How district cooling works


District cooling is the most reliable, cost-effective way to provide comfort cooling for thew whole spectrum of residential, commercial and industrial facilities. District cooling is based on the centralised production of chilled water and its distribution for a wide range of indoor air cooling purposes.

Chilled water is produced in a central plant and distributed via a system of pipes that can run underground, on the surface or over rooftops. Inside the buildings, these transmission pipes are normally connected to a conventional air handling unit or fan coil that allows the water to chill the air passing through. This means multiple chiller units placed locally are no longer required. Once the required thermal energy has been extracted from the cold water, this water is returned to the central plant to be re-chilled and re-circulated through the closed-loop piping system. A cooling set-up of this kind is very flexible and also operates with greater efficiency (under all load conditions) than traditional distributed chiller installations.

The energy to produce the chilled water is often taken from the sea, rivers or lakes. The surplus heat from district heating or industrial waste energy can also be used as input to an absorption cooler. Traditional electrical chillers are a third source of thermal energy that can be used to chill water for district cooling.

Energy efficient
District cooling is extremely energy efficient. For example, a district cooling network can exploit the low temperatures that nature itself produces free of charge, or use waste thermal energy from the heat pumps used in district heating production. Different methods of storing this thermal energy make it possible to even out district cooling production round the clock and to counterbalance seasonal fluctuations. This boosts efficiency still further.

Refrigeration technology in district cooling
Large-scale conventional refrigeration equipment is rarely used to chill the water needed for district cooling. This is because the efficiency of such set-ups depends on the outside temperature, and is at its lowest when it is hot outside. An alternative to conventional refrigeration equipment as a way to chill water is the so-called absorption refrigeration, powered by heat energy. Instead of a compressor, an absorption refrigeration system has an absorber, a circulation pump and a generator.

The water used in district cooling is cooled in the evaporator, where the water is evaporated at about 3°C, using an extremely low pressure of about 0.01 bar. The heat energy for the evaporator is taken from the water to be cooled.

Heat pumps and waste cold
In many district cooling networks, heat pumps are used to exploit thermal energy from purified sewage and waste water. A heat pump normally provides three units of heat for every unit of electricity consumed.

When a heat pump extracts heat from sources such as waste water or sewage, this water becomes relatively cold. Exploiting energy that would otherwise go to waste makes this process particularly energy-efficient. A well-tuned heat pump can produce three units of heat and two units of cold for each unit of electrical energy consumed by the pump.

Cold depots balance out fluctuations
Warm temperatures result in high demand for so-called convenience cooling. However, this demand can fluctuate widely in the course of any 24-hour period. One way to tackle this is special storage depots in which the cold thermal energy accumulates when the need for district cooling is low. It can then be used later, when the need for cooling increases again. Such storage depots also pave the way to more rational production, which in tum means that refrigeration plants can be run more efficiently and with smaller equipment.

A "cold depot" might be a large insulated water tank filled with cold water. But nature can also be used as an accumulator, in the form of so-called aquifers. A typical aquifer might be a natural underground reservoir in which slow-moving water can store energy.

"Free cooling"
Nature's own sources of low-temperature thermal energy are a valuable resource. The cold water found in lakes, the sea and other watercourses is ideal for use for district cooling. Water at a temperature of 3-4°C is pumped up and cools the water circulating in the district cooling network, using heat exchangers. The water, which by then has often reached temperatures of 12-16°C, is subsequently returned to the lake in question or the sea.