Energy Saving Between Air-Cooled and Water-Cooled Chiller Water System in Hong Kong - Essay Example

Study on energy saving between air-cooled and water-cooled chiller water system in Hong Kong CHAPTER FOUR DATA ACQUISITION AND ANALYSIS 4.1 Introduction The core objective for this study was the determination of the means by which building owners in Hong Kong could integrate both air-cooled and water-cooled chillers in their conditioning systems. This was considerate of the effects that the global climate changes have had on the sectors of the global economy including the housing sector in Hong Kong (Manske, Reindl & Klein, 2001.p.676-691). However, quantitative analysis and understanding of the housing sector in Hong Kong would be required so as to define the viability of the choice made for a chiller over another (Chan & Yu 2004.p.113-127). Such kind of data shall be obtained from past recordings that have been made on the wet-bulb globe temperature, which was the main instrument in this study. Subsequently, considering that this study was piloted in order to define the effectiveness of the water-cooled chillers and the air-cooled chillers in the hope of establishing which of the two systems is more conservative towards energy consumption in commercial residences, in Hong Kong. Greater emphasis for the data analysis shall be on the conditioning aspects such as the cooling rate of the refrigerants, the energy consumption rates of the chillers in the two systems, and the coefficient of performance (COP) for the systems (Langley 2000; Chan & Yu 2002b.p.31-41). This chapter shall be concerned with the addressing of the data acquisition methods and techniques that have been detailed in this study, as well as provide the detailed results for the collected data in appropriate formats and presentation formats as discussed in the earlier chapters of the study. 4.2 Identification of data sources The main sources of data for such a study that seeks to relate the energy consumption of a given chiller system to its adaptability by commercial building owners in Hong Kong would be the use of primary data from the collected temperature readings using the WBGT instrument. This would be supplemented by the collection and analysis of secondary data available in various government agencies relating to the building and construction sector (Kimmel, 2007). Subsequently, experimental data on the efficacy and efficiency of the WBGT instrument as used in measuring humidity and temperature rates would be obtained from the meteorological department, as well as from publications related to the current study. The nature of such data to be obtained may vary; however, through the adequate analysis and presentation using the appropriate graphical techniques, it is possible to attain a conclusive report on the study’s objectives (Babbie, 2011). Table 1 indicates the temperature readings obtained using the wet bulb globe temperature (WBGT) instrument. A comparison for this data is made when data was collected using the dry bulb temperature indices. These data were successively maintained for one month, upon which the following analysis is obtained. The readings obtained below are in degrees Celsius (0C). Table 1: Readings from the WBGT instrument Days Air Temperature (ta) (0C) Natural Wet Bulb Temperature (tnw) (0C) Globe Temperature (tg) (0C) 1 32 28 33.2 2 32.2 28.2 33 3 32.5 28.4 33.4 4 32.9 28.6 33.5 5 33.2 28.3 33.6 6 32.8 28.4 33.3 7 32 28 33.1 8 33.1 27.6 33 9 32.6 28.5 33 10 32.3 28.7 33.2 11 32 28.3 33.5 12 31.9 28.7 33.6 13 32.2 28.4 33.8 14 32.4 28.1 33.4 15 32.8 28.2 33.1 16 33.1 28.6 32.8 17 33 28.3 33 18 32.7 28.1 33.2 19 32.5 27.6 33.4 20 33.1 27.9 33.7 21 33.5 28.2 33.5 22 33.9 28.5 33.6 23 33.3 28.8 33.4 24 32.9 28.6 33.3 25 32.9 29.1 33.2 26 32.9 29.5 33.1 27 32.4 29 33.5 28 32.8 28.7 33 29 33 28.4 33.2 30 33.4 28.2 33.5 The determination of the WBGT values was obtained through the performance of the following calculations. The variations in the calculations were based on the exposure and experience that resulted from the solar radiation. For the temperature readings taken inside the building, as well as outside the building but, without exposure to solar radiations, the following formulae was adopted. WBGT1 = 0.8tnw + 0.4tg (1) The outcomes for the WBGT are as shown in the table below. Table 2: WBGT1 results taken without exposure to solar radiation Days Wet Bulb Globe Temperature (WBGT1) Index (0C) 1 35.68 2 35.76 3 36.08 4 36.28 5 36.08 6 36.04 7 35.64 8 35.28 9 36 10 36.24 11 36.04 12 36.4 13 36.24 14 35.84 15 35.8 16 36 17 35.84 18 35.76 19 35.44 20 35.8 21 35.96 22 36.24 23 36.4 24 36.2 25 36.56 26 36.84 27 36.6 28 36.16 29 36 30 35.96 The information in the table above when presented in a chart has the behaviour of a wavy curve as shown below. For the case of measurements taken outside the building and in the exposure to solar radiations or in a condition where a radiant source of heat was present indoors, the adopted formula was: WBGT2 = 0.8tnw + 0.4tg + 0.1ta (2) The measurements obtained in table 1 were calculated using formulae 2 and the following results were obtained for the WBGT in the case of solar radiation exposure used in this study. The obtained results, which were compared to the dry bulb temperature indices, also taken alongside with the WBGT readings, as shown in table 3 below. Table 3: WBGT2 results taken with exposure to solar radiation Days Wet Bulb Globe Temperature (WBGT2) Index (0C) 1 38.88 2 38.98 3 39.33 4 39.57 5 39.4 6 39.32 7 38.84 8 38.59 9 39.26 10 39.47 11 39.24 12 39.59 13 39.46 14 39.08 15 39.08 16 39.31 17 39.14 18 39.03 19 38.69 20 39.11 21 39.31 22 39.63 23 39.73 24 39.49 25 39.85 26 40.13 27 39.84 28 39.44 29 39.3 30 39.3 The data in a graph setup results in the following chart that has the behaviour of a wavy curve The generalised form of the data used in this study takes into consideration the fact that the air-cooled and water-cooled conditioning systems are regarded as high esteem and considering the high esteem with which people contemplate the power consumption rate incurred to support the systems. As such, a comparison of the two systems of air conditioning chillers in this study seeks to determine the heat rejection rates of the systems independently, through the use of the wet-bulb globe temperature (Love et al., 2005.p.24-29). This is because the wet-bulb globe temperature (WBGT) has the potentiality of decoupling both humidity and temperature controls while operating under low condensing temperatures as determined by its operating mode. Therefore, the recognition of data sources in this study underscores the necessity for the optimal control of the given chiller systems (Niu, Zhang & Zuo, 2002.p.487-495). The results obtained in the optimal control were compared to a standard design case in this study. In the WBGT, a slight improvement in the test results can be obtained through the allowance of variations in the temperature settings (Turner & Doty 2006). Appropriate data for analysis in this study was obtained from the regular humidity and temperature data collections that were made through the use of the WBGT, as a key instrument for this study. Subsequently, this data was supplemented by data obtained from previous related study and data from the housing sector in Hong Kong in relation to the adaptability of house owners to the conditioning system installed and the energy consumption rates in terms of cost factors. Furthermore, noting that the refrigerants to be used in the conditioning systems in houses shall always be cooled to their lowest temperature rates, this study shall rely on obtainable comparative data on the energy consumption rates of refrigeration and conditioning systems used in most of the commercial buildings in Hong Kong (Yang, Chan & Wu 2009.p.2204-2211). General description Considering the harsh weather conditions that at times are experienced by workers in Hong Kong, the use of conditioners is rapidly adopted among commercial buildings. However, most buildings are installed with air-cooled conditioners, which are costly in management and consumption of electric energy (Yu & Chan 2006.p.628-648). As such, the adoption of water-cooled conditioners is expected to curb this situation through the reduction in the electric energy consumption. However, sufficient data is needed to validate this advantage of the water-cooled chillers over the air-cooled chillers. In this study, this is being facilitated through the use of the wet-bulb globe temperature (WBGT) in numerous laboratory tests. The collected data shall be subjected to statistical analysis by measuring of central tendencies, percentages and frequencies by the use of MS Excel (Panneerselvam, 2004). The generalised form of the data used in this study as shown in the table below. Table 4: Generalised data showing comparison between various conditions Days Air Temperature (ta) ) (0C) Natural Wet Bulb Temperature (tnw) (0C) Globe Temperature (tg) ) (0C) Wet Bulb Globe Temperature (WBGT1) Index (0C) Wet Bulb Globe Temperature (WBGT2) Index (0C) Dry Bulb Temperature Index (0C) 1 32 28 33.2 35.68 38.88 28.2 2 32.2 28.2 33 35.76 38.98 27.5 3 32.5 28.4 33.4 36.08 39.33 27.3 4 32.9 28.6 33.5 36.28 39.57 27.9 5 33.2 28.3 33.6 36.08 39.4 27.6 6 32.8 28.4 33.3 36.04 39.32 27.2 7 32 28 33.1 35.64 38.84 28.1 8 33.1 27.6 33 35.28 38.59 28 9 32.6 28.5 33 36 39.26 26.9 10 32.3 28.7 33.2 36.24 39.47 27.6 11 32 28.3 33.5 36.04 39.24 27.6 12 31.9 28.7 33.6 36.4 39.59 27.9 13 32.2 28.4 33.8 36.24 39.46 27.8 14 32.4 28.1 33.4 35.84 39.08 27.5 15 32.8 28.2 33.1 35.8 39.08 28.1 16 33.1 28.6 32.8 36 39.31 28.3 17 33 28.3 33 35.84 39.14 28.6 18 32.7 28.1 33.2 35.76 39.03 28.8 19 32.5 27.6 33.4 35.44 38.69 28.1 20 33.1 27.9 33.7 35.8 39.11 27.7 21 33.5 28.2 33.5 35.96 39.31 27.3 22 33.9 28.5 33.6 36.24 39.63 27.8 23 33.3 28.8 33.4 36.4 39.73 27.4 24 32.9 28.6 33.3 36.2 39.49 28 25 32.9 29.1 33.2 36.56 39.85 27.9 26 32.9 29.5 33.1 36.84 40.13 28.3 27 32.4 29 33.5 36.6 39.84 28.5 28 32.8 28.7 33 36.16 39.44 28.1 29 33 28.4 33.2 36 39.3 27.8 30 33.4 28.2 33.5 35.96 39.3 27.4 Encounters faced with the sources of data collection In most instances, consumers of the air conditioning systems have had no variations or disparities in their selection of chiller systems given that it is often hard to determine the difference in performance and energy consumption of the air-cooled and water-cooled chillers in retail outlets. Subsequently, owing to the humid and hot weather conditions in Hong Kong, most of the air conditioning systems are chosen based on the comfort derived from them and not on the efficiency in energy consumption (Smith & King, 1998.p.47-48). As such, it becomes quite hard to form a solid and compact database on the chiller systems, even in the event that surveys are conducted in that line. In another context, it may prove challenging to obtain established data of the two chiller systems by the use of the measuring instrument WBGT given that a few of such studies have been conducted before. Moreover, the research studies that have been directed in this regard have always been diversified into determining other concepts of air conditioning separate from those objectively being sought for in this study. Still, most researchers have perceived the use of the WBGT as a cumbersome method; hence, inadequacy in the data collected and documented are about the same, in relation to air conditioning systems (Yang, Chan & Wu, 2009.p.2204-2211). Integration of the optimisation plan An understanding of the study’s conditioning system begins with the identification of the input variables and constraints of the systems. Data on the chillers’ energy consumption and efficiency are integral parts of the water-coolers operational definition, and this is predisposed by elements such as the compressors’ efficiency at part and full loads, the temperatures of the water condenser, the temperatures of the chilled water supply and the operation and maintenance practices adopted by the researcher (Erechtchoukova, Khaiter & Golińska, 2013). In assessing or measuring the performance of water chillers, it can be noted that the actual chillers’ energy efficiency may be different to that stated by manufacturers, in terms of the performance data. This reason is because such data on chiller efficiency are mostly based upon the idealized condition in which the experiment is conducted. According to Niu et al., (2002.p.487-495), very few or particular studies have been concentrated on the total efficiency of a manufacturing plant and its auxiliaries such as the water pumps and the cooling tower fans of the condensers. Instead, a lot falls into the energy efficiency of the chillers, as the sole aspect, when it should be conjoined with the previous aspects. The mentioned auxiliaries may have substantial negative impacts on the energy efficiency in most cases; the pumps and fans are driven by speed motors that are fixed, instead of variable frequency drives (VFDs) (Chow et al., 2002.p.103-109). Considering data collected from different cooling systems and case studies and integrating the same into the perspective of this study; the following table can be derived. This table provides a summary presentation of the various auxiliaries that have a deterministic effect on the energy efficiency of water-cooled chillers (Whitman, Johnson & Tomczyk, 2005.p.126-134). Table 5: Summary of case study data on the efficiency of cooling systems Type Size (kW) Efficiency Unit Efficiency Average system load Water cooled 600 CoP 2.7 _ Air cooled 620 CoP 2.5 _ Variable speed crew 1500 CoP 2.9 _ VSD, ultra-efficient oil-less compressors 2620 CoP 6.28 _ Air-cooled chillers and fan coils 1250 EER 1 to 1.5 21% Water-cooled chillers and fan coils 1275 EER 0.8 to 1.7 20% DX Split 8 EER 1.3 to 1.8 40% Source: International Energy Agency (2008) From the table, it is shown that even though there is a possibility of attaining seasonal energy efficiencies that are higher than five as designated by the CoP, it is required that this acquisition of data be supported by the operation of a well-designed and managed Variable Speed Drive (VSD) plant (Underwood, 2007). Performance of the compressors The water cooled chillers may be set up using three different main types of chiller compressors. These include a reciprocating, rotary and centrifugal forms of compressors. These compressors also have variations in sizes with the reciprocating one ranging from 175 to 800kW; rotary 240 to 1,400kW; and centrifugal compressor between 700 to 8,800kW. Given the efficiency levels of these compressors, when ranked, the centrifugal compressor becomes the most preferred in relation to efficiency. In this study, the choice of the compressor that made for the identification of the air conditioners in various commercial buildings sampled within Hong Kong, preference was made to those that had centrifugal compressors in-built (Yu & Chan, 2007.p.3816-3829). This choice of a compressor to be analysed was based on the significant improvements that have been noted in this compressor type in relation to its chillers’ load efficiency and variable speed, which at its maximum would ensure that the obtained energy consumption efficiency is achieved at about 50% loading (Yik et al., 2012.p.263-279). As indicated in the figure 1 below, loading efficiency of the chiller compressors of below 40% would result in a rapid reduction of the CoP of the chillers. Also represented in this figure is a graph of the CoP curve. Figure 1: Load performance curve of a chiller operative in head pressure control Source: Yu & Chan, 2007 With reference to the graphical data, the first impression would be that of chillers running at part load; hence, the likelihood of increased energy efficiency in the conditioners. However, this performance cannot be relied upon given that it does not provide considerations for the operations of the part load operations in the pumps and the fans within the circuit of the condenser (Fong, Hanby & Chow, 2006.p.220-231). Therefore, the result is a reduction of the total efficiency significantly in the conditioner, especially in the event that the speed is not variable. In a study of the variations in the efficiency of either water-cooled or air-cooled conditioners such as this, the key concern in data acquisition and analysis is the efficiency metric that is achieved given that the designing is always expected to be representative of the seasonal performance of the chillers (Swider, 2003.p.539-556). A representative data of this nature would be based on the determination of the efficiencies of the chillers at full and part loads, and then estimating the time proportions taken by the chillers to run each load founded on the presumed average climate, as represented in the graph above. Condenser temperatures of the chillers It was recognized that the obtainment of favourable work conditions based on the conditioning of the room is dependent on the condenser temperatures obtained. In this manner, those chillers that were found to be more efficient were those that were operated under lower heat sink temperatures that occurred during the times when the air temperatures were lower. Subsequently, it was discovered that the obtainment of a reduction in the temperature of the condenser water by 10C led to a saving of the energy by the water-cooled chillers of about 3.6%. These savings in energy consumption were realised to be even much greater with the use of the VSD for the centrifugal chillers. In water-cooled chillers, the installment of cooling towers as was discovered aided in the provision of lower condensing temperatures, a case that was different from the air-cooled chillers. However, the main challenge of the towers was the high consumption rate of water and the risk of Legionella (Yu & Chan, 2007.p.3816-3829). As such, considering the health risks posed by this bacterium, and the stringent regimes of maintenance that the water-cooled chillers required, the study determined that there was a likelihood of declining interests for cooling towers in most offices in Hong Kong. However, as an alternative to the cooling towers that was discovered from the secondary data as have been commonly used in commercial buildings, evaporative cooling heat rejection units were identified. For these, their performance fell between that of the cooling towers and dry-air heat rejection units, since no mist of water droplets was produced; thus, minimising the risk of Legionella (Yu & Chan 2005a.p.143-148). Determination of temperature of the chilled water The efficiency of chillers is enhanced when the water that leaves the chiller is of a higher amount. This is because, under normal circumstances, the chilled water being supplied is at an average temperature of 60C for dehumidification purposes. The reasoning of this is for the coil inside the condenser to be freezing such that any water vapour found in the air will be condensed on it (Hartman, 2001.p.43-51). However, it was also discovered that this may not be necessary for all occasions hence, prompting the increase of the temperature to about 130C. Running water at 130C is more essential in the case that a condensation of the same is avoided from taking place along the ceiling systems and chilled beams (Yu & Chan 2008.p.931-950). Additionally, in the event that ‘free cooling’ is needed, then it would be essential to increase the temperature of the chilled water supply to match to that of the condenser. If this were to be attained, then it would be relatively fine to switch off the compressors (Grabon & Wahl 2001). Incorporation of a chilled water storage In this study, the principle of chilled water storage, which refers to the process of generating chilled water overnight when a building’s cooling demand is lower; hence, spare capacity in the chillers, was applied (Sebzali & Rubini 2007.p.975-984). This chilled water generated during the night would then be stored in insulated tanks to be used in the following day, especially, when the cooling period is at its peak, such as in the mid-afternoon (Huan, Shijun, Hongxing & Jianlei 2000.p.213-217). The interpretations from the wet-bulb globe temperature should be able to indicate that with the hot, humid afternoons, chilled water supply would be more efficient in the water-cooled chillers than if the conditioners were to be air-cooled. The effect of this would be a reduction in the energy consumption by the conditioners installed in commercial buildings; hence, attainment of reduced operation costs for the owners. This is the reason why water-cooled chillers were more preferred than the air-cooled chillers (Yu & Chan 2010.p.203-209). In other instances, the cost-benefits of this method are such as a reduction in the number of chillers that are required in a building; hence, savings on the capital and maintenance costs. There is also a reduction of the demand charges of electricity at the peak times, and a reduction of the energy tariffs since the water-cooled chillers will be running during the night or off-peak times, charging the chilled water that will only be used during the peak afternoons for cooling of the offices (Chan & Yu, 2002a.p.565-581). Subsequently, it is determined that the use of chilled water storage helps in the reduction of the CO2 emissions. This is because the lower temperature of the air at night enables the running of the chillers more efficiently. This chiller utilisation efficiency can be approximated to peak at approximately 60% as shown by the figure 2 below (Sebzali & Rubini 2007.p.975-984). This is the case of a normal chiller curve, while the contrast of the same is as represented in figure 3 below. Figure 2: Normal chiller curve In this case of a normal curve, the efficiency of the chiller utilisation is approximated to be at about 60%. This occurs with peak electricity demand being equal to the peak cooling demand. However, this scenario changes in figure 2 below. Figure 3: Graph of a chilled water storage In this case, the utilisation of the chiller is approximated at about 85% in which case the peak cooling demand is attained by the chilled water tank. Peak electricity demand is only attained at night when the second chiller recharges the tank. SUMMARY TABLE This study centered around the analysis of energy consumption of the water-cooled chillers and the air-cooled chillers in Hong Kong, with the main concern being on the determination of the amount of heat savings for the two systems (Chow et al., 2002.p.103-109). Particularly, considering a case study of Sino Plaza, a commercial building in Hong Kong, that in its earlier stages of operation had air-cooled chillers installed into its system. However, the more energy efficient water-cooled chillers later on replaced these. Based on the data obtained from the company’s records, this study produced the following data represented in a table format, showing the variation in energy consumption and saving attained before and after introduction of water-cooled chillers. Table 6: Summary table of energy consumptions before and after introducing water-cooled chiller system Before (Hypothetical) After Period-(Weeks) Average Energy Consumption rates of Air-cooled chillers (kWh) Average Energy Consumption rate of Water-cooled chillers (kWh) Energy Saved (kWh) Percentage Saved 1 825 723 102 12.36% 2 842 748 94 11.16% 3 912 841 71 7.79% 4 838 780 58 6.92% Information presented in this table shows that there was a comparative decline in the energy consumption by the building’s chillers with the introduction of the water-cooled chillers, as opposed to when only the air-cooled chillers were being used. In period 1, 12.36% of energy was saved, 11.16% in period 2, 7.79% in period 3, and 6.92% in period 4. There was a decline as well in the rates, an indication to the low usage rate due to the favourable climatic conditions witnessed for the period; hence, low humidity and temperature rates (Yu & Chan 2005b.p.1747-1758). 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Optimization of water-cooled chiller system with load-based speed control. Applied Energy 85(10): 931-950. YU, F. W., & CHAN, K. T. (2010). Economic benefits of optimal control for water-cooled chiller systems serving hotels in a subtropical climate. Energy and Buildings 42(2): 203-209. Read More