Office air conditioning design with dedicated outdoor air system (DAOS) is a developing practice in humid climates to curb IAQ issues while also reducing energy consumption.
DOAS overcomes the traditional practice of over-cooling large recirculating airflows to remove moisture. The use of desiccant dehumidification for this is particularly beneficial as it has the ability to deliver 50C dew point on fresh air, reducing the threat of mould. This is readily achievable even in the tropics.
This dry condition is adequate to treat 7.5 litres per second per person with the personal moisture load of 70W latent (0.1 litres per hour). Offices can receive a Green Star credit for humidity control below 60 per cent rh which can be achieved with a desiccant DOAS on minimum outside air.
In what could be a world first, solar heat is used for dehumidification of a ventilation air device under a subtropical climate at Central Queensland University in Rockhampton.
The project is supervised by PhD candidate Ali Baniyounes and Associate Professor Mohammad Rasul.
Seibu Giken DST dehumidifiers, supported in Australia by Air and Aqua Tech (AAAT), have a specialist desiccant which is effective at using waste heat and converting it into dehumidification capacity sufficient to work in the tropics.
Other recent commercial HVAC applications in Australia to use the Seibu Giken technology include swimming pools (60 per cent rh), supermarkets (below 50 per cent rh), libraries (below 50 per cent rh) and data centres (below 55 per cent rh).
The CQU project engineered out various IAQ problems including volatile organic compound (VOC) transfer through the air system. Evacuated tubes were eventually selected to transfer the heat; but roof heated water, as used by the University of Southern California, Berkeley, was also a serious consideration.
Gas hot water or other hot water sources were also considered.
The effect of the desiccant is adiabatic (enthalpy in less enthalpy out equals zero) which travel down the enthalpy line with the result of a sensible heat gain as well as some heat transfer from the heaters. To reject this heat, the use of an indirect evaporative cooler consisting of fire-rated 95 per cent efficient evaporative cooling media from AAAT with a crossflow/counterflow sensible plate heat exchanger rejected the heat.
While the unit is a small office design, it is scalable for much larger systems already on the drawing boards of other partner universities. Coincidentally, the CQU campus library has recently upgraded its humidity control with large desiccant DOAS systems providing a space condition below 50 per cent rh, as per the National Library of Australia guidelines. These units run off rejected refrigeration heat but are also easily upgradable to run from solar thermal panels.
Where exhaust air is available from the space, Seibu Giken DST also offers energy recovery ventilators which are resistant to VOC transfer and provide efficiency of 70 to 80 per cent for latent and sensible heat with its 100 per cent counter flow devices.
www.aaatec.com.au.