There are a whole range of options available to designers when considering the application of cooling for a building, but unlike Europe, radiant cooling systems are rarely selected in the UK. In some ways this is surprising since they are more efficient, more comfortable, more attractive and healthier than systems which circulate cooled air. They are also less costly to run and maintain, and use less energy.

The term ‘Radiant Cooling’ is in some ways misleading in that it is not possible to radiate ‘cool’. What the term refers to is that a large chilled surface directly absorbs the radiant heat emitted from other surfaces in the room, including heat emitted from occupants. The result is that there is less asymmetric radiation in the room, and the room feels a couple of degrees cooler than the air temperature which is the reverse of the radiant heating effect. This has a corresponding reduction in energy required to maintain a comfort temperature as the actual cooling load is reduced.

The radiant cooling effect is not the only reason why radiant cooling systems are more efficient. they are all hydronic systems. Water has about 3500 times the energy transport capacity of air, and even allowing for the water pumps to overcome the pressure drops, will use about 5% of the energy used to move chilled air around a building. The space taken by pipe work is also considerably less than air ducts, not to mention the lack of dirt build up within air ducts and associated health risks and maintenance costs. The noise generated in moving the air, is also avoided. Any additional ventilation would involve around about 20% of the volume of air, and in many modern buildings natural ventilation is used, which is entirely compatible with a radiant cooling system.

Because the water temperature is controlled so that dew point is never reached, chillers for radiant conditioning systems tend to operate at higher temperatures than chilled water for fan coil systems. This results in increased chiller efficiency and lower operating costs. Moreover, the water flow temperatures (typically 16 - 18ºC) are very suitable for groundwater or surface water cooling, which enables cooling with just the energy cost of the water pumps, if such a source is available. Staying above dew point also means that energy is not wasted condensing water, and issues associated with the drying of air in many conditioned buildings are avoided. Of course it is possible to use dehumidification, either in the room space or in supplied ventilation air, in which case it is possible to operate at cooler temperatures and increase the cooling capacity, or decrease the area of cooling surface.

Very often moving chilled air from conventional cooling systems or wholly convected cooling systems such as chilled beams can cause discomfort to occupants in the direct air flow. Using radiant cooling systems eliminates this issue, making the room generally more comfortable throughout.

There are, on the other hand, a couple of drawbacks to radiant cooling systems, for example, because surface temperature is limited by dew point, a large surface area is required to provide cooling. With a capacity of about 50 – 60W per square metre of ceiling (less for walls and less again for floors), effectively the whole ceiling is used. This means that radiant cooling is insufficient for rooms with a high cooling load. Another potential drawback is that integrating the cooling into the structure can sometimes be complex. This issue can be reduced by, for example, using standard SPC Thermatile radiant ceiling panels supplied with chilled water integrated into a suspended ceiling, or by using the SPC Nest system which is insulation-backed plasterboard with embedded pipe work.

Another option, which eliminates these disadvantages, is to use radiant conditioning sails. The term ‘Sail’ is well recognised in Europe as a whole, in the UK we would tend to use the term ‘floating ceiling’, as a sail is always suspended below the soffit. An SPC ThermaSail is manufactured from a patented composite aluminium sheet. The sheet is strong and rigid, requiring no other structure than the flat panel itself. This feature of the sail is crucial to its performance, as without sidewalls or other strengthening structures above the ThermaSail, the air above the sail is cooled, and allowed to freely convect down around the edges of the sail. The addition of a sidewall would reduce the output by 15 – 20%.

ThermaSail Radiant Conditioning Sails fill the gap between chilled ceilings and passive chilled beams in almost every aspect. They provide all the benefits of a radiant cooling system, yet achieve roughly double the output of a conventional chilled ceiling due to the considerable amount of convective output. This convective output however, will diffuse around the sides of the sail, without creating any uncomfortable draughts.

The whole concept of Radiant Conditioning Sails is simple and straightforward, therefore, reliable and attractive.

However the advantages of using SPC’s ThermaSails do not stop with their efficient cooling performance. Warm water can also be passed through the coil to provide very efficient heating as well. The warm water heats the sail to provide radiant heating in a similar way to a conventional radiant panel, with the radiant heating effect saving energy. The difference is that with no insulation on the top surface of the sail, there is additional convective heat output, because the sail is always suspended below the soffit in the room. This is useful output warming the room space. Once the Sail has been sized for cooling, low warm water temperatures can be used to make the most of condensing boiler efficiencies, but there is also low system inertia, so the performance is very controllable, wasting less energy in warming up and cooling down times.

Control of the cooling and heating can be done in a couple of ways: -

Firstly, three pipe or four pipe systems can be used to control the cooling and heating performance, using the sails in a similar way to fan coils. The only difference is that dew point sensors need to be fitted to ensure that the temperature stays above dew point.

Secondly, a separate heating coil can be installed on the Sail, making control simpler. In this instance, the area of sail for heating would have to be added to the area of sail required for cooling.

For temperature sensing, black bulb temperature sensors are recommended, because they pick up the comfort temperature. When radiant heating and radiant cooling are being controlled, a normal air temperature sensor would have to be compensated in opposite directions for each mode, which would generally prove unsatisfactory.

An additional use of a ThermaSail is to provide acoustic attenuation in a room. The sail can optionally be perforated with a variety of perforation schemes, which can meet most sound reflection requirements. This can also give an interesting variation to the finished appearance of the sails.

A Radiant Conditioning ThermaSail is very slim, and the smooth finished surface can be just 60mm below the soffit, whilst still allowing for convection air movement above. This means that there is very little ceiling height lost, and sails can be used in low ceiling applications.

With the strength and simplicity of the composite aluminium structure, a ThermaSail can be made to almost any shape, size and colour. Other services can easily be integrated, which could include for example down-lights, up-lights, or lights suspended from and below the sails. Other services such as ventilation grilles or sprinklers could also be integrated in to the sails.

This leaves the architectural designer free to modify and shape the sails to create attractive yet fully functional ceiling features. Designer ThermaSails, provide attractive ceiling features with silent and efficient cooling and heating.

Even the most basic Thermasail options provide an attractive ceiling appearance, yet at the same time provide extremely comfortable indoor working conditions for demanding applications.

The simplicity of a ThermaSail also makes the installation easy. Weighing less than 8Kg per square metre including water, the sail is normally suspended from the soffit on suitable adjustable hangers (usually wire hanging systems), and the connections made either horizontally or vertically. This makes them very suitable for both new build and refurbishment projects, and also makes it straightforward to adjust layouts should office spaces be divided differently at a later stage.

In summation, SPC’s ThermaSail Radiant Conditioning Sails are an innovative, yet simple, attractive and extremely efficient option for comfortable cooling and heating for a whole variety of building types, giving the designer a whole range of new options.

SPC offer a CPD seminar approved by the CPD Certification Service to discuss the benefits and design principles of Radiant Conditioning Sails, and a design service to assist with selection of sails for specific applications.

SPC will be showcasing both their Thermatile Radiant Panels and their ThermaSail Radiant Conditioning Sails along with their award winning MINIB trench heaters at Interbuild 2007.

Interbuild Details: Hall 3/3a Stand M22

Email: neil.hudson@spcoils.co.uk