Reading time ( words)
Solar systems that capture, store and transmit solar power are a major focus of research across the world. However, the role of solar power systems used for solar-powered satellites that orbit our planet have received far less attention. As the role of satellites for communications, navigation, research and defense continues to grow, researchers from Khalifa University of Science and Technology (KU) and the Massachusetts Institute of Technology (MIT) say there is a need to address a number of unique challenges faced by satellite power systems.
“Benchmarks play an important role in comparing systems to develop standard designs that achieve desired performance efficiencies. Small, ground-based electrical grids, otherwise known as microgrids, have benchmarks, but we cannot apply them to satellites,” explained Dr. Hatem Zeineldin, Professor of Electrical and Computer Engineering.
He is part of a collaborative research team working to address this gap, made up of KU research engineer Samrat Acharya, KU MSc student Fatama Alshehhi, KU research engineer Omair Khan, KU research engineer Alex Tsoupos, KU Professor of Electrical and Computer Engineering Dr. Mohamed Elmoursi, KU Assistant Professor of Electrical and Computer Engineering Dr. Mohamed Al Hosani, KU Associate Professor of Electrical and Computer Engineering Dr. Vinod Khadkikar, MIT Professor of Electrical Engineering and Computer Science Dr. Steven Leeb, MIT Professor of Electrical Engineering Dr. James Kirtley, and MIT Professor of Electrical Engineering Dr. David Perreault.
“The loads for microgrids on the ground and in the sky are different, the photovoltaic irradiance is different because the PV panel is receiving solar irradiance very differently. That is why our team is hoping to be among the first in the power community to invest substantial research into developing a benchmark for satellite power systems,” Dr. Zeineldin added.
A preliminary paper on their initial work authored by the KU research team is to be presented at the upcoming Smart Energy Systems and Technology 2018, to take place in Sevilla, Spain, on 10-12 September.
Space and satellite technology are of particular interest to the UAE’s economic development. The UAE has a goal of sending an unmanned mission to Mars by 2021, making it one of only nine countries in the world with a program to explore Mars. Space is also one of the seven sectors targeted in the UAE National Innovation Strategy. The UAE Space Agency estimates that investments in space technology in the country have exceeded $5.4 billion.
Al Yah Satellite Communications (Yahsat), which is a wholly owned subsidiary of Abu Dhabi’s strategic investment company Mubadala, plans to expand its satellite fleet from 3 to 7 by 2020. With Yahsat’s announced acquisition of homegrown mobile satellite company Thuraya this year, it will have 5 satellites by end of 2018.
Microgrids In The Sky
“Essentially a satellite is a flying microgrid – a standalone power system. Our team has great experience in power electronics, power systems, control systems, electric machines, and communications, and we are extending our expertise with some of the concepts from the microgrid power system domain to the space domain to create an efficient and systematic approach to powering the satellites,” Dr. Elmoursi shared.
Satellites are built to perform various specialized and complex functions for government, industry, usually at a cost of millions of dollars. After being launched from earth they are positioned at low-, high- or medium-earth orbits, depending on their intended function, and are expected to perform for up to 15 years.
However, many do not achieve their intended goals, and often due to the electrical power system (EPS) failure. Data from insurance claims show that around 40% of satellite failures on-orbit are related to EPSs, which over a 22 year analyzed period, has resulted in losses totaling approximately $400 million.
In a satellite, the large horizontal wings that extend from its sides are photovoltaic panels that collect solar energy for the operation of various payloads and instruments. Some of the solar power also has to be stored for the periods when the satellite is ‘in eclipse’, out of sight from the sun.
Two energy storage technologies are typically used for this – batteries and fuel cells. The satellite’s EPS needs to reliably provide electricity directly from solar power and these storage systems. If anything goes wrong, the satellite is tens of thousands of kilometers away from the nearest engineer, and cannot be fixed.
Mini-Satellites and Major Power Challenges
“EPS failure accounts for the largest mission failure percentage for CubeSats in later stages of the mission, after 30 days of launch, and second largest in the earlier stage of the mission, before 30 days,” Acharya explained.
A CubeSat is a mini-satellite that has standard dimensions of multiples of 10×10×10 centimeters and a mass less than 1.33 kg. These devices are used for space research with specialized purposes like atmospheric research or radio communication. As small, modular, and relatively inexpensive satellites, they can be more easily carried into space, where they can be deployed independently or combined with other CubeSats to make larger satellites.
As part of the UAE’s strategic focus on developing its space sector, the Mohammad Bin Rashid Space Centre (MBRSC) and American University of Sharjah developed and launched its first CubeSat in 2017 - Nayif-1. Khalifa University is working with the UAE Space Agency and the American University of Ras Al Khaimah (AURAK) to design another CubeSats – MeznSat 3U. The Masdar Institute, which later merged to become part of KU, designed its own CubeSat – MYSAT-1 – in 2017.
CubeSats face additional EPS challenges beyond normal satellites. Due to their small size, they are more susceptible to radiation that can interfere with electrical and other systems. Having the PV panels placed around the cube’s six sides means that sun’s rays will not always fall directly on the panel faces, resulting in reduced solar power absorption. This issue is compounded by the fact that a CubeSat spins on its axis while it circles the earth, so the PV panels are constantly moving in and out of solar irradiance.
Shooting for a Standard
The team’s early research has already illustrated the impact of factors that affect the design of the ideal satellite EPS, including the nature of the satellite mission and the satellite orbit. They are further exploring how to integrate self-healing technologies into the satellite EPS, allowing it some ability to repair itself in case of malfunction. They are also looking to capitalize on innovations in communications, to implement the master-slave approach in satellites, where one satellite helps in the management of data and communication for a set of satellites.
“The goal is to develop a satellite EPS that is modular, so that it can fit multiple missions. The benchmark will be the starting point for looking at the power electronics design, fault management, recovery systems, actuator design, as part of developing new power electronic topologies that are more efficient. This will help us optimizing the design of the electrical system. Once we have a model, and we publish it, it can serve as a benchmark for space applications,” Dr. Zeineldin concluded.
With a modular EPS, satellite and CubeSat developers will no longer have to create customized power systems for each mission, and can build off of a basic plan that is optimized for the space environment and satellite requirements. This may help to reduce the high rate of EPS-related satellite failure, and improve mission success and efficiency.
The collaborative MIT-KU research’s team’s benchmarking efforts should provide the local and global satellite technology sector with a qualitative approach to electrical power system design in satellites of all sizes. Additionally, developing this satellite EPS expertise within the UAE will support the UAE’s space sector by training talented students that graduate ready to solve practical problems and innovate.