Wireless Sensor Networks (WSNs) may be considered as the third wave of a revolution in wireless technology. It makes the life of humans more secure, easy and beneficial. A sensor is an electronic device used to detect or measure a physical quantity and convert it into an electronic signal. In a wireless sensor network, sensors play an important part, as sensing is one of its central roles. They sense data from various environments, process the data locally with some computation and communicate the data among the sensor nodes. The most important feature of wireless sensor networks is the elimination of wires in communication. Without wiring requirements, they can be deployed in a range of remote monitoring applications where running wires is prohibitive or impossible.
Sensor networks are formed from a collection of sensing nodes, which communicate with one another, typically through wireless channels, in order to collect spatially distributed data about their environment. Such networks have the potential to provide better quality data than single or small numbers of individual sensors in applications such as natural and built environmental monitoring, process monitoring, security and surveillance.
In order to be cost effective in many applications, the sensor nodes must be low cost and low maintenance. Sensor networks unshackled from the mains, or battery power, open the possibility for greater reliability, lower maintenance costs, improved safety, & widespread deployment. WSN systems have been varying power needs from a few microwatts of power in sleep mode to 100 milliwatts in transmit mode. Thus, these systems require a means to store energy. Batteries are the most commonly used energy-storage medium for WSNs. With new low-power WSN systems, nodes can operate for up to three years at a one-minute sample interval on four AA batteries.
Fig 1: wireless sensor node power cycle…
Wireless sensors are ubiquitous and very flexible devices that can adapt using harvested energy. Since wireless sensor nodes are commonly placed in hard-to-reach locations, changing batteries regularly can be costly and inconvenient. It is now possible to implement wireless sensors using harvested energy because of the off-the-shelf availability of ultra-low-power, single-chip wireless microcontrollers (MCUs) capable of running control algorithms and modern communication algorithms. Power consumption can be minimised by optimising the relative amount of time spent in low-power sleep mode.Thus, most of the time wireless sensor nodes spend their time in sleep mode. The only subsystem that stays awake is the Real-Time Clock (RTC), which keeps time and wakes up the wireless sensor node to measure a sensor input.
Energy harvesting
Wireless Sensor Networks could involve billions of sensors, which are needed on variety of applications but adoption has been hindered by conventional primary batteries, which will eventually need replacing – and therefore cannot be fitted and forgotten for a long period.
While the performance of battery technology is gradually improving and the power requirements of electronics are generally dropping, these are not keeping pace with the increasing demands of many WSN applications. For this reason, there has been considerable interest in the development of systems capable of extracting useful electrical energy from existing environmental sources. Such sources include ambient light, thermal gradients, vibration & other forms of motion.
As energy storage technologies, such as super capacitors and Thin-Film Batteries (TFBs), have become more cost-effective, energy harvesting technology has become more sophisticated and efficient in recent years. These power management devices are to fully exploit the capabilities of the respective energy transducer elements and the sensor networks electronics that are ultimately powered by them. The following graph (Fig: 2) shows the power density of various energy harvesting technologies.
Fig: 2 The power density of various energy harvesting technologies…
Energy sources suitable for scavenging
- Solar energy harvesting
As an energy source for WSNs, solar cells are the most mature technology and supported by developed markets. Solar cell technology is one of the promising technologies, which are most studied and suited for various applications where availability of sun light is good. The photoelectric effect is a property of some materials to release electrons when exposed to light. These free electrons can be captured and used as a source of power for many applications. There are so many types of factors, which should be considered before using Solar Cells as an energy source like availability of bright sunlight, days of sunny and cloudy days in a year, environmental conditions of deployment, power requirements etc.
But higher manufacturing cost and relatively low efficiency of solar cells, presents considerable limitation to the use of solar cells over wide range of applications.
- Thermoelectric harvesting
Thermal gradients are one of the oldest techniques for generating electricity. A simple thermocouple is a junction of two dissimilar wires with a temperature difference between the junction and the wire ends. The thermoelectric effect is the direct conversion of thermal gradient to electric potential and vice versa. A thermoelectric generator creates a voltage when there is difference of temperature on each side (known as the Seebeck effect). Conversely, when a voltage is applied to it, it creates a temperature difference (known as the Peltier effect).The phenomena of creating electric voltage with a temperature difference and vice versa are termed as thermoelectricity.
- Mechanical vibration
Mechanical vibration can be converted to electrical energy. Basically, there are three mechanisms for the conversion from mechanical vibration energy to electrical energy conversion: piezoelectric, electrostatic and electromagnetic.
- Piezoelectric energy harvesting
There are certain materials that can generate an electric charge or voltage when they are under mechanical stress; this is a direct effect of piezoelectricity. Alternatively, the same materials are able to produce a mechanical deformation (or force) when an electric field is applied to them, this is inverse/converse effect of piezoelectricity.
- Electrostatic (Capacitive) energy harvesting
A capacitor broadly defined as two conductors separated by a distance that can hold opposite charges. Change in capacitance caused by change in distance or relative position between two conductors by vibration is the basic principle of capacitive energy harvesting. An electrostatic energy harvester uses the force between charges stored on electrodes to couple the mechanical energy into the electrical energy.
- Electromagnetic Energy Harvesting
There is a large amount of electromagnetic energy all around us transmitted by communication devices; we just need to tap into it. Some scavenging devices can capture this energy, convert it from AC to DC, and then store it in capacitors and batteries or can be used as power source.
Challenges
Thus, in situations when there is no ambient energy from which to harvest power, the secondary power reservoir must be used to power the WSN. Of course, from a system designer’s perspective, this adds a further degree of complexity since they must now take into consideration how much energy must be stored in the secondary reservoir to compensate for the lack of an ambient energy source. Just how much they will require will depend on several factors. These will include:
- The length of time the ambient energy source is absent
- The duty cycle of the WSN (that is the frequency with which a data reading and transmission has to be made)
- The size and type of a secondary reservoir super capacitor or battery)
- Is enough ambient energy available to act as both the primary energy source and have sufficient energy left over to charge up a secondary reservoir, when it is not available for some specified period?
Conclusions
In the wireless sensor networks, the use of renewable and non-renewable energy source is a promising technology with harvesting to overcome the limitation of battery source.
Although, the current harvesting technology does not provide sustained energy supply, harvesting energy from the environment offers an exciting promise for long-term WSN deployments. Energy harvesting or scavenging is possible from sources such as waste heat from industrial plants, vibrations and temperature differentials in aircrafts and automobiles, and even from human action such as walking, lifting and pressing. The self powered WSNs find their applications where battery replacements are difficult – both when the sensor nodes are not easily accessible, or because they are deeply embedded. Energy harvesting technology is now certainly ready for the prime time. By combining this technology with a wireless sensor network, both wide area and narrow field sensors can provide the data needed to detect threats more accurately and for longer periods of time across larger areas.
Although power is for free from numerous ambient energy sources, system designers and systems planners must prioritise the specific requirements of their power management systems from the onset – in order to ensure efficient designs and successful long-term deployments.
If you want to share thoughts or feedback on this article, please leave a comment below.
