Mechanically Storing Renewable Energy at a Residential Scale

Abstract

Today, more and more households are generating their own solar power. This helps us to come closer to a circular economy, since less fossil fuels are required to meet our energy needs. However, excessively generated energy is often wasted, since transporting or storing it for later use is challenging. Lithium ion batteries provide a solution, however their short lifespan and environmental problems that are caused during production make them far from green.
A mechanical storage system was proposed, minimizing the environmental problems of Li-ion while providing the household with a storage solution for excessively generated solar energy. After exploring multiple energy storage methods and analyzing their potential suitability to residential energy supply & demand, Flywheel Energy Storage was chosen as the applied storage technology due to its high energy density and mainly mechanical components.
ScriptEssential calculations were extended into a full simulation script, used to analyze different scenarios of use. It is capable of the following:• Confirming chosen rotor dimensions• Determining required rotational speed• Characterization of rotor losses & spin-down times • Characterization of torque losses• Reading supply/demand data from external source • Visualizing 24h supply/demand/storage profilePrototypeThe script was partially validated using a functional model and performing tests concerning spin-down times with two different rotors and vacuum levels.LEFtAs a final deliverable, a full mechanical storage system was designed. LEFt, which stands for Leftover Energy Flywheel technology, is a mechanical battery that stores an excess of residential solar power in the form of kinetic energy by spinning a flywheel in a vacuum. It comes in three main form factors; Flat, Slender and Extra Slender. These types all suit different scenarios and therefore different households. LEFt was designed using a subsystem approach to cope with all co-dependent aspects of the system.The most essential part, the flywheel rotor, was dimensioned according to the script.RotorDifferent versions of LEFt include differently dimensioned rotors. A large height over radius ratio makes LEFt suitable for short term storage. It can be applied to store electricity that is generated during the afternoon for evening use.Changing the application and storage limit result in different configurations and dimensions. A rotor with a small height over radius ratio can be suitable for longer term storage. A setup with a certain supply & demand makes this type potentially capable of 24h storage and might allow off-the-grid living in the future.SuspensionThe flywheel rotor is suspended nearly frictionless in the vacuum, by levitating it using a magnetic bearing system.The main vertical thrust is supplied by a Halbach Array of passive magnets, whereas radial displacement is corrected by two Active Magnetic Bearings that are handled by an advanced control system.Motor/GeneratorDriving the rotor and regenerating electricity is done by one machine; an electric motor that is positioned outside the vacuum. Using a single phase motor allows easy installation without the need for a transformer.Magnetic couplerTo drive the rotor from outside the vacuum, a magnetic coupler was designed, making use of two discs with a pattern of passive magnets. A control system allows smart coupling and decoupling, resulting in a freely spinning rotor in idle situations.Vacuum housingEnclosing the flywheel rotor is done by a depressurized housing.This has proven to reduce resistances, increasing storage times and therefore the applicability to longer term storage.Market implementationSelling LEFt is done best by a lease plan, in collaboration with solar panel suppliers. The full retail price of over €10,000 will be too high for a one-time investment.Sustainability assessmentThe environmental impact of the design is done by comparing it to a competing lithium ion battery. The results of an Eco Audit show that the impact of LEFt is lower, but still significant because of the large amounts of steel that are needed. ConclusionA conceptual design for a flywheel energy storage system was proposed and partially validated. It was concluded to be a better alternative for lithium ion batteries in residential energy storage, since it minimizes social and environmental problems. Further development and extensive analysis is required to fully validate and make the design ready for production.