6 ms·

I always wondered why compressed air storage systems work against ambient pressure instead of having two tanks at high and higher pressures. This would greatly

I always wondered why compressed air storage systems work against ambient pressure instead of having two tanks at high and higher pressures. This would greatly increase the density of the gas, as well as lowering the temperature differential. It would take a long time to get it up to initial pressure, as there would be a lot of heat to dissipate, but then it differential mode, the gradient would be much better.

ssl-3 3y ago
Suppose we are driving a turbine. Does having an increase in the density of gas present an advantage over having a higher pressure delta by dumping to ambient for a given volume of compressed-gas storage? Why would I want one tank at "high" pressure, and another tank at "higher" pressure, when I could just have one tank of "higher" pressure to begin with? Or, better: Two tanks of "higher" pressure in even less space than one of "high" and one of "higher" pressure? (If the answer is "Because turbines work better with higher densities," then: Do they work more-betterer-enough to make up for the size and complexity?)
bluGill 3y ago
Because with the tanks are not infinite size. That means as you release pressure the differential between the two tanks equalizes in the middle. Mean while in the current system the low pressure vessel is effectively infinite and so you have more usable volume to work with. Plus of course you can use both vessels to store energy instead of one.
amluto 3y ago
Several reasons. The first is fundamental: density is not really helpful for this sort of application. The work done in expanding material (including gas) at a given pressure is P dV, and the work done in moving material across a pressure difference is V dP. Notably, mass doesn’t appear at all here, so adding more mass or density doesn’t add energy storage capacity in and of itself. You can compute this more explicitly, and, for an ideal gas, the useful energy extractable from a tank of gas at pressure P (under ideal, isothermal conditions) is proportional to the change in log P. So it’s actually rather more important to achieve low pressure than high pressure. On top of this, there’s a practical consideration: the atmosphere is an effectively infinite source of gas at 1 atm. If you are working between high and higher pressure, you need two reservoirs. All you’re gaining is less temperature change per unit pressure change, but it’s probably the same amount of heat for any practical purpose, so you still need an intercooler of some sort for good efficiency.
pjc50 3y ago
I would have thought this is like the Carnot cycle: the greater the difference across which you're generating energy, the better. So you want the low side at as low a pressure as possible.

ssl-3 3y ago

Suppose we are driving a turbine. Does having an increase in the density of gas present an advantage over having a higher pressure delta by dumping to ambient for a given volume of compressed-gas storage? Why would I want one tank at "high" pressure, and another tank at "higher" pressure, when I could just have one tank of "higher" pressure to begin with? Or, better: Two tanks of "higher" pressure in even less space than one of "high" and one of "higher" pressure? (If the answer is "Because turbines work better with higher densities," then: Do they work more-betterer-enough to make up for the size and complexity?)

bluGill 3y ago

Because with the tanks are not infinite size. That means as you release pressure the differential between the two tanks equalizes in the middle. Mean while in the current system the low pressure vessel is effectively infinite and so you have more usable volume to work with. Plus of course you can use both vessels to store energy instead of one.

amluto 3y ago

Several reasons. The first is fundamental: density is not really helpful for this sort of application. The work done in expanding material (including gas) at a given pressure is P dV, and the work done in moving material across a pressure difference is V dP. Notably, mass doesn’t appear at all here, so adding more mass or density doesn’t add energy storage capacity in and of itself. You can compute this more explicitly, and, for an ideal gas, the useful energy extractable from a tank of gas at pressure P (under ideal, isothermal conditions) is proportional to the change in log P. So it’s actually rather more important to achieve low pressure than high pressure. On top of this, there’s a practical consideration: the atmosphere is an effectively infinite source of gas at 1 atm. If you are working between high and higher pressure, you need two reservoirs. All you’re gaining is less temperature change per unit pressure change, but it’s probably the same amount of heat for any practical purpose, so you still need an intercooler of some sort for good efficiency.

pjc50 3y ago

I would have thought this is like the Carnot cycle: the greater the difference across which you're generating energy, the better. So you want the low side at as low a pressure as possible.