1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Solar energy storage systems

  1. May 20, 2015 #1
    I have seen a lot of effort go into finding cheap, safe, high energy density phase change materials to store energy in the form of heat so power produced during the daytime may be delivered at night time. But I have an issue with storing energy in phase change materials.

    They rely on chemicals for the most part that are somewhat-cheap to not-cheap-at-all, and require some sort of storage tank or building and heat exchange system integrated throughout their volumes (except in liquids).

    For solar photovoltaics, using phase change is generally out of the question, leaving batteries as the only viable storage method.

    Overall, the amount of energy stored with phase change materials is on the order of a few hundred Kj/Kg meaning a huge amount of phase change material is needed for solar concentrator systems.

    But is phase change and batteries really the way to go?

    I can see a few simple systems that would reduce the cost of energy storage, though simplistic, is likely more efficient overall.

    In the case of photovoltaic systems, one could convert a portion or all of the produced power to kinetic energy and store it in a couple of simple ways. Of course scaling to the size of the power plant is required. One way is to use a pump to push water up a nearby hill to a pond on top, (or up into a tower with a tank on top,) and then release the water when power is desired through a secondary water power generating system such as a pelton wheel generator.

    In this case a water storage tank is more expensive to build probably than a pond at a higher elevation than the power plant, but still would likely be less expensive than a large tank and the costs of phase change materials or batteries to store the same amount of energy the tank could hold. The losses in the system would be the inefficiency of the electric powered pump to pump the water up, the friction with the walls of the pipe, evaporation losses if it is a pond that is not covered, friction with the pipes on the way back down to the generator and the final inefficiency of conversion to electricity again in the hydro-generator.

    A second way to accomplish this would be to lift a huge weight, using a motor and gear reduction drive system, that has an output to a generator driven by gearing. There are a lot of ways to do this, including a huge block of concrete with a wire attached being lifted up in a tower, to pulling an old derelict train car or train engine (with cars loaded with dirt if you needed more storage) up an incline on a short section of railroad. When power is needed you allow it to roll down the track and drive the generators (possibly directly attached to the wheels... driving the onboard diesel-electric generators of the engine itself perhaps?)

    In the case of a solar concentrator, you would change the motor (that either lifted the weight or that drove the pump that lifted the water) from electric to a steam powered one.

    That's about it. Relatively simple and no phase change materials required. Water for the storage medium (salt water is heavier) or mass of some kind.

    I am sure there are multiple variations on these themes, and it is likely someone has already done some of these things, but I haven't seen them being seriously considered or implemented, and since one of the downfalls of solar power is contribution of power at night, it seems that this would be an important problem to solve. And the simpler the solution, usually, the better the results.

    Are these methods efficient enough to be useful?

    Ultimately how well do they compare to the best phase change schemes?

    Are these methods a better way to go? (Now obviously building a man-made mountain on a perfectly flat plain so you can build a pond on top may not be cost effective where there are no mountains, so it might not be a perfect fit everywhere.)

    Will be interested in seeing the responses from an engineering point of view, because obviously there are a lot of ways to 'skin this cat' and a lot of factors involved.

    (NOTE:After reading some other threads, I found mention that water pumping is used by grid tied utilities and the efficiency runs on the order of 70 to 85%)
    Last edited: May 20, 2015
  2. jcsd
  3. May 20, 2015 #2


    User Avatar
    Gold Member

    You've obviously given this a lot of thought and your analysis seems cogent and certainly well presented. My only comment would be that there are a LOT of knowledgeable people out there thinking about this stuff a lot and if there are good solutions, I have to assume that they are at the very least being tried out if not already in at least small-scale use. I can't point you at anything specific and clearly you've done a lot of reading on this (certainly more than I have), but my point is simply that there is money is this and where there is money innovation follows for sure so if solutions are out there, they have likely already been thought through.
  4. May 20, 2015 #3
    At the bottom of this post is a picture of steam coming from one of my inexpensive yet high efficiency parabolic trough designs . So I AM one of those people thinking about and working in the area. One would think what you said is true, that it's probably all been thought of and tried. I am also the inventor of a very efficient gold sluice, a vortex drop riffle sluice known as a Gold Well. I read an article by a PhD regarding the physics of the Hungarian Riffle sluices some years back. In essence he made the claim (and substantiated it by a bunch of equations, studies, etc.) that a more efficient sluice than the Hungarian Riffle type was really not possible because the Hungarian Riffle type sluices were at THE theoretical maximum for recovery in a gravity-water-riffle type system.

    My sluices get a higher % of gold, and far smaller gold than Hungarian Riffle types. And the difference is not small, it is a big difference, so it's not a case of splitting hairs. So I guess it wasn't all tried before ...

    Regarding the solar power storage systems I was mentioning above, I saw another thread in which someone had a similar question and the response was 'well imagine loading and unloading several 1 cubic yard blocks per second of iron, for example, and how difficult that would be. There was no further comment, and it left one to believe that the only way solid mass can be used to store power is with some complex and impractical method of loading and offloading solid materials. The original question was why couldn't solid materials be used to store energy instead of water or some liquid. But one doesn't need to load or offload blocks, simply make a mechanism similar to the ones I mentioned in the first post. And I kind of like using an old derelict train ready for the train junkyard or something like that.

    At OMSI (Oregon Museum of Science and Industry) in Beaverton, OR, I recall having visited there when I was younger and they had a wonderful display there as to what 1000 Watts of electricity was. You could go up and crank a crank handle on this gizmo and a weight similar to a weight lifters weight was slowly elevated to 33 feet (I believe it was 33 feet) and then it tripped a lever and it came down very fast along the guide rails and the cable from it was wrapped into a pulley system which in turn turned a generator and lit up a whole bank of light bulbs and buzzers that amounted to 1KW total. It was a lot of work to get it up there but it was a fun way for people to get a real feel for what a kilowatt of energy was.

    And just as in my sluice design, which is in reality not that complex, but was overlooked all these years, perhaps some of the mechanical means of power storage of electricity should have a fresh look taken to see if some really easy and cheap method of storing energy has been missed.

    But here in this thread I was hoping to get a handle on the real world efficiencies of the processes, such as the real losses that would be experienced by the different methods. Example: Electric motor to drive the pump to pump the water uphill, best case, 87% efficient or 13% losses of input energy. Friction with pipe walls, best case if they were lined with teflon, 3% loss. Conversion of kinetic energy in a pelton wheel generator to electricity, 4% to 11% depending on size of generator. Etc.

    Although I know that there are losses at every stage, I do not have a handle on where the largest losses are, or what scale of losses might be encountered. And if I knew that I might find some way of getting around the larger losses in the schemes, and push up the efficiency of storing energy in this way.

    For example, in a parabolic trough concentrator, a high temperature is developed, parabolic dishes even higher. In the old days the furnace and boiler was in the basement, and steam went up a pipe (heat rises obviously) through a cast iron radiator which dissipated heat into the room, and then the liquid water returned by the same pipe as liquid, obviously taking up far less space after condensing, to the boiler to be heated up again to steam.

    Now if this method were employed on a huge scale (for large power plant that puts out megawatts you may need a 20 foot diameter pipe) the steam would rise to a condenser at the top of a nearby hillside, air moving through would remove most of the heat and the steam would condense to liquid and drop into the pond above, bypassing the need for a pump. Of course the heat losses would be huge in this system, so it's efficiency wouldn't be optimum. However if the pipe were surrounded by another pipe that held a vacuum, there would be very little heat losses and efficiency would greatly improve. Further a partial vacuum at top in the heat exchanger, maintained by condensation of the steam to liquid there, would make the mass flow rate even better.

    Is it easy to make a 20 foot diameter pipe surrounded by another even larger pipe (wouldn't have to be a lot larger, just enough to make a space for a vacuum between the two) that contains a vacuum? NO. Is it cheap enough? Unknown I think, as it all depends how it is done and from what materials and what the return is at the end of the day. But current parabolic trough systems do NOT have a vacuum between the solar absorber pipe and the glass envelope because there is no easy way to maintain a vacuum between them. But I accomplished just that, and cheaply, field serviceable as well in the design I have come up with. (Picture attached to this post.)

    So I guess what I am really looking for is to identify the major efficiency issues in different mass lifting schemes that could be workable and see if they can be fine tuned with a little creative thinking to be less lossy rather than chasing the phase change materials. After all, if the issues with this method can be solved, the reward would be solar energy having a much more practical role in overall power production, namely able to supply power at night, giving a way to get away from fossil fuel and nuclear power plants.

    Attached Files:

    Last edited: May 20, 2015
  5. May 20, 2015 #4


    User Avatar
    Gold Member

    Just to mention, the method of lifting water is commonly used at hydroelectric dams, which are a good source of electricity storage (with decent efficiency, something like 70 % ). But with a tank in lieu of a reservoir, the capacity is limited : if I believe google, a 1000 cubic meter tank at 100 meter elevation stores a paltry 0.3 MWh. Whether this can be useful / better than another in a specific application, I don't know - many different solutions are used for different use cases.
  6. May 20, 2015 #5
    A tank might be useful for a home or commercial building running on solar to store energy instead of using batteries. And .3MWh would be awesome to store here at my place in the desert in Arizona that I run entirely off solar (it's off grid far from power lines.) Actually I had toyed with the idea of pouring a 100 ton block of concrete and rock with a couple huge eye bolts in and a framework tower to lift it up. You see my batteries are fully charged by 11AM even with high power usage in the house. And it bothers me that I can't store the rest of the power being created by the system and put it to good use. Since I don't live near power lines, I really can't sell it on the grid. But a lot of people could sell it on the grid, and if they could sell it at night their solar power becomes much more useful.

    More batteries are expensive and their life is limited. Your .3MWh of power stored in 12V batteries would require about 250 100Ah batteries, or somewhere around $20,000 in batteries, which will last a few years before needing replacement. Of course you can get bigger batteries that last longer but that is even more expensive.

    And once they are full I'm back where I began. But the same is true of the lifting system, except I think per KWh it would be far cheaper than batteries, the life span is a lot longer, and it wouldn't go "dead" sitting around for long periods of time.

    But, once I have the block at the top of the tower, it's full too, just like the batteries, except it would be storing a lot more energy. The amount of power stored by the weight lifted up in a tower (whether water pumped up to a tank there or a weight suspended by cable and lifted up) is huge compared to what I could store in batteries. Just can't figure out what to do with all the extra power ... maybe a Rube Goldberg Power Wasting Machine is the ticket to waste it off in a creative way!

    I couldn't imagine building a tower for a 1GigaWatt power plant!
    For a GigaWatt power plant one would probably prefer to build their power plant at the bottom of the Grand Canyon and put the reservoir on top nearby, or use a train storage idea, track on a good steep incline. Need more storage? Add train cars full of rocks or dead batteries for even more mass!
    Last edited: May 20, 2015
  7. May 20, 2015 #6


    User Avatar
    Gold Member

    Glad to see someone as energetic and inventive as you working on this. Hope you come up w/ good stuff. Anything that increases the use of environmentally friendly power is likely to be a good thing.
  8. May 20, 2015 #7


    User Avatar
    Science Advisor

    If you want to store 0.3 MWh by lifting 100 ton the structure would need to be between 1 & 1.2km tall... (dependant on what type of ton you use)


    The investment required for such a project boggles the mind (billions), It makes $20k for batteries look like an absolute bargain.
  9. May 20, 2015 #8
    kiev tower.jpg I doubt that this tower cost billions. It is a television station tower I saw while I lived in Kiev, Ukraine for a few months, and I would venture a guess that it is strong enough to lift much more than 100 tons, (say 1000 tons) given the right modifications (and cables and pulleys). That would yield over 3.4 billion Joules (around 940,000 watt hours or nearly 1000KWH) It stands over 1263 Feet tall and cost nowhere near even $100 million let alone billions. There is a 3 story office building built into it. So I think that a stripped down no-frills tower 1km tall if that's what was wanted, capable of supporting a few million pounds is doable for a few million to tens of millions of dollars.

    But I am not advocating this in particular, just as an example. The pond on top a hill or mountain is in my opinion a much better and cheaper way to go.
  10. May 20, 2015 #9


    User Avatar

    Staff: Mentor

    One of the first things I learned in undergrad is that engineers do not build in overdesign for no good reason. So that tower is not designed for the extra loading that you are proposing. You should learn more about structural loading and safety factors to approximate what kind of structure would be required for the gravitational PE storage that you want.

    One hint -- use a mountain-type structure to help with the strength of the structure you design. Even if you don't use water for the mass of the energy storage, the extra strength of a mountainside will help to lower the cost of a mass-lifting energy storage system.
  11. May 20, 2015 #10


    User Avatar
    Science Advisor

    The tallest ever man made structure for a few million dollars?
    Even if it was possible, it's still orders of magnitude more expensive than batteries (or flywheels etc).

    Wiki has a good overview

    Two wiki sources that have non-pumped gravity storage products:

    Using weighted trains!

    Using bucket chains and gravel!
    And their prototype:
  12. May 21, 2015 #11


    User Avatar
    Gold Member

    Your place must have amazing night sky : )

    Just to be sure, my example 0.3MWh was with 1000 ton (a cubic tank 10 m a side) not 100. In essence a water tower plus the energy storage / recovery system. I suspect the cost of such a water tower might be closer to $ 1M or more than $ 20K though.
    Last edited: May 21, 2015
  13. May 21, 2015 #12
    Well, I think you are all picking apart my lifting weight plan as if I really had some inclination to do it (which I don't.) and pointing out the flaws in its design. Since I never actually put much effort into it past the concept, it's bound to have issues or be unrealistic. I would prefer to know what kind of losses would be in such a system. Such as losses through drive trains to lift the weight (provided a suitably low cost and strong tower was feasible) and what kind of motor or engine would give the highest efficiency for the design. Likewise with the pumping of water up a hill to a pond, the losses of energy due to friction of the water with the walls of a pipe or the losses in the pumping system.

    There are a lot of mines around here. One mine in particular, Vulture Mine, just a few miles from here, has a vertical shaft 3200 feet deep (actually runs at 45 degrees and has rails like a train track.) In mining they make a large lift capacity headframe that is in essence a crane positioned over the shaft to lift ore out. So there is the 'tower' built out of a mountainside. No cost of framework to support the crane on top needed, and a good 1 km in drop. The headframe could be made from huge bridge steel beams positioned over the opening, and the weight could be on ore cars riding up and down on the track. I think the ship building cranes have huge lift capacity, 1000 tons being well within their capabilities.

    Again I am not planning on the tower, or even a shaft type system. They were just thrown out as scenarios of alternative ways to use mass to store energy other than using water. I like the bucket line and gravel one. I had considered a conveyor. Same thing really.

    Maybe I should ask direct questions like:

    Using a pump to lift water a total of 1000 feet in elevation, what is the maximum efficiency I can expect (best case) from the pump (how much energy will be lost in the pump, not including the drive motor for it) So what would be the most efficient pumping system that I could use for that purpose and how much energy could I expect to lose by pumping water that height.

    Using a pulley system to lift a solid weight, what is the maximum efficiency I can expect. How much energy would be lost in the friction of the bearings, etc. Of course there are a lot of kinds of pulley systems, and a lot depends on how many bearings and pulleys. Or drive gears, or chains, etc. Just want to kind of home in on a range of realistic best case efficiencies here. This not including the drive motor efficiency either.
  14. May 21, 2015 #13


    User Avatar
    Gold Member

    With a pond and 1000 ft elevation, your setup is starting to look more like a that of a hydroelectric dam, and they reportedly achieve 70-80 % round-trip efficiency - and a pipe and reversible turbine must be less expensive to build than a tower : )
    Have you had a look at the pumped-storage article from wikipedia (linked above) and references therein (including some about using this technology for solar power storage) ? Given that this system is widely used (albeit at bigger scales) there must be quite a few resources on the engineering issues faced.
    Looks like an interesting project, I'd love to see the pictures if you go that route.
  15. May 21, 2015 #14
    Well my thought was the water being pumped to a pond would be about the most effective solution for costs and efficiency too, but I don't have the actual data to prove it. If price is set aside, which is in fact more efficient? Pumping water up a (nearby) mountain to a lake there or dead lifting a huge weight (regardless of cost of the tower to lift it up with.)

    If I take the information you quote (and I have seen other places) where the overall efficiency runs somewhere between 70-85%, that means best case is around 15% loss of input energy. I would assume that part of that is lost in the motor running the pump and the pump friction in the bearings as well as the pipe friction and turbulence.

    Steam moving through a pipe has a lot more turbulence and friction, but if a parabolic trough or dish concentrator creates steam power as it's output, then steam moving up a pipe to a condenser at top with a partial vacuum there might or might not be more efficient than an electric motor and pump, given that a higher loss can be tolerated there because there is no additional losses by motor and pump. But will it come out to be less than 15%?

    I saw once calculations of a coffee maker where the input energy wasn't really sufficient to lift the water from the lower reservoir, heat it the given temperature rise needed and lift it to the top to trickle through your morning coffee into the pot (there's no pump in a coffee maker). The reason I saw explained that it could work was due to small bubbles moving through the small tubing pushing water ahead of it as the bubble rose.

    So if the ultimate efficiency of just using a lower pressure at top of the system and a condenser moving steam to the top of the mountain turns out to be too inefficient, perhaps the coffemaker principle (a lot of smaller pipes with bubbles helping to lift the water) could be employed. Again, that may or may not work as there may be a limit to how far bubbles could push a liquid in front of it (I have a feeling it would be a function of the weight of the water column above it.)

    Another interesting thing I have seen regarding lifting water in unusual ways, is the capillary action of trees, able to lift water to the top of a huge redwood, hundreds of feet tall. Again maybe not practical for huge volumes and probably very very expensive if a scheme could be developed, but nonetheless very interesting. I have cut tall trees and seen the water almost explode out of the base when the centers snapped when cut.
  16. May 23, 2015 #15
    Well now considering the mine near here, with the 3,000 foot + shaft, the cost of a tower is avoided. Using a bucket line dropping water down to the bottom and using a pump to bring it back up might not be so expensive. Plus it could be done small scale. They also have several truly vertical shafts (the 3200 foot shaft is at 45 degrees) that are six hundred or so feet deep. Even if the main shaft wasn't usable, the shallower shafts would work well for a small scale 'proof of concept' and test setup.

    I could see a motor/generator as being an option. I had a friend that was working with pushing a motor out of phase to generate power. Will have to check what kind of motor it was. He had a pelton wheel on the shaft and just tied the motor to the power grid at his meter.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook