Encapsulated Pumped Storage, Part 12: Water

Series Introduction

EPS is built around moving water, in the desert. There are bound to be some challenges. (Of course, it uses the same water over and over, and it doesn’t kill any fish.)

Water Required

I’ve said that the amount of water for the Capitan site is roughly a billion gallons. Let’s refine that number.

The amount of water it takes to fill up the facility and get it ready for operation is the capacity of one of the two storage fields. When all the water is in the lower field, the “battery” is completely discharged. Once you’ve pumped it all to the top, the battery is fully charged. But as I’ve explained, there should always be some water in all the bags, because that’s the anchoring mechanism that keeps them in place.

How much “anchoring water” do we need? The usable volume of a Capitan bag is 3,600 cubic meters (951,000 gallons). That’s the amount that needs to move in or out of an individual bag in order for the overall site storage capacity to meet its spec. Let’s say we add another 10% as non-usable volume to anchor the bag. That’s 360 cubic meters (95,100 gallons) with a mass of 360 metric tons, and a weight (at 8.3 pounds per gallon) of 789,000 pounds, or 394 imperial tons. (In the last post I estimated the weight of the bag itself at 3.25 metric tons; it’s remarkable how small this is compared to the water being contained.)

With a 27 x 27 meter nominal footprint, the bag’s area is 729 square meters, so 10% “anchoring water” would equate to about half a ton per square meter. That sounds sufficient to hold the bag down in any conceivable situation, and may be overkill. But keep in mind that the residual water has another purpose, which is to make sure we don’t draw down the bag to the point where the hydraulic friction starts to significantly increase; so we’ll stay with a 10% “anchoring water” spec for now. If we can trim that to 5% later, so much the better.

This 10% allowance adjusts our water requirement from 1000 * 3,600 cubic meters, to 1000 * 3,600 * 1.2 = 4.32 million cubic meters (1.14 billion gallons).

New Mexico’s Water Usage

To put that amount into perspective, how much water is used in New Mexico (a large, not very populous, arid state) every year? The 2015 report from the Office of the State Engineer1 states “In 2015, withdrawals for all water use categories combined totaled 3,114,255 acre-feet (AF).” That’s 1.01 trillion gallons a year, or an average of 2.78 billion gallons per day. So the Capitan site’s initial fill uses about 1.14 / 2.78 or roughly 41% of one day’s statewide water use—just one time, at the start of the facility’s multi-decade life. (Some “make-up water” will be needed due to spillage and other unexpected events, but the amount will be trivial in comparison.)

Irrigation is by far the dominant consumer of water in New Mexico, as in many Western states. From the same report, “Irrigated Agriculture accounted for 2,376,065 AF (76.30%) of the total withdrawals.

What Is The Market Price Of That Much Water?

All water usage is, of course, subject to water rights. There are several ways in which the right to use this water could be obtained. It might be possible for the state, realizing the benefits the project will bring, to grant the water right at no cost. Or, an existing water right could be leased from its owner for the one-time acquisition of the water.

Irrigation users in the West generally pay lower rates than other classes of users. What would the amount of water we need cost at irrigation rates? An idea of this comes from a report published by The New Mexico Water Resources Research Institute at New Mexico State University2. This states that the mean price of an acre-foot of irrigation water in New Mexico is $2,129. (This is an average over the period 1986-2007. A newer reference would be desirable.)

The 1.14 billion gallons (3,499 acre-feet) we need for Capitan, times $2,129, comes to $7.45 million for the water. That’s one percent of the tighter $750 million project budget, if we do have to pay for the water.

Water Quality Requirements

The water doesn’t need to be potable, and nothing will be living in it. It just cycles back and forth in a closed system. Our only requirements are that it should be:

  • free of suspended particles (silt, algae, etc.), to prevent wear of the turbines and pumps. The particle size limit would be much larger than for drinking water, and putting the water through something like a sand-bed filter before adding it to the system may be good enough.
  • close to neutral pH, to prevent corrosion to metal parts.
  • low in salinity, because salt water is also hard on metal parts. The pump and turbine manufacturers will have data on what’s acceptable.
  • low in solvent residues, globs of oil, or other contaminants.

These fairly lax requirements may open up some alternative water sources. Treated effluent would be one possibility. Another would be brackish (somewhat salty) water; in many rural parts of the Southwest, well water is available, but is too saline for human consumption without costly treatment.

Another possible source, relevant to this part of New Mexico, is recovered fracking water, and/or “produced water,” which is the large quantity of water that comes up mixed with the oil pumped out of oil fields that have been fracked.

A useful article on this topic3 says, “In 2018 the total water used for fracking was about 4.2 billion gallons. But this constitutes less than five percent of the total water used in Lea and Eddy counties, the oil and gas counties in southeastern NM. For comparison, irrigated agriculture accounted for 85% of water withdrawals in these counties.” Another excellent article4 notes: “In 2018 alone, New Mexico’s share of the Permian Basin generated 42 billion gallons of oil and gas wastewater, according to the New Mexico Environment Department.[…] Produced water is not only contaminated with salts; it also contains bits of crude oil, naturally occurring heavy metals and radioactive materials, and trace amounts of fracking chemicals — the identities of which companies do not have to disclose to the public, aside from a few exceptions, because the information is considered proprietary.

The counties mentioned (Lea and Eddy) are in the Permian Basin oil region, in far Southeastern New Mexico. Fracking water and produced water would need treatment (which is already a growth industry in the region) before use in a EPS facility, primarily to reduce the high salinity. It remains to be seen whether the recently booming fracking industry will bounce back from a wave of bankruptcies due to low oil demand and prices, but the idea of using water that’s otherwise an environmental hazard is appealing. Other potential EPS states, like California, also have large amounts of this water to get rid of. (The oil and gas industry has recently been winning approval to spray it on crops after treatment, in spite of the contaminants mentioned above.)

How Do You Get The Water There?

With conventional open reservoirs in wet regions, once we’ve built a reservoir, we can just wait for nature to fill it. For an EPS facility, we need to get the water there ourselves. By far, the best way to do this is by drilling one or more water wells, either at the site itself, or close enough that we can run a temporary plastic pipeline from the wells to the site. (Water rights can be traded, so we could use a water right acquired in a different area.)

If we can’t use local wells, things get harder. It’s about 85 miles from the Rio Grande to the Capitan site, so a temporary pipeline would be a major and expensive project in itself. Finding agricultural water users closer to the site would help, as water rights could be acquired from them during the off-season when crops aren’t being grown.

Would trucking in the water be feasible? Given that the initial fill is a one-time expense, possibly, but the logistics aren’t good. On interstate highways, trucks are limited to 80,000 pounds gross weight5 (vehicle plus payload). Regulations are different and sometimes more forgiving on non-interstate highways, but the best we could hope for is about 11,000 gallons per truckload6. At 1.14 million gallons per bag, it would take about 104 truck trips to fill each bag. The dollar and CO2 (diesel fuel) costs would add up quickly.

I don’t want to sound too pessimistic, because I know practically nothing about the details of how over 1 trillion gallons of water are distributed around New Mexico each year (most of it to rural regions for irrigation). As with so many other aspects of EPS, there’s no substitute for domain expertise.

Time Scale For Water Delivery

1.14 billions of water can’t be moved all at once. The water will initially be delivered to the lower storage field, which in virtually every case will be flatter, more spacious, closer to roads, and easier to build on. When construction starts, deployment of lower storage bags should be highest priority (with other work proceeding in parallel). As soon as a bag is in place, and attached to at least the beginnings of a distribution piping system, it becomes available to hold water. We can bring in water at a steady pace as the site is built, and by the time it’s ready to operate, it will be also be fully loaded with water.

If we allow six months for this phase, with 1,000 bags to deploy and fill, we need to fill an average of 5.5 bags per day. The equivalent rate of water flowing into the site, around the clock, would be 260,000 gallons per hour, or 4,340 gallons per minute, or 9.7 cubic feet per second. With one or more wells located at or near the site, this seems achievable.

Can EPS Run On Hot Water?

Water in bags in the desert will get hot at times, as discussed in the previous post. Assuming the bags themselves can tolerate the high temperatures, what effect will hotter water have on the operation of the turbines and pumps?

This research paper7 studies the effect of water temperature on pump performance. The results are not too surprising. In Part 6 of this series I discussed cavitation, a pump-damaging effect that happens when water entering the pump forms vapor bubbles. The cure is to place the pumps at a lower elevation than the lower storage bags, to keep them under positive pressure. When the water is hot, it’s closer to its boiling point, and so its tendency to form vapor bubbles will be increased. That means it will take more positive pressure to protect the pumps than when the water is cold. This will have to be taken into account during the design process, when specifying the elevation for the pumps.

Another concern is the effect on pump seals8. These are not insurmountable problems; they’re just issues that have to be planned for.

At this point, we don’t really know how hot the water will get; this is site- and climate-specific, and only detailed computer modeling or real-world prototypes could tell us. It helps that we draw water from the bottom of each bag, where it’s coolest in summer (and warmest in winter). In this respect, EPS is no different from the large dams on the Colorado River, which discharge cold water from the bottoms of their reservoirs, to the detriment of the relatively warm-adapted fish that live downstream. Fortunately, we have no fish to protect, but we do need to know how much the effectiveness of an EPS system will be affected during extremely hot weather.

Previous: Encapsulated Pumped Storage, Part 11: Bags

Next: Encapsulated Pumped Storage, Part 13: A First Look At Benefits


  1. https://www.ose.state.nm.us/WUC/wucTechReports/2015/pdf/2015%20WUR%20final_05142019.pdf
  2. https://nmwrri.nmsu.edu/wp-content/uploads/2015/technical-reports/tr356.pdf
  3. Oil, gas and water in New Mexico: The complicated relationship between important natural resources in New Mexico
  4. Wastewater, wastewater everywhere: In the Permian Basin, a new kind of boom
  5. https://en.wikipedia.org/wiki/Semi-trailer_truck
  6. http://transcourt.com/blog/how-much-gas-does-a-tanker-truck-hold
  7. Effect of Water Temperature on Centrifugal Pumps Performance under Cavitating and non-cavitating Conditions
  8. https://www.pumpsandsystems.com/avoid-system-damage-when-pumping-hot-water

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