ASHRAE Journal - December 2014 - 40

TECHNICAL FEATURE

The total amount of electrical usage is made up of
many sources: lighting, equipment, plug loads, computers, fans, pumps, as well as chiller loads. There was no
separate metering of chiller usage so the effect of using a
more efficient chiller is diminished by adding the other
sources of electrical usage.

TABLE 3 Cooling degree days (base 65°F) - Las Vegas.
CDD PER ACCOUNTING YEAR

CDD PER CALENDAR YEAR

AVERAGE ANNUAL CDD - 3,568

ACCOUNTING YEAR

-
1998: 7/97 - 6/98
-
2000: 7/99 - 6/00
-
2002: 7/01 - 6/02
2003: 7/02 - 6/03
2004: 7/03 - 6/04
2005: 7/04 - 6/05
2006: 7/05 - 6/06
2007: 7/06 - 6/07
2008: 7/07 - 6/08
2009: 7/08 - 6/09
2010: 7/09 - 6/10
2011: 7/10 - 6/11
2012: 7/11 - 6/12
2013: 7/12 - 6/13
Average

Results
Cooling Degree Days
Before looking at the energy and maintenance
costs, it is useful to look at the cooling degree days at
base 65°F (18°C) (CDD) for Las Vegas. These are given
in Table 3. The average annual CDD = 3,568 is taken
from a local source in Nevada. From the Principles
of Heating, Ventilating and Air-Conditioning, which has
weather data taken from the ASHRAE Weather Data
Viewer 5.0, gives a value of 3,486 CDD for McCarran
International Airport. The average CDD from NOAA
for 2000 to 2013 for the accounting year for the Clark
County School District, July through June, averaged
3,789. The accounting year is the basis for the school
district's reported annual energy usage and costs. The
average for the calendar years 2000 through 2013 was
3,793.
It is clear that, on the average, either basis yields the
same number of CDD: 3,790. Notice that for 1998 the
number of CDD was 2,908. This is a 23% lower value in
this comparison year than the average of the 14 years
over which the new equipment has been compared.
Therefore, this yields a conservative estimate of the
energy savings for the new systems compared to the old
systems that were in use in 1998. The range of reported
CDD during 2000 to 2013 is a high of 4,091 to a low of
3,566 or +8% to -6% of the average.
The deviations in CDD (23%, +8%, and -6%) are not
unusual. CDD is not a very precise indicator of the
severity of the summer. It is only a single parameter indicator-temperature-and does not take into
account variations in solar intensity fluctuations,
wind velocity variations, nor humidity load variations.
However, it is a readily available number that is in
common use.
The results of the evaluation of this CDD data is that
there are no "unusual summers" during the data collection time period. Also, the 14 years of data have a higher
number of CDD than the reference year of 1998, providing a conservative estimate for energy savings.
40

ASHRAE JOURNAL

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D ECEM BER 2014

CDD

2,866
3,635
3,755
3,601
4,054
3,495
3,842
3,898
3,743
3,918
3,550
3,682
4,000
4,087
3,789

YEAR

CDD

1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013

3,300
2,908
3,241
3,773
3,917
3,663
3,846
3,755
3,595
3,755
4,091
3,818
3,818
3,668
3,566
4,045
3,791
(14 years) 3,793

Electrical Energy Usage
Table 4 provides the total electrical energy used in the
11 elementary schools for the 1998 year (last year that all
schools were operating with the old HVAC system) and
for 2002 through 2013 when all schools were operating
with the new HVAC systems. The data was converted
from kWh to kBtu per square foot.
In the last column on the right, the average for the
years 2002 to 2013 is given for each school. The bottom row is the average for all schools for each year.
The average for all schools and all years from 2002 to
2013 is given in the lower right corner and was 51.0
kBtu/ft2. From Principles of Heating, Ventilating and AirConditioning, Page 8, the typical annual goal for an
annual electrical energy budget for a building is 15
kWh/ft2, which converts to 51.2 kBtu/ft2 (580 MJ/m2),
very close to what was experienced in the Las Vegas
schools.

Discussion
The average annual electrical energy consumption per
square foot per year for the 11 schools for the 12 years of
usage following the renovations was 51 kBtu/ft2 (580 MJ/
m2). For the 11 schools in 1998 prior to renovation, the
average was 59.4 kBtu/ft2 (675 MJ/m2). This is a 14.1%
reduction in energy usage for the 11 schools for 12 years.

ASHRAE Journal - December 2014

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