Compressed-Air Energy Storage: Salt Cavern, Intercooled Compression and Heat Store (2D Schematic)

Presets

Live readouts

Graphs

Cavern pressure against time

Cavern gas and heat-store temperature against time

Electric power (+ charging, - generating)

Round-trip efficiency of a full cycle against the number of stages

Numerical checks

running...

How to use

Pick a preset and press play. The cavern fills from the minimum to the maximum pressure (charge), waits (hold) and is emptied again (discharge); the round trip of every completed cycle is read at the bottom of the readout. Try: (1) Ideal machines and note the round trip is 100 percent - then switch off one thing at a time; (2) lower the exchanger effectiveness and watch heat leak into the cavern; (3) raise the heat-store decay and lengthen the hold; (4) change the number of stages and compare with the sweep graph; (5) switch on the isothermal cavern. Space pauses, R resets, hover a part for its numbers.

The science

What is stored. In adiabatic compressed-air energy storage (A-CAES) surplus electricity runs a multi-stage compressor. The air leaves each stage hot; instead of throwing that heat away, an intercooler moves it into a thermal store. The cooled air goes into an underground salt cavern. When power is needed the air is released, reheated from the store and expanded through turbines.

Compression stage. For an ideal gas with k = (γ-1)/γ = 0.2857 and a total pressure ratio π = p_cavern/p₀ split equally over N stages, r = π^1/N:

T_out = T_in [ 1 + (r^k - 1) / η_c ], w = c_p (T_out - T_in)

The intercooler has effectiveness ε: T_cooled = T_a + (1 - ε)(T_out - T_a). Heat c_p(T_out - T_cooled) per kg is stored. The electric power is P = ṁ Σw / η_m.

Cavern. A rigid cavern of volume V holds mass m at temperature T. The first law for an open system with inflow enthalpy c_pT_in and heat flow hA(T - T_wall) to the rock is

m c_v dT/dt = ṁ (c_p T_in - c_v T) - hA (T - T_w) (charge), m c_v dT/dt = - ṁ R T - hA (T - T_w) (discharge), p = m R T / V

The pressure therefore slides between p_min and p_max and the machines work at the instantaneous ratio. Charging heats the gas, discharging cools it, which is why the wall heat transfer matters.

Discharge. Air from the cavern is reheated to T_in = max(T, T_a + ε(T_store - T_a)) before each turbine stage and expands to T_out = T_in[1 - η_t(1 - r^-k)], giving w = c_p(T_in - T_out). The store is stratified by cavern pressure: the heat taken from stage j at a given pressure is returned at the same pressure, as in a real packed-bed or oil store. It also decays toward ambient with time constant τ.

Round-trip efficiency is η_RT = E_out/E_in over a charge-hold-discharge cycle. In the reversible limit (η = 1, ε = 1, no store loss, isothermal cavern) every stage is run backwards, so η_RT = 1. Everything below that is lost to machine irreversibility (η_c, η_t, η_m), to heat that the exchangers fail to capture (the (1 - ε) share dissipates in the cavern and the rock) and to store decay.

Checks (run in the page). (1) the isentropic compression work from the integral of v dp against c_pT_a(π^k - 1); (2) the cavern first law over a charge: the change of internal energy against the integrated enthalpy inflow minus wall heat; (3) the reversible limit giving η_RT = 1; (4) p = mRT/V after a full cycle.

Simplifications. Air is a calorically perfect ideal gas (c_p = 1004.7 J/kg/K); pipe and valve pressure drops, humidity, compressor surge limits, a part-load efficiency curve, thermal stress on the salt and brine creep are ignored; the cavern is one lumped node at uniform T; the thermal store is a stratified tag table, not a full heat-transfer model; the equal pressure ratio per stage is an assumption (optimal for equal efficiencies). Real projects target 60 to 70 percent. The model is educational and is not affiliated with any plant or vendor.