Implementing Self-Powered WSNs for Building Automation
By Tim Bradow, VP of Marketing, Infinite Power Solutions (IPS)
Advanced solid-state batteries together with ambient energy harvesting provide a permanent eco-friendly power solution for deploying wireless sensor networks in Smart Buildings.
Wireless Sensor Networks (WSNs) are self-organizing, self-healing networks of small sensor nodes and are positioned to transform a variety of industrial sensing applications, including building automation (occupancy detection, HVAC, lighting controls, etc.) and machine–to-machine communications (machine health monitoring). These and other similar WSN solutions are highly attractive because of their ease of use and lower cost of installation compared to wired solutions. The global market for WSNs is estimated to grow to several billion $US over the next few years, but this is predicated on industry adoption and implementation of certain wireless standards and the availability of suitable battery power solutions.
Individual sensor nodes within a WSN communicate their measured data wirelessly via low power radio protocols such as WirelessHART and Zigbee, and often operate in the 2.4GHz ISM band using IEEE 802.15.4 standard radio ICs. In 2010, the Bluetooth 4.0 specification introduced Bluetooth Low Energy (BLE) technology, which was designed primarily to target health and fitness applications, but is gaining traction in a variety of simplified WSN applications, including low cost building automation. A variety of other radio protocols are also competing for market share, but a dearth of industry consensus on which protocol to use has had some impact on the adoption rate of WSNs to date.
Reliable battery power is critical
Another challenge for a viable WSN implementation is the ability to keep the sensing nodes reliably powered. WSNs are usually off-grid, meaning they don’t have access to wired power, or the cost of providing wired power (e.g., grid-access, cabling, installation and maintenance labor) is prohibitive. Therefore, such sensor nodes are usually battery-powered and minimizing power consumption is critical. Sensor nodes typically operate with a limited duty cycle, meaning that the active mode (often the measurement and transmit mode in the case of wireless sensors) is limited and the device may be in low power sleep mode most of the time to maximize battery life. Active operation is initiated in response to an outside stimulus or triggered by an internal timer. This type of Off/Active operation is very effective to maximize battery life. But what happens if the battery itself is not up to the task of providing decades of service to match the expected lifetime of the sensor node?
Despite a design engineer’s best intentions and usage of low-power design techniques, conventional batteries (primary or secondary cells) generally will not last for 20 years. Conventional rechargeable batteries degrade with use and simply wear out with repeated charging and primary batteries such as alkaline or lithium thionyl chloride (Li-SOCL2) cells typically don’t hold enough energy for the life of the system. Thus, conventional batteries require periodic replacement and can limit product lifetimes. In remote sensing applications, batteries may not be easily accessible for maintenance or replacement. This can result in extended periods where the system is down and data may be permanently lost. Inactive sensor nodes resulting from dead batteries can cost thousands of dollars in service calls and equipment downtime. Therefore, a more reliable and permanent power solution is needed for successful WSN implementation—one that employs a better, longer lasting rechargeable battery technology and leverages ambient energy harvesting—creating a much longer system life (measured in decades) and achieving the lowest total cost of ownership.
Leveraging Ambient Energy and Energy Storage
Energy harvesting systems capture ambient energy from the environment surrounding the system and may power the application when sufficient energy is available, and accumulate and store this energy for later use when the ambient energy may not be sufficient. Common energy harvesting technologies that convert ambient energy into electricity include photovoltaic (light), thermoelectric (heat), electrodynamic (motion) and piezoelectric (vibration/strain). In addition, directed RF energy can be used to continuously trickle charge batteries used in sensor nodes, particularly when the nodes can be in close proximity to a directed RF energy source, such as in building automation applications.
Ambient energy sources often provide variable energy levels. For example, the energy harvested from mechanical vibration using piezoelectric elements can be intermittent with variable voltage impulses, depending on the source of vibrational energy. Irregular supply of the mechanical vibration results in inconsistent energy output by the piezoelectric element, ranging from zero to hundreds of volts. This requires an energy storage device to store the energy when available and then power the application when needed. Most ambient energy harvesting systems also produce very low current and variable voltage, which energy storage devices can address by serving as an “energy buffer” to provide the proper voltage and current required by the load (e.g., the sensor, microprocessor, radio IC). Energy storage devices can also provide peak current to the load when needed – something that is often not possible with today’s low power energy harvesters.
Not all energy storage devices are created equal
Several important considerations must be made when evaluating energy storage devices. For example, the energy storage device’s state of charge will fluctuate between various partial levels of charge, which can be problematic for some battery chemistries. In addition, the application may experience long periods of inactivity with no available ambient energy. Thus, the storage device must have extremely low self-discharge characteristics to ensure stored energy is not lost, and that sufficient energy is available when needed. In addition, batteries provide “instant-on” capability because sufficient energy is stored and always ready for use. Supercapacitors are sometimes considered as battery alternatives due to their high cycle life. However, they have very low capacity and very high self-discharge rates compared to batteries of similar size, making them a poor choice for storing the low charging currents typically generated by energy harvesters. They will leak most of their stored energy within a few days of the last recharge, so they must be recharged often to be effective.
Most design engineers are familiar with conventional battery types, both rechargeable and non-rechargeable, used in consumer electronics and some have applied these with limited success to WSNs. Example primary (non-rechargeable) cells include the classic CR2032 coin cell and cylindrical AA/AAA or larger cells. However, conventional batteries use an organic wet chemistry that is inherently unstable with typical lifetimes of less than 10 years. Since they must store a lifetime of energy, these batteries are bulky and can be larger than the systems they power. They can also leak their corrosive electrolyte material and damage the sensor node. Some are considered harmful to the environment, prompting government regulations for their proper disposal.
Rechargeable batteries such as lithium ion, lithium-polymer, nickel-metal hydride and lithium coin cells typically exhibit poor cycle life (limited recharge cycles), high self-discharge rates, limited operating temperature range, and low power capability when scaled to small form factors. Due to their organic electrolytes, they can sometimes overheat and cause a fire. In addition, conventional rechargeable batteries generally have insufficient service lifetimes to serve the intended life of the sensor node. This requires periodic maintenance and replacement, which is a hassle at best, and often cost-prohibitive.
Solid-state batteries offer unique benefits
In a growing number of applications, such conventional batteries are now being replaced by longer lasting solid-state batteries. These eco-friendly batteries are typically much smaller in size and don’t hold as much energy in a single charge. However, because they can be recharged faster and more frequently than conventional batteries, they can often provide a better, longer lasting power solution. Like other technologies, solid-state solutions tend to win over time. For example, vacuum tubes were replaced by solid-state circuits, making radios much smaller and more reliable. The same is true with photographic film, which was replaced by digital media. Now we see conventional light bulbs being replaced by light emitting diodes (LEDs). The solid-state metamorphosis is happening now in the battery space. The benefits for batteries include smaller size, higher performance, and longer life.
Infinite Power Solutions, Inc. (IPS) is an industry leader in manufacturing rechargeable thin power solutions and has developed a new class of solid-state batteries optimized for high power delivery in a tiny form factor, high cycle life, and unusually low self-discharge, making them ideal for storing energy harvested from ambient environments. This unique energy storage device, combined with ambient energy harvesting transducers, enables sustainable, autonomous power for a variety of devices, even under challenging thermal conditions. These THINERGY® Micro-Energy Cell (MEC) products (Figure 1) are all-solid-state, flexible and paper-thin. Optimized for the variable charging scenarios typical of ambient energy harvesters, these MECs can be fully recharged more than 10,000 times, ~10x more cycles than what typical rechargeable batteries offer today.
Figure 1 – THINERGY MECs are paper-thin, rechargeable, 4V batteries that last the life of the application and are ideal for storing energy harvested from ambient environments.
THINERGY MECs are fabricated using a sophisticated vacuum sputter deposition process to deposit thin layers of inorganic battery materials onto a thin metal foil substrate. The cathode material is Lithium Cobalt Oxide (LiCoO2), and the anode material is metallic Lithium (Li); together they form a powerful and highly rechargeable 4V cell. A solid-state electrolyte called Lithium Phosphorus Oxy-Nitride (LiPON) is used to provide high discharge rate capabilities, such as 40mA continuous current (70 C-rate) from a paper thin cell the size of a postage stamp. In addition, LiPON acts as a highly effective barrier to electrons to ensure an extremely low self-discharge current (measured in single-digit nano-amps) that enables decades of shelf life with no need for recharge. The combined battery and flexible metal foil packaging materials form a total cell thickness of only 0.17mm, making these micro-energy cells far thinner than other thin-film batteries, printed batteries, coin cells, and supercapacitors, which can range in thickness from 0.4mm to 5mm in thickness. MECs provide a safe, reusable and clean energy source that delivers decades of power to electronic devices and systems without requiring maintenance or replacement as is common with conventional batteries. Operating from -40°C to +85°C, MECs offer near loss-less energy storage and the lowest total cost of ownership.
Figure 2 – The IPS-EVAL-EH-02 Wireless Environmental Sensor Energy Harvesting Evaluation Kit is a complete reference design for autonomously powered indoor Building Automation Wireless Sensing applications.
IPS also offers development kits to assist product development engineers in designing their own energy harvesting-based wireless sensor systems. One such kit is the IPS-EVAL-EH-02 Wireless Environmental Sensor Energy Harvesting Evaluation Kit (Figure 2), which features a variety of common indoor sensors (occupancy, humidity, temperature, and ambient light) and transmits the sensor data via a 2.4GHz IEEE Std. 802.15.4 compliant MiWi™ RF transceiver module from Microchip. This kit includes a solar panel for ease of use, but also allows the user to connect other harvesting transducers to harvest available ambient energy, such as thermal, vibrational, and RF energy, instantly enabling the designer to enter the wireless sensor market for building automation.
October 1, 2012