However, a recent purchase at our office included a number of 100W CPUs (some AMD, some Intel), and a 95W 19" monitor. A 17" flat panel display came in at 41W. So even with a flat panel display, the new workstation used more power than 1999 models. Also a larger fraction of the workstation total, the 100W CPU, comprises the continuous(typically) load fraction. Suns and Macs with the RISC chips can be much less (40W), but are far less common.
The most recent LBNL report assumes 560 kWh/year for a server. This corresponds to 63W average. The last server I Fluked here clocked in at 150W, and certainly never turns off. The newest main copier here seems to generate far more heat without any greater output, even with all Estar features enabled.
In simulations, I usually assumed 150-200 W/employee was a typical overall average occupied power density accounting for all office equipment, fridges, coffee makers and a few task lights and occasional absences. This was borne out by some floor-wide measurements. For sizing purposes, it's important to note that employees and office equipment are very unevenly distributed in most buildings. We've seen large open plan areas with occupant densities of 70sf/person yielding 3 W/sf-conditioned in a zone. This is within buildings with average overall densities of 250 sf/employee; note these actual average builiding-wide occupant densities are typically 50% or less than design occupancies.
Energy analysts need to clearly differentiate design equipment power densities from average overall occupied equipment densities. While designers may have good reason to worry about 3W/sf equipment loads for office zone airflow sizing, this does not mean the chillers or central AHUs need to be sized for 3W/sf over the entire building. Designers have long recognized that the sun does not shine on the E & W at the same time, they could recognize the diversity in equipment also. And if arguing this point isn't worth the brain damage, we can take comfort in the fact that slight oversizing (given good VSD-driven fans, pumps, chillers etc.) can reduce peak HVAC energy use. For example, the AHU designed for 550 fpm never sees more than 400 fpm. The real trick becomes assuring that flows, particularly zone airflows, can successfully modulate down with stability and to avoid unwanted reheat.
One minor request for those engaged in these types of surveys: A square
foot is a pretty squishy number in the buildings world. We've got gross,
GBA, GMA, net, NUSF, rentable, occupied, conditioned, etc., and people
interpret these numbers in divergent ways unless they are specifically
defined by the owner or a code. These quantities can diverge by 40%. So
please define which sf is used to normalize the power or energy. That's
one reason I started using W/employee for offices, in addition to the smaller
deviations of that metric.
Fred W. Porter
Senior Engineer
Architectural Energy Corp.
2540 Frontier Ave. Suite 201
Boulder CO 80301
email: fporter@xxxxxxxxxxxxxx
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