Low cost passive cooling

Citation

Barnard, N. (2001), "Low cost passive cooling", Facilities, Vol. 19 No. 3/4. https://doi.org/10.1108/f.2001.06919caf.001

Low cost passive cooling

Low cost passive cooling

Keywords: Air conditioning, Refurbishment

This article describes refurbishment work at Stevenage Borough Council’s Offices undertaken to address a summertime overheating problem using passive measures, including storage of night cooling in the building structure. The work is part of the Council’s "Best Value" approach to dealing with office comfort conditions. The night cooling storage is achieved without exposing the structure by means of an innovative technical solution called "Airdeck" (Figure 1). Monitoring of the building before and after the refurbishment confirms that the measures have had a significant thermal benefit, reducing internal summer temperatures by around 5°C. This has been attained at a fraction of the cost of installing an air conditioning system.

Figure 1 Airdeck element

Background

As in many existing offices, increasing IT gains were creating overheating problems in many parts of the building. Rather than resort fo a fully air conditioned solution it was decided to use passive measures where possible and effective. The passive design solution was based on solar blinds and the use of thermal mass in combination with night cooling. The thermal mass is used to store cooling introduced into the building at night by circulating relatively cool ambient air through the building – "night cooling". This stored cooling is then released the next day to offset heat gains and limit internal temperature rises.

The simplest way to achieve thermal access to the slab would have been to expose it directly to the space. However this had drawbacks in terms of aesthetics and acoustics. In addition, these exposed surfaces could have acted as a store for unwanted heat losses during the winter and significantly increased heating demand. Therefore the solution proposed was based on maintaining the integrity of the false ceiling and using fans to circulate air between the space and void above to create and control the thermal link to the slab. The fan energy penalty is relatively low as pressure drops are small and can be at least partially offset by storage of unwanted heat during cooler periods.

Simply circulating the air through the ceiling void provides only very limited heat exchange between the air and the slab. Therefore performance was improved by passing the air through narrow air paths between the slab and Airdeck sheeting elements fixed to the soffit (Figure 2). The turbulent air flow created in these paths will improve heat exchange between the slab and the circulating air so improving the night storage and day discharge of the cooling. The depth of the air path is set as a function of air flow rate to create turbulent flow and high surface convective heat fransfor ( ~ 15W/m²K) without incurring undue pressure drop penalties (~ 30Pa) (refer to CIBSE Building Services Journal, March 1999, "Keeping cool after dark" for report on testing and development work).

Figure 2 Thermal access

Installation

The Airdeck elements have initially been dimensioned at 1,800mm x 450mm to integrate into floor/ceiling voids and for ease of handling. A 150mm spiggot connection is located centrally for connection to the fan system. The Airdeck element is constructed from sheet steel with 12mm returned edges along the length for rigidity. Spacers/seals are fixed along the underside edges to form and seal the air path from the spiggot connection to the ends.

The Airdeck is installed in an existing ceiling void approximately 220mm deep. The system has been designed fo provide thermal access commensurate with an exposed soffit. A modular arrangement has been adopted with four Airdeck elements per circulating fan (Figures 3 and 4). In this application air is extracted from the space by the fan via an extract grille in the false ceiling and ducted to the Airdeck element spiggot connections. The air then flows under the elements exchanging heat with the slab, out of the ends into the ceiling void, and is finally returned to the space via grilles in the false ceiling.

Figure 3 Typical design module

Figure 4 Installed module

In summer cool outside air is introduced into the space by window fans at night. The Airdeck fans also operate to store the cooling in the slab. During the day the Airdeck fans operate to release the stored cooling from the slab. Ventilation is provided by windows under occupant control. In mid season the Airdeck fans can also be used in a heat recovery mode to store any excess heat gains during the day. In winter the Airdeck fans will be off, acting to isolate the thermal mass of the slab from the space.

Monitoring results

Monitoring was undertaken in areas throughout the building in the summer of 1998 before the refurbishment works were undertaken and again in the summer of 1999 with the remedial measures in place. (Note: during this initial operating period the Airdeck system was under manual control.) Results for one of the West facing areas with Airdeck installed are shown below to give a comparison of thermal conditions "before" (Figure 5) and "after" (Figure 6). These indicate that a reduction in the region of 5K in internal temperatures has been achieved relative to ambient temperatures. Approximately 12K of this is due to the solar blinds that were also installed in this area.

Figure 5 Comparison of internal/ambient temperatures prior to refurbishment

Figure 6 Comparison of internal/ambient temperatures with Airdeck and solar blinds

The monitored results are inline with those predicted by the design analyses. They demonstrate that effective use can be made of thermal mass in existing (and new) offices fitted with conventional false ceilings and that direct exposure of the mass is not necessary.

Further information and enquiries: www.airdeck.co.uk

Nick BarnardOscar Faber