Been thinking about energy storage, so here's something to kick around some.
These days we're getting ever better at harvesting the energy that's all around in the wind, sun, water and even tides (although more work needs doing in the latter) but they all have one massive problem, they don't always happen when we want to turn on the kettle for a cuppa.
So, what we need is a good way to store the spare power for when we want to use it and it's a still dry night and the tide is inconveniently not flowing for another 3 hours.
But I hear you cry we can do that, we put in a battery bank and an inverter. Sure, that works, especially small scale - and with modern LED lighting, for example, you can remove the inverter and use low voltage DC, thereby removing one of the losses. But batteries are expensive to make and can be tricky to keep working long term, and 'most all of them degrade over time, even with good charging systems.
Large scale is harder - and we need large scale, if it's gonna save the planet. So I got to thinking - we already have, in some places, pumped storage hydro - and that has a lot of merit; when there's surplus power at any time you pump the water uphill, then when you need power you can generate with it. However, it needs the right geography and takes a lot of space to do it on any scale, and when it's hot and dry your water evaporates, also the pumps, pipework and turbines and stuff are costly to install and need a fair amount of maintenance
So here's my idea, starting from the pumped storage, which I hope to build a small scale prototype of sometime. Rather than water, I propose to lift a mass. The mass can be anything that's handy and cheap - if it's in a desert, you'd use sand. If you have rocks, you can use rocks, if you can afford it, make 'em into concrete. When you have spare power, a motor winds the weight up, when you need power, you let the weight down, driving a generator. It's all simple and mechanical and thus cheap and easy to build and maintain. It's also readily scale-able - you can double the capacity by making the weight twice as big or lifting it twice as far or just by making 2 of them - and where the terrain is suitable it can go underground - in the basement of your house, say - so needn't be visually obtrusive. You can of course also do mechanical energy storage with a big flywheel - but that involves either crazy speeds or a lot of mass spinning or both, because you're using the speed to store the energy. It is more effective physically if you make the mass move faster, because energy is proportional to speed squared, but high speed has it's own issues. Gravitational potential energy though is given by mgh: so a 1 tonne weight (1000kg) lifted by only 1m has a potential energy of somewhere around 9.8KJ. (sorry, not going to make it into BTUs for you!) If you make it 5m high instead of 1, then you get 5x the capacity, and obviously, you can make the mass bigger (although making it too big will increase the cost for whatever holds it up.
It could be fully mechanical, with a cable hanging the weight; or hydraulic, with a ram underneath or maybe some other method I've not thought of yet. It's not going to be super-efficient, but then every system is inefficient somewhere and they key point is if it's cheap enough to build, and uses power which would otherwise be wasted, it doesn't matter if it's not so efficient.