Storing energy for rainy days

One of the problems with renewable energy is that for wind, wave and solar power the supply is not constant or predictable on a day to day basis.  Statistically we can figure out what we're likely to get from any one site based on observation over a period of time and extrapolating that into the future, but we can't predict accurately a long time in advance (i.e. more than a few days based on weather forecasts) how many kW will be available.

Unless we want to build huge amounts of extra capacity, then we need to consider the problem of storing energy for when we need it - in the case of solar power, literally putting something away for a rainy day.

This takes me back to my school days when we were learning about converting energy from one form to another.  For example converting electrical to chemical energy in batteries and converting between kinetic (movement) energy and potential energy by raising things up hills and letting them run down again.

What are the options?  More to the point, what are practical solutions?

Batteries are plainly useful - but to store the amounts of power we would need to cover for cloudy days with no wind will take a lot of them.  One of the solutions being touted is what is known as V2G - vehicle to grid.  The theory is that we hook all our electric cars up to the grid to charge - but in such a way that the grid can take electricity from all the plugged in cars when demand is high or supply is low.  That will require the kind of smart grid I talked about in a previous blog entry.  There are a few practical problems to overcome with this as well.  Batteries that are designed for powering cars may not be the best kind of battery for retrieving electricity from rapidly.  Then there's the problem of being able to rely on one's vehicle.  If you plug it in at night you expect it to be ready to drive in the morning - but if the grid has emptied the battery of all its charge then you won't be going anywhere quickly.

Another option is hydro electricity.  Hydro power stations already use off peak electricity that would otherwise be wasted to pump water back up the hill so's it can be used again.  There is obviously a net energy loss due to inefficiency, but it's still a pretty good solution.  In that sense our hydro power stations are already primarily storage units.  It is however very questionable as to whether we can have enough hydro power on tap to meet all our storage needs.

Given that we will likely be challenged to construct enough storage facility to cope with all the lulls and increases in demand in cold weather or when Scotland are about to win the world cup and everyone has their tv's on we need to look at alternatives.  For example, it is unlikely that the whole world will be becalmed at any one moment - so trading arrangements supported by HVDC lines for moving electricity between and around countries would seem to be a sensible way to go.  The only down side perhaps is that we would be at the mercy of another country in maintaining our electricity supply - but if you consider that we are already pretty much in that state for oil and gas then we'd be no worse off - and perhaps if we have enough renewables, in times of plenty the boot would be on the other foot.