Thursday, December 13, 2007

Water Conservation and LEED

....Again, I love LEED and the USGBC because without them we would not be writing about a lot of the 'stuff' we are talking about here. That being said I have recently had a frustrating experience in calculating our water usage on a project.

Very early in the project the engineers suggested designing a cistern to collect rain water from the roof, condensate from the air handlers and environmental rooms, and the backwash of specialty equipment. Since this water should be relatively clean they wanted to use is as make-up water for the cooling tower, as opposed to irrigation. [A few weeks ago I wrote a note about how much water electrical plants use, so hold on to your hat.] For our ~280,000sf lab project we are estimating that the cooling towers will require 8.5MILLION gallons of water per year. [Don't think your 500,000sf office building is any better by the way.] The cistern collecting water from the three sources listed above should collect ~2million gallons per year. Even though this is a 24% reduction in our cooling tower usage, it does not count towards our water efficiency credits under the LEED 2.2 rating system. Fear not, we are going to get at least a 40% reduction by using low flow fixtures and the like, but this seems a bit odd to me.

All of this being said I am going to try and explain some of the stuff in the previous paragraph. [Disclaimer, I am a designer not an engineer so please correct me on my technical errors.]

A cooling tower is essentially a big radiator on top of a building. It's sole purpose is to reject heat from the building to facilitate the refrigerant cycle enabling the 68 degree, low humidity environment we have become addicted to.

Roof run off is pretty easy to understand. Rain falls, it hits the roof, goes into a roof drain, and normally goes to a storm sewer in the street. There is growing concern over this because in large urban areas this leads to two major problems. First the water table never gets replenished. All of the water goes into the sewer instead of percolating back into the ground. This was a huge problem a few years ago in places like New Jersey where there was a drought. Everybody kept watering their fields and lawns and such. All of a sudden the water wells for the public water supply were dry and there was no water to drink let alone take a bath. Second the water is diverted into major waterways such as streams and rivers. This water is warmer and flows more quickly than normal, so it destroys sensitive habitats and accelerates erosion downstream.

Condensate water is water that condenses on the cold mechanical equipment in the building. Most large buildings have air handling units [AHU] to push the air around to all of the space in a building. These AHUs have coils in them that are either hot or cold depending on the weather outside. In most office building and such, you are cooling almost year round due to the heat gain from the people, equipment, lighting, and other heat sources from within the building. [By the way this is a pretty energy efficient way to do it. Many of the buildings you see every day have DX units for heating and cooling which are much less energy efficient.]

Certain equipment, especially in lab buildings, are backwashed to clean the filters. Think of a Britta filter on your kitchen sink, but instead of wanting a glass of water you need tens of gallons per minute. Obviously that little filter would need to be replaced very frequently. So in lab buildings instead of replacing those filters they backwash them to clean the filters out. That 'reject water' is usually put into the sanitary sewer, refer to the roof run off paragraph above to see why this is an issue.

Tossing millions of gallons of water into the air is not great, but that is where we are today. Even if we are not going to get a coveted LEED point for cisterns we should still try to conserve water. Oh, by the way there are cooling towers available that reclaim the evaporated water and recycle most of the water. But like most cool, pun intended, building technologies they are only widely available in Europe, right now.


Green-A said...

You can still borrow the Cooling Tower Water Management credit (WE 4.1-4.2) from LEED-EB for an Innovation point, bro. When LEED goes to the much-anticipated 'bookshelf credit system' you'll be able to pull down credits from any rating system for any project (as I understand it). Rock on.

Anonymous said...

I don't know much about the actual credit differences between LEED-NC and LEED-EB, but I love the discussions and posts about it all. I'm studying for my LEED-NC exam which I'm taking in about 3 weeks. Great blog, keep the great posts coming!

Anonymous said...

I would have thought you would qualify for an exemplary performance point for WE Credit 3 for having greater than a 10% reduction in your process water.


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