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Replacement of Existing Air-cooled Chiller Systems by Water-cooled Chiller Systems in Commercial Buildings in Hong Kong
According to F.W.H. Yik, J. Burnett & I.Prescott, the air-cooled chillers in Hong Kong are usually rated at an outdoor temperature of 35 oC and COP of the air-cooled chillers including the condenser fan power is ranging from 2.6 to 2.9. For a direct seawater-cooled chiller plant with seawater entering temperature of 27 oC, COP of the water-cooled chiller plant could achieve 4 to 5. As the electricity consumption for air-conditioning system in Hong Kong often accounts for a dominant portion of the operating cost of the shopping complexes, water-cooled air-conditioning systems are more preferable than air-cooled air-conditioning systems when space is sufficient for such installation and cooling water is available at low cost.
In the past years, portable water supply was mainly imported from China and the reliability of this crucial water supply has been a major concern in Hong Kong. The use of fresh water in air-conditioning system was banned by Waterworks Regulations in Hong Kong and this discouraged the use of cooling towers in most commercial buildings including shopping complexes. Hence, air-cooled air-conditioning systems were prevalently installed in Hong Kong in the old days. In order to conserve electricity and to reduce the emission of greenhouse gases by electricity generation, the Hong Kong Government has put effort and emphasis on exploring the feasibility and viability of facilitating buildings to use water-cooled air-conditioning systems instead of air-cooled air-conditioning systems. Pilot Scheme for Wider Use of Fresh Water for Evaporative Cooling Towers was launched in June 2000 by the Hong Kong Government. The scheme aims to promote the energy efficient water-cooled air-conditioning systems and to assess the impacts on infrastructure, health and environmental effects with an ultimate aim to facilitate territory-wide implementation of water-cooled air conditioning systems in Hong Kong.
Technology of Water-cooled Chiller Systems in Commercial Buildings
The air-conditioning systems in buildings work on refrigeration principles by using cooling medium to decrease the indoor air temperatures. In air-cooled air-conditioning systems, heat absorbed by the refrigerant is directly rejected to the ambient; whereas in water-cooled air-conditioning systems, either fresh water or seawater is used as a heat rejection medium. And heat absorbed by the refrigerant is rejected to the ambient by evaporation through cooling towers or by seawater discharging into the sea. There are three major schemes in water-cooled air-conditioning systems, namely, the cooling tower scheme, the central sea water scheme, and the district cooling scheme.
In the cooling tower scheme, the air conditioning system uses evaporative cooling tower for heat rejection. Water in the cooling tower will be lost due to continuous evaporation, bleed-off and wind drift. The water lost would be replaced by water coming from the city water mains.
In central sea water scheme, the air conditioning system uses seawater for heat rejection. A dedicated central sea water supply distributes seawater from the sea to the user building. The rejected warm sweater from the condenser will be returned to the sea via dedicated pipe.
In district cooling scheme, chilled water is produced by central chilled water plant. Individual user purchases chilled water for their building from the district cooling scheme operator and do not need to install their own chiller plants. For this scheme, a central chiller plant, a pump house and a central distribution pipeline network would be required.
Water-cooled air conditioning system rejects heat depending on the ambient wet-bulb temperature rather than the dry-bulb temperature, so the refrigerant can be cooled to a lower temperature. This results in a better system coefficient of performance (COP) and thus more energy efficient. The District Cooling Scheme and Cooling Tower Scheme are more efficient than conventional air-cooled system as much as 35% and 20% respectively in accordance with a study commissioned by the Electrical and Mechanical Services Department (EMSD).
Pros and Cons for Application of Water-cooled Chiller Systems
Equipped water-cooled chillers and cooling tower with VSD and optimize their operation by automation control system could effectively trim down the peak demand charge, optimize the chiller efficiency in off-design condition, and lead to a more efficient operation of the overall water-cooled air-conditioning system.
Variable speed drive chiller compressor can be considered as replacement of traditional chiller in the future, as its cost has been gradually reduced. The VSD chiller compressor will allow the compressor to run at lower speed under part-load conditions, thereby yielding a lower compressor kW/ton rating under such situations than using conventional centrifugal chillers where part-load control is by controlling the inlet guide vanes.
In the conventional chiller plant automation control system, it controls the cooling tower to open the valves and start the tower fan on one-to-one basis even in common header system. When the condensing water temperature drops, the required compressor head will reduce. The efficiency of the water-cooled chiller equipped with VSD will improve by 4 to 5% while the entering condensing water temperature drops by 1 oC. It, therefore, would be better to operate the idle cooling towers in lower speed in order to further lower the condensing water temperature for the water-cooled chillers so as to increase the efficiency of the chillers. Lower total fan power consumption and lower condensing water temperature are resulted. As a result, optimization of the chiller and cooling tower operation with automation control system as above would further increase efficiency of the water-cooled chiller plant.
The operating strategy of the multiple chillers is also crucial to achieve efficient operation of the chillers. For multiple chillers operating at a part-load condition, the second chiller should not be brought on-line until the first one is up to a pre-determined capacity. Generally, the least energy is used by one chiller operating at 90% capacity as compared with that used by two chillers each operating at 45% capacity. Retrofitting the existing air-cooled chiller plant with new water-cooled chiller plant could usually rectify the problems of load mismatching, low reliability of the existing chiller plant.
Additional benefit from the conversion of air-cooled to water-cooled chiller plant would be the improvement of system reliability and minimization of system downtime when all the water-cooled chillers are furnished with variable speed drive as the starters. In case of power loss, the restart time of chiller could be reduced from 30 minutes to 5 minutes when compared with the conventional and typical EM starter. Moreover, after the conversion of the water-cooled chiller, less power would be consumed which means less CO2 emission. This would reduce the green house effect.
There are nonetheless some limitations and potential risks for replacing the existing air-cooled air-conditioning system with new water-cooled cooling tower system. Noise from cooling towers, stagnant water in dead legs of water pipe or in idle system, nutrient growth due to contamination from surrounding areas and exposure to direct sunlight, poor water quality such as Legionella count, deficiencies in cooling tower system, separation of the cooling towers and access to existing building/residents, and occupational safety and health issues are all have to be dealt with carefully during the design stage, the installation stage as well as the operation and maintenance stage. Appropriate cooling tower system design, regular and proper maintenance including water treatment to the cooling tower system, and annual audit are all necessary to minimize the potential risks from the cooling tower system.
Also, conversion of the existing air-cooled chiller plant to water-cooled chiller plant takes up more spaces as the additional air-conditioning equipment including cooling towers, condensing water pumps, water tanks, condensing water pipes, etc. shall be incorporated into the system and all of the equipment and the structural supporting frames for cooling towers and water tanks require additional spaces. Like places in Hong Kong where it is so densely populated and space is very limited with very high land price, optimal utilization of building spaces is a very important factor which the landlords would consider.
Model to Access Efficiency Improvement - Implementation of Load-based Speed Control for System Optimization in Water-cooled Chiller Systems
The system COP means the chiller load output divided by the total input power of the chiller, condenser water pump and cooling tower fan. For conventional operation of cooling towers, the fans are cycled on and off, or controlled at variable speed to maintain the temperature of cooling water leaving the tower at its set point. The condenser water pump is staged continuously to provide the chiller operating with the rated flow of condenser water for all loading conditions.
In accordance with the studies performed by F. W. Yu and K. T. Chan, load-based control could be applied to enhance the energy performance of water-cooled chiller systems. Thermodynamic-behavior chiller and cooling tower models were developed to find out how the energy use varies for a chiller system operating under various controls of condenser water pumps and cooling tower fans. The optimum operation of the water-cooled chiller systems could be obtained via the load-based speed control which the speed of the cooling tower fans and the condenser water pumps is regulated as a linear function of the chiller part load ratio. It resembles the typical sequencing of chillers based on their load conditions and without the need of high quality humidity sensors to reset the cooling water temperature. The system COP under the optimal control could increase by 1.4% to 16.1% when compared with the equivalent system of fixed temperature and flow rate control for the cooling water leaving the cooling towers.
Improvement in system performance could be achieved by applying variable speed control to the condenser water pumps and the cooling tower fans. To optimize the system, the condenser water flow rate would vary in direct proportion to the chiller load. This results in the control algorithm of pump speed (Spump,op) shown in Equation (1), given that speed is directly proportional to flow rate in accordance with the pump laws. The minimum speed is set at half of the full speed (Spump,full) to ensure the minimum condenser water flow required when the chiller load in terms of part load ratio (PLR) drops to below 0.5.
Spump,op = (1)
Following the traditional control of cooling water temperature, the controller for tower fan speed modulation has to evaluate the optimum set point (Tctwl,op) and operates the fan at the right speed to meet that set point. Based on the analysis by F. W. Yu and K. T. Chan, it is possible to apply load-based speed control for cooling tower fans so as to achieve optimum system operation. Figure 4 shows data of the optimum fan speed at which the maximum system COP took place for a set of operating conditions in terms of various combinations of PLRs from 0.2 to 1 at 0.1 intervals and wet-bulb temperatures from 16 to 28 DegC at 4 DegC intervals. Using regression analysis, a linear relationship between the optimum fan speed (Sfan,op) and chiller PLR can be obtained as Equation (2) with the coefficient of determination (R2) of 0.9215. Sfan,full denotes the full speed of the tower fans and the constant coefficients would be different for each specific design of the system.
Sfan,op = (0.7281PLR + 0.1776) Sfan,full (2)
It is expected that the load-based speed control is generic for all types of multiple-chiller systems with full or partial use of variable speed drives for the system components. The optimal control of the whole system could be highly simplified in this way as the sequencing of chillers, pumps and tower fans and their individual speed controls can be based entirely on the chiller load conditions only. The system COP under the optimal control could increase by 1.4% to 16.1% when compared with the equivalent system of fixed temperature and flow rate control for the cooling water leaving the cooling towers.
Retrofitting the existing air-cooled chiller plant with new water-cooled chiller plant could usually rectify the problems of load mismatching and low reliability of the existing chiller plant. A better system coefficient of performance (COP) and thus more energy efficient would be achieved. The application of water-cooled chiller system is more efficient than the conventional air-cooled system for as much as 35%. Implementation of the load-based speed control for the system could further increase the system COP by as much as around 16%.
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