Water-cooled Air-conditioning Systems Research Articles

First of its kind in Hong Kong - innovative reuse of treated effluent and enhanced energy efficiency for air-conditioning systems

In Hong Kong, air-conditioning (A/C) systems are major consumers of electricity. Although water-cooled air-conditioning systems (WACSs) are more energy efficient, the need for water towers, water treatment and other operation and maintenance (O&M) demands restrain their application in most buildings. This paper reviews a pilot application of treated effluent from sewage treatment works as a cooling medium for the Direct Expansion type of A/C system within the treatment Plant. The Drainage Services Department (DSD) installed a WACS with a self-assembled effluent-cooled condenser to study the energy efficiency, cooling performance and actual O&M requirements of the new system over a one-year pilot. It revealed that an energy saving of up to 27.6% per year could be achieved when compared with a conventional air-cooled air-conditioning system (AACS). This innovative effluent cooling A/C system achieves innovative reuse of treated effluent as well as reduction in carbon footprint as a result of higher energy efficiency for sustainable development.

Field-based Analyses on Approach Temperatures for Performance Evaluation of Centralized Air Conditioning System in a Shopping Mall Building

This paper presents field analyses on the influences of condenser and evaporator approach temperatures of water cooled air conditioning system for a shopping mall. The mall building comprises food and non-food retail stores and uses a centralized air conditioning (AC) system to condition the entire building. The AC system comprises three chillers with the same cooling capacity. Chillers’ performance parameters included power consumption, condensing and evaporating temperatures, evaporator-condenser approach temperatures, pressure drop and cooling capacity parameters were hourly recorded for a period of one year. Performance characteristics of each chiller were evaluated at different condenser and evaporator approach temperatures and regression analyses were also established to examine the correlations and influences of the approach temperatures on the chillers’ performance. The analyses results showed that the increase of condenser approach temperature could increase the power consumption and reduce the coefficient of performance (COP) of the chillers. While evaporator approach temperature was found to have insignificant effect to the chiller performance. The results also showed that for the investigated chillers with condenser and evaporator approach temperatures maintained below 2 K, the approach temperatures had negligible correlation to the power consumption and the overall performance of the chillers. At this operational conditions, the water cooled air conditioning systems could operate at their best possible performance.

Assessing the energy performance of VAV and VRF air conditioning systems in an office building located in the city of Florianópolis

Abstract The objective of this study is to analyze the energy performance of two types of water-cooled air conditioning systems, variable air volume (VAV) and variable refrigerant flow (VRF), in terms of their cooling energy use through building simulation. These systems were designed to operate in an office building located in the city of Florianópolis, Brazil. The analysis involved the application of two building use schedules: a) constant and b) variable. Moreover, an analysis of the coefficient of performance (COP) and partial load ratio (PLR), and the percentage of operating hours for each range of cooling COP and PLR for each air conditioning system, allowed the system cooling efficiency to be assessed and the results to be related to the annual energy consumption. The coefficient of performance of VAV and VRF is 6.7 and 5.0, respectively, but the VRF system presented the lowest energy consumption for both schedules. The difference in the cooling energy consumption values for the VRF and VAV systems, for the variable schedule compared with the constant schedule, is mainly influenced by the partial load performance during the hottest period of the year.

The Power Simulation of Water-Cooled Central Air-Conditioning System Based on Demand Response

With rapid development of smart grid, terminal power load is paid more and more attention, especially that of air conditioning system, which constitute an integral part of summer peak electricity load. In this paper, through controlling indoor air temperature deduced by equivalent thermal parameters-model, daily energy consumption and energy consumption during peak hours can be adjusted and reduced to realize demand side management. To reduce peak load of grid and ensure stable and reliable operation of grid, four temperature control strategies, including constant setting temperature control strategy, pre-cooling control strategy, curtailment control strategy and improved curtailment control strategy were proposed. Meanwhile, three operation schemes of the chiller were also employed. Furthermore, considering the power consumption and curtailment, five indicators including hourly average load, hourly average load ratio, energy saving ratio, load-shedding ratio during peak hours and coefficient of performance (COP) have been proposed to evaluate the performance. Results showed that the operation scheme of operating numbers control obtained better effect. Energy saving ratio was highest of 32.5% with curtailment control strategy, while load-shedding ratio during peak hours was optimal with improved curtailment control strategy. However, COP was maximum at 3.66 when pre-cooling control strategy was employed. This study can guide the operation of water-cooled air conditioning system through energy saving ratio and COP, also it has more benefits on economy of power system by hourly average load and load-shedding ratio.

Effects of anthropogenic heat due to air-conditioning systems on an extreme high temperature event in Hong Kong

Anthropogenic heat flux is the heat generated by human activities in the urban canopy layer, which is considered the main contributor to the urban heat island (UHI). The UHI can in turn increase the use and energy consumption of air-conditioning systems. In this study, two effective methods for water-cooling air-conditioning systems in non-domestic areas, including the direct cooling system and central piped cooling towers (CPCTs), are physically based, parameterized, and implemented in a weather research and forecasting model at the city scale of Hong Kong. An extreme high temperature event (June 23–28, 2016) in the urban areas was examined, and we assessed the effects on the surface thermal environment, the interaction of sea–land breeze circulation and urban heat island circulation, boundary layer dynamics, and a possible reduction of energy consumption. The results showed that both water-cooled air-conditioning systems could reduce the 2 m air temperature by around 0.5 °C–0.8 °C during the daytime, and around 1.5 °C around 7:00–8:00 pm when the planetary boundary layer (PBL) height was confined to a few hundred meters. The CPCT contributed around 80%–90% latent heat flux and significantly increased the water vapor mixing ratio in the atmosphere by around 0.29 g kg−1 on average. The implementation of the two alternative air-conditioning systems could modify the heat and momentum of turbulence, which inhibited the evolution of the PBL height (a reduction of 100–150 m), reduced the vertical mixing, presented lower horizontal wind speed and buoyant production of turbulent kinetic energy, and reduced the strength of sea breeze and UHI circulation, which in turn affected the removal of air pollutants. Moreover, the two alternative air-conditioning systems could significantly reduce the energy consumption by around 30% during extreme high temperature events. The results of this study suggest potential UHI mitigation strategies and can be extended to other megacities to enable them to be more resilient to UHI effects.

Energy and exergy analysis of a tri-generation water-cooled air conditioning system

In order to optimize the energy utilization of an air conditioning system, a tri-generation system has been introduced. Energy in the form of heat is inserted to the prime mover of the system, and the total cooling demand of the building would be produced by utilizing a compression and an absorption water-cooled chiller. The prime mover can either be a micro-gas turbine (MGT), a gas turbine (GT), or a solid oxide fuel cell (SOFC). All the system components, such as the absorption chiller and the cooling tower have been modeled and analyzed through the energy and exergy approaches. Performance parameters of the system, such as energy utilization factor and second law efficiency, have been calculated in different operating conditions. The results would reveal the best operating conditions of the system and the most critical components would be highlighted.

Proposal of an eco-friendly high-performance air-conditioning system. Part 1. Possibility of improving existing air-conditioning system by an evapo-transpiration condenser

Air-conditioning (AC) system consumes high energy and releases waste-heat. In the present study, we propose a method to improve its performance and minimize waste-heat by replacing existing air-cooled condenser by an evaporation and transpiration, evapo-transpiration, condenser. The improvement is confirmed by performing experiment for a conventional air-cooled AC system and a water-cooled AC system. Condenser temperature in the air-cooled system is higher than outdoor-temperature by 5–10 °C, while it is −5 to 5 °C in case of the testing system. From simulation results, saving energy consumption is expected to reach up to 30% in summer with the testing system. Based on these results, an evapo-transpiration heat-exchanger was developed as a new condenser. Heat-transfer coefficient of the testing heat-exchanger is at least 4 times higher than that of air-cooled condenser. Even hot fluid is used inside copper-tubing, its outlet-air temperature is as nearly as outdoor temperature.

A case study on the application of air- and water-cooled oil-free chillers to hospitals in Hong Kong

A comparison study on using air-cooled and water-cooled conventional and oil-free chillers in hospitals in Hong Kong is reported. The study commenced with a comprehensive literature review to reaffirm the effectiveness of recommended precaution measures for containing the risk of Legionnaires’ disease (LD) associated with using cooling towers. Numerical performance models for oil-free chillers established for use in the study are presented. Based on recorded cooling demands of two existing hospitals, the life-cycle energy consumption and cost of system options involving different combinations of heat rejection method and chiller type were evaluated, compared and discussed. The results show that water-cooled air-conditioning systems with conventional chillers and cooling towers are still more energy efficient than air-cooled systems with oil-free chillers. Water-cooled oil-free chillers may become an economically viable choice when their price is lowered in future. Pilot installation is recommendable, as it would allow the reliability and maintainability of the new oil-free chillers to be ascertained. Practical applications: Findings of the literature review reaffirmed the effectiveness of measures recommended in contemporary guidelines for LD prevention, which would ease the mind of owners, designers and operation and maintenance personnel in regard of using cooling towers in hospitals. The numerical models for oil-free chillers presented in the paper are useful for evaluation of the energy consumption of this new type of chiller and for comparison with other types of chillers. The study approach described in the paper, including the energy consumption prediction and life-cycle cost evaluation methods, provides a useful framework for similar studies.

Critical Factors in Adopting Cooling Media for Central Air-Conditioning Systems in Hong Kong

There has been a dilemma about whether to adopt water-cooled or air-cooled air-conditioning systems at various times for different developments in Hong Kong, strongly influenced by the local government. Fresh water has been a scarce resource as a cooling medium; however, in recent years, the local government has advocated water-cooled air-conditioning systems, using fresh water or sea water as a cooling medium in the condenser units. Although such water-cooled air-conditioning systems use less energy than air-cooled systems, they can be potentially harmful to the environment and end users if not adequately designed and maintained. There are obvious pros and cons to both systems. Moreover, sea water as a cooling medium has its limitations if one expects to achieve more sustainability, inevitably at a higher initial cost compared with other systems. In this research, detailed studies are carried out on water-cooled and air-cooled air-conditioning systems in buildings in Hong Kong, aiming to achieve more sustainable design while comparing the cost implications of adopting an appropriate system or converting an existing system. In addition, the government's water supply policy, operation theory of water-cooled and air-cooled condenser units, environmental implications, difficulties in conversion and professionals' opinion are explored. Major stakeholders such as developers, facilities managers, engineers, surveyors and architects could benefit from this research into air-conditioning systems with respect to fulfilling further sustainability criteria and providing environmental protection at optimal costs. This research will be useful in sustainable green building design and also in reducing energy consumption and adverse environmental impacts such as global warming, air pollution and climate change.

Modeling the performance characteristics of water-cooled air-conditioners

Water-cooled air-conditioning systems (WACS) are widely used in the commercial sector for energy efficiency, but not in the domestic sector. It has been found that there are no mathematical models and energy simulation programs to enable detailed investigation and evaluation into the energy performance of water-cooled air-conditioners. To improve the applicability of water-cooled air-conditioners in the domestic sector, the development of a prediction model for energy performance analysis is needed. This paper addresses the development of an empirical model for predicting the operational performance and energy consumption for the use of water-cooled air-conditioners. The model consists of four sub-models which has taken into account the energy consumption of the condenser water system. A prototype WACS was set up and tested in an environmental chamber to validate the resultant model. The overall COP of the WACS was found to be greater than 3 at 90% rated capacity. The predictions from the model compared well with the experimental results, with RMS error within 11%.

Analysis of a novel absorption refrigeration cycle using centrifugal separation

Water-cooled air-conditioning systems (WACS) are, in general, more energy efficient than air-cooled air-conditioning systems (AACS). The Laws of Hong Kong prohibit the use of fresh water from town mains for comfort air-conditioning, which rules out the use of cooling towers to most air-conditioned buildings. In the absence of lakes or major rivers in Hong Kong, only those buildings that are situated near the harbor can adopt WACS. As a measure to cut down greenhouse gases emissions, the Hong Kong Government is currently exploring the feasibility and viability of widening the use of WACS by the provision of centralized, district-wide water supply systems, which include seawater supply systems for once-through condenser cooling or for making up of water losses at cooling towers, and district cooling systems. Evaluation of the likely energy, economic and environmental benefits of such capital-intensive infrastructure developments requires estimation of the simultaneous cooling demand of a large group of buildings on a district scale, and the energy use for air-conditioning such buildings. This paper describes the method used in these estimations. Comparisons of the results estimated by this method with results obtained by detailed simulation, and with building energy use data obtained from surveys and audits are presented.

A study on the energy performance of three schemes for widening application of water-cooled air-conditioning systems in Hong Kong

A study on the energy performance of three schemes for widening application of water-cooled air-conditioning systems in Hong Kong

Predicting air-conditioning energy consumption of a group of buildings using different heat rejection methods

Water-cooled air-conditioning systems (WACS) are, in general, more energy efficient than air-cooled air-conditioning systems (AACS). The Laws of Hong Kong prohibit the use of fresh water from town mains for comfort air-conditioning, which rules out the use of cooling towers to most air-conditioned buildings. In the absence of lakes or major rivers in Hong Kong, only those buildings that are situated near the harbor can adopt WACS. As a measure to cut down greenhouse gases emissions, the Hong Kong Government is currently exploring the feasibility and viability of widening the use of WACS by the provision of centralized, district-wide water supply systems, which include seawater supply systems for once-through condenser cooling or for making up of water losses at cooling towers, and district cooling systems. Evaluation of the likely energy, economic and environmental benefits of such capital-intensive infrastructure developments requires estimation of the simultaneous cooling demand of a large group of buildings on a district scale, and the energy use for air-conditioning such buildings. This paper describes the method used in these estimations. Comparisons of the results estimated by this method with results obtained by detailed simulation, and with building energy use data obtained from surveys and audits are presented.

A study on the energy performance of three schemes for widening application of water-cooled air-conditioning systems in Hong Kong

The Hong Kong Government intends to reduce greenhouse gases emissions from power stations through widening application of the more energy efficient water-cooled air-conditioning systems (WACS) in buildings. Consideration is being given to the development of centralized, district wide systems for supplying seawater for once-through condenser cooling (the CPSSCC scheme), or for make-up of water losses at cooling towers (the CPSSCT scheme) such that buildings that are restricted to using air-cooled air-conditioning systems (AACS) may adopt or convert to WACS. Development of district cooling systems (DCS) for supplying chilled water to buildings is another feasible option. This paper describes a study into the energy benefits of implementing these schemes in Hong Kong. Application of the analysis method to a territory-wide study for Hong Kong showed that these schemes were all technically feasible and economically viable. It was estimated that compared with all buildings using AACS, the economic saving that could be realized through implementing these schemes in large scale are around 6% for the CPSSCC scheme, 2–3% for the CPSSCT scheme and 20–23% for the DCS scheme.

Water-cooled air-conditioning systems for energy and power saving

The paper presents the results of an experimental investigation to compare the performance of water-cooled and air-cooled air-conditioning systems. Two identical chilled water air-conditioning syst...

Choice of sink for air conditioning systems. Part 2

The technical superiority of water-cooled air conditioning systems for different applications in terms of overall power rating have been established by the authors in an earlier publication. This paper deals with the generalized approach for evaluating the techno-economic choice of sink for an air conditioning system. A single parameter of unit cooling cost in fils/kWh c (100 fils = 1 Kuwaiti dinar = US$3.25), combining the system's technical performance and economic aspects, is an effective cost-benefit criterion for making the choice for the type of sink. The results of an analysis, applied to example case studies in Kuwait, favour the water-cooled system for both the direct expansion (mostly used in the residential sector) and the chilled water (commonly used for non-residential applications) air conditioning systems.