Introduction
District cooling has been a commercial alternative to traditional A/C and refrigeration technologies since the mid-90s. It started on a small scale in Sweden and France but spread after just a few years to all Western Europe, and must now be considered as a very strong competitor in most European countries.
A District Cooling system can be based on one, or most often several of the following technologies:
Electricity-driven mechanical chillers
Absorption or adsorption chillers are driven by District Heating or waste heat
Free cooling from the air, water, or geothermal energy.
District cooling operators are very often backed up by strong financiers such as energy companies, municipalities, or large industrial conglomerates. Local AC&R contractors therefore experience difficulties in getting their voice and arguments heard when debating the pros and cons of local vs. centralized AC&R systems.
“District cooling is considered to be all forms of cooling in which the cooling production is centralized, and the product is offered to a broader market.”
“The main idea of district cooling is to use local sources for cooling that would otherwise be wasted or not used, in order to offer the local market a competitive and highly efficient alternative to the traditional solutions.”
How does District Cooling work?
General
District cooling refers to cooling that is commercially supplied through a cold/heat carrier medium against payment on the basis of a contract. District cooling can be a network serving several customers; it can also refer to the local production and distribution of cooling to supply the needs of an institution - business centres, airports, hospitals, universities, and public buildings. Experience demonstrates that this type of block cooling can be the starting point for a district-cooling network when new users are added.
Centrally-produced district cooling can reach an efficiency rate often 5 or even 10 times higher than that of traditional local electricity-driven equipment. This is achieved by a combination of free cooling and traditional cooling. It also offers great flexibility, tailored to users’ needs, to combine cooling production with different possibilities such as:
Deep-sea or deep lake water “free cooling”
Absorption chillers (in combination with surplus heat production from industrial sources, waste burning plants, or cogeneration production plants)
Heat pumps in combination with heat demand (i.e. district heating systems)
To increase efficiency and reliability, these cooling sources and production techniques are often combined with different kinds of storage solutions, such as:
Seasonal storage where free cooling in winter is stored for use during the summer period
Night-to-day storage facilities where overcapacity during the night is stored for use during daytime.
District Cooling systems
The centralization of cooling production is a prerequisite for high efficiency insofar as it makes possible the use of “free cooling” or surplus heat sources, and thereby offers the benefits of large-scale energy production. A distribution network is, therefore, necessary to provide the cooling supply to customers.
There are two main schematics that are used in district cooling systems:
“Real” district cooling systems, in which “cold” pipework is used to distribute the cold fluid from a central cooling station to the final user.
“Hot” district cooling system, in which a district heating network is used to provide thermal power to local sorption chillers (i.e. absorption or adsorption units), which provide cooling power to the user.
In the “real” district cooling system a combination of free cooling (from underground water, lake, or sea), compression chillers, and sorption chillers can be used to produce cold water. Dedicated pipework is needed to transport the cold fluid (generally @ 6-7°C) from the central cooling station to the user. This pipework needs particular care in insulation to avoid condensation on the metallic surfaces, from which corrosion can occur. Insulation is also needed to minimize thermal losses. In most applications, the main cooling network is coupled with the end-user by means of a heat exchanger normally referred to as a substation or energy transfer station (ETS).
It is necessary to take into account that in most situations buildings and HVAC equipment already exist. Therefore, when introducing DC systems, it is important to consider how to match these requirements in the future as the DC system must provide cooling power according to temperature, mass flow, and nominal power. This can be done by lowering the supply temperature that enables the use of existing building-bound systems. An alternative is to upgrade or rebuild the end-user system by increasing the heat exchanger surface or introducing additional high-temperature equipment such as baffles. Despite the additional investment, the upgrading alternative is normally the most common as it enables higher efficiency for the system with long-term energy savings.
Unlike DH networks, DC temperature differences between supply and return are much lower (6°-10°C compared to 20°-50°C). This means that in order to provide the same thermal power, for a DC network you need a much higher flow rate and consequently the diameter of the pipe must be larger and so the net cost will be significantly higher than for a DH network. Therefore DC systems are never used on small capacity installations or in an area of low cooling power density as the investment will be much too high.
Production
In the production plant, one or a combination of the following production and storage techniques are the most common.
Combining district heating/cooling
"Surplus cooling" can be used from heat pumps that are originally intended for the production of district heat, operating on, for instance, seawater or wastewater. By connecting the cold side of the heat pumps to a district-cooling system, the heat pumps can be used for simultaneous production of heat and cold.
Absorption chillers
Absorption chillers use heat as their primary energy and not electrical power as is the case for conventional compression chillers. The benefits of this technology compared to conventional chillers are that electrical power consumption is dramatically reduced and primary energy is used more efficiently. Surplus heat from, among others, municipal waste incineration, industrial processes, and power production may be used for cooling production by the integration of an absorption chiller into the plant.
Cooling production can also be distributed in local areas or buildings using the district heating system as the distributor of the waste heat to the local absorption chiller. This can imply higher distribution temperatures in the district heating system during the summer. This consequence must be fully analyzed before using this distribution solution.
As heat demand is seasonal and low during the summer, cooling production through an absorption chiller enables an increase in the efficiency of the plant by using excess heat that is available while displacing less environmentally friendly alternatives.
"Free cooling"
This refers to the extraction of available cold water. It can be compared to the use of geothermal energy in district heating systems. The cold water required to cool buildings can be found in oceans, lakes, rivers, or aquifers.
Using heat exchangers the cold is transferred to the distribution network and delivered to the customers where it is used in the cooling infrastructure of the building. The maximum cooling temperature delivered to customers can be guaranteed with - if needed - additional cold from different sources.
Such a system can be developed when the water temperature is cold enough and when the plant is close to the buildings where the water is transported. The advantage of free cooling is that it offers cooling on a renewable basis.
Such schemes exist in Europe (Stockholm, Helsinki) and North America (Toronto).
"Industrial chillers"
Highly efficient industrial chillers can be added as part of the production mix to secure outgoing temperatures and redundancy, and/or for peak capacity.
Storage
To increase efficiency and reliability, these cooling sources and production techniques are often combined with different kinds of storage solutions, such as:
Seasonal storage
Often performed as an aquifer solution, where free cooling in winter is stored for use during the summer period.
Night to day storage
Often performed as ice or water storage solutions. Overcapacity during the night is stored for use during the daytime.
Distribution
In a district cooling network, the chilled water is distributed to buildings where it loses its cold content, thus cooling down the building temperature; the warmed-up water is then returned to the central production facility. The supply temperature is normally between 6°C and 7°C, but an ice mixture of 0°C is used in some cases. The typical temperature in the return pipe is 12°C-17°C. The supply of cooling to the user can also be done through a district heating system coupled with absorption chillers at the user’s location.
Substation or Energy Transfer Station (ETS)
The customer interface or the “substation” is usually an indirect connection via a heat exchanger - the same technology as for district heating. The substation is not only the connection point and the contract boundary between the supplier and the customer but also a digital connection for the measurement of the cooling delivery.
Why choose District Cooling?
Maximization of diversities. Energy Efficiency.
Deferring Capital costs.
Increase in net lettable areas in buildings.
Reduction in O&M costs for the consumers.
Reduction in municipal utility infrastructure.
Minimization of environmental impact.
Thank You...!