Brendon writes in:
Heating a sidewalk section has climate change implications. I calculate the 26-year cost of your section at $8,722 at the low end and $9,708 at the high end (depending on the discount rate you assign to the future impacts of climate change. I tend to lean towards the higher end). This means your break-even point is 8% to 20% higher, meaning maybe 173 to 192 pedestrians per day. Of course with a carbon tax in place, there would likely be more walkers in some places, meaning heating the sidewalks become feasible in more places.
Now, if you could use waste heat that hasn't been previously captured to heat sidewalks, as they are proposing to do with the new "interchange" plaza and HERC steam, the carbon footprint becomes effectively zero additional. Much less per kWh/BTU.
Other interesting facts, heating all the sidewalks in Minneapolis with electricity from the grid for one year would produce more greenhouse gases than the disposal of all our solid waste and wastewater does over the same time period. The additional energy consumption would be equal to about 1/3 of the current annual consumption in all residential properties in the city. It would increase the city's annual electricity consumption by 8%.
He nicely identifies a feedback effect, heating up sidewalks will create more emissions, which will heat the atmosphere, which will eventually negate the need for heating up sidewalks. There must be an equilibrium point here.
More seriously, the use of waste heat is a great idea, especially near the HERC. The problem would be building infrastructure to distribute that more broadly. There might also be waste heat from wastewater (which is still liquid in the winter, and thus warmer than the ground around it) which we don't capture, or let go to roads, by running sewers under the streets rather than the sidewalks.