Raising the chilled water temperatures to save energy !
Raising the chilled water temperatures to save energy !
(OP)
Is there a simple method to determine the energy saved by raising the central plant's chilled water operating temperature ? The assumption here is that the new environment served by the building air handlers, can withstand higher (temperature and relative humidity) operating conditions.





RE: Raising the chilled water temperatures to save energy !
RE: Raising the chilled water temperatures to save energy !
Alternatively, there may be information out there that shows the excess flow cost is generally cheaper than the excess chiller operating cost. I'd be very interested if anyone else can add to this. -Chas
RE: Raising the chilled water temperatures to save energy !
As Chas points out, when dT goes down, m goes up for a constant Q. However, it IS possible to raise the chilled water temp at the same dT; as long as the impact on RH and increased wet bulb is acceptable ... which it was stated is O.K.
So instead of the coil operating at 45 - 55 dT, adjust the settings to operate at 47 - 57 dT.
The power savings comes in due to the reduced "lift" across the compressor: at the higher chilled water temp, the compressor suction will be higher.
If you have access to the system manufacturer's modeling program or the compressor power vs suction and head curve, you can compute power savings. The compressor is typically the major power consumer in the chiller system.
Likewise: you can lower the lift (and compressor power) further by decreasing the condensing temperature by either installing a larger tower or increasing tower flow.
RE: Raising the chilled water temperatures to save energy !
RE: Raising the chilled water temperatures to save energy !
Russ is also correct! I gave the 2F change merely as an example: there is no set rule! It CAN be simple or it CAN be complex. It really depends on your system. For instance: do you have a primary CW loop and a secondary loop; or just a primary one? Is it a constant volume (water and/or air) with bypass valves/dampers or modulating, variable flow?
If this were a new design and you had a lot of options, you could also save money by LOWERING the CW temp; the savings coming from reduced pumping costs.
It surely can be complex and the best solution might be to contact someone who has proprietary software that can model ANY system.
From personal experience, I know that Trane company has such software and their engineer salespeople can be very helpful in helping you analyse and optimize your system. Check your yellow pages for the nearest office. Or if you don't like Trane, I suppose Carrier might have similar; but I can't vouch for that.
RE: Raising the chilled water temperatures to save energy !
RE: Raising the chilled water temperatures to save energy !
As a Spanish teacher once said: "I ONLY REPEAT myself ONCE!" (upon which I was thrown out of class for laughing at what seemed an absurd comment!.... Actually he was a good guy and was giving us another opportunity to learn!!):
"It surely can be complex and the best solution might be to contact someone who has proprietary software that can model ANY system.
From personal experience, I know that Trane company has such software and their engineer salespeople can be very helpful in helping you analyse and optimize your system. Check your yellow pages for the nearest office. Or if you don't like Trane, I suppose Carrier might have similar; but I can't vouch for that."
I would appreciate knowing how Energybuff brings this thread to closure!
RE: Raising the chilled water temperatures to save energy !
Simple answer: 1.5% reduction in energy (for the same cooling load) per degree increase in CHWS temp.
You can get even better additional savings by dropping the CW temp, let the towers run flat out, with a few caveats.
Chillers are more efficient when allowed to produce higher chillwater supply temps. Set your chiller at 47, if no complaints, set it at 48. Beyond that the humidity control gets a little out of hand, but you could try. Maybe in summer you will need to back down to 45 or even 44. Control the chiller to CHWS temp.
RE: Raising the chilled water temperatures to save energy !
If its a process cooler, noooooo' problem, as long as the process can stand it..
Comfort Cooling & Human beings that "Know" when they are comfortable are not going to tolerate it...
Want to save the money???
Select a chiller that can get down to .2kw/ton and run on 55deg/f water....The condenser water pump could be very easily pulling more power than the chiller it supplies..
Nowwwwwww' you saving money....
RE: Raising the chilled water temperatures to save energy !
RE: Raising the chilled water temperatures to save energy !
Part 2: pumping energy. In a constant volume CHW loop, there will no appreciable change in pumping energy, the 3Way valves act to keep constant head, or so goes the thoery. In a primary/secondary with variable flow, you may have to pump a little more, but maybe not because the load itself (and therefore flow requirements) has actually dropped.
Added benefit, the chiller compressor is more efficient as higher CHWS setpoint, and there is less overall load, therefore it is rejecting less heat to the CW system, therefore allowing the towers to send even cooler water back to chiller, making the chiller more efficient.
In my book, the bottom line is to raise the CHWS temp as high as you can get away with, without lots of complaints or molding or process problems. I sure appreciate any feedback or clarifications. The real analysis of these things gets complicated real fast, as as true system approach must be done.
RE: Raising the chilled water temperatures to save energy !
In the end, I agree with you and think this would best be aswered by doing a whole-plant analysis (CHW and CW pumping included) before and after raising the CHW temp. The end result might also vary system to system.
RE: Raising the chilled water temperatures to save energy !
1. As others have mentioned, more CHW flow is required, which partially offsets the chiller kw saving.
2. As CHW temp rises, less dehumidification is done. This cuts energy consumption further, but at the expense of comfort. For the usual case, dehumidification just about ceases when CHW temp gets above 48 deg F, and the environment turns cavelike.
3. As CHW temp rises, the cooling capacity of the coils in the air handlers drops sharply, because of lower LMTD between air and CHW. This is made even worse if the occupants set their thermostats lower than design temperature, a very common condition.
If you really want to save energy, why not pull the disconnect switches on the cooling cycle components. This is a 100% reduction. You can't do better than that, if energy saving is the lone goal. Seriously, before you do much tinkering with settings, think about the function of the system, and what it is designed to do, and why it is being operated in the first place.
RE: Raising the chilled water temperatures to save energy !
I really like that approach: look at the limits (most savings = chiller off; least savings = "by the book, worst case for particular site")
Somewhere in-between is reality... what the "process" (keeping people comfortable, computers on line, a production line up and operating or whatever)needs... which might be a dynamic function of time.
In actual application, a smart control system can "transparently" react to the dynamics. Overtime, actual accrued savings can be read from the monthly electricity bill!
But if you're trying to make an economic decision on paper, say a life cycle cost analysis, then software can be used.
I guess it all depends on what you're trying to accomplish, how large the relative $ are, how critical the process.
A BIG FAT rule of thumb (ouch!! ... I've slammed that thumb so many times!!!
This has been a good discussion!