The idea behind it is that there is a lot of heated water being dumped into the sewers of Paris. Water from hot showers, washing machines etc. A steel plate is placed in the water and it contains a liquid that conducts heat up into the school's cellar and then distributed around the school.
Scientists say that not only does this reduce other energy consumption, it also doesn't smell. One of the few drawbacks of this system is that it can only be used within 200m of the sewage, which means that only 10% of Paris can be heated through sewer energy. The other big drawback is the initial cost. For example, the school project will cost more than US$550,000 but will pay off in the long term, because it will generate 70% of the heat necessary.