A method of operating an energy storage apparatus, comprising: (i) providing an energy storage apparatus that comprises: a first fluidic circuit containing a first phase change material, the first fluidic circuit including a first storage vessel and a first expander ; a second fluidic circuit containing a second phase change material having a boiling point greater than a boiling point of the first phase change material, the second fluidic circuit including a second storage vessel and a second expander ; and a heat pump having a cold side heat exchanger thermally coupled to the first fluidic circuit and a hot side heat exchanger thermally coupled to the second fluidic circuit characterized by the steps of: (ii) operating the energy storage apparatus in a charging mode by energising the heat pump to cool the first phase change material and heat the second phase change material; (iii) operating the energy storage apparatus in a storage mode by storing the first phase change material in the first storage vessel and storing the second phase change material as a pressurised vapour in the second storage vessel ; and (iv) operating the energy storage apparatus in a discharge mode by vaporising condensed first phase change material and expanding the vaporised first phase change material in the first expander , and/or expanding the vaporised second phase change material in the second expander . (i) providing an energy storage apparatus that comprises: a first fluidic circuit containing a first phase change material, the first fluidic circuit including a first storage vessel and a first expander ; a second fluidic circuit containing a second phase change material having a boiling point greater than a boiling point of the first phase change material, the second fluidic circuit including a second storage vessel and a second expander ; and a heat pump having a cold side heat exchanger thermally coupled to the first fluidic circuit and a hot side heat exchanger thermally coupled to the second fluidic circuit characterized by the steps of: a second fluidic circuit containing a second phase change material having a boiling point greater than a boiling point of the first phase change material, the second fluidic circuit including a second storage vessel and a second expander ; and a heat pump having a cold side heat exchanger thermally coupled to the first fluidic circuit and a hot side heat exchanger thermally coupled to the second fluidic circuit characterized by the steps of: (ii) operating the energy storage apparatus in a charging mode by energising the heat pump to cool the first phase change material and heat the second phase change material; (iii) operating the energy storage apparatus in a storage mode by storing the first phase change material in the first storage vessel and storing the second phase change material as a pressurised vapour in the second storage vessel ; and (iv) operating the energy storage apparatus in a discharge mode by vaporising condensed first phase change material and expanding the vaporised first phase change material in the first expander , and/or expanding the vaporised second phase change material in the second expander . <EOS>