District heating
District heating is a system for distributing heat generated in a centralized location through a system of insulated pipes for residential and commercial heating requirements such as space heating and water heating. The heat is often obtained from a cogeneration plant burning fossil fuels or biomass, but heat-only boiler stations, geothermal heating, heat pumps and central solar heating are also used, as well as heat waste from factories and nuclear power electricity generation. District heating plants can provide higher efficiencies and better pollution control than localized boilers. According to some research, district heating with combined heat and power is the cheapest method of cutting carbon emissions, and has one of the lowest carbon footprints of all fossil generation plants.
District heating is ranked number 27 in Project Drawdown's 100 solutions to global warming.
History
District heating traces its roots to the hot water-heated baths and greenhouses of ancient times, perhaps today most known in the Roman Empire. A hot water distribution system in Chaudes-Aigues in France is generally regarded as the first real district heating system. It used geothermal energy to provide heat for about 30 houses and started operation in the 14th century.The U.S. Naval Academy in Annapolis began steam district heating service in 1853. MIT began coal-fired steam district heating in 1916 when it moved to Cambridge, Massachusetts.
Although these and numerous other systems have operated over the centuries, the first commercially successful district heating system was launched in Lockport, New York, in 1877 by American hydraulic engineer Birdsill Holly, considered the founder of modern district heating.
Generations of district heating
Generally, all modern district heating systems are demand driven, meaning that the heat supplier reacts to the demand from the consumers and ensures that there is sufficient temperature and water pressure to deliver the demanded heat to the users. Each generation has a defining feature that sets it apart from the prior generations. The feature of each generation can be used to give an indication of the development status of an existing district heating system.First generation
The first generation was a steam-based system fueled by coal and was first introduced in the US in the 1880s and became popular in some European countries, too. It was state of the art until the 1930s. These systems piped very high-temperature steam through concrete ducts, and were therefore not very efficient, reliable, or safe. Nowadays, this generation is technologically outdated. However, some of these systems are still in use, for example in New York or Paris. Other systems originally built have subsequently been upgraded.Second generation
The second generation was developed in the 1930s and was built until the 1970s. It burned coal and oil, and the energy was transmitted through pressurized hot water as the heat carrier. The systems usually had supply temperatures above 100 °C, and used water pipes in concrete ducts, mostly assembled on site, and heavy equipment. A main reason for these systems was the primary energy savings, which arose from using combined heat and power plants. While also used in other countries, typical systems of this generation were the Soviet-style district heating systems that were built after World War II in several countries in Eastern Europe.Third generation
In the 1970s the third generation was developed and was subsequently used in most of the following systems all over the world. This generation is also called the "Scandinavian district heating technology" because many of the district heating component manufacturers are based in Scandinavia. The third generation uses prefabricated, pre-insulated pipes, which are directly buried into the ground, and operates with lower temperatures, usually below 100 °C. A primary motivation for building these systems was security of supply by improving the energy efficiency after the two oil crises led to disruption of the oil supply. Therefore, those systems usually used coal, biomass and waste as energy sources, in preference to oil. In some systems, geothermal energy and solar energy are also used in the energy mix. For example, Paris has been using geothermal heating from a 55–70 °C source 1–2 km below the surface for domestic heating since the 1970s. Especially in the former Eastern Bloc nuclear energy has been used for district heating and new systems keep being installed in China. The source of the heat of nuclear district heating is virtually always waste heat from power reactors, but proposals to build dedicated heating reactors or to use the waste heat from repurposed spent fuel pools have been brought forth.Fourth generation
Currently, the fourth generation is being developed, with the transition to the fourth generation already in process in Denmark. The fourth generation is designed to combat climate change and integrate high shares of variable renewable energy into the district heating by providing high flexibility to the electricity system.According to the review by Lund et al. those systems have to have the following abilities:
- "Ability to supply low-temperature district heating for space heating and domestic hot water to existing buildings, energy-renovated existing buildings and new low-energy buildings."
- "Ability to distribute heat in networks with low grid losses."
- "Ability to recycle heat from low-temperature sources and integrate renewable heat sources such as solar and geothermal heat."
- "Ability to be an integrated part of smart energy systems including being an integrated part of 4th Generation District Cooling systems."
- "Ability to ensure suitable planning, cost and motivation structures in relation to the operation as well as to strategic investments related to the transformation into future sustainable energy systems".
Fifth generation/cold district heating
A fifth-generation district heating and cooling network, also called cold district heating, distributes heat at near ambient ground temperature: this in principle minimizes heat losses to the ground and reduces the need for extensive insulation. Each building on the network uses a heat pump in its own plant room to extract heat from the ambient circuit when it needs heat, and uses the same heat pump in reverse to reject heat when it needs cooling. In periods of simultaneous cooling and heating demands this allows waste heat from cooling to be used in heat pumps at those buildings which need heating. The overall temperature within the ambient circuit is preferably controlled by heat exchange with an aquifer or another low temperature water source to remain within a temperature range from 10 °C to 25 °C.While network piping for ambient ground temperature networks is less expensive to install per pipe diameter than in earlier generations, as it does not need the same degree of insulation for the piping circuits, it has to be kept in mind that the lower temperature difference of the pipe network leads to significantly larger pipe diameters than in prior generations. Due to the requirement of each connected building in the fifth-generation district heating and cooling systems to have their own heat pump the system can be used as both a heat source and a heat sink for the heat pump, depending on if it is operated in heating or cooling mode. As with prior generations the pipe network is an infrastructure that in principle provides open access for various low-temperature heat sources, such as ambient heat, ambient water from rivers, lakes, sea, or lagoons, and waste heat from industrial or commercial sources.
Based on the above description it is clear that there is a fundamental difference between the 5GDHC and the prior generations of district heating, particularly in the individualization of the heat generation. This critical system has a significant impact when comparing the efficiencies between the different generations, as the individualization of the heat generation moves the comparison from being a simple distribution system efficiency comparison to a supply system efficiency comparison, where both the heat generation efficiency as well as the distribution system efficiency needs to be included.
A modern building with a low-temperature internal heat distribution system can install an efficient heat pump delivering heat output at 45 °C. An older building with a higher-temperature internal distribution system, e.g. using radiators, will require a high-temperature heat pump to deliver heat output.
A larger example of a fifth-generation heating and cooling grid is Mijnwater in Heerlen, the Netherlands. In this case the distinguishing feature is unique access to an abandoned water-filled coal mine within the city boundary that provides a stable heat source for the system.
A fifth-generation network was installed in 2016 at two large buildings of the London South Bank University as a research and development project.