What is energy harvesting?
The energy problem
Fossil fuels are finite and environmentally costly. Sustainable, environmentally benign energy is be derived from nuclear fission or captured from ambient sources. Large-scale ambient energy (e.g. solar, wind and tide), is widely available and large-scale technologies are being developed to efficiently capture it.
At the other end of the scale, there are small amounts of ‘wasted’ energy that could be useful if captured. Recovering even a fraction of this energy would have a significant economic and environmental impact. This is where energy harvesting (EH) comes in.
What is EH?
Definition: Energy harvesting (also known as power harvesting or energy scavenging) is the process in which energy is captured from a system’s environment and converted into usable electric power. Energy harvesting allows electronics to operate where there’s no conventional power source, eliminating the need to run wires or make frequent visits to replace batteries.
EH also has the potential to replace batteries for small, low power electronic devices. This has several benefits:
- Maintenance free: no need to replace batteries
- Environmentally friendly: disposal of batteries is tightly regulated because they contain chemicals and metals that are harmful to the environment and hazardous to human health
- Opens new applications: such as deploying EH sensors to monitor remote or underwater locations
Successfully developing EH technology requires expertise from all aspects of physics, including:
- Energy capture (sporadic, irregular energy rather than sinusoidal)
- Energy storage
- Material science
- Systems engineering
Where can energy be harvested?
Energy is lost in every industrial process and everyday technology that you can think of, e.g.:
- Power stations:All the world’s electrical power is generated by heat engines. These are gas or steam-powered turbines that convert heat to mechanical energy, which is then converted to electricity. Approximately two-thirds of the energy input is not converted to electrical power but lost as heat.
- Computers and microwaves (in fact all our electronic gadgets): lose energy through heat and/or vibration.
How can we harvest waste energy?
Different types of waste energy can be captured using different EH materials. The most promising microscale EH technologies in development include:
- Vibration, movementand sound, can be captured and transformed into electrical power using piezoelectric materials
- Heat can be captured and transformed into electrical power using thermoelectric and pyroelectric materials
Energy Harvesting Overview
All processes that involve energy conversion are, to some degree, inefficient. Motors get hot, as do power transistors, automobile engines, and light bulbs; in each case energy is wasted as heat. Radio stations put out megawatts of RF but their signals reach antennas as microwatts. Energy harvesting devices capture some of this wasted energy, convert it to electricity, and put it to work.
The best-known energy harvesting collectors are large solar panels and wind generators, which have become major alternative energy sources for the power grid. But small embedded devices must rely on energy scavenging systems that can capture milliwatts of energy from light, vibration, thermal, or biological sources.
Since the output from energy harvesting devices is usually small and intermittent, a system must be carefully designed that may include a boost converter, a charge controller for a rechargeable Li-Ion or thin-film battery, a regulator for the MCU and other loads, an MCU, sensors, and a wireless connectivity module. The closer an energy harvesting device can come to supplying the overall demands of an embedded system, the closer that system can come to being battery free.
The most widely used energy harvesting devices rely on solar, thermal, RF, and piezoelectric sources of energy.
Photovoltaic (PV) or solar cells convert light energy into electricity. Photovoltaic cells have the highest power density and highest power output of the various energy harvesting devices.
Thermoelectric energy harvesters convert heat into electricity. They consist of arrays of thermocouplers that generate voltage in response to a temperature differential across their bimetal junctions (the Seebeck effect). The reverse is also true: impressing voltage on a thermocouple junction heats one junction while cooling the other which is the basis for heat pumps (the Peltier effect).
RF energy harvesters capture ambient RF radiation, rectify it, boost it, and use it to power ultra-low-power embedded devices. RFID works on that principle, though by reacting to a strong RF field that is directed at the sensor and not by harvesting ambient RF.