How energy efficent are low-temp systems?

[Badbird2000]

Ok, i couldn't sleep last night and got to thinking... At my very first engineering job in Tampa back in 1994, my company was designing a middle school using low-temp systems.. the SA was 44 degrees to the space. Not sure of the CHWS temp, but i believe it was high 30's.. i wasn't involved with the project, it was already on it's way out the door. It got constructed and was commisioned and worked as designed. Fast forward, today i am doing energy audits and got to wondering if a low-temp system would be more efficent in the right climate. With colder air, you supply less cfm to the space, so supply fans are smaller, ductwork is smaller, but i would think the coils would be larger in the AHUs and the chiller might have to be larger. Any input?

 

[ChrisConley]

My personal design philosophy is generally the opposite.

The closer your heating/cooling medium is to room temperature, the few parasitic losses you have. Warmer chilled water (say in a radiant cooling project) allows for a much more efficient chiller, and less energy loss (piping loss, etc,.). Cooler heating water allows for condensing boilers and less energy loss. Ventilation and dehumidification are separated from heating and cooling to allow for separate processes (desiccant dehum, or dedicated DX for instance).

The issue is capital cost, and I'd argue, capital cost only. In low exergy system, you end up with a lot of heat exchanger area (radiant floor, radiant ceiling, large coils, etc.). Finding a balance on pump energy can be an issue as well (nothing that more money can't fix).

 

[KiwiMace]

The chiller efficiency and capacity drops substantially with lower LWT's, and you risk condensation on the grilles. I would guess being Tampa, that this design was primarily aimed at the high latent component, coupled with the higher ventilation requirements of the school.

I would propose that something like a dedicated OA unit might be more appropriate with conventional supply temps to the space, in the light of today. The other ways of doing it equate roughly to the number of HVAC Engineers currently working in Tampa plus the logged in users here.

I am personally very skeptical of elaborate or unusual systems, on the grounds that the 'next' engineer/technician/commissioning agent will try and solve one problem in a way that has always worked in the past, and mess up the whole deal. Good documents are as rare as the people who read them.

 

[Badbird2000]

I know they are special grilles they used that would disperse in a slightly differnt pattern since there was less airflow to the room. There were no problems with them dripping. Also, my boss at the time later on became the head engineer for the school board, and worked out a formula to reduce the oa to each classroom based on intermittent occupancy, with the kids changing class every hour, the door opening, lunch schedules, etc. I can't recall if the oa was handled by a decoupled OA unit or not.

 

[Waramanga]

Oh yeah, low temperature VAV is all the rage here in Australia. They run very low air volumes to infuser diffusers (3-1 secondary to primary air ratio) and the story goes that this is a workable concept that beats chilled beams on price and efficiency. More flexible than chilled beams and the ducting etc is smaller than conventional VAV. They reckon that the extra chiller power is more than compensated for by the lower fan power used and you get lower slab to slab heights. I think that it may be good where you dont get much free cooling but in cooler climates the smaller economy cycle would kill it I think. It really pays to try the options on a location by location basis.