Whereas the new SANS Standard vary around 100 w/sq m for the electrical connections for air- conditioning systems for Office Blocks, this article looks into the benefits of 2 Stage Evaporative Cooling as the cooling force with the inherent feature that the annual electrical usage can be drastically reduced. The question arises what do you require to make the system fully operational and comparable with full compressor driven chilled water air- conditioning systems?
Background
More than 10 years ago the first 2 Stage Evaporative Cooler Systems were installed in office buildings around the Sandton Area designed by Toon Hermanand Associates which have been monitored IRO their performance on an informal basis, since.
Since the original installation there has been an evolution in the design and control philosophy to use 2 Stage Evaporative Cooling for Office buildings and the current general guidelines are as follows:
a. Minimize the building facade load – i.e. work in close association with the designer and client with the view of achieving the following results: smaller ducting for ease of installation; reduce the cooling load difference between the North (West) and South offices which will be the glass carrying facades to prevent undercooling of the Southern Offices; reduce the installed costs.
b. Design a full VAV system with duct riser and branch static pressure control, slightly varying supply air temperature (Varying with the ambient dry and wet bulb relationship, but normally less than 1oC/hour) at between 15 and 19 oC as the supply air control temperature. Supply VAV terminals with reheaters which then become the heating system for winter operation and design a full return-and-relief-air system for winter operation.
c. Zoning is not required any longer:
The variance in office load to keep within comfort limits which a good VAV system can be handled is between 30 percent and 120percent of the maximum design load to a typical installation at Tsa = 18oC with the room controlled at 23 oC ± 1oC.
Zoning is thus not a pre-requisite any more as the difference in room temperature between a South Office which can go down to 30% load and a North office which can simultaneously rise to 120% is now only 2oC. Machine sizes can thus be optimised for the most economical selecting.
d. Is the terminal reheat not wasteful and can heat pumps not be employed to increase economic efficiency?
No, due to the following reasons:
• In Summer the heaters are switched off since a good VAV system would keep the space temperature within comfort limits (22-24 oC) at (50-60 percent RH)
• In winter on minimum air flow, and thus minimum fresh air (due to the return air system) the building will only accept the heating it requires to keep the space at the selected floor temperature (around 20 oC) and no waste heat is employed. (The feature to do terminal reheat for spot on temperature control in summer is even available but should not be used due to energy wastage). Although heat pumps have the inherent capacity to reduce the electrical input to 30 – 40 percent of the required heating output, the heating energy is not available to spread throughout the building to the VAV outlets where it is actually required.
Trying to preheat the supply air at the air handling units give rise to the complication that can lead to overheating in low load areas and thus discomfort and energy wastage will result since no futher reduction in the air flow volume is possible on minimum air flow. Example 50 watt/sq.m at 33 percent supply air at 2l/sec.m results in 45 oC supply in temperature.
Zoned and constant volume systems with little variance in the floor load could use this technology for winter heating. The big picture shows that for 2 Stage Evaporative Cooling the winter demand load is identical to a chiller air conditioning system which could be as low as 28 VA/m2 for a well designed building, but summer air cooling could be as low as 9 VA/m2 for 2 Stage Evaporative Cooling compared to around 26 VA/m2 for full Air- Conditioning
2. Energy input comparison
By how much is the energy input then reduced for 2 Stage Evaporative Cooling Systems?
The following factors come into consideration when estimating the electrical input requirements for 2 Stage Evaporative Cooling.
• The air flow requirements for cooling and heating.
This is directly related to the building design and client usage of the interior eg percent glass, type of roof insulation, glass type, structural mass and insulating properties, floor planning, lighting levels and heat generating characteristics of lighting and computer loads and other equipment heat load policy usage.
• The geographical area under consideration. Whereas 2 Stage Evaporative Cooling is suitable for the largest part of South Africa excluding the lowveld and Kwazulu Natal Coastal Areas; the off-pack temperature varies between 17 oC and 20 oC throughout the country at 17 oC for the more arid areas eg Bloemfontein, Kimberley to 20 oC for the higher humidity areas of Rustenburg and Polokwane.
3. Floor design temperatures should be in the region (22-24oC) with the RH variance between 50 and 65 percent. Winter floor temperature around 18-22 oC and whilst humidification is possible, the associated energy requirements normally excludes this.
- The energy of moving and cooling the primary air in summer.
- Typical offices should vary between 30 and 60 watt/sq m as RSH and with a TD between 4 oC (19 off – 23 room) and 6 oC (17 off – 23 room) and the air volume should vary between 5 and 10 l/sec.
- The air moving energy of 2 Stage Evap Cooling = 1 watt per l/sec (700 Pa @ 70 percent fan eff)
- The cooling energy for 2 Stage evap cooling = 60 percent of the air moving energy
- The cooling energy for Single Stage Evap Cooling = 3 percent of the air moving energy (fixed)
- The total energy requirement for 2 Stage Evap Cooling in the cooling mode = 1,63watt /l/sec. Thus the Cooling energy input varies between 8,2 and 16,3 watt/sq.m for 5 to 10l/sec supply air volume.
- Comparison between the systems: 2 Stage Evap Cooling and Chillers.
The corresponding energy input for a chilled water system should be between 23 to 49 watt/sq.m. For winter heating the power requirements are exactly the same for both systems and depending on the building design could vary between 25 and 60 watt /sq.m. Comparing the 2 systems then gives rise to the following:
3. Conclusion
2 Stage Evaporative Cooling can reduce the Summer cooling demand substantially to the effect that a standby power generator can be considered if load shedding is considered. There is however no change in winter heating demand but the annual power consumption is actually halved. Surely in terms of Green Building Design 2 Stage Evaporative Cooling warrants serious consideration as a cost and energy saving method without impeding end-user comfort conditions.
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