What Are The Primary Clean Energy Sources For Cities Of The Future?

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As an obvious statement, cities aren’t energy islands which can or should try to be self-sufficient. Cities are specialized areas for economic growth through trade and innovation, not net producers of raw materials or energy. That said, there are obvious paths for cities to exploit more wind, solar, hydroelectric, geothermal, and other forms of clean energy.

From an urban-centric perspective, exploiting natural fuel sources has different requirements, so I’ll explore each separately.

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Wind Energy

Regardless of the real, utility-scale wind in urban areas worldwide, as documented by Paul Gipe, cities aren’t the best places for wind energy.

Energy density is a fictitious problem in 2017, but regardless, wind turbines need space around them and a lack of obstructions in front of them in order to be most cost effective. That’s not what cities are good at.

What is required for wind and cities are good transmission grids from areas with excellent wind resources to urban areas, and efficient energy markets that enable the electricity to get there. That’s pretty much what exists or is being built globally, so this is on track despite typical NIMBY reactions to new power lines.

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Solar Energy

There’s a somewhat different answer for solar energy. There are two scales of rooftop solar that are viable in urban areas, home or small-building solar and small utility-scale solar farms on big box stores (aka commercial solar), on warehouses, and as shading over sprawling parking lots.

But of course cities aren’t necessarily where the best solar resource is, have a lot of taller buildings which shade lower buildings, and have a lot of roofs which either aren’t aligned to the sunshine or have other things like HVAC on top of them. The numbers I’ve seen suggest that perhaps 25% of urban buildings could have solar on the rooftops, and my calculations suggest that only in sprawling southern US cities could more electricity be generated than a household uses, on average.

Cities need more electricity than they can get from solar inside their boundaries. As a result, like wind energy, they require good transmission grids and efficient energy markets to bring electricity from high-solar-resource areas to urban areas. These interconnections are also under development around the world, so this issue is being resolved.

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Hydroelectric & Water

There are very few hydro dams in urban areas. There are some flood-control dams and locks, but few generating dams. The reservoirs would just take up too much valuable real estate, so the big dams are typically in the middle of nowhere. Once again, transmission and markets.

Water heat exchange systems are very viable in cities on larger bodies of water, however. Toronto, for example, provides district heating and cooling to multiple buildings by pumping heat exchange fluid (water) through pipes running moderately deep into Lake Ontario. This process requires that buildings be set up to exchange fluids for heat exchange, however, so it has limits in terms of retrofitting. If a new district close to water is transforming from industrial land to commercial and residential, this is an excellent piece of infrastructure to invest in.

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Geothermal

This starts to get interesting. There are two types of geothermal: generation and heat-exchange for cooling and heating. There are very few geothermal generation plants in cities simply because the best sites are very tectonically active and building cities on potential volcano and earthquake zones doesn’t happen that often.

Iceland is the exception rather than the rule, as it can’t escape the highly active areas in that island country. Japan is an exception in a different way, as it has hotsprings throughout the country which have high geothermal potential, but citizens are strongly protective of those springs, as they are cultural sites that have been used for bathing for centuries, so are much less exploited than those in Iceland.

Heat-exchange for cooling and heatingis much more interesting inside of cities. It’s a lot cooler underground in the summer in most cities and a lot warmer in the winters. Drilling holes and circulating a heat-exchange fluid through them can cool buildings in the summer and warm them in the winter with very low operating costs.

However, you can’t really set up drilling sites in cities unless you are starting a major new development shifting an industrial site to commercial and residential, or you’ve torn down an existing building. That’s why geothermal in most cities is restricted to new developments. For example, one luxury condo in downtown Toronto uses geothermal heat exchange to minimize its heating and cooling bills. Nice looking building too.

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In summary, it’s mostly about building transmission and markets to get electricity to cities and exploiting natural heat / cooling opportunities inside cities as they become available.