As we make the transition from fossil fuels to clean, renewable energy sources, one fact that becomes clear is that while fossil fuels provided both an energy source and a storage medium in one, in the new world of renewables, these two functions will have to be provided separately. The ability to store energy at times when it is available and not needed as well as those times it is needed but not available will be a key to the effective utilization of renewable energy.
Harvesting the energy resource at a steady rate will generally result in the most efficient operation. But as we move towards more electrified systems, there are further efficiencies that can be realized by the ability to provide very focused energy delivery mechanisms that are intelligent and interactive with the loads themselves itself in ways that bring previously unimaginable levels of efficiency. Hybrid electric vehicles are probably the most obvious examples, though there are numerous others in existence and countless more in development.
Storage could take the form of charged electric batteries, compressed air, mechanical springs, rotating flywheels, pumped water, heat, ice, electrolytic production of hydrogen, super-capacitors or numerous other methods. All of these can help ease the integration of renewables into the grid and take advantage of synergies that might not be readily apparent. For example, using night-time, off-peak electricity to produce ice that can be used for air conditioning the next day, is a form of thermal energy storage, that not only saves money by taking advantage of lower electric rates, but it also runs the equipment in cooler conditions when it is more efficient, and operates primarily at a time of day when the percentage of renewables producing power is higher, resulting in a lower carbon footprint.
Greatest Synergy in Options That Produce Electricity
But it is those storage options that can directly produce electricity that probably offer that greatest synergy in deliberately and carefully designed systems containing such elements as: distributed renewable power sources, flexible base load infrastructure, a smart grid, demand response and rapid response energy storage systems. When integrated fully, these elements will perform together like discrete components in a circuit whose primary function is to reliably provide the cleanest and most efficient form of power possible, around the clock, day after day.
Many people consider a perfect battery to be the energy storage Grail. Batteries are good because not only are they clean and extremely efficient, but they can provide stored energy instantaneously. The development of new, very large, highly efficient batteries, suitable for utility scale storage, has become very big business. AES Energy Storage, for example, has, at this writing, some 76 MW of storage in operation or construction, with 500 MW more in development. EOS is another such company, just bringing its first products to the market this year. Its website makes the following:
- Electricity off-peak/peak time shifting
- Frequency regulation, spinning reserve and other ancillary services
- Capacity payments (reliability incentive)
- Load firming for renewables
- Transmission and distribution capital expenditure deferred
Batteries Have Limitations
But batteries have their limitations. Since they depend on a chemical reaction, they are limited in the number of charge-discharge cycles they can go through before wearing out. They also are limited in the amount of power they can deliver at once, and their output tends to decline in cold weather.
Many of these shortfalls can be addressed by either substituting super-capacitors, also known as ultra-capacitors, for batteries, or using them together. According to Chad Hall, founder of ultra-capacitor maker Ioxus, ultra-caps can withstand millions of cycles, they can perform at temperatures down to 40 below, they can deliver enormous bursts of power (albeit for a very short time), and they are super-efficient (over 90 percent, as compared with batteries at around 70 percent).
They are great for start-stop vehicles, such as hybrid electric buses that can accelerate from zero to 30 mph in about 2.5 seconds. They also work well in utility applications. Considering that 90 percent of all voltage dips and sags last for less than two seconds, these can all be handled with efficient ultra-caps, while batteries handle the other 10 percent.
When used together, ultra-caps can significantly extend battery life, by handling the power surges that take such a toll on batteries. Buildings and heavy equipment can use systems up to 4MW that can provide short-term ride-through for critical applications such as pharmaceuticals, silicon manufacturing, glass-making, even metal fabrication.
As we move forward into a clean energy future, you can expect to see energy storage systems playing an increasingly important role in helping to provide smart, energy-efficient systems that will make the most of every drop of energy.
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