alt Hacker News"Low grade heat", less than the boiling point of water (often), can be stored in a sand system and recovered efficiently and distributed later.
But low grade heat cannot efficiently drive a heat engine to produce other, more useful and transferable versions of energy, like electricity. For that you need "high grade" heat.
The reason is, heat engines do not operate on heat. They operate on heat flow. You must have a high temperature side and a low temperature side, and you extract energy by taking heat out of the high side and injecting it to the low side, through various processes. By thermodynamics, this process has hard efficiency limits of around 35%, with that efficiency heavily affected by the delta between the hot and cold side.
The "low" heat side is often a great source of low grade heat, for industrial or communal purposes, but cannot be used to run an efficient heat engine. It's already "used up" essentially.
The "grade" you refer to is simply temperature. To efficiently extract energy from a heat flow you want a really hot hot side and a really cold cold side. On earth we can't get a cold side much colder than outside air temperature (not without wasting more energy than you gain) but luckily that is cold enough when the hot side is thousands of degrees. But you can't get much energy from roomtemp and roomtemp+10. A warm cup of coffee will run a Stirling engine for a party trick, but it's not going to charge your phone, much less your car.