District heating, also known as heat network, is a system for distributing heat generated in a centralised location for multiple residential and commercial heating requirements. District heating plants provide higher efficiencies, lower costs and better pollution control than localised boilers, reducing carbon emissions. District cooling on the other hand, working on broadly similar principles, delivers chilled
water to buildings which require cooling. The source for the cooling can be sea water, which is cheaper and more sustainable than using electricity to run air conditioners for cooling.
Today in France, there are over 500 district heating district heating and 17 district cooling systems. The heat networks serve approximately an equivalent of 2 million residential units, including 2/3 in the residential sector. This represents 6% of national heating needs. Present in dense urban areas, 40% of these networks are powered by renewable energy sources. The French government has set the ambitious goals of increasing the share of renewable energy to 75% and tripling the number of connected residential units by 2020. (Syndicat national du chauffage urbain et de la climatisation urbaine, 2014)
A district heating system includes one or more heat generating units, a primary distribution system wherein heat is transported by a heat transfer fluid, and a set of exchange substations, from which the buildings are serviced by a secondary distribution network. Heat can be obtained from several sources such as a fossil fuels, waste incineration, biomass incineration (essentially wood), geothermal heating, heat pumps, solar power or nuclear power. Some heat production units operate also in cogeneration mode, simultaneously producing electricity and heat.
The primary distribution network is a loop which leads the heat transfer fluid from the heat generating unit to the exchange sub-station. This fluid can be water or steam, conveyed by pipes made of a jacketed system: an outer sheath steel inside which there is another steel sheath surrounded by a insulation thickness and carrying the heat transfer fluid.
Generally located at the foot of buildings, exchange sub-stations consist of a heat exchanger that transfers heat between the two circuits. Substations also have smart heat meters which enable to know the building’s energy consumption.
District Heating: Example of the Parisian Heat Network
In Paris, the district heating was widely developed during the 20th century, even though its origins can be traced back to the 14th century. It is operated by the Parisian Urban Heating Company (Compagnie Parisienne de Chauffage Urbain or CPCU) which produces heat as a vapour in eleven production sites from multiple energy sources. The network operates in closed circuit: once the hot fluid (steam) has delivered its calories, it returns in the form of chilled water (condensate) to heating stations which reprocess it.
The network is 480 km long, has a total power of 300 MW and serves 17 municipalities of the Paris metropolis. In 2014 it delivered to customers 4.4 TWh of heat.
The functioning of a district cooling system is the exact opposite of that of a district heating system: the cooling network collects heat in served buildings and evacuates it at a cooling station including a heat discharge point. Heat is transported by a cooling fluid (usually water, whose temperature is between 1 and 12 ° C in the first leg, and between 10 and 20 ° C return). Although less common, the cooling networks have advantages over individual air conditioning systems: lower environmental impact, reduction emissions of greenhouse gases, ability to use diverse sources of energy including renewable ones, lower costs, etc. France has 17 district cooling systems, the difference between the numbers of heating and cooling networks being of course explained by the climate. The dominant technology in cooling networks being the compressor (95%), which is basically the same technology as individual air conditioning systems or fridges, but at a larger scale. However, some other technologies do exist to power district cooling systems, such as exploitation of deep water or underground coldness.
District cooling: Sea Water Air Conditioning project in Réunion Island
Réunion Island is an overseas region of France located in the Indian Ocean. This small volcanic island (2,500 km²) is situated about 900 km east of Madagascar and 175 km southwest of Mauritius, the nearest islands. As of 2014, its population numbered about 850,000 inhabitants. The climate there is tropical, which explains the need for cooling systems.
Sea Water Air Conditioning (SWAC) project is an urban scale infrastructure project for cold production from deep marine waters alongside with a distribution network providing air conditioning to Saint-Denis, the main city of Réunion Island. The project consists of offshore pipes, a pumping station and a distribution network, which will be designed to deliver a power of 40 MW. The cost of this project is estimated at about €150 million. Once approved, its implementation could be as early as February 2018.
The cold source will be provided by seawater collected at a distance of about 6 km from the coast and at a depth of 1,100 m by offshore pipes. Thanks to a deep polar marine current coming from the South, the water temperature at this depth is 5°C all year round. The cold water distribution network will consist of 23 km of insulated pipes, serving over 60 sites including large consumers, such as the hospital centres, the university, the airport, several state departments, the Regional Council’s headquarters, residential buildings, etc. In total, over 400,000 m² of floor surface could be supplied. Servicing points will be equipped with smart meters enabling real-time adjustment of supply to demand.
This technology, using a renewable source of energy, needs up to 90% less electricity compared with conventional cold production systems. Eventually, the network will remove up to 40 MWh/year from the island electric consumption, which is equivalent to the greenhouse gases emissions of 16,000 vehicles.
This innovative project will allow Réunion Island to significantly reduce its carbon footprint and generate jobs. Moreover, it will be a global showcase, opening the way for other similar projects in the intertropical belt.
Enquête annuelle nationale sur les réseaux de chaleur et de froid, Syndicat national du chauffage urbain et de la climatisation urbaine, 2014
Les réseaux de chaleur à Paris et en petite couronne, Atelier Parisien d’Urbanisme, 2006