Archive for the ‘LEED’ Category

Hotel Dual Electrical Services for LEED Credits

Friday, January 2nd, 2009

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.

ASHRAE 62.1 LEED Credits for Hotels

Tuesday, December 9th, 2008

Summary: Example of how to apply ASHRAE 62 to achieve a LEED credit. 

Applying ASHRAE 62 to a hotel to get LEED credits is very easy to do because hotel spaces generally have separate air handlers serving each type of space.  Therefore, to achieve ASHRAE 62 ventilation is as simple as setting the air handler minimum outside air to the ventilation requirements for the respective zones computed according to the ASHRAE 62 method.  This includes an area factor and a occupancy factor, of course.  The only situation that truly challenges the designer is where a single outside air unit is used to serve several air handlers each serving zones with different ventilation requirements.  For those cases, the ventilation minimum outside air setting must be computed according to the involved formulas.  ASHRAE has a spreadsheet available to help with these calculations, or you can use a design program such as Design Master which has this feature integrated into the load calculations.

Below is an example of an ASHRAE 62 computation performed for a simple building with diverse zones served by common air handlers.  The example is offered to assist in understanding what is involved.  The computations are performed using the ASHRAE 62 spreadsheet and Design Master for comparison.

Sample Analysis:

ASHRAE 62 ventilation concept is a method for setting the minimum outside air for an air handler, which serves rooms that have different percentage outside air requirements.

The building consists of five zones, each zone served by a separate HVAC unit. Within each zone are a number of rooms ranging from one room to eight rooms. The single room zones are a trivial case of the ASHRAE 62 concept. For those zones, the minimum outside air setting is simply the ventilation rate prescribed by the ASHRAE 62 ventilation amount based on occupancy and room area.

For the zones with more than one room per zone, there are three possible methods of calculating ventilation rates that could be used, each with a different effect on air quality and energy consumption. The first method is to simply add up the individual room ventilation rates and set the HVAC unit to a minimum outside air based on that sum. This results in an amount of outside air at the low end of the spectrum. This is the most energy efficient selection, since this minimizes the outside air heating and cooling load on the system. However, it may not provide sufficient ventilation for the room, which requires a higher percentage of outside air compared to other rooms in the zone. For example, if a conference room and an office are both served by the same HVAC unit, and the conference room requires 50% outside air and the office requires 20% outside air, then the office will not receive sufficient ventilation based on this method.

One method to absolutely guarantee that every room receives the required ventilation is to set the outside air at the HVAC unit to the percentage matching the room with the highest percentage ventilation requirement. The drawback to this approach, however, is that the rooms such as offices compared to conference rooms would receive far more ventilation than necessary. Thus, energy would be wasted.

The ASHRAE 62 approach is to find a middle amount of ventilation between the two extremes described above. Based on extensive research and analysis, the ASHRAE approach gives credit to the mixing effect of return air from one room to another. For example, if an office needs 20% outside air, but it is being served with 25% outside air, then a portion of the air returned to the HVAC unit is still “fresh” and could be counted as outside air when recirculated. The trick is to determine just how to translate this general concept into actual usable outside air values. This is what the formulas of ASHRAE 62 accomplish. ASHRAE 62 computations result in a setting for the outside air that is a proper compromise between the most efficient energy setting, and the maximum case ventilation setting.

The following table shows the five zones, the supply air for each room, and the amount of ventilation required for each room within the zones. In the least ventilation amount case, the sum of the room ventilation cfms is shown for each zone. In the maximum ventilation amount case, the ventilation for each room is computed based on the percentage of ventilation of the worst case room applied to all the other rooms. This percentage is then applied to the zone to show the worst case outside air setting. The column between the least and the worst-case ventilation rates is the ASHRAE 62 rate as computed using the ASHRAE 62 formulas. Note that this amount is between the two extremes in all cases except the cases where the zone has only one room. In those cases, the ventilation rate simply matches the minimum amount required for the room.

The ASHRAE 62 calculations were performed using Design Master HVAC software, which combines load calculations, ductwork design, and the ASHRAE 62 calculations. As a further check of this method of computing the AHSHAE 62 values, the official ASHRAE spreadsheet was used. The values matched and confirmed the correctness of the solutions. 

 The following exhibits are included for reference:

  1. Floor plans showing the five zones and rooms. Each room is labeled with the occupancy and ventilation requirement: LINK
  2. Summary of the outside air settings recommended for each HVAC unit. Note, this is the minimum setting and any economizer cooling operation may and should increase these values during economizer operation: LINK
  3. Design Master printouts showing the ASHRAE 62 results: LINK
  4. ASHRAE 62 spreadsheet showing a comparison analysis: LINK