Gravity and storage

My firm was contacted a bit over a year ago by a start-up firm called "Genergy, LLC" with what appears to be an innovative technology for utilizing buoyancy to generate electricity. This post is NOT about that concept. Rather, it's a side trip down a road that originated in a post by the founder of the firm and inventor of the patented technology, Mr. Kurt Grossman, in a thread in a Linkedin group he started called "Gravity Energy." The specific thread discusses pumped hydro storage.

Photo Credit: Mitsui & Co., Ltd.

Pumped hydro storage is one of a very few currently viable methods to store energy at so-called "utility scale" or "grid scale." The idea is to use energy (typically generated by intermittent sources such as wind and solar) to pump water from a lower to an upper reservoir and then to re-capture the potential energy imparted by this process by running the water back down to the lower reservoir through a hydroelectric turbine generator (diagram at left - click to enbiggen).


The technology is, in principal, capable of turning an intermittent source into one suitable for base load power. Of course, you might recognize that hydropower from dams utilizes this principle with mother nature providing the pump via the hydrologic cycle.


In the thread, I mentioned that geography places limits on the wide-scale adoption of pumped hydro storage because of the necessity for two very large reservoirs with a large elevation difference in close proximity. This is something that we don't find naturally around every corner and that, to construct, would entail an extraordinary infrastructure expenditure. Of course, given that one of my Company's main Divisions is dedicated to quality assurance and quality control of infrastructure projects, I'm not at all opposed to such an undertaking!


But I want to "bring this storage method home," so to speak. Were I to install a solar array and utilize pumped hydro in my household, what would this entail? Let me assume that I'd like to have the ability to store sufficient energy to provide electricity to my home for three days. Such a need will most likely arise in winter when I won't need air conditioning - hot summer days usually provide plenty of sunshine and the three days' worth of winter storage should also suffice for summer nights when the sun is down.


As it happens, I've calculated my home's average continuous energy use before, and it's an embarrassingly large 2.1 kilowatts. Therefore, I need to provide 2.1*24*3 or 151.2 kilowatt hours of energy in the system's water tank (that my homeowners association will never let me build no matter how the numbers come out). Energy units are energy units, so I can convert these 151.2 kilowatt hours to 5.443*10^8 or 544.3 million joules. My system is likely to run, optimistically, at about 75% efficiency, so I'll need to store 7.258*10^8 joules. As a side note, this is the total thermal energy available from burning about six gallons of gasoline. To actually use this energy and assuming a Home Depot generator runs at 25% efficiency, I'd need to have 24 gallons available. Keep this number in mind!


Now there are two ways to increase the energy stored in a pumped hydro system: increase the quantity; and increase the height to which it's pumped. A cubic meter of water is 1000 kilograms and raising this 1000 kilograms one meter will add 1000*9.8*1 or 9800 joules. Let's say instead that I could raise it 10 meters. Note that this is 33 feet so that the average height of the water in the tank is above the tops of the roofs in my neighborhood. Now I've added 9.8*10^4 joules. This means I need to raise (7.258*10^8)/(9.8*10^4) or 7406 cubic meters of water. This is just shy of two million gallons (1.96 million) so I'll go with the two million gallon tank. This is the part where I suggest recalling the 24 gallons of gasoline.

Photo credit: San Patricio Municipal Water District

As it happens, we have a swimming pool. It's likely on the small side of average at around 19,000 gallons. So the water in about 100 of those will just do the trick! To the left is a photo of such a tank. Perhaps this explains why those who go "off grid" use batteries to store energy captured by their solar arrays and wind turbines and keep fossil fuel powered generators for those times where the batteries don't suffice.


There's much more on the practicalities (or lack thereof) for utilizing pumped hydro storage on a wide scale to store energy from intermittent sources at one of my favorites sites. It's called "Do the Math" and is published by Dr. Tom Murphy, a professor of physics at UC San Diego.