Energy efficient by design
Chilled beams and ceilings are generally accepted as an energy-efficient way to cool buildings, but the design of chilled beam and ceiling systems is vital to their effective performance. Andrew Jackson highlights important points to consider when applying these technologies.
Chilled beams and ceilings are an increasingly popular technology with clients who want predictable space cooling delivered in an energy-efficient way. Chilled beams are by now a well-known technology and have been applied in a range of commercial buildings. Good design and installation have proved very important in maximising the benefits they can bring to a project.
Chilled beams are a flexible, low-energy choice for both new-build and refurbishment projects, and the same can be said of chilled ceilings. Passive chilled beams consist of an exposed chilled-water coil suspended from the ceiling. The convection process drives warm air up through the occupied space. This air passes over the chilled beam, is cooled and returns to the occupied space.
An active chilled beam operates on the same principles, but with the addition of nozzles delivering air near the water coil to increase the effect of natural induction — causing more air to be drawn up from the occupied space.
Air systems, often at floor level, deliver fresh air into the space in combination with the beams.
Passive chilled beams have cooling outputs in the region of 300 W/m of length, and active chilled beams can achieve cooling outputs of 500 W/m.
The spacing of chilled beams depends on the width, height and design of the unit. Often beams incorporate elements such as lighting, or are specifically shaped for a particular project. The density of beams in the occupied space will therefore determine the maximum cooling output of the system. Another important design aspect is that both active and passive beams can allow the exposure of the thermal mass of the building soffit, which can be exploited for cooling. In general, it is accepted that an exposed thermal mass can provide additional cooling potential of 24 W/m2.
A chilled ceiling incorporates a single-piece copper element into the rear of a standard metal ceiling tile. Chilled ceilings achieve outputs of around 45 to 65 W/m2, which is adequate for most office buildings. However, they can be used in conjunction with chilled beams to meet additional cooling requirements, for example where there is solar gain around a glazed elevation. A major design advantage of chilled ceilings is that they can be installed in voids of less than 100 mm owing to the narrow diameter of the cooling element, which is particularly relevant when refurbishing 1960s and 1970s buildings.
Water supply and return temperatures for chilled beams are around 14 to 17°C. Avoiding condensation has long been recognised as an important design factor for beams, and is usually tackled by dehumidifying outside air, combined with careful monitoring to maintain occupant comfort.
For today’s designers, it should be noted that the operating temperatures of chilled beams and ceilings make them ideal for linking with ground-source systems. In fact, this is a growing trend among building owners who are keen to make an environmental statement. One recent example is the London School of Hygiene & Tropical Medicine. Here, the beams are supplied with ground water drawn from geothermal boreholes beneath the building, tapping into a natural aquifer.
Paul Downie, of Downie Consulting engineers who worked on the project, says, ‘The environmental systems are designed to balance the building’s significant thermal gains with inherent passive construction and renewable sources. This includes groundwater extracted from 70 m below existing pavement vaults, which provides a totally renewable form of cooling for the building.’
With refurbishment now playing an important role in the work of consulting engineers, chilled beams are demonstrating that retrofitting these systems can be successfully achieved, and bring a number of advantages, particularly where space is at a premium.
One example of a particularly large refurbishment project is at Lewins Place office building which used over 1580 m of active integrated service chilled beams. This low-energy cooling solution was an important element in helping the building achieve a BREEAM ‘Very Good’ rating.
As with most technology choices for refurbishment projects, a number of factors lead to the choice of beams. Physical building constraints affected decisions about opportunities to maximise the available space and which energy efficient services to specify. Dr Brian Atkins, Associate, Hulley & Kirkwood Consulting Engineers, said, ‘In striving for the maximum net floor area and maximum floor to ceiling height, a number of different thermal emitters were considered: fan coil units; perimeter induction units; thermal ceiling and active beams.
‘Given the spatial constraints, appearance, the existing riser locations and the calculated cooling/heating loads, a 4-pipe top discharge active beam option was selected to satisfy these criteria.’
With year-round equipment and people loads, many buildings do not need a heating system for large parts of the year. However, heating still needs to be considered, and perimeter trench heating systems are often used to satisfy this demand.
The design of active chilled beams and integrated service modules already incorporates fresh air distribution as part of the ducted air supply. A heating element can be incorporated into an active beam, removing the need for an additional heating system — reducing both capital costs and long-term energy use.
Andrew Jackson is marketing director with SAS International.
