Supermarkets, Hospitals. Pools, Data centres and other areas with a level of humidity expectation are usually controlled by commercial air conditioning, and can benefit from Preconditioning outside air in humid climates.
Supermarkets are a complex environment and standard air conditioning systems are often not effective for the various store configurations. Humidity can effect refrigeration operation and customer purchasing decisions if the store is too cold or because frozen products are not visible or iced up. Humidity is a hidden energy issue. Often over half the store energy is refrigeration and air conditioning, and designs based on first cost can leave a substantial energy legacy for the owner. Desiccant dehumidification offers an array of benefits to store operation energy and comfort, which can outweigh the initial investment. Up to 80% refrigeration case energy is consumed by the condition of the air around the evaporator, and ambient moisture is a significant component. Generally it can be said a reduction in store humidity of 10%RH would save 10% of case energy. (ref ASHRAE handbook 2003 2.3 (fig 1)).
- Reduced energy input
- Reduced latent load (ice ) on evaporators
- Reduced defrost cycles
- Reduce strip or anti fog heaters
- Less ice and condensation on product
- Reduced product loss due to defrost
DATA CENTRES , HOSPITALS and COMMERCIAL BUILDINGS
The design practice of (DOAS) Dedicated outdoor air systems, with desiccants is now prominent with technology advancement and over 100,000 systems in commercial buildings.(ref AHRAE May 2008)
DOAS avoid humidity problems and save energy by removing the latent load of outside air, and over drying the outdoor air sufficiently (i.e. 9Cdp which is 4°Cdp below room setpoint) to absorb the internal (moisture) latent load. This avoids the cost of dehumidification of the entire recirculated air stream.
Improved IAQ Generally maintaining surface humidity below 60%rh will maintain a neutral mould and bacterial growth rate. Humidity control impact to buildings is acknowledged by the following relevant standard. ASHRAEs design window between 40-60%rh, and a designated energy modelling parameter of 65%rh by ASHRAE 62.1-2007(5.10.1). Japans’ sanitation law of 40%-65%rh is supported with efficient building standard CASBEE with credits for 55%rh to 45%. Australia’s Green Star also credits below 60%rh in space and 80%rh in duct. 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 (or 30%rh at 23°C) and 11-14°C temperature increase when regenerated with 50-60°C. And need less preconditioning when reactivated with 120°C. In central Air-conditioning systems, the sensible heat ratio (SHR) mismatch occurs when the outdoor %rh is high and neutral temperature. This is neutralised by having desiccants having the latent duty and a separate devices for sensible, providing independent control capacity.
Simply providing the room dew point doesn’t work as any internal load and infiltration forces the humidity over set point and over cool and reheat to be wasteful.
Reduced Air Conditioning Coil Work The design practice of DOAS with desiccants is now prominent with technology advancement and over 100,000 commercial buildings systems.(ref ASHRAE May 2008) DOAS avoid humidity problems and save energy by removing the latent load of outside air, and over drying the outdoor air sufficiently (ie 5°Cdp or 30%rh @ 23°C) to absorb the indoor (moisture) latent load. This avoids the cost of dehumidification of the entire recirculated air stream. The use of Exhaust Air in and Energy Recovery Ventilator ERV option also reduces this energy substantially.
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, so overcooling is becomes more wasteful. This practice was restricted in 1999 and later banned in USA by building energy standard ASHRAE90.1 and countries like Australia have banned the use of electric elements for reheat.
Reduced Plant Size, Increase COP DOAS allows the used of chilled beams and smaller more efficient plant. Due to the SHR being substantially different to that of what commercial air conditioning is designed for, the cooling plant is oversized to deal with the latent component. With standard operating temperature differences across cooling plant the recirculation airflow can be double what is needed. ie an operating scenario may have external latent load of 50kWR and internal latent of 50kWR, (ie 199kWRlat) and with total sensible of 100kWR a SHR of 0.5 results, where coils are often comfortably designed for 0.7. Commercial Buildings building have various configurations.. When assessing a design or refit for DOAS, the following considerations will influence the system selection.
- Minimum ventilations required (exhaust)?
- Exhaust air availability for use on ERV?
- Services such as chilled water, and COP Coefficient of performance? (ie kW input = kW refrigeration/COP)
- Hot water, gas to 50C or waste heat temp?
- Customer priority CO2 cost energy IAQ