Enhancing the commercial viability of Renewell’s energy storage devices with drivetrain reduction

To avoid the worst effects of climate change, we need to transition to renewable sources of energy and reduce our reliance on fossil fuels. Traditional grid-scale energy storage necessary for renewable energy generation faces a myriad of challenges, so alternate storage methods are essential. Independently, the remnants of old infrastructure create problems throughout the United States as 2.6 million abandoned oil wells leak methane into the atmosphere. Renewell seeks to remedy both issues concurrently by repurposing abandoned oil wells as gravity energy storage devices. The device suspends a heavy weight inside an abandoned oil well with a wire rope. It takes excess energy received from the grid to power a motor and raise the weight storing potential energy. When the energy is needed for use, the weight is lowered. The spool unwinds off a drum which turns a motor and generates electricity. As the weight is raised, wire rope layers accumulate on the drum. This increases the distance from the outer layer of wire to the center of the drum resulting in greater torque. Subsequently, the maximum power needed to lift the weight increases.

Our team developed a system to enhance the commercial viability of Renewell’s energy storage devices by reducing the maximum power needed to lift the weight allowing Renewell to purchase a less expensive motor. To do this, we replaced the spooling motor with a motor-driven capstan, providing a constant radius and therefore reduced lifting torque. As a result, we reduced the maximum power needed to lift the weight by 25% and decreased the total system footprint to within a 30-foot radius of the well, allowing the devices to be deployed in urban areas with limited space. This is achieved by placing a powered capstan above the oil well that decouples the spooling and tensioning aspects of Renewell’s original prototype and using a separate spooling motor to guide de-tensioned wire onto a drum.

The capstan design brought about three safety concerns relating to the rapid release of the weight which we address with five experiments. The safety concerns addressed were progressive degradation of the capstan’s friction characteristics, wire degradation, and wire overlapping. From our scaled-down testing, we determined that a capstan successfully detensions the wire rope and reduces the force required to lift the weight. However, extended lifetime tests showed that steel wire results in degradation of the aluminum capstan and six wraps of wire rope around the capstan result in wire overlapping and catastrophic failure. The guard, a component we designed to help guide wire on and off the capstan and prevent wire overlapping, was both unnecessary with fewer wraps and required additional motor power for its operation. Friction characterization ensured that an aluminum capstan and steel rope require only ~3.25 wraps to maintain neat spooling. However, the aluminum capstan experienced significant wear, so future testing is required to ensure safe implementation for Renewell. We suggest three tests: (1) a lifetime wear test performed on a capstan made of the same material that will be used on the actual system, most likely steel, (2) a full friction characterization for steel rope on a steel capstan before and after lifetime testing, and (3) an adjusted lifetime wear test when lubricant is added to the steel capstan.