A tool for grid-scale storage that compresses air into an underground cavern with surplus electricity and recovers it through a turbine when needed. Adjust the cavern volume, pressures and efficiency to see the extractable air mass, ideal stored energy and deliverable electricity update in real time, using an isothermal estimate.
Parameters
Cavern volume V
m³
A sealed underground space — salt cavern, depleted gas field or rock mine
Maximum pressure P_max
bar
Cavern pressure at the end of charging
Minimum pressure P_min
bar
Cavern pressure at the end of discharging
Air temperature T
°C
Representative temperature of the cavern air (assumed isothermal)
Round-trip efficiency
%
Fraction of input power recovered. About 70% for adiabatic CAES
Results
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Extractable air mass (t)
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Ideal stored energy (MWh)
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Deliverable electricity (MWh)
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Energy density (kWh/m³)
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Pressure ratio P_max/P_min
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Efficiency loss (MWh)
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CAES plant — charge / discharge cycle
Surplus electricity drives a compressor that pumps air into the underground cavern (charge); when needed, high-pressure air spins a turbine to generate power (discharge). Watch the cavern pressure gauge and the air flow.
Extractable air mass m and ideal stored energy E. V: cavern volume, R: specific gas constant of air 287 J/(kg·K), T: absolute temperature, P_avg: average cavern pressure, P_atm: atmospheric pressure. This is an isothermal estimate; the electricity actually recovered is set by the round-trip efficiency.
Deliverable electricity E_out (η_RT: round-trip efficiency) and energy density per volume ρ_E. The energy density of compressed air storage is only slightly above that of pumped hydro, so a huge cavern volume is the key to large capacity.
What is the Compressed Air Energy Storage Simulator?
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"Compressed air energy storage" — does it really store electricity by compressing air? Can air hold that much power?
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It does — CAES for short. The electricity grid has a deep problem: supply and demand must match instant by instant. But wind and solar generate on their own schedule — driven by the wind and the sun, not the grid. So you need grid-scale storage. When electricity is cheap and abundant, CAES drives compressors that pump air up to tens of atmospheres into a huge underground cavern. The cavern itself becomes, in effect, a giant pressurised "battery".
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Do they dig the underground cavern on purpose? That sounds like a huge effort.
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Conveniently, most plants use a "solution-mined salt cavern". You inject water into a rock-salt layer to dissolve the salt out, and you get a large, very airtight cavity. Depleted gas fields or rock mines are sometimes used too. When the grid needs power, the high-pressure air is released, and on its way out it spins a turbine coupled to a generator. The strength is that, like pumped hydro, it gives enormous capacity and long discharge duration — without needing two reservoirs or a mountain. Try raising the "cavern volume" on the left; you'll see the stored energy climb sharply.
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You're right, more volume gives more energy. So where is the hard part?
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The central engineering difficulty is thermodynamic. Compressing air heats it up; expanding it cools it down sharply. Older "diesel" CAES plants simply threw the heat of compression away and burned natural gas to reheat the air before the turbine. That hurt both efficiency and emissions. So modern "adiabatic" CAES designs capture and store the heat of compression and give it back during expansion. That pushes the round-trip efficiency — the fraction of the electricity put in that is recovered — up toward 70%. The efficiency slider on the left is exactly that number.
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I see. So the higher the efficiency, the more electricity you can take back out.
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Exactly. This tool's calculation is an "isothermal estimate" — it assumes the cavern air stays at a constant temperature — but even so the key levers are clear. A bigger cavern and a higher pressure store more energy. Meanwhile, how much of that comes back as usable electricity is set by the round-trip efficiency. The gap between the ideal stored energy and the deliverable electricity is the "efficiency loss" — the part lost as heat. Play with the three levers — capacity, pressure and efficiency — and get a feel for them.
Frequently Asked Questions
Compressed air energy storage (CAES) banks energy by using surplus electricity to drive compressors that pump air, at pressures of tens of atmospheres, into a huge underground cavern — most often a solution-mined salt cavern. When the grid needs power, the high-pressure air is released and spins a turbine coupled to a generator. CAES offers the enormous capacity and long discharge duration of pumped hydro without needing two reservoirs or a mountain. This tool estimates the stored energy with an isothermal model.
First, the air mass that can be withdrawn as the cavern pressure falls from P_max to P_min is found as an ideal gas at fixed volume and constant temperature: m = (P_max − P_min)·V / (R·T), where R is the specific gas constant of air, 287 J/(kg·K), and T is the absolute temperature. The ideal stored energy is then estimated as the isothermal expansion work of that mass from the average cavern pressure down to atmospheric pressure: E ≈ m·R·T·ln(P_avg / P_atm). The deliverable electricity is this value multiplied by the round-trip efficiency.
Round-trip efficiency is the fraction of the electricity put in that is recovered as usable electricity. The central engineering difficulty of CAES is thermodynamic: compressing air heats it up, and expanding it cools it down sharply. Older diesel CAES plants threw the heat of compression away and burned natural gas to reheat the air, which hurt efficiency. Modern adiabatic CAES designs capture and store the heat of compression and give it back during expansion, pushing round-trip efficiency toward 70%. In this tool, that efficiency sets the electricity actually recovered.
On the electricity grid, supply and demand must match instant by instant, yet wind and solar generation come and go on their own schedule, not the grid's. Grid-scale energy storage bridges that gap, soaking up surplus power and giving it back when it is needed. CAES, alongside pumped hydro, is one of the few technologies that can do this at very large scale and for many hours, supporting the large-scale deployment of renewable energy.
Real-World Applications
Commercial CAES plants: The first plant to enter commercial operation was the Huntorf plant in Germany in 1978, which uses salt caverns and supplies about 290 MW. The McIntosh plant in Alabama, USA, commissioned in 1991, is a 110 MW-class plant; both are "diesel" types that throw away the heat of compression and reheat with natural gas. An isothermal estimate like this tool helps you grasp the scale by entering the cavern volume and pressure range of such existing plants.
Supporting large-scale renewable deployment: In grids with a high share of wind and solar, the core challenge is how to store the surplus power generated on sunny, windy days. CAES can secure a far longer discharge duration than lithium-ion batteries (hours to a dozen hours or more), and, unlike pumped hydro, does not require specific terrain — so wherever salt layers exist, large "intraday-shift" storage can be built. It is used to smooth the output fluctuation of wind farms and to shift surplus night-time power into the day.
Demonstration of adiabatic CAES (A-CAES): Adiabatic CAES stores the heat of compression in a thermal medium and returns it during expansion, pushing round-trip efficiency toward 70% without burning natural gas. Demonstrations toward commercialisation are advancing, including Europe's ADELE project and 100 MW-class salt-cavern A-CAES plants now operating in China. Set the efficiency near 70% in this tool to feel how the deliverable electricity grows for the adiabatic type.
Comparing storage technologies: When designing grid storage, engineers compare lithium-ion batteries, pumped hydro, CAES and flywheels by capacity, discharge duration, cost and siting constraints. The strength of CAES is "large capacity, long duration, low cost and long life", and this tool's energy density (kWh/m³) and deliverable electricity provide material for comparison with other technologies and for the early stage of plant concept design.
Common Misconceptions and Pitfalls
The first thing to keep in mind is that this tool is an "isothermal" estimate. In a real CAES plant, the air is heated strongly by compression and cooled sharply by expansion. The cavern temperature varies through the charge and discharge process, with near-adiabatic behaviour mixed in. The isothermal m·R·T·ln(P_avg/P_atm) is an educational estimate that shows the "order of magnitude and how the levers act" for the stored energy — it is not a design value for a real machine. Detailed design needs a thermodynamic cycle analysis that accounts for multi-stage compression, intercooling, heat storage and heat loss from the cavern.
Next, do not assume the round-trip efficiency is a fixed number. Efficiency varies strongly with the adiabatic efficiency of the compressor and turbine, the quality of the heat storage, the cavern heat loss and the operating conditions. A diesel type that throws away the heat of compression is around 40-54%, while the adiabatic type that stores and returns the heat only reaches about 70%. Moreover, the diesel type burns natural gas, so it involves fuel input and CO₂ emissions separate from the pure "electricity-to-electricity" efficiency. Comparing storage technologies on the efficiency figure alone overlooks this difference in assumptions.
Finally, a cavern cannot have its pressure raised without limit. The allowable cavern pressure is set by the mechanical strength of the rock or salt layer and the depth below the surface (overburden pressure). Raising the pressure too far causes cavern failure, creep deformation, or volume contraction from salt creep. Lowering the minimum pressure P_min too far also destabilises the cavern, so in practice the plant operates while keeping P_min reasonably high. In this tool, bringing P_min close to P_max drives the extractable air mass toward zero and the stored energy disappears, revealing the essential point that the pressure difference is what produces the stored amount.