A method for operating an electrochemical direct heat to electricity converter, the method comprising: providing an electrochemical direct heat to electricity converter comprising: a primary thermal energy source; a working fluid (12; 26); an electrochemical cell comprising a first membrane electrode assembly and a second membrane electrode assembly, each including a first porous electrode (16, 24), a second porous electrode (20, 22) and at least one membrane (23; 25), wherein the at least one membrane (23; 25) is sandwiched between the first and second porous electrodes (16, 20, 22, 24) and is a conductor of ions of the working fluid (12; 26); a counter flow heat exchanger transferring heat from high temperature, low pressure working fluid leaving the second membrane electrode assembly to low temperature, high pressure working fluid leaving the first membrane electrode assembly, an energy storage reservoir (36a; 240) which is a heat storage reservoir (36a; 240); and an external load, wherein the primary thermal energy source is coupled to at least one of said membrane electrode assemblies (5; 6) and the electrochemical cell operates on heat to produce electricity, characterized in that when thermal energy available from the primary thermal energy source is greater than necessary to meet demands of the external load, the converter stores excess energy in the heat storage reservoir (36a; 240), and when the thermal energy available from the primary thermal energy source is insufficient to meet the demands of the external load, the converter uses at least a portion of the excess energy stored in the heat storage reservoir (36a; 240) to supply power to the external load. providing an electrochemical direct heat to electricity converter comprising: a primary thermal energy source; a working fluid (12; 26); an electrochemical cell comprising a first membrane electrode assembly and a second membrane electrode assembly, each including a first porous electrode (16, 24), a second porous electrode (20, 22) and at least one membrane (23; 25), wherein the at least one membrane (23; 25) is sandwiched between the first and second porous electrodes (16, 20, 22, 24) and is a conductor of ions of the working fluid (12; 26); a primary thermal energy source; a working fluid (12; 26); an electrochemical cell comprising a first membrane electrode assembly and a second membrane electrode assembly, each including a first porous electrode (16, 24), a second porous electrode (20, 22) and at least one membrane (23; 25), wherein the at least one membrane (23; 25) is sandwiched between the first and second porous electrodes (16, 20, 22, 24) and is a conductor of ions of the working fluid (12; 26); a counter flow heat exchanger transferring heat from high temperature, low pressure working fluid leaving the second membrane electrode assembly to low temperature, high pressure working fluid leaving the first membrane electrode assembly, an energy storage reservoir (36a; 240) which is a heat storage reservoir (36a; 240); and an external load, an energy storage reservoir (36a; 240) which is a heat storage reservoir (36a; 240); and an external load, wherein the primary thermal energy source is coupled to at least one of said membrane electrode assemblies (5; 6) and the electrochemical cell operates on heat to produce electricity, characterized in that when thermal energy available from the primary thermal energy source is greater than necessary to meet demands of the external load, the converter stores excess energy in the heat storage reservoir (36a; 240), and when the thermal energy available from the primary thermal energy source is insufficient to meet the demands of the external load, the converter uses at least a portion of the excess energy stored in the heat storage reservoir (36a; 240) to supply power to the external load.