The thermodynamics of converting wasted heat energy to reusable energy is available quite easily. Roof farms simply locate near a bakery, cleaners, pizza store, large building or local brewery since these establishments tend to generate a lot of wasted heat from their continuous operations.
For example, a bakery or pizza shop ventilation system transfers heat to a thermal fluid. The system uses this heat as an energy input to heat or to store water for use in bathrooms, dishwashing, or heating residential or commercial spaces.
Like the Victory Gardens under President Truman, these rooftop greenhouses will increase the supply of food for the poor, as well as middle class families. The result will be better health outcomes from eating fresh food, as well as a reduction in prices due to the increased supply. In addition, rooftop farms can contribute toward gainfully employing local people living in the inner cities.