The world is in critical need of a solution to the current sustainability crisis stemming from our gluttonous overuse and abuse of non-renewable energy resources. The social, economic and environmental ramifications of not taking heed of the warning signs now are immense. We are in urgent need of advanced technologies capable of drastically reducing our carbon dioxide gas (CO2) and other greenhouse gas emissions into the atmosphere. Fortunately an alternative energy solution is emerging out of nanochemistry - hopefully just in time.

         Electricity is clean, silent, and easily transported, stored and converted into work. It is the most useful and desirable energy form currently available to humanity. It comes as no surprise, then, that our 'modern' world runs increasingly on electric power. Electricity, however, like hydrogen, is not an energy source, it is an energy vector. What this means is that primary energy sources must be converted into electric power in order to be effectively utilized. At present, approximately 15% of the world's electricity comes from nuclear (fission) power. Sadly, the vast majority (close to 80%) is still generated by burning hydrocarbons such as oil, gas, and much cheaper coal. In 2006, nearly half (~49%) of the 4.1 trillion kWh (kilowatt-hours) of electricity generated in the United States came from coal.

        China alone is expected to add around 25 GW (gigawatts) of extra capacity every year for the next decade in order to meet demand. This is equivalent to one additional new large coal-fired power plant every week, for the next ten years. Unfortunately, coal, along with producing immense amounts of climate-altering CO2, also contains mercury. The combustion of coal is causing contamination of the oceans and food chain with this highly toxic metal.

Our Star - The Sun        In order to avert this imminent yet avoidable planetary catastrophe, we must harness the ultimate source of energy, and begin to fulfill almost all of our power requirements directly from the sun. In order to turn a profit, businesses are usually forced to choose cost competitive sources of power, even if they mean environmental destruction. Historically, these energy sources have been those with the most negative impact on the environment. Fortunately, due to steadily declining costs, and since the cheapest option wins, solar is expected to become the predominant primary energy source of the 21st century. The earth receives 3.9 x 1024 joules of energy from the sun every year - enough to satisfy our yearly global energy demand in less than one hour. Solar is the future of power generation, as it is non-polluting (and CO2 neutral), freely available, and essentially unlimited. Nanotechnology-enabled photovoltaic (PV) solutions will be instrumental in effecting our transition away from the polluting, unrenewable power sources of present. Solutions will be achieved through direct exploitation of advances made possible by nanoscience. In order to end the world's dependency on coal and oil, massive deployment of nanotechnology-based photovoltaic materials, and new electric energy storage devices such as new batteries and capacitors will be required, at a cost level below that of burning fossil fuels (~$1 per watt). Nanotechnology will directly enable economically and environmentally feasible, sustainable solar electricity generation on a global scale.

Concentrated Solar Power        Cheap and abundant electricity from the sun will come from inventing new technologies, not by taxing carbon dioxide emissions. More than 100 years after Albert Einstein described the "photoelectric effect" (whereby electrons are freed from their respective atoms by absorbing energy in the form of photons of light, thereby producing an electric potential), nanoscience & technology are beginning to make inexpensive solar power generation a reality. Only a small fraction - about 12 GW of the world's power - was generated by solar-electricity in 2009. Newly installed capacity, however, is currently increasing at about 30% per annum, and is accelerating. The cost per watt of electricity generated by photovoltaic modules has dropped from almost $100 in 1975 to less than $1 in 2010 - achieving the benchmark of "grid parity", and marking the beginning our solar-powered future.

        Several technologies have been identified as having the potential to inexpensively convert the sun's light into electricity on a large scale, satisfying the world's insatiable, growing demand for energy. Concentrated solar power (CSP) is one such technology. CSP uses mirrors or lenses to focus a large area of sunlight onto a much smaller area, where it is used to either heat a transfer fluid, or produce electricity directly via photovoltaics.

Concentrated Solar Power

        Thin-film photovoltaic (TFPV) systems are another, rapidly growing, emerging technology. There are three major TFPV technologies, amorphous silicon (a-Si), cadmium telluride (CdTe), and copper indium gallium diselenide, (CIGS) all of which use single micron or thinner films instead of the hundreds of microns thick silicon wafers used in traditional silicon (c-Si) PV modules. Silicon panels are fragile, bulky, heavy, and expensive. Thin-film modules currently have around a 20% (and growing) market share. They are less expensive to manufacture and use, flexible, light weight, and do not require expensive silicon.

        Governments, initially in countries like Germany, France, Italy, Spain, and Greece have instituted a "feed-in" tariff scheme that aims to incentivise and expedite the switch to solar electricity production. Skeptics may see this as just one more example of the poor (through another tax on their electric bills) financing the rich (who own the solar panels.) However, the incentives were instituted by the governments of these countries in order to develop a large, distributed, and modern photovoltaic industry. The large profits financed by the incentives were intended to fuel innovation, lowering the overall cost of solar electricity. As a result, a large number of new commercial PV technologies have emerged thanks to these incentives programs, and the cost per watt has recently been reduced to below the $1 per watt milestone. Many other countries have recently followed suit with similar incentives, having seen the potential benefits.

Nanosolar Thin Film PhotovoltaicsNanosolar        Nanosolar has pioneered a technology for depositing a super-thin semiconductor layer, using a high speed printing process. Nanoparticle containing CIGS 'ink' is printed directly onto mile-long rolls of thin aluminum "solar-electric foil." The result is a flexible solar cell 100 times thinner than a silicon wafer, produced in 1/100th the time. A single production line at the company is able to assemble panels at a rate of 640MW per year. Printing is a fast, simple, effective and robust thin-film deposition process that further decreases PV manufacturing costs. Nanosolar was established by an heir of a German family that became wealthy, somewhat ironically, by selling electricity produced by burning coal, since the 1920s.

        In 1954, the inventor of the silicon solar cell foresaw that thin films would one day become the future of solar electricity generation. Current day thin film solar cells are a direct result of advances in the field of nanochemistry. It is our ability to manipulate matter at the nanoscale with the stability required for practical industrial applications that has made this possible. Over the past two decades, chemistry has quietly continued to refine its techniques for producing nanoscale material building blocks of precise shapes, sizes, composition, and surface structure.

Our Star        Current PV technologies are reliant on the quantum nature of light (which is the theory that light exists as discrete packets of energy called photons.) Semiconductors are fundamentally limited by band-gap energies - the amount of energy, an absorbed photon in this case - required for an outer shell electron to be freed from its orbit around the nucleus of an atom, and become a mobile charge carrier). Revolutionary increases in electricity production are anticipated due to materials designed for particular functions at the nanoscale. One such revolutionary approach based on the wave nature of light, first proposed by Professor Robert Bailey in 1972, is only now becoming a reality thanks to advances made possible by nanochemistry. The idea is to directly convert solar radiation into usable electricity via broadband rectifying nanoantennas. Solar radiation is electromagnetic in nature, meaning it consists of waves of oscillating magnetic and electric fields. Multi-walled carbon nanotubes (MWCNTs) several hundred nanometers in length (matching that of a specific frequency of light) are capable of acting like finely tuned antennas for all of the individual frequencies (wavelengths) of the incoherent solar spectrum. These antennas also need to be arranged in a somewhat scattered pattern in order to collect incident light of either direction of polarization. Such rectennas would be able to altogether avoid the fundamental limitation of semiconductor band-gap.

        Increased thickness of conventional, monocrystalline silicon solar cells improves light absorption, but it also makes it more difficult for electrons to be freed. This trade-off can be avoided by using nanowires, whose scale (10-100 nm wide by ~5 μm long) permits more effective collection of electrons (perpendicular to the absorption of light). Plastic and dye polymer-based solar cells are two other new nanotechnological PV technologies. Organic solar cells (OSCs) are another low cost, light weight and flexible PV solution that can be printed in large volume by roll-to-roll inkjet technology. Future OSCs are expected to achieve an efficiency close to their theoretical maximum of 15%.

         Solar is an inherently intermittent energy source (its main drawback), which must often be stored during periods of peak collection in order to fulfill demand during periods of low irradiation. By contrast, hydrogen is an excellent energy storage vector, capable of storing solar energy. Electricity collected from solar cells can be used to electrolyze water (via a high-efficiency membrane electrolyzer) to produce portable (renewable) hydrogen which is then stored for later use.

         A much simpler to implement solution involves the use of a pump, reservoir, and hydroelectric generator. Electricity generated by a solar PV system would pump water from a lower reservoir to a higher one, thereby converting the electricity to potential energy stored in the water. When the sun is not shining, and power is required, the top tank could be used to fill the bottom while turning over a hydroelectric generator, converting the potential of the water back into electricity (hydro). The main drawbacks to this system are losses through inefficiencies, the size of the tanks required, and the fact that the tanks' size limits the amount of power that can be stored.

         Overall, the future is bright for solar. The sun is the best candidate for reliably supplying the bulk of our growing energy demands. Best of all it is clean, renewable, and available almost everywhere for part of the day. Solar collection in space could even potentially provide 24/7 constant power delivery beamed back to earth.

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