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The concept of dry cooling is all about delivering sensible cooling from a hydronic circuit. It might sound simple, but it has a prerequisite - the temperature of the system’s circulating water has to be kept above dew point at all times. Once this criterion is met, there is a clear potential to save energy and be cost efficient in the operation.
As the concept of dry cooling is all about delivering sensible cooling to an indoor space from a hydronic circuit, it is a must that the temperature of the system’s circulating water is kept above dew point at all times. This to eliminate the risk of condensation. Active chilled beams and climate ceilings are designed to operate without the upcoming of moisture, which is why these product alternatives, in general, are designed without filters or drainage systems that otherwise would take care of any such water. Normally, this allows for a significant cost savings over the lifespan of an installation in regards to service and maintenance. When filters are used, they need to be cleaned or replaced in order to ensure proper functionality of the unit, and drainage systems need proper care to avoid growth of bacteria and mould.
What about humid conditions?
The concept of dry cooling may seem a bit risky for designers who are not used to create heating, ventilation and cooling (HVAC) solutions of this kind. The fear of ending up with condensed moisture dripping from the terminal units is what is the most frightening. However, in several parts of the world, it is not a problem, certainly not as long as the supply water temperature is controlled correctly.
A dry comfort cooling system relies on adequate ventilation rates to handle the latent loads in the building. In highly humid conditions, the air handling unit (AHU) must also dehumidify the outdoor air before it is distributed in the building. This means that all the condensed moisture is taken care of in one spot which, if we return to the cost of maintenance, will require only one point of service. Dehumidification in the AHU can be efficiently accomplished by using sorption treated rotors, meaning that the treatment of the rotors helps transfer moisture from the outdoor air, the supply air, to the exhaust air with the need of minimal energy consumption. However, infiltration of humid air from the outside can be an issue, and has for sure been in the past, but to ensure that new as well as renovated buildings are energy efficient, the levels of infiltration are kept to an absolute minimum these days.
And temporarily increased levels of humidity?
If there is a fear of temporarily getting increased levels of humidity in a room, consequently causing a risk of condensation, there are a few ways to go about to mitigate the risk of ending up with water dropping from the installation. The easiest way, is to use sensors attached to the cold water pipes which can register changes in conductivity. When a thin layer of moisture is formed on the sensors they will trigger a signal to the room thermostat to close the water valve. A more advanced method is to measure the relative humidity in the room on a continuous basis and let the thermostat close the water valve when the dew point in the rooms is closing in on the supply water temperature.
Both of the above mentioned types of condensation protection will stop the hydronic circuits’ heat removal from the room which may cause an uncomfortable indoor climate for the people inside. The most advanced method of condensation protection is therefore developed to sense the risk of condensation in the room and will send a signal to the system to, instead of closing the water valve, increase the temperature of the supply water. This way, the water circuit system will continue to remove heat from the room, obviously with a lower capacity due to the increased water temperature, but the reduced capacity is still quite often more than enough for a temporary period of time.
Advantages of dry cooling
From a system perspective there are several advantages of operating under dry cooling conditions. The temperature of the cold water is significantly higher than in a traditional fan coils system, which relies on wet cooling. Meaning, the water can be produced using a chiller operated with a very high seasonal energy efficiency ratio (SEER). In general, as it is the outside air only that is dehumidified, the peak cooling capacity is lower in a system operating under dry cooling conditions. This further means that the size of a chiller can be reduced. Further, it also means that free cooling by direct heat exchange with the ambient air is possible during a larger part of the year. All in all, advantages that may have a noticeable impact on energy consumption, hence cost.
Moreover, savings emerge as fans in each terminal unit, used to overcome pressure drops due to the use of filters, are unnecessary. An active chilled beam solution makes use of the existing duct pressure to create a flow of room air through the coil. The energy requirement to provide that pressure in regards to the AHU, is minimal compared to what fans in each unit otherwise would consume. The same applies to a climate ceiling solution, it is not dependent on any forced convective flow and does therefore not need any additional fan energy.
The Nordics and ground source energy
In the Nordic countries it is very popular to use ground source heat pumps for heating, and often the heating requirements become dimensioning for the depth of the bore holes. However, the bore holes can also be used to obtain cooling from the ground during the cooling season, the meteorologically warmer period of the year. Using a system solution dependent on dry cooling then allows for the depth of the bore holes to be reduced, subsequently, the initial investment cost is reduced. A further energy benefit is that the heat removed from the building during the cooling season can be stored in the ground and be recovered during the heating season. The efficiency of this kind of energy storage depends primarily on the geological conditions.
In summary, to make use of dry cooling, a system design is required where the supply water temperature and relative humidity of the indoor air is controlled. When correctly controlled, it is ensured that the dew point is kept above the supply water temperature at all times. The different ways of controlling water temperature and humidity can be seen as condensation guards and provide extra security in case of temporarily increased humidity levels. The compelling benefits of reduced requirements of service and maintenance, lower energy consumption and enhanced overall efficiency make the careful design of dry cooling solutions worth the while.
At Swegon we are happy to discuss any questions in regards to dry cooling solution with either chilled beams or climate ceilings. It can also be useful to turn to our guide for further insights about dehumidification by the means of sorption treated rotors.
