Building Blocks for Energy Harvesting - Madebydelta

Building Blocks for Energy Harvesting A short guide

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Energy Harvesting guide

Building Blocks for Energy Harvesting – a short guide The number of smart things is on a rise. Applications like smart city, smart home, Industry 4.0 require a lot of devices to be connected to the internet. Some estimates forecast 26 billion devices online by 2020. One major issue with wireless smart devices is their power supply. The use of batteries can have some significant shortcomings. Firstly, batteries have limited amount of available energy. In some applications, the size of the battery required to cover the lifetime of the product would be too large and too expensive. The cost of exchanging batteries in thousands of devices, especially if they are in hard-to-reach or hazardous places, can make the entire sensor deployment cost prohibitive. Secondly, in US alone over 3,000,000,000 batteries are used each year. As the number of IoT devices reaches tens of billion units even more batteries would be required and later discarded, thus resulting in a significant environmental footprint. This guide will provide you with the basic information you will need to comprehend the basics of how energy harvesting is conducted. For further information please visit www.idemolab.com.

2 / Building Blocks for Energy Harvesting – a short guide


Energy harvesting as an alternative energy source In application scenarios where a typical battery cannot fulfill the lifetime requirement an alternative powering solution is needed. The process of collecting energy from the ambient environment and converting it to electrical power is referred to as energy scavenging or energy harvesting(EH). With the advancement of materials and technologies a wide spectrum of energy sources can be utilized for powering IoT devices. Energy harvesting can be used to supplement the existing battery or it can be used as a sole power source for the device. Either way the first step in evaluating the EH solution is to determine the power consumption of the application. The next step is determining how much energy is required for the running the application for the targeted lifetime. Based on the results from the first and second step an approximate size and complexity of the energy harvester could be determined. These calculations would result in the cost and size of the energy harvesting solution which can be then compared to the battery solution in the terms of size and cost.

Sensor

It should be noted that the total cost of the battery should include the cost of holders, connectors, mounting during assembly and integration, as well as the cost of the battery replacement if it is planned in the products lifetime. The battery replacement cost is typically a driving factor and it includes the price of a new battery, the cost of time and effort to replace the battery as well as the cost of lost productivity while the battery is being changed. There is also the environmental impact of disposing of batteries that has to be taken into account. In order to design an energy harvesting powered system the following steps are required: 1. Selecting the type of harvester 2. Selecting the power management for the harvester 3. Selecting intermittent storage This guide has a goal to introduce the various building blocks required to power a system from energy harvesting, as shown in the figure beneath, as well as the design parameters of these blocks.

MCU

Comm.

Energy Mgmt

Energy Harvester

Figure 1. Buildingblocks for Energy Harvesting


Power Mgmt

Storage


Selecting the type of harvester The environmental conditions where the application is deployed is going to determine the type of harvester to be used. The Table 1 summarizes the typical characteristics of several types of EH. There are other types of harvesters that are been reported in research that are using triboelectric effect, biochemical potentials, radioactive decay etc. however they are still in development phases and are hence not directly useable in end products. Our focus in this section is going to be primarily on commercially available energy harvesting power sources that can be used in commercial products.

Kinetic

Thermal

Solar

Power output

uW range for low small devices (1x1x1cm) mW range for larger devices (5x5x5cm)

1 mW/cm2 for 5 °C 400mW/cm2 for 100 °C difference

2uW/cm2 indoor living room 20uW/cm2 office space 20mW/cm2 outdoor direct sunlight

Energy source characteristics

Harmonic or stochastic vibrations Vibration spectrum Variability of amplitude Variability of Frequency

Thermal resistance of material attached to the TEG on hot and cold sides Cooling capabilities Temperature range of the source

Light levels available Spectrum of the light Angle of the light beams Existence of Partial shading

Pros

Easy installation Not dependent on outside factors, can be contained inside the power source

No moving parts

Very high energy density in outdoor applications No moving parts

Cons

Large size Frequency dependence Doesn’t scale easily Complex design High price

Low efficiency Can require big heatsinks to maintain large difference Requires proper thermal contact to the source

Needs to be kept clean Light levels change exponentially resulting in very large cells for indoor use

Examples

Suspension on a train Railroad monitoring Machine health monitoring

Recovery from exhaust heat Hot pipeline monitoring

Calculators Solar farms Battery chargers

Table 1 Comparison of EH power sources



Using light as a power source

Light is a very common power source. However, the light intensity, which directly corresponds to the amount of generated power, varies significantly. Under direct sunlight, depending on the location on the earth, about 130,000+ lux illuminates the solar panel which is equivalent to 1kW/m2. On the other hand, in a typical indoor illuminance levels are in the range of 30-50 lux and go as low as 5 lux, which results in low microwatt power outputs. The amount of light energy converted to useable electric power depends on the efficiency of the solar cell and the spectrum of incoming light. The efficiency of the solar cells on the market is currently directly tied to their price. Cheap amorphous silicon solar cells typically have efficiencies in the range of 8-11 % while more expensive crystalline solar cells can reach 23%. However, it should be noted that the solar cells don’t have the

Figure 2. Solar cell power output as a function of available light levels 5 / Building Blocks for Energy Harvesting – a short guide

same efficiency over the entire light level range. The efficiency advertised for the solar cell is typically measured at optimal conditions. In indoor conditions, a cheap amorphous solar cell can outperform a higher nominal efficiency monocrystalline solar cell as shown in the graph below. The spectrum of the light source can have a significant effect on the solar cells. For example, amorphous silicon solar cells are better at harvesting visible portion of the light spectrum while crystalline solar cells are more efficient typically in the red and near infra-red part of the spectrum. This is especially important when designing systems for indoor use, as the output of the solar cell under indoor light conditions can change almost two times depending on the source of light. For example, and indoor amorphous silicon solar cell at 150 lux level can produce 2x more output under LED light compared to a halogen light source.


Using movement as a power source

In the applications where light is not present or the solar cells cannot be used, like in an industrial setting or clothing other power sources are required. One such source is converting kinetic energy into electrical power. This can be done in many ways, most widely used methods are: 1. 2. 3.

electromagnetic induction piezoelectric effect electrostatic and triboelectric effect

Electromagnetic induction has been the primary method for converting motion into electric power and can be seen in range of sizes from bicycle dynamos to large scale generators. The electromagnetic is typically used in commercial vibration energy harvesters due to its large power output, robust design and high reliability.

Another interesting use of the electromagnetic induction is in impact harvesters. Here a coil and a lever are connected in a way that pressing on a lever makes a sudden change in the magnetic field through the coil thus generating power. The generated amount of power is typically sufficient to power a simple sensor that can do some sort of a measurement and send a wireless transmission of the sensor data. Beside electromagnetic induction there are other methods for converting mechanical energy into electrical energy. Piezoelectric generators are based on the piezoelectric effect where the strain/pressure is converted into electric power. One of the benefits of piezoelectric generators is that they can be built smaller and cheaper compared to electromagnetic harvesters. However, they also produce smaller amount of power.

The downside of the electromagnetic energy harvesting is that typically the harvesters are large and expensive. Electromagnetic generators cannot typically scale down to small sizes since magnets don’t scale down well. Power generated as a function of acceleration for several commercial harvesters can be seen in the figure below. Some of the manufacturers of vibration harvesters are: Perpetuum, Kinergizer, and ReVibe. Figure 3. Piezo element.

Figure 4. Power output per volume of different electromagnetic harvesters as a function of input acceleration. 6 / Building Blocks for Energy Harvesting – a short guide


Commercial energy harvesters based on piezoelectric effect are entering the market however they are not as prominent as electromagnetic harvesters. The lack of piezoelectric options is due to reliability issues, after all this is a ceramic that is being compressed and stretched tens and hundreds of times each second. An example of application powered by piezoelectric generator is a remote control by Philips that uses the button presses to generate power to communicate with a TV set. The company EnerBee has developed a rotational harvester that is combining magnets and piezoelectricity to build a generator that converts rotational motion into electric power. An interesting feature is that this harvester operates in steps producing a fixed amount of power step in rotation allowing power generation even for very low RPMs. The device is producing 0.5mJ per one full rotation. A new type of kinetic energy harvesting method that has received a lot of attention lately is triboelectric harvesting. The triboelectric effect is a type of contact electrification in which certain materials become electrically charged after they come into frictional contact with a different material. Rubbing glass with fur, or a plastic comb through the hair, can build up triboelectricity. The strongest benefit of this type of harvester is their low price, high durability, flexibility and possibility to scale from cm2 device to 10’s of m2. These generators have been reported to reach as much as 300W/m2 and volume power density of 400kW/m3. There are still no commercially available triboelectric harvesters, however there is a large interest in them as they can be used in disposable electronics, powering of smart fabrics, floor power generation, ocean wave power generation, and other applications.

Furthermore, if the kinetic energy is distributed over several vibration frequencies the harvester would typically only be able to collect energy form one of those. Major research effort is placed into designing broadband harvesters or harvesters that can be tuned in order to remove this limitation of the system. Discontinuous energy harvesters are not limited by the frequency, however their output is tied to certain events in the environment. For example, pressing a button, turning a wheel 90 degrees, stepping on a floor tile etc. These harvesters are designed to deliver as much energy as possible during the occurrence of the event and can benefit applications where these events need to be detected, or to power a sensing system continuously if the event repeats often enough. Using temperature differences as power source

When no light nor movement are available we can look for a thermal gradient to produce power. Thermoelectric effect occurs when a temperature difference over a junction of two different materials produces a charge disbalance thus providing electric current. It should be noted that this type of energy harvesting is very inefficient, typically 2-5 %. Although inefficient, thermoelectric generators (TEGs) can still provide sufficient power for an IoT application from even low thermal differences of few degrees that can be found between the ambient air, warm pipes, human body, machines, etc.

Operation of kinetic energy harvesters

The kinetic energy harvesters can be divided in two main groups based on their method of operation: ·· Continuous (resonant) energy harvesters ·· Discontinuous (impact) energy harvesters Continuous energy harvesters are typically designed to extract the energy by being at resonance with the vibration of the body they are connected to. In this way maximum mechanical power is transferred from the body to the harvester, thus maximizing the power output. However, this typically results in a narrow bandwidth of the frequencies where the harvester generates power output. This is a major drawback of resonant harvesters because even minor changes in the vibration spectrum would have a significant impact on the power output of the harvester. 7 / Building Blocks for Energy Harvesting – a short guide

Figure 5. The temperature difference between your hand and the air can provide enough energy to send out small information packets from a wireless sensornote.

At higher temperature differences TEGs can provide hundreds of watts. Example applications would be generating power by recovering waste heat from internal combustion engines, exhaust pipes, industrial processes with metal melting, and metal works etc. Typical power outputs of TEGs depending on the thermal gradient over them can be seen in figure 6.


Figure 6. Power output of thermoelectric generators for several different TEGs exposed to various thermal differences.

There are two main challenges when designing the system powered by TEGs. First is size second is voltage output from the TEG. Maintaining the temperature difference over the thermoelectric generator typically requires large heatsinks resulting in a bulky product. This is one of the main challenges when it comes to using TEGs for powering wearable devices, although there are heat powered watches available on the market and a heat powered flashlight these are more exceptions than typical cases. Another challenge when working TEG devices at low temperature differences is their low voltage output. At a few degrees kelvin over TEG, the output is typically in the 10’s of mV range. Such small voltages require specialized circuits to boost the voltage to a useable voltage levels. We will address this issue in the power management section.


Using ambient radio energy as power source

Harvesting of the ambient radio waves present from the WiFi and mobile phone networks has been shown as feasible. However, the amount of energy is very limited, depends strongly on the environment and typically requires large antenna surfaces which might not be practical for majority of applications. This type of harvesting typically provides small amounts of power, equivalent to the amount of power that a small coin cell battery can provide for years. Therefore, the RF harvesting requires rather special applications where size is not an issue and the target lifetime is beyond 15 or 20 years. Wireless transfer of power, as in RFID, is not considered here as it is not based on ambient energy. In these systems there is an active transmitter that is powering the sensors.


Selecting the power management for the harvester Utilizing energy harvesting generators comes with a set of challenges that prevent it from being a simple drop in replacement for the battery. The output characteristics of the generator vary significantly depending on the type of the generator used and availability of ambient energy. Furthermore energy harvester output is seldom directly useable by sensors or microcontrollers hence different types of electrical interface circuits are required. These interface circuits should convert the output voltage from the harvester to levels useable by typical electronic circuits. The outputs from different EH have drastically different voltage characteristics. For example TEGs at low thermal differences generate 10s of mV requiring elaborate step up converters if there is no battery to pre-charge to step up converter. On the other hand, TEGs exposed to high thermal differences can produce voltages of up to 10V requiring step down converter in order to interface with typical electronic circuits. Some harvesters can be designed to have outputs that are

Figure 7. Energy harvester power management interface circuits. 9 / Building Blocks for Energy Harvesting – a short guide

closer to the operating voltages of application circuits. For example, by stacking multiple solar cells the array can have its maximum power point in the range of 2-3V which coincides with the operating voltage of majority circuits. Piezoelectric generators, especially the discontinues ones, can produce short bursts of very high voltage that require a completely different approach to handling compared to previously discussed scenarios. Additionally, in order to maximize the power generated by the harvester, the interface circuit should maintain the output of the harvester at the maximum power point. In some applications, it is also necessary to track the change of the maximum power point as the environmental conditions change. An energy harvesting interface circuit can be built out of discrete components however PCB real-estate for such circuits are typically high and require significant number of components. There are energy harvesting power management integrated circuits that have been designed to work with certain types of energy harvesters available from Texas Instruments, Linear Technology and STmicroelectronics, to name a few. An overview of some circuits, and the harvesters they are aimed to work with, is presented in the figure below.


Selecting intermittent storage

storage compared to supercapacitors. Batteries have lower leakage and a flat discharge curve providing a more stable voltage source and ability to discharge them completely with minimal efficiency penalty.

In a typical scenario energy harvesting generators cannot provide sufficient power to cover the active power consumption of an IoT application. Therefore, the power should be accumulated before the application task can be executed. Typically, two types of storage are used: Supercapacitors or rechargeable batteries.

On the other hand, batteries are coming with several drawbacks that have to be mitigated in the design. First, battery voltage must be controlled as under voltage and overvoltage conditions can permanently damage the battery. The depth of discharge has a significant impact on the number of times the battery can be recharged thus dictating the lifetime of the battery. Depending on the application needs the lifetime of the battery can be traded with the storage capacity as batteries that are charged to 60% can withstand several times more chargedischarge cycles compared to batteries that are charged to 100%. Lastly, majority of Lithium based batteries have safety issues to thermal runaway in the case of damage or short circuit that results in the battery catching fire.

Supercapacitors are used in applications where long lifetime is expected as they can be charged and discharged over 100 000 times without performance degradation. Another benefit of using supercapacitors is no shipping restriction and they are safe, compared to lithium batteries. Supercapacitors typically have low series resistance allowing for high discharge currents required for example during radio transmissions.

Another option is to use the energy harvester to supplement the battery. In this scenario, a primary battery can be used as a power source while the energy harvester is providing power whenever ambient energy source is available. The level of battery extension depends on the environmental conditions. Energy neutral operation

Due to unpredictable nature of the energy source availability, the energy harvesting system can seldom be used as a direct battery replacement. The variable availability of the energy is in stark contrast to the constant availability of energy in a battery powered system.

Figure 8. Supercapacitor.

One of the drawbacks of supercapacitors are relatively high leakage current that can be in μA to mA range, depending on the voltage and capacitance. Another drawback is the linear discharge curve, typical of a capacitor, which results in not all energy being available for use without additional converter circuits which is effecting the efficiency of the stored energy use. Furthermore, supercapacitors have a low energy density requiring about 10x more volume compared to a battery with the same energy storage. Rechargeable batteries provide a good solution for size constrained applications and are cheaper per unit of energy


In order to utilize the maximum potential of the energy harvesting source the application should be aware of the available energy and schedule its execution accordingly. The complexity of scheduling can vary depending on the application. In the simplest form the application is executed each time a certain amount of energy threshold is reached. A more complex scenario can have the application waiting for onset of high ambient energy availability in order to do an energy demanding task. An example is a solar powered system where computationally heavy tasks are executed during the day when the light is available, while less demanding tasks are done during night when running on stored energy.


Conclusion Implementing energy harvesting systems is significantly more complex compared to battery systems, however the removal of the battery and reduction/removal of maintenance can have benefits that outweigh the design and implementation cost. Energy harvesting has a solid potential to provide long lasting IoT systems that can operate unattended for extended periods of time.


This guide was aimed at introducing the basic building blocks of an energy harvesting system and introducing some of the design challenges and decisions that needs to be taken to develop a successful energy harvesting powered system. If you want to hear more about how to get you and your device started on Energy Harvesting feel free to contact us for an informal conversation. Please visit www.idemolab.com for contact information.

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