Hotel Dual Electrical Services for LEED Credits

Summary: Requesting two service transformers with two voltages from the utility service is energy efficient and cost effective.

Large hotels generally have a 480/277 volt service provided by the utility and step-down transformers for 208/120 volt loads.  Since a hotel is largely 120 volt loads, the size of the step down transformers is about half of the capacity of the service.  This results in dual transformation of a large amount of power.  The energy efficient approach is to request two service transformers from the utility:  One transformer at 480/277 volts, the other at 208/120 volts.  The lower voltage transformer will eliminate all the losses from stepdown transformers in the hotel while keeping the utility losses the same.  For a large hotel, this is generally allowed by the utility.  The following is an example of the analysis applicable for computing energy savings from dual service transformers.

Based on an actual hotel of 333 rooms, the following LEED energy analysis was performed:

Assumptions:

Hotel Room Count:  330 guestrooms

120/208 volt load per NEC:  948 kva (Say 1000 kva)

Heat losses from distributed dry-type transformers  throughout the building:  2.0% of NEC load

Cost of electricity:  $0.08 per kwh

Load Factor:  Verified load factor data for hotels was not available at the time of this study.  However, load factor data for other facility types was available, and the data indicates a load factor of 30% to 50% is probable.  Since the load factor is an important part of this analysis, a continuing effort is in progress to obtain better load factor data specific to hotels.  However, we know that the magnetizing losses are a constant regardless of load and represent about 1% of the transformer losses based on nameplate data.  Therefore the I squared R losses will vary with load factor, but not in some fraction of a proportion.  For this study, the load factor is assumed to be included with the transformer loss number of 2%.  Peak losses for small transformers are commonly stated at 3%, which is the sizing criteria for cooling equipment.

Cooling Efficiency: Transformers located within the hotel require mechanical cooling to remove the heat generated by the transformers.  Since utility transformers are allowed to be cooled with ventilation air, there is an additional energy cost for dual transformation that must include the cost of mechanical cooling of the transformers.  This study is based on 1 kw per ton of cooling.

Computation of Savings:

Transformer energy savings per year in dollars =  1000kva x 0.02 efficiency x 8760 hrs/yr x $0.08 /kwh  =   $14,000 savings

Associated Cooling Savings =  1000kva x 0.02 efficiency x 3413BTUH/kw / 12000 BTUH per ton x 1 kw/ton x 8760 hrs/yr = $4,000 savings

Total Savings = transformer savings + cooling savings = $14,000 + $4,000 = $18,000 per year.

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