State of charge sensor for phase change material thermal energy storage

1 . A thermal energy storage system, comprising:

a heat exchange module having a housing with an open interior and a quantity of phase change material in the open interior of the housing, the phase change material having a liquid phase and a solid phase, the liquid phase having a first liquid volume and a liquid density and the solid phase having a first solid volume and a solid density different from the liquid density, the combined volume of the liquid phase and the solid phase defining a total volume of the phase change material, wherein a change in the heat of the phase change material causes a change in the relative amount of the phase change material in the liquid phase and the amount of phase change material in the solid phase and a change in the liquid volume and the solid volume, such that the phase change material undergoes a change in total volume, the heat exchange module further comprising a heat exchange module reference gas creating a heat exchange module reference gas vapor head over the phase change material;

an expansion container comprising a housing and an open interior, the expansion container being in fluid communication with the heat exchange module, the expansion container further comprising a diaphragm hermetically dividing the open interior of the expansion container into first and second portions, wherein the first portion and the diaphragm are in fluid communication with the heat exchange module reference gas vapor head of the heat exchange module through a port located in the first portion and the second portion is fluidically isolated from the heat exchange module and hermetically sealed from the surrounding atmosphere, the first portion of the expansion container exchanging the heat exchange module reference gas with the heat exchange module through the port;

a quantity of the heat exchange module reference gas being in fluid communication with the phase change material, the heat exchange module reference gas having a first pressure and first volume before a change in the heat of the phase change material, and a second pressure and second volume after a change in the heat of the phase change material;

a quantity of an expansion container reference gas in the second portion of the expansion container, the expansion container reference gas being different that the heat exchange module reference gas, the expansion container reference gas being fluidically isolated from the heat exchange module reference gas and having an expansion container reference gas pressure and an expansion container reference gas volume;

a sensor for sensing the expansion container reference gas pressure and generating a pressure signal that is related to the expansion container reference gas pressure, wherein at least a portion of the sensor is within the second portion of the expansion container;

wherein with the exchange of heat with the heat exchange module there will be a change in the heat of the phase change material, a change in the amount of phase change material in the liquid phase and in the solid phase, and a change in the total volume of the phase change material;

wherein the change in the total volume of the phase change material will cause a change in the volume of the heat exchange module reference gas from the first volume to the second volume and a change in the pressure of the heat exchange module reference gas from the first pressure to the second pressure, and wherein the change in volume and pressure of the heat exchange module reference gas will cause a change in the position of the diaphragm and thereby a change in the expansion container reference gas volume and the expansion container reference gas pressure; and,

wherein the sensor will detect and generate a signal of the expansion container reference gas pressure after the exchange of heat, and wherein the sensor continuously monitors the pressure of the expansion container reference gas as the phase change material changes phase between an entirely solid phase and an entirely liquid phase.

2 . The thermal energy storage system of claim 1 , further comprising a processor for receiving the signal of the reference gas pressure, the processor determining from the expansion container reference gas pressure the change in total volume of the heat exchange module reference gas and of the phase change material.

3 . The thermal energy storage system of claim 2 , wherein the processor determines from the change in total volume of the phase change material the amount of phase change material in the liquid phase and the amount of phase change material in the solid phase after the exchange of heat with the phase change material, and the state of charge.

4 . The thermal energy storage system of claim 1 , wherein the thermal energy storage system is hermetic and of fixed volume.

5 . The thermal energy storage system of claim 1 , wherein the sensor is attached to the diaphragm.

6 . The thermal energy storage system of claim 5 , wherein the sensor senses the position of the diaphragm.

7 . The thermal energy storage system of claim 6 , wherein the sensor comprises a strain gauge for measuring the strain of the diaphragm.

8 . The thermal energy storage system of claim 1 , wherein the sensor continuously monitors the pressure of the expansion container reference gas as the phase change material changes phase between an entirely solid phase and an entirely liquid phase.