reproduction, not for fuel production. The efficient steps that are relevant for fuel production might conceivably be isolated and connected directly to one another to produce fuels such as H 2, CH 4, or alcohols. Hybridizing nature in that way takes advantage of the elaborate molecular processes that biology has evolved and that are still beyond human reach, while eliminating the inefficient steps not needed for fuel production. (Crabtree & Lewis, 2007) w There are three routes to improving the efficiency of photosynthesis-based solar fuel production: 1. breeding or genetically engineering plants to grow faster and produce more biomass, 2. connecting natural photosynthetic pathways in novel configurations to avoid the inefficient steps, and 3. using artificial bio-inspired nanoscale assemblies to produce fuel from water and CO 2. w w The first route - occupation of GMO industry The second and third routes, which involve more direct manipulation of photosynthetic pathways, are still in their early stages of research. (Crabtree & Lewis, 2007)

Artificial photosynthesis w Artificial photosynthesis - using inanimate components to convert sunlight into chemical Artificial photosynthesis w Artificial photosynthesis - using inanimate components to convert sunlight into chemical fuel. (Gust et al. , 2001) w “Increased understanding of photosynthetic energy conversion and advances in chemical synthesis and instrumentation have made it possible to create artificial nanoscale devices and semibiological hybrids that carry out many of the functions of the natural process. Artificial light-harvesting antennas can be synthesized and linked to artificial reaction centers that convert excitation energy to chemical potential in the form of long-lived charge separation. ” (Gust et al. , 2001) w Image: (Crabtree & Lewis, 2007)

Nonbiological ways of creating fuels w Solar fuels can be created in fully nonbiological Nonbiological ways of creating fuels w Solar fuels can be created in fully nonbiological way based on semiconductor solar cells rather than on photosynthesis. (Khaselev & Turner, 1998) w In photoelectrochemical conversion (“Photovoltaic. Photoelectrochemical Device”), the charge-separated electrons and holes are used to split water or reduce CO 2 at the interface with an electrolytic solution, rather than being sent through an external circuit to do electrical work. (Crabtree & Lewis, 2007; Khaselev & Turner, 1998) w Hydrogen was produced at the electrode–water interface with greater than 10% efficiency by Adam Heller in 1984 and by Oscar Khaselev and John Turner in 1998, but the fundamental phenomena involved remain mysterious, and the present devices are not practical. (Crabtree & Lewis, 2007)

Conversion to Heat w The first step in traditional energy conversion is the combustion Conversion to Heat w The first step in traditional energy conversion is the combustion of fuel, usually fossil fuel, to produce heat. Heat produced by combustion may be used for heating space and water, cooking, or industrial processes, or it may be further converted into motion or electricity. (Crabtree & Lewis, 2007) w The premise of solar thermal conversion is that heat from the Sun replaces heat from combustion; fossil-fuel use and its threat to the environment and climate are thus reduced. (Crabtree & Lewis, 2007)

Concentrating Sunlight for Heating w Unconcentrated sunlight can bring the temperature of a fluid Concentrating Sunlight for Heating w Unconcentrated sunlight can bring the temperature of a fluid to about 200 °C, enough to heat space and water in residential and commercial applications. Many regions use solar water heating, though in only a few countries, such as Cyprus and Israel, does it meet a significant fraction of the demand. Concentration of sunlight in parabolic troughs produces temperatures of 400 °C, and parabolic dishes can produce temperatures of 650 °C and higher. (Crabtree & Lewis, 2007) w Image from: (Wikipedia, 2007 b)

Power towers w Power towers, in which a farm of mirrors on the ground Power towers w Power towers, in which a farm of mirrors on the ground reflects to a common receiver at the top of a tower, can yield temperatures of 1500 °C or more. (Crabtree & Lewis, 2007) w The high temperatures of solar power towers are attractive for thermochemical water splitting and solar-driven reforming of fossil fuels to produce H 2 (Steinfeld, 2005) w The 11 megawatt PS 10 solar power tower produces electricity from the sun using 624 large movable mirrors called heliostats.

Conversion to Heat Concentrated Sunlight w The temperatures produced by concentrated sunlight are high Conversion to Heat Concentrated Sunlight w The temperatures produced by concentrated sunlight are high enough to power heat engines, whose Carnot efficiencies depend only on the ratio of the inlet and outlet temperatures. w Steam engines driven by solar heat and connected to conventional generators currently supply the cheapest solar electricity. Nine solar thermal electricity plants that use tracking parabolic-trough concentrators were installed in California's Mojave Desert between 1984 and 1991. Those plants still operate, supplying 354 MW of peak power to the grid. Their average annual efficiency is approximately 20%, and the most recently installed can achieve 30%. w Although those efficiencies are the highest for any widely implemented form of solar conversion, they are modest compared to the nearly 60% efficiency of the best gas-fired electricity generators. w Achieving greater efficiency for solar conversion requires large-scale plants with operating temperatures of 1500 °C or more, as might be produced by power towers. Another alternative, still in the exploration stage, is a hybrid of two conversion schemes: A concentrated solar beam is split into its visible portion for efficient photovoltaic conversion and its high-energy portion for conversion to heat that is converted to electricity through a heat engine.

Conversion to Heat Thermoelectric materials w w w Thermoelectric materials, which require no moving Conversion to Heat Thermoelectric materials w w w Thermoelectric materials, which require no moving parts to convert thermal gradients directly into electricity, are an attractive possibility for reliable and inexpensive electricity production. Charge carriers in a thermal gradient diffuse from hot to cold, driven by the temperature difference but creating an electric current by virtue of the charge on each carrier. The strength of the effect is measured by thermopower, the ratio of the voltage produced to the applied temperature difference. Although thermoelectric effect has been known for nearly 200 years, materials that can potentially convert heat to electricity efficiently enough for widespread use have emerged only since the 1990 s. w Efficient conversion depends on minimizing thermal conductivity of a material, so as not to short-circuit thermal gradient, while maximizing the material's electrical conductivity and thermopower. Achieving such a combination of opposites requires the separate tuning of several material properties: the bandgap, the electronic density of states, and the electron and phonon lifetimes. w The most promising materials are nanostructured composites. Quantum-dot or nanowire substructures introduce spikes in the density of states to tune thermopower (which depends on the derivative of the density of states), and interfaces between the composite materials block thermal transport but allow electrical transport. Proof of concept for interface control of thermal and electrical conductivity was achieved by 2001 with thinfilm superlattices of Bi 2 Te 3/Sb 2 Te 3 and Pb. Te/Pb. Se, which performed twice as well as bulk-alloy thermoelectrics of the same materials. The next challenges are to achieve the same performance in nanostructured bulk materials that can handle large amounts of power and to use nanodot or nanowire inclusions to control thermopower. Figure 5 shows encouraging progress: structurally distinct nanodots in a bulk matrix of thermoelectric material Ag 0. 86 Pb 18 Sb. Te 20. Controlling the size, density, and distribution of such nanodot inclusions during bulk synthesis could significantly enhance thermoelectric performance. 15 (Crabtree & Lewis, 2007) w w w

Thermoelectric materials w A nanodot inclusion in the bulk thermoelectric material Ag 0. 86 Thermoelectric materials w A nanodot inclusion in the bulk thermoelectric material Ag 0. 86 Pb 18 Sb. Te 20, imaged with high-resolution transmission electron microscopy. Despite a lattice mismatch of 2– 5%, the nanodot (indicated by the dotted line) is almost perfectly coherently embedded in the matrix. The arrows show two dislocations near the interface, and the white box indicates the unit cell. The nanodot is rich in silver and antimony relative to the matrix. (Crabtree & Lewis, 2007)

Storage and distribution w IN addition to efficient conversion, another challenge related to solar Storage and distribution w IN addition to efficient conversion, another challenge related to solar eergy usage is energy (electrical or heat) storage. w Access to solar energy is interrupted by natural cycles of day–night, cloudy–sunny, and winter–summer variation that are often out of phase with energy demand. w Solar fuel production automatically stores energy in chemical bonds. Electricity and heat, however, are much more difficult to store. Cost effectively storing even a fraction of our peak demand for electricity or heat for 24 hours is a task well beyond present technology. w Storage is such an imposing technical challenge that innovative schemes have been proposed to minimize its need. w Sci-Fi solutions n n Baseload solar electricity might be generated on constellations of satellites in geosynchronous orbit and beamed to Earth via microwaves focused onto ground-based receiving antennas. A global superconducting grid might direct electricity generated in sunny locations to cloudy or dark locations where demand exceeds supply. w But those schemes, too, are far from being implemented. Without cost-effective storage and distribution, solar electricity can only be a peak-shaving technology for producing power in bright daylight, acting as a fill for some other energy source that can provide reliable power to users on demand.

Storage w (Krauter, 2006) Storage w (Krauter, 2006)

Outlook w The Sun has the enormous untapped potential to supply our growing energy Outlook w The Sun has the enormous untapped potential to supply our growing energy needs. w The barrier to greater use of the solar resource is its high cost relative to the cost of fossil fuels, although the disparity will decrease with the rising prices of fossil fuels and the rising costs of mitigating their impact on the environment and climate. w The cost of solar energy is directly related to the low conversion efficiency, the modest energy density of solar radiation, and the costly materials currently required. w The development of materials and methods to improve solar energy conversion is primarily a scientific challenge: Breakthroughs in fundamental understanding ought to enable marked progress. w There is plenty of room for improvement, since photovoltaic conversion efficiencies for inexpensive organic and dyesensitized solar cells are currently about 10% or less, the conversion efficiency of photosynthesis is less than 1%, and the best solar thermal efficiency is 30%. The theoretical limits suggest that we can do much better.

Conclusion w If solar energy is to become a practical alternative to fossil fuels, Conclusion w If solar energy is to become a practical alternative to fossil fuels, we must have efficient ways to convert photons into electricity, fuel, and heat. (Crabtree & Lewis, 2007) w The need for better conversion technologies is a driving force behind many recent developments in biology, materials, and especially nanoscience. (Crabtree & Lewis, 2007)

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