Despite the rapid growth of today’s solar industry, a fundamental question remains: what happens when produced solar energy outweighs its demand at any given time? The intermittent nature of solar energy (and wind, for that matter) is such that we have no control over peak production periods. In other words, the sun shines when it wants, not when we want. As a result, we need to find efficient and economical methods to store this energy so that it can be consumed later when we need it.
Currently, the majority of energy storage is done through a process called pumped-storage hydropower. Excess solar energy pumps water to a holding reservoir of higher elevation to be stored as potential energy only to be released upon demand and generate electricity. This accounts for 93% of energy storage worldwide, with the United States boasting upwards of 20 GW of capacity – good for 20.6% of the world share. Hydropower storage, however, requires a vast amount of water, and as demand increases for renewable energy storage in water-strapped places such as California this means becomes unfeasible. Therefore, we need to think of other ways to store energy on a large-scale for a growing renewable energy industry. As Paul Tullis writes in Slate, the method needs to be 1) power dense, 2) cheap, 3) efficient (at least 80% conversion,) 4) durable and 5) quick on the draw (instant energy release.)
Some methods floating around are batteries, compressed air and flywheels. Compressed air systems are not yet efficient enough (only 70% conversion rate at best,) flywheels are expensive and iron phosphate batteries are not as energy dense as desired. We need to do better.
Advanced Rail Energy Storage
Enter Jim Kelly of Advanced Rail Energy Storage. While Kelly hasn’t exactly solved the problem yet, he does have a pretty cool idea. His company is set to begin testing a system next month that uses excess solar power to move boxcars full of gravel up a small slope (7-8 degree incline) via an electric third rail. The gravel trains are then stored as potential energy for downhill release upon demand. This idea claims to work better than hydropower-storage in arid places (such as California), which also tend to be hotbeds of solar energy production. The system has a near total conversion rate while also being able to store the energy indefinitely.
One drawback to ARES’s model is its relatively large footprint – a 500 MW facility would require eight miles of track. Transmission is another issue, as the energy is produced and stored far away from its consumption point. This fact, argues Tullis, is a case for battery storage as that method allows for energy storage close to its consumption point without requiring major upgrades to transmission technology.
As states scramble to increase renewable energy generation to meet a network of renewable portfolio standards, more and more utilities will begin the search for vast swathes storage energy space. Southern California Edison, for example, just contracted for 50 MW of energy storage – the first utility to do so on such a scale. Without a clear storage technology to meet the coming demand increase, though, we’ll be waiting to see if innovators such as Kelly can figure out a solution. It could be as simple as pushing rocks up a hill.
About Owen Teach
Owen Teach is an environmental policy major at Middlebury College. He is member of the Mosaic Blog Leadership Team, and developed a strong interest in renewable energy after spending a summer at the Office of Energy Efficiency and Renewable Energy (EERE) of the Department of Energy. He is currently a member of Middlebury’s 2013 Solar Decathlon team, and also spends a lot of his time writing for the Middlebury Campus newspaper. Owen also is a man of many languages, as he studies both Spanish and Portuguese.
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