What are the advantages of dry cooling?

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 to efficiently operate the building.

As the concept of dry cooling is all about delivering sensible cooling to an indoor space from a hydronic circuit, the temperature of the system’s circulating water is always kept above dew point. This is to eliminate the risk of condensation. Chilled beams are designed to ventilate, cool and heat without condensation during the cooling mode, which is why chilled beams are designed without drainage systems that otherwise would take care of any such condensation. Fan coil units are also used to provide hydronic cooling, heating and ventilation. Because fan coil units are designed to operate with moist coils, they require filters to prevent dust from adhering to the moist coil surfaces. Filters must be cleaned or replaced to ensure proper unit functionality. Since chilled beam coils do not get moist, filtration is not required – particles have no moist surface to adhere to – they pass through the chilled beam. Room air is kept clean via the Supply Air, filtered at the air handling unit.

What about humid outdoor air conditions?

The concept of dry cooling may seem a bit unusual, or even risky for designers who are not used to sensible-only cooling solutions of this kind. Coil condensation resulting in moisture dripping from the terminal units is the most common concern. Chilled beams are successfully applied whenever the chilled water supply temperature is maintained above dewpoint, which is simple to design and control.

Chilled beam systems rely on the air handling unit to adequately dehumidify the outdoor air before it is distributed in the building. This means that all the condensed moisture is drained from one central location, which is easier and less costly to maintain. Dehumidification can be efficiently accomplished by first using a DOAS (dedicated outdoor air system) unit with sorption-coated rotor type heat exchanger to reject moisture from the outdoor air, without the operating cost associated with cooling coils. Only when the rotor capacity is exceeded are mechanical cooling coils required for dehumidification. And because the rotor takes care of part of the dehumidification load, the mechanical cooling coil may be smaller, saving more first cost.

Moisture also enters buildings through infiltration. It is important to intentionally design buildings with strict airtightness strategies, including envelope design and DOAS unit control. The Passive House concept has made airtightness practical and affordable for any building type. Airtightness reduces both heating and moisture loads. Although now airtightness measure is 100% effective, designing to 0.6 ACH is easily achievable. And with such low levels of infiltration, a DOAS unit controlling Supply and Return Air flow rates within 10% of each other, the ventilation system serves to prevent building under-pressurization which might otherwise compound the infiltration of moisture.

What about temporary humidity increases?

Chilled beams have been successfully applied in buildings since the 1980’s. The conditions under which condensation can occur are well-understood, and several design steps are regularly used to prevent condensation in rooms that have become temporarily humid.

First, the dewpoint of the conditioned zones should be measured, and used to control the chilled water temperature, so that the chilled water temperature is always above the dewpoint. This method allows for cooling even while design conditions have been exceeded.

A second step is to use dewpoint sensors attached to the chilled water pipes in the chilled beam. When dewpoint is approached, a signal from the sensor can be used to trigger the closing of the chilled water valve until the risk of condnesation has passed. Meanwhile, the control system can also trigger a temporary increase in dry supply air to expedite dehumidification.

Reduce fan energy consumption

From a system perspective dry cooling offers several advantages.

The temperature of the cold water is significantly higher than in a traditional fan coils system. The chilled water supply for chilled beam systems can be produced using a chiller operated with a very high seasonal energy efficiency ratio (SEER). Since it is mainly the outdoor air that is dehumidified, the peak cooling capacity is lower in a system operating under dry cooling conditions. This also means that the size of a chiller, multi-function or reversible heat pump can be reduced. Further, it also means that free cooling by direct heat exchange with the ambient outdoor air is possible during a larger part of the year. All in all, these advantages may have a noticeable impact on energy consumption, hence cost.

Chilled beam system savings also emerge from the absence of individual fans in each zone. Unlike fan coils, chilled beams do not require fan energy to overcome filters in each zone.

An active chilled beam solution makes use of the ventilation duct pressure to recirculate room air through the coil. The energy requirement to provide that pressure at the DOAS unit is minimal compared to what fans in each unit otherwise would consume.

Ground source energy

In some regions 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 local geological conditions.

Conclusion

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, so the chilled water temperature is always above dew point. The control of condensation is well-understood and easily applied, in response to seasonal outdoor air and temporary indoor air humidity conditions. The additional benefits of reduced filter and fan service and maintenance, resulting in lower energy consumption and enhanced overall efficiency make the careful design of dry cooling solutions a compelling option.

Contact us to discuss our chilled beam solutions.  At Swegon we are happy to discuss dry cooling solutions, including the application of our DOAS air handling units, chillers, heat pumps and chilled beams.