mobile energy



Mobile or on-board applications have special constraints on energy supply compared to traditional equipment. Due to their mobility, these systems often cannot be wired to conventional power supplies during operation. Therefore, it is necessary to place the power supply on the processor's board, but this adds bulk and weight to the system. It is this main reason that drives the demand for micro-energy.What are the energy requirements for mobile applications?

Several criteria such as energy level, power level, primary energy (power generation) and secondary energy (conversion), energy storage and charging with ambient energy, and remote power supply can be used to classify micro-energy.

We can divide micro-energy sources into two categories according to the power requirements of the system on the power source.

1. Energy requirements for mobile applications of "micro" power (su-Watt)

The power consumption of the extremely small power supply system can be as low as microwatt level, and it is often used in distributed or onboard sensors to capture and transmit information, that is, to detect the system status and transmit it to the remote monitoring center in real time. Due to the development of microelectronics technology (now developed to nanotechnology), the power consumption of sensors has been greatly improved, and now some commercial sensors consume only 10~100mW, the energy requirements for mobile applications of "micro" power are also very large.

Micro sensor

The miniature sensors currently being developed are not only greatly reduced in size (the size of the entire chip is reduced from 1mm2 to 0.1mm2), but also the power consumption is also greatly reduced. The current deformation detection technology can reduce the average power consumption of the designed sensors (such as accelerometers, gyroscopes, Hall sensors, etc.) to the microwatt level. Electronic technologies such as information processing and transmission have also made corresponding progress, but the main problem at present is whether the collected information should be processed locally and then sent intermittently, or the information detected by the sensor should be continuously sent to the remote monitoring Center?

Different application requirements determine different energy requirements,  which need to correspond to relative energy requirements for  mobile, and sometimes simple changes in application conditions can cause problems in power supply. We illustrate this problem by taking the automatic switch as an example. This switch only needs to transmit "on/off" information to the controlled device, and it can obtain energy from the environment by converting the energy generated when a person presses the operating switch into electrical energy. At first, the user might simply send a "start/stop" message to the device; then, the user would encode the switch to distinguish the actions of other switches; and finally, it might even manage to have the switch receive feedback from the system to Verify that the system has received the "start/stop" message and is able to resend the message if necessary. This last case requires a larger power supply than the first, because it not only consumes more energy, but also requires energy storage devices to operate differently when necessary.

Obviously, the power supply methods of practical application systems are varied, and are not limited to sensor systems. Various biomedical devices, such as insulin micropumps or pacemakers implanted in the human body, are typical applications. The figure below shows the power range required for different systems to operate.

Power range required by different electronic devices

Some onboard systems may require large amounts of power (several watts), but operate in a pulsed or intermittent fashion. For example, a mobile phone needs to consume 1~3W of power when talking, but only needs a few milliwatts when it is in standby. For equipment that operates in an intermittent manner, power is often not the most important constraint, but the energy required for continuous operation.

2. Energy requirements for  mobile applications of "high" power (a few watts)

Larger application systems require greater energy requirements, often requiring tens of watts of power, such as micro robots, micro drones, and various high-tech products (such as notebook computers, etc.). In the field of microelectronics technology, the performance of components and the development of Moore's Law, with the rapid development of notebook computer functions, its sales are also increasing. Unfortunately, Moore's Law doesn't apply in batteries. Although some alternatives to batteries (such as micro-fuel cells) have emerged, their performance is only an order of magnitude better than current batteries. Therefore, the development of mobile devices in accordance with Moore's Law (or "Super Moore's Law") is problematic, unless electronic components can improve performance without additional energy consumption.

3. Energy requirements

In the previous two subsections, energy requirements were expressed in terms of power. However, for mobile systems, energy is often a more important parameter than power because it determines how long the system can continue to operate. The energy requirements of mobile systems span several orders of magnitude, from nanojoules (nJ) to megajoules (MJ). In addition, for different application systems, the energy units used are also different.

micro drone
Technological performance evolution process of IT industry

For example, nanojoules are often used in wireless communication, and in the battery of notebook computers, it is expressed in ampere-hour or milliamp-hour and voltage.

Thus, in system applications with low power consumption levels, We can calculate the energy requirements for transmitting information. For example, in a wireless communication system, the current power consumption is about 5~20nJ per 1 bit of information transmitted. In some industrial equipment, it is often necessary to transmit several 8-bit coded information with redundancy, and the transmission distance is tens of meters, and the energy requirement is about 500µJ. This level of energy requirement is often found in industrial wireless communication systems such as ZigBee.

There are some electronic devices that, although consuming very little power, often require a lot of stored energy. For example, a pacemaker consumes 25 μJ of energy per pulse. But in its life cycle of more than ten years, the energy requirement of the battery is about 20kJ (2.8V, 2A h).

For unmanned aerial vehicles (UAVs) or laptops, depending on their operational requirements, a compromise needs to be made between the power and energy storage characteristics of batteries. A notebook computer usually needs to work continuously for 2~6 hours, and the corresponding energy is several hundred kilojoules (the battery of Dell D600 notebook computer is 4.7A·h, 11.1V, 180kJ). A drone needs to fly for 10min~2h depending on the mission and terrain, and there is a weight limit on the battery, which is more demanding than a laptop. In this application condition, a very important performance index of the battery is the specific energy.

Taking lithium polymer batteries as an example, under the extreme conditions of use, the currently available specific energy is about 140W·h/kg. When the specific energy of this level is obviously insufficient, and the increase of energy cannot be obtained by charging, other batteries with higher specific energy, such as zinc-air batteries, can be used, and the specific energy is as high as 370W·h/kg (zinc-air batteries). Batteries are now used in some hearing aids)

4. Duration of meeting specific power requirements

As we have seen, the appropriate one should be selected from a number of possible power supply technologies, which may vary depending on the specific application. Among them, the selection principle based on the expected continuous working time is very worthy of attention.

Take the electric system of a micro drone, for example, which requires 15W of continuous power to operate. The choice of power source is different depending on whether the working time must last 10min, 30min or 1h. For a specific battery energy storage technology, power and stored energy are directly proportional to the mass or volume of the battery. For the energy that uses fuel, the power required by the system determines the mass and volume of the power converter, but the continuous working time determines the amount of fuel that needs to be carried. Depending on the conversion efficiency of the fuel and the converter, as well as the type of fuel on board, the equipment filled with fuel can be more or less bulky.

Unmanned aerial vehicle UAV

The diagram below illustrates such an energy supply selection idea that if you simply keep flying, you only need to consider the total mass of the generator and converter capable of delivering 15W of power. Next, depending on the efficiency of the system, the amount of fuel required to meet the continuous operating time must be considered. In this example, we noticed that drone lithium-ion batteries are more suitable for short-duration (several minutes) missions; for 8-20min missions, it is more reasonable to use micro-turbines and generators for energy supply, while For missions longer than 20 minutes, it is more effective to use micro thermoelectric generators. Here we perform a simplified theoretical calculation based on continuous runtime. This example is not intended to categorize the different energy supply technologies listed, but rather presents some parameters that must be considered when selecting an energy supply technology that matches system needs.

The relationship between the mass (g) required by different power supplies and the continuous running time

The figures given in the figure are only schematic, and the data used for the calculation (eg power, efficiency, volume, volume) refer to the results of a brief analysis in a book published in 2003. Therefore, these data appear to have changed a lot today, and depending on the current state of development, the above options may also be very different.