Many systems that use renewable energy heat sources have large thermal storage tanks. Examples include systems using biomass boilers and solar thermal collectors.
I call these heat sources “wild” since they can operate over a wide range of conditions and often at temperatures significantly higher than required by a modern low-temperature hydronic distribution system. The label might seem a bit embellished, but I use it to emphasize that managing these heat sources within a system is very different from just turning a gas-fired boiler on and off.