Response to Pump Up The Storage

In this article I will briefly respond to Dr Murphy’s article entitled Pump Up The Storage. In that article, Dr Murphy shows, through straightforward calculations, that pumped hydropower storage is nowhere near sufficient to compensate for the intermittency of renewables. He assumes that renewables need 7 days of backup power to compensate for prolonged overcast wind lulls. He then calculates the amount and size of dams necessary to provide 7 days of storage and finds that it would take triple the annual concrete used in the United States and would be much larger than any construction project completed so far. As a result, Dr Murphy concludes that it would be impossible, or at least very difficult, to use pumped hydro storage to compensate for the intermittency of renewables.

In his article, Dr Murphy provides all kinds of calculations and physics. However, he makes some assumptions too. In this article, I will make different assumptions and see the results.

First and foremost, I will assume we use a combination of storage mechanisms. I will assume that we use pumped hydropower for the first 12 hours of storage (nighttime) so that solar power is essentially a 24-hour energy source on sunny days. Of course, this would not be enough for long overcast periods. For longer periods of storage, hydrogen would be used. As a result, I will assume 12 hours of pumped hydro storage, not 7 days, which means only 1/14th the amount of storage is required from pumped hydro. I will address the inefficiency of hydrogen later.

It makes no sense to use a single form of storage for all purposes. Instead, storage would be divided into short-term and long-term solutions. The short-term storage is used every single night, so we would use a more expensive, smaller, and more efficient form of storage for that (like pumped hydro). The long-term storage is used only occasionally but is prolonged, so we would use a less efficient form (like hydrogen) which allows large volumes of storage.

Second, I will assume that the dam size needs only to be 5% of the size which Dr Murphy assumes. In his simple physics model, Dr Murphy proposes constructing a dam right through the middle of the reservoir. This is never done in practice. A simple physics model will not suffice here. We need to dig into the details of dam construction and siting. Dams are not built haphazardly. Instead, a crew of surveyors will spend years finding a naturally-occurring “choke point” or narrow section between mountains or in a valley. For example, the Hoover Dam is not built through the middle of Lake Mead. Instead, it is built in a tiny choke point, so it's much narrower. I obtained the figure of 95% narrower by looking on Google Maps, observing the three largest dams in North America (Daniel Johnson, Hoover, and Glen Canyon) and then estimating the width of the dam versus the width of the reservoir using a ruler. Those three dams would be more than sufficient by themselves for 12 hours of storage for the whole country.

Next, I will assume that electrification reduces total energy demand by approximately 50%. Electric vehicles, heat pumps, and so on, are far more efficient than their fossil fuel counterparts. For example, an electric vehicle travels almost triple the distance per unit of energy as a gasoline-powered car. As a result, I will assume a 50% reduction in energy usage from electrification.

Finally, I will assume that the dams are built in a staggered fashion over 50 years. Dr Murphy calculates that it would take triple the annual concrete production in the United States to build his dams. However, the dams wouldn’t all be constructed in a single 3-year period.

If we use the assumptions above, then the dams would take 0.01% of concrete production continuously (3/50/20/14/2 = 0.01%). This would be sufficient for 12 hours of storage for the whole country, until the sun comes up again.

Incidentally, far more dams have already been constructed than would be needed for this purpose in North America. It would be necessary to increase the maximum power of those dams by adding turbines. However, the size of the dams is already sufficient.

I should also point out that we could use any combination of short-term storage technologies, including pumped hydro, compressed air, sodium-sulfur batteries, iron-air batteries, flow batteries of various chemistries (vanadium, iron, organic, and others), pumped heat, gravimetric, and others. Storage is only an unsolvable problem if all of the above solutions combined are insufficient. However, it appears that any one of these solutions by itself is sufficient to provide our 12-hour short term storage.

Finally, this scheme relies upon a long-term form of storage also (hydrogen). This kind of storage is necessary to keep the lights on during prolonged wind lulls when it’s also overcast in the desert. Dr Murphy points out in another article that hydrogen has large round-trip energy losses, so he subsequently dismisses it. However, those large round-trip losses would be incurred only occasionally. Periods when it’s overcast in the desert, and there’s no wind in Texas, are fairly rare. Whereas nighttime happens every single day (and would use our efficient storage above), overcast wind lulls happen only a few times per year. Let's assume that 10% of total energy comes from hydrogen storage with a 67% round-trip energy loss. In which case, it would be necessary to overbuild our solar and wind farms by 20% (0.9+0.1*3) to generate enough hydrogen the rest of the time to cover those periods. This compares with coal power plants which use steam turbines and lose 60% of their energy as waste heat all the time, so the coal mines had to be overbuilt by almost triple to keep the coal power plants running.

In conclusion, it appears to be relatively easy to compensate for the intermittency of renewables. The trick is to find an optimal combination of locations and storage technologies. It cannot be done using a simple physics model of a single solution. We must try various things in combination to find something which works. Once this is done, it becomes clear that a renewable future is totally feasible and (in fact) fairly straightforward using currently-available technologies.