# How do you make a 1 kilometer seal?

by 0xDEADBEEF
Tags: kilometer, seal
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 P: 825 There is a German professor for whom the polls are still open on which side of the genius/lunatic line he is. Since the main problem with renewable energy is saving it for the time, when there is no sun or wind, he wants to build a giant energy storage. Available water reservoirs are scarce so his idea is to use the potential energy of a giant plug of granite that would be lifted with water pressure. Here is his website: http://eduard-heindl.de/energy-storage/index-e.html But let me summarize, what I think I understood: - The plug has the same hight as its diameter and would be lifted up to half that hight. Therefore the energy capacity scales as a power of 4! (Volume times hight) - Apparently there are are steel rope cutting techniques employed in South African mines that can be used to cut such a plug in place. - A plug of 500 meters would be enough for Germany's electricity needs for a few days - I don't remember were one would take the water from but it doesn't seem to be a big issue I did some maths on the pressure involved and it seems to pan out. So my question is mostly how would you go about sealing such a plug? What other problems do you see from an engineering point of view? Is is geologically sound?
 P: 145 That's a neat concept. I have no idea about a seal. BUt I would think it would need to be mechanical and not rubber or some other synthetic material. The seal would probably relate very much to how the plug was cut from the earth.
 P: 669 I am not sure if it is worth the trouble. Granite has a density of 2.7 kg/cm^3, water has 1 kg/cm^3. So if you used just water without the granite plug on top, you would get a ~3 times smaller energy density, but much much less hassle with seals and so on. The scaling works just the same. In case of doubt, you can imagine that a water "plug" is frozen solid at the top, and you lift that with water fed in below. As an additional bonus, ice floats, no seal is needed. With the scaling to the forth power you "only" need to increase the diameter and lifting height by 30% to make good the factor 3 lost to density. So the equiv. of 500m granite is 640m water which makes 275 million cubic meters (0.275 cubic kilometers). A massive granite plug has to be cut and either be used used in place - there cannot be many places where this is possible, or it has to be cut, extracted, and transported. I doubt that this will cover the energy demands for a very long time. For comparison, the Walchenseekraftwerk is a hydroelectric power plant that uses the ~200m height difference between two lakes (~2/3 of the proposed lifting distance converted to water) with a reservoir volume of 110 million cubic meters (~half of what I calculated above). The hydroelectric power plant produces 124 MW, mostly used during peak hours. http://en.wikipedia.org/wiki/Walchen..._Power_Station (the German language page has a lot more information)
 P: 825 How do you make a 1 kilometer seal? I would guess that a big advantage is, that you can make a hole in the ground at any level. While you would need to make a reservoir for all that water above ground to drain it to ground level, which would require massive walls. Of course the advantage is lower if you need energy to pump the water up from underneath the ground (it remains the weight difference between granite and water).
 P: 669 If the granite plug is lifted by water pressure, then you need to get the water to and from wherever the plug is located - most likely underground at some depth. To extract power from the lowering of the plug, you will most likely use turbines just as in conventional hydro storage. This is also written in the proposal that you linked above. If the plug is supposed to be 500m high and to be lifted by half its height, then you need a hole that is 750m deep. That is a lot, about the size of an open pit mine. This sounds completely unfeasible to me. The old world trade center towers were about 500m high. The other example they mention is a height of 150m. That scale looks much more realistic, but it still quite huge, the pit needs to be 225m deep assuming that the whole installation is kept underground. Realistically you will need two pits, one for water storage and one with the granite plug for energy storage. The water volumes involved are on the scale of a sizable lake and you cannot assume that this much water can be extracted from and fed back to the environment whenever you have spare energy or peak loads. Water is transferred from one to the other by pumps for energy storage, and through turbines for energy extraction. The energy you can store and extract is then (turbine efficiency)*(pump efficiency)*(density of water - density of granite)*(lifting height)*(cross section) For each meter you lift the granite, you will lower the water level in the storage tank by 1m and vice versa - hence the difference in density in the formula. In traditional hydroelectric storage you displace air, the density of which is so much lower than water that it can be neglected. The relative altitude of the two reservoirs does not matter. But for traditional hydroelectric storage you need the two reservoirs at different height, typically two lakes (artificial or natural) at different altitude, and you have problems like evaporation that would not occur in a closed underground setup. I cannot see any way that the seal could be avoided. Obviously and unfortunately, the granite plug cannot be replaced by something that floats.
 P: 825 He plans on having the top above ground. Apparently people have asked him after a talk if one could build an hotel on top I guess the real question is really where you get the water from. You would gain very little if you need to make a second giant hole for all the water.
 Mentor P: 22,301 Since area scales as a square function of diameter, but circumference is only a proportion, I'm not sure I'd even bother trying to seal it tightly. The water loss through a few mm gap would be insignificant compared with the water volume being pumped into the reservoir.
 P: 490 Could you not use some of the electricity produced to warm the plug slightly to get a nice fit.
P: 669
 Quote by russ_watters Since area scales as a square function of diameter, but circumference is only a proportion, I'm not sure I'd even bother trying to seal it tightly. The water loss through a few mm gap would be insignificant compared with the water volume being pumped into the reservoir.
If you assume r=500m and a gap of 0.1m, then the ratio of areas of the plug and the gap=leak is 2500:1. That is not bad. In absolute terms, the leak is still ~300m^2, so you have to make sure that you are pumping water in and out much faster than the leak rate. In other words (this is a very rough approximation), the feed pipes and the pipes to the turbines need to be larger than 300m^2.

Can anybody calculate the "holding time", i.e. the time it would take the plug to sink to the bottom because of the leakage?

Another "small detail" to consider is how to guide the plug so that it does not jam.
 P: 26 Extremely ridiculously impractical.... But I love the idea! The forces at work here are too massive for any economical seal.... IF the "cylinder is put into place and displaces the water below it...Yes! the energy would be amazing... But....Then what? I dare anyone to lift the piston back up. SO....for the initial seal.... Too massive.... WIthout steel or concrete cylinders..... This is ridiculous....How would you lift the piston from TDC back to the bottom of the cycle?
 Engineering Sci Advisor HW Helper Thanks P: 7,172 It's probably not worth asking too many technical questions, since his home page describes the inventor as "Professor für e-Business Technologie", not physics or engineering.

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