Knowing how much electricity is being used, and for what purposes, is the first step towards reducing that amount. Unfortunately, most consumers of electricity have no way of knowing how much energy is being consumed by each of the many electric circuits in their home or business.  This research project aims to solve that lack of information by developing a simple and easily installed system for measuring the energy consumption of a building's individual electric circuits. The system uses convenient stick-on sensors. These sensors are simply attached to the circuit breakers inside a breaker panel, needing no wiring or electrician, and wirelessly report the individual electric currents — from which the energy consumption of each circuit can be inferred — to a secure repository on the Internet for easy access anywhere. These fine-grained, real-time energy consumption data will allow residential energy consumers and commercial building managers to make better decisions about their electricity usage.

Motivation

The Demand Response (DR) initiative uses dynamic electricity pricing as an incentive to encourage commercial and residential customers to shed loads when electricity demand threatens to exceed supply. To inform customers regarding which of their electrical loads might be temporarily shed, we have researched “stick–on” wireless AC current sensors that can be easily and inexpensively attached to individual circuit breakers in office building or dwellings. These sensors, which could be installed by non-technical personnel, could indicate which equipment or appliances are drawing electrical energy heavily at a given time.

Typical Circuit Breaker with Stick–on Sensor

Opened Breaker Box with Sensors and Wireless Radio

Current sensors have been attached to four circuit breakers (a). Sensor output voltages are conducted down the white ribbon cable to a circuit (b) that converts each sensor’s voltage to an average (root–mean–square) value. The average value is sent to a radio (c) that broadcasts the sensor voltages, which are proportional to the sensor outputs and thus to the breaker currents, to a receiving device, such as a laptop (d), outside the box.

Data on the Internet

The data received outside the closed breaker box—the radio signals go through the cracks in the box’s door—can be displayed locally or be put on the Internet for access anywhere.

How Does the Sensor Work?

The sensor, in its protective polymer enclosure, contains permanent magnets attached to a cantilever beam coated with a piezoelectric film. The stick–on sensors are located so that their magnets are opposite the closest current–carrying conductor inside each commercial circuit breakers. The AC magnetic field produced by a breaker current exerts a time–varying force on the nearby permanent magnets, causing the cantilever beam to vibrate. The piezoelectric film on the beam produces a voltage whose amplitude is proportional to the current flowing through the breaker.

Meso–scale and MEMS Current Sensors

Meso–scale stick–on AC current sensor for use on a circuit breaker. MEMS AC current sensor that can be used on circuit breakers or included in appliance cords and in various electrically powered devices.

Average Currents Measured at a Circuit Breaker

This breaker supplied a 1300–watt space heater and a 250–watt incandescent light bulb. The AC current sensor output voltage has been converted to its average (root–mean–square) value.

Other Points

Meso–scale sensitivity: 307–150milliwatts/ampere (rms)

Linearity: The sensor’s mechanical resonant frequency was set at 80 Hz, while the frequency of the breaker current is 60 Hz. The mismatch of driving frequency and its resonant frequency causes the sensor’s output voltage amplitude to be linearly proportional to the breaker’s current amplitude.

Sensors as energy harvesters: At present, each breaker sensor can harvest up to ~20 microwatts of power, which could be used to supply the associated circuitry and a low–power radio.