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Green Engineering

Energy Harvesting and Power Balance in Wireless Sensor Networks

Gathering the energy needed for operation locally rather than relying on line power or batteries, can enable the use of wireless sensor networks in more locations and reduce the cost of maintenance in such locations as older buildings where wired installations would be prohibitive.

MARTIN R. JOHNSON, ILLUMRA AND EUGENE YOU, ENOCEAN

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Wireless, energy-harvesting technologies are making waves in terms of sustaining building automation and energy conservation controls. By becoming versed in energy-harvesting and wireless strategies, OEMs can quickly develop energy management devices (sensors, switches, controllers, relays, gateways) that overcome the confines of hardwired solutions and the maintenance issues inherent to battery-dependent devices. Customizable, application-specific functions (primarily for lighting and HVAC) are embedded inside radio and energy-harvesting modules. This article will discuss the science behind the wireless, energy-harvesting technology, then delve into how to budget miniscule amounts of energy sufficient for managing building energy usage.

Buildings account for 40 percent of all energy (electricity and fossil fuel) consumption in the United States, and according to the AIA, approximately 50 percent of all GHG (greenhouse gas) emissions. Climate conditioning (HVAC) and lighting account for most of the energy consumed by buildings. A lot of that energy is routinely wasted. By automating control of lighting and HVAC energy management, OEMs can quickly develop solutions that satisfy a historic market demand for energy-saving instruments. 

Older buildings are less likely to be energy efficient than current construction. Building Automation Systems (BASs) have been shown to be reliable in reducing energy consumption in buildings on average of 40%; however, most buildings in the U.S. do not integrate BASs. Upgrading energy-inefficient buildings with BASs has traditionally been hindered by many factors, primarily the following:

 

  • Existing buildings are expensive to retrofit (installation costs, slow payback)
  • Retrofitting existing buildings with BAS is invasive, often complicated and potentially risky (e.g., building closures, unknown variables behind walls/ceilings and exposure to asbestos)

 

Integrators are overcoming these traditional barriers by using battery-less, self-sustaining, wireless sensors and controls. The controls reduce the amount of energy unnecessarily wasted in our buildings and bypass many obstacles inherent in hardwired equivalents. 

Energy Harvesting & Wireless

At the center of energy harvesting and wireless is the EnOcean wireless standard. EnOcean, a spin-off of Siemens AG, invented energy-harvesting wireless sensor networks. Today, the technology has evolved and 130 member companies of the EnOcean Alliance now subscribe to the open wireless standard. The Alliance is a consortium of companies advocating the use of self-powered wireless monitoring and control systems that serve as catalysts to sustainable building energy management (Figure 1).

Figure 1
Figure 1: The EnOcean Wireless Standard and Alliance

Energy-harvesting technology stems from a simple observation. Where measurable sensor values reside, ambient energy exists sufficient to power sensor radio communications. For example, when a switch is pressed, temperature changes or luminance level varies, energy is produced. These rudimentary operations generate enough energy to transmit radio signals that are useful in terms of sustaining wireless communications between sensors, switches and controls within a building automation system. Instead of batteries, EnOcean-based controls use miniaturized energy converters and capacitors that supply power to building energy management devices. The bottomless power generation (or energy conversion, to be more precise) stems from various sources of ambient power: linear motion converters, solar cells and thermoelectric converters (Figure 2).

Figure 2
Figure 2: Radio & Energy Harvesting Modules—Radio modules powered by ambient sources of energy

Creating an energy-harvesting-powered device requires more than simply cutting the wire from the mains supply or replacing batteries with an energy-harvesting module. It requires a system engineering approach to make it work. Given a limited energy budget, every component in the system is critically important. A market-ready product is viable only when all aspects in the system are considered along with how they operate as a whole in the system.

What does a complete solution entail? An energy-harvesting wireless sensor is comprised of building blocks, each of which has been optimized specifically for energy harvesting and to work in harmony with each other. The balance between energy generation and energy consumption is a critical consideration in developing sustainable solutions. When factoring the amount of ambient energy available in buildings, continuous operation is only feasible when all of the building blocks are optimized for low power consumption. In order to power devices within the naturally enforced limits of energy availability, sensors must transmit infrequently, execute procedures within the shortest possible time, and be able to switch off all blocks when not required for operation (Figure 3).

Figure 3
Figure 3: Energy Harvesting Building Blocks

Different wireless energy-harvesting applications require varying amounts of energy for routine operations. For example, a magnetic contact window sensor requires less energy than an occupancy sensor. The magnetic window sensor senses and signals an isolated event while the occupancy sensor must sense continuously or at intervals and send data periodically. In addition, there may be sleep modes interspersed with periodic actions of sensing and transmitting data with corresponding different levels of energy consumption (Figure 4). 

Figure 4
Figure 4: Typical Wireless Node Operation

Designing for Micro Energy Budget & Balance 

When moving forward with a wireless, energy-harvesting design, many variables come into play including the actual harvesting, the storage and budgeting of available energy. Careful consideration of the variables is vital to a successful design. 

With almost any energy-harvesting device, it is possible and probable that the source of ambient energy will not always be present. For instance, in the case of solar harvesting, light may not always available. Thus, a means of storing energy is necessary. The stored energy is used to bridge the gap during the time when the ambient energy supply fades away. There are various means of storing electrical energy: capacitors, primary (non-rechargeable) and secondary (rechargeable) batteries, supercapacitors, etc. Batteries are becoming increasingly unpopular because they must be transported, installed, replaced and properly disposed of and recycled.  Because most battery chemistries contain toxic chemicals, all of these activities are unpopular, unsafe, expensive and/or environmentally unfriendly. Capacitors simply don’t store enough energy in a small space, so they are not well suited to carry the burden when ambient energy is unavailable for harvesting.

An increasingly popular energy storage reservoir for energy-harvesting applications is the supercapacitor or ultracapacitor. These electrochemical capacitors have the fortunate characteristic of relatively large volumetric energy density as compared to traditional ceramic, electrolytic or tantalum capacitors, as shown in Figure 5. They are routinely used in a wide variety of applications ranging from automobiles to cameras to wireless, battery-less sensors, particularly those based on the EnOcean wireless standard.

Figure 5
Figure 5: Volumetric Energy Density

Supercapacitors possess the desirable trait of tolerating many hundreds of thousands, even millions of charge-discharge cycles, which is two to three orders of magnitude better than rechargeable batteries. They can also be charged/discharged very quickly. Their volumetric energy density is a couple of orders of magnitude less than typical primary lithium cells, but when combined with an energy harvester, such as a solar cell, they never have to be replaced. Another benefit is that they don’t contain toxic chemicals. Thus, they are quite useful in low-power wireless electronics.  

Special attention should be given to the selection and sizing of a supercapacitor for a given application. The performance of supercapacitors is affected by time, temperature, voltage and charge cycling. The designer should not select a capacitor whose initial capacitance and equivalent series resistance (ESR) just barely meet the requirements of the application. The capacitance and ESR degrade as time progresses. For example, a supercapacitor charged to its maximum rated voltage and held at its maximum rated temperature can lose approximately 30% of its capacitance in as little as 1000 hours; its ESR can increase over 200%.  Charge/discharge cycling also affects their capacitance.  

A supercapacitor can lose 15% of its capacitance during the first 100,000 cycles, and worsens if it is charged up to its maximum voltage each time. After 100,000 cycles the reduction in capacitance levels off to about 20% at 1 million cycles. Lowering the temperature by 10°C tends to reduce the degradation by a factor of two. Thus, the designer should determine the minimum voltage and energy requirements of the application and then derate the initial capacitance of the supercapacitor appropriately. Each manufacturer will provide more specific information to assist the designer in sizing a supercapacitor for any given application.

Solar and Thermal Energy Harvesting

One of the most common energy harvesters today is the solar cell. Solar radiation represents the largest energy resource of the terrestrial ecosystem. Unfortunately only about 0.1% of the sunlight level is available indoors. Technologically speaking, there are two major solar cell types. One type is typically used for outdoor applications while the other is used for indoor applications. Crystalline silicon solar cells, also called “outdoor,” reach their best efficiency under sunlight (peak sensitivity at 800 nm). Amorphous (non-crystalline) silicon cells, also called “indoor,” with peak sensitivity at 500 nm, are ideally suited for poor light and fluorescent light (FL) conditions. While conventional crystalline solar cells are about two times more efficient by optimum light conditions (outdoors), amorphous cells outperform them by far under poor indoor lighting conditions and win on the 24-hour energy cycle because they can also use poor artificial light and early morning or evening light.

As a rough, conservative estimation, indoors and for small area amorphous solar panels (a few cm2), an operating current in the range 8.5 μA/cm2 @ 200 lx (FL) can be considered. This value can be roughly linearly extrapolated using a derating factor for lower illumination and/or smaller area. That corresponds to 4 μA/cm2 @ 100 lx. A similar extrapolation approach is shown in Figure 6.

Figure 6
Figure 6: Approximate constant output current

For best performance, the solar panel output voltage should be near the required application operating voltage. The amount of output voltage mainly depends on the number of cells connected in series. Take care to ensure that the voltage delivered by the solar cell is above the minimum required by the system (at the lowest expected light level) and that it is below the maximum the system can tolerate (at the highest expected light level).

These are rough estimates that depend on many variables. First, carefully check the specific environment, define worst-case application requirements and add an additional reserve of 20% by the solar panel dimensioning. Verify your assumptions by measuring the real solar panel values in the worst case. Generally consider 25 lx as a lower brightness limit for designing ambient light powered devices. Below this limit solar cell efficiency drops dramatically.

Alternative to the use of a standard solar panel (e.g., in applications with no or not enough light), a radio module can also be powered by other external power sources such as a thermo-electric generator based on a standard Peltier element as shown in Figure 7. Table 1 summarizes the amount of current required by an STM300C radio module. With a wake cycle of 100 seconds, and transmitting each time it wakes, the long-term current required for continuous operation is 1.6 uA. With a 2.6V supply voltage, this works out to be about 4.4 uW of continuous energy.   This amount of energy can be delivered by an ECT300 EnOcean module with as little as 2 kelvin temperature differential. A capacitor provides a reservoir for harvested energy storage and provides the short-term burst current required by the radio module.

Figure 7
Figure 7: Block Diagram of radio module powered by Peltier thermal energy harvester.

By harvesting ambient energy sources and coupling that with energy-optimized wireless technologies, it is possible to realize a wide variety of wireless lighting and HVAC sensors and controls. These devices can be easily installed in almost any building, with minimal invasiveness, to help conserve energy and save costs.

ILLUMRA
Orem, UT. 
(801) 349-1200. 
[www.illumra.com].

EnOcean
Salt Lake City, UT. 
(801) 943-3215. 
[www.enocean.com].