Technical Article

August 19, 2019 by Francesco Orfei

## Energy harvesting is a way to obtain electrical energy from the one already available in the environment. Where is energy harvesting appropriate in designs?

Energy harvesting is a way to obtain electrical energy from the one already available in the environment. Where is energy harvesting appropriate in designs?

One of the main issues in designing modern devices is the pervasive requirement for extremely low power, especially for wireless sensor network applications.

When dealing with such power requirements, there are at least two main factors:

• The amount of time a system must remain ON with respect a hypothetical period of work (duty cycle)
• The number of components that compose a system

Additionally, an engineer must consider factors such as budget. For example, non-rechargeable batteries discharge and must be properly disposed at the end of their life, which represents a cost. Meanwhile, rechargeable batteries or capacitors are a valid alternative because they can be recharged.

In this article, we'll talk about the broad strokes of energy harvesting, a concept wherein a system can "harvest" energy from its environment.

### What Is Energy Harvesting?

Energy harvesting is a way to obtain electrical energy already available in the environment. This concept represents a valid solution to provide energy to electronic systems and requires an energy converter (energy harvester) to function.

Perhaps the most famous example of energy harvesting is the use of light, one of the most diffused sources of energy where a photovoltaic cell is the corresponding energy harvester.

Among the other sources, kinetic energy harvesting is another important technology where a vibration energy harvester is the corresponding transducer.

It is important to note that only a portion of the available energy can be converted into electrical energy because of the dissipation during the conversion during which an amount of heat is produced.

#### Energy Harvesting vs. Energy Stealing

Energy harvesting represents the will to recover the energy already spread in the environment, and this is totally different from the concept of subtracting energy from, say, the motion of a vehicle.

If we place energy harvesters under the asphalt of a road to harvest electricity from passing cars, we define this concept as "stealing" rather than "harvesting". This is because we are subtracting energy from the motion of the vehicle. In this way, the vehicle will consume more fuel because of the energy "stolen" by the harvester. We can think about this as the vehicle making a very slight climb.

This can be explained with the first principle of thermodynamics, i.e., energy cannot be created or destroyed but it can be transferred from one location to another and converted to and from other forms of energy.

There are four renewable sources of energy: thermal, solar, electromagnetic and kinetic. Vibration energy harvesting converts kinetic energy into electric energy. It is not the most available in nature, but it can be a valid alternative to the solar one. During the night or inside a tunnel no light is available, but a machine while running can vibrate and these vibrations can be converted into electricity.

#### Using Energy Harvesting in an Electronic System

Dealing with energy harvesters is never easy. They inexorably impact the cost and the performance of electronic systems.

When an energy harvester is the only source of energy in a system, it generally means that a very efficient energy management system is required. Compared to the cost of a battery, an energy harvester is more expensive by orders of magnitude.

If we consider a coin cell battery-powered sensor, the cost of typical battery, such as a CR2032, is around one dollar and it gives 3 V. In order to replace this battery with an energy harvester like a solar cell, we have to take into account that the flux of energy is not constant and that energy storage is required. This storage can be represented by a supercapacitor or by a rechargeable battery but, in both cases, we need a charger and a voltage regulator.

This is the reason why an energy harvester powered system costs more and is more complex. But on the other side we have a theoretically infinite work life.

In order to understand why energy harvesting is important, imagine a very big bridge where many sensors are placed for structure monitoring. They should be energetically autonomous, small, light and capable of wireless communication.

These requirements are very common today because of the hassle associated with wired and connectivity for a sensor. Of course, no one wants to change the batteries, either, because maintenance is a cost.

Or imagine being in a very large and wild area where no power lines are available. Or imagine having to insert a sensor inside a structure (e.g., a column made of concrete or under the asphalt) so that you cannot extract it to change the battery.

The only economical way to power an electronic system for a long time in these situations is to use an energy harvester.

There are also some disadvantages to energy harvesting.

For example, the cost of an energy harvester can be high when compared with the overall cost of a wireless sensor.

Another con is that is not always easy to have a small converter. If we think about the size of a coin cell battery, today it's not easy to build an energy harvester with the same footprint that can provide a useful amount of energy. For the sake of comparison, a typical deep sleep current of a wireless sensor can be around one microamp. A vibration energy harvester the size of a AA battery can provide tens or hundreds of microamps at the most with accelerations of around 1 g. (These values vary a lot depending on the technology of the harvester, on the materials used, on the frequency distribution of the vibrations and on their peak to average ratio).

Moreover, generally energy conversion efficiency increases with the size of the generator. This is due to several factors, one of which is related to the fact that energy harvesters often produce an AC current which must be rectified. If we use diodes to rectify the current, we have to deal with the threshold voltages of the junctions; these represent an energy loss. The bigger the input voltage to the rectifier, the higher is the conversion efficiency.

Generally, we can say that efficiency can be evaluated with the following formula:

$$Efficiency = \frac{Output Energy}{Input Energy} \leq 1$$

### Energy Sources in the Environment

When we need energy for our system, we have to choose among several sources and we need to take into account several other parameters such as the cost, the availability of components, the impact on the environment, the energy density, the transportability, the possibility of energy storage, and the safety situation.

Generally, as a starting point, it is easier to divide sources of energy into two categories: renewable and nonrenewable energies.

#### Renewable vs. Nonrenewable Energy

Renewable energy sources can be easily defined as those that are naturally replenished regularly or over a relatively short time scale: biomass, hydropower, geothermal, wind, solar, etc. Other energy sources are nonrenewable: petroleum, natural gas, coal, uranium, etc.

Energy harvesting converts wasted energy from all available energy sources (renewable or nonrenewable) into electricity.

In all energy transformation, there is a certain amount of wasted energy because the efficiency of each energy converter is lower than 1. Everybody knows that a solar panel heats up when exposed to the sun in order to produce electricity. This heat comes from the light, itself, and it represents the wasted energy (together with the reflected from the surface of the panel).

By coupling a thermoelectric generator to the solar panel, a portion of this heat can be converted into electricity. This is mainly because it is not easy to establish large temperature differences from one side to the other of the thermoelectric generator.

### Energy Density for Different Energy Harvesting Technologies

The following table, from a Texas Instruments whitepaper, summarizes the density of energy for different sources and technologies.

TI states that “The most promising micro-harvesting technologies extract energy from vibration, temperature differentials, and light. A fourth possibility—scavenging energy from RF emissions—is interesting, but the energy availability is at least an order of magnitude less than that of the first three.”

Table 1. Energy harvesting estimates, from Texas Instruments

### Harvested power

Vibration and motion

Human

4 μW/cm2

Industry

100 μW/cm2

Temperature difference

Human

25 μW/cm2

Industry

10 mW/cm2

Light

Indoor

10 μW/cm2

Outdoor

10 mW/cm2

RF

Mobile phone

0.1 μW/cm2

Wi-Fi

0.001 μW/cm2

For example, for an application in a wild remote area, the most readily available energy source may be the sun, so a solar panel may represent the ideal solution for the majority of the situations. On the other hand, in a mine, there is almost no light and the temperature is almost the same between the rocks and the air, so it is impossible to use solar and thermal energy harvesting. But what about vibrations? If your purpose is to monitor mine carts, the vibrations of the carts moving on their rails could be converted into electricity to power their sensors.

Of course, it is completely possible to use more than one energy harvester at a time.

In my next article, I will go into more depth on the subject of vibration energy harvesting.