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In Response To "Data Centers are Adopting Green Initiatives but are Wary of Vendors' Marketing Messages"
Dr. Sidd Mukherjee
March 19, 2008
The article surveys green initiatives in datacenters. Some important
points are made:
a) almost a fifth of those organizations with green
initiatives do not include the data center
b) of those who did, 44%
suggested cooling a center to cut energy use
c) most respondents were
unconvinced of vendors claims of environmental friendliness.
This is disconcerting, and suggests that vendors and clients are
not communicating well.
A better way to exhibit the gains achievable through energy efficiency
might be to illustrate savings made possible by specific technologies.
A substantial share (30% and increasing) of the power used in a modern
datacenter is used for cooling. Electricity rates are increasing throughout the
world, and it is clear that carbon taxes will hasten the rise in the near
future. There are two components to electricity tariffs, the Kilowatt-Hour
(KWH)charge, and the demand charge, measured by the peak demand over a certain
time period in KiloWatts (KW). The first is the total energy used, while the
second is based on the maximum rate of energy use. For an illustration of
these tariffs, one may visit the ConEd site in Ref 
The Kilowatt-Hour charge is based on the total energy consumed during
the billing period, and is obviously reduced by more efficient AC. A more
efficient air conditioner will eat fewer KWH. But the second demand charge
is not as easy to minimize. This is due to time sensitive pricing, which
makes electricity during peak hours much more (as much as a factor of two)
more expensive than during off peak hours. At the same time, the power
drawn by any air conditioner will increase with the external temperature,
thus in the afternoon (when the demand tariffs are the highest) the
air conditioning will draw the most electricity. This is the part that
is not addressed by merely improving efficiency.
This variable pricing gives rise to opportunities for arbitrage. One such
technology is that used in Thermal Storage Systems. These systems use cheaper
off peak power to store ice, chilled water, or a suitable phase change material
such as Glauber's salt at night and use it up during the day to meet cooling
needs. Another beneficial feature is that the chillers and ancillary equipment
need not be sized for peak load, since a large fraction of the cooling during
high demand periods is delivered by the previously stored ice. This reduces
the initial cost, and thus any associated financing charges.
The leading candidates for Thermal Storage media are chilled water and ice.
Although phase change materials promise better specific storage capacity,
the technology of phase change materials is not as mature as the water or
ice based systems. Chilled water based system require larger storage tanks
but achieve a better chiller efficiency and lower chiller cost than ice based
systems. Ice storage systems can be retrofitted to some existing installations.
Thermal Storage Systems may be designed for full or part storage. Full
storage systems displace all of the peak demand by generating a sufficient
quantity of chilled medium during off peak hours to meet all of the cooling
load during peak hours. The system pulls no power for the chiller during
peak hours. Partial storage systems do not eliminate chiller load but they
reduce it substantially during peak, meeting a part of cooling demand
with previously chilled medium.
Substantial savings are possible as may be seen in the references below ,which
document a reduction in demand by 3MW and cost savings of almost a quarter
of a million US$ per year at the Dallas Veterans Affairs Medical Center.
The cost savings pays for the project in seven years at current energy
prices. Given the projected rise in electric costs, the payback period will
probably decrease. Several other projects are detailed in the references
- Con Edison - PSC No. 9 Electric Tariff
- WSU Energy Efficiency Fact Sheet On Thermal Energy Storage
- Storage Cooling Systems
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