Thermodynamic cycle apparatus and method

A thermodynamic energy storage apparatus has a heat pump 1 which stores energy by cooling a heat storage medium in a first thermal store 2, and heating a second thermal storage medium in a second thermal store 3. A first thermodynamic circuit 4 with a first working fluid is able to absorb heat 9 from an auxiliary (e.g. environmental) heat source 10 to produce pressurised vapour, generate energy by expanding the vapour 6, and condense the fluid 7 by rejecting heat to the cold store 2. A second thermodynamic circuit 5 with a second working fluid can be heated 14 by heat from the hot store 3 to produce pressurised vapour, generate energy by expanding the vapour 11, and condense the fluid 12 by rejecting heat to an auxiliary (e.g. environmental) heat sink 15. The two thermodynamic circuits are thermally independent. The thermal stores may comprise phase change material (PCM) which may be encapsulated.