Part 1
Richa Dham and Sachin Gupta, Cypress Semiconductor
Without any doubt, Internet of Things (IoT) is evolving to make human life easier. Technology is enabling human to connect to objects that may be sitting miles away from them. With this number of electronic devices around us are increasing exponentially. Some of these devices have luxury of wall power. On the other hand, other lack availability of wall power as either they are portable or laying electrical wiring is not economically viable. Majority of these devices are generally powered using a battery.
This article is a two-part series and in the first part we will discuss the challenges in using batteries in an IoT system and a battery-less solution by using energy harvesting system. We will talk about various Energy Harvesting devices and the blocks that constitute the Energy Harvesting System. In the second part of this article we will take an IoT example and run through the design challenges for same.
Challenges in operating IoT devices using batteries
Battery replacement– A lot of IoT devices for example wireless sensor nodes are installed at locations that are not easily accessible. Also, some applications like beacons that are used for asset tracking in a warehouse or to advertise promotional offers in a retail store involve several devices. So, either physical limitations or volumes make it challenging to replace batteries regularly.
High running cost–Initial cost of product may look attractive in a battery powered device. However, running cost may go very high due to regular cost incurred by new batteries. Also, if batteries are replaced by some maintenance engineer, it would invite additional expenses.
Waste management–As most batteries use hazardous material, they are not environment friendly when it comes to their disposal. Precautions shall be taken based on the battery type that need to be disposed-off. Based on country, additional cost may be levied on new batteries to take care of disposal expenses. Irrespective of the cost, with the number of devices going high and hence number of batteries; environment is getting polluted.
The only solution to these problems is the use of renewable energy. Usage of energy harvesting devices (EHD) can help in designing battery-less system, replacing primary battery with a rechargeable battery that would need to be replaced less frequently than primary batteries or using supercapacitor for holding charge that does not need to be replaced.
Energy Harvesting Devices
Energy harvesting devices convert other form of energy i.e. solar energy, kinetic energy, thermalenergy etc. into electrical energy. If any of these sources are available, a device can be powered without using a battery.
Some of the commonly available energy harvesting devices that are usable for IoT applications are:
Solar cells– Solar cells are one of the most commonly used energy harvesting devices not just for IoT but also for electricity generation for commercial purpose/powering buildings. Each solar cell produces very small output voltage. So, solar cells are generally connected in series to increase the output voltage. Output power from each solar cell depends on the illumination. Series connected solar cells modules are available off-the-shelf and can be integrated in a system easily. Solar cells are useful in applications where light is available for example beacons in a retail store that need to transmit only when store is open and hence illuminated. Another example can be wireless sensor nodes that either have sunlight available or are placed in environment that has sufficient artificial light. Figure 1 shows a solar powered Beacon that uses a 15 x 15 mm solar cell that can power the beacon at as low as 100 lux illumination.
Thermoelectric generator – Thermoelectric generator (TEG) produces a voltage difference based on the temperature difference between two surfaces. These are solid state devices that work on the principle of the Seebeck effect. Most of the devices around us, including human body have a temperature difference with respect to ambient/room temperature. This temperature difference can be used to generate usable electrical energy to power IoT devices.
Piezoelectric generator – Piezoelectric generator produces a voltage difference based on the mechanical stress across its surface. So, if there is a source of vibration, it can be used to produce electrical energy using piezoelectric generator.
Electromagnetic Generator – Electromagnetic generator also produce electric energy based on the vibration/movement. This type of transducer uses a coil and a magnate.
Each of the energy harvesting devices are capable of generating different amount of output power. For example, an indoor solar cell can produce just few micro-Watt while a thermoelectric generator or an electromagnetic generator may produce 10s of mW power. On the other hand, solar cell may be easy to install in most application while vibration or heat based energy harvesting devices may bring mechanical challenges. So, one must carefully look into energy requirement, available source of energy and mechanical challenges, if any.
Energy Harvesting System (EHS)
Energy harvesting devices generate intermittent power that cannot be fed to a load directly. Several other components are required to obtain a stable output at required voltage. Figure 2 gives the high-level representation of an energy harvesting system. Out of the three blocks shown in figure 2, we have already discussed about energy harvesting devices. Output of these energy harvesting devices is fed to an energy harvesting power-management integrated circuit (PMIC). Output of these energy harvesting PMICs is a stable output power that is then stored in an energy storage device.
Energy harvesting PMIC – Along with energy harvesting devices, it is important to choose right energy harvesting PMIC and energy storage device to reach to a solution that meets cost and power requirements. Energy harvesting PMICs are available in several variants and some of them are optimized for a particular type of energy harvesting device. So, selection of an appropriate energy harvesting PMIC can generate stable power efficiently with possibly fewer components and hence lower cost. At high-level, energy harvesting PMICs can be divided based on the convertor topology that they use – buck convertor and boost convertor.
Buck convertor – PMICs based on this topology are mainly used with series connected solar cells, piezoelectric generator that produce output voltage that is generally much higher than the required supply voltage of the load.
Boost convertor – This type of PMICs are mainly used with thermoelectric generator or single solar cell. In these transducers output voltage is just few volts. Moreover, at lower temperature gradients and lower illumination levels respectively, this voltage can be 0.5V or lower for thermometric generator and solar cell. So, a boost convertor topology is used to bring this output to a level that can power system load.
Some of the PMICs come with integrated full-wave rectifier that are essential for piezoelectric and electromagnetic generators that produce an AC output. It eliminates the need of external components for these types of transducers. Also, it is important to examine input and output voltage range, maximum input and output current, start-up voltage and quiescent current specifications.
Some energy harvesting PMICs can switch between a primary battery and an energy harvesting device output to yield a hybrid solution that offloads battery when alternate power source is available and hence increase battery life. Also, energy harvesting system can be used to charge the battery and then battery can be used to provide uninterrupted supply when energy harvesting device is unable to produce sufficient output. Some PMICs have dual bridge-rectifiers and allow more than one type of device to be connected at the same time that can be useful in some applications where only one source is available at one time. Even if both sources are available most of the time, effective output increases.
Energy Storage Device – Stable output from PMIC is stored in an external storage device that is a capacitor in most cases. Selection of capacitor impacts the overall performance of power-supply system. Major factors that impact the selection of a capacitor in energy harvesting system are:
Capacity – Large capacity capacitors are required in application where abundant energy is available at some times with long breaks of unavailability. Another example is a system that requires large chunk of energy in one-go and instantaneous energy output from energy harvesting device is too small to meet that need.
Leakage – Leakage becomes an important parameter when capacitor needs to hold energy for long period of time.
Equivalent series resistance (ESR) – A higher ESR can cause significant drop in output voltage as load current increases and may not be acceptable in some systems. So, ESR shall be looked into carefully while selecting a capacitor. Some PMICs allow two type of capacitors to be connected in such a way that load is driven by one capacitor and second comes into picture only to charge first capacitor. So, first can be a low ESR capacitor and second can still be a high ESR capacitor.
Cost – Not to mention, cost plays an important role and it is important to keep a balance between end system’s performance and cost.
For low ECR and low leakage, ceramic and tantalum capacitors are a good choice. Ceramic capacitors have lower cost than tantalum capacitors. However, they are available in smaller capacity than tantalum. Multiple capacitors can be connected in parallel to increase overall capacity. Supercap become necessity if energy harvesting device’s output may be absent for long time. Though they offer maximum capacity, they have very high leakage and ECR. If Supercap needs to be used, it is good to use a PMIC that allows two storage capacitors. The load can be connected to low-ECR capacitor and Supercap can be used to store surplus charge and then later use that to charge low-ECR capacitor.
Conclusion
We discussed the general battery challenges that are leading to a need of battery-less solution in IoT domain and the fundamental Energy Harvesting system components.
In IoT applications, primary batteries increase running cost, maintenance cost and pollute environment. Energy harvesting system come handy to help eliminate primary batteries from the system or extend their life. There are choices of energy harvesting devices available and can be selected based on available energy source. Appropriate selection of energy harvesting power management integrated circuit can help in reducing component count and keep system cost. A capacitor is used to store the energy and selection greatly depends upon available energy, energy requirement and cost.
In the next part of this article we will discuss about how an Energy Harvesting System can be integrated in an IoT solution with an example.
About Authors
Richa Dham is Sr Marketing and Applications Manager with Cypress Semiconductor. Her interests lie in defining new solutions especially in the connectivity and IOT area. She completed her Masters in Technology in Communications Engineering from Indian Institute of Technology, Delhi (IITD).
Sachin Gupta is working as Staff Engineer Product Marketing with Cypress Semiconductor. He holds diploma in Electronics and Communication from Vaish Technical Institute and Bachelors in Electronics and Communication from Guru Gobind Singh Indarprastha University, Delhi. He has 8 years of experience in SoC applications and marketing.
