Solar Research Shines

sunshine
Courtesy of Creative Commons

Everyone loves the idea of solar power — heating and cooling your home using the sun as a clean, free source of power. It sounds like the ultimate way to lower your carbon foot print! However, solar cells are expensive and typically only about 15% efficient, as I discussed in an earlier blog.

In order to make solar power more practical on a wide scale, a lot of research is underway to increase solar power efficiency. Stanford researchers have just reported a significant breakthrough in such solar power research, as described in their new paper in Nature Materials. They have developed a novel solar technology that uses both the light and heat of the sun to generate electricity. This new technology could double solar power efficiency and make it more affordable.

When most people think of solar power, they think of rooftop solar panels. These sort of solar panels (or arrays of photovoltaic solar cells) use expensive semiconductor materials to convert photons of light into electricity. The photons from sunlight are absorbed by the semiconductor material, so the energy from the photons is given to the electrons in the semiconductor. The energy given to an electron can “excite” it from the valence band to the conduction band, where it is free to move around within the semiconductor to produce electricity. Solar panels basically convert solar energy into direct current electricity. However, these types of solar panels aren’t very efficient. If an excited photon doesn’t absorb enough energy, then it can’t make it to the conduction band to produce electricity. On the other hand, if an excited photon absorbs more energy than needed (to make it to the conduction band) then the excess energy is lost as heat. In silicon solar panels, half of the solar energy that hits the solar panel is lost due to these two processes. Ideally you would like to somehow harvest the energy that is lost as heat, in order to make solar cells more efficient.

Solar power can also be generated by a thermionic energy convertor, which directly converts heat into electricity. A thermionic converter produces electricity by causing a heat-induced flow of electrons from a hot cathode across a vacuum gap to a cooler anode. However, only a small fraction of the electrons gain sufficient thermal energy to generate this kind of electricity, and very high temperatures are needed for efficient thermionic conversion.

The Stanford researchers have recently developed a new process that exploits the benefits of both solar and thermal cell conversion. The research was led by Nicholas Melosh, as a joint venture of Stanford and SLAC National Accelerator Laboratory. Melosh’s group coated a piece of semiconducting material with a thin layer of metal cesium, demonstrating that this allowed the material to use both light and heat to generate electricity. This new PETE (photon-enhanced thermionic emission) device used the same basic architecture as a thermionic converter except with this special semiconductor as the cathode.

Although the physical process of this PETE device is different than the standard solar cell mechanisms, the new device gives a similar response at very high temperatures. In fact, the PETE device is most efficient at over 200 C. This means that PETE devices won’t replace rooftop solar panels, since they require higher temperatures to be efficient. Instead, they could be used in combination with solar concentrators as part of a large scale solar power plant, for instance in the Mojave Desert.

Melosh’s initial “proof of concept” research was performed with the semiconductor galium nitride to demonstrate that the new energy conversion process works, but galium nitride isn’t suitable for solar applications. They plan to extend their research to other semiconductors, such as gallium arsenide which is commonly used in household electronics. Based on theoretical calculations, they expect to develop PETE devices that operate with a 50 percent efficiency at temperatures exceeding 200 C. They hope to design the new PETE devices so they can be easily incorporated into existing solar power plants, significantly increasing the efficiency of solar power to make it competitive with oil.

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Solar-Powered Drip Irrigation May Save Lives in Africa

Americans spend on average 12.4% of their paycheck on food according to the U.S. Department of Labor’s latest survey. In contrast, sub-Saharan African communities spend 50-80% of their income on food, even though they are engaged in agricultural production as their main livelihood. These communities rely on rain-fed agriculture for crop production, despite having a short annual rainy season of only 3-6 months. Traditionally women and girls are responsible for hauling water by hand from very long distances in order to grow some crops, particularly during the long dry season.

Only 4% of cropland is irrigated in sub-Saharan Africa. Clearly irrigation could help improve quality of life for these food-insecure communities, if a water source is available. The most efficient type of irrigation for such a dry climate is drip (micro) irrigation, which delivers water and fertilizer directly to the roots of a plant. Low-pressure drip irrigation systems require only 1 m of pressure to irrigate plots of up to 1,000 square meters (0.25 acres). However, this irrigation technology requires access to a reliable water source.

One solution is a photovoltaic-powered drip irrigation system that combines the efficiency of the drip irrigation with the reliability of a solar-powered water pump. In such a system, a photovoltaic solar array powers a surface or submersible pump (depending on the water source) that feeds water into a reservoir. The reservoir then gravity-distributes the water to the low-pressure drip irrigation system. Energy is stored via the height of column of water in the reservoir. These systems can be configured so that no batteries are required. The pump only runs during the daytime and the system passively self-regulates. Namely, the volume of water increases on clear hot days when plants need the most water.

This kind of solar-powered drip irrigation system was tested in two rural villages in Northern Benin. The systems were installed and financed by an Non-governmental Organization, Solar Electric Light Fund, with the goal of boosting vegetable production from communal gardens in order to combat high malnutrition and poverty levels. The research was performed in collaboration with Stanford University. This NGO-academic research team scientifically evaluated the impact of the irrigation system on the community through a randomized controlled project that was rigorously studied and analyzed. The study results were recently published by Stanford University in the Proceeding of the National Academy of Sciences.

Three solar-powered drip irrigation systems were installed in two villages. Each irrigation system was used collectively by an agricultural group of 30-35 women, who each farmed her own 120 square meter plot and some additional shared plots used for group expenses. Researchers monitored these communities, as well as two “control” villages in which women’s agricultural groups grew vegetables by hand watering. This allowed a comparison between the solar-powered drip irrigation system to traditional watering method.

Each of the solar-powered irrigation systems supplied on average 1.9 tonnes of produce per month — including high-valued crops such as tomatoes, okra, peppers, eggplants, carrots, and greens — without displacing other agricultural production. The women farmers kept on average 18% by weight of the vegetables and sold the rest at local markets. As a result, vegetable intake across all villages increased by about 1 serving (150 g raw weight) per day during the rainy season. For the villages with irrigation systems, the vegetable intake rose to 3-5 servings per day even during the dry season. Overall the users of the irrigation systems showed remarkable benefits even in the first year, compared with the control households. The article states, “Their standard of living increased relative to the non-beneficiaries (by 80% of the baseline), their consumption of vegetables increased to the Recommended Daily Allowance, and the income generated by production of market vegetables enabled them to purchase staples and protein during the dry season.”

Hardly anyone is going to argue against the potential benefit of irrigation in Africa. However, one question remains — is the expense of a solar-powered system really necessary? The Stanford researchers would argue that it is, despite the expensive up-front costs. They compared their irrigation system with a hypothetical alternative system that used a liquid-fuel (gasoline, kerosene, or diesel) engine-driven pump, instead of the photovoltaic array and pump. This alternate pump can have significant problems, because fuel supplies can be unreliable and fuel prices volatile. According to their analysis, the solar-powered irrigation system is actually more cost effective in the long run, particular when fuel prices are high. It is also better for the environment since it doesn’t cause carbon-emissions.

The solar-powered drip irrigation system in the Benin project cost approximately $18,000 to install ($475 per 120 square meter plot) and requires about $5,750 ($143 per plot) per year to maintain. Based on the projected earnings of the farmers, the system should pay for itself in about 2.3 years. In addition, the cost of the photovoltaic arrays is expected to lower for larger-scale projects.

The project in Benin isn’t the only one underway. Solar-powered drip irrigation systems are also being installed by other groups in different areas of the world. For instance, the Sustainable Agriculture Water Management Project has installed solar-powered drip irrigation systems to 5,000 farmers in Sri Lanka’s dry zones. The hope is that these international efforts can provide substantial economic, nutritional, and environmental benefits to food-insecure impoverished communities.

Making Diesel at Solar Plants

Normally biofuels and solar power are considered to be competing alternative energy sources. However, some researchers are merging these technologies, trying to use the best of both to create “solar fuels.”  This includes the researchers at a small start-up company from Cambridge Massachusetts, Joule Unlimited, which was recently listed as one of the world’s ten most important emerging technologies by MIT’s Technology Review 2010 TR10. It was also selected as part of the TR50 in February, the only company besides Google that was chosen for both honors.

Joule Unlimited has manipulated and designed genes to create photosynthetic microorganisms. These microorganisms use energy from the sun to convert carbon dioxide and water directly into ethanol or hydrocarbon fuels (such as diesel). The photosynthetic microorganisms are designed with a genetic switch that limits growth. They are allowed to multiply for a couple days, then the genetic switch is flipped to divert their energy into fuel production. The microorganisms excrete the fuel, which is chemically separated and collected using conventional technologies.

The goal of this direct, continuous process is to achieve high fuel production with minimal land use. The microorganisms are grown in water inside transparent bioreactors, where they are circulated to make sure that all the microorganisms are exposed to sunlight. Different kinds of non-potable water can be used in this process, including brackish water, waste water or seawater. The microorganisms are fed concentrated carbon dioxide and other nutrients. The long term hope is to use carbon dioxide from polluting facilities such as coal plants.

Joule Unlimited claims to have specifically designed both their microorganisms and bioreactors to work in harmony together, in order to maximize fuel production. For instance, the company carefully designed the bioreactor to keep the heat within the limits required by their microorganism. In the long term, the company is hoping to produce 25,000 gallons per acre per year of ethanol and 15,000 gallons per acre per year of diesel at the competitive price of $30 per barrel. They are planning to scale up from demonstration facilities to building a commercial facility in 2012, in order to start producing diesel in 2013. However, their engineers still need to improve the performance of the microorganism to meet these targets, as well as address whatever issues arise during scale-up.

Joule Unlimited isn’t the only one working in this research area. Others working on solar fuels include:  (1) Synthetic Genomics in La Jolla, CA, (2) BioCee in Minneapolis, MN, and (3) University of Minnesota BioTechnology Institute, St. Paul, MN. Hopefully the race is on, and the winner will be all of us.

Joule facility
A diagram of how a Joule facility would work with bioreactors growing micro organisms with sunlight and CO2 in water. A separator removes the end product -- liquid fuel or chemicals. (Courtesy of Joule Unlimited)

Want A Net-Zero Energy Home?

Berkeley Lab presents 3 talks on “The House of the Future.” Come get a preview of tomorrow’s zero-energy home with cool roofs, smart windows, and computer-driven control systems. These talks are for a general audience, on May 10 at 7-9 p.m. at the Berkeley Repertory Theatre. Free admission. More information at http://www.lbl.gov/LBL-PID/fobl/.

Cheap Solar Using Plastic Electronics?

In order to make solar power cost-effective without governmental subsidies, we need to dramatically reduce the manufacturing costs of solar cells. (I discussed solar costs in a previous blog.) A lot of research is underway to do just that. One area of solar research focuses on finding a low-cost alternative to indium tin oxide (ITO), which is an expensive conducting material currently used in standard solar cells. ITO is a rare by-product of mining that is also used in flat-screen televisions, mobile phones, and other devices with display screens. As the demand for these popular devices increases rapidly, the price and availability of ITO for solar cells has become a real problem.

A team of chemical engineers think they’ve found the solution — plastic electronics. This collaboration of chemical engineers from Princeton University, University of Texas, and University of California Santa Barbara reported their latest results in the Proceedings of the National Academy of Sciences on March 30, 2010. Their research focuses on conductive polymers (plastics).

Conductive polymers have been around for a long time, but their ability to conduct electricity degraded when manufactured into devices. Basically the manufacturing process caused their structures to be trapped in a rigid form and that prevented electrical current to travel through them, thus severely limiting device performance.

The multi-institutional collaboration has overcome this problem, by treating the conductive polymers with dichloroacetic acid (DCA) after they are processed into the desired form. This “postdeposition solvent annealing” with DCA dramatically rearranges the structure of the polymer, resulting in a smooth film with high conductivity. As a result, they are able to make polymers that are translucent, malleable and highly conductive. These materials could have wide reaching applications as electrodes in transistors, anodes in solar cells, and light-emitting-diodes.

One amazing thing about this research is the simplicity of production. This collaboration made a transistor (a very basic device used to amplify and switch electronic signals) by printing the polymer onto a surface, using a method similar to that used by a standard ink-jet printer. In the future, they hope to distribute the conductive polymers in cartridges like printer ink.

What does this mean for solar power? An important thing for solar is that these conductive polymers are translucent. Although they are less transparent than ITO (e.g., transmissivity of 73% v.s. 84% respectively), they are dramatically cheaper. So the newly developed conductive polymers are still a promising low-cost alternative as anodes for solar cells. Hopefully this research will translate into cheap, commercially available solar cells soon.

Researchers have developed a new way to manufacture electronic devices made of plastic, employing a process that allows the materials to be formed into useful shapes while maintaining their ability to conduct electricity. In the transistor pictured above, the plastic is molded into interdigitated electrodes (orange), allowing current flow to and from the active channel (green). Courtesy of Loo Research Group.

Science At The Theater

I want to let my Bay Area readers know about an upcoming cool science lecture for the general public. Berkeley Lab presents “Just Say No To Carbon Emissions” on April 26 from 7-9 pm at the Berkeley Repertory Theatre. Admission is free. There will be three dynamic speakers from Berkeley Labs discussing renewable energy topics. Ramamoorthy Ramesh, a material scientist, will describe current research efforts to make cheap solar. Nan Zhou will discuss efforts to increase energy efficiency and reduce carbon dioxide emissions in China. Lastly, the geologist Curt Oldenbury will explain a strategy to reduce carbon emissions from coal and natural gas, by storing it deep underground. This event is co-sponsored by “Friends of Berkeley Lab” and “Berkeley Energy and Resources Collaboration.” For more information, check out their website.

Obama’s Carbon Tax Cools Down

Although there is some controversy over the issue, human-induced green house gas emissions are generally considered to be the primary cause of global warming. Carbon dioxide is considered to be the most important of these “greenhouse” air pollutants, and the burning of fossil-fuels a main source. This is because fossil fuels have carbon atoms that are released as carbon dioxide when they are burned. For example, gasoline consists of atoms of hydrogen and carbon (about two hydrogens per carbon). When it burns, the hydrogen combines with oxygen to make water, and the carbon combines with oxygen to make carbon dioxide. If the combustion is incomplete, it also makes carbon monoxide.

In contrast, renewable non-combustible energy sources (such as wind or sunlight) do not convert hydrocarbons into carbon dioxide. That means that renewable energy technologies have a better chance at competing with fossil fuels (such as petroleum and coal) if you take into account the full energy cost, including the amount of carbon dioxide emitted during energy production. Solar plants cost more to construct than coal plants, but they don’t pollute. So economists argue that solar might actually be cheaper if you include the damage done by carbon dioxide pollution in the cost of coal. Basically, if you give a monetary value to the cost of polluting the air, then emissions become an internal cost of doing business that is visible on the balance sheet.

In order to address this issue, President Obama has pushed to establish carbon emission trading (known as “cap-and-trade”). The basic idea of this carbon exchange market is that the government sets a limit (cap) on the total amount of greenhouse gases that can be emitted nationally, and the market sets the price. In other words, a business would have to pay a carbon tax, buying the right to emit carbon dioxide from the government. The basic idea is to create a financial incentive to reduce greenhouse gas emissions, while boosting energy efficiency and renewable energy efforts.

President Obama specifically proposes a 14% emission reduction from 2005 levels by 2020 (and 83% reduction by 2050). He also proposes that companies buy an allowance, or permit, for each ton of carbon emitted at an estimated cost of $13 to $20 per ton to start. Rather than following Obama’s proposed system of “full permit auctioning,” the House of Representatives passed a bill last June that would establish a variant of a cap-and-trade plan for greenhouse gases. I’m not going to get into the details of this American Clean Energy and Security Act of 2009, which is still under consideration in the Senate. However, I did want to note that this bill’s cap-and-trade program allocates 85% of allowances to industry for free, auctioning only the remaining 15%. There is a tremendous amount of debate on the best system to curb emissions of climate-changing gases. There are other competing bills currently in the House and Senate. However, it appears that none of these bills are likely to pass Congress. “Realistically, the cap-and-trade bills in the House and the Senate are going nowhere,” said Senator Lindsey Graham, who is trying to create bipartisan climate and energy measures. “They’re not business-friendly enough, and they don’t lead to meaningful energy independence.”