Designing Smart Gas and Water Meters for the Utmost in Energy Efficiency
Electronic water and gas meters represent vexing low-power design challenges for embedded control systems requiring RF connectivity. The nature of these applications requires them to be battery powered (i.e., electricity is rarely provided at the point of service for gas or water utilities). The expected battery life for these metering systems is often greater than 20 years to avoid costly truck rolls for a simple battery replacement. Because of this long-life design requirement, most water and gas meters use lithium thionyl chloride (LiSOCl2) battery chemistry. This chemistry is chosen because of its very low self-discharge behavior and resulting ability to last for up to 20 years in some applications. However, these batteries are very expensive (as much as $1.5/A-hr) resulting in battery bill of material (BOM) costs of up to $10 to $15 per water or gas meter.
Many smart meter providers have determined that they can differentiate their products by extending their communication range. In their system network topology, a fixed number of meters would communicate usage and billing information to a single repeater mounted on a utility pole through a sub-GHz proprietary network. The repeater would aggregate and transmit the collected information back to the utility provider over a cellular network modem or other backhaul channel. A single repeater could support approximately 1000 meter nodes. However, the cost of the repeater can be anywhere from 10 to 100 times greater than a single meter node. Metering suppliers often face pressure from their customers to reduce the number of repeaters in a given network. This can be most readily achieved by improving the robustness of the transmitter (TX) link.
The system design challenge is clear: Increase the power within the TX budget without increasing the total power budget. The reductions would have to be found in other functional areas, namely the receiver (RX), active mode and sleep mode budgets. In my next blogs, we’ll consider several strategies for designing smart meters to achieve the utmost in energy efficiency.