Exploring HAPS' Long-Duration Flight from an Energy Perspective
Energy Balance of HAPS
HAPS (High Altitude Platform Station) developed by SoftBank aims to provide high-speed, low-latency communications by flying unmanned aircraft equipped with communication devices in the stratosphere at an altitude of 20km, directed towards the ground. HAPS is not a satellite, but an aircraft, therefore requires energy to maintain altitude. In order to fly for long periods, power generation during flights, such as solar power, is necessary. This article will explain how HAPS operations are enabled from an energy perspective.
Solar Power Generation in the Stratosphere
The stratosphere, in which HAPS flies, is above the clouds therefore unaffected by weather. However, because it is closer to the earth than satellites, the earth hides the sun during nighttime. Until now, there has been almost no detailed measurement data on the amount of solar power generated in the stratosphere, which is halfway between space and the ground. In cooperation with several partners, SoftBank measured the amount of electricity generated and the solar spectrum* in the stratosphere. As a result, it was confirmed that the attenuation of sunlight due to the atmosphere was negligible. The stratospheric sunlight has approximately 1.36 times more light energy than that of the earth's surface (Air Mass 1.5**) and is particularly intense in the ultraviolet region below 400nm. This behavior is almost the same as in outer space (Air Mass 0) even though there is still enough atmosphere.
*The solar spectrum represents the distribution of light emitted from the sun at different wavelengths. This spectrum is divided into three main ranges: ultraviolet (UV), visible (Visible), and infrared (IR), and is an important component of sunlight-using technology and research, especially in solar cells, meteorology, and agriculture.
**AirMass is a measure of the thickness or distance of the atmosphere through which sunlight passes. Basically, AirMass 1 (AM1) is defined as the earth's surface when the sun is directly above (at zenith), and the value of AirMass increases as the sun goes lower. As this value increases, more sunlight is scattered and absorbed by particles and moisture in the atmosphere. Typical daytime conditions in mid-latitude regions are considered to be AirMass 1.5 (AM1.5), which is used as a standard for evaluating solar cell performance. Space has no atmosphere at all and is therefore AirMass 0 (AM0).
Simulation of Power Generation
Since HAPS is entirely powered by solar energy, lack of power generation is a serious risk which would affect flight continuity. To simulate the amount of electricity generated by solar cells, an onboard solar power simulator for vehicles can be used. The atmospheric and solar radiation models are modified to be appropriate for the stratosphere, shading effects such as those that occur on the ground are excluded, and 3-D motion and airframe deflection are added to the calculation. For high accuracy, calibration with actual measurements is required. SoftBank estimates the amount of electricity generated when flying in the stratosphere at any given point with high accuracy, based on actual measured data in the stratosphere collected independently, taking into account the aircraft shape, solar cell specifications, latitude, longitude, date and time.
Simulation of Power Consumption
While most aircraft aim to maximize flight speed and distance traveled, HAPS is intended to be optimized for circling flight, also known as loitering, in which the aircraft stays in a certain airspace for a long period of time. Therefore, the airframe is designed to minimize propulsive energy, similar to human-powered airplanes, high-performance gliders, and competition model airplanes.
For example, to calculate the power consumption when the flight altitude is fixed at 20km, the specifications of the aircraft could be set as follows. Wingspan is 80m, wing area is 200㎡, lift-drag ratio is 32, lift coefficient is 1.0, and electro-mechanical conversion efficiency is (0.85*0.95). Furthermore, the altitude is 20,000m, the air density is 0.0726, the airspeed is 34.5m/s, and the total weight of the aircraft is 1000kg.
The average power consumption of the aircraft is approximately 14kW, resulting in a daily power requirement of 336kWh. To enable flight, it would be necessary to acquire additional energy beyond this amount at the specific coordinates and time of flight. If there is a shortage, it would be necessary to reconsider the specifications of the aircraft.
Energy Balance Diagram
Below is a diagram of the energy balance. Here, the amount of electricity generated and the remaining battery power are represented. During the daytime, the altitude is maintained while generating electricity, and excess energy is used to charge the battery. After sunset, discharge from the battery begins (time A) and continues until sunrise. At sunrise (time B), the battery is not fully discharged to maintain the remaining capacity C in case of unexpected strong winds or other risks.
This article focused on the basic energy balance for operating HAPS in the stratosphere. The HAPS aircraft developed by SoftBank is equipped with multiple solar panels on the wing surfaces of the solar plane to enable sustainable long-term autonomous operations. In the past, there were similar aircrafts based on a similar concept, but at that time, the energy balance was challenging with the technology available, making it difficult to reach the practical stage. However, advances in solar panel and battery technologies, along with energy-efficient design of the aircraft and the use of advanced lightweight materials, have significantly improved overall energy efficiency. Thanks to these achievements, providing services through HAPS has become a realistic option.
SoftBank will continue to pursue further innovation and conduct research and development towards the early realization of HAPS.