A group of English engineers have built an advanced heat pump and connected it to an energy storage system using two silos full of plain old gravel that they say is as cheap as pumped hydro, as location-agnostic as a battery - and is super efficient. Using the heat pump as the key, the team built an energy storage system that compresses argon gas to produce a temperature differential and deposits heat and cold into two separate large silos of gravel. Energy is stored in the gravel and when the process is reversed, it can be released.