Sustainable Energy for a High-Powered World

Professor Afridi with Ph.D. students Maida Farooq (left) and Firehiwot Gurara

How the High Frequency Power Electronics Group incubates environmentally positive innovations

As global warming increases and countries seek to lower carbon emissions, finding ways to access sustainable energy is paramount. Associate Professor Khurram Afridi and his lab, the High Frequency Power Electronics Group, are at the forefront of this quest to create methods and technologies that will harness the energy we need through environmentally positive innovations.

Afridi has garnered attention recently for his work on wireless recharging of electric vehicles while they are in motion. His research in this area has the potential to revolutionize transit in years to come. But electric vehicle recharging is not the only impactful project focusing on energy and sustainability coming out of the High Frequency Power Electronics Group. Several lab members are working on research projects with similarly far-reaching potential. 

“Maida Farooq and Firehiwot Gurara are both working on the forefront of high frequency power electronics,” Afridi said. “They are developing new power electronic technologies appropriate for MHz-frequency operation while also addressing important societal problems. Firehiwot and Maida are role models for the next generation of engineers driven by the passion to improve lives.”

Smaller, Cooler, More Efficient UPSs to Keep Data Centers Humming

As the world moves more and more to online applications—everything from cloud computing to Bitcoin and block chains—data centers are growing both in importance and in energy consumption. To make data centers more dependable and sustainable, Ph.D. student Maida Farooq has turned her attention to the systems that funnel energy to the servers.

“With the rise of artificial intelligence and machine learning and the shift to a cloud-based system, data has been growing exponentially over time,” Farooq says. “All that data is stored in the servers, and so the storage requirement is growing in proportion to the data. It’s projected that data centers will be consuming most of the energy of the planet in the coming years. Obviously all those servers will need power supplies.” 

Data centers count on Uninterruptable Power Supplies (UPSs), which supply power to critical loads by decoupling the servers from direct reliance on the power grid. A UPS will take power from the grid and process it so that it is smooth and acceptable for the servers. In addition, if an outage should happen, the UPS will continue to supply uninterrupted power via its built-in battery. “If we didn’t have UPSs, then data would be lost when servers lose power,” Farooq says.

Image of the proposed prototype
Side views of the proposed high power density online UPS prototype built in the lab, capable of achieving a power density of 26.4 W/in3. Image Provided.

Current UPSs have a number of problems, Farooq explains. One is their insatiable need for power. Another is their size; they take up a lot of space in the server racks that could better be utilized for the servers themselves, saving the data centers money. On top of that, the heat generated by all those UPSs means data centers need an inordinate amount of cooling. “If we are able to make more efficient, smaller UPS systems, we will be able to save a lot on energy consumption,” Farooq says.

Farooq has set her sights on reducing the size of the UPS by decreasing the size of the energy storage elements—the inductors and capacitors—inside it. “Their size scales down when we increase the frequency of the switches,” she says. “But it’s not that straightforward. The switches typically used are silicon based, and silicon-based transistors are not able to switch that fast. They have a lot of parasitics that come into play.”  

Parasitic capacitance and inductance—when unwanted capacitance and inductance effects are formed in a circuit—are essentially unavoidable in electronic devices. However, the recent emergence of gallium nitride (GaN) semiconductors has introduced a possible solution. “GaN devices are very good at switching at high frequency because they have much lower parasitics,” Farooq explains. “They allow us to switch much faster, and by doing that we can envision a way to miniaturize the UPS.” 

Online UPS inside a chassis
Online UPS inside a chassis for testing under practical conditions of a data center. Photo provided.

Using GaN switches, though, is only the beginning of Farooq’s UPS redesign. When switching to higher frequencies, the layout of the circuit board, and the control of the UPS, has to be compatible as well. “We can replace silicon switches with GaN switches in the circuit boards currently on the market, but they still won’t be able to switch at high frequencies because the way things are placed is not compatible with high frequency operation,” she says. “We need a new topology.”

Working with Afridi, and following the preliminary work of her collaborator Danish Shahzad Ph.D. ‘21, Farooq created a new UPS circuit topology and associated controls that are compatible with high-frequency operation. The new circuit topology utilizes half-bridge structures comprised of GaN switches instead of the four-quadrant structures comprising silicon switches which made high-frequency operation challenging. The novel control approach developed by Farooq enables the new UPS topology to deliver high quality power. The result is a smaller, much more efficient UPS that needs much less cooling. 

“The innovative work that Maida is doing in UPS design will have direct impact on our ability to sustainably scale data center workload,” said Afridi, “making it possible for everyone on the planet to have access to information.”

The researchers have created a lab prototype and are now working with UPS manufacturing companies to test it. After that, Farooq will work on further topological innovations that will allow her new UPS design to work on different line voltages across the world. “In Japan, for instance, you have low line voltage, while in Europe you have a very high line voltage,” Farooq says. “I want to explore online UPS topologies that can support universal input voltage while allowing usage of GaN switches to enable miniaturization via high-frequency operation.  This will enable us to deploy the online UPS anywhere in the world.”

A Solar Cookstove that Generates and Stores Electricity Via Waste Heat

As part of her graduate work, Ph.D. student Firehiwot Gurara is co-designing a solar cookstove that can also generate electricity for applications like lighting or phone charging. The system, which is funded by the Cornell Atkinson Center for Sustainability, concentrates solar energy for cooking and also has a thermoelectric generation unit which extracts the waste heat from the stove and uses it to generate electricity for the electrical applications. In addition, a thermal and electrical storage unit allows for energy storage so that a user can cook food even at night, as well as store energy for the electrical applications.

“Firehiwot’s work on the solar cookstove and the conversion of its waste heat into useful electricity has the potential to change the quality of life of millions of people around the world,” Afridi said.  

Gurara is working on the electronics for the system, while a group of students from Professor Zhiting Tian’s lab in the Sibley School of Mechanical and Aerospace Engineering are creating the cookstove itself. “We’re still in the initial prototype stage,” Gurara says. “I’ve designed the power converter to accommodate the power requirements of both energy storage and charging and lighting devices. We’ve tested the electronic system, but it still has to be refined to integrate with the cookstove system.”

Designing the power converter required Gurara to juggle a number of key design considerations. These included handling the thermoelectric generators’ wide input voltage range caused by variations in temperature, as well as managing the wide output power requirement for battery and loads. The space constraint in the stove itself proved a challenge, as well. 

Gurara hopes the cookstove will provide an alternative energy source for people in regions or situations where the electrical grid infrastructure is nonexistent or underdeveloped, she says. “Over 3 billion people don’t have access to clean cooking, for example,” she explains. “This especially impacts the health of women in developing countries, who suffer a lot from indoor air pollution caused by using wood and other biomass-based fuels for cooking. I’d like for the system to be applicable for them, but the thermal generators are a bit expensive, so the work needs a lot of modifications before that can happen.” 

The project began back in late 2019, when Gurara and her colleagues sent out surveys to contacts in India, China, and Pakistan to assess the energy demands for specific applications, willingness to pay, and challenges related to energy or electricity access for people in those regions. 

“The survey was not conclusive, since it only involved a few people from each country, but it helped us with narrowing down the areas of application,” she says. “We identified cooking as a major challenge, especially for people in rural areas where they don’t have access to the electrical grid or sometimes even to other energy sources. And the application is not just limited to energy access in developing countries but could also be used in parks in the United States, for instance, where there is no convenient access for cooking or for phone charging.”

Once they had started the project, Gurara, Tian and undergraduate mechanical engineering student Mike (Quanhuan) Liao participated in the National Science Foundation’s Innovation Corps (I-Corps) program, which helps academic inventors learn about entrepreneurship and the ins and outs of founding a startup business. As part of the I-Corps experience, the researchers conducted phone interviews with people from Pakistan, Ghana and Ethiopia, to understand the challenges they have with traditional biomass-based cooking, as well. 

Gurara originally came to Cornell as an undergraduate transfer student interested in power electronics. The chance to work on sustainability related projects, among other things, drew her to the university. She joined Afridi’s lab, then stayed on after graduating to pursue a Ph.D. in Electrical and Computer Engineering. 

“I really like that Cornell has such a collaborative environment,” she says. “Professor Afridi is also very supportive. He’s one hundred percent committed to making sure we succeed at whatever we’re doing. That’s the main reason why I’m here.”

Top: Associate Professor Khurram Afridi and Ph.D. students Maida Farooq and Firehiwot Gurara in the High Frequency Power Electronics Group lab. Photo by Eric Laine. 

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