energy harvesting, wireless sensor networks & opportunities for industrial applications

7/30/2019 Energy Harvesting, Wireless Sensor Networks & Opportunities for Industrial Applications

1/6

Energy harvesting, wireless sensor networks &

opportunities for industrial applications

Sebastien Boisseau and Ghislain Despesse, CEA-Leti

2/27/2012 8:45 AM EST

What can we do with less than a 100W-source?

With the will to increase the number of sensors around us and to respect several economic and

environmental constraints, researchers and R&D engineers are looking for new green and unlimited

energy sources that will allow to remove batteries or wires and to develop autonomous wireless sensor

networks with theoretical unlimited lifetimes. These new sources are based on ambient energy.

Unfortunately, ambient energy is not very powerful -100W/cm is a good order of magnitude for energy

harvesters- but is enough for many applications and especially in industry.

From thousand to million sensors in our environment

More and more sensors, this is a general will to increase the amount of information collected from

equipment, buildings, environments allowing us to interact with our surroundings, to predict failures or

to better understand some phenomena. Many fields are concerned: automotive, aerospace, industry,

habitat. Some examples of sensors and fields are presented infigure 1.

Figure 1: Million sensors in our surroundings

We have chosen to focus our study on industry, which is one of the most economically attractive areas. In

order to reduce machine downtimes, costs of maintenance and costs of broken parts replacements, more

and more industrialists are interested in developing (wireless) sensor networks able to collect many

information (pressures, vibrations, temperatures) from their equipment to implement predictive

maintenance.

Unfortunately, it is difficult to deploy many more sensors with todays solutions, for two main reasons:1- Cables are becoming difficult and costly to be drawn (inside walls, on rotating parts)

2- Battery replacements in wireless sensor networks are a burden that may cost a lot in large factories

(hundred or thousand sensor nodes).

http://eetimes.com/General/PrintView/4237022

r 6 21/09/2012 12:03


2/6

As a consequence, industrialists, engineers and researchers are looking for developing autonomous

wireless sensor networks able to work for years without any human intervention. One way to proceed

consists in using a green and theoretically unlimited source: ambient energy.

Ambient sources

Four main ambient energy sources are present in our environment: mechanical energy (vibrations,

deformations), thermal energy (temperature gradients or variations), radiant energy (sun, infrared, RF)

and chemical energy (chemistry, biochemistry).

These sources are characterized by different power densities (figure 2). Energy Harvesting (EH) from

outside sun appears to be the most powerful (even if values given infigure 2 have to be weighted by

conversion efficiencies that rarely exceed 20 percent in photovoltaic cells). Unfortunately, solar energy

harvesting is not possible in dark areas (near machines, in warehouses). Similarly, it is not possible to

harvest energy from thermal gradients when there is no thermal gradient or to harvest vibrations when

there is no vibration. As a consequence, the source of ambient energy must be chosen according to the

local environment of the WSN node: no universal ambient energy source exists.

Figure 2: Ambient sources power densities before conversion

Figure 2 also shows that 10-100W is a good order of magnitude for 1cm or 1cm-EH output power.

Obviously, 10-100W is not a great amount of power; yet it can be enough for many applications and

especially Wireless Sensor Networks.

Autonomous wireless sensor networks (aWSN) & needs

A simple vision of aWSN nodes is presented in figure 3a. Actually, aWSN nodes can be represented as 4

boxes devices: (i) sensors box, (ii) microcontroller (C) box, (iii) radio box and (iv) power box.To power this device by EH, it is necessary to adopt a global system vision aimed at reducing power

consumption of sensors, C and radio.

Actually, significant progress has already been accomplished by microcontrollers & RF chips

manufacturers (Atmel, Microchip, Texas Instruments) both for working and standby modes. An

example of a typical sensor nodes power consumption is given infigure 3b. 3 typical values can be

highlighted:

- 1-5W: standby modes power consumption

- 500W-1mW: active modes power consumption

- 50mW: transmission power peak


r 6 21/09/2012 12:03


3/6

Figure 3: (a) aWSN node and (b) sensor nodes power consumption

First of all, this diagram gives a minimum EH output power more or less necessary to build viable

EH-powered sensor nodes. This limit can be fixed to 1-5W that corresponds to a good order of

magnitude for microprocessor and RF chips standby modes.

Secondly, this diagram highlights the fact that todays EH devices cannot supply aWSN in a continuous

active mode (500W-1mW power consumption vs 10-100W for EH output power). Fortunately, thanks

to an ultra-low power consumption standby mode, EH-powered aWSN can be developed by adopting anintermittent operation mode as presented infigure 4. Energy is stored in a buffer (a) (capacitor, battery)

and used to perform a measurement cycle as soon as enough energy is stored in the buffer (b & c).

System then goes back to standby mode (d) waiting for a new measurement cycle.

Figure 4: WSN measurement cycle

Therefore, it is possible to power any application thanks to EH, even the most consumptive one. The main

problem is to adapt the measurement cycle frequency to the continuously harvested power. To illustrate

possibilities given by EH for aWSN, one needs only to look at the link between power, energy and

measurement cycle frequency.

Power, Energy, Measurement cycle frequency Is an EH-based source viable?

Sensor nodes average power consumption (P) corresponds to the total amount of energy needed for one

measurement cycle (W) multiplied by the frequency of this action (f).

This simple link between P, W and f can be illustrated byfigure 5. By using log-log scales, with energy in

abscissa and measurement frequency in ordinate, average power consumption is represented by straight

lines of slope -1. Obviously, power sources can also be represented in this diagram. It allows to compare

limited sources (batteries, lithium, wood) and ambient energy sources (e.g. 100W green line). For

example, harvesting 100W during 1 year corresponds to a total amount of energy equivalent to 1g of


r 6 21/09/2012 12:03


4/6

lithium.

Moreover, by taking this approach of looking at energy consumption for one measure instead of an

average power consumption, it appears that:

1- Sending 100bits of data consumes 5J

2- Measuring acceleration consumes 50J

3- Making a complete measure: measure+conversion+emission consumes 250-500J.

Therefore, with 100W harvested continuously, it is possible to perform a complete measure every 1-10

seconds (0.1-1Hz). This is in agreement with many industrial needs and especially with predictive

maintenance topics.

Figure 5: Power, Energy and Frequency PWf diagrams

EH Technological offers and actors

According to a study market performed by CEA-Leti, EH-powered aWSN is a field of growing interest.The technological offer is being improved and diversified. For the same reasons, the number of industrial

actors increases. Figure 6 presents several industrial actors working on energy harvesting (except

photovoltaics). Obviously, many research centers also work on EH (University Of Southampton (UK),

MIT (USA), Peking University (CN), Holst Centre (D), Berkeley (US), INSA (FR), LETI (FR),

Fraunhofer(D))

Figure 6: Industrial actors on EH (except photovoltaics)

Unfortunately, except for PV cells, EH is still perceived by industrialists as a non-mature technology that

requires much improvements before being really interesting and widely used. Nevertheless, its advantages


r 6 21/09/2012 12:03


5/6

compared to wires or batteries are already well perceived. Table 1 presents some industrialists visions of

EH (CEA-Leti study market). We compare these results with standard solutions, i.e. batteries and wires.

Table 1: Industrialists visions of EH

Needs from industry Key success factors

Actually, this market survey has raised the main reasons why EH is still not in agreement withindustrialists needs. They are summarized intable 2. We also present EH main limits and propose some

improvements, focusing on vibration energy harvesting (VEH) and thermoelectricity.

Table 2: Industrialists needs and EH limits

Conclusion: Still an emerging technology but with great opportunities

Even though many developments have taken place over the past 10 years, energy harvesting except for

PV cells is still an emerging technology that has not yet been adopted by industry.

Nevertheless, improvements of present technologies that are currently under investigation should

enable to meet the needs expressed by industrialists. For vibration energy harvesters, the most important

focal area of research is probably the increase of the working frequency bandwidth that is still a

technological bottleneck preventing this technology from being a viable and versatile supply source. For

thermoelectricity, improvements mainly concern materials to increase EH output power even on small

thermal gradients. As for robustness and impacts of aging, only time will bring more information.

To conclude, energy harvesting can be and will be a viable solution to develop autonomous wireless

sensor networks. Its adequacy with sustainable development is a great opportunity. Obviously,photovoltaic cells are probably the most advanced and robust technology today, but it cannot work in all

situations and especially for industry applications; thermoelectricity and vibration energy harvesting can

be suitable for these environments. Nevertheless, both of them should be improved and prove their worth

before it is fully adopted by industry.


r 6 21/09/2012 12:03


6/6

About the authors

Sebastien Boisseau and Ghislain Despesse are researchers at CEA-Leti, a French institute focused on

micro- and nanotechnologies and their applications. CEA-Leti is part of CEA, French Atomic Energy and

Alternative Energies Commission.

The authors would like to thank F. Pinaud and S. Joly for the market survey, and their VEH

coworkers, B. Ahmed Seddik, J.J. Chaillout, A.B. Duret, P. Gasnier, P.D. Berger, S. Rich and S.Dauv for their contribution to this article.

Visit CEA-Leti

Follow us on SmartEnergy Designline


energy harvesting, wireless sensor networks & opportunities for industrial applications

Documents