They can't deliver the high power, high energy density, and fast recharge that modern electrical usage demands.
Batteries store energy but can't provide high power or fast recharge. You have to connect a lot of batteries to achieve high power.
Electrochemical capacitors can generate high power but have low energy storage density.
And electrostatic capacitors deliver high power and fast recharge, but like their electrochemical counterparts, suffer from low energy storage density.
High power, high density, fast recharge. You need all 3 -- but can only have 2.
Still, all is not lost. Nanotechnology comes to the rescue.
Researchers at the University of Maryland's NanoCenter have built electrostatic nanocapacitors out of billions of nanostructures, increasing the energy storage capacity of the capacitors by a factor of 10 over that of conventional devices.
According to the story in InTech, a publication of the International Society of Automation:
This advance brings electrostatic devices to a performance level competitive with electrochemical capacitors and introduces a new player into the field of candidates for next-generation electrical energy storage.These nanocapacitors could be mass-produced as energy storage panels layered one on top of the other. Multiple panels could stack together inside a car battery system or solar panel.
Long term, nanotech could give us new energy capture technology that would integrate with storage devices used in manufacturing.