Battery vs. Water Tank

I was discussing solar panels and renewable energy a few weeks ago. One of the new alternatives that came up was the super-heating of water using mirrors.  There is some REALLY cool stuff being built. Crazy stuff. I wish I had a $40M grant to go tinker...
Solar Tower System by Abengoa Solar located in Spanish province of Sevilla

So freakin cool. Basically you turn all the mirrors toward the tower to heat up water to 250C at 40bar (580 psi)!

After using a solar panel and a deep cycle marine 12V battery system at Burning Man, I wondered what it would take to replace my sealed lead acid battery with a tank of water. If I could heat the water and use it as my energy storage device, could I get rid of the need for a car battery? Let's assume I've got some solar apparatus that can heat the water - the thing I want to know is the size of the vessel of water it would take to replace the battery. If the required size of the vessel is equal to an Olympic-sized swimming pool, then the system is not feasible.
Tanks holding pressurized super heated water

Super heating water is a complex thing that requires pressurized vessels. Because I'm trying to replace a system used at Burning Man (think simple and portable), I don't want to heat the water past ~120F. At 120F, if water spills, it'll only discomfort a human and not physically burn or harm them.

Get the calculation excel file here. It would take roughly 9 US gallons of water, heated from room temp (25C) to 50C to equal the power behind a very heavy, filled with lead (Pb) and sulfuric acid, expensive, car battery. If my calculation is correct, 9 gallons is not so bad! That's like two water coolers (5 gallon carboys). Neat. So now I just need a way to heat the water - maybe with some mirrors and a control system, some motors, probably some batteries...

Wait - once I have 50C water, what do I do with it? Here's the problem - how does one convert thermal energy efficiently to mechanical energy, and then to electrical energy? The great thing about a 12V battery is that you can immediately 'hook-up' devices that do work for you. With hot water, I don't how to do anything but take a nice shower. How would you convert hot water to usable work?

Comments 32 comments

  • I think everyone missed the little detail of where this experiment is taking place ? its Europe people ? nothing has to make sense or provide return on investment there. Its socialism ? you get a grant and you might as well spend it on strippers. I totally agree ? the mirror farm looks hella cool but there is no way it will ever produce even the slightest return of that 40 mil, not to mention the time value of money and the expected life cycle of the project.
    This stuff isn?t trivial and conventional academia BS will not do ? otherwise we would have had something viable by now. IMHO I am far more inclined to look into traditional solar panel systems. As that technology matures the cost of the panels will drop and combined with the rebates offered by most utilities the whole package might become viable for the average Joe.

    • Solar thermal systems have two big advantages over photovoltaics. First, they have a significantly better EROEI (Energy Return on Energy Invested) than PV systems – better in the best demonstrated cases than US oil production during the 1970s. Second, they don’t require rare elements (such as indium) to reach best efficiency as high-efficiency PV systems do. It’s all aluminum, concrete, and steel, which has major implications for future scale-up.
      Put your ideology back in your pants and pay attention.

    • By Traditional of course you mean the Solar systems which have produced the most electrical power to date?
      That would be solar trough systems which similarly concentrate light to boil water, create steam, and turn turbines.

  • 1 gallon of water weighs around 8.3 pounds,
    so ignoring the container, insulation,
    power conversion equipment, etc you are
    lugging around over 75 pounds of
    weight just for the water.

  • You might want to look into using thermoacoustic stirling cycle (or more precisely a lamina flow stirling engine) - Wikipedia - Thermoacoustic Stirling Engine
    See one in action:
    Video on Youtube
    You can convert solar thermal energy into rotary motion, which can be hooked to a generator. maybe.

  • More on Solar in the Southwest deserts in the US.
    I remmber driving past Solar One near Barstow …. you could see that thing glowing from miles away.

  • Thermodynamics suck. the reality is while you can extract energy from the water, with such a low difference in temperature you wont be able to extract much of the energy. all system have inefficiencies and in general the the higher the delta T the more efficient the system is, but at best you looking at system efficiencies of 60%. as far as directly extracting the energy in the water the only direct approach without going into extremely exotic materials is thermocouple but the efficient with thermocouples is very low 6%@1000C they’re not practical. a more efficient approach would be a sterling engine based system but again they’re not very efficient at low temperature.

  • In the 70’s I did some work with Barber-Coleman for remote solar powered well pumps. They had a small amonia or freon turbine. The idea was to use a flat plate tracking solar water heater (which we already made) as the hot side and the well water as the cold side to recondense the fluid.
    Something like that could work for you. You will need a heat dump. A radiator pointed at the night sky with a reflector to block it from the ground does pretty good. It is looking at about 3 degrees K if there was no air. Otherwise a big bag of water kept in the shade but also exposed to the open sky where the sun ain’t.
    The other ideas about pumping uphill are probably the easiest. Maybe your storage tank could be built into the top of Burning Man. It could power a massive array of white LEDs before it burns, like a solar powered Las Vegas Burning Man!

  • I am building a system that runs off of compressed air, rather than water. This water system looks interesting, though!

  • FYI, I sourced a huge fresnel lens from a discarded rear-projection TV.

  • The problem with heat energy is that it isn’t very useful. Calculations have been done to show that the upper efficiency limit for converting it to mechanical or electrical energy (a less entropic, and thus more useful form) is something like 30%. And you get less efficiency the lower the temperature of your stored heat is. That’s why you’d be better off superheating your water far beyond boiling inside a pressurized container.
    Of course, if all you wanted to do with your hot water was make coffee, warm a bowl of soup, or take a shower, then there is no efficiency loss. Indeed, the most efficient way to collect solar energy intended for use as heat is to collect it directly as heat, as opposed to turning it into, say, electricity. For this reason, hydronic solar panels are much more adept at collecting solar energy that PV panels, although there’s no sense in using the energy they collect for anything other than household heating needs.

  • Okay, nice idea. One thing you could do is let the steam spin a Tesla turbine ( Use most of the electricity for your fun but then have a little to convert H20 to H via electrolysis. That way you can store some energy for later use.
    Or just let it run and convert all the energy to H. That is much more portable and will last.

  • What you need is carnot knowledge.

  • Hmmm, having been to burning man many times I have often contemplated creative power options. Along with a way to keep food and beer cold.
    One of the previous posts mentioned using refrigerant to create electricity. You could at least build a solar powered cooler. Refrigeration systems all contain a compressor that compresses refrigerant so that it can be decompressed for cooling. De-humidifiers work the same way.
    If you could create enough pressure via heat via focused sunlight you would have no need of ice. One option for focusing sunlight would be to use a large fresnel lens. They sell one at American Science and Surplus, that purportedly melts pennies in seconds and has a 42" focal length.
    I’ve considered buying one to encourage my gray water to evaporate and use the resulting steam to spin a turbine :)

    • FYI, for experimentation: I sourced a huge fresnel lens from a discarded rear-projection TV.

  • Can you use a phase-change material instead of water?
    I don’t know if they make phase-change materials that work in the 120 degree range. Although I suppose you could use wax. According to this:
    Bee’s wax melts (phase-change) at 45C/113F, paraffin between 47C/116F & 65C/149F and carnauba between 78C/172F & 85C/185F.

  • Occam: _
    More on Solar in the Southwest deserts in the US.
    I remember driving past Solar One near Barstow …. you could see that thing glowing from miles away.

  • Its a shame you had to eliminate the pressure requirement. turning pressure into work is a piece of cake.
    does this thing have to be portable? or transportable?
    If you are willing to carry around 75lbs of liquid, would lemon juice work instead of water? ;)

  • bau, Solar energy systems like this are under construction at various places across western USA, at a cost of hundreds of millions of dollars.
    See here, for example :

  • Well I think you have two seperate things going on here.
    First the solar water heater, second the storage of the heated water.
    as far as conversion is concerned, the less steps the better, peltier may be the way to go.
    The storage of water should happen in a well insulated container. you could go high tech and use co2 filled glass foam.
    Heating could be performed in small glass tubes, 8 x 8 array of glass tubes fed with water.
    after all this, cost, technology, efficiency and effort. Battery is looking pretty good.

  • 120F is not useful since the air temperature there is often 100F. Why not focus the sunlight on a very small area, trickle water in and convert that to steam in amounts just enough to run a turbine. If you route the exhaust of your turbine through a radiator, you even have a closed system that doesn’t require a lot of water. Solar energy in, mechanical energy out. Then add a generator. You’d still need a battery to store energy, unless you compressed an air tank.

  • Very cool idea, but as others have said there is a lot of energy wasted converting from thermal to mechanical then to electrical. However, I’m thinking a pickup truck with a water tank, some mirrors and a few small hydro-electric turbines might not sound so crazy compared to recent gas prices!!

  • You’ll also have problems with heat leaking out. Since the volume-surface area isn’t as great with a smaller tank, you’d have to seriously consider insulation for the tanks.
    And of course the thermodynamics are nasty, as someone else pointed out. More delta t!
    Unless I’m mistaken, the majority of solar farms these days actually use this mirror/water tank setup, not PV panels. In fact, I think PV solar panels areused mostly for smaller setups. The water tank system might actually more (cost?) efficient at larger scales than PV panels.
    And traditional solar panels are not going to go down in price–if you look at the trends in industry and cutting edge research, “traditional” (now that I think of it, I think the superheated water technology came before PV panels) silicon PV panels have basically hit a lower bound in terms of production cost. All that’s helping right now are some tax rebates here and there, but that doesn’t actually make solar cheaper. Most of the new efficient solar technologies are mostly dye-based panels.

  • A pound of water weighs 8.33 pounds. 9 gallons of water would weight about 75 pounds.
    How much does the battery weigh?
    The solar cell can charge the battery for use at night.
    How fast would the water cool at night making the device useless? I think this device would be pretty useless in cold climates or climates that get cold at night.

  • This months issue of Popular Mechanics has an article about a new solar installation that uses mirrors in a parabolic configuration aimed at a Stirling engine.
    One dish/engine can provide enough power for up to 12 households. You could try to build something similar. You’ll be in the desert with lots of sun power.

  • If you can come up with a temperature differential, then 50C will probably work just fine.
    Chena Hot Springs in Alaska has done some cutting edge work on low temperature geothermal power. The idea is if you can’t boil water, why not boil R-134a instead?
    Chena Hot Springs spins a generic air conditioner backwards (a Carrier infact!), which turns a generator with some very slight modifications. A 165F hot spring well and 45F cold water well generates 400kW.
    Its been known for years, when air conditioners are shut down, the units spin backwards “by themselves”. It took years for someone to put two and two together here.
    I have always wondered if a solar hot water system, generic air automotive air conditioner (and plumbing), alternator, and buried copper tubing to chill the water, etc. would be enough to generate a usable amount of energy. This would probably be your best starting point.

    • A “small” correction here for caseyh: Compressors do not spin backwards during an off cycle in a refrigeration system, either residential Heat Pump or Air Conditioner or a screw chiller. During the first second or two after shutdown, it may spin backwards as it unloads the pressure on it (this is only true for scroll compressors, not reciprocating ones), however, it does not continue to spin while the rest of the system equalizes. Equalization happens as the refrigerant expands through a metering device.
      I like the concept, though. It is a resourceful use of already existing technologies. I would love to see more of this type of innovation.

  • I don’t think you’re asking the right questions. The battery can hold the energy for a long period of time, but the water will cool off. It’s not an efficient long term storage given that thermal bleed off is much higher than electrical bleed off. With only a small temp difference and low temps, you’re also only looking at extremely low Carnot efficiencies, only 7.7% in this example. Given a stirling engine, you’re talking about 3.1% Carnot efficiency. Expect to only get out 3% of the power you put into the water in other words, or 2700 gallons.
    You might have better luck with a pump and waterwheel/turbine setup and use potential energy. At least then you only have to worry about leaks and evaporation. If you seal the upper reservoir the pressure build up from daytime heating can then be used to boost the generator energy instead. I’m not saying this is going to be all that efficient either. It also depends on having good valve sealing and a high pressure pump!

  • Since you’re worried about spilling, You could use concrete rather than water, it won’t spill. Then you have to bring the temperature up a good deal. Above ~600F, you’ll need molybdenum or other rare metal for piping - so you’ll want to keep it under that.
    Run some pipes through the concrete. You can extract power with a steam-cycle engine.
    But you already realize this isn’t a good fit for burning man (except in name alone).

  • Sounds like it would be the perfect thing to hook up to a Stirling Engine. See if you can get enough power out of it to run a small project…

    Use a series of Peltiers. Convert from heat directly to energy. (Not the most efficient system in the world. but portable, solid state, and safer then dealing with steam.)

  • Would it be cheating to heat up a smaller container of water (3 gallons?) to 100C and then use the steam to rotate a turbine?

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