Stupid question: why can't you store all the energy in warm water, eliminating the problematic (big, dangerous) pressure vessel altogether?
I envision a compressor, a heat exchanger (mist or otherwise) and an expander. The expander and compressor are connected mechanically (they can be the same device like in a piston engine or can be rotating machinery connected via shaft like in a turbine).
During storing, or water warming, the expander produces less power than the compressor so you need electrical energy to spin the system.
During energy release, or water cooling, the expander produces more power than the compressor needs and you can use a generator to extract that.
This is pretty much a standard heat pump or refrigerator arrangement.
I assume this is not efficient because of some not at first sight obvious quality of thermodynamics. It'd be cool to get a little bit insight into that.
By the way, your diagram's first picture with the piston is off: either the shaft should be thicker or it should depict sealing between piston edge and cylinder, now the volume of the pressure vessel changes very little when the piston moves.
The trouble with your arrangement is that if there ∆T is low, there must be very many cycles between mechanical and heat energy into order to store an equivalent unit of energy. This gives very many opportunities to lose efficiency.
There's a company out there, Isentropic Systems, that's trying this. A steep mountain to climb.
Stupid question: why can't you store all the energy in warm water, eliminating the problematic (big, dangerous) pressure vessel altogether?
I envision a compressor, a heat exchanger (mist or otherwise) and an expander. The expander and compressor are connected mechanically (they can be the same device like in a piston engine or can be rotating machinery connected via shaft like in a turbine).
During storing, or water warming, the expander produces less power than the compressor so you need electrical energy to spin the system.
During energy release, or water cooling, the expander produces more power than the compressor needs and you can use a generator to extract that.
This is pretty much a standard heat pump or refrigerator arrangement.
I assume this is not efficient because of some not at first sight obvious quality of thermodynamics. It'd be cool to get a little bit insight into that.
By the way, your diagram's first picture with the piston is off: either the shaft should be thicker or it should depict sealing between piston edge and cylinder, now the volume of the pressure vessel changes very little when the piston moves.