Could a "gravity battery" work in a residential home?
Sustainable energy sources like wind and solar present a challenge: how do you store excess energy during periods of overproduction for when you really need it? Some large-scale utilities have turned to mechanical energy storage: lifting heavy weights or pumping water uphill into a reservoir. Once that energy is needed, the weight is released and its mechanical energy powers a generator that produces electricity. These mechanical batteries have been proven to work on a large scale, but never at the scale of a single residential home. A team of Purdue University undergraduates undertook a study to investigate whether a “gravity battery” could be made small enough to fit into a single-family home.
“A gravity battery stores energy just like a AA battery would, except it’s more environmentally friendly,” said Caden Jarausch, a senior in mechanical engineering, and lead author of the study. “Instead of using chemicals like lithium, it uses the potential energy of a heavy weight to turn gravity into electricity.”
To test their theory, they used the DC Nanogrid House: a residential house in West Lafayette that serves as a testbed for sustainable energy projects. The house has solar panels on the roof that collect energy throughout the day. The idea was to design the battery as if it were being built in a wall of the house, reaching from the attic all the way to the basement. As the solar panels gathered a surplus of energy, the weight in the basement would slowly start to lift to the attic. Once in the attic, it would store the energy until it was needed; then the weight would be lowered, feeding the energy back into a motor which would act like a generator to produce electricity.
The students’ research mainly focused on the feasibility of building the battery. They collected and analyzed the energy consumption of the DC House to better understand when peak energy use is. Then, they investigated how to make it structurally sound while also keeping it cost-effective. They compared different types of wood and weights, landing on southern pine and high density concrete. However, after accounting for all aspects (including safety features), the battery turned out to be far too expensive for the small amount of energy it could actually store. Therefore, they concluded that a gravity battery wasn’t feasible for residential homes.
However, they still decided to write and publish a paper on their findings.
“We still wanted to publish the paper because I think there’s a bigger message,” said Jarausch. “Sometimes when you’re doing research, you’re not going to get the expected result, and sometimes that result is just as important as if you were to get the expected result.”
Jarausch had never conducted research before, but he decided to reach out to Eckhard Groll, William E. and Florence E. Perry Head of Mechanical Engineering and Reilly Professor of Mechanical Engineering, who suggested this project. “Most professors at Purdue are more than willing to talk with their students,” said Jarausch. “I encourage students to just reach out to any professors they’re interested in working with.”
He emphasized that research challenges you in ways that classes and extracurriculars don’t. This summer, he had the opportunity to present this research at the Herrick Conferences. As the youngest person there, he was pretty nervous going into it; however, multiple people congratulated him after his presentation.
“I had one person come up to me and say the research we did on the gravity battery opened his eyes to something in his research that could help solve a problem he’d been dealing with,” said Jarausch. “That made me feel really good. Even though we didn’t get the results we wanted, the research we did and the information we shared is still helpful to people.”
Writer: Julia Davis, juliadavis@purdue.edu
Feasibility of Gravity Batteries in Residential Homes: A Case Study
Caden L. Jarausch, Thomas B. Avery, Enrico Setiawan, Andreas J. Hoess, Haotian Liu, Eckhard A. Groll
ABSTRACT: Sustainable energy generation and storage are key factors in the transformation of society towards a carbon-free future. While great progress has been made in the development of renewable energy generation systems, there is still a mismatch between the global energy supply and demand. Renewable energy sources, such as solar and wind energy, are subject to variable efficiencies that depend heavily on local weather conditions. Thus, energy storage is necessary for a sustainable energy grid to meet the demand of high usage phases during periods of lower energy production. Although many systems currently depend on chemical batteries for energy storage, these systems face issues regarding the limited availability of materials needed for fabrication as well as the energy intensive production and recycling processes tied to such systems. Thus, the feasibility, scalability, and use cases of simplified and environmentally friendly alternative energy storage options must be investigated. Subsequently, a feasibility study on the use of a gravity battery as a form of domestic energy storage was conducted in Purdue University's DC Nanogrid House, an ongoing project that aims to convert a residential property to run solely on DC power whilst operating predominantly independent of the grid. Gravity batteries store energy in the form of potential by lifting a weight using a motor-winch combination. When needed, the battery is discharged by lowering the weight and utilizing a generator to convert the potential energy back into electricity. This is an attractive form of energy storage for its simplicity and longevity without the need for chemical components. In the present paper, the energy consumption data of the DC Nanogrid House was first analyzed to set goals for the required storage capacity of the system, followed by the development of an initial design that was later modeled using CAD. This design went through a techno-economic analysis and was optimized to meet the building and safety specifications. The case study and the techno-economic analysis performed were all used in determining that while gravity batteries continue to show great promise in industry, the insufficient volumetric energy density and efficiency of the systems make the technology currently unviable to be effectively utilized on the residential scale.