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Gravel Batteries – As green energy proponents address its intermittent nature, good old-fashioned gravel may provide a solution.
Renewable energy is becoming more and more popular these days. We recently jumped on the band wagon and had solar panels installed on our home in Anthem, Ariz. On average, we have 299 sunny days per year, so it is a pretty darn good investment.
The down side to energy from solar panels and wind turbines is their on-off nature. When the wind stops blowing or the sun stops shining, the energy production stops. That is not a problem for us, because we are still connected to the grid and can get power even when the sun doesn’t shine. But believe me — they know how to charge rate payers who have solar!
In order to make solar and wind commercially viable, there needs to be some method to store excess energy production for use when there is no sunshine, no wind, or during peak demand. Electricity cannot be stored easily, but construction of a new battery gigafactory in the United States, as well as other high-tech methods on the horizon, may be part of the solution.
While we wait for new technology to catch on, there are some pretty good solutions already in place. Some environmen-tally friendly methods use — you guessed it — gravel. In terms of supply chain, handling, and construction, few materials are as cost effective, easy to obtain, and simple to use as gravel.
The most common method to store energy is pumped hydro storage. During excess solar or wind production periods, water is pumped uphill into a reservoir. During low or non-production periods, the water flows down through a generator to a lower reservoir. Very simple; very easy. However, hydro storage takes up a lot of space. An idea is being batted around where the water and reservoirs would be replaced by huge buckets filled with gravel. Excess energy produced will be used to haul the rock uphill in a ski-lift kind of contraption. When energy is needed, gravity will carry the rock downhill, producing electricity on its downhill trip.
There are a few somewhat more sophisticated ideas in the works, where excess energy is converted to thermal energy and then stored in giant gravel “batteries,” thus evening out the intermittent nature of wind turbines and solar panels.
One example is in Steinfurt, Germany. Rather than build an expensive tank, the battery is constructed underground in a covered pit. The storage material is a mixture of gravel and water. The side walls, top, and bottom are heat-insulated. The pit has a double-sided polypropylene liner with a vacuum control to identify leaks, and the liner is protected from the gravel by a layer of fleece.
When excess energy is available, heated water (195 degrees F) ‘charges’ the battery, either by direct water exchange (right side of the illustration) or via plastic pipes (left side of illustration). The hot water is stored until it is needed, at which time the water flow is reversed.
The use of rocks for thermal storage is attractive because rocks are not toxic, non-flammable, and inexpensive. The main problem I see with gravel batteries is convincing my wife to allow me to tear up our entire backyard landscaping and fish pond so I can replace it with a big hole filled with gravel and pipes. Is that really too much to ask? AM