Combining air conditioning and passive ventilation into an energy-efficient solution
Fig. 2: First-floor office space on the corner of the first floor of Passivent’s building in Manchester has provided a successful trial of mixed-mode. The inlet grills for the passive ventilation are just visible above the window line, and the ventilation stack can be seen just above the roof line.
Air conditioning and passive ventilation both have important contributions to make to the comfort of buildings. Now those contributions have been combined and increased by two companies working together to exploit the benefits of mixed-mode cooling.Not only does the new Part L2 of the Building Regulations demand cuts in the energy used by buildings, it also requires that buildings do not overheat. Compliance requires computer-model demonstrations to show that reasonable temperatures can be maintained. Guidance on maximum indoor temperatures from CIBSE recommends that internal temperatures should not exceed 25°C for more than 5% of the occupied period and that they should not rise above 28°C for more than 1% of the occupied period.
Limitations Strategies based on natural ventilation and mechanical ventilation are widely considered — but have limitations. Natural ventilation, for example, can cope with heat loads of around 40 W/m2 — about half that of a typical modern office building. Night cooling using natural ventilation makes a valuable contribution by reducing peak internal temperature by 2 to 3 K the following day. Donald Daw of air-conditioning company Mitsubishi Electric explains that despite their environmental credentials, naturally ventilated buildings are prone to overheating. He, quite naturally, is committed to cooling being provided by air conditioning — but he is equally committed to energy-efficient air conditioning. He points out that the energy efficiency of air-conditioning has increased by 50 to 60% in the last three years — largely due to inverter drives, which have made COPs of 4 to 5 at part load commonplace.
Mixed mode Such jumps in performance will not been seen over the next three years, which has led Mitsubishi to investigate how the benefits of air conditioning and natural ventilation can be combined. Mitsubishi’s expertise in air conditioning and controls has been combined with the natural-ventilation capabilities of Passivent to develop a mixed-mode solution which achieves a step change in the efficiency of achieving comfortable buildings. The system is now being brought to market. A small project in Manchester tells the story. It compares mechanical cooling with fans to distribute air around a building with a mixed-mode system using just natural ventilation. The reduction in energy consumption is 41%.
Project The project is in Passivent’s own office building in Manchester (Fig. 2), which suffered from uncomfortable temperatures in summer. In the area in which the trial was carried out, temperatures exceeded 25°C for 13% of the occupied time and were above 28°C for 3% of the occupied time — considerably longer periods than the CIBSE guidance referred to above. This particular office gets the Sun virtually all day, and opening the windows had very little effect. Because there was no way of removing heat from the building at night, summer mornings were a particular problem. The office temperature rose above 31°C on one day in July. The approach is based on a combination of passive ventilation with spot comfort cooling. Using night cooling to flush the office of hot air contributes 19% to the overall 41% reduction in energy consumption compared with a full mechanical system.
How mixed mode works… Winter operation is shown on the left, and summer on the right. 1. First floor in winter: reverse-cycle air conditioning heats the space, with natural ventilation providing fresh air. 2. Ground floor in winter: high heat loads can be handled by passive ventilation alone. 3. First floor in summer: air conditioning cools the space, and fresh air is supplied by natural ventilation. 4. Ground floor in summer: the ambient temperature is low enough to provide free cooling using natural ventilation, and the air conditioning does not run.
During high summer, night cooling comes into its own. On a typical day in July, there is a 57% reduction in energy consumption compared with a full mechanical systems — 39% of which is directly attributed to night cooling. Overnight cooling delays the use of spot cooling the following day. Internal air temperature is used to control the system. Passive ventilation is preferred, but when the indoor temperature rises above setpoint, the building-management system switches to spot cooling. As temperatures start to fall again in the late afternoon, the spot cooling is turned off and passive ventilation is restored. Air for passive ventilation enters the building through grilles above the windows and into the ceiling void. It enters the occupied spaces through grilles in the ceiling and is exhausted through a Passivent Aircool ventilation stack.
Refurbishment The success of this small project has prompted Mitsubishi to investigate similar ideas as part of refurbishment work of its office building at Hatfield, Herts. This 3000 m2 building dates from the 1980s and has a totally glazed façade. Three systems will be installed and their performance monitored. One will be a mixed-mode system like that at Manchester. Another will be based on Mitsubishi’s Lossnay heat-recovery ventilation systems. The third will be an AHU system. This refurbishment is due for completion in November 2005. Donald Daw sees no great difficulties installing passive ventilation in existing buildings. He points out that many older buildings have a stairwell at each end for safety reasons and that with suitable fire precautions they can provide the stack for passive ventilation. Neil Rideout, technical director with Passivent sees spot cooling as an important complement to passive ventilation. He explains that with outdoor temperatures above 21°C, no free cooling is available and that below 7°C no cooling is required. Cooling and heating are both needed — and both can be provided by reverse-cycle air-conditioning units. He concludes, ‘With new Building Regulations on the energy efficiency and air tightness of buildings, it has never been more important to introduce fresh air into the occupied spaces. ‘The Mitsubishi Electric and Passivent initiative has created a system that will satisfy both the Building Regulation requirements on ventilation, whilst helping conform to Government emissions targets through the sensible use of minimum energy.’ Pasisvent and Mitsubishi Electric have produced a guide to mixed-mode cooling systems (tel. 01707 278915)
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