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Figure 1. Giant gravel “batteries” can convert excess energy to thermal energy.
Figure 1. Giant gravel “batteries” can convert excess energy to thermal energy.

Gravel batteries and renewable energy

Bill Langer discusses the merits of renewable energy – and how few materials are as environmentally friendly to use as gravel.

Renewable energy is becoming more and more popular these days.

We recently jumped on the bandwagon and had solar panels installed on our home in Anthem, Arizona. On average, we have 299 sunny days per year, so it is a pretty darn good investment.

The downside 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 ratepayers who have solar!

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 the construction of a new battery gigafactory in the US, as well as other high tech methods on the horizon, may be part of the solution.

The downside to energy from solar panels (pictured) and wind turbines is their on/off nature.
The downside to energy from solar panels (pictured) and wind turbines is their on/off nature.

While we wait for new technology to catch on, there are some pretty good solutions already in place. Some environmentally 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 (90oC) “charges” the battery either by direct water exchange (Figure 1, right side) or via plastic pipes (Figure 1, left side). 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?

Bill Langer
Independent Research Geologist

Bill Langer is a freelance writer and retired Senior Research Geologist for the U.S. Geological Survey. Bill is now an independent researcher specialising in aggregate resources. Click here to email Bill or visit his website.
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Thursday, 17 October, 2019 2:04pm
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