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D.4 SAMPLE CASE STUDY: A PARTIAL PUE ANDDCIE DETERMINATION FOR THE CRITICAL POWER PATH WITHIN THE DATA CENTER

D.4 SAMPLE CASE STUDY: A PARTIAL PUE ANDDCIE DETERMINATION FOR THE CRITICAL POWER PATH WITHIN THE DATA CENTER

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©2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org).

For personal use only. Additional reproduction, distribution, or transmission in either print or

digital form is not permitted without ASHRAE's prior written permission.



Real-Time Energy Consumption Measurements in Data Centers



Figure D.4 - Measuring points for

PUE or DCiE methods of efficiency

calculation



244



©2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org).

For personal use only. Additional reproduction, distribution, or transmission in either print or

digital form is not permitted without ASHRAE's prior written permission.



Appendix D—Uninterruptible Power Supply (UPS)



For this example, the critical AC power path starts at the electrical

distribution panels. Instruments at these panels measure and display

current, voltage and power in each feeder to the respective downstream

UPS systems. Each UPS further connects to a downstream PDU, from

which originate the branch circuits to the IT loads.

The IT load consists of ten computer racks. Two separate 208/120

volt three-phase AC power paths (A & B) are distributed to each rack via

separate branch circuits to rack-mounted power distribution units

(RPDU), so that each rack has two RPDUs. Dual-corded IT equipment

(e.g., servers) connect one cord to source ―A‖ and the other cord to

source ―B.‖

With this information we can calculate both Data Center

infrastructure Efficiency (DCiE) and Power Usage Effectiveness (PUE).

Note that measurements at all points in the critical power path must be

taken as nearly simultaneously as possible. This is possible with

automated, real-time measurement. Manual measurements may require a

team of people in order to minimize the time required to gather data.

STEP 1 - Measure input power to the data center

This can be obtained by careful measurement at the electrical

distribution panels, ensuring that the power supplied only to the critical

power path is included in the measurement. In this example, there are

four feeders to each UPS. Power measurements in watts are assumed to

be displayed on meters for each feeder, and are tabulated in Table D.3

below:



245



©2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org).

For personal use only. Additional reproduction, distribution, or transmission in either print or

digital form is not permitted without ASHRAE's prior written permission.



Real-Time Energy Consumption Measurements in Data Centers



STEP 2 - Calculate the total IT load power.

This is most easily accomplished by taking power measurements

from the output of the PDUs and aggregating them. The PDU meter

accuracy is typically more precise than the RPDU meter accuracy. If, for

some reason, that is not possible, or if the user wishes to know the exact

power from each source at each rack, power consumption can be

measured at the RPDU. The data in Table D.4 represents measured

currents at the rack RPDUs and calculated power. Note that in this

example current is measured but power is calculated using available data.

In an actual data center, power (watts) might actually be displayed on a

meter, thereby negating the need to calculate the power from the current

measurements. Power must be calculated for each source (A + B) and

aggregated to get the true power consumption.

Power (W) =



Σ(V · I ·



= (208 · IA ·



3)

3 ) + (208 · IB ·



3)



(D.4)



where V is the three-phase voltage and I is the current for each

source.



246



©2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org).

For personal use only. Additional reproduction, distribution, or transmission in either print or

digital form is not permitted without ASHRAE's prior written permission.



Appendix D—Uninterruptible Power Supply (UPS)



STEP 3 (Optional) - Calculate RPDU losses.

In most cases the efficiency of the RPDU itself is considered to be an

integral part of the IT load. If one wishes to count RPDU as part of the

infrastructure losses, it may be necessary to use calculated losses

provided by the RPDU manufacturer.

For this example, the IT load will be adjusted to obtain just the

aggregate computing losses, assuming an RPDU efficiency of 98.5%.

Therefore:



Net IT Load



Server Rack PDU Load RPDU Efficiency

247



(D.5)



©2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org).

For personal use only. Additional reproduction, distribution, or transmission in either print or

digital form is not permitted without ASHRAE's prior written permission.



Real-Time Energy Consumption Measurements in Data Centers



Net IT Load



66,354W 0.985 65,359W



STEP 4 - Calculate Critical Power Path PUE

By taking the reciprocal of DCiE, the Power Usage Effectiveness

(PUE) can be calculated. In this example:



PUE(CPP )



1

DCiE (CPP )



(D.6)



where PUE(CPP) is the Power Usage Effectiveness for the Critical

Power Path.



PUE(CPP )



1

1.152

0.8678



STEP 5 - Calculate DCiE

For this example, a reading was obtained of 76,470 watts of threephase power at the input to the data center and 66,354 watts were

consumed by the IT loads.



DCiE (CPP )



Total IT Load

100%

Critical Power Path Input Power



(D.7)



where DCiE(CPP) is the Data Center Infrastructure Efficiency for the

Critical Power Path.



DCiE (CPP)



66,354W

100% 86.78%

76,470W



Further analysis or breakdown of element efficiencies within the

critical power path including the UPS and power distribution elements

can be conducted. This example just provides an overall value of PUE

for the entire critical power path.



248



©2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org).

For personal use only. Additional reproduction, distribution, or transmission in either print or

digital form is not permitted without ASHRAE's prior written permission.



E.1



OVERVIEW



Onsite power generation has the potential to benefit a data center in

numerous ways. The most direct method is by reducing high energy

costs by eliminating or reducing the demand for power from the

electrical grid. Although there is a cost associated with any onsite

generation methods, overall energy cost savings can be achieved by

reducing the peak demand from the grid, which can often result in

reduced peak usage fees and ratchet charges. A ratchet charge is a utility

rate provision under which the demand charge for each month (or other

period) is based on the highest measured demand (or its percentage) over

the previous year (or other period). Depending on the electrical

generation method, peak shaving (reducing peak demand) or base

loading (constant electricity generation) schemes may be appropriate.

Onsite power generation can be categorized into two types; methods

that are stand-alone or methods that can be part of a Combined Cooling

Heat and Power (CCHP) system. The first type is normally associated

with Green technologies, or power generation methods that do not utilize

fossil fuels.

Alternative energy sources include solar, wind,

hydroelectric, and geothermal. These environmentally friendly power

generation technologies have advanced in recent years to the stage where

implementing these systems can be cost effective, with paybacks in just

years instead of tens of years.

In addition to the local economic and data center infrastructure

benefits that can be obtained, the displacement of inefficiently generated

power off the utility grid provides real environmental gains in the area of

emissions and reduces the burden on natural reserves.



©2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org).

For personal use only. Additional reproduction, distribution, or transmission in either print or

digital form is not permitted without ASHRAE's prior written permission.



Real-Time Energy Consumption Measurements in Data Centers



The second category of power generation technologies all generate

heat as a byproduct, and can therefore be used with a CCHP system.

CCHP refers to a system that couples power generation equipment with

equipment designed to do additional work using the leftover heat energy

from the electrical generation process. These systems are located at the

point of use and are a subset of the industry category of Distributed

Generation (DG) or Distributed Energy (DE). In data center applications,

CCHP systems not only generate electricity for use, but also use the

remaining heat for water-cooling of the data center, and may include

heating and steam generation components for use outside the data center

and for dehumidification.

CCHP systems can utilize prime movers such as gas turbines, gas

engines, diesel engines, and microturbines to power generators. These

technologies can be used with a number of different fuel sources,

providing the opportunity to optimize the availability, cost, as well as

performance of the power generation fuel to suit the needs of the data

center facility.

From an input power perspective, CCHP systems directly displace

power provided by the utility grid while simultaneously displacing the

electrical load associated with the chiller systems being displaced by the

cooling capacity of the CCHP system. In a perfectly sized system, the

cooling output of the CCHP system will be sized to eliminate 90% to

95% of the electric power required to run conventional cooling

equipment and will displace about 50% to 55% of the IT load, meaning it

will not be metered at the utility grid meter. Figure E.1 illustrates a

typical application of CCHP in the data center.

Onsite power generation alone does not affect the calculation of PUE

or DCiE. These metrics are based on electrical power consumption, and

are not biased by how the electricity is being supplied. Regardless of the

source, the power consumed by the IT equipment as well as the power

consumed by the data center facility (counting the power generation

outside of the facility) both remain the same.

However, when calculating PUE and DCiE for a CCHP system, a

non-electric energy source is used to operate an absorption chiller. The

reuse of what would otherwise be waste heat improves the PUE and

250



©2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org).

For personal use only. Additional reproduction, distribution, or transmission in either print or

digital form is not permitted without ASHRAE's prior written permission.



Appendix E—Onsite Power Generation and CCHP in Data Center Applications



DCiE ratios by reducing the total metered power into the data center by

the following equation:

Pdp = Pgen + Pcool



(E.1)



where Pdp is the total power displaced by CCHP at the utility meter,

Pgen is CCHP electric output power as metered at the combined output

point (collector bus) of the CCHP system, and Pcool is displaced cooling

power (power displaced by the absorption chiller providing cooling to

the data center).

E.2



CCHP



Key to all applications for CCHP is using the exhaust heat for useful

purpose. A data center is an excellent application since there is a

relatively constant thermal load requirement.

Sample Data Center Application of CCHP

This section introduces a real-world application of CCHP to the data

center environment, introducing critical design guidelines for maximum

efficiency and availability as well as serving as the basis for the followon instrumentation discussions.

A CCHP system used in the data center could be set up with grid

independent capability allowing it to not only provide economic benefit

(return on investment), but to also operate in the event of a grid outage,

just as an emergency generator set would. The CCHP system could also

be used as an extra layer of redundancy in addition to a conventional

diesel generator(s). When configured in this fashion, the system can

sustain the combined power and cooling requirements of IT equipment

indefinitely, provided there is no interruption in CCHP fuel supply.

Figure E.1 illustrates how a CCHP system would be used in this

configuration. This layout is microturbine based but can essentially use

any CCHP prime mover technology. The CCHP system serves a data

center with a UPS and electric driven cooling. The bold lines represent



251



©2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org).

For personal use only. Additional reproduction, distribution, or transmission in either print or

digital form is not permitted without ASHRAE's prior written permission.



Real-Time Energy Consumption Measurements in Data Centers



paths of normal or possible power flow. This design requires inclusion

of a Dual Mode Controller (DMC).



Figure E.1 - CCHP schematic for data

center operation



The DMC is a device that seeks to remain closed at all times when

the grid is present. This allows the microturbines to flow surplus power

to the entire building while the CCHP system is running in parallel with

the grid. In this mode the system is running in a maximum, base-loaded

condition. If the grid goes down, the DMC will immediately open the

252



©2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org).

For personal use only. Additional reproduction, distribution, or transmission in either print or

digital form is not permitted without ASHRAE's prior written permission.



Appendix E—Onsite Power Generation and CCHP in Data Center Applications



connection to the grid. This is a system safety interlock and protection

feature that prevents inadvertent power flow back to the grid. For a brief

period of time, the data center operates on UPS, as the CCHP system

resets itself to run independent of the grid. Once reset, the CCHP system

feeds power to the critical data center loads.

The automatic transfer switch (ATS) logic is set up so that the CCHP

system is the normal source. To prevent a false start of the diesel, a brief

time delay is introduced into the logic of the transfer switches. After the

delay, if the CCHP system is not online, the transfer switch will close its

starting contacts activating the diesel backup system.

A key element to a successful design of the cooling plant is the use

of a hybrid chiller plant. The hybrid chiller plant includes both electric

driven chillers and waste heat driven chillers in combination for

reliability and energy savings. The layout of the equipment will depend

on loading, rate structure and system requirements. Typical decoupled

systems can be utilized as well as side stream applications and series

flow applications.

The series flow arrangement can provide additional energy savings

over normal operations by reducing the inlet temperature of the chiller

water to the electric chiller. This arrangement can provide additional

savings if the electric chiller is sized to provide full load without an

absorption chiller but is operated most of the time in a part load

condition. This provides not only superior energy savings but also

reliability.

Absorption Chillers

Absorption chilling is a mature, stable technology that uses heat

instead of mechanical energy to provide cooling. The single-effect

absorption chiller system consists of an evaporator, an absorber, a

condenser, a generator, and a solution heat exchanger. Water is typically

used as the refrigerant in vessels maintained under low absolute pressure

(vacuum). In cooling mode, the chiller operates on the principle that

under vacuum, water boils at a low temperature. Under typical operating

conditions, this occurs at approximately 40 °F (4.4 °C), thereby cooling

the chilled water that circulates through the evaporator tubes. A

253



©2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org).

For personal use only. Additional reproduction, distribution, or transmission in either print or

digital form is not permitted without ASHRAE's prior written permission.



Real-Time Energy Consumption Measurements in Data Centers



refrigerant pump is used to spray the refrigerant water over the

evaporator tubes to improve heat transfer. A single-effect absorption

chiller system diagram is shown in Figure E.2.



Figure E.2 - Schematic representation

of a single-effect absorption chiller

system



254



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D.4 SAMPLE CASE STUDY: A PARTIAL PUE ANDDCIE DETERMINATION FOR THE CRITICAL POWER PATH WITHIN THE DATA CENTER

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