Increased ventilation to improve Indoor Air Quality (IAQ) is often in conflict with the challenge of energy reduction.
A comparison of ventilation technologies is often analysed at peak design, as it is difficult to model all relevant climatic scenarios and commercially available modelling is not available. The DST Modelling Method has evolved due to the need for makeup air modelling and the emergence of Dedicated Outside air System (DOAS) technology.
To support this, we detail the psychometrics of conventional baseline HVAC compared with five alternate technologies: Energy recovery ventilators (ERV), Desiccant DX, with Chilled water module (CHW) and ERV options. Weather Data sourced from ASHREA are modelled with Visual Basic (VB).
These methods can then be ranked on suitability bases on peak demand, operating cost, and ventilation cooling performance requirements of various cases. We demonstrate a manual calculation method of three prominent technologies, being Baseline with ERV’s (passive desiccant) and Desiccant DX package. The options are compared with the capital cost to model financial expectations of ventilation in various Australian climates.
All DOAS systems provide viable energy results as long as there is a humidity control expectation. Typically OA needs to consider internal latent loads which needs to be 3g/kg below setpoint (i2 Room 21C 60%rh 9.3g/kg, OA needed for 7.5l/s per person is 6.1g/kg or 7Cdp, without this overcool and reheat requirement occurs.
DST Dryer manager, Gordon Lacey, has worked with Desiccant design since 1986 and developed a VB version in 2010.
He has provided substantial energy savings for over 200 supermarkets, libraries, cleanrooms, hospitals, hotels and a range of other humidity related applications.
Benefits of Dedicated Outdoor Air Dehumidification
Improved IAQ Generally maintaining surface humidity below 60%rh will maintain a neutral mould and bacterial growth rate. Humidity induced problems can occur in typical building design practices that ignore the substantial ”part load” hours when little cooling is required but humidity is still considerable.
Better Temperature & RH Control The effect of a preconditioned desiccant can dry the fresh air to below 5Cdp. This is neutralised by having desiccants
Reduced Air Conditioning Coil Work The design practice of DOAS with desiccants is now widespread and the use of Exhaust Air in and Energy Recovery Ventilator ERV option also reduces energy use.
Reheat Avoidance Reheating of overcooled air is a traditional low cost practice to allow air-conditioning plant to dehumidify to dewpoint (ie 12°C) and re-heat the air back towards room temperature. Part load conditions often see ventilation loads creating a need for dehumidification but little or no sensible cooling.
Reduced Plant Size, Increase COP DOAS allows the used of chilled beams and smaller more efficient plant. With standard operating temperature differences across cooling plant the recirculation airflow can be double what is needed.