As many as 50 billion devices will be online by the end of the decade. Along with smart thermostats and appliances, this so-called Internet of Things (IoT) includes swaths of tiny sensors that track everything from steps and calories to humidity and light. A web of power cords would undercut its usability. Thus, universities and companies alike are refining energy-harvesting techniques to free the IoT from plugs—for good.

Piezoelectric
This past summer Rochester, N.Y.–based MicroGen Systems rolled out the Bolt, a quarter-sized generator that converts ambient vibrations into usable power. A subtle rumble, perhaps produced by an air conditioner or microwave, causes a flap in the device to flutter, which in turn creates a current that goes into either a capacitor or a small rechargeable battery.

The Good: Scalable. Vibration sources readily available.
The Bad: Produces only enough energy for low-power devices, such as sensors.

Solar
SunPartner Technologies, a French company, has developed transparent solar panels that can cover screens and other surfaces. An array of microlenses on the photovoltaic material bends light around the strips to make them invisible. The company is already producing displays for smartphones and watches and is finalizing a prototype of an embedded sensor.

The Good: Virtually invisible panels can be incorporated into a wide array of devices.
The Bad: Will not work in inconsistent light or typically dark areas, such as basements and under sinks.

Wi-Fi Backscatter
A prototype by University of Washington researchers harvests power from existing wireless transmissions, such as television and radio signals, to send messages over a local Wi-Fi network. The device selectively reflects Wi-Fi signals, encoding data that other devices on the network can then decode. The team's start-up aims to bring the first products to market within a year.

The Good: Can both charge devices and transmit data.
The Bad: Wi-Fi transmissions typically come in bursts, making connectivity unpredictable and power draw relatively low.

Thermoelectric
By taking advantage of electrons' natural flow from the hot side of a conductive material to the cold side, a thermoelectric generator can convert body heat into power. A team at the Korea Advanced Institute of Science and Technology recently demonstrated a compact version encased in flexible glass; it is capable of producing 40 milliwatts of power at room temperature.

The Good: Potential to continuously charge a battery as long as the device is in contact with a warm body.
The Bad: Requires a large temperature differential (about 31 degrees Celsius) to work. Small power yield. Best suited for wearables, not ambient sensors.