I always liked flywheels
Batteries and inverter systems suffer from losses: I2R (I Squared R or copper losses), Iron Losses depending upon the magnetic medium and harmonics losses due to the quality of the waveform.
They also are expensive to produce(I know enconomy of scale etc) and contain lots of nasty chemicals.
Flywheels are an old mechanical method of storing energy and coupled with a Motor/Genny power unit they could become very accessible ways of energy storage.
Some electronic energy conversion/inverter will be necessary as the Flywheel maybe spins 2000-30,000 rpm to get 50hz 230V AC and it make everything easier to do. (unless you want a commutated DC system for the house).
Another big strength is the discharge rate, they can spend all day charging up on solar cells or wind, water whatever you want to use a the prime source and then release a lot of the stored energy over a short time period.
I think its important when talking about energy storage to look very closely at energy usage.
If your using solar power, well while the suns shining your out at work.
The household demand is very low, the device stores at a trickle rate.
But when you come home you switch on the TV, kettle for a cup of tea, electric shower etc and the demand is very high but for a short period of time.
I remember a lecturer telling me that power station engineers used to use a copy of the radio times to plan for peak energy demand (to avoid transient instability).
If the world cup was on they would crank everthing up 5 mins before the commercial break as people would
put the kettle on, flush the toilet(this is a big often forgotten energy usage-pumping water), the UK power demand could increase by 30% for 5 minutes.
Batteries are really a portable solution, just look at electronics CMOS was developed for battery powered devices due to it's Non-Quiescent power usage.
The big driving force in electronics has been for portable devices but little has been invested in heavy power.
An interesting point is that if you look at the HV heavy current devices that came out of the collapse of russia, they had SF6 circuit breakers about the quarter of the size of rest of the world because our world concentrated on the sony walkman and was consumer driven whereas russia continued with a heavy current bias (build it big).
Incidently if you think Dyson is a genius, It was basically a russian design - just look at the timing !
Flywheels have the potential of being very accesible to a mass market and a low tech solution.
Maybe you could buy a "Flywheel Kit" and B&Q, dig a hole in your garage, drop in the specialist parts like the bearings and control box then just fill the Flywheel container with concrete, follow the balancing instruction (this might be the weak link) and hey presto cheap easy storage at 90% plus efficiency.
I can image the future where your every day Dave has a couple of wind turbines, a few solar cells and lightly buzzing Gyro Inc Flywheel storage unit humming away in his garage inspection pit as he flips steaks and sausages on his Green Electric barbecue.
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