The cooling demand in our buildings is increasing. District Cooling can offer a sustainable and cost-efficient solution for the increasing demand. Attention needs to be paid to passive measures for reducing loads as a priority and subsequently to free cooling and sustainable production.
By Lars Sønderby Nielsen, Managing Director, Enexio Solutions ApS
Large heat pumps and chillers are gradually being integrated into the energy system to utilize free energy sources and waste energy from low-temperature sources. A key driver for the development is the green transition in district energy. Much attention is paid to heat pumps for displacing fossil fuels in district heating. But at the same time, similar benefits can be obtained with district cooling solutions, or in the optimal case by simultaneous heating and cooling production.
The drivers for increasing cooling demand are numerous. Strict building codes help reducing heating demand in the winter but can as an unwanted side effect leads to overheating in the hot summer months. In residential construction individual cooling systems often lack efficiency and are, in many cases, impossible to implement within the increasingly strict energy code requirements. In commercial construction, the energy required for cooling is often higher than the energy required for heating. I addition we seem to experience a change in climatic conditions with longer periods with more severe temperature conditions. This calls for robust and sustainable solutions for maintaining an adequate indoor thermal environment in the warm summer periods.
The first and foremost measure in meeting increasing cooling demand should always be passive measures. Such are obtained with good architectural design including for example shading and natural ventilation of buildings. Such actions also help reducing peak loads and can thus help to reduce the capacity cost for the cooling system.
The second measure should be the use of free cooling solutions, like ground source water, seawater, or similar. These can preferably be integrated into district cooling systems for efficiency reason as well as shared and thus lower investment costs among the users. Free cooling solutions are however seldom sufficient to meet the demand.
Also, a district cooling system needs production capacity from one or more traditional sources based on large scale chillers of either compression or absorption type. Each chiller type has its distinct advantages and disadvantages, and the final choice of the cooling source is often a combination of different types.
Use of free sources and waste energy
Free sources should always have priority for cooling purposes. Free cooling can be obtained from ground sources, seawater, lakes, or similar depending on local conditions. Besides, the warm summer period often offers free high-temperature sources from the surplus heat, waste incineration, or similar.
High-temperature thermal sources often have limited use in the summer but can be used for cooling purposes by converting the heat to cooling with absorption chillers. Using absorption for district cooling purposes is the ideal way of upgrading low-quality waste heat in the summer period.
The operation principle of absorption cooling is a machine with 4 large heat exchangers, which together act as a chiller. The machine uses water as a refrigerant and a salt (LiBr) to absorb water vapor at low pressure. Unlike a conventional electrical chiller, the absorption chiller is driven by thermal energy, resulting in minimal electricity consumption and very low operating costs.
The machine uses water as a refrigerant, which means that the GWP value (Global Warming Potential) is practically zero. The technology is mature and well-proven, with hundreds of thousands of commercial installations worldwide.
Absorption is largely used already in, for example, Sweden, Finland, Germany, and France. In Gothenburg, Sweden, for example, absorption chillers are widely used for district cooling purposes, converting surplus heat from waste incineration at typically 90°C to district cooling that can be delivered at 6°C to commercial buildings connected to the cooling network of the city.
In warm hemispheres, absorption cooling can be combined with thermal solar collectors. A thermal solar field can feed the generator of an absorption chiller, which then transforms the hot solar production (90-100 °C) to chilled water (typically 6-12 °C) that can be used in a district cooling system.
The rejected heat needs to be deposited in traditional cooling towers, with a temperature set of typical 37 °C/30 °C. Naturally, this requires that the solar heat source for driving the absorption chillers is reliable and has high availability. Solar cooling is preferably driven with temperatures above 90 °C, but the absorption process can work with temperatures down to 70-80 °C. This requires however machines of so-called double or triple effect which will impact the capital expenditure.
What about the temperatures?
The basic laws of thermodynamics of course apply in district cooling and absorption chiller design. In the cooling plants, good efficiency is obtained when the supply water temperature is as high as possible combined with a district cooling system that can operate at a high delta T.
However, this represents a few challenges, both in the grid and on the building level. Each building needs to be equipped with an HVAC system that can operate at the temperatures provided by the cooling grid. If one individual customer requires 6 °C, this will determine the design temperatures for the whole grid. This again will impact chiller design, piping, required flow, and thus the total cost. That is a frequent issue for the design of cooling systems in existing buildings.
In new developments, buildings can be designed for integrated high-temperature cooling, which lowers the cost and eases the integration of free cooling and optimized chiller design. Absorption technology can provide a supply water temperature down to 6 °C without problems. This requires that the extracted heat is rejected to cooling towers at temperature levels typically in the range of 37°C /30°C. But if there is a wish to upgrade and utilize the rejected energy, the thermal design and temperature levels need more careful consideration.
Co-production of heating and cooling
If a simultaneous requirement for heating and cooling, the cold-water circuit of the absorption chiller can be used for cooling, and at the same time, the heat rejected in the absorber/condenser circuit can be used for heating. An example of such a solution is the University Hospital, AUH, in Denmark, where an absorption chiller delivers 3 MW of cooling to the hospital’s center for particle therapy.
The absorption chiller is powered by hot water at 105°C from the district heating transmission network. This converts 4.3 MW heat to 3 MW cooling and 7.3 MW district heating, used simultaneously. Chilled water is supplied at 12°C while the rejected heat, supplied at 52°C, can be further boosted and used in the heating system.
The AUH absorption chiller is installed in conjunction with mechanical cooling from compression chillers. This means that production can be planned according to energy prices. The heat source price for operating the absorption chiller generator typically varies between 0 and 20 EUR/MWh, whilst the electricity spot price sees big variations with typical seasonal averages of 180-200 EUR/MWh.
Such price schemes would on average give an advantage for absorption over mechanical cooling. But the real advantage is the flexibility of operation that allows selecting the production scheme according to actual energy prices. This can be done without much extra capital expenditure since the total installed cooling capacity needs to match peak load demand, which typically occurs for only short periods annually.
There is not one cooling solution that solves all problems. District cooling systems allow the integration of several different production methods, where free cooling sources can be prioritized and integrated alongside conventional chiller solutions of both compression and absorption types.
The advantage of absorption technology is that it can convert thermal sources like solar, waste heat, and surplus district heating to cooling in the warm summer periods. Such cooling sources are green and sustainable and use pure water as a refrigerant with a global warming potential factor (GWP)=0. Most benefits are naturally found in systems where there are simultaneous heating and cooling demands. If the system and chiller design is done with care, the energy extracted from cooling purposes can be upgraded and utilized in a heating system.
For further information please contact: Lars Sønderby Nielsen, email@example.com