packaging sunlight

Methods under study aim to capture solar energy where it is abundant and deliver it where it is needed.

By Robert Palumbo, Anton Meier, Christian Wieckert,
and Aldo Steinfeld

There's hardly a place the sun doesn't shine. But most people typically don't think of solar energy as the solution to a potential oil crisis. It's difficult to imagine driving into the local gasoline station and filling the gas tank with sunlight.

Still, a number of scientists and engineers from around the world are intrigued by solar energy's potential. With solar collectors that operate with a collection efficiency of merely 20 percent, the sunlight falling on a mere one-tenth of a percent of the Earth's land area could supply enough energy to meet the current needs of all the citizens of the planet. Furthermore, the solar energy reserve is essentially unlimited and its use is ecologically benign.

Good enough reasons to expect the vast utilization of solar energy—were it not for some very serious drawbacks. The solar radiation reaching the Earth is very dilute, with a maximum of only about one kilowatt per square meter. It's also intermittent (available during daytime and under clear skies) and inconveniently distributed over the surface of the Earth.

This solar reactor was used to recycle dust from electric arc furnaces at temperatures above 1,500 K.

These issues have spurred the search for recipes to concentrate, store, and eventually transport solar energy from the sunny and sparsely inhabited regions of the Earth's sunbelt to where much of the energy is required, the world's industrialized population centers. These tasks can be accomplished by converting concentrated solar energy into chemical energy carriers, in the form of solar fuels, that can be stored for long periods of time and transported over great distances to meet customers' energy demands.

Producing fuel from sunlight may sound like some futuristic technology, but the means can be found in the writings of two prominent scientists of the 19th century, Sadi Carnot and Josiah W. Gibbs. They created the discipline of thermodynamics, which is the study of how energy can be converted from one form to another—for example, from solar energy to chemical energy. In very simple terms, thermodynamics tells us that the higher the temperature at which we supply energy to a process, the more creative we can be with what comes out as a final product.

When people use sunlight in a typical flat-plate solar collector, they can produce warm water that could be used for taking baths or supplying space heat. This type of device can make a great deal of sense for certain local conditions. If, however, the same solar energy powers a chemical reactor at very high temperatures, near 2,300 K, then this opens up a whole new possibility: Solar energy collected in the Australian Outback can heat homes, supply electricity, propel cars, and more—in Tokyo.

Parabolic mirrors can concentrate the dilute energy of sunlight into a small area, and this energy can be captured efficiently, with the help of suitable receivers, to produce heat at high temperatures for driving an endothermic chemical transformation. Regardless of the nature of the fuel, the theoretical maximum efficiency of such an energy conversion process is limited by the Carnot efficiency of an equivalent heat engine. With the sun's surface as a 5,800 K thermal reservoir and the Earth as the thermal sink, 95 percent of the solar energy could, in principle, be converted into the chemical energy of fuels.

It is up to engineers to design and develop the technology that approaches this limit.

SOLAR HYDROGEN

Almost everyone has had firsthand experience with concentrating solar energy. Children often play with magnifying glasses, focusing sunlight onto a point to ignite paper and leaves. Increase the size of the concentrator, and you can perform even more impressive stunts. Sunlight focused with a large parabolic mirror can drill a soccer ball-size hole through a quarter-inch-thick piece of steel in less than 10 seconds.

At the Paul Scherrer Institute in Villigen, Switzerland, we have a parabolic dish that follows the sun as it moves across the sky, and reflects and concentrates the sun's rays to essentially a small circle. The energy brought to this circle is equivalent to the surface seeing 5,000 suns.

We experimentally study industrially attractive thermochemical processes that can be driven by this concentrated thermal energy. One such process is for the production of hydrogen. Depending on the pressure, the temperatures achieved in our solar furnace can split water into its constituent parts—hydrogen and oxygen—in a process known as thermolysis. (Two molecules of water become one of molecular oxygen and two of molecular hydrogen.)

Although conceptually simple, the usefulness of this reaction has been reduced by the absence of an effective technique for separating H₂ and O₂ at high temperatures. In order to avoid their recombination or ending up with an explosive mixture, we are developing several approaches.

This facility at PSI, the Two Stage on Axis Solar Concentrator, is a solar furnace comprising two large mirrors—a flat 120-square-meter reflective surface and an 8.5-meter-diameter parabolic dish. The dish concentrates sunlight into a point that receives a peak flux of 5 MW per square meter.

One approach that we follow to keep hydrogen and oxygen from recombining is to form each gas at a different step in the process. A two-step cycle based on reduction-oxidation (redox) reactions begins with a solar, endothermic step: the high-temperature solar thermal dissociation of metal oxides. Then, a second, non-solar, exothermic step hydrolyzes the metal to form H₂ and the original metal oxide. The net reaction is still two water molecules becoming one oxygen and two hydrogens, but since the hydrogen and oxygen are formed in different steps, the need for high-temperature gas separation is eliminated.

Such a process does not require exotic material. In a sunny region of the world, common zinc oxide can be brought to the focus of a solar concentrator and decomposed at 2,300 K to metallic zinc. (The oxygen can be vented to the atmosphere.) In a second step, and without solar heating, the pure zinc can be reacted with water and form hydrogen that can be further processed for heat and electricity generation. Zinc also can be employed directly in zinc-air batteries of fuel cells for electricity generation.

The chemical product from such power generation processes is zinc oxide, which can be recycled. Once the hydrogen is expended, it will convert back to water. It is a cyclic process in which no material is consumed and no material is discharged. The only energy that enters into the process is sunlight. The energy available in the hydrogen used to produce electricity or power is solar energy in disguise.

Specifically, work at PSI, at the Swiss Federal Institute of Technology in ZŸrich, and at Valparaiso University in Indiana (funded by the Swiss Federal Office of Energy, the Swiss Federal Office of Education and Science, and the U.S. National Science Foundation) links fundamental physical science studies to process engineering, aimed at developing solar chemical reactors that convert solar energy efficiently into chemical fuels. The solar chemical reactor features a windowed rotating cavity-receiver lined with zinc oxide particles that are held by centrifugal force. In such an arrangement, zinc oxide is directly exposed to high-flux solar irradiation and simultaneously serves the functions of radiant absorber, thermal insulator, and chemical reactant.

SUNLIGHT INTO FOSSIL FUELS

Metals are attractive candidates for storage and transport of solar energy. Furthermore, the replacement of fossil fuels by solar fuels, such as solar hydrogen and solar metals produced from sunlight alone, is a long-term goal. It requires the development of new technologies, and it will take time before these methods are technically and economically ready for commercial applications. That makes it strategically desirable to consider mid-term goals aiming at the development of hybrid solar/fossil-fuel endothermic processes in which fossil fuels are used exclusively as chemical reactants and solar energy as the source of process heat.

The solar parabolic dish at the Paul Scherrer Institute tracks the sun. The dish concentrates the rays of the sun into a small circle at its focus.

The carbothermal reduction of metal oxides using coke, natural gas, and other carbonaceous materials as reducing agents brings about reduction of the oxides at more moderate temperatures. For example, metal oxides react with methane to create hydrogen, carbon monoxide, and pure metal. This reaction combines, in a single process, the reduction of metal oxides with the reforming of natural gas for the co-production of metals and syngas, all without the need for catalysts. With proper optimizations, the process may produce high-quality syngas with a hydrogen-to-carbon monoxide ratio suitable for synthesizing methanol—a promising substitute for gasoline.

The products of such hybrid processes are cleaner fuels whose quality has been solar-upgraded: Their calorific value is increased by the solar input in an amount equal to the enthalpy change of the reaction. Increased energy content means extended fuel life and reduced pollution of the environment.

The mix of fossil fuels and solar energy creates a link between today's fossil fuel-based technology and tomorrow's solar chemical technology. This approach also builds bridges between present and future energy economies: Solar energy has the potential to become a viable economic resource once the cost of energy begins to account for the environmental externalities (such as the emission of greenhouse gases) from burning fossil fuels. By incorporating solar technologies into the production of fossil fuels, the transition from fossil fuels to solar fuels can occur smoothly, and the lead time for transferring important solar technology to industry can be reduced.

SOLAR ENERGY AS PROCESS HEAT

Hybrid solar/fossil fuel thermochemical processes for hybrid production also include cracking and reforming/gasification. The solar cracking refers to the thermal decomposition of natural gas, oil, and other hydrocarbons to co-produce primarily hydrogen and carbon black. Carbon black, a solid, can either be sequestered without CO₂ release or used as a material commodity or metallurgical reducing agent under less severe CO₂ restraints. The steam-reforming of hydrocarbons, and the steam-gasification of coal and other solid carbonaceous materials yield syngas, which can be further processed to hydrogen via the catalytic water-gas shift reaction followed by the separation of hydrogen and carbon dioxide.

Some of these processes are practiced at an industrial scale, with the process heat supplied by burning a significant portion of the feedstock. Internal combustion results in the contamination of the gaseous products, while external combustion results in a lower thermal efficiency because of the irreversibility associated with indirect heat transfer. Using solar energy for process heat offers three advantages: The discharge of pollutants is avoided, the gaseous products are not contaminated, and the calorific value of the fuel is upgraded.

Making environmentally benign fuels isn't the only ecologically friendly process a solar furnace can perform. The industrial commodity production of metals from ore carries severe environmental consequences. It's an energy-intensive activity, and when that energy is provided by fossil fuels, the result is a massive discharge of greenhouse gases and other pollutants.

This solar chemical reactor can raise temperature of zinc oxide to 2,300 K.

These emissions can be substantially reduced, or even completely eliminated, by using concentrated solar energy as the source of high-temperature process heat. Using this heat in the commodities industries creates a potential path for these industries to be more sustainable.

Similarly, by replacing fossil fuels with solar energy, part of the CO₂ emissions in the lime and cement industries can be reduced and the industries' dependence on conventional energy resources can be eliminated. The thermal decomposition of calcium carbonate (limestone) to calcium oxide (lime) at temperatures above 1,300 K is the main endothermic step in the production of lime and cement. Substituting concentrated solar energy for fossil fuels as the source of high-temperature process heat is a means of eliminating the dependence on conventional energy resources and reducing CO₂ emissions in the lime and cement industry by 20 to 40 percent, depending on the fossil fuel source.

An indirect-irradiation solar calcinator consists of a refractory-lined rotary kiln containing a set of axially arranged silicon carbide tubes. Due to the rotational movement of the kiln, the reactants are transported through the tubes from the preheating zone in the back (feeding side) to the high-temperature zone in the front (discharging side). The well-insulated rotary reactor is tilted and works in a continuous mode of operation.

Environmental regulations have created the need for technologies that recycle hazardous waste materials into useful commodities, rather than deposit them in dumpsites for an indeterminate period of time. Commercial recycling techniques by blast, induction, arc, and plasma furnaces are major consumers of electricity and high-temperature process heat, which makes them major contributors of greenhouse-gas emissions. Concentrated solar radiation can supply clean thermal energy at high temperatures to drive these complex processes that usually involve gases, solids, and melts.

High-temperature thermal processes are well-suited for the treatment of complex solid waste materials derived from a wide variety of sources—municipal waste incineration residuals, discharged batteries, dirty scraps, contaminated soil, dust and sludge, and other byproducts from the metallurgical industry.

Thermal pyrolysis and gasification can convert carbon-containing waste materials into syngas and hydrocarbons that can be further processed into other valuable synthetic chemicals.

Waste materials containing metal oxides may be converted by carbothermal reduction into metals, nitrides, carbides, and other metallic compounds. The chemical products from such transformations are feedstock for a variety of manufacturing processes and may also be used as fuels.

The business end of a solar furnace: Devices like this could convert desert sunlight into fuels that can be shipped to industrialized regions.

These applications just scratch the surface of what can be done with concentrated solar radiation. It is an energy resource that can be used as the source of process heat for driving energy-intensive chemical reactions at high temperatures, thus avoiding emissions of greenhouse gases and other pollutants derived from the combustion of fossil fuels for heat, and for electricity generation. Such thermochemical transformations offer an efficient path for the conversion of solar energy into storable and transportable chemical fuels.

Prospects are favorable that solar thermochemical technologies for producing fuels and chemical commodities will become competitive with other production technologies—provided that the cost of fossil fuel-based materials and processes involved in those technologies account for the externalities of burning fossil fuels, such as the cost of CO₂ mitigation and pollution abatement.

Solar reactor concepts have been experimentally demonstrated with 5 to 10 kW prototypes, tested at PSI's solar furnace. The experimental results indicate that the solar chemical reactor technology can be further scaled up and developed for industrial applications.

Our research into high-temperature solar chemistry is forging the connections between old and new technology by mixing fossil fuels and sunlight. And, in the longer term, we hope to develop the scientific and technological know-how for a radically new recipe that mixes sunlight and water to produce fuels from solar energy.

Either way, the ultimate goal remains the same—to create the means by which we can be supplied by a clean, universal, sustainable energy resource.

Robert Palumbo, Anton Meier, Christian Wieckert, and Aldo Steinfeld are researchers at the Laboratory for Solar Technology of the Paul Scherrer Institute in Villigen, Switzerland. Palumbo is also a professor at Valparaiso University in Indiana, and Steinfeld is also a researcher at the Swiss Federal Institute of Technology in Zürich.

Return to Index