It can be stored as heat in stone or sand. There are companies building and selling both solutions. They hold the heat for months, long enough to last through a winter.
It appears you haven't taken a thermodynamics or heat transfer class: Heat has a property called "quality", and low quality (roughly, low-temperature) heat such as you're describing is not really useful for much outside of warming water a bit - you're certainly not going to be generating power that way!
All current heat engines as defined by the 2nd law of thermodynamics require a DELTA T to run, and the small delta T provided by low-quality heat sources cannot generate much power, and drastically slashes the efficiency possible from the system. (Interestingly, there does appear to be a part of the 2nd law which only applies in the quantum realm and has no classical heat engine analog - if that turns out to be true, then it changes things up quite a bit...)
All current heat engines as defined by the 2nd law of thermodynamics require a DELTA T to run, and the small delta T provided by low-quality heat sources cannot generate much power, and drastically slashes the efficiency possible from the system. (Interestingly, there does appear to be a part of the 2nd law which only applies in the quantum realm and has no classical heat engine analog - if that turns out to be true, then it changes things up quite a bit...)