Ambient energy stands ready to serve
With electronic circuits now capable of operating at microwatt power levels, it is feasible to power them from non-traditional sources. Hence, the rise of energy harvesting, which provides the power to charge, supplement or replace batteries in systems where battery use is inconvenient, impractical, expensive or dangerous.
Energy harvesting can eliminate the need for wires to carry power or to transmit data. It can power smart wireless sensor networks to monitor and optimize complex industrial processes, remote field installations and building HVAC systems. And otherwise wasted energy from industrial processes, solar panels and internal combustion engines can be harvested for useful purposes.
Ambient energy sources include light, heat differentials, vibrating beams, transmitted RF signals and any source that can produce an electrical charge through a transducer. Such "free" energy sources can be converted into electrical energy by using a suitable transducer, such as thermoelectric generator (TEG) for heat, a piezoelectric element for vibration, a photovoltaic cell for sunlight (or indoor lighting) and even galvanic energy from moisture. These energy sources can be used to power electronic components and systems autonomously.
Despite their complexity, energy-harvesting systems have already been deployed in transportation infrastructure, wireless medical devices, tire pressure sensing and building automation. In building automation systems, elements such as occupancy sensors, thermostats and light switches can eliminate the power or control wiring normally associated with their installation and instead use localized energy harvesting. A wireless network using an energy-harvesting technique can link sensors in a building to reduce HVAC and lighting costs by turning off power to nonessential areas when the building is vacant. The cost of enabling energy-harvesting electronics is often lower than that for running supply wires, so there is an economic gain to be had by adopting a harvested power technique.
Many of the advantages of a wireless sensor network disappear if each node requires its own external power source. Through power management developments have enabled electronic circuits to operate longer for a given power supply, that approach has its limitations. Energy harvesting provides a complementary approach, powering wireless sensors nodes by converting ambient energy into usable electricity.
A typical energy-harvesting configuration or wireless sensor node comprises four blocks (See Figure 1): an ambient energy source; a power conversion component to power the rest of the node; a sensing component, comprising a microprocessor or microcontroller that processes measurement data and stores the data in memory; and a communications component, consisting of a short-range radio for wireless communications with neighboring nodes and the outside world.
Once the electrical energy has been produced, it can be converted by an energy-harvesting circuit and then modified into a suitable form to power the downstream electronics. Thus, a microprocessor can wake up a sensor to take a reading or measurement.
The collected data can then be manipulated by an analog-to-digital converter for transmission via an ultralow-power wireless transceiver.
Of course, the energy provided by the energy-harvesting source depends on how long the source is in operation. Therefore, the primary metric for comparison of scavenged sources is power density, not energy density. Energy harvesting is generally subject to low, variable and unpredictable levels of available power, so a hybrid structure is used that interfaces to the harvester and to a secondary power reservoir. The harvester, because of its unlimited energy supply and deficiency in power, is the energy source of the system. The power reservoir, either a battery or a capacity, yields higher output power but stores less energy, supplying power when required but otherwise receiving a charge from the harvester.
Consider the breakout of energy usage in the United States. Buildings are the No. 1 user, accounting for 38 percent of total energy consumption, closely followed by the transportation and industrial segments, at 28 percent each.
Moreover, building energy use can be categorized into commercial and residential consumption, representing 17 and 21 percent, respectively. A further breakdown of the residential figure reveals that heating and cooling account for 76 percent of total energy consumption in that domain.
With energy usage forecast to double between 2003 and 2030, energy savings of up to 30 percent could be attained via building automation.
Ambient energy sources
State-of-the art and off-the-shelf energy-harvesting technologies, for example in vibration energy harvesting and indoor photovoltaics, yield power levels in the milliwatts under typical operating conditions. While such power levels may appear restrictive, the operation of harvesting elements over a number of years can render the technologies broadly comparable to long-life primary batteries in terms of both energy provision and the cost per energy unit provided.
Further, systems incorporating energy harvesting will typically be capable of recharging after depletion. The same cannot be said for systems powered by primary batteries.
The laggard in this chain has been the energy harvester.
Existing implementations of the energy-harvesting circuit typically consist of low-performing discrete configurations, usually comprising 3D components or more. Such designs have low conversion efficiency and high quiescent currents, compromising end-system performance.
The low conversion efficiency will increase the amount of time required to power up a system, which in turn increases the time interval between taking a sensor reading and transmitting the data. A high quiescent current limits how low the output of the energy-harvesting source can be, since it must overcome the current level needed for its own operation before it can supply power to the output.
Power management is the key aspect to enabling remote wireless sensing, but it must be implemented starting at the concept of the design. System designers and planners have to prioritize their power management needs from the onset in order to ensure efficient designs and successful long-term deployments.
Integrated solutions are available that can overcome the deficiencies of current discrete energy harvester solutions.
The LTC3109 is a dc/dc that takes a "system level" approach to solving a complex problem. It converts the low-voltage source and manages the energy between multiple outputs.
The part can harvest and manage surplus energy from extremely low-input voltage sources such as thermoelectric generators, thermopiles and even small solar cells. It operates from input sources as low as 30 mV, regardless of polarity.
The circuit shown in Figure a uses two compact step-up transformers to boost the input voltage source to the LTC3109, which then provides a complete power management solution for wireless sensing and data acquisition. It can harvest small temperature differences and generate system power instead of using traditional battery power.
About the author:
Tony Armstrong is director of product marketing for power products at Linear technology Corp.
This article was originally published in EE Times’ special digital issue entitled: "Alternative energy: From the unsustainable… to the unlimited."