District cooling, large-scale cooling system that works according to the same principle as district heating. A central unit cools down a coolant which then passes through the environment to be cooled. District cooling is used for air conditioning (so-called comfort cooling) but also for cold rooms and the like for, for example, food storage.
As the efficiency of refrigeration units increases with their size, the operation of district cooling systems is cheaper than individual air conditioners. In addition, in a large system, for example, seawater can be used for cooling the brine. The disadvantage is the large construction costs, as it is expensive to run district cooling lines in small-scale systems. For the customer, district cooling can be described as a form of outsourcing, as he does not have to operate his own cooling system and also avoids taking responsibility for refrigerants that are often hazardous to health and the environment.
In large district heating systems with relatively cheap fuel, absorption heat pumps are often used. These units are operated in contrast to conventional electric cooling systems on heat. Due to the lower efficiency, cheap heat is required to be profitable.
In the case of waste incineration, the fuel (garbage) cannot be stored during the summer when the need for heating is low, but you must burn even though the heat is not needed. To avoid venting the heat without using it for something useful, you can use the unwanted heat in an absorption system and produce cooling that is in extra demand during the same period.
The compressor cooling machine is the dominant technology for cold generation and is also common for the production of district cooling, especially in countries with low electricity prices.
In a compressor cooling machine, or heat pump, you add 1 part electricity and get out about 3 parts heat and 2 parts cooling, or physically correctly expressed 1 part electricity plus 2 parts heat in gives 3 parts heat out. In small plants, it is normally either the cold or the heat that is sought. In combined plants for both district heating and district cooling, both parts can be sold. This means that the heat that is removed with the district cooling system can be sold as heat in the district heating part with an electricity input corresponding to approximately one third of the heat obtained. If heating and cooling were instead provided on a small scale, it is very likely that there would be two separate plants, one of which would only produce heat and the other would supply cooling. The need for electricity for the same amount of heating and cooling is halved thanks to a combined plant, to the benefit of both the economy and the environment. In addition to halving electricity consumption and getting the same amount of useful energy, large heat pumps are slightly more efficient than small ones for thermodynamic reasons.
In cases where you have access to deep lakes or seas, cold deep water can be used as a source for the cold. Thanks to the water's ability to have a maximum density at 4 ° C, water settles with that temperature at the bottom of lakes and seas. At these temperatures, only heat exchange between the seawater and the district cooling network is required, which means that the energy input in the form of electricity to provide the cooling is very low.
District cooling is growing in Sweden despite the comparatively cool climate. The temperate climate gives rise to the need for both heat and cold.
In many places, coal condensate and nuclear power are a dominant form of production for electricity. These production plants have large excess heat that is vented away and could very well operate absorption cooling plants. In Sweden, you "cool" off the heat in the district heating network and thus benefit from the excess heat or produce district cooling with the heat. If one chose to invest in district cooling networks in warmer countries with current production facilities as a heat source, this would have very positive effects on energy use. In the same way that Swedish electricity production greatly increases with falling outdoor temperature, the need for electricity rises in many hot places instead with increasing outdoor temperature.
Calculation example with coal power plant
To illustrate the benefits of using district cooling in places where controversial coal power is used, we can look at the following calculation examples:
We assume that this is a city where there is a need for cooling, which is common. The cooling is produced with conventional heat pump technology which provides about 2 parts cooling of 1 part electricity. All production with condensing power plants provides about 1 part electricity and 2 parts heat. Thus, an energy supplement to a coal condensing power plant of 3 parts coal is required to produce 1 part electricity, the rest are normally losses which also give rise to controversial greenhouse gas emissions. When this coal is used in an often small-scale conventional refrigeration plant, 2 parts of 3 parts of coal are produced (via 1 part electricity), or 2/3 parts of cooling per 1 part of coal energy.
If we instead use the excess heat from the coal condensing power plant for absorption cooling, we receive about 1/2 part of cooling per 1 part of coal energy, but have 1/3 part of electricity left, and can thus get another 2/3 of cooling via electricity, a total of about 5/6 parts cooling per 1 part carbon energy.
Alternatively, we can use heat and cooling separately, then it will be about 2/3 parts heat plus about 2/3 parts cooling per 1 part carbon energy, a total of about 4/3 parts heat plus cooling per 1 part carbon energy.
While district heating on commercial grounds has existed since 1877, it took until the 1960s before district cooling began to be built. The first out was Hartford in 1962. In Japan, district cooling was established in the 1970s, starting with the world exhibition in Osaka in 1970. Growth was for several years concentrated in the USA and Japan. In Europe, systems had been installed in La Défense in 1967 and Hamburg in 1968, but it took until the 1990s before use took off, then mainly in France, Sweden and Germany.
Sweden's first district cooling plant was put into operation in Västerås in 1992. In recent years (approx. 2012… 2016), about thirty facilities in Sweden provided district cooling corresponding to approximately 1'000 GWh of heat output per year . However, that figure is far from its potential. Studies show that the total demand for district cooling amounts to the equivalent of 2'000… 5'000 GWh per year .
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