Mark Saussure blogged about Green Data Centers last week. One of the technologies he wrote about was "stored cooling." He writes:
There was some really cool stuff (literally) regarding techniques of stored cooling. The most interesting was the "Cool Battery". This is a device that uses a synthetic phase change material (don't ask me what it is) that freezes at up to 43 degrees F. Basically a big honkin ice cube. Up to 3000 ton/hr units are available. The material can be frozen by your chillers during off peak times and the chillers can be turned off at peak times thus saving on energy costs or credits the institution would be in line to receive.
I've been thinking about the thermodynamics of this a little and I guess I have to rethink what "green" means.
Here's a Washington Post article about the MetLife Building in NYC which uses ice as the phase change material to "store the cold." In this case both money and energy are saved because the system which freezes the ice using off-peak power is more efficient than the regular air conditioning system. During peak times, the ice melts and the cool air circulates throughout the building. There are days when this alone is sufficient to cool the MetLife building. The cycle renews every night. According to the numbers, this not only reduces the energy consumed during peak energy consumption times, but it also achieves the environmentally friendly goal ("green" -- I think), by reducing the overall energy consumption of the building. While the separate system was expensive to install, it has very little in the way of moving parts (if the system "breaks," the ice just melts).
If we cool the data center with the same system that freezes the phase change material (PCM), as I think Mark's post implies, can this be "green?" We can save money provided we have different rates for off-peak and peak power or get credit for using off-peak power. I use off-peak power as the variable here, because from what I can see there is no off-peak data center usage anymore, particularly when I see how our Research Computing and Cyberinfrastructure folks are keeping the clusters busy.
Assuming that we are using the same chillers, I have to believe, we are fighting the Second Law of Thermodynamics ("In an isolated system, a process can occur only if it increases the total entropy of the system," i.e., "There's no such thing as a free lunch.") not once, but twice. The system would use more energy than just chilling the building in the absence of the PCM "cool battery."
Could this still be green? Maybe... When speaking with Dave Beyerle, he suggested that this process would still lower the peak power required by the data center. So, while it might require more energy, that energy is more spread out over time and perhaps we don't need as big of a power plant to supply it. Perhaps we build fewer power plants as a result. Perhaps it still is green.
Regardless, it's always interesting to think about something you haven't thought about in a while (It's been over 20 years since I took Statistical Mechanics). Just because something says it's "green," often we need to look at the bigger picture to determine if it really is. Coming soon: my rant about compact florescent lights.
Comments (1)
One really big item that is being overlooked is that as we develop more technology that makes use of 'off peak' power, we will in effect be creating a new peak time.
The hydrogen economy was originally based on the idea that during off-peak times all that excess electricity would go towards the production of hydrogen. Our cheap nuclear electricity was supposed to be the engine that drove this model. We all see how well that worked out.
What we really need for data center cooling is a way to store winter's cold for use during the summer. Back in the 1970's U Alabama was experimenting with creating 'hot' and 'cold underground reservoirs by 'moving' heat from one pile of earth to another. I lost the reference to this but I wonder 30 years later how well that experiment worked out.
What I think is more reasonable is co-gen. Heat in a data center is a fact of life but victory will go to someone who finds a way to use that heat. I figure there has to be a way to take all the server generated heat in the Computer Bldg and transport it across the street to the Nat and use that excess heat to preheat the pools. In winter we should have a way to use that heat in our reheat coils.
A combination os successful co-gen and off-peak 'cold storage' is the real future but I don't see anybody exploring that model.
Posted by Jeff N | December 19, 2007 9:09 PM
Posted on December 19, 2007 21:09