Abstract
In many cases, Heating, Ventilation, and Air Conditioning (HVAC) equipment is not backed up with factual data and practical recommendations on energy efficiency and thermal comfort in buildings as well as alternatives for operation and control settings. As a result, operational decisions are not adequately informed by appropriate expertise in energy-saving techniques. This study aimed at using experiments to assess the energy consumed by the Chiller system, as well as numerically examine the potential of optimizing consumption while improving thermal comfort in office buildings.
To begin with, the Taguchi design of experiment was employed for choosing control factors that affect the response parameter. An experimental design of the chilled water temperature setpoint, the operation time of the chiller plant, and the air velocity of the fan coil were conducted with the Taguchi method. The aim was to study the effect of these variables on the chiller plant energy consumption and to determine the variable that influences the response variable the most. The results revealed that chilled water temperature setpoint (CWTS) was the highest and most significant (Rank:1) with a 52.2% contribution. Time of operation was least significant (Rank:3) with a 1.2% contribution.
To evaluate the energy performance of the Chiller system and the indoor environment conditions, this study experimentally analysed the influence of chilled water temperature setpoints on energy consumption and indoor thermal comfort conditions of an office building. Three chilled water temperature setpoints (10 °C, 12.5 °C, and 14 °C) were studied. The indoor environment variables (temperature and relative humidity) which are considered indicators of thermal comfort were recorded with data loggers for three consecutive days for a maximum of 9 hours and twenty (20) minutes for each chilled water temperature setpoint (CWTS). Similarly, energy consumption was used as a metric to determine the system’s efficiency.
The lowest CWTS of 10 °C recorded the highest power consumption of 90.14 kW in an hour as against 12.5 °C which yielded 87.45 kWh, and the 14 °C, which registered the minimum power consumed per hour of 86.54 kW. The results confirmed that the higher the CWTS, the lower the power consumed will be (increasing the CWTS from 12.5 oC to 14 oC). The total maximum cost of energy consumed by the Chiller system was $ 149.33 at 10 °C CWTS per operational day, followed by $ 147.62 at 12.5 °C CWTS per operational day. The minimum total cost of energy consumed per operational day was $ 145.48 at 14 °C CWTS. The results
vii
obtained when varying the chilled water temperature setpoints reveal that increasing the chilled water temperature setpoint (by 12%) reduces energy consumption per hour by 2% without compromising thermal comfort.
Subsequently, to analyse the energy consumption of the chiller system under different CWTS, a numerical model of the chiller system was developed and implemented in TRNSYS 17 software to perform simulations of the energy utilization rate. The developed mathematical model (TRNSYS) was validated by comparing the mathematical model results against the experimentally measured values using some error validation models. The error validation models employed in this study were the percentage bias (PBIAS), mean absolute error (MAE), root mean squared error (RMSE), and mean square error (MSE). It was found that the power consumption results from both the experimental data and the TRNSYS model exhibited consistent trends, indicating a reasonable level of accuracy in the TRNSYS model. Afterward, predictions were performed to compare the energy consumption of the chiller beyond the CWTS with the experimental data at different CWTS (10 oC, 12.5 oC, and 14 oC). The results showed that the maximum and minimum energy consumed daily by the chiller was at 10 oC (77.4 kWh) and 18 oC (73.7 kWh) CWTS. Compared to the 10 °C CWTS, the Chiller showed a low energy savings of 3% (at CWTS 14 °C), while it showed an energy savings of 5% (at CWTS of 18 °C).
Similarly, to evaluate the effect of CWTS on thermal comfort, CFD modeling was carried out to study the indoor environmental conditions of the office building at different chilled water setpoint temperatures (10 °C, 12.5 °C, and 14 °C). The CFD modeling and computation were carried out using the Fluent analysis system, which is available in the ANSYS Workbench version 2020 R1 software package. The indoor environmental temperature for the experimental and predicted (CFD simulation) at the various CWTS was studied. The results showed that all the various CWTS registered a mean indoor temperature value ranging between 24 °C to 26 °C. The relative humidity and temperature varied according to the CWTS and the time of the day. The experimental and predicted results were within the range perceived as comfortable by ASHRAE Standard 55. The relative difference (RD) values obtained for the predictions for all CWTS studied fall between 0.00 % and 9.07 % of the corresponding experimental values. The results revealed that the CFD simulation code could be described as reasonably good. It can
viii
thus be used to compare indoor environmental characteristics for the Chiller system and subsequently serve as a basis for thermal comfort optimization studies.
Furthermore, to evaluate the thermal comfort conditions in the office space (test room), the PMV-PPD was calculated based on the average temperature and relative humidity values with the metabolism rate, clothing insulation rate, and air velocity derived from literature based on the kind of work and clothing the occupants were wearing. The Predicted Mean Vote (PMV) and Predicted Percentage Dissatisfied (PPD) models shown under the ASHRAE thermal comfort criteria were employed. Using the PPD model index, the quantitative prediction of the percentage of thermally dissatisfied people who feel too cool or too warm was established. The mean indoor temperature and relative humidity values of the comfort zone and air movement on the psychrometric chart were analysed. At CWTS of 10 °C, the PMV was computed as -0.40 (neutral) which translates to a PPD of 8 %. A PMV of -0.05 (neutral) corresponding to 5% was calculated at CWTS of 12.5 °C. Also, a PMV of 0.29 (neutral) was calculated at CWTS of 14 °C which corresponds to a PPD of 7%. The PPD and PMV indices indicate that thermal comfort conditions were within the standard's prescribed limitations (below 10% and in the range of -0.5, 0, or +0.5 respectively) and hence were acceptable.
Moreover, the thermal comfort (air temperature, relative humidity, and PPD) and energy efficiency (Power consumption and COP) metrics for the test room and Chiller system were considered to predict the CWTS that improves the efficiency of the Chiller system for both energy savings and thermal comfort. Quadratic regression models were developed for the prediction of the Power Consumption, air temperature, and relative humidity based on the Chilled water temperature setpoints. The results showed that from 10 oC to 15 oC, the PPD of the room was within the comfort zone (from 5 - 9%) with reductions in power consumption (from 87.84 kW to 84.4 kW) and an increase in COP. An increase in CWTS (from 16 oC CWTS upwards), resulted in a steady increase in PPD from 12% to 24% but with a decrease in power consumption. Thus, the CWTS could be reset between 14 oC and 15 oC to reduce energy consumption and maintain thermal comfort. It was also confirmed that the CWTS can be increased by various degrees from 10 to 18 oC for energy efficiency for a commercial office building in the tropics. This increase in CWTS would result in a daily energy saving potential of about 5% of the chiller as compared to the existing operational settings without any extra cost. Conversely, the daily energy consumption by the fan coil would increase by about 5.5%
ix
by this increment in the CWTS. It was determined that the chiller system can provide comfort even when the CWTS is increased to 14 °C.
Again, the Taguchi approach was used to predict the optimum values of the controlled variables that will minimize the energy consumed by the chiller system. The results revealed that to further reduce the energy consumed by the chiller system from 86.54 to 82.8 kWh, the chiller system could be operated at 14 oC CWTS, an airspeed of 2.2 m/s for shorter hours (1 hr).
Finally, the design of experiment (DOE) approach was employed using a statistical two-level non-randomized factorial design in Minitab to study the effects of high CWTS, number of rows, and tube diameter on the heat and mass transfer performance of the fan coil unit. The DOE analysis showed that under the condition of high CWTS (14 o C), energy consumption that is less than the current energy consumption may be expected from the fan coil system when the number of rows increased from 3 to 6 and the tube diameter increase from 7 to 9 mm.
Overall, the research revealed information about the Chiller system's energy use and office building thermal comfort. It showcased the application of mathematical modeling to depict how a Chiller system can effectively reduce energy consumption while also highlighting the impact on the comfort conditions within the office building. The water-cooled water chiller-fan coil system (Chiller system) was mathematically modelled to gain a better understanding of its performance characteristics. The following modeling methodologies were used: TRNSYS modeling, Computational Fluid Dynamics (CFD) modeling, and the building energy thermal comfort/ASHRAE-55 modeling Approach. The power consumption and thermal comfort dynamics were studied using the TRNSYS and CFD modeling techniques. Using the CFD modeling approach, the uneven thermal dynamics (temperature and relative humidity impacts) were also graphically examined. The ASHRAE-55 Thermal Comfort Modeling Approach was used to apply the Predicted Mean Vote (PMV) and Predicted Percentage Dissatisfied (PPD) models from the ASHRAE thermal comfort standards. Also, quadratic regression models were developed for the prediction of the Power Consumption, air temperature, and relative humidity based on the Chilled water temperature setpoints. Finally, the study also employed the Taguchi Approach to demonstrate how Chiller system energy use may be kept to a minimum while maintaining a comfortable indoor environment.
x
A significant input of this research to the corpus of knowledge is the provision of experimental facts regarding the enhancement of water-cooled water chiller-fan coil systems in office buildings using an experimental approach. Until the current study, no experimental investigation in the country (Ghana) had substantiated the aforesaid statement. Another significant input of this research to the body of knowledge is the establishment of adequate grounds to affirm that the procedure for minimizing the energy consumption and maintaining comfort zone by the TRNSYS method and the building energy thermal comfort/ASHRAE-55 Modeling Approach could be used for tropical climates where higher relative humidity and reasonable temperatures are normally acceptable. Simulation models were validated by experimental data in a comparative analysis of the performance of the chilled water-fan coil systems. As a result, another important contribution of this study is the development of validated simulation models for energy assessments of chilled water and fan coil systems and the prediction of power consumption, air temperature, and relative humidity based on the CWTS. This serves as a baseline for future verified simulation research in office buildings involving water-cooled water chiller-fan coil systems. Furthermore, this is the first study of its kind in Ghana.
Keywords: Chilled water temperature setpoint, CFD, chiller-fan coil systems, energy saving, PMV-PPD, Taguchi Approach, thermal comfort, TRNSYS simulation