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M&V Project Case Study: Variable Speed Drives on Air Conditioning System

at a Large Office Block in Johannesburg

A. R. Kenny, Energy Research Centre, University of Cape Town

ABSTRACT

At a very large office complex, beginning in 2004, lights and variable speed drives for the air-conditioning were changed for more efficient ones. To calculate the energy and power savings

for the motors, a representative 7.5 kW motor was run in the laboratory using both the old andthe new drives over a range of loads. A relation was then found between the power consumption

of the old and new drives. 22 of the new drives were metered over a period of months. The

power that the old drives would have used was derived from the power that the new onesactually used. Savings in power, energy and money were then calculated.

Introduction

This project in increasing energy efficiency at the office complex in Johannesburg began in

2004. It consisted of two parts: (i) the replacing of lighting with a total power rating of 2237kW with more efficient lighting and (ii) the replacing of variable speed drives on motors driving

fans and pumps in the air conditioning system with a total power rating of 1473 kW with more

efficient variable speed drives. This case study only considers the latter.

There are 72 motors in total. The motors themselves were not changed. 60 of them were driven

by thyristor-based voltage controllers and 12 of them, on the cooling tower fans, were fixed

speed drives and used dampers to vary the airflow. All were changed to variable speed controlusing pulse width modulation (PWM) frequency converters.

This case study describes the methods used to measure the savings in power and energy on the

72 motors. It gives the results of energy and power saved. It explains the problems encountered

and sums up the lessons learnt from this project.

Method

One method considered was to measure and log for several months the power used by the motorsbefore the changes and then do the same after the changes. This was dismissed as impractical

and inaccurate. The change to more efficient lighting would considerably reduce the heatreleased into the office spaces and therefore the air-conditioning load before and after would bedifferent. It would be impossible to match the performance of the motors before and after the

changes because they would be operating under different loads depending on weather, time of

year and other conditions before and afterwards.

In the case of the replacement of thyristor control with pulse width modulation (PWM)

frequency control, it was decided to take a representative 7.5 kW motor and in the laboratory


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measure its power consumed over a full range of loads, using first the thyristor control and then

the PWM control. A relation was then found between the power used by the old drive and thenew drive for each load. 22 of the motors were metered after the changes and logged

continuously. Using the relation, the power that would have been used by the old motors was

calculated from the power actually used by the new. The savings were then calculated.

In the case of the replacement of the fixed speed drives with dampers on the cooling tower fans

with PWM speed control, an attempt was made to measure the power drawn by a fixed speed

motor with the damper and a motor with the PWM speed control on the same type of coolingtower at various air flows. However, it proved impossible to get reliable measurements and this

method was abandoned. Instead it was decided to use a theoretical calculation to derive the

power drawn by the old motors from actual measurements of the power drawn by the new.

Using standards curves of power against air flow for a fan with a VSD and a fan with a damperfrom the 2000 Ashrae Handbook, HVAC Systems and Equipment (p40.9), a relationship was

found between the power drawn by the VSD and that drawn by the damped system at the same

air flows.

22 of the 72 motors were metered after the changes and logged continuously. Using the relations

described above, the power that would have been used by the old motors was then calculated

from the power actually used by the new. The savings were then calculated and scaled upproportionally to get the savings of all 72 motors.

Results for the Power Relation between Old and New Drives

In the laboratory, measurements were made on the 7.5 kW motor, using first the thyristor control

and then the PWM control. It was found that the former was unstable at loads lower than 55%.Accordingly, measurements could only be made from 55% load to 100% load. The results of

these measurements are shown in Table 1.

Table 1. Power Consumption Comparison between Old and New Drives

% Power kW kW

New Old

55 3.313 5.308

60 3.754 5.563

70 4.656 6.09280 5.472 6.832

90 6.508 7.279

100 7.683 7.674

(Source: Analysis and Comparison of the Performance of Two Adjustable Speed Drives, M.

Snow and L. Dosiek, University of Cape Town Research May-August 2004)


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Various curves were fitted to these points and it was found that a linear curve had a good fit with

a maximum error of 2.2% between 100% power and 55% power. The equation for this line, withmeasurements in kW, was:

OLD POWER = 0.5767 * NEW POWER + 3.4503

This line was extended to 0% power. This gives a conservative value of the savings below 55%

power since in this region the difference in the power consumed by the old and new motors

would be greater than that shown in the linear relation. (It should be noted that this extrapolationis not scientifically correct but it does provide a practical way of viewing savings which could

not otherwise be determined). The constant in the linear equation was scaled up to get the

equations for the 11, 15, 22 and 45 kW motors that were metered on site.

In the case of the fixed speed motors with dampers being replaced by PWM speed control and no

dampers, the relation between the two was:

OLD POWER = 2.5 x LN (1.667 x NEW POWER + 1) + 4.5

Results for Savings

Meters were put on 22 of the 72 motors and readings were logged every half hour from the 30th

November 2004. The power used by the new motors was converted into the power that would

have been used by the old motors using the relations described in the previous section. Figure 1shows the power consumed by the new and old motors for the month of April.


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Figure 1. Power Consumption of New and Old Motor Drives for April 2005

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04/13/2005

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0.0

50.0

100.0

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kW

kW totals for 22 motors: Apr 2005

Blue: new. Brown: old.

The figures for the 22 motors were scaled up proportionally to get the figures for all 72 motors.

Calculations were then made for energy, power and money saved. Table 2 shows these for the

month of April 2005.

Table 2. Savings in Energy, Power and Money for the Motor Drives for April 2005

kWh kW Money kWh kW Money kWh kW Money

Rand Rand Rand

Before After Savings

359,241 893 86,260 202,143 498 48,323 157,098 395 37,937

Conclusions and Comments

The savings shown in Table 2 are slightly conservative for reasons given above. The savings aremuch less than that predicted by the Esco but this is simply because it had been assumed that themotors would be running at high loads whereas most of them actually run at low loads. Most of

the motors seem oversized. A complicating factor is that the more efficient lighting releases less

heat and so reduces air conditioning loads.


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A lesson from this project is that a careful and accurate estimation of the actual loads should be

obtained beforehand for the equipment to be changed. This would give all parties a clear valueof the likely savings.

References

Savings Report. February to April 2005. A. R. Kenny. Project Number 2003039. Energy

Research Centre, University of Cape Town.

Analysis and Comparison of the Performance of Two Adjustable Speed Drives. M. Snow and L.

Dosiek. University of Cape Town Research May-August 2004.