Raising the chilled water temperatures to save energy ! 3

[energybuff]

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.

 

[Goorah]

Look at you chiller's capacity vs power consumption characteristics. Then, with capacity directly proportional to delta T, do a revised load profile and calculate revised power consumption.

 

[ChasBean1]

Funny you mention this - we have a client who does this and I was asking why. Eng. 101 tells me that the demand will determine the use. Q = mc dT - if I lower my dT, well guess what? My m will go up! Under either condition, I'm using the same Q because energy demanded is by the HVAC units and terminal loads. If I drop the HVAC units' delivery temps, I'll demand more energy from the water. By just raising the CHW temp, all cooling coils will start to open up, which means more pumping power. It's a lose-lose. The thermostat settings will be the principle factor determining cost savings.

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

 

[Goorah]

Whoops! I need to clarify my earlier response.

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.

 

[RussP]

Both Goorah and ChasBean1 have valid points. The real answer to your origianl question is that it is a complexe and not easily answered question. Saving chiller energy by raised CHWS temp COULD actually waste more energy by increasing the flow requirements of the system, or it may not. And I don't believe simply assuming you can raise the a 10 degree delta T up 2 degrees is accurate either. Whether or not you can extract the same amount of energy from the air at the unit is a function of the unit's coil performance characteristics. There have been studies in the HVAC magazines that have modeled sample buildings and determined that resetting CHWS temp is a valid way to save energy, but the amount of energy is not as significant as one would preliminarily assume or it may be a break even situation. Of course, all of this assumes you have the ability to vary the water flow. A constant volume system would be a very valid reason to reset CHW temps as no extra energy would be wasted in pumping energy anyway.

 

[Goorah]

Hi All:

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.

 

[JimD1]

Not being and engineer myself, but a building operator. You would also decrease dehumidification of the air,your clients may complain.

 

[Goorah]

Hello all:

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!

 

[PacificSteve]

I will love to see the follow responses to this one!

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.

 

[YorkMech1]

JimD1 has put up the stumbling block of any idea of saving money by raising the lvg. water a couple of degrees...The space humidity jumps by several points as the dehumidification capbility of the chill water coil is compromised...
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..
No

http://wwwwwww'

you saving money....

 

[ChasBean1]

PacificSteve - you sounded so sure that I didn't want to question, but does the 1.5% per degree take into account the extra CHW pumping energy that will be needed to satisfy the same load (system opens as CHW valves go open to maintain the AHU discharge air setpoints, etc., causing more flow demand)... Is this savings representative of chiller efficiency, or whole system efficiency? Tx, CB

 

[PacificSteve]

Ouch, I knew I couldn't get away with a simple 1.5%/degF. As Energy buff stated, its OK to let the RH and even the room temp swing up. When you drop the CHWS temp, you will do less dehumidifying, thus you are not just getting better chiller efficiency, you are reducing the load.

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.

 

[ChasBean1]

Steve, I wasn't trying to sharp-shoot at all so don't take the question the wrong way. My point was more focused on a typical two-way CHW valve system; raising CHW temp and as you said, do less dehumidifying at the coil. The two-way valves at the coils will open more to meet the load setpoints, causing the central pumps to increase flow (travel down their impeller diameter curve to a new, higher flow value), which would raise the pumping power by flow change-cubed. If the central pumps have speed drives (again assuming a two-way valve system), the speed drives will operate at a higher level to maintain loop differential pressure. This could be a significant added operating cost. As you said though, with a three-way valve system the difference might not be so significant or might even improve with more flow redirected to the coil, which would generally add more friction (raise the system curve) than if the valves were in full bypass.

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.

 

No doubt the chiller kw/ton will drop when CHW temp is higher. BUT consider the following:

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.

 

[Goorah]

AMEN! to what Bert says: "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."

I really like that approach: look at the limits (most savings = chiller off; least savings = "by the book, worst case for particular site&quot

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!!! :-( ) .... for COMMERCIAL, COMFORT COOLING is: HVAC represents 50% of a buildings operating cost (direct energy, maintenance, debt reduction, etc.) and of that number: APPROXIMATELY 50% is the chiller compressor energy. But you have to look at the process: is it a factory that has a $250,000 per hour downtime opportunity cost associated with it? ...Then even the most inefficient HVAC system would "make sense".

This has been a good discussion!