Some of the larger-scale options (pumped hydro, CAES, FLES etc.) presently being considered for storing intermittent renewable energy rely on the existence of holes in the ground, often man-made ones, to make them work. In this post I take as a hypothetical example the world’s biggest man-made hole (the Bingham Canyon Copper Mine, Utah, shown as viewed from space in the inset) and fill it with water from the Great Salt Lake 25km to the north to get an idea of how much untapped hydro storage potential Bingham and other holes like it might offer. I find that Bingham has the potential to store about 3TWh, which would make it by far the largest pumped hydro facility in the world. 3TWh of storage, however, is nowhere near enough to support an all-renewables world, and there just aren’t that many more big man-made holes like Bingham around.
The three basic ingredients of a pumped hydro project are an upper reservoir, a lower reservoir and pipelines (penstocks) connecting the two. Bingham already has two of these ingredients – a large hole suitable for an upper reservoir and the Great Salt Lake, which I assume, optimistically, could be used as a lower reservoir.
First we will look at the potential size of the upper reservoir. The Bingham Canyon deposit has been mined more or less continuously since 1906 and here’s the resulting hole:
Figure 1: The Bingham pit looking northeast towards Salt Lake City. The original Bingham Canyon, or what’s left of it, is marked for reference. The benches are either 50ft (15m) or 100ft (30m) high. Data from Google Earth.
The sheer scale of Bingham is difficult to grasp. People high up in the visitor’s gallery, for example, see ant-like mine trucks crawling around the base of the pit:
Figure 2: Truck activity in base of Bingham Canyon pit.
But on closer inspection they turn out not to be very ant-like at all:
Figure 3: The Cat 797 H haul truck, with a payload of 400 short tons.
I can’t find a detailed estimate of the total volume of material excavated from Bingham, but a ball-park estimate using Google Earth indicates around 10 billion cubic meters of material weighing approximately 25 billion tonnes. This is over a thousand times the 6.7 million cu m of excavation contemplated by the Flat Land Energy Storage project discussed by Euan Mearns in his FLES post.
Of more direct concern, however, is the capacity of the upper reservoir to hold water. As shown in Fig. 1 the limiting water elevation occurs where Bingham Canyon itself intersects the pit, at which point the elevation is about 1920m above sea level. Using 1900m as the limiting water surface elevation gives the reservoir shape shown in Fig. 4. We have a roughly circular reservoir with an average diameter of about 5,000m, a maximum depth of 500m and an average elevation of 1,650m. Treating this shape as an inverted cone yields a volume of 3.3 billion cubic meters, a thousand times as much as the amount stored in the disused Glenmuckloch pit discussed in Euan’s recent eponymous post.
Figure 4: Upper reservoir extent, water level at ~1,900m. The discolored areas around the reservoir are pit slopes containing waste material or unmined ore, access roads, leach dumps etc. Data Google Earth.
The next question is what to use as a lower reservoir. As shown in Figure 5 we can either go about 25km north to the Great Salt Lake (average elevation 1,280m) or roughly the same distance south to Utah Lake. The question, however, largely answers itself because Utah Lake is only a third the size of the Bingham upper reservoir (1.1 billion cubic meters) and this would limit the upper reservoir to a third of its capacity, like Gorona del Viento on El Hierro in the Canaries. The Great Salt Lake, on the other hand, contains about 20 billion cubic meters. The problem is its salinity, which can range from 5-27%:
Figure 5: Location of Bingham upper reservoir relative to the Great Salt Lake and Utah Lake. The metro area to the east is Salt Lake City. Data Google Earth.
The next question is where does the 3 billion cubic meters of water needed to fill the upper Bingham reservoir come from? To make the project work I must assume that it will be filled with water from the Great Salt Lake – preferably from a deep intake that taps the less saline water at depth (can water with 5-10% salinity be used to drive hydro turbines? I have no idea, but sea water at 3.5% does not seem to pose any insuperable problems.) Kennecott, the current property owner, also extracts maybe as much as 20,000 gpm from wells, drains and collection basins for use as cooling and tailings water in the mineral processing facilities adjacent to the Great Salt Lake shown in Figure 5, and this could also be used to fill the pit if minerals production is no longer an issue. There is also some chance that the pit would eventually fill itself, although this would take a long time. Elevations over 2,000m at this latitude can receive over 1,000mm of annual precipitation, mostly falling as snow, but copper deposits tend to be highly permeable and much of this percolates into the ground. Because of this a large plume of contaminated groundwater has spread downhill into the Salt Lake Valley over the years.
To keep the upper reservoir full and aid in the process of filling it we therefore need to line the pit sidewalls with an impermeable liner and/or cover them with an impermeable clay layer. How much will this cost? Again I have no idea, but I have to assume that the cost won’t be prohibitive or the project won’t be viable.
So having established the basic design elements of the Bingham Canyon pumped hydro project and forced it to work whether it wants to or not, how much energy does it store? According to Engineering Toolbox storage capacity in joules (watt-seconds) is given as:
- Volume of the upper reservoir (3.3 billion cubic meters)
- times the average elevation difference between the upper and lower reservoirs, (1650 – 1280 = 370m)
- times the density of water (1000kg/m3)
- times the gravitational constant (9.81 m/s-2)
= 12,000,000,000,000,000 watt-seconds, or
3,300,000,000 kWh, or
3,300,000 MWh, or
3,300 GWh, or
What emerges is a truly monstrous pumped hydro storage system which at ~3TWh exceeds the capacity of any existing system in the world by orders of magnitude (Bath 0.024, Dinorwig 0.010 TWh). However, it will still supply US demand for only about 7 hours and will also be far too small to store any significant fraction of the world’s seasonal solar or wind surpluses.
And I haven’t even discussed the project’s fatal flaw. It’s not problems with Great Salt Lake brines, nor costs, nor environmental impact, nor any of the thousand-and-one other potential problems that I’ve glossed over, but this:
Figure 6: The Great Bingham Canyon Landslide of April 10, 2013.
There’s nothing about the Bingham Canyon Mine that isn’t big, so it’s to be expected that the largest “artificial” landslide ever recorded (150 million tonnes) occurred there on April 10, 2013. There were no injuries because the event had been predicted some days in advance, but one can imagine what would have happened if the pit had been full of water up to the 1900m level (just below the buildings in the left foreground) at the time. And yet more landslides can be expected in the future. Open pit mines and other large man-made holes in the ground are not designed for long-term stability. They fight gravity, and gravity always wins in the end.