Pumped hydro energy storage uses reversible hydro electric turbines which employ electricity to run backwards and pump water from a low elevation reservoir to a high elevation reservoir. At some later time the water from the high elevation reservoir can be run back through the turbines to produce electricity. The round trip efficiency (i.e. the percentage of the input electrical energy which is recovered during forward operation of the turbines in the range of 70% to 85%.Pumped hydro storage is the most extensively used storage method for utility scale electrical energy. The gravimetric energy density of this storage method is not high. For example if the elevation difference between the low an high reservoirs is 600 meters then the gravitation potential energy per kilogram of water (ignoring efficiency losses) is 9.8*600=5880Joules/kg=1.6Watt-hours/kg (Lead acid batteries have an energy density of 25Wh/kg). However, in spite of this low energy density it is genrally claimed that pumped hydro energy storage has a relatively the low levelized cost compared to battery storage (for a contrary opinion see here). Potential reasons for this relatively low cost are three. First the storage medium (water) is brought to the storage facility in large part by natural processes and does not have to be manufactured and transported. Second, the storage 'container' is a natural basin which needs to be damned but does not otherwise require construction. Finally the cycle life is extremely long. A pumped hydro storage facility can operate for many thousands of cycles without needing to be rebuilt.
In spite of the potential long term economic advantages of pumped hydro storage facilities the rate of new construction is not high. Even though the long term costs may be relatively low, the up front capital cost are high, and often significant land and water use issues are involved in building a large dam. Almost no one but a national government can undertake the construction of such a facility. Furthermore locations with appropriate high head basin and sufficient rainfall are comparatively rare.
One possibility for expanding the number of locations where pumped hydro storage can be used is to create the low elevation reservoir by excavating an underground chamber. A Canadian company called Riverbank Power is planning to develop such underground pumped hydro facilities. Their web site is not overly informative, but a few extracts are given below:
Riverbank’s Aquabank™ is an underground alternative power generation facility that produces electricity from turbines located underground near a suitable water source with a combined installed capacity of 1,000 MW.
The Aquabank™ system temporarily diverts water from the source using the force of gravity down 600 meter shafts to an underground powerhouse, where it travels through four massive turbines, thereby creating emission-free electric power. This newly generated power is then harnessed by a transformer and sent to the power grid to help accommodate peak consumption periods in urban communities.
Once through the turbines, the water is then temporarily stored in enormous reservoirs at approximately the same depth as the powerhouse before being pumped back to its original source using lower cost power from traditional and renewable power sources.
Toronto, Canada – April 15, 2009 – The test results from the first bore hole to establish suitable geology for Riverbank Power Corporation’s Wiscasset Energy Center have yielded excellent results according to a report issued today by the Company. The underground alternative energy pumped storage facility would produce 1,000 MW of energy to be used during peak consumption periods.
The results indicate the existing rock at the site is favorable for the construction of the powerhouse and that additional “packer tests” demonstrated very little water absorption into the surrounding area.
“While these initial results are very encouraging, we will need to conduct additional test borings to further confirm the geology of the entire project area that will provide technical data for the design engineers,” said John Douglas, President and CEO of Riverbank Power, “For now however, these are tremendous results that firmly establish Wicasset within our portfolio of pumped storage projects.”
The objective of Riverbank Power’s drilling program is to confirm that the site has the appropriate geology required to construct the innovative, underground alternative energy generation facility. John Hellert, CEO of the firm that conducted the testing, Continental Placer Inc., said, “We are pleased that the initial drill hole confirmed our preliminary opinions of the site. The project location is underlain by very competent crystalline rocks which testing indicates will be suitable for the pumped storage project and use as construction aggregate.”
The comment about the geologic suitability of the proposed pumped hydro facility near Wicasset Maine is interesting. I have heard it claimed before that underground pumped hydro can be located anywhere since a hole in the ground can be dug anywhere. Apparently this claim is not correct, or at least the economics are unattractive if you have to line the underground chamber with a manufactured waterproof coating.
Presumably the capital costs of excavating large underground reservoirs are higher are higher than building dams across existing geologic formations capable of holding large amounts of water. Exactly how much this expense may be is not dicussed by RiverBank, but presumably if the cost is absurdly high no one will agree to pay for such a project.
Underground pumped hydro facilities would directly consume water only through leakage and evaporation from the reservoirs on the surface and beneath the ground, since otherwise the water is used over and over. Of course in a time of drought pressure to release water from the storage facility would be high.
The question arises as to how much water would have to be held in storage in order to provide a useful amount of energy storage. I will examine this question by considering energy usage in my home county of Santa Clara in the San Francisco Bay area. Below are some relevant numbers concerning population, reservoir capacity and electricity consumption per capital in Santa Clara County:
I will consider the case of 24 hours worth of storage at average electricity consumption rates which is equal to 26kWh×1.7×106= 44.2 million kWh. Other assumptions I make are:
The energy stored per liter of water (taking account of efficiency losses) is 9.8m/s2×1kg/liter×600m*.89=5233 Joules/liter or 1.45kWh/m3. The required amount of water for storing one average day's worth of electricity is then given by:Water Storage = 44.2million kWh/1.45kWh/m3 = 30.5 million mm3
30.5 million m3 is 14% of the county's current total reservoir capacity. Two comments should be made about this percentage. First the reservoir system is rarely ever full (only after an exceptionally rainy winter) so that the percentage of average water reserves would be substantially higher than 14%. Secondly this water would be consumed only through evaporation and leakage so that the yearly supply of new water required would be significantly lower than the percentage of average water reserves represented by the 30.5 million m3 in the pumped hydro system.
It certainly seems conceivable that such a storage system is economically viable in terms of water use, although it might come under pressure during periods of extended drought.
Twenty-four hours of storage would obviously improve the economic performance in intermittent renewables since load following over this time scale would be possible. It is not clear however that 24 hours of storage is sufficient to give intermittent renewables the same long term (i.e. days, weeks or months) load following capabilities as fossil fuel power generation. Longer storage times would put higher demands on water supply. Obviously wet climates would have less constraints in this respect than relatively dry climates like California.
Finally I should mention the fact that in the case of Santa Clara county we are near the ocean so that we could potentially use salt water instead of fresh water and so avoid conflict with other uses of water. Naturally the maintenance cost of a salt water system would be higher because the highly corrosive nature of salt water.
April 28, 2020Energy Storage News
rogerkb [at] energystoragenews [dot] com