fantasy charging system

[dbartram]

Someday I'd like to have solar panels; I'll have to move to a different house (not least so that I can have a driveway, for the charging component of this scenario).

I gather that solar goes well with a big battery for the house. But that's expensive, in part because the battery doesn't last forever.

So, here's an alternate proposal, one that requires a certain amount of land. It's partly inspired by the Niagara Falls hydroelectric system. Components:

1) solar panels, to power
2) a pump, connected to a
3) lower tank -- the pump raises water to
4) an upper tank, which (as the water descends back to the lower tank) powers
5) a turbine generator, used to supply electricity to the car and/or the house.

Of course some additional equipment is required -- e.g. regulators, and a means of keeping the water clean.

I don't suppose anyone here is already operating something like this?

 

[Tronguy]

Speaking in terms of What Could Possibly Go Wrong (yes, I'm an engineer, I always think like that):

1. Big systems get efficiencies that little systems can only dream about. So, a pipe that moves water up/moves water down will be a heck of a lot more efficient in terms of resistance to flow if said pipe is Really Big in diameter. But a 50' diameter pipe isn't going to work well in somebody's back 40, particularly. Water has viscosity, you know.
2. Speaking of that: A generator/turbine working with high water pressure and high flow is going to have an efficient time of extracting the potential energy from the water going from on high to down low. The bigger (more MW) the generator, I strongly suspect the more efficient it's going to be. Small guys.. maybe not so much.

So, I'm an electrical engineer, not a hydodynamicist, and so could easily be proven wrong on the above by somebody more knowledgeable. However, I guess I've heard of a half-dozen or so store-and-generate systems like this, and they've all been HUGE: Like, serious lakes and big turbines. I.. don't think I've heard of somebody generating, say, 100 kW from water flow, although I guess there were such things since the 1900's on to roughly the 1950's, at which point (guessing) it may be that amortized cost of the electricity for small water generators was 3X how much it would cost from getting the energy off the grid, so people plowed their money into bigger installations which would actually give a return on investment.

And, yeah, batteries don't last forever. Neither do ball bearings and grease for turbines and generators with moving parts, although it's a gimmie that the costs of maintaining a Niagra Falls power operation is probably less than the equivalent of a coal plant.

But I guess my point on this is that, now that the world is moving towards electrical storage in efficient batteries, the cost per kW-hr of said batteries is rather rapidly dropping, as is the capacity. As people on TMC know, there's a new battery technology announced on average once a month or so, but some of the bigger ones (molten sulfur, I think? Or maybe it's molten salts) are going into production, and they store huge amounts of energy and don't have obvious wear-out mechanisms.

For now I think the solar panel/powerwall (or equivalent from other vendors) is probably the cheapest way to go off-grid and, if one can stand the capital costs, cheaper than staying connected to one's local power company. As time moveth on and costs drop, it'll become even more likely that people will start putting these systems in every time they build a house, just like, well, people are doing today.

 

[Casss]

I'm not an engineer so this is a hypothetical question, not a suggestion...

But assuming you could get one working, would a thermal mass / 'sand battery' type energy storage solution be better for individual, consumer applications like this to reduce complexity and failure points?

Your water energy storage system would basically be a small-scale replica of industrial water energy storage systems, but replacing the national grid with your own solar. Which is 'proven', but I'm sure there is some critical math you could find out there that will tell you whether it's feasible or useful on a small scale.

 

[GeorgeSymonds]

If my A level maths serves me right...

4 metric ton of water at 3 meters has circa 120k joules of potential energy

And 120k joules of energy is approx 0.03kwh

So... if you were thinking of adding 30kwh to your car, you'd need to either raise the water to 3km high, or increase the amount of water to 4000 tonnes at 3m - I guess you could do a bit of both

Oh, and the above assumes 100% efficiency

 

[Casss]

Oh, and the above assumes 100% efficiency

Which you wouldn't get. 70-80% at most?

I'm guessing DIY'ing this would be the only non-insane way to do this from a financial standpoint.

At the risk of losing some efficiency, building something like the lynmouth cliff railway (but PV powered on the upward trip and capturing on the downward trip) using a really basic, easy to maintain (comparatively) combination of two tanks, on tracks and a cable system might be more realistic. It still won't beat a conventional battery though unless the scale is huge. Maybe having no pumps to look after and just a simple electric motor connected to your PV would be easier with off the shelf parts.

Edit: actually you'd still need pumps because the tank doing the upward trip on your energy capture run needs to be empty, unless you just do one track / tank as you don't really need two unless you need more to increase storage capacity.

 

[tsh2]

Batteries are pretty good for scaling. It seems like almost everything else is quite bad. Maybe there is a way of storing low quality heat, extending the thermal mass of the building into the subsoil (the traditional name I can't find now, and from Wikipedia it seems maybe not all that efficient anyway). Remember with a battery system, it is only the batteries which need replacing every ~10 years, the invertor should last longer.

 

[Mr Miserable]

There is a similar type of system in existence which is probably more efficient and has less potential points of failure.
It involves moving a weight up and down a disused mineshaft.
A kinetic battery?

 

[davidmc]

Wales has a similar system

Dinorwig Power Station - Wikipedia

en.wikipedia.org

 

[GeorgeSymonds]

I know the title is "fantasy charging system", but did anyone check my maths earlier? I recon you need to lift 10 metric ton 4 meters to get just 0.1kwh. That would make this type of thing pretty impractical for most

Dinorwig drops water 500 meters at the rate of 390 metric tons every second which is somewhat bigger than a domestic install.

 

[pgkevet]

A small scale system would be a sad waste of effort. You can look at micro-hydro websites for some idea about how little you'd generate. I have a small river and a stream crossing my land but early sums on either proven how pointless it'd be. I even have a hill field that rises over a hundred feet but the size of pond to give a useful supply..let alone the piping and turbine and power line back to the house give no sane payback. Good luck with reinforcing your house to fill the roof space with enough tankage to run a couple of lightbulbs....
Now generating hydrogen instead....

 

[arg]

I know the title is "fantasy charging system", but did anyone check my maths earlier?

It looked OK to me.

Another comparator to get a feel for these "lift something heavy" storage ideas is to imagine driving your Model S up a hill - how much energy that takes, and how much you get back from the regen. The car is a couple of tonnes, and you have to drive it up a really big hill to make a noticeable dent in the battery (and need an even bigger hill to get the "fuel gauge" to go backwards on the way down).

This clearly shows just how densely the energy is stored in the modest volume of battery under the floor, compared to these schemes involving pumping water or running wagons up tracks etc.