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Air conditioning world: Mitsubishi puts mix mode system in head office

Mitsubishi is putting its money where its mouth is. Last year, it linked with ventilation specialist Passivent in a move which could save energy costs of up to 40%. It has recently refurbished its Hatfield offices and installed the system there. Philip Ord, VRF product manager, explains the workings of the mixed mode system
Air conditioning world: Mitsubishi puts mix mode system in head office
MITUSBISHI Electric has refurbished its Hatfield headquarters and installed a range of air conditioning systems, including the revolutionary mixed mode system, as a test bed for its advanced technology.

The 5,000m2, 3-storey, 1980s building at Travellers Lane, Hatfield has been transformed in the process and is now being used to demonstrate the effectiveness of the company's range of energy efficient, environmental control systems.

The mixed mode unit, which has been developed in conjunction with industry expert, Passivent, combines the best of natural ventilation with totally flexible air conditioning to offer environmental control that uses fresh air as much as possible and switches on the air conditioning only when necessary. The mixed mode system has been installed in a ground floor, 500m2 open-plan office used by the company's PLC division. Heat recovery air-conditioning provides up to 67kW cooling when the ambient temperature of the office rises above 21°C in addition to delivering up to 72 kW of heating in the colder months of the year.

'The Passivent system gives us free cooling when ambient conditions allow and both the air conditioning and the natural ventilation are controlled by a Mitsubishi Electric PLC (programmable logic controller) which allows us to ensure the whole system uses as little energy as possible,' explains Philip Ord, VRF product manager.

The PLC links Mitsubishi Electric's G50 controller with the Passivent louvre system to modulate the amount of fresh air allowed into the system.

The new Part L2 of the Building Regulations demands cuts in the energy used by buildings at the same time as placing a requirement that buildings do not overheat, so the Mixed Mode system answers both of these criteria, by using natural ventilation as much as possible and switching to the air conditioning only when the Passivent system cannot cope alone.

The natural ventilation system took only a couple of days to install and uses the ceiling voids as a natural plenum to deliver air to grilles in the ceiling. On the outside of the building, six units of the glazing have been modified to incorporate the external Passivent vents.

These are opened and closed automatically according to carbon-dioxide concentration, indoor and outdoor temperatures, and rain and wind. Air leaves the office through more grilles in the ceiling and is discharged into a stairwell.

Being warm, it rises up the stairwell and passes to the atmosphere through a louvred turret on the roof. For the air conditioning DX ducted units serve the open-plan areas of the office with 4-way-blow ceiling cassettes installed in two meeting rooms.

'We finished the installation at the end of November and have been continually monitoring usage but it won't be until spring before we start to see the real benefits of the free cooling,' adds Philip.

Overall, the system is predicted to consume up to 40% less energy a year than full mechanical air conditioning, allowing Mitsubishi Electric to recoup the additional cost of the Passivent system within two years through savings in energy bills.

The system has already been tested and monitored in Passivent's own offices in Manchester, which suffered from uncomfortable indoor temperatures during summer months.

'The Passivent headquarters are next to a busy road and a tramline so opening windows to introduce fresh air in the summer is not an option as the road and transport system makes too much noise,' explains Philip.

The mixed mode system is simple to use, yet provides some of the most advanced control and monitoring available in the industry today, with the highly advanced PLC ensuring that all of the system elements work in complete harmony.

Six units of the glazing have been modified to incorporate the external Passivent vents.


Energy consumption is reduced because fresh air is supplied to the indoor environment via the natural ventilation system without the need for mechanical power consumption and this provides the building with free cooling from outdoors when required.

In addition to this, a special 'night cooling' mode removes unwanted heat and can reduce the need for energy consumption by almost 40% on the following day. The overall effect for the occupant is a vastly improved air quality and comfort level, with minimum energy consumption.

Passivent's Manchester office was tested by Brunel University where research found the annual energy consumption of the system was reduced by an astonishing 41%, when compared with a full mechanical system.

As well as removing any indoor pollutants, the mixed mode air conditioning will also provide accurate control of CO2 levels, keeping them well within the recommended CIBSE guidelines.

The application is also ideal for schools and lecture theatres as it can purge a space and fill it with fresh air when it is known to be unoccupied so that a new group entering the room comes into a fresh and natural space.

Mitsubishi Electric has also produced a free CPD-accredited guide to mixed mode cooling systems.

Contact Mitsubishi Electric on 01707 278915.
1 March 2006

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