Industry 4.0 has increased the demand for embedded systems deployed in the industry. These systems monitor and control different parameters in an industrial setup and come up as an important factor for the industries to function. These electronic devices should work 24-7 without any interruption, which means the power supply to these systems should not be interrupted. The embedded systems generally run on a DC supply.
Although batteries are a reliable source of DC, they cannot be used in remote applications like mines, underground pipelines, etc. due to frequent replacement because of the power drain. Radioisotopes and other radioactive elements can provide energy for a long period of time but wider adoption of the same can cause damage to the environment. Hence, harvesting energy from conventional resources efficiently is very important for an electronic system’s smooth functioning.
Renewable energy sources like wind and light have been used to harvest clean energy for decades—but predominantly on a large scale. Scaling down these conventional methods of harvesting is possible, but careful considerations must be taken with respect to the complete system so that gain at one level does not come at the loss of efficiency at another; that may result in reduced efficiency of the complete system. Another disadvantage of these renewable sources of energy is the stability. The sources like wind, sunlight, or flowing water cannot produce the same magnitude of energy throughout as per the demand of the load, so a hybrid solution consisting of both batteries and energy-harvesting techniques can be used for optimal performance.
Understanding Energy Management Techniques
Energy-harvesting techniques can be used to derive power from renewable and environmentally friendly sources. These harvesters provide energy that is discontinuous, hence the power-management devices and control system must be designed keeping this in mind. A power management circuit is needed to interface the harvesting circuit with the electronic circuit. The function of this circuit is to control the power supplied by the harvester to the needs of the load. This includes changing voltage levels, maintaining current and voltage profiles, etc.
The power management circuit tracks the maximum power point to maximize the derived power from the harvester circuit. MPPT (maximum power point tracking) circuits are often used to set the transducer conditions such that maximum power is transferred from source to load. MPPT is generally implemented by a dedicated DSP that continuously monitors the system parameters like voltage, current, source, and load impedance to increase power transfer. Dedicated MPPT circuits are used for high-power circuits while low-power circuits use techniques like the hill-climbing method, fractional short circuit, and fractional open circuit to determine the maximum power point.
Storing Harvested Energy in a Hybrid Electrical Energy-Storage System
Hybrid electrical energy storage (HEES) is a mixture of electrical energy-storage devices like batteries, supercapacitors, etc., and energy-harvesting techniques to generate energy from sources like light, wind, movement, etc. HEES is an efficient way to power embedded systems for a longer period of time with greater uptime and efficiency. In this method, if the energy harvested is used by the embedded systems and the extra energy is stored in energy-storage devices like batteries—while on the other hand the energy harvested is less than the requirement of the load the excess energy is provided by the storage device—the amount of charge that can be stored and the life of the energy-storage elements depends on factors like the efficiency of the harvesting module, the input/output voltage level of the DC-to-DC converter, and the characteristic of the connected load.
The energy-storage unit generally consists of two different devices: batteries and supercapacitors. Nickel-metal hydride (NiMH) and lithium-ion (Li-ion) are the two most popular types of batteries. Li-ion batteries have energy density, moderate self-discharge rate, and suffer from no memory effect because of partial charging, but they require complex circuits for control. NiMH batteries suffer from memory effects but can be used directly without the need for complex circuitry. Supercapacitors provide a high charging and discharging limit suitable for applications requiring high currents in less amount of time.
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When harvesting is abundant, the goal of the system is to store as much energy as possible while supplying the varied requirements of the load. Task scheduling and dynamic voltage and frequency scaling are energy-efficiency techniques used to control the parameters of the load and the converter to maintain efficiency during the charging process. On the other hand, when the energy harvested is less, the loads need to be supplied from the storage elements. Task scheduling is again used to reduce the power requirements of the load as much as possible to extend the life of energy-storage elements.
Recent Developments
Researchers have been focusing on finding novel materials for the transducer design of energy harvesters as well as converters and sensor systems to increase the overall efficiency of the system. Another major drawback on which researchers have been focusing is the discontinuous nature of energy harvesters. Artificial intelligence and machine learning models can be used for predicting the availability of sources like light and wind for efficient and timely harvesting and storage of required energy. More advanced models based on exponentially weighted moving averages (EWMA) are being developed to predict the weather conditions more accurately.
In the case of wireless mobile devices, the transceiver module consumes the largest amount of power as possible to stay active, even when no communication is carried out, to check for incoming packets. To reduce power consumption, asynchronous wake-up transceivers can be used to activate only when they sense incoming packets.
Modern-day mobile electronics and wearables are characterized by a trend of size reduction, in particular the sensors that are implanted in the human body. Smaller storage devices are being researched to provide similar efficiency as conventional-sized devices. The recent advancements in the field of energy harvesting are providing great momentum to Industry 4.0 to provide a clean and long-lasting source of energy.