Rotary vs. Static UPS Operating Costs Analysis
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STATIC VS. ROTARY UPS
LIFE CYCLE COST ANALYSIS
PREPARED FOR EXODUS COMMUNICATIONS
By
Peter Gross, PE
Einhorn Yaffee Prescott
April 14, 2000
A.Assumptions
The facility gross floor area is 300,000 sf
Computer equipment load density: 75 W/sf
Total computer load: 22,500 KW
Two topologies were considered:
- Rotary UPS configuration consisting of two systems, each with 11 primaries and two reserves
Number of 1600 KVA/1280 KW modules equipped with integrally mounted 2000 KW diesel engines: 22 primaries and 4 reserves
Total number of rotary units:26 modules
- Static UPS configuration consisting of five systems each with four primary buses and one reserve. Each primary bus consists of three stand alone 500 KVA/400 KW modules, while each reserve bus supports three 500 KVA/400 KW modules in parallel.
Number of primary units: 72
Number of reserve units: 15
Total number of static units:87 modules
Other assumptions
Life cycle analysis duration10 years
Interest rate7%
Cost of energy$0.05/KWHr
B.Cost Analysis
- Equipment cost
The equipment cost components are summarized in the attached spreadsheet. Installation costs are included.
Rotary system$46,540,000
Static system$43,595,000
There is a $2,945,000 saving in equipment cost if the static system is installed.
- Energy costs
Efficiency of the rotary system at full load:95.7%
Efficiency of the static system at full load94.0%
Energy lossesPo(1-eff)/eff,
where Po is the system output power and eff is the system efficiency
Rotary system losses 22,500(1-0.957)/0.957=1011 KW
Static system losses 22,50091-0.940)/0.940=1436 KW
Yearly cost of energy (a 30% allowance for the mechanical system losses was used):
Rotary system 1011 KW x 1.3 x 24 hrs x 365 days x $0.05= $575,663
Static system 1436 KW x 1.3 x 24 hrs x 365 days x $0.05= $817,658
The additional yearly cost of energy for using a static system is:
$817,658 - $575,663=$241,995
The present worth of the energy savings over the ten-year period is:
((1+i)n – 1)/(i(1+i)n)
where i is the interest rate (7%)
n is the life of the system (10 years)
NPV=$241,995 ((1+0.07)10)-1)/(0.07(1+0.07)10)=$1,698,804
There is a $1,698,804 saving in energy costs if the rotary system is installed.
- Maintenance costs
The yearly cost of maintenance for the rotary equipment is $14,000 per unit. Assuming a 3% yearly escalation, the present worth of the maintenance cost, including the $420 uniform gradient is:
$14,000 ((1+i)n –1)/(i(1+i)n)+ $420 ((1+i)n)-1)/(i2x (1+i)n))-(n/(i(1+i) n))= $109,788
In addition there is a $35,000 cost for replacing the induction coupling every six years. The present value of this activity is:
$35,000 ((1+i)-n)= $23,322
Total maintenance costs for the rotary system:
26 x (109,788+23,322)= $3,694,860
The maintenance cost for the static equipment is $20,548 per module (including the batteries) for five years. Assuming a 15% escalation rate every five years, the present worth of the static equipment maintenance is:
87 units x ($20,548 + 1.15 x $20,548 x (1+i)-n)=$3,253,452
There is a $441,407 saving in maintenance costs if the static system is installed.
- Battery replacement cost
The static system uses VRLA batteries that last approximately four years, therefore the batteries will have to be replaced twice during the life of the system. The cost of the battery is $22,050 plus $5,000 for installation. Assuming the cost escalation matches the interest rate, the total cost of battery replacement is:
87 units x 2 x $27,050 = $4,706,700.
The cost of AGM batteries that have a longer life expectancy is about $100,000, therefore, the total battery cost will be higher. There are no batteries used with rotary equipment, with the exception of the diesel engine starting batteries.
There is a $4,706,700 saving in battery replacement costs if the rotary system is installed.
- Cost of additional A/C equipment
The rotary equipment does not require dedicated cooling other than ventilation to remove the heat from enclosure. The additional cost needed to cool the static UPS system is;
1436 KW x 3.4/12 x $1,000/ton=$406,887
where 1436 KW are the static UPS losses. A $1000/ton installed cost for the A/c equipment was assumed.
There is a $406,887 saving in A/C equipment costs if the rotary system is installed.
- Cost of displaced space
The majority of the rotary equipment will be installed in outdoor enclosures. The potential revenue generated by the additional space inside the building, recovered by the use of rotary UPS systems is about $250/sf.
A static UPS block consisting of four primary buses and one reserve requires about 4000 sf, therefore the total space occupied by the static system is 20,000 sf. The rotary UPS indoor installed switchgear requires about 500 sf per module or a total of 13,000 sf.
The potential revenue gained if rotary equipment is used is therefore:
$250 x (20,000 sf – 13,000 sf)= $1,750,000
The present worth of the total revenue gained over the ten years is:
$1,750,000 x ((1+i)n )-1)/(i(1+i)n)=
$1,750,000 x ((1+0.07)10 )-1)/(0.07(1+0.07)10)= $12,285,000
There is a $12,285,000 additional revenue generated by the recovered floor space if the rotary system is utilized.
C.Conclusions
The following table summarizes the cost benefits of each system. Negative numbers indicate a cost advantage for the static UPS.
Initial equipment cost / -$2,945,000Energy costs / $1,698,804
Maintenance costs / -$441,407
Battery replacement costs / $4,706,700
Additional A/C equipment costs / $406,887
Displaced floor space cost / $12,285,000
Total / $15,710984
Based on these considerations, it is clear that there are major cost benefits in using rotary UPS systems at the Exodus mega-centers. Even if the recovered floor space revenue is not considered, there is a $3.4 million saving generated as the result of using the rotary system. If the revenue generated by the displaced space is taken into account, the cost benefit of the rotary system increases to almost $16 million.
MISSION CRITICAL FACILITIES GROUP
11835 WEST OLYMPIC BOULEVARD, SUITE 1265, LOS ANGELES, CA90064
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