If you're curious, try googling for "SandShifter", here's an energy.gov profile: https://www.energy.gov/eere/solar/project-profile-csp-energy-storage-solutions

If you're curious, try googling for "SandShifter", here's an energy.gov profile: https://www.energy.gov/eere/solar/project-profile-csp-energy-storage-solutions-multiple-technologies-compared https://www.energy.gov/eere/solar/project-profile-csp-energy... https://www.osti.gov/biblio/1183706-solar-holdings-final-technical-report https://www.osti.gov/biblio/1183706-solar-holdings-final-tec...