Seeing my Holiday Cactus (photo at right) in full bloom reminds me that the light coming into my house from one of its 50 windows provides energy that this plant is able to capture and use to generate flowers. Plants exemplify complex, highly evolved, and efficient systems for capturing and storing energy. Unlike the cactus or other plants, I can’t absorb light and its energy to create food. As I think about renewable energy, I am aware of my human dependence on biological and technological systems for energy, food, and habitat support.
What are the options available for human-created systems to capture and store energy? Just as plants require sunlight, adequate temperature, soil, and water, solar cells and windmills can’t capture energy when the sun isn’t shining and the wind doesn’t blow.
A recent article by Davide Castelvecchi in the March 2012 issue of Scientific American (Gather the Wind, pp. 48-53) discusses a variety of ways to make renewable energy storable and thereby more practical. Since portable solar-powered devices are fairly common, many of us are used to holding up our calculators to the light to instantly recharge the battery-controlled device. Those of us driving hybrid vehicles are familiar with the alternating solar-charged battery and gasoline fueled systems. But understanding how renewable energy can be captured and stored becomes less clear when I think about large-scale renewable systems like the converted Texas cattle-grazing grasslands and former Indiana wheat fields that now generate energy from acres of windmills. How can this uneven and unpredictable wind-turbine system provide a reliable and consistent energy source?
The technical expert review panel that helped to guide and review Castelvecchi’s renewable energy storage research suggests five different energy storage systems: pumped hydro, compressed air, advanced batteries, thermal storage, and home hydrogen. Below is a short description of the first four of these systems. In lieu of the at home hydrogen system, this discussion closes with a look at artificial photosynthesis, which can produce several types of fuel storage systems resulting from the collection and regeneration of energy from the sun.
Pumped Hydro: This technique, commonly recognized as hydroelectric dams, has been used for at least a century in many countries that have naturally occurring geographic topographical elevation variations. The U.S. has 38 pumped-hydro facilities, but future pumped hydro projects planned could double this number within the near future. Pumped hydro system growth is tied to landscape topography. Areas that feature a large elevated basin with an impermeable ground surface are natural settings for pumped hydro systems. Working from a natural topography well suited to the storage capacity and topography requirements greatly reduces the cost and safety issues that are the largest inhibitors for using this energy storage method. However, like the blooming holiday cactus featured above, finding a naturally occurring physical setting can make pumped hydro technology function almost like a potted plant bearing fruit in a household engineered setting.
A country like Ireland may have a unique opportunity for economic growth by offering several large-scale pumped hydro facilities in its western coastal area as shown in a current photo and artist's rendering (at right) in the two images below.
Image Caption: Reserve power. Once this valley in western Ireland (left) is dammed (artist's rendition, right), stored seawater behind the dam will provide renewable power when it's needed for the U.K. electrical grid. Image Credit: Artist’s rendition of what a pumped hydro facility on the western Ireland coast would look like. Image published in news.sciencemag.org
Igor Shvets, a materials scientist at Trinity College Dublin and founder of the Natural Hydro Energy Company, proposes that the coast of western Ireland is a perfect, natural setting for combining large-scale pumped hydro storage with wind energy generation. The western Ireland coastal landscape features glacial valleys lined with impermeable schist and basalt that are scarcely populated and positioned above a steep drop-off to the ocean.
In the Science NOW online story Massive Energy Storage courtesy of West Ireland (Ferber, 18 Feb 2012), Shvets is reported to have said, “90% of what you need for energy storage is already made for you by nature,” along the western coast of Ireland. The image (above) comparing the current provides an artist’s rendering of this Irish company plan to use the natural setting for a large-scale ocean-water dam that could potentially provide energy for the U.K. electrical grid.
Compressed Air: Surplus renewable energy is used to highly pressurize and compress air, which is then injected and stored in underground caverns. The stored, high-pressure air is released and blasted through turbines, as energy for the grid is needed.
|Image Description: In compressed air energy storage, off-peak power is taken from the grid and is used to pump air into a sealed underground reservoir to a high pressure. The pressurized air is then kept underground for peak use. When needed, this high pressure can drive turbines as the air in the reservoir is slowly heated and released; the resulting power produced may be used at peak hours. More often, the compressed air is mixed with natural gas and they are burnt together, in the same fashion as in a conventional turbine plant. This method is actually more efficient as the compressed air will lose less energy. Image Credit: Ridge Energy Storage & Grid Services LP
Advanced Batteries: Batteries have been the ideal means for supporting intermittent energy demands because they turn on and off instantly and can be scaled up easily. For example, the photo (below) shows how AES Energy has installed 30 megawatts of lithium-ion batteries in Elkins, WV to back up it 98 megawatts of wind turbines.
Image Description: This lithium-ion battery installation can smooth out variability in the adjacent wind farm near Elkins, WV. Image Credit: AES Energy Storage
Thermal Storage: Regions that have steady sunshine may choose concentrated solar power stations as an economical way to generate power and store solar energy. At this time, energy from concentrated solar power plants (like the two shown below that were taken 12 June 2009) is about twice as expensive as energy generated by natural gas power plants. Future projections, however, suggest that improvements to the chemistry of the solar cell fluids and enhanced design and system efficiencies could make concentrated solar power cost equivalent to fossil fuels within 10 years.
Image Description: PS20 and PS10 Solar Power Plant, Andalusia, Spain. Image Credit: Koza 1983
Artificial Photosynthesis: Like the real photosynthesis process, what is referred to as artificial photosynthesis produces a potentially storable fuel or live electrical current—similar to what is distributed by the commercial energy grid. Artificial photosynthesis is similar to the natural plant-based photosynthesis process that uses light to induce a photoelectrochemical reaction between CO2 and water to produce a clean fuel. In the case of plants, the fuel produced is glucose (C6H12O6). Artificial photosynthesis can be controlled to produce liquid Hydrogen, methanol fuel (CH3OH), or can be funneled into a fuel-cell system to generate electricity by combining hydrogen and oxygen into water. Artificial photosynthesis sounds like the perfect answer to many energy challenges, as it does not require mining, plant growth, or drilling, and it even removes large amounts of harmful CO2 from the air in the process.
Image (at right) Description: A researcher at Australia's Monash University checks equipment that mimics the way plants convert sunlight into fuel- so-called “artificial photosynthesis. A variety of research studies investigating the potential for artificial photosynthetic systems are under way in the United States, Europe, Japan and Australia. Read more here: http://www.mcclatchydc.com/2008/10/23/54687/scientists-seek-to-make-energy.html#storylink=cpy
Investigations of artificial photosynthesis for energy production and storage systems are in their early stages in the United States, Europe, Japan and Australia. Human engineered photosynthesis systems have worked in bench-top settings, but are not successful or efficient as large-scale, functional systems. Some of the problems yet to be resolved before artificial photosynthetic systems can be mass-produced include the following. Manganese works as an effective catalyst for photosynthesis in plants, but human engineered systems have not been able to control the unstable qualities of manganese, and since it is not a water-soluble substance, it is difficult to work with and not a practical catalyst to use. Other challenges holding back development of artificial photosynthesis are finding the right formulas for an electrolyte solution to absorb protons made available from the split water molecules.
Storing Renewable Energy...Naturally
Each of the five energy storage systems featured are fascinating and show great potential for providing ways to store the excess energy periodically created by renewable energy sources. I continue to be impressed, though, by plants (like my Holiday Cactus) that absorb sunlight to create beautiful food and delicious oxygen that has allowed human life to evolve and survive and which makes Earth such a beautiful green energy planet.
For further reading about storing renewable energy: