District cooling energy can be produced in several alternative ways:
• free cooling (utilisation of nature’s own energy resources)
• absorption cooling equipment
• compressor technology
• heat pumps
In absorption technology, the operating energy is industrial waste heat or heat produced together with the electricity generation of a power plant, which cannot be sold as district heat during the summer due to low consumption levels.
These production methods can be combined according to local conditions so that the energy required at any given time will be produced with the most favourably priced method. The storage of district cooling energy is essential even in smaller systems to improve the profitability of operations. Storage also brings flexibility and extra reliability in the supply of cooling energy.
Cooling energy is delivered to customers in the flow pipes with cooled water. Cooling water is treated in the same way as district heating water. Water heated in the customer’s heat exchanger is transmitted in the return pipe back to the cooling plant where the same water is cooled again.
The temperature of cooling water delivered to the customer varies in different district cooling systems. When producing cooling energy with absorption technology or free cooling, the temperature of cooling water is, for example, 8 oC. When producing energy with compressor cooling equipment, the water temperature may fall to 6 oC. In the measurement situation, 8-10 oC is used as the temperature difference between flow and return water.
With simple arrangements, cooling energy can be transmitted from a cooling plant located close to the customers. A district cooling system covering a wide customer area requires careful planning in terms of the production plants, location of the distribution network, dimensioning and the functioning of the entity.
A building can be connected to a district cooling network using either a direct or indirect connection. In direct connection, the water in the district cooling network circulates in the cooling system of the building. In indirect connection, the district cooling pipes and the cooling system of the building both form a separate circuit. The heat exchanger located in the equipment room of the building is between the circuits. Indirect connection is recommended to be used in district cooling. Direct connection should be used only in exceptional cases in small district cooling systems of a maximum of 2-3 customers.