Energy Harvesting for Medical Devices and Remote Sensors
Energy harvesting from the environment is a technique that uses natural or man-made energy sources, and is now an increasingly popular technique for powering electrical devices. Solar-powered lights and pumps for decorative ponds are cheap, plentiful, and commonplace. Widespread exploitable energy sources include solar energy, natural geothermal and solar heating, man-made heating, and vibrational energy as a by-product of machinery, natural or artificial movement of gases or fluids, man-made electromagnetic fields from wi-fi, radio and television broadcasting.
There is now a wide range of integrated circuits available that exploit a range of energy sources that we’ll be discussing. Some energy sources such as geothermal heating are available to exploit around the clock and provide a reliable and consistent supply throughout the year. However, they are only available in specific geographic locations and not particularly practical for many electronics applications. Other energy sources such as solar power are available in just about any geographic location, but only for part of the day and subject to the seasons and the weather. However, the availability is reasonably predictable over the long term. Alternatively, engines powered by petrochemicals provide a reliable source of thermal and vibrational energy that can be harvested as a byproduct while operating. However, as soon as the engine stops running, the vibrations stop, and the thermal source quickly decays. Which energy source is best for you will come down to your energy requirements, how your device operated, and the environment you’ll be operating in.
Common Energy Sources
Fluid flow is one of the oldest forms of energy harvesting, the flow of fluids has long been captured using windmills powered by the wind and waterwheels powered by streams and rivers. Turbines can be used to capture energy from the flow of any available gas or liquid, from hydroelectric power stations down to medical implants powered by blood flow.
Photovoltaic technology is now one of the most widespread technologies for harvesting solar energy, used in everything from small self-powered electrical devices to roof-top panels that supplement domestic and commercial energy usage and now reasonably common wide-area solar farms provide renewable energy as part of national power generation strategy. Typical implementations use amorphous silicon or dye-sensitized solar cells to generate electricity directly from solar energy. The power generated is proportional to the ambient lighting levels. However, the nature of sunlight in terms of brightness and frequency range means that it can typically be converted into greater energy levels than comparable artificial lighting.
The piezoelectric effect is the conversion of mechanical movement into electrical energy using crystals. While this can be driven by any form of movement where there is a continuous rate of change of strain, the energy collected is relatively small. This makes this technique only practical for devices with very low power requirements that are exposed to continuous mechanical movements, such as an exercise monitor attached to a person or a sensor attached to rotating machinery. One disadvantage of this technology is that the effect degrades over time, limiting the device’s operating life.
The triboelectric effect, commonly observed as static electricity, is caused by pulling apart different materials to create an electric charge, an effect that can be maximized by rubbing two such materials together to create a continuous cycle of contact and separation. However, the effects of friction and that inevitable wear that will occur in the two triboelectric layers will significantly limit the device’s useful lifetime.
Thermoelectric generation is the conversion of thermal gradient found at the junction of two dissimilar conductors into electrical energy, known as a thermocouple. A thermoelectric generator can be created from a series of semiconductor PN junctions linked together in a relatively compact component. However, the relative inefficiency of the process and low currents generated limits the applications for this technology. It is most commonly utilized in wearable medical health monitoring devices where body heat provides a dependable energy source.
The pyroelectric effect is the conversion of temperature change into electrical energy. While more efficient than thermoelectric harvesting, a continuous change of temperature is required to generate meaningful energy levels. The application of this technology is currently minimal.
Electrostatic or capacitive harvesting converts mechanical energy from vibrations acting on a capacitive device to change its capacitance into electricity energy. This is achieved by having one fixed plate and a second moveable plate that the external vibrations operate on. However, the capacitive device must have a significant potential difference applied before the vibrational energy can be harvested. This pre-charging of the capacitor will require an additional energy source.
Magnetic induction harvesting converts mechanical energy from movements or vibrations acting on a magnetic material to change the magnetic field into electrical energy. Most commonly used in generators and alternators to convert rotational energy from a mechanical engine or a fluid flow, on a smaller scale, vibrations acting on flexible magnetic materials can be used to generate small levels of electrical energy in a solid-state device.
Electromagnetic radiation can be collected using an antenna optimized for a particular frequency band or for a broad range of frequencies. Due to relatively small energy levels available, radiation can be harvested from ambient wi-fi, radio, and television signals or from electromagnetic radiation directed explicitly to the collector for harvesting.
The biological process for using enzymes to break sugars down into constituent water and carbon dioxide to release energy offers potential applications in medical implants powered by harvesting blood sugar. However, it has limitations in terms of requiring access to a supply of suitable enzymes to sustain conversion.
Why Harvest Energy?
The advantage of energy harvesting is the ability to place remotely located devices quickly and cheaply without needing to provide power over wires or use replaceable batteries. Energy stores in the form of batteries and capacitors can manage energy sources that are not available when the device is required to operate.
Energy harvesting lends itself to remote sensing devices that typically require small amounts of energy to maintain operations while being used in locations where providing an external power source can be problematic. A particularly ideal application is in health monitoring systems. The continuous monitoring of a mechanical system’s status can provide valuable fault prediction data to reduce maintenance costs, replacing periodic checks based on theoretical failure rates with preemptive actions performed as-required, based on actual performance. Such mechanical systems are usually brimming with suitable energy sources in terms of heat, movement, vibrations, and electromagnetic fields.
A typical remote sensor requires a sensing function, a data processing function, a wireless communications capability, and a power supply. The development of efficient integrated solutions for sensing, processing, and communications have facilitated replacing traditional battery-based power supplies with energy harvesting. The availability of energy harvesting devices as an integrated circuit has simplified their incorporation into compact and rugged devices that can be deployed almost anywhere.
What’s the Best Source?
When looking to identify the best energy source to harvest from machinery, the search starts with internal and external sources. For internal sources, these typically come in the form of heat or motion. Heat can come from inefficient components generating excess heat due to losses when operating or from friction where moving surfaces are in contact with each other. Heat can also come as the by-product of exothermic reactions such as in a chemical manufacturing process or as the result of ignition of fuels such as in an engine or furnace. Fuel-powered engines are typically less than 50% efficient, generating significant heat levels that can be exploited. Motion can come from the rotation of machinery or from vibrations caused by the movement of machinery. Motion can also come from the movement of fluids (gas or liquid) that are directly generated, such as in a pumped pipeline or indirectly generated by the movement of a component in its environment.
For external sources, these typically come in the form of light, heat, electromagnetic radiation, or motion. Light can be collected from sunlight or from any artificial lighting present. Heat can be collected from the indirect effect of sunlight heating a surface or from geothermal activity. Electromagnetic radiation can be collected from any ambient electromagnetic energy in the environment, found in high densities in the majority of urban areas and in lower levels where radio or television signals are broadcast. Motion can come from any naturally occurring movements of fluids such as wind or flowing water.
Implementing Energy Harvesting
A range of off-the-shelf integrated circuits are now available that incorporate energy harvesting functions with power supply circuitry that can be dropped into a design in place of the usual power supply logic. A typical chip includes a cold-start circuit for initial power-up, an ultra-low-power boost converter to take the voltage generated by the energy harvesting components, a suitable voltage for the energy store, and a low-dropout regulator to provide usable power supply to the device being powered. Integrated functions include energy storage management for fast charging, low power warnings, and selectable output voltage. Commonly available integrated circuits from the usual chip suppliers include:
- Solar-powered power management units with buck converters, buck-boost converters, and low-dropout regulators
- Thermoelectric driven power management units with boost converters, buck-boost converters, and low-dropout regulators
- Magnetic driven power management units with buck converters and buck-boost converters
- Piezoelectric driven power management units with buck converters and buck-boost converters
Hybrid energy harvesting integrated circuits are also available where a single energy source will not meet the circuit design requirements. For example, combining different semiconductor materials into a single package can bring together photovoltaic cells and a thermoelectric generator in one chip. Similarly, organic materials are available that can be used to generate electrical charges for both photovoltaic and thermoelectric effects.
When implementing an integrated energy harvesting chip, additional components in the form of the energy collection and storage components will be necessary. The collection element will typically be photoelectric cells, a radio frequency receiving antenna, or a thermocouple. The storage element will typically be a lithium-ion battery or a super-capacitor. It is also common to need to add external capacitors, inductors, and resistors in accordance with the data sheet. A typical chip will require less than ten passive discrete components in addition to the collection and storage elements.
The availability of integrated circuits that have energy harvesting functionality built-in makes the adoption of this power source straightforward when designing remote sensing devices with a ready source of energy available to be tapped into. Eliminating the need for batteries or a wired power supply can offer significant advantages over the long-run in terms of through-life costs and potentially simplifying the device’s design by removing the need for a battery compartment or a power socket.
Where availability of the energy source cannot be guaranteed, or in the case of sunlight where it is only available for set periods, an energy store will be required. The harvested energy provided to the store over a set period obviously must equal or exceed the energy taken from the store by the device over that period.
The main design question then becomes, which is the best source of energy to harvest, and of course, this doesn’t have to be a single source. Tapping into a couple of different energy sources may be preferable if no single source provides a dependable energy supply. It all depends on the operating environment, the operational requirements, and any budgetary constraints.