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Air-cooled chillers for Las Vegas revisited.

In the first article, "Air-Cooled Chillers for Hot, Dry Climates," (1) four alternatives for system replacement were considered and evaluated using a reliable life-cycle cost analysis program. The alternatives were:

1. New rooftop units;

2. Multizone systems with air-cooled chillers and gas heat;

3. Multizone systems with water-cooled chillers and gas heat; and

4. Multizone systems with air-cooled chillers and electric heat.

The multizone systems with air-cooled chillers and gas heat provided the lowest 20-year life-cycle cost. The school system had done some replacement of rooftop units with multizone systems and experienced significant savings in maintenance costs over a period of five years. The expected savings in maintenance and electrical energy costs for these schools for the 20-year life expectancy of the new HVAC systems was $4.5 million. Table 1 shows the significant results from the first article. (1)

In the second article, "Safe Bet for Vegas Schools," (2) the reasons for considering the alternatives and changes are discussed.

"HVAC maintenance costs for the retrofitted Garrett Middle School in Las Vegas were less than half of the costs to maintain two similar schools with rooftop units. The difference is attributed to Garrett's replacement of its RTU multizone units with an air-cooled chiller, closed loop chilled water system and multizone air-handling units."

Table 2, extracted from the second article, shows the results for the 20-year life-cycle cost analysis for two of the 11 elementary schools being considered.

The second article (2) concluded:

"Looking only at the chiller energy consumption would lead one to install a water-cooled chiller, rather than an air-cooled chiller in a desert area. However, considering all of the factors: chiller efficiency, maintenance costs, cooling tower need, water treatment, etc., the air-cooled chiller system would save CCSD about $100,000 throughout the life of the system. The savings from using the air-cooled system over just the simple replacement of the original direct expansion rooftop units was approximately $275,000 per school for the 20-year life of the new system.

These renovations have been completed and are operating as expected with significant savings in operating and maintenance costs. The total savings are expected to reach about $1,100,000 throughout the 20-year life of the HVAC systems in these 11 Clark County schools."

Data Collection

The data that has been retrieved from the CCSD data center includes:

* Electrical energy usage in kBtu/[ft.sup.2];

* School areas annually in square feet; and

* Maintenance costs in dollars.

Also collected were the 65[degrees]F (18[degrees]C) cooling degree days (CDD) for the Las Vegas area on both an annual basis and an accounting period basis.

The factors that affected the energy usage include:

1. Was the school operated on a nine or 12-month basis?

2. The area of each school changed from the original 43,000 [ft.sup.2] to up to 68,000 [ft.sup.2] (4000 [m.sup.2] to up to 6300 [m.sup.2]). This was done in some cases by permanent additions and, more commonly, by adding modular classrooms. During the time of this data collection, the area change was done with modular units. Between 2002 and 2013 the 11 schools had area changes ranging from 55,000 [ft.sup.2] to 68,000 [ft.sup.2] (5100 [m.sup.2] to 6300 [m.sup.2]). The average area for all schools for all years was 61,000 [ft.sup.2] (5667 [m.sup.2]).

3. Some schools had outside activities taking place after school hours.

4. During the period of analysis, some schools hosted evening classes for variable lengths of time.

5. Some schools had computers added.

6. Some schools hosted summer school during some years.

7. Some schools added chiller operation 24/7 for a two-week period.

All of these factors were not easily accounted for because they took place at unknown times for unknown periods, and increased energy usage. However, averaging the energy usage over all the years and all the schools helped smooth out these effects.

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.


Cooling Degree Days

Before looking at the energy and maintenance costs, it is useful to look at the cooling degree days at base 65[degrees]F (18[degrees]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 3,793.00

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.

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/[ft.sup.2]. From Principles of Heating, Ventilating and Air-Conditioning, Page 8, the typical annual goal for an annual electrical energy budget for a building is 15 kWh/[ft.sup.2], which converts to 51.2 kBtu/[ft.sup.2] (580 MJ/[m.sup.2]), very close to what was experienced in the Las Vegas schools.


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/[ft.sup.2] (580 MJ/ [m.sup.2]). For the 11 schools in 1998 prior to renovation, the average was 59.4 kBtu/[ft.sup.2] (675 MJ/[m.sup.2]). This is a 14.1% reduction in energy usage for the 11 schools for 12 years.

Following the retrofit, two schools, Harmon and Smith, are higher in kBtu/[ft.sup.2] when compared to the original energy usage. Harmon was higher at 56.3 kBtu/[ft.sup.2] (639 MJ/[m.sup.2]) after renovation compared to 53.6 kBtu/[ft.sup.2] (609 MJ/[m.sup.2]) prior to renovation, an increase of 5%. Smith was higher at 52.8 kBtu/[ft.sup.2] (600 MJ/[m.sup.2]) compared to 46.2 kBtu/[ft.sup.2] (525 MJ/ [m.sup.2]), an increase of 14%. This could be due to many unknown factors such as building usage, extra lighting load, higher computer usage, extra classes being taught, etc. Further investigation indicated that these two schools had control issues. These involved oversized hot deck gas heaters and undersized electric damper actuators for the low leakage zone dampers installed for improved energy efficiency. This resulted in simultaneous heating and cooling at times. However, nine of the 11 schools had lower average energy use in 2002 to 2013.

Figure 1 shows the average of all schools each year following renovation compared to the reference year of 1998. 2003 was slightly higher than 1998 due to damper issues. Figure 2 is a plot of the individual schools showing the before and after renovation energy usage values. Harmon and Smith are highlighted to show their lack of energy savings.

School     Abbreviation

Disken         DIS
Dondero        DON
Edwards        EDW
Ferron         FER
Harmon         HRM
Harris         HAR
Smith          SMI
Tate           TAT
Tomiyasu       TOM
Ward           WAR
Wengert        WEN
Average        avg

There is some maintenance cost data available for six of the 11 schools. In the other schools, HVAC maintenance costs other than those for HVAC systems were not separated. These appropriate data are given in Table 5.

In the 2011ASHRAE Handbook--HVAC Applications, studies of HVAC maintenance costs have been reported for two studies, one in 1983 and the other in 2004. The median value for 2004 was $0.44/[ft.sup.2] x yr ($4.74/[m.sup.2] x yr). Since the data presented in Table 5 was from 2002 to 2013, the $0.44/[ft.sup.2] x yr ($4.74/[m.sup.2] x yr) from this reference would be a reasonable estimate for comparison. The schools in this study were performing better than what was reported in the Handbook, which includes something about reports of maintenance costs: "The maintenance costs of mechanical systems varies widely depending upon configuration, equipment location, accessibility, system complexity, service duty, geography, and system reliability requirements." There appeared to be some lack of proper training for maintenance needs with the new HVAC systems.

From Table 2 the maintenance costs were reported at $0.181/[ft.sup.2] x yr ($1.95/[m.sup.2] x yr) in 1998 along with an estimate for the renovated system, with limited data, at $0,085/ [ft.sup.2] x yr ($0.92/[m.sup.2] x yr). Using the 1998 value of $0.181/[ft.sup.2] x yr ($1.95/[m.sup.2] x yr) compared to the value given in Table 5 of $0.127/[ft.sup.2] x yr ($1.37/[m.sup.2] x yr) a savings in maintenance costs of about 29.8% was achieved.


Using the results from 2002 to 2013 and extrapolating to a 20-year life results in the following savings. An average electrical cost was found by taking the 1998 value for CCSD and extrapolating for half of the life, yielding a value of $0.121/kWh. The average area was 61,000 [ft.sup.2] (5667 [m.sup.2]).

From Table 4: 59.4 - 51.0 kBtu/[ft.sup.2] x yr results in an average of 8.4 kBtu/[ft.sup.2] (95 MJ/[m.sup.2]) annually per school.

8.4 kBtu/[ft.sup.2] x yr per school (61,000 [ft.sup.2] x 11 schools x 20 yrs) = 113 million kBtu or 33,038,000 kWh

At $0.121/kWh that gives $3,997,600 savings in 11 schools for 20 years.

From Table 1, the estimated savings were $3,226,000. So, the actual savings exceeded the estimated savings by $771,600 or 24%.

From Table 1, the maintenance costs were $0.181/[ft.sup.2] x yr ($1.95/[m.sup.2] x yr) per school prior to renovation. From Table 5, the representative actual maintenance costs following renovation were $0.127/[ft.sup.2] x yr ($1.37/[m.sup.2] x yr) per school.

Savings = ($0.181/[ft.sup.2] x yr - $0.127/[ft.sup.2] x yr) (61,000 [ft.sup.2] x 11 schools x 20 yrs) = $724,680

From Table 1, the savings in maintenance costs were estimated at $1,231,450 or an over estimate of 41%, which is $506,770.

Combining electric usage costs and maintenance costs, the savings were under estimated by $264,660.

The total savings for electric and maintenance costs for 11 schools for 20 years was $4,722,280. In addition, the life-cycle cost estimate was 5.9% better than expected.

There was a significant savings in water usage by not selecting water-cooled chillers over air-cooled. Water use was a significant concern in the original decision to go with air-cooled chillers. This turned out to be a wise decision considering that a 14-year drought has dropped Lake Mead's water levels to their lowest point ever this year.


H. Richard Cuppett, P.E., provided all energy, maintenance, use, and cost data for the 11 Clark County School District elementary schools included in this analysis. A memoriam to Elton Dale Scheideman, AIA, who died in February 2006, during his 15thyear as Director of Planning and Engineering for CCSD. He authorized the first air-cooled chiller project at Garrett Middle School in August 1991. He would have been pleased with the results presented here for the 11 elementary schools.


(1.) Howell, R.H., D.W. Land, E.D. Scheideman. 2003. 'Air-cooled chillers for hot, dry climates." ASHRAE Journal, 12.

(2.) Howell, R.H., D.W. Land, E.D. Scheideman. 2004. "Safe bet for Vegas schools." ASHRAE Journal, 5.

(3.) Howell, R.H., W.J. Coad, H.J. Sauer. 2013. Principles of Heating. Ventilating, andAir-Conditioning, 7th edition. Atlanta: ASHRAE.

(4.) 2011 ASHRAE Handbook--HVAC Applications, Chapter 37.


Ronald H. Howell, Ph.D., P.E., is retired. Donald W. Land, P.E., is facility engineer at University of Nevada at Las Vegas. John M. Land is a retired data analyst.

TABLE 1 Dollar savings In operating costs after retrofit.

AVERAGE                         MAINTENANCE      SAVINGS FOR
MAINTENANCE                     SAVINGS FOR          20 YEAR
COSTS FOB                       ALL SCHOOLS         EXPECTED
THE 11 SCHOOLS                     (641,380        EQUIPMENT
                                [FT.sup.2])             LIFE

Before              After

$0.181/           $0.085/   $61,572/Year at       $1,231,450
[ft.sup.2]     [ft.sup.2]           $0.096/
Per Year         Per Year        [ft.sup.2]

AVERAGE                          ELECTRICAL      SAVINGS FOR
ELECTRICAL COSTS                SAVINGS FOR          20 YEAR
                                   (641,380   EQUIPMENT LIFE
                                [FT.sup.2])   (EXTRAPOLATED)

Before              After

59,422 Btu     50,480 Btu     $161,308/Year       $3,226,170
/[ft.sup.2]   /[ft.sup.2]    at $0.1002/kWh
Per Year         Per Year

17.4 kWh/       14.9 kWh/
[ft.sup.2]     [ft.sup.2]
Per Year         Per Year

TOTALS FOR                          FOR ALL           FOR 20
MAINTENANCE                     ABO SCHOOLS        YEAR LIFE

                              $222,800/Year       $4,457,620

                                 PER SCHOOL

Maintenance                     $5,597/Year         $111,950
Electrical                     $14,664/Year         $293,288
Total                          $20,262/Year         $405,238

TABLE 2 Life-cycle cost analyses for Edwards and Wengert.



1      150      $487,715       Base        $1,302,187
2      150      $646,053    7 Years        $1,067,334
3      150      $648,648   12 Years        $1,172,814
4      150      $701,600   11 Years        $1,166,525



1      169      $717,160       Base        $1,799,652
2      169      $952,265    8 Years        $1,495,759
3      169      $924,718   11 Years        $1,591,332
4      169    $1,008,700   11 Years        $1,605,235

TABLE 3 Cooling degree days (base 65[degrees]F)--Las Vegas.


CDD--3,568        CDD--3,568

ACCOUNTING YEAR    CDD    YEAR               ODD

--                        1997             3,300
1998:7/97-6/98    2,866   1998             2,908
--                        1999             3,241
2000:7/99-6/00    3,635   2000             3,773
--                        2001             3,917
2002:7/01-6/02    3,755   2002             3,663
2003:7/02-6/03    3,601   2003             3,846
2004:7/03-6/04    4,054   2004             3,755
2005:7/04-6/05    3,495   2005             3,595
2006:7/05-6/06    3,842   2006             3,755
2007:7/06-6/07    3,898   2007             4,091
2008:7/07-6/08    3,743   2008             3,818
2009:7/08-6/09    3,918   2009             3,818
2010:7/09-6/10    3,550   2010             3,668
2011:7/10-6/11    3,682   2011             3,566
2012:7/11-6/12    4,000   2012             4,045
2013:7/12-6/13    4,087   2013             3,791

Average           3,789          (14 years) 3,793

TABLE 4 Electrical energy use per unit area.


SCHOOL   1998   --   2992   2003   2004   2005   2006   2007

DIS      56.9   --   50.1   77.0   50.5   50.3   50.1   49.7
DON      59.5   --   51.7   63.4   51.0   45.4   50.0   55.4
EDW      54.4   --   51.1   57.0   44.2   48.6   48.7   51.6
FER      61.7   --   55.6   65.0   50.9   59.3   61.4   59.5
HRM      53.6   --   43.1   55.0   45.7   54.9   58.0   67.3
HAR      60.0   --   44.2   48.4   44.8   45.6   44.2   52.0
SMI      46.2   --   44.1   55.1   47.3   53.7   59.9   56.4
TAT      69.9   --   54.5   76.0   49.5   57.9   53.7   53.3
TOM      56.4   --   50.3   47.0   46.9   51.2   50.7   51.3
WAR      72.5   --   55.9   54.9   48.3   47.8   46.1   51.6
WEN      62.7   --   60.5   63.0   46.3   47.9   53.7   57.6
avg      59.4   --   51.0   60.1   47.8   51.1   52.4   55.1

SCHOOL   2008   2009   2919   2911   2912   2013   AVERAGE

DIS      47.8   48.2   47.0   49.1   42.6   40.1      50.2
DON      49.8   50.9   49.3   46.1   45.6   43.1      50.1
EDW      48.9   53.7   60.8   50.4   44.2   44.1      50.3
FER      50.5   50.0   47.7   55.3   51.4   50.0      54.7
HRM      67.7   60.2   56.4   56.4   55.4   55.9      56.3
HAR      41.1   41.9   40.1   46.1   43.3   50.8      45.2
SMI      56.0   54.3   51.5   54.1   53.0   48.1      52.8
TAT      51.5   53.4   49.6   49.0   48.5   40.6      53.1
TOM      49.1   50.0   42.5   44.0   39.9   43.3      47.2
WAR      42.8   43.3   41.5   43.7   41.0   41.6      46.5
WEN      56.3   59.9   60.3   59.5   48.1   47.6      55.0
avg      51.0   51.4   49.7   50.3   46.6   45.9      51.0

TABLE 5 Representative maintenance cost data.

SCHOOL                HVAC           AREA   $/[FT.sup.2]/
           MAINTENANCE ($)   ([FT.sup.2])              YR

Edwards             11,882         62,962           0.189
Ferron               1,300         59,852           0.022
Tate                12,747         64,029           0.199
Tomiyasu             7,258         58,241           0.125
Ward                12,232         65,050           0.188
Wengert              2,906         62,761           0.046
Average                                             0.127
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Author:Howell, Ronald H.; Land, Donald W.; Land, John M.
Publication:ASHRAE Journal
Geographic Code:1U8NV
Date:Dec 1, 2014
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