A thermodynamic cycle apparatus comprising: (i) a first reservoir containing a first storage medium; (ii) a second reservoir containing a second storage medium; (iii) a heat pump having a cold side thermally coupled to the first reservoir for cooling the first storage medium and a hot side thermally coupled to the second reservoir for heating the second storage medium; (iv) a first thermodynamic circuit of a first working fluid, the first thermodynamic circuit comprising: a first evaporator for evaporating the first working fluid to create a first pressurised vapour; a first expander arranged to expand the first pressurised vapour; and a first condenser arranged to condense first working fluid received from the first expander and provide first working fluid to the first evaporator, the first condenser being thermally coupled to the first reservoir; (v) a second thermodynamic circuit of a second working fluid, the second thermodynamic circuit comprising: a second evaporator for evaporating the second working fluid to create a second pressurised vapour, the second evaporator being thermally coupled to the second reservoir; a second expander arranged to expand the second pressurised vapour; and a second condenser arranged to condense second working fluid received from the second expander and provide second working fluid to the second evaporator; (vi) an auxiliary heat input means thermally connected to the first thermodynamic circuit so that auxiliary heat may contribute to the creation of the first pressurised vapour; and (vii) an auxiliary heat output means thermally connected to the second thermodynamic circuit so that the second working fluid can lose heat to an auxiliary heat sink; wherein the first thermodynamic circuit is thermally independent of the second thermodynamic circuit, and the apparatus is operable in a charging mode, a storage mode, and a discharge mode; wherein in the charging mode the heat pump is energised to cool the first storage medium and heat the second storage medium; in the storage mode cooled first storage medium is stored in the first reservoir and heated second storage medium is stored in the second reservoir; and in the discharge mode, the first pressurised vapour is expanded by the first expander and/or the second pressurised vapour is expanded by the second expander. (i) a first reservoir containing a first storage medium; (ii) a second reservoir containing a second storage medium; (iii) a heat pump having a cold side thermally coupled to the first reservoir for cooling the first storage medium and a hot side thermally coupled to the second reservoir for heating the second storage medium; (iv) a first thermodynamic circuit of a first working fluid, the first thermodynamic circuit comprising: a first evaporator for evaporating the first working fluid to create a first pressurised vapour; a first expander arranged to expand the first pressurised vapour; and a first condenser arranged to condense first working fluid received from the first expander and provide first working fluid to the first evaporator, the first condenser being thermally coupled to the first reservoir; a first evaporator for evaporating the first working fluid to create a first pressurised vapour; a first expander arranged to expand the first pressurised vapour; and a first condenser arranged to condense first working fluid received from the first expander and provide first working fluid to the first evaporator, the first condenser being thermally coupled to the first reservoir; (v) a second thermodynamic circuit of a second working fluid, the second thermodynamic circuit comprising: a second evaporator for evaporating the second working fluid to create a second pressurised vapour, the second evaporator being thermally coupled to the second reservoir; a second expander arranged to expand the second pressurised vapour; and a second condenser arranged to condense second working fluid received from the second expander and provide second working fluid to the second evaporator; a second evaporator for evaporating the second working fluid to create a second pressurised vapour, the second evaporator being thermally coupled to the second reservoir; a second expander arranged to expand the second pressurised vapour; and a second condenser arranged to condense second working fluid received from the second expander and provide second working fluid to the second evaporator; (vi) an auxiliary heat input means thermally connected to the first thermodynamic circuit so that auxiliary heat may contribute to the creation of the first pressurised vapour; and (vii) an auxiliary heat output means thermally connected to the second thermodynamic circuit so that the second working fluid can lose heat to an auxiliary heat sink;