Water heaters and other thermal energy storage devices increase the temperature of a medium above an ambient or normal temperature and store the warmer medium. Water heaters in particular must store the heated water in such a way that it is available for immediate use on demand. PNNL’s thermal energy storage system is designed to monitor the amount of heated medium so that consumers and business owners know what is available for various applications such as showering, washing dishes, or laundering. It may also increase appliance efficiency, reduce consumption, and save costs.

A water heater typically has three regions: hot water in the top of the reservoir, cold water near the bottom, and a thermocline in between. The position of the thermocline corresponds to a boundary for the cold and hot water. This boundary moves down as more water is heated and up as hot water is used. PNNL’s thermal energy storage system follows the thermocline as it moves up and down in the reservoir and provides information on its location and the current state of the thermal energy storage device (for example, heating, not-heating) to alert users as to optimal discharge times.

The system comprises four components: a reservoir to hold the medium, a control system that can adjust the temperature of the medium, a location system to follow the thermocline, and a monitoring system that provides information on the state of the system. The thermocline location system includes a position-indicating object that moves through the reservoir with the thermocline and sends a signal to the monitoring system. The four components work together to identify an optimum heating approach, decreasing energy consumption while providing sufficient heated medium as needed. The system could be used to create better thermal energy storage devices such as water heaters.