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This paper describes the evaluation and improvement of a volumetric method of measuring state of charge for ice storage systems. The volumetric method was evaluated using measurements from a well-instrumented thermal storage system. The baseline for the evaluation was an energy balance method that is highly accurate but prohibitively expensive for commercial use. The volumetric method of determining state of charge tends to overestimate the "true" state of charge during discharge because it does not account for sensible heating of water trapped between the ice and heat transfer tubes. This sensible effect can lead to errors of up to 8% at the end of tank discharge when it is most important to have an accurate indication of state of charge. A correction factor was developed that accounts for sensible energy storage during discharge and reduces the maximum error to about 2% at the end of discharge. The correction uses volumetric state of charge and secondary fluid inlet and outlet temperatures as inputs and does not significantly increase the complexity or cost. The correction factor is particularly important for high discharge rates that may occur in conjunction with the use of real-time pricing of electricity.

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