“Our hybrid system can be implemented in any kind of building but the energy content of the gravity storage system increases with the height of the building, which means high buildings offer the largest potential for its development,” researcher Mayurakshi Mondal told pv magazine.
The hybrid system consists of a conventional rooftop PV system, a solar tree, two gravity power modules for building (GPMBs), and a vanadium redox flow battery (VRFB), with the three storage systems being charged exclusively by the two solar installations. “The proposed system emphasizes on supplying the essential load demand of a building through renewable energy as much as possible, especially focusing on the reduction of the demand from the grid at peak hours,” the scientists explained. “The system is also focused to flatten the load demand curve of the grid.”
They estimated the levelized cost of energy (LCOE) of the redox flow battery at $0.424/kWh and that of the GPM at $0.148/kWh. Due to this difference, the battery is used only as a secondary storage system, the said. During the charging phase, the GPMBs work via a motor that drives a pump to push water from the top of a piston in a larger pipe into a smaller one. It then pushes it back to the larger pipe below the piston.
“This pushes the piston upward and the operation terminates when the piston reaches the topmost position of the larger pipe, thereby storing energy in the mechanical form,” the Indian group said, noting that both GPMBs should have a diameter of 1.5 meters each for the larger pipe.
In the discharging phase, the piston in the larger pipe comes down and pushes the water downward, into the smaller pipe. The water flows through the pump/turbine unit operating in the turbine mode and generates electricity with the electrical device operating in the generator mode, the academics said. “The discharging operation ends when the piston reaches the bottom-most point in the larger pipe.”
They considered a 110-meter building with 39 floors and a total floor area of 750 square meters. The two GPMBs have a storage capacity of 96.2 kWh each and the redox flow battery is able to store 300 kWh. The 75.4 kW PV system occupies a surface of 450 square meters, which corresponds to around 60% of the roof area, and five solar trees totaling 57.7 kW are installed in the exterior open-air space of the building.
“After meeting the residential load demand and charging both GPMBs, any extra generated energy is used to charge the VRFB. Thereafter, any remaining energy generated is sold to the grid,” they said.
In the proposed system configuration, two GPMBs only supply electricity within the peak hours 6 pm to 9 pm and 9 pm to 12 pm, in order to reduce the burden of peak hours over the grid.
“The proposed system is able to supply 47.77% of renewable energy to the building as a yearly average and is capable of injecting 48.89% renewable energy to the building along with grid,” the researchers said.
They described the hybrid system in “Solar PV driven hybrid gravity power module—Vanadium redox flow battery energy storage for an energy efficient multi-storied building,” which was recently published in the International Journal of Energy Research.
“Further studies on GPMB are required before practical implementation, like the effect of high starting current devices such as an air-conditioner, water pump and so on over its operation,” the scientists said. “The system also requires a cost optimization analysis.”
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