District Cooling - An Underrecognized, Clean, and Efficient Solution for Cooling Our Homes
By Dejan Ivezić, PhD, Faculty of Mining and Geology, University of Belgrade
District heating is a well-established technical system in Serbia, present in approximately 60 cities and municipalities. However, the situation is entirely different with district cooling systems, which not only do not exist in our country but are also a concept unknown to a large part of the public.
This is unfortunate because, in densely populated urban areas, district cooling can be an excellent solution for residential and commercial spaces with growing energy needs in the context of climate change. In principle, the operational mechanism is similar to that of district heating systems. District cooling (DC) refers to the centralized production and distribution of cooling energy.
Chilled water is produced in a central cooling plant and distributed to consumers. This water is cooled to a relatively low temperature (4-10 °C) in a centralized plant and then distributed to consumers through underground pipelines. At the consumer's end, this water absorbs and removes heat from the interior of the cooled buildings (either by direct passage through the internal installation or via an additional heat exchanger), heats up, and is then returned to the central plant for re-cooling.
This is a closed-loop district cooling system. Open-loop district cooling systems also exist, where water from deeper parts of lakes, rivers, and seas (with a temperature below 10°C) is transported through the distribution network. After use, or after "absorbing" thermal energy in heat exchangers at the consumer's premises, it is returned to the natural source.
"The efficiency of district cooling systems is 5 to 10 times greater than that of split systems."
- Dejan Ivezić, PhD.
Waste heat from industrial processes, waste incineration, or cogeneration plants, or heat pumps in combination with district heating systems, can also be used as energy sources for preparing chilled water.
The efficiency of district cooling systems is 5 to 10 times greater than that of split systems. The experiences of cities that use district cooling show that the urban temperature during the summer months is 1 to 2 °C lower compared to the situation where split systems are used. This is particularly important for potentially the largest systems in Serbia, such as those in Belgrade or Novi Sad.
A District Cooling System Can Be a Cleaner and More Economical Alternative
District cooling systems offer a range of advantages for society, property owners, and users, as well as for energy service providers.
Among other things, these systems are environmentally friendly: they do not contribute to local air warming and the creation of heat islands, there is no noise at the user's location, and their use reduces CO₂ emissions and the use of harmful refrigerant gases such as chlorofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs), which are widely used in traditional cooling systems.
Simultaneously, district cooling systems offer a secure supply and avoid investments in the production, transmission, and distribution of electricity during summer peak loads – which is particularly important in a time of rising summer consumption due to increasingly high temperatures. They are also a more economical solution for property and building owners, offering lower operating costs compared to alternatives such as local cooling with split systems, a clear cost structure, reliable service, flexible adaptation of supply to demand, as well as space savings, greater architectural freedom, and quality. These are particularly important factors given the new (and difficult to implement) restrictions on the installation of external air conditioning units.
District cooling is an innovative service that can serve as a way for service-providing companies to attract new and retain existing customers; it is a competitive product that enables a long-term stable and profitable business. Such systems also offer a special advantage for energy service providers: the possibility of energy storage.
The Operating Principle of a District Cooling System
A conventional district cooling system with centralized production of cooling energy is shown in the figure below.
Source: Sustainable District Cooling Guidelines. IEA DHC/CHP Report, 2019.
The cooling plant (CP) produces chilled water to provide cooling services. It includes chillers, cooling towers, pumping stations, and other auxiliary systems. The chiller (compressor or absorption) is the main driving force of the district cooling system; it enables the transfer of thermal energy from a low-temperature heat reservoir (the temperature of the chilled water) to a high-temperature heat reservoir (usually the environment), thereby producing low-temperature water that is distributed to consumers.
The distribution network is used to distribute chilled water from the CP or energy storage to substations. The pipes in this network can be placed above ground or below the ground surface. Most of them are underground and are made of pre-insulated or even uninsulated pipes (due to lower costs) with corrosion protection.
The substation represents the connection between the district cooling network and the local cooling system at the consumer's premises. The substation contains a heat exchanger that enables the transfer of thermal energy from the buildings that need to be cooled.
Energy storage is used to optimize production in the cooling plant during periods of maximum consumption, as well as to ensure the continuous supply of chilled water when the cooling plant is not in operation. Energy storage is usually an integral part of the district cooling system and can take advantage of different electricity tariffs for consumers (in cases where this is sensible).
District Cooling is an Underutilized Solution, and Not Only in Serbia
District cooling is not yet sufficiently developed even in Europe. Sweden has the largest district cooling network among European countries, with an installed capacity of 5.7 GW. Currently, district cooling systems operate in 36 major cities across Sweden. The district cooling system installed in Stockholm is the largest and accounts for 50% of the total sales of cooling energy in Sweden.
Networks in France have been developed in major urban areas, including Paris and its suburbs, Marseille, Grenoble, Toulouse, Bordeaux, and Montpellier. The total installed capacity of district cooling systems in France is 761 MW. Countries with more than 100 MW of installed capacity in district cooling systems include Austria, Finland, Germany, Italy, Norway, and Spain.
In the European Union, there is no specific legislation regarding district cooling, nor a special institutional framework for the development and operation of district cooling systems. The district heating and cooling markets, as well as the regulatory frameworks, consider heating and cooling integrally.
"The basic idea of introducing district cooling systems in Serbia should be connected with the use of locally available energy sources (waste heat, cold water from rivers and lakes, industrial waste heat), which would otherwise remain unused."
- Dejan Ivezić, PhD.
In Serbia, there is no experience in the application of district cooling systems. The cooling market in the residential sector is mainly based on individual electrically powered split systems. Local, centralized air conditioning and heating systems or heat pumps for heating and cooling are generally reserved for larger commercial buildings and only exceptionally for residential buildings.
The fundamental idea for introducing district cooling systems in Serbia should be linked to the utilization of locally available energy sources (waste heat, cold water from rivers and lakes, industrial waste heat) that would otherwise remain unexploited. For example, the planned construction of new residential complexes at Ada Huja and Makiš in Belgrade, which are located near the Danube and Sava rivers, opens up the possibility of considering the introduction of district cooling systems in the early stages of design and planning.
Inset: Obstacles to Establishing District Cooling Systems
The primary parameter for the application of a district cooling system is the thermal load density. High demand in a small area makes district cooling systems a better solution compared to traditional systems. If consumers are too dispersed, a district cooling system will not be cost-effective due to large energy losses during the transport of cooling energy and the high costs of network construction. At the same time, as with other utility systems, the construction of a district cooling system is more difficult in already established urban areas.
Previous research on network infrastructure systems (e.g., natural gas, district heating, electricity, railways, or broadband internet) has shown that there are five key problems that need to be addressed when establishing a new large-scale technical system such as a district cooling system:
Technical uncertainty: for example, the risks of the technology in terms of reliability and environmental impact.
System inertia: meaning that time is needed from the initiation of the system to its full use; the distribution network can have a lifespan of up to 100 years. If doubt arises about the system's potential for expansion, this can be a major threat to its survival.
Economic conditions: characterized by large initial investments in production and distribution capacities and significant uncertainty about future profitability.
Organizational form of business: especially the question of whether cities should be responsible for the construction and operation of the system or leave it to private partners.
The legal relationship between the supplier and the user: with great uncertainty in user behavior and their strong interdependence. Therefore, it is very important to involve a large user in the initial phase of establishing a network system.
In addition, numerous studies indicate that consumers are often poorly informed about market conditions, technology characteristics, and their energy consumption in relation to district cooling systems. A lack of adequate information can lead to these solutions not being considered at all when making decisions. This problem can be mitigated through information campaigns which, if properly implemented, can enable the application of district cooling systems.
Source: Klima101