District cooling is the provision of space cooling needs through the distribution of cold water through a distribution grid to individual buildings. Cold is generated at a single or multiple central locations and can be delivered directly into the building, or through heat exchangers at the building. The centralised production of cold at a large-scale, perhaps in a number of different locations in the network to reduce distribution losses, allows the design of highly efficient systems using the latest cooling technologies. The distribution network for district cooling is essentially the same as for a district heating network, and indeed some studies have looked at the feasibility of using district heating networks in summer as cooling networks. Significant energy is required for pumping, around 1% of the cooling energy supplied may be typical, but given district cooling systems tend to be small a lower value may be achievable. At the same time, distribution losses can be significant. However, in temperate climates, if insulated pipes can be buried, ground temperatures won’t be significantly different from the supply distribution temperature. However, where it is too expensive to underground much of the network, or in hot climates, losses can be more significant and may be in the order of 2-10%.
In a district cooling system cold production can be from multiple sources, including:
CHCP systems consist of a CHP system coupled to a thermally driven chiller. The CHP units includes, the prime mover, a generator and a heat recovery system. The prime mover can be a steam turbine, gas turbine, internal combustion engine, stirling engine or fuel cell. For the district cooling network, thermally driven chillers produce cold for space conditioning which requires a temperature level of between 6 °C and 16 °C depending on whether or not humidity control is also required. Absorption chillers and adsorption chillers can provide the cold required, although the temperature requirement at the inlet of the district cooling system will need to be less than that required at the residential building due to losses. District cooling systems can also take advantage of waste heat from industrial processes to drive thermally driven chillers to provide cold water for a district heating network. The driving temperatures for thermally driven chillers are low compared to typical industrial process exhaust temperatures. The real limitation the use of waste heat for cooling in district cooling networks is the close proximity of the industrial site to the residential and commercial sector loads. This will limit the opportunities to take advantage of waste heat from industrial processes.
The use of conventional electric chillers to produce cold water for district cooling networks is a mature solution. The key technologies are conventional large-scale chillers vapour compression chillers. Centrifugal chillers are the state of the art and the pooling of cooling loads means that these systems can save 25% to 50% of energy and peak electricity demand by allowing optimal system design and operating conditions.
The economies of scale that come from district heating networks allow free-cooling sources (these can be closed or open loop systems) such as deep sea or lake water and geothermal to be economically tapped to reduce energy consumption for cooling. The use of the ground as source of cooling through closed loop systems either laid or drilled can be particularly efficient and economic in conjunction with heat pumps to provide heating and cooling. In this case, the “hot” wells can be charged during summer when cooling is required with the rejected heat of the heat pump in cooling mode. This will raise the surrounding ground temperature into winter, where the higher ground temperature will reduce the temperature lift of the heat pump and improve efficiency (see figure below). These geothermal systems are referred to as aquifer thermal energy storage systems (ATES).
Deep ocean or lake water can often provide cold water at 6°C. In some cases, it will be possible to utilise free-cooling for most of the networks cooling needs, with energy consumption reduced to that required for pumps and controls. Such systems can achieve 80% energy savings or more, but will require other sources of cold water when water temperatures rise, or if the system isn’t designed to rely heavily on one source to ensure continuity of supply. Deep sea or lake cooling systems require the following components:
A good deep sea water or lake cooling system needs to be located close to a cold water source, have a large geographically dense cooling load to service, be able to achieve high utilisation rates and be able to deploy a compact district cooling network (Jensen, 2011). The supply and discharge pipe is typically made out of HDPE and is available in large diameters. They are rugged, have good flexibility, have few joint problems and don’t corrode. Particular care must be taken in designing the path and loads on the pipe to avoid stresses and that anchoring is adequate. Pipe anchoring with pre-cast concrete shoes will keep the pipeline off the sea or lake floor and negates the natural buoyancy of the pipes. Once onshore, pumping and heat exchangers are required to circulate the cold sea or lake water through the heat exchangers and then to pump the chilled water in the district cooling system to individual buildings.
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