In the IDEC cooling discussion, I made the point that not only do we get a reduction in dry-bulb temperature as our airflow passes through the IDEC unit, but we get a reduction in wet-bulb temperature, also. And since we now have seen that the direct evaporative cooling process depends critically on the wet-bulb temperature of the air it is cooling, it seems we should get some advantage by running the air through the IDEC section, and then running it through the direct section. And we do:
(click for larger image)
As you can see, the resultant leaving dry bulb is on the order of 64º, which is better than the resultant of 72º from the indirect section alone, or 69º for the direct evaporative section alone. Now 64º degrees may not seem cool enough for typical cooling applications--and for most projects it probably isn't (although it is important to not that ASHRAE comfort conditions can be met with this leaving air condition in a predominantly sensible load application given enough air). But keep in mind that this is the performance on a design day. How many hours a year would you be able to meet the traditional supply air temperature of 55º? Lets look at psychrometric chart with Seattle bin data loaded into it:
(click for larger image)
A quick note of explanation: The vertical line at 55º is the economizer line--any climactic conditions to the left of that line can be used to create cooling air directly using OA alone (or mixing OA with RA) and thus require no additional cooling at all. The blue diagonal line along the 53º wet-bulb line is a conservative mapping of the direct evaporative regime. At any bin hours under this line, direct evaporative cooling can be applied to the ambient OA to achieve cooling air directly. And lastly, the red diagonal line above that is the indirect-direct evaporative cooling regime, where the application of both cooling techniques will provide acceptable supply air conditions (assuming about 70% effectiveness on the IDEC). And above that line, the indirect evaporative system can still be applied to greatly reduce the load on any supplemental mechanical cooling system, if used to meet the same 55º leaving air condition.
Two things jump out of this analysis: First, the vast majority of the hours are satisfied without using mechanical cooling. In fact, in Seattle, most hours are met with simple economizers--which explains the emphasis in our local codes on this cooling technology. You can even think of evaporative cooling as simply an enhancement to the standard economizer. The second takeaway is that there are still quite a few hours that are not met. How can we address this?
Well, one way is to play around with the leaving air temperature. If we supply some more air to the zone, we can provide warmer cooling air. Let's look at that same chart, only this time lets use a supply air temperature of 60º:
(click for larger image)
By simply providing for a little more air to the zone, we meet a much higher percentage of the bin hours; so much so you that can now consider a system without mechanical cooling, as long as the occupants are willing to accept a few more hours outside of standard comfort conditions a year. Granted, this additional comfort comes at an energy cost--the cost of moving that additional quantity of air. This cost is, of course, offset by the avoidance of mechanical cooling. But, additionally, we know from the previous chart that this additional air is not needed all of the time. A variable speed control on the fans would naturally bring the air volumes down during periods where colder air is achievable.
One of the things that should be obvious is that this analysis is greatly dependent on the local climate and elevation of the project. To evaluate how effective this cooling method is, you need to create similar plots for each project locale. And where you are in the state has a great effect on how well you do. For example, a cool-wet climate like that on the Olympic Peninsula sounds like it might be a good candidate. So let's see how it compares to Seattle:
(click for larger image)
It looks pretty similar to Seattle, as we might guess. How about a hot, dry climate like Spokane?:
(click for larger image)
That's a real winner! There's only a small fringe of hours outside of the range where indirect/direct evap works alone. So if Spokane works, surely Yakima must also be a great candidate:
(click for larger image)
Hmmm... There's quite a few hours outside of the indirect/direct evap zone. Good thing we did this analysis before committing to a evaporative-only system!
Indirect-direct evaporative cooling, either as the main cooling technology or as an enhancement to the economizer cycle is a technology that has wide application in the Pacific Northwest, even in rainy Seattle. But it is a technology that requires careful analysis--it's not as simple as throwing compressor tons at a cooling problem. With today's emphasis on energy efficiency and sustainability, it is a technology that deserves a second look.
There is certainly more to talk about on the subject. Future topics will include integrating compressorized cooling with an evaporative system, indoor comfort conditions, water treatment and maintenance, control of evaporative systems. and the role of return air in these systems.
Resources you may find useful:
Energy Labs Indirect/Direct System Performance Calculator (Simply the direct and indirect calculators linked together
Energy Labs Direct/Indirect Evaporative Systems Engineering Guide (booklet format)
No comments:
Post a Comment