Power Usage Effectiveness (PUE) is not the best metric in the world, and attempts to change it crop up periodically. The latest idea would utterly change the calculation, and overturn the results of today’s PUE by rating compressors better than evaporative cooling.

PUE Revised was suggested on Jack Pouchet’s blog at Emerson Network Power, and discussed at DatacenterDynamics. It adds a large “correction” factor to account for water used in the cooling system. This factor is so large that a 1MW data centers using evaporative cooling with a PUE of 1.1, would now have a PUE Revised of 1.4. Pouchet calculates that if it uses indirect evaporative cooling, its PUER would shoot up above 2.0.

Will the Green Grid buy it?

The proposal will be put to the Green Grid which created PUE, as a possible update, Pouchet, a long term Green Grid participant, told me: ”We are moving forward with this via the Green Grid processes.” 

Virtually everyone I’ve spoken to is dubious about the prospects. Why would the Green Grid change a metric which has become ubiquitous, and is well on the way to becoming an international standard, having been published as a draft standard (DIS) by ISO.

But even if PUE was set in stone, any possible change should stand or fall on its merits. So what is the idea behind PUER?

Pouchet believes it’s needed because PUER is distorting the data center sector: “Although it was clearly launched as a tool to be used wholly within the confines of a single facility PUE has none-the-less become the de facto universal metric for comparison shopping when evaluating data centers,” he says on his Emerson blog. “This behavior has led to numerous unintended consequences not the least of which is the staggering increase in the use of water by data centers.”

PUE favors evaporative cooling, says Pouchet. But is this true? Evaporative cooling requires less electrical energy than mechanical cooling, so data centers that are able to use it will be cheaper to run, and will be responsible for lower emisisons.

However, not everyone can rely on evaporative cooling. It wants a cool dry climate. Countries like Dubai, Singapore, Mexico and Brazil are too hot and/or moist to rely on evaporation alone.

Even if evaporation works, water may be expensive, DatacenterDynamics’ Stephen Worn told me. In China and India, data centers may need to make their own water in desalination plants. Even California has a long-running drought, which data centers are having to contend with.

Critics of PUE say it is a metric which unfairly favors Northern Hemisphere data centers. To get the PR benefit of hosting in “greener” low-PUE facilities, some people in less favourable climates are opting for hosting at US or European sites, and local data centers are losing out.

Evaporation energy

The PUER proposal adds a big factor to account for water use, but chooses a factor which is difficult to justify. PUER simply adds in something called “water equivalent energy” or WEE, which is the “evaporative energy” of the water, or the amount of energy the water takes with it when it evaporates (the proposal also adds SGE, a correction for the energy used in the site’s backup generators).

The WEE addition looks like false accounting to me. The evaporative energy is not another cost, and is not associated with more emissions. It’s been arbitrarily moved form the credit side of the account to the debit side. While there is an energy cost per liter in providing the water for the evaporative system, it’s not equal to the evaporative energy - and it will vary from place to place. 

Another odd thing is that PUE already includes a factor for water, ”The ‘missing’ part that Jack suggested has ALWAYS been included in PUE - but no one takes any notice of it,” Professor Ian Bitterlin pointed out on our LinkedIn discussion page. The Green Grid v2 standard and the ISO 30134-2. draft international standard, already include proxies for embedded energy including that of the water used, he notes.

And Bitterlin raises another perplexing thing. Rejigging PUE to make adiabatic coolers less attractive might look like a commercially-oriented proposal.. except Pouchet’s employer Emerson has invested heavily to produce some highly regarded evaporative cooling products.