Key application issues and solutions of micro energy harvesting technology

When it comes to powering handheld terminals, portable devices, and stationary equipment that is miles away from an outlet, is there a better solution than batteries ?

The answer to this question will always depend on the development of application technology. However, energy harvesting technology, which extracts unused energy from the environment , is becoming a strong competitive solution in various application fields. In the past few years, energy harvesting technology has moved out of the laboratory and onto the workbench of design engineers. In the short term, although energy harvesting technology will not completely replace batteries in all application fields, it has shown many advantages, such as sensors that can operate for years without battery replacement or maintenance, low energy consumption, green environmental protection , and long-term low-cost benefits for end users.

For decades, large-scale energy harvesting from wind and solar plants has been a small but growing part of the world’s energy mix. In 2007, the global photovoltaic market was about $1.2 billion, with fewer than 500,000 inverters shipped. Now, micro-harvesters that extract milliwatts of power from vibration, temperature differences, light, and other ambient sources are also moving toward commercial applications. A few milliwatts may not be much, but they are well suited for ultra-low-power technology products developed by IC companies such as Texas Instruments (TI). Figure 1 shows the difference between large-scale and micro-energy harvesting.

Figure 1: Comparison of large-scale and micro-scale energy harvesting technologies.

Energy harvesting opens up new vistas in engineering in many ways. It also requires engineers to revise their thinking from an energy perspective, especially in terms of strategies for energy management design. While we cannot yet say that energy harvesting technology has rewritten the rules for achieving optimal energy efficiency in circuit design, many best practices are counterintuitive to many engineers.

Application fundamentals: Market

Broadly speaking, harvested energy includes various energy sources, such as kinetic energy (wind, waves, gravity, vibration, etc.), electromagnetic energy (photovoltaic, electromagnetic waves, etc.), thermal energy (solar thermal energy, geothermal energy, temperature changes, combustion, etc.), atomic energy (nuclear energy, radioactive decay, etc.) or bioenergy (biofuels, biomass energy, etc.).

Because energy harvesting technologies are broad and diverse, few attempts have been made to estimate the size of the overall market, and many applications remain undiscovered. Currently, examination of the market for micro-energy harvesting technologies tends to focus on the niche where the technology can clearly replace batteries.

According to statistics from market research firm Darnell Group, 200 million energy harvesters and thin-film batteries will be in use by 2012. The energy harvesting application market in the automotive, home, industrial, medical, military and aerospace fields will grow from 13.5 million units in 2008 to 164.1 million units in 2013.

Wireless sensor networks, where remote nodes are required to operate autonomously for years, are the primary target applications. Depending on their location, these sensor nodes can harvest energy from light, vibration, or other sources. For example, clocks, calculators, and Bluetooth headsets are all potential applications for photovoltaic cells. In addition, Seiko's Kinetic watch uses technology that converts motion energy into electricity; Freeplay's EyeMax broadband wireless radio uses vibration energy to power the radio system.

One of the most attractive technologies is energy harvesting from body heat, a solution used in Seiko's Thermic watch. A new generation of biometric sensors that can count key data from simple pulse rate to ECG waves may even use body heat as an energy source.

The conversion technology is only part of the whole system. A typical energy harvesting system includes many components, such as registers in thin-film batteries, a large number of complex energy management circuits, analog converters, and ultra-low power microprocessors (MCUs). A very important design goal is to match the power supply circuit with the application circuit to achieve the best overall performance. Once the designer is confident that the harvesting technology will support the product, the application can be developed.

Application Basics: Energy Access

At the initial stage of the study, it is necessary to estimate the energy availability. Figure 2 gives an approximate figure of the energy per unit that can be provided by micro-energy harvesting in four environments.

Figure 2: Energy harvesting estimates for four environments.

The next step will be to evaluate the energy that can be harvested by a viable system.

Solar photovoltaic collection is a highly efficient collection technology due to the use of large solar panels. Each 100 square millimeters of photovoltaic cells can generate an average of about 1mW of electricity. The general energy efficiency is about 10%, and the capacity ratio (the ratio of the average electricity generated to the electricity that will be generated when the sun is continuously shining) is about 15%~20%.

Commercially available kinetic energy harvesting systems can generate milliwatts of power. The energy is most likely generated by an oscillating body (vibration), but electrostatic energy harvested by piezoelectric cells or elastomers also falls into the kinetic energy range. Buildings such as bridges and many industrial and automotive structures can generate vibration energy. Basic kinetic energy harvesting technologies include: (1) a body on a spring; (2) a device that converts linear motion into rotational motion; and (3) a piezoelectric cell. The advantage of (1) and (2) is that the voltage does not depend on the power source itself, but on the design of the conversion. Electrostatic conversion can generate voltages as high as 1,000 V or more.

Thermoelectric harvesting technology exploits the Seebeck effect, which generates a voltage when a temperature difference exists between two metals or semiconductors . A thermoelectric generator (TEG) consists of a thermopile connected in thermal parallel and electrical series. The latest TEGs produce an output voltage of 0.7V under matched load, which engineers often use when designing ultra-low power applications. The amount of electricity generated depends on the size of the TEG, the ambient temperature, and (when harvesting heat from the human body) the level of metabolic activity.

According to research by Belgian research firm IMEC, at 22°C, a watch-type TEG can generate an average of 0.2-0.3 mW of useful power during normal activity. Typically, a TEG can continuously charge a battery or supercapacitor , but advanced power management is required to optimize performance.

The three major micro-energy harvesting sources mentioned above have several things in common. They all usually generate unstable voltages, rather than the 3.3V stable voltage that is still widely used in current electronic circuits. In addition, all three technologies provide intermittent power or sometimes no power at all. Therefore, design engineers need to use power converters and hybrid energy systems to solve these problems.