ES1.3 · Air storage and thermal process boundaries

Learning objectives

• Explain why 70% and 60% are not directly comparable RTE claims.
• Calculate A output divided by total stated energy input.
• Can either commit 5 MW?

Allow 30–45 minutes including derivations and exercises. All numerical values are teaching assumptions.

1. Compression stores a thermodynamic state

Compressed air inventory is defined by pressure, temperature, mass and the allowed operating window. A pressure vessel volume alone is not usable electricity. Compression releases heat; expansion cools the gas. The heat balance and minimum pressure determine how much electricity can be returned.

2. Separate combustion and recovered heat

A fuel-assisted cycle may discard compression heat and burn fuel before expansion. A heat-recovery arrangement aims to store and reuse compression heat. Their inputs and performance are different. Record fuel energy separately and do not label a fuel-assisted generation ratio an electricity-only round-trip efficiency.

3. Understand the liquid-air alternative

Liquid-air storage uses refrigeration, a cryogenic tank, liquid pumping and warming/expansion. Cold recovery and external waste heat can affect performance. Availability of those resources must coincide with the cycle; an attractive performance point relying on unavailable waste heat is not a site result.

4. Treat the store and machine independently

Cavern integrity, leakage and pressure cycling constrain inventory; compressor and expander maps constrain power. Startup and part-load behaviour may limit dispatch. Thermal-store depletion can become the first constraint even when pressurised air remains. A constant efficiency cannot represent every pressure and load.

5. Prepare the process evidence pack

Request mass, heat, electrical and fuel balances on compatible conditions. Include standby and recovery states, hazardous interfaces, pressure/cryogenic permissions and civil scope. Quantities and specific machine data are needed before dispatch or financial optimisation can be trusted.

Worked answers and reasoning

Attempt the exercises before opening each answer.