Introduction

The application of Passive Downdraught Evaporative Cooling (PDEC) to the central atrium space within the new stock exchange in Malta demonstrates the viability of its wider application in southern Europe, both to new buildings and in refurbishment projects. PDEC meets approximately 25 percent of the cooling requirement and the residual load is met by chilled water cooling coils. The system avoids the need for conventional air conditioning and saves approximately 50 kWh/m²/year. The building was completed in 2001.

The project for the new Malta stock exchange involved the refurbishment of an existing listed building on a very prominent site in Valetta. The client wished to demonstrate their commitment to environmental responsibility. The new Malta stock exchange incorporates a five-story atrium surrounded by perimeter cellular offices. While the cellular offices and the lower ground-floor meeting room are air-conditioned, the implementation of passive downdraught cooling for the control of temperature within the central 14 m high atrium avoids the need for conventional air-conditioning and reduces energy and maintenance costs.

The new stock exchange is located in the Mediterranean Island of Malta within the existing Garrison Church at Castille Place, Valletta. Malta experiences mild winters (9.5°C mean minimum in January) and warm summers (30.6°C mean maximum for August), with recorded extremes of 1.4°C and 42.1°C. As a small island, the relative humidity is generally high, exaggerating the perception of cold in winter and increasing discomfort in summer. Wind direction is generally from the northwest, although warm winds from south occasionally bring dust in summer.

Objectives

At the design stage of the Malta stock exchange – as a refurbishment of an existing building – concerns were expressed that proposals for cooling the main large volume may lead to either very intrusive ductwork or large fan coil units within the spaces themselves. The objective was to propose a cooling strategy for this main large space with minimum reliance on mechanical and electrical (M&E) equipment. During summer days the proposal was to exploit downdraught cooling which was induced in a column of air by either evaporative cooling by means of hydraulic nozzles (‘micronisers’) or direct cooling by means of chilled water coils. Previous work suggested that a combination of these techniques was both technically and economically viable for many locations in southern Europe. The distribution of cool air with both techniques is from high level to low level and the pattern of resulting air temperatures and air velocities within the space is strongly influenced by the geometry of the major spaces. In addition, during summer nights the project exploits convective night ventilation whereby air at low night-time temperatures removes heat built-up during the day within the walls, floors and roof of the building. The objective in combining these ventilation and cooling strategies is to provide a comfortable internal environ-ment with mini-mum reliance on M&E equipment. The air volume flow rate is influenced by the size of openings at high and low level and by the resistance imposed on these openings by louvers, motorized dampers, cooling coils, etc.

Process

The project started in 1998 and was completed in 2001. As part of the cooling and heating strategies for the large central space, five complementary specifications have been implemented. These figures show diagrammatic sections through part of the building indicating the air distribution path according to the cooling strategy.

Automatic ventilation openings: The 18 high level vents (1.5x0.85m) are top hung windows at the ridge, with a maximum throw of 0.5m. These window vents are subjected to considerable wind pressure, so each vent is operated by two 24v actuators linked to rack and pinion gear operated by three-phase motors. The vents at lower ground level consist of motorized dampers mounted in the east, west and south walls. These ventilators have edge seals to minimize uncontrolled ventilation.

Cooling coils: Two circuits run from the header along each side of the walkway. The copper coils are inclined at approximately 70 degrees to encourage airflow through the coils, and 300 mm-wide custom-made trays were installed to collect condensate. The cooling coils call for chilled water in response to variations of both internal temperature and internal relative humidity. The control strategy adopted assumes that the cooling coils are linked to the automatic dampers and vents and operate in conjunction. When the coils are switched on, the actuators at high level and the dampers at low level will both be set to the minimum aperture.

PDEC system: The PDEC system relies on 18 hydraulic nozzles at one pair/bay in four sets. Each set is controlled by motorized valves linked back to the Building Management System (BMS). The total water volume flow required is approximately 90 l/hr with a pressure of 25 bar. The pipework is attached to the balustrade and the micronisers have been located at the centre of the gap between the cooling coils. The PDEC system also operates in conjunction with the motorized dampers. When the system is on, both banks of motorized dampers and high level vents are set to fully open. The aperture of openings may be overridden by the wind speed/direction sensor. The dampers are set to the minimum aperture when the wind speed is above 5 m/s. It was paramount to minimize the risk of microbial growth within the PDEC system, hence the system was designed to comply with the Australian new standards (AZ/NZ 3666 Parts 1,2 and 3 of ‘Airhandling and water systems of buildings – Microbial control’) developed to reduce the risk of Legionella microbes from developing in cooling water systems. When evaporative cooling is operating (Fig 1) the energy costs of cooling are negligible. Of course when the chilled water coils are operating (Fig 2) the energy cost of chilling must be met.

However, because both techniques rely on buoyancy to drive the airflow through the building, fan power (which can be 30–40% of air-conditioning running costs) will be avoided. Figure 1Summer day cooling strategy using PDEC

Figure 2Summer day cooling strategy using cooling coil

Night Time Convective Cooling: It was anticipated that during the day in summer, internal temperatures would rise to 25 or 26°C. Useful convective cooling is therefore promoted when the external temperature drops below 23°C, which occurs frequently at night during the summer period.

The movement of air (in the absence of wind) is driven by buoyancy forces (Fig 3) reversing the daytime air movement pattern under downdraught cooling. The night cooling is controlled via the motorized dampers at lower ground level and the vent actuators at the ridge level. Convective cooling relies on large air change rates. When night ventilation is initiated, all low and high level vents are fully opened. In this way the benefits can be genuinely regarded as ‘free’ cooling. Figure 3Summer night cooling strategy using convective night ventilation

Heating: Fan-coils have been used with the heat pump chillers for a maximum water temperature of 50°C. Simulations indicated a peak demand of 25–30 kW to bring the central space up to 21°C from an ambient low of 8°C on a Monday morning in February. However, once a building is occupied the demand for heat will be very low. It was important to make sure that the location of these fan-coil units avoids the obstruction of the airflow path under natural ventilation. Simulation results show that temperatures in the space remain at an average of 24–25°C but these vary according to localized internal gains across the plan. Warm air rises from the lower ground-floor through the voids on the north side of the building. On the south side the downdraught effect from the cooling coils is more emphasized although with low velocities (between 0.3–0.4 m/s). The general result is a good mixing of the air across and along the central space and a fairly even temperature distribution. Despite the outside temperature being defined at 38°C, the downdraught cooling effect from the coils appears to be able to maintain relatively low internal temperatures with no signs of overheating. This is also helped by the more even distribution of the cooling coils at high level.

Financial Resources and Partners

Cost/benefit studies were not carried out for this project. The total costs are not available.

Results

The potential savings for the atrium space alone represent over 60 percent of the predicted energy consumption for large fan coil units. The general building improvements (shading, insulation, night ventilation) also benefit the cellular office accommodation, reducing the energy consumption of the cassette units by approximately 23 percent. The total cooling energy consumption for the whole building using fan coils was estimated at approximately 103,924 kWh, compared with 54,139 kWh for the built solution using cooling coils and PDEC, resulting in a total energy saving of 48 percent. This represents an annual saving of £3,080/year based on £0.06/kWh. The economic benefits are capital and running cost savings and the environmental benefits are reduction in CO₂ emissions by approximately 28,152 kg CO₂.

Lessons Learned and Replicability

The downdraught process drives the airflow pattern inside the building avoiding the need for fans, ductwork and suspended ceilings. Downdraught cooling and night-time convective cooling strategies have therefore clear benefits in the increased energy efficiency of the building and in the comfort perception of its occupants. Adequate time must be allowed for commissioning and demonstration of the control strategies. The success of PDEC applied to the central atrium space within the new stock exchange in Malta, demonstrates the scope for replication throughout southern Europe, both on new buildings and in refurbishment projects.

Advantages of Passive Downdraught Evaporative Cooling