What may be the cleanest and most effective way to heat cities is something you've probably never heard of: a process called district heating, or DH.
District heating is an integrated heat distribution network that, through an organized underground piping system, eliminates the need for wasteful individual boilers and furnaces. By connecting buildings and services to a DH network, DH also can capture and make use of heat that would otherwise be released as waste from various structures, such as factories and public transportation services. Its greatest promise lies in its piping infrastructure, which provides the groundwork for the wide-scale deployment of distributed renewable energy systems.
Denmark’s original DH power plant went online in 1903. It now covers more than 60% of the country’s space and water heating capacity.
After oil prices quadrupled in response to the 1970's oil embargo, Denmark decided to refit and expand its DH network, and actually enacted laws requiring residences and businesses to connect to it. These laws were passed for a few reasons. First, at the time of the crisis, Denmark could not afford to waste any more precious oil on inefficient individual heating units. The country already had begun holding "car-free Sundays" — and passed a sales tax of 180% on any new car to discourage the use of vehicles — so this was the next natural step. Second, in order for the project to be cost-effective and worthwhile, the Danish government had to be sure that residences and businesses would physically connect to the DH system. Lastly, in response to the crisis, Denmark began refitting old power plants to capture the useful heat that is released as a byproduct of electricity generation, heat would otherwise be released in cooling towers as waste. In order to direct the captured heat to several different points of end-use, these combined heat and power (CHP) plants require the infrastructure that a district heating network provides.
Having installed its first CHP plant back in 1904, Denmark is one of the countries leading the way in developing CHP, and even CCHP — combined cooling, heat and power — plants. These systems are twice as efficient as conventional power plants, and they significantly reduce carbon emissions, electricity bills and fuel poverty.
This simplified diagram shows the fuel trajectory in both a CHP plant (top) and conventional power plant (bottom).
However, most CHP plants continue to run on coal and natural gas. Until renewable "fuel" sources become commonplace for CHP plants, their efficiency improvements will only buy us more time with a dirty, depleting resource. This is a classic case of eco-efficiency versus eco-effectiveness. Capturing waste heat from coal- and natural gas-fired plants is a necessary improvement with serious benefits, but by no means are conventionally fueled power plants the end-all solution. It is necessary to recognize this.
Through organizations like Biogas Taskforce, many of Denmark’s CHP plants have begun integrating biomass and organic biogas into their fuel systems in order to begin completely phasing out fossil fuels from heat and power production. Waste is another primary source of fuel for CHP plants in Denmark, and is now considered a “resource” that the country actually imports from England. This, again, is only an intermediate step toward weaning off fossil fuels, and one that must not be seen as a final solution as it does not address the colossal issue of waste reduction.
District heating systems do not require CHP plants, though. Heat from DH systems can be sourced from geothermal heat, solar thermal heat, and large and small-scale heat pumps. When wind turbines are generating more electricity than is demanded, as has been the case many times in Denmark, a DH system might even be receiving some of its heat from wind-powered electric heaters. Thus, not only do these systems distribute heat in an efficient and clean way, they also directly promote the growth of certain renewable energies that depend on that vast piping infrastructure.
Connecting various renewable systems into one central, integrated network will increase the reliability of renewable energies, while also improving grid flexibility and resilience. Of course, the infrastructure is expensive, but the payback is certain, and the benefits are enormous.
(Image at top: The tunnel between Rigshospitalet (National Hospital) in Copenhagen and Amagerværket (Amager Powerplant) in Amager. The tunnel transfers heated water and steam for the city. / Bill Ebbesen)