Solar Energy Harvesting System Fuels Expanded Capabilities

By Marlys Amundson

In use around the world, solar technologies provide a number of valuable resources, including light, electricity, and cooling. Now thanks to breakthrough research at UCLA, solar energy also can power a class of tiny, environmental sensors.

Scientists studying water resource management in India and tracking local animal populations are utilizing the solar harvesting system developed by researchers at UCLA’s Networked and Embedded Systems Laboratory (NESL) that provides unlimited energy for embedded sensor systems.

“Using solar cells to generate power is not new, obviously, but I think we have provided two key things,” says electrical engineering professor Mani Srivastava, who directs NESL. “We have developed tools to convert a standard platform for tiny sensor nodes into one that can operate off solar cells. The other part, which was really defined by one of my students, Aman Kansal, is how you can exploit the spatial and temporal variations in energy and availability for system management.”

Often deployed for long-term studies of specific environmental factors including light, humidity, and temperature, simple non-mechanical sensors, or motes, have very low energy requirements and spend most of their life cycle in “sleep” mode. However, running only intermittently even the most power-efficient motes will exhaust their batteries in just over a year.

“Energy supplies are a key problem in sensor networks, especially those which are embedded and expected to last for years without any user intervention,” explains doctoral student Kansal. “For nodes that are located outdoors, we have developed a way to use the sun to recharge the sensors.”

The UCLA electrical engineering team has designed and fabricated Heliomote, a module that provides an inexpensive and reliable source of energy. The harvesting circuit designed by the NESL researchers draws power from commercial solar panels, manages the use and storage of available energy, and routes power to the attached sensor.

The most mature wireless energy harvesting system available to researchers, Heliomote operates independently to extend the lifetime and capabilities of large-scale embedded sensor networks.

“By making even small efficiency gains in the solar powered circuit, we can potentially reach a point at which the system could operate indefinitely,” explains graduate student Jonathan Friedman. “The circuit would produce more energy in a day than it needs to power the sensor, ensuring that the power supply is never exhausted.”

For most large-scale systems, researchers are interested not in how the work is divided but in the end-performance of the network and its ability to deliver critical information. To enable the system to operate more efficiently, the UCLA team has developed algorithms that allow a multi-node system to determine when solar energy is available, and adjust the overall system demands on any given sensor accordingly.

“Because these are event-driven systems,” says graduate student Sadaf Zahedi, “we want to maintain a specific energy level in each of the sensors. So we’ve created a system that can adjust on the fly and look to the nodes in direct sunlight to run at a higher duty cycle, limiting the energy demands on nodes at night, or those located in partial light or shadow.”

The team recently completed a three-month deployment of several Heliomote-powered sensors, and verified that the sensors can collect and send data at 10-second intervals without running out of energy. In addition to demonstrating the system’s success, they will be able to use the collected data for testing new algorithms.

“We also discovered problems that you’d never learn in the lab that we can apply to future installations,” says electrical engineering graduate student Jason Hsu. “But we also verified that it can operate steadily over a long period of time—and that isn’t possible with just batteries.”

The team is now working with the third generation of the solar circuit, having redesigned it to operate more efficiently in a wide range of conditions. They also are exploring a redesigned system that can use both a battery and a capacitor to increase the device’s overall efficiency by adding short-term energy storage.

“Typically in a university setting,” says Kansal, “people think about the design, try to optimize it, and then build the system. We have been building and optimizing simultaneously on this project, and have learned a lot more than we could have thinking about it only on paper.”

Vijay Raghunathan (MS ’02, PhD ’05), a member of NESL, was also instrumental in Heliomote’s design. Building on their success, the NESL team is exploring other uses for the solar energy harvesting unit, including medical applications. They are designing a new system that will use Heliomote to power wireless electrocardiographs for monitoring patients’ health when systems powered by electricity are unavailable, as in the aftermath of Hurricane Katrina.

Two members of NESL, Friedman and graduate student David Lee, have founded ATLA Labs and already have released a commercial edition of Heliomote that has been deployed by university researchers around the world.

“Not many companies are building devices for university researchers,” explains Lee. “We wanted to manufacture the hardware for researchers so they don’t have to reinvent the wheel each time but can concentrate their research focus.”

The commercial version of Heliomote is extremely rugged, able to withstand underwater submersion and high temperatures. The system is compatible with a wide range of off-the-shelf motes and requires no modification prior to deployment. In addition, the unit can operate efficiently over a wide range of temperatures, and automatically controls recharging, so that the batteries are never under or overcharged.

Researchers from EPFL (Ecole Polytechnique Federale de Lausanne) in Switzerland are studying water resource management and conservation in agricultural lands in India, and scientists at other institutions have contacted ATLA about using the devices to track lizard populations and vineyard conditions.

“We have the technology, we know it works, we know its performance characteristics and what it can do, and we know what we expect it to do when deployed,” says Friedman. “The question then becomes, what devices should the Heliomote power? Do we need to collect information on temperature, light, or seismic data?”

At UCLA, Dr. Eric Graham, a member of the Center for Embedded Networked Sensing, is interested in using Heliomote to power devices that will track the effects of relative humidity on the life cycle of moths through a deployment at the James San Jacinto Mountains Reserve.

Main Image: A heliomote. Photo by NESL.