Figure 1 shows the three main stages of the methodology to assess the impact of the cooling tower system's energy efficiency improvement: installation and inspection, measurement and data processing, and results and analysis. Each stage was designed to evaluate and obtain a comparison of both systems, standard efficiency motors and higher efficiency motors with adjusted load and variable speed drive.

Figure 1.

Flowchart of the methodology

The installation and inspection stage starts with revising the available technologies and installing new technologies. Each technology is passed through the same process to obtain the same results that are compared and analysed. The measurement and data processing stage can be divided into five steps: data collection, measurement, characterisation of energy consumption, and power quality analysis. After running this process on the previous and proposed system, the results can be compared and obtained the different analyses such as comparison of energy consumption, comparison of energy saving, and variation of power quality parameters.

Regarding data collection, this step takes the necessary data to perform the study. Some information collected is related to the nameplate, loads, flow control methods, and motor starters. Regarding the energy consumption measurement, this step measures energy and daily power flow consumption at the common connection point and each load. The energy consumption data and the daily flow were obtained from the records of the energy management system for the previous system (from December 12, 2020, to May 31, 2021) and for the new system (from July 1, 2021, to December 30, 2021). All measurements were taken in the totaliser and the pumps.

The energy data obtained in the new measurements are tabulated. Then, the cooling tower operation baseline with energy consumption data (in kWh) versus the water flow demanded by the production plants is obtained. In addition, the energy parameters are studied, and the software NEPLAN is used to model the electrical system with the new characteristics. Finally, a power quality analysis was performed considering the evaluation of the total and individual voltage and current harmonics.

The cooling tower object of study is the induced mechanical draft type. It is part of a closed-cycle cooling system that extracts the most outstanding amount of heat energy generated in the distillation towers' heat exchangers and the soap plant's vacuum systems.

Figure 2 shows the system comprising a two-cell cooling tower, four motor pump assemblies, and two extractors at the top of the cooling tower. The tower cells are made of polymer (plastic). They are responsible for uniformly distributing the water as a thin film, thus facilitating its cooling as the air passes through them.

Figure 2.

Cooling tower flowchart

The air is sucked through the filling of the tower using two fans located in the upper part, establishing a flow against the current with the water, thus facilitating heat exchange between the two media. The extractors introduce 800 m³/h of forced air from the bottom to the top of the tower at an average speed of 15 km/h. Each fan is driven by a 25 HP, 1700 rpm motor and 5 kNm gearbox. The temperature of the water passing through the tower's interior drops by approximately 6 °C. The water reaches the bottom of the tower, where it is stored in a basin of approximately 8,000 l volume. At this point, the water is sucked in by four centrifugal pumps and sent through pipe networks to the different places required for cooling. The working pressure required by the processes must be between 50 and 70 psi (3.45 to 4.83 bar), and the temperature must be around 30 ºC. Once the water has exchanged heat, it returns to the cooling tower to be cooled again.

The operation of the cooling tower pumping system was controlled by the operator, who maintained the flow and pressure constant. Depending on the demand, the operator activates the necessary pumps to maintain the pressure and flow parameters required by the processes. The last pump did not work at 100%, and the flow parameters were regulated by throttling the main valve.

For the calculation of energy savings, the consumption of the system before and after the technological change. Power consumption is calculated according to eq. (1). $C_m$ corresponds to the monthly consumption, $C_d$ is the daily consumption, and $d_c$ is the number of days of that consumption.

$C_m=C_d\times d_c$ (1)

The annual consumption is calculated as in eq. (2). The term $C_a$ corresponds to the annual consumption, $C_m$ is the monthly consumption, and $m_c$ is the number of operating months.

$C_a=C_m\times m_c$ (2)

Energy saving can be calculated according to the subtraction of the consumption before and after the technological change, as expressed in eq. (3). The term ES refers to energy saving, $C_{before}$ is the energy consumption before the technological change, and $C_{after}$ is the energy consumption after the technological change.

$ES=\frac{C_{before}-C_{after}}{C_{before}}\times 100\%$ (3)

The technological change consists of substituting four motors for others with less power (adjusted load) and greater efficiency. Furthermore, the flow control method changed from a manual to an automatic system with a flow and pressure loop closed with variable speed drives (the pumps start automatically depending on the demanded flow load and control the speed of the pump to regulate flow). These significant changes have a great advantage regarding the rational use of energy; however, non-linear loads (four variable speed drives) must be considered when evaluating energy-saving measures because they produce harmonics that affect power quality. Next, it explains how the variable speed drives affect power quality and the equipment used. The methodologies for measuring the harmonics injected into the network by each installed equipment will be described for the induction motors.

Organisations today set not only production goals but also energy savings goals. To comply with international commitments regarding what has to do with climate change, the Colombian Congress has approved laws such as 697 of 2001. This law invites the industry to embrace the culture of rational energy use by implementing administrative systems such as the ISO-50001 standard [19]. This standard allows companies to establish an energy policy, objectives, goals, and action plan ultimately aimed at protecting the environment and achieving savings that help the operating cost of organisations.

Table 3 shows monthly data: the power consumption measured in each motor of the cooling tower, the estimated work time of these motors and the energy consumed.

After installing the new motor-pump systems with the variable speed drive and an automatic control system to control flow and pressure, the measurements were made again, obtaining the results in Table 4.

Figure 3 shows the baseline built based on the available data. This unfiltered line has a determination coefficient of 0.69.

Figure 3.

Unfiltered baseline

To improve $R^2$, the data are filtered to remove unrepresentative data from the indicator. For filtering, two lines parallel to the baseline are used as a limit with the models $\left(E_t+N\times Sxy\right)$ and $\left(E_t-N\times Sxy\right)$ obtained from the data's mean dispersion. Values outside these lines are considered outliers and are removed. Figure 4 shows the indicator's baseline and the filters' limits delimited by the dispersion value N.

Figure 4.

Baseline limits for data filtering

The data extracted from the sample does not represent more than 30% of the sample; therefore, the representative sample is not affected. With the data that remained from the sample, the value of $R^2$ was determined again, and it verified that its value is more significant than 0.9, which shows that the indicator is very strong [22]. Figure 5 shows the baseline with the filtered data.

Figure 5.

Baseline with filtered data

Figure 6 shows the baselines of the system before and after the technological change. The baseline is obtained from the linear fit of the scatter diagram of the cooling tower system's consumption data (in kWh/day) vs. production (in m³/day). This indicator comprises the relationship between the flow rate of the pumping system and energy consumption. The high correlation between these variables (0.9242) and (0.9883) before and after the technological change is the significant relationship between these variables and the validity of the indicator [22].

As shown in Figure 6, when the change is made, the energy performance indicator improves considerably concerning the abovementioned conditions, reflected in the calculated energy savings. A significant result of the new pump-motor systems is that the indicator has less dispersion, which shows that the system has improved considerably, not only in efficiency but also in the quality of operation.

Figure 6.

Baseline comparison of the system before and after the technological change

With the new pumping system, energy consumption not associated with production due to cavitation and throttling was minimised due to the flow control strategy by variable speed drive [24]. In the new baseline, the dispersion of points is less, and the energy is used more efficiently, taking better advantage of the work of the pump according to the flow needed for the process.

The performance indicator and the baseline of Figure 6, the energy efficiency of electric motors before and after the technological change, can be evaluated. These tools offer an alternative solution to the difficulty of directly measuring electric motors' mechanical power and efficiency under industrial conditions raised in different studies [17].

The Pareto chart shown in Figure 7 has been constructed. On the x-axis, (M1, M2...M18) represents the motors shown in Table 3. The primary y-axis units indicate the power consumption of each motor. The secondary y-axis units allow the observation of the accumulated power consumption of motors.

Figure 7 shows that the three motors of the clean water basin consume almost 50% of the energy consumed by the plant. These 55 kW (75 HP) equipment pieces were selected for the change for lower power motors, higher efficiency, and speed control utilising variable speed drive.

Figure 7.

Pareto diagram of motors before the technological change

Figure 8 shows that the loads decreased considerably with the change of equipment. Now, five motors consume 55% of the system load, and among them, three of the motors are those of the replaced pump motor assembly.

Figure 8.

Pareto diagram of motors after the technological change

Figure 9 shows the electrical network diagram used to evaluate the technological changes, and Table 5 shows the data of the motors before and after the technological change. This network has a main feeder, the main bus, a transformer, a secondary bus, and 13 motors. The system is used to validate the power flow with all the motors connected in the network and the response of the load to the power quality events.

Figure 9.

Electrical network of the industrial plant

The electrical system of the cooling towers is made up of a 600 kVA transformer, with a primary voltage of 13.2 kV and a secondary voltage of 460 V that feeds an MCC. The MCC comprises a totaliser, 12 electric cells with soft starters, a variable speed drive that controls the glycerin basin pump, and 18 motors with capacities between 7.4 kW and 55.8 kW. Four motors, 55.8 kW driven by soft starters, were changed for the same number of 33.5 kW machines driven by variable speed drives. These drives feature a closed control loop to keep flow and pressure constant at varying speeds.

The installation of new Grundfos brand pumping systems was proposed, with pumps designed to work under the operating conditions of the process, with which greater efficiency is obtained and, therefore, greater energy use. Other motors of the system will be maintained as in the initial configuration.

Table 5 shows the data of the changed motors and those that were maintained. In the simulation, the nameplate data of the equipment (motors and transformers) and the measurements made with the network analyser were used.

All the scenarios consider the old and new technologies. The measurement is installed on the buses (Bus 1 and Bus 2) to evaluate the behaviour of the different signals. The software NEPLAN was used to simulate the load flow of the electrical system. The software Power Log Classic was used to analyse data from the network analysers.

Table 6 shows the cooling tower system's power flow results with the currently working motors, four 55 kW motors, four 22 kW motors, two 25 HP motors, two 20 HP motors, and a 50 HP motor. The load flow is simulated with all the motors in operation, resulting in the transformer's installed load capacity of 88%. However, the four pumps of the dirty and clean water basins only work three, and the fourth pump remains in reserve for contingency and maintenance events. Under these conditions, the load factor of the transformers is 72%, and the power factor is 0.846. The reason for this is that the company corrects the plant's power factor on the medium voltage side (13.7 kV), giving less importance to correcting the power factor on the low voltage side.

Table 7 shows the power flow results of the cooling tower system after the technological change. With variable speed drives, the harmonic measurements are entered for the simulation giving a THDI (total harmonic current distortion) of 11.09% at the common connection point. The transformer was left with an installed load of 78%. The four motors of the new system pumps are highly efficient, starting with a variable speed drive each. Energy measurements were made to determine the harmonic components the variable speed drive injects into the network.

This section analyses the energy savings obtained with the technological change in the cooling tower pumping system, comparing the data before and after the change. In addition, the baselines in each stage and the system's energy quality parameters are evaluated.

Power consumption comparison.

Figure 10 shows the power demand behaviour before and after the technological change, with records every minute during a typical cooling pump system working day.

Figure 10.

Active power before and after the technological change

From the behaviour of the curves, the evident reduction in energy consumption can be observed when replacing the motor-pump systems, reflected in the space between both curves. In the upper curve corresponding to the demand before making the change, it is observed that in the first moments, the trend is variable with high peaks due to the continuous start-up of plants while the flow stabilises. When stabilisation is reached, there are peaks of powers associated with cavitation in the pumps, which causes starts for a short period. In the curve corresponding to the demand after the technological change (lower curve), as in the upper curve, the load does not initially stabilise while the plants start up, and the flow stabilises.

Power quality parameters.

This section compares the behaviour of current harmonics before and after the technological change. Figure 11 shows the average THDI of each phase, while Figure 12 shows the behaviour of individual average current harmonics from the 2^nd to the 13^th order.

Figure 11.

THDI before and after the technological change

Figure 12.

Individual harmonics before and after the technological change

According to Figure 11 and Figure 12, the total and individual harmonics increased significantly by including the variable speed drive. It shows that although the change brought benefits in energy savings, it caused a deterioration in the power quality of the electrical system. This aspect should have been considered when planning the inclusion of variable speed drives.

Concerning the individual harmonics, there is a predominance of harmonics of the fifth, seventh, and eleventh order, the product of the characteristics of the six-pulse variable speed drive [27]. The 5^th- and 11^th-order harmonics are negative sequence harmonics, which generate a torque opposite to the rotation of the motor, causing torque pulsations, motor vibrations, etc. Also, the 7^th-order harmonics is a positive sequence, generating torque in the same direction as the motor. In both cases (positive and negative harmonic sequences), additional currents are generated, which increase both the losses, such as energy consumption [28].

Considering that studies show that current harmonics can produce losses of up to 4% [29], these results show the need to install harmonic filters to mitigate the unwanted effects of these phenomena.