Enabling cold compressed air energy storage through pressure vessel manufacture with autofrettage

Compressed air is an attractive energy storage solution that can address many of the problems associated with operating large electricity grids with high levels of renewable penetration. The mature nature of the technology makes compressed air a robust and cheap alternative to batteries that is particularly applicable to offshore generation. Storage tanks (pressure vessels) must be utilised if geological alternatives, such as solution mined salt caverns, are not available or cannot be excavated in a particular deployment. Tanks are typically expensive, however it is possible to realise significant improvements (over 50%) in cost per unit exergy stored if 'real gas effects' of air are exploited. Economic benefits resulting from realistic air property dependencies rely on storing air at low temperatures, circa -40°C. In this temperature range concerns are raised over the integrity of common pressure vessel materials; a transition from ductile to brittle failure modes is observed in many BCC (body centred cubic) steels that limits the size of 'safe' (non-propagating) flaws in the vessel and increases the potential for fast fracture/catastrophic failure. Autofrettage is a manufacturing process in which a beneficial compressive stress state at the internal wall of a pressure vessel is induced by over pressurising the cylinder during manufacture. Autofretteage allows larger flaws or defects (such as cracks) to be safely accommodated in a design, compared to an identical vessel that has not undergone autofrettage. In this work autofrettage is investigated as a method which can allow cold compressed air energy storage to be realised. Safe operating pressures and temperatures are determined for vessels that have undergone autofrettage. These are then compared to similar calculations for more 'simple' vessel designs (i.e. without autofrettage) and economic arguments are developed for the adoption of cold compressed air storage. Costings (cost per unit exergy stored) are not significantly sensitive to the additional effort required to autofrettage a vessel (due to the pressure levels involved).