Harvesting Energy From Carbon Dioxide Emissions

Energy: Device generates electricity from the entropy created when the greenhouse gas mixes with fresh air

An electrochemical cell could someday generate electricity from carbon dioxide emitted by power plants as the gas wafts into the atmosphere. Researchers demonstrate that the cell harvests energy released by the entropy created when CO2 mixes with fresh air (Environ. Sci. Technol. Lett. 2013, DOI: 10.1021/ez4000059). The device could help power plants increase electricity output without producing additional CO2.


Electricity From CO2            
 A new electrochemical cell generates electricity from carbon dioxide dissolved in water solutions. When dissolved, the gas forms carbonic acid (H2CO3), which then dissociates into H+ and HCO3 ions. These ions adsorb selectively onto one of the two electrodes (left and right), depending on the type of membrane on the electrode (yellow and red). This process generates a current between the electrodes.            Credit: Environ. Sci. Technol. Lett
Bert Hamelers of Wetsus, a research center focused on water treatment technology in Leeuwarden, the Netherlands, and his team developed the new device based on one they created to tap energy released when seawater and freshwater mix. The previous cell consisted of electrodes coated with ion-exchange membranes. As seawater and freshwater flowed through the cell, the membranes absorbed and released sodium and chloride ions, creating a current.

Hamelers realized that the same cell design could harvest the energy released when two gases mix. To do so with CO2, the team first mixed it with a liquid, using either deionized water or a 0.25 M water solution of monoethanolamine (MEA), which is often used to remove CO2 from exhaust gases. In water, the CO2 forms carbonic acid, which then dissociates into H+ and HCO3 ions. These ions act like the sodium and chloride ions in the previous entropy-harvesting device. As the solution passes through the cell, ion-exchange membranes on the cell’s electrodes absorb the ions, H+ on one electrode and HCO3 on the other. This process produces current between the electrodes.

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                       Mixing Gases            
To harvest energy from mixing CO2 and fresh air, researchers first must dissolve the gases in water solutions (CO2, right; air, left). The water then passes by membranes in an electrochemical cell (rectangular block in the middle) in alternating pulses. The cell generates electricity as ions in the solutions adsorb onto and desorb from the electrodes.    Credit: Bert Hamelers/Wetsus        
Then water with dissolved fresh air flushes through the cell. Since this water is mostly ion free, the membranes release the H+ and HCO3 ions into the water, producing current in the opposite direction as before. This now ion-laden water leaves the cell and gets flushed with air. The CO2 gas reforms and is then released. The fluidics system continually repeats this cycle, sending alternating pulses of the dissolved CO2 and dissolved air through the cell.

With the small-scale system the researchers built in their lab, they could harvest 24% of the energy released when they used deionized water and 32% when they used MEA. At its most efficient, the lab setup generates only milliwatts of power. But with a scaled-up system, the researchers calculate that power plants could produce megawatts of power using CO2 emissions. They estimate that flue gases from power plants worldwide contain enough CO2 to generate 850 TWh of energy every year.

But the system has a few obstacles to overcome before it can be used in such large-scale applications, the team and outside experts say. For example, impurities in a power plant’s flue gas, such as sulfur dioxide or nitrogen oxides, could foul the cell’s membranes. The immediate problem is getting CO2 emissions dissolved into a liquid upon exiting the stacks. With current technology, dissolving that much gas in liquid would require more energy than the researchers’ system could generate. So it will take more research to find the optimal process to dissolve CO2 using as little energy as possible, Hamelers says.

Still, the concept is “marvelous,” says Volker Presser of the Leibniz Institute for New Materials in Germany. Now the researchers “need to envision a system that can take up tonnes and tonnes of CO2,” over multiple cycles, he says. With such a system generating extra electricity, Presser says, coal plants could produce energy more efficiently, without emitting more CO2.

Chemical & Engineering News
ISSN 0009-2347
Copyright © 2013 American Chemical Society

New record for photovoltaic solar cells






28.09.12 – This week, EPFL’s Institute of Microengineering presented in Frankfurt “hybrid” photovoltaic cells with an energy conversion efficiency of 21.4%, the highest obtained for the type of substrate they used. This breakthrough will contribute to lower the cost of solar cell based installations.

In the medium term, an investment of only 2000 francs in photovoltaic cells would suffice to provide more than enough electricity for the consumption of a four people household. This promising scenario has been made possible by the innovations accomplished by EPFL’s Institute of Microengineering in Neuchatel. The team of prof. Christophe Ballif, director of the Photovoltaics Laboratory (PVlab), presented their work at the European Photovoltaic Solar Energy Conference and Exhibition that just took place in Frankfurt.

The PVlab specializes in thin film solar cells and has been interested for several years in “hybrid” technologies, better known as heterojunction technologies, designed to enhance solar captors’ performance. “We apply an infinitesimal layer – one hundredth of a micron – of amorphous silicon on both sides of a crystalline silicon wafer,” explains Christophe Ballif. This “sandwich” conception contributes to increase the sensors’ effectiveness.

For this assembly to be efficient, the interface between the two types of silicon requires to be optimized. Antoine Descoeudres managed to achieve this feat together with Stephaan DeWolf and their colleagues. They chose the commonest – and therefore cheapest – crystalline cell (called “p-doped silicon”), took care of its preparation and improved the process of application of amorphous silicon. They obtained a 21.4% conversion efficiency, which had never been achieved before with such type of substrates: nowadays, the best quality monocrystalline cells only attain an energy conversion efficiency of 18-19% at best. In addition, the measured open-circuit voltage was 726 mV, which constitutes a first-time accomplishment as well. Last but not least, they broke the 22% efficiency barrier on a less common substrate.

Close to the market
These results, validated by the Fraunhofer Institute for Solar Energy Systems (ISE) in Germany, will soon be published by the IEEE Journals of photovoltaics.
To bring these innovations to a stage of industrialization may only take a few years. This research was partly financed as a commission for Roth & Rau Switzerland, whose parent company, Meyer Burger, has already started the commercialization of machines built for assembling this type of heterojunction sensors. “Within three to five years, we expect to reach a production cost of 100 francs per square meter of sensors, estimates Stefaan DeWolf. In Switzerland, with the conversion efficiency achieved, such a surface will be able to produce between 200 and 300 kWh of electricity per year. “