Tuesday, April 07, 2009

Underground pumped hydro storage to increase renewables and the efficiency of thermal power plants

Illustration - Principle of the underground pumped hydro storage proposed by Pierre Couture. It can store power and regulate variations in renewable energy, among others, and is an essential component of intelligent electricity networks of tomorrow. (Illustration: Paul Berryman, from my book Driving Without Oil)

On 5 March 2009, Steven Chu, the United States Secretary of State for Energy, stated before a Senate committee his government's priorities in research and development. Five areas were identified and the storage of energy on a large scale is one of them. Storage units of appropriate size would compensate for daily variations of solar and wind, and increase the efficiency of thermal power plants. Energy storage is a key element to reduce greenhouse gas emissions as well of toxic emissions associated with electricity production.

Currently, the most used technology for large-scale storage of energy is pumped hydro. In its traditional version, there are two water tanks located one above the other, connected by an underground tunnel which houses a turbine that can operate in both directions. During periods of low electricity demand (eg at night) the electric motor-generator coupled to the turbine pump water from the lower reservoir to upper reservoir. In periods of high demand, we let the water flow from upper to the lower reservoir, which causes the turbine to generate electricity. In a well designed power plant, the movements of water between the two reservoirs cause a loss of about 20% of stored energy.

But to regulate the daily fluctuations of wind and solar power or to control the variations in demand throughout the day, we do not need to store more than 25% of the energy produced. Hence, regulation leads to a loss of only 5% of the total energy produced.

Illustration - Principle of a traditional pumped hydro storage facility. (source: Wikimedia Commons)

There are currently about 200 pumped storage facilities on the planet, totaling 90 GW of power, or about 3% of the installed capacity at the global level, according to the ESA (Electricity Storage Association).

The obligation to have a significant height difference between the two water tanks has brought many people to say that pumped hydro storage is a technology that could not operate on a large scale. As proof, the following statement

"Expanded use of this technology depends on the availability of suitable geography"

found in the "National Energy Policy Recommendations" of the IEEE-USA (Institute of Electrical and Electronics Engineers), dated 15 January 2009 (download HERE under the heading Energy and Environment).

But it seems that such an assertion is a lack of imagination, because one can very well build pumped hydro storage in the midst of vast plains or in the heart of a city. Just dig a deep well in the rock and build galleries at the bottom to get the second tank.

This is the concept which was proposed by Pierre Couture, a researcher for Hydro-Quebec and inventor of the modern wheel motor (previous post). Louis-Gilles Francoeur, journalist at Le Devoir has unveiled the project in an article dated 22 January 2004.

To avoid too large excavation for the galleries, it is expedient to settle at a greater depth. Pierre Couture recommended digging a well about 2 meters in diameter and three kilometers deep. Turbines that can be reversed and also act as pumps are placed at every kilometer going down, with a buffer cave behind each group of turbine-generators assembly (see illustration at the top of the post).

Calculations show that for a 1 GW of power lasting 10 hours, one must have 3 kilometers (1.9 miles) of galleries with a 20 meters x 20 meters (65 feet x 65 feet) opening, that is 1.2 million cubic meters, to store water at the bottom. The cost of such a plant would be in the range of $ 700 million to $1,000 million, and could regulate power plants with nameplate power of 3 to 4 GW.

If the facility lasts 50 years, we arrive ultimately at a cost below 0.2 cents / kWh of energy produced and regulated, which is only a few percent of the production cost.

Such pumped hydro storage facilities can be used in multiple ways. One can, of course, increase the percentage of renewable energies on a grid by regulating their inherent fluctuations. To reduce the need for too large storage facilities, we need to set up power lines to connect wind farms over thousands of kilometers, because there is always wind somewhere. The high voltage DC power lines are particularly interesting in this regard since they generate only 3% loss per 1000 km (600 miles). As for the solar power plants, they follow quite well the daily demand for electricity (more sun at noon). By placing them in desert areas, one ensures a minimum of cloud cover, which requires less storage of energy for the fluctuations. Most of the storage would be used to postpone to the night a part of the energy produced in the day.

Furthermore, pumped hydro is also interesting to increase the efficiency of thermal power plants. We know, for example, that gas-fired combined cycle plants can achieve an efficiency of 60%. Unfortunately we can not vary significantly the power of such plants to follow daily demand. We must use for that gas power plants whose efficiency is less than 40%. Thus we see all the benefits of coupling a pumped hydro storage to one or more gas-fired power plants. We could then use the most efficient combined cycle gas-fired power plants operating at constant optimal conditions. The daily fluctuations would be managed by the pumped hydro facility. In doing so, we would obtain 50% more electricity with the same natural gas!

With the additional electricity recovered one could close much dirtier coal power plants, waiting to close also, over time, the gas power plants and replace them with renewable energy.

Pumped hydro storage is an essential element of any smart energy policy! And with the underground concept proposed by Pierre Couture, it will become more and more interesting.


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