Mixed-mode refurbishment

in office
At Mitsubishi Electric in Hatfield, 500 m2 of office on the ground floor are served by a mixed-mode system comprising VRF air conditioning and passive ventilation.
Delivering comfort conditions using a mix of air conditioning and natural ventilation is much more energy-efficient than air-conditioning alone. A refurbishment project at Mitsubishi Electric shows how.When Mitsubishi Electric and Passivent launched their mixed-mode air-conditioning system during the summer, it was billed as having an annual energy consumption 40% less than full mechanical air conditioning. The concept of this mixed-mode approach is to use air conditioning when outdoor temperatures exceed 21°C and natural ventilation at lower temperatures. When the outdoor temperature is below 7°C, it is unlikely that any cooling will be required. Back in the summer, the only demonstration project was a small installation in Passivent’s own building in Manchester. Refurbishment Now, Mitsubishi Electric has completed a much larger project as part of refurbishment in its 3-storey building at Hatfield, which dates from the early 1980s. An area of 500 m2 on the ground floor has been refurbished in this way. A full air-conditioning system has been installed to provide cooling when the ambient temperature exceeds 21°C. This heat-recovery VRF system uses R410A and can provide 67 kW of cooling and 72 kW of heating. 4-way-blow ceiling cassettes are installed in meeting rooms, and DX ducted units serve open-plan areas. The Passivent natural ventilation system is in addition to the air-conditioning system and is used only when ambient conditions enable the benefits of free cooling to be exploited. The extra cost of this system is expected to be paid back in two years by eliminating the need for air conditioning when ambient conditions permit, and thereafter the energy cost savings will continue. Natural ventilation Compared to the air conditioning, the natural ventilation system was very simple to install, and took only a couple of days. Externally, six glazing units of this fully glazed building have been modified to incorporate the external vents (Fig. 1). These vents are opened and closed automatically according to indoor and outdoor temperatures, carbon-dioxide concentration, rain and wind. Air is delivered into the ceiling void (Fig. 2), which acts as a plenum to deliver air to grilles in the ceiling. Air leaves the spaces through other grilles in the ceiling and is discharged into the fire-escape stairwell (Fig. 3). This warm air rises up the stairwell and passes to atmosphere through a louvred turret on the roof, which replaces a skylight (Fig. 4). The operating regime is to use the passive system as much as possible to provide comfort conditions inside the building.
Anti-clockwise from top left
Fig. 1: The external visible face of the natural-ventilation system in Mitsubishi’s Hatfield premises is six grilles incorporated into the glazing units.
Fig. 2: Air from the external grilles enters the ceiling void, which acts as a plenum.
Fig. 3: Air exhausted from the occupied spaces by the natural ventilation system enters the full-height stairwell.
Fig. 4: Natural-ventilation air leaves the building through this turret on the roof.
Overnight cooling During the peak of the summer, when the passive system cannot achieve the internal temperatures required, it still has an important role to play by operating overnight to cool the interior as much as possible before the following working day — thereby delaying the starting time of the VRF system. Philip Ord, VRF product manager with Mitsubishi Electric, explains, ‘It is very important that the operation of the air conditioning and the passive ventilation do not conflict with each other. Control is the key, and this is achieved using a Mitsubishi PLC [programmable logic controller] with code specially written to communicate with our G50 controller.’ Philip Ord tells us that the number of I/O points is probably too many for a building-management system, hence the use of PLC. The installation at Hatfield is intended as a demonstration project. Consulting engineers are encouraged to contact Mitsubishi (information below). The development of this mixed-mode system by Mitsubishi Electric and Passivent is a direct response to the need to reduce carbon emissions while accepting that there is a definite need for air conditioning at the height of the summer. Mitsubishi Electric is at Travellers Lane, Hatfield, Herts AL10 8XB. Tel. 01707 276100
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