Air-cooled Chiller Research Articles

Improved energy performance of air-cooled water chillers with innovative condenser coil configurations – Part I: CFD simulation

In condensers of large air-cooled chillers, VV-shaped finned-tube condenser coils are usually configured with the upper fan, often resulting in unevenly distributed flow and varying wind speed. This may causes the heat transfer rate of the condenser to fall below the intended level and negatively affects the energy efficiency of the chiller. This first part of the study attempted to investigate the effects of various configurations of condensing coils, three types of variant fin configuration (VFC) and four types of variant row configuration (VRC), on reducing the mal-distribution of airflow across compact air-cooled condensers. Using CFD airflow simulation and heat transfer analysis, the results of this study showed that VRC-designed condensers improve airflow distribution and enhance heat transfer performance. The best case (Case C4) increased the average air velocity and overall heat transfer rate of the base case (Case A) by approximately 10.3% and 5.3%, respectively.

Climatic influence on the design and operation of chiller systems serving office buildings in a subtropical climate

Climate change is a hot topic which would influence the design and operation of building systems. Many simulation studies focus mainly on the impact of climate change on the overall energy consumption of buildings. This study investigates climatic influences on chiller systems commonly used to provide cooling energy in commercial buildings but with considerable electricity consumption. Climate change scenarios in 2020, 2050 and 2080 were considered to generate forecast weather data based on a typical meteorological year weather file for subtropical Hong Kong. The building simulation program EnergyPlus was used to model an office building with air-cooled and water-cooled chiller systems. Hourly simulations were carried out to examine how different climate change scenarios influence the capacity of the systems and their electricity consumption. The findings indicate that a 3.9–4.2% extension in capacity should be considered for a system to meet the increasing cooling demand over its 15-year functional life. The electricity consumption of the system could increase by 6.8–8.0% over its functional life when operating for the warmer future climate. The ways to enhance the sustainability of chiller systems are addressed.

Development of New Air-Cooled Heat Pump Chiller 'Compact Cube'

Development of New Air-Cooled Heat Pump Chiller 'Compact Cube'

Influence of air conditioning management on heat island in Paris air street temperatures

Projections of future climate suggest increases in extreme temperatures particularly in mid latitudes. In addition, the effect of heat waves, which are becoming a major “summer killer”, is exacerbated in urban areas owing to the heat island effect. Air conditioning (A/C) is a key parameter for health problems in case of heat waves since, on one hand, it reduces mortality but, on the other hand, depending on the heat management, it can increase street temperature therefore increasing the air cooling demand. Results of a meso-scale meteorological model (MESO-NH), coupled to an urban energy balance model including a simplified building model (TEB), are used. Simulations based on a realistic spatial cartography of air-cooled chillers and cooling towers in the city of Paris and surroundings have been performed. The simulation period corresponds to the extreme heat wave in Paris: 9–13 August 2003. Five scenarios will be discussed: firstly a baseline without air-conditioning (NO-AC scenario); secondly the actual situation including individual air dry coolers, wet cooling towers and an urban cooling network relying on free-cooling (water-cooled A/C with the river Seine) (REAL scenario). A third scenario will assume that all the heat is rejected as sensible heat in the atmosphere (DRY AC scenario). Two other scenarios correspond to a prospective where A/C is doubled. Scenario 4 assumes that all the heat is rejected as sensible heat in the atmosphere (DRY ACx2 scenario). On the opposite, scenario 5 assumes that all the heat is rejected underground or in the river Seine (NOREJ scenario). Results show that A/C affects the UHI depending on its management. A detailed analysis on selected districts shows that the local temperature variation resulting from heat island is proportional to the sensible heat rejected locally by A/C, indicating that a clever A/C management is all the more important to provide comfort and to mitigate heat island. Moreover, the incidence of the sky view factor is also discussed.

Energy and Exergy Analysis for Improving the Energy Performance of Air-Cooled Liquid Chillers by Different Condensing-Coil Configurations

This study constructed a parameter analysis for improving the energy performance of air-cooled water chillers by altering the angle configuration of the condenser coils. The mathematical models for energy and exergy analyses of the individual components and overall system of air-cooled water chillers are presented. This study investigated the potential enhancement of performance efficiency in air-cooled chillers and the energy conversion efficiency of each component, in order to determine how the angle configuration of condenser coils influences chiller performance. This study found that the overall performance of an air-cooled chiller could be improved by approximately 3.4%, and the total irreversibility could be reduced by approximately 2.7%. With each 1% increase in average wind speed over the condenser coils, the overall performance of an air‑cooled chiller was found to be enhanced by approximately 0.43%, and its total irreversibility was reduced by approximately 0.35%. The results of this study can be effectively applied to air-cooled condenser units, and can provide an important basis of reference for developing and enhancing the energy efficiency of air-cooled chillers.

Performance enhancement of air-cooled chillers with water mist: Experimental and analytical investigation

Abstract This paper investigates how water mist evaporative pre-cooling can be applied to air-cooled chillers to improve the chiller efficiency by on-site experimental studies. There is a lack of detailed experimental studies on the application of water mist system on air-cooled chillers. The experiment was conducted on a chiller plant with water mist system in a subtropical climate. The experimental results showed that the dry bulb temperature (DBT) of entering condenser air with water mist pre-cooling could drop by up to 9.4 K from the ambient air temperature, and the approach could be as low as 0.5 K. A thermal effectiveness of up to 0.91 was obtained in using the water mist system. The pre-cooled condenser air enabled a drop of the condensing temperature by up to 7.2 K, and the chiller coefficient of performance (COP) could be improved in varying degrees by up to 18.6%. This study demonstrates that the water mist system coupled to air-cooled chillers is an energy-efficient and environment friendly technique, which has significant potential to improve the efficiency of air-cooled chillers and reduce the electricity demand for the commercial and industrial sector.

Experimental evaluation of a low-power direct air-cooled double-effect LiBr–H 2O absorption prototype

This paper describes a new small air-cooled double-effect LiBr–H 2O absorption prototype directly powered by fuel and discusses the experimental findings for some tests carried out in Madrid in 2007, with natural gas as energy source. The prototype, which has been designed to supply 7 kW of cooling power, was able to chill water up to 7–18 °C under extreme outdoor temperatures. A new flat-sheet adiabatic absorber was used allowing it to operate at outdoor temperatures about 45 °C without any sign of crystallization. A mean daily coefficient of performance (COP) of about 1.05 was obtained. Since this absorption machine does not need cooling tower, there is neither water consumption nor Legionella pollution. Moreover, it is a quite compact unit. The ratio of cooling power over volume is about 6.0 kW/m 3, while for the only air-cooled absorption chiller, Rotartica 045v, in the marked until 2009 this ratio is 4 kW/m 3. When comparing with electric chillers presently on the market, this prototype was found to have a cooling cost approximately 15.9% higher and an environmental impact 16.7% lower. The absorption prototype is a more environmentally friendly solution as it does not emit fluorinated refrigerants.

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.

SPC and Kalman filter-based fault detection and diagnosis for an air-cooled chiller

Buildings worldwide account for nearly 40% of global energy consumption. The biggest energy consumer in buildings is the heating, ventilation and air conditioning (HVAC) systems. In HVAC systems, chillers account for a major portion of the energy consumption. Maintaining chillers in good conditions through early fault detection and diagnosis is thus a critical issue.

Solar space heating and cooling for Spanish housing: Potential energy savings and emissions reduction

Abstract An experimental solar energy facility was designed to meet as much of the heating demand in a typical Spanish dwelling as possible. With a view to using the facility during the summer and preventing overheating-induced deterioration of the solar collectors in that season of the year, an absorption chiller was fitted to the system to produce solar-powered air conditioning. The facility operated in solar space heating mode in the winter of 2008–2009 and in cooling mode during the summer of 2008. The design was based on a new type of flat plate vacuum solar collectors that delivered higher efficiency than conventional panels. This type of collectors can reach temperatures of up to 110 °C in the summer and up to 70 °C on the coldest winter days. The solar facility comprised a 48-m2 (with a net area of 42 m2) solar collector field, a 25-kW plate heat exchanger, a 1500-l storage tank, a 4.5-kW (Rotartica) air-cooled absorption chiller and several fan coils. The facility was tested by using it to heat and cool an 80-m2 laboratory located in Madrid. As the average area of Spanish homes is 80 m2, the findings were generally applicable to national housing. The solar facility was observed to be able to meet 65.3% of the space heating demand. For air conditioning, the system covered 46% of the demand, but with high indoor temperatures. In other words, the collector field was found to be able to air condition only half of the home (40 m2). Lastly, the savings in CO2 emissions afforded by the use of this facility compared to conventional air conditioning were calculated, along with its amortisation period. These results have been extrapolated calculating the potential energy savings and emissions reduction for all the Spanish households.

Performance analysis of a trigeneration system based on a micro gas turbine and an air-cooled, indirect fired, ammonia–water absorption chiller

The objective of this paper is to experimentally determine the efficiency and viability of the performance of an advanced trigeneration system that consists of a micro gas turbine in which the exhaust gases heat hot thermal oil to produce cooling with an air cooled absorption chiller and hot water for heating and DHW. The micro gas turbine with a net power of 28 kW produces around 60 kW of heat to drive an ammonia/water air-cooled absorption chiller with a rated capacity of 17 kW. The trigeneration system was tested in different operating conditions by varying the output power of the micro gas turbine, the ambient temperature for the absorption unit, the chilled water outlet temperature and the thermal oil inlet temperature. The modelling performance of the trigeneration system and the electrical modelling of the micro gas turbine are presented and compared with experimental results. Finally, the primary energy saving and the economic analysis show the advantages and drawbacks of this trigeneration configuration.

A Preliminary Research on Operation Mode of Condensing Heat Recovery Hot Water Supply System for Water-Cooled Chillers

The objective of the present paper is to investigate the condenser heat recovery in water-cooled chiller system. According to the technology characteristics of vapor compression refrigerating cycle, the viewpoint of condensing heat reclaim was proposed, and heat recovery system can be used in buildings. Air-cooled and water-cooled chillers with heat reclaim capability were discussed. Three optimized water system designs were introduced for more heat recovery or higher heating water temperature. The results show that it is very useful for designing and developing the new heat recovery unit with high efficiency, energy conservation, and environmental protection by analyzing the feature feasibility of condenser heat recovery system. The control of entering-condenser water temperature is recommended for higher chiller efficiency and operating steadily, the water system designs of preferential loading and side stream are for more heat recovery.

New method for COP optimization in water- and air-cooled single and double effect LiBr–water absorption machines

In this work a new method to optimize the COP in water- and air-cooled single and double effect LiBr/H 2O absorption chillers is proposed. This method determines the effect of condensation temperatures and the solution concentration variation on COP, clearly defining the crystallization limit for different scenarios. This limit is especially important in the design of air-cooled chillers. Taking this into account a simulation program has been developed to calculate the optimum COP. In the case of parallel flow double effect chillers this program estimates not only the optimum COP but also the percentage of refrigerant vapour generated in the high and low temperature desorbers in terms of the condensation temperature. Additionally it provides the mass flow of the solution that should be distributed to each desorber to attain the desired variation in solution concentration.

Energetic and exergetic analysis of a commercial ammonia water absorption chiller

The results of an energetic and exergetic analysis of a commercial water/ammonia air-cooled absorption chiller with a cooling capacity of 10 kW are presented and discussed. The comparison with a conventional-single effect chiller operating under the same conditions shows that to reach similar performances to those of pre-absorber equipped installation an additional heat transfer area in the air-cooled absorber is necessary to ensure the absorption of totality of the refrigerant. The second law analysis reveals that the highest exergy dissipation (75%) is located in the driving compartment and that the irreversible absorption process is responsible for an important part (45%) of the COP degradation.

Modelling and adaptive control of small capacity chillers for HVAC applications

Abstract An adaptive controller for single and uneven tandem scroll compressor, packaged air-cooled water chillers is described. The designed controller allows to substantially increase the energy performance of the system, as well as to achieve excellent regulation performances in process applications. The controller parameters are adapted on the basis of estimates of the plant thermal load, that are computed by using a Kalman filter. An ad hoc approach is used to define a relay control logic, which is based on a numerical optimization procedure. To support controller design, a detailed simulation environment is developed and validated on an experimental facility. Performance of the algorithm is described with both simulation and experimental data. At low part load ratio values the proposed algorithm grants a 0.6–0.7 K improvement in the regulation performance in terms of supply water temperature standard deviation and mean supply water temperature deviation. The unit energy efficiency improvement with respect to supply water temperature control varies from 7.3% to 3.0% while the experimental seasonal energy efficiency rating improvement is 9.1%.

Improved energy performance of air-cooled chiller system with mist pre-cooling

This study investigates how to lower the dry bulb temperature of outdoor air via mist pre-cooling to enhance the coefficient of performance (COP) of an air-cooled chiller system. Based on standard chiller models in commercial simulation software DOE-2E, a modified model was made to analyse how the chiller COP varies under two different condensing temperature controls with mist pre-cooling of air entering the air-cooled condenser. Drawing on the model calibrated by manufacturer and operating data, it was estimated that around 18% decrease in the electricity consumption could be achieved by coupling floating condensing temperature control with an optimal mist generation rate, with regard to a chiller system serving a hotel in a sub-tropical climate. Remarks have been made on how optimal mist control can be implemented to maximize electricity savings while reducing surplus mist carrying over the condenser coil.

An integrated model for the design of air-cooled chiller plants for commercial buildings

Cooling load calculation is the first step in designing the air-conditioning system of a building. The calculated cooling capacity with appropriate buffer is then used to select the number and size of chillers in the system. N + 1 is a common formula used by designers to size the chiller plants in Hong Kong buildings, where N is the actual number of chillers required and 1 is a redundant chiller provided to ensure reliability. This paper reviews the problem of excess capacity and discusses the risk exposure of chiller systems without redundant chillers. The cooling load profiles of the chiller plants of four medium-sized commercial buildings in Hong Kong are reviewed. The risk exposure of chiller systems without redundant chillers can be minimized by applying risk-based preventive maintenance. The just-in-demand design reduces capital cost of the building and frees up funds for continuous energy measurement and improving the energy efficiency of chiller plant systems. This paper presents a model for designing chiller plants that improves the energy efficiency of the plant in a cost effective and thoughtful manner. It is designed with consideration of the life cycle of the plant and real-time continuous commissioning, monitoring, measurement, comparison and execution for better energy management.

An improvement of airflow and heat transfer performance of multi-coil condensers by different coil configurations

Mal-distribution of airflow is an important factor for the performance of air-cooled multi-coil air-cooled condensers. This study is an attempt to investigate the effects of different included angles between the coils of the condenser. It has been found in this study that it can be a mean to improve the performance of multi-coil condensers without using larger heat transfer surfaces. A commercially used four-coil condenser of an air-cooled water chiller was used as the base case in the tests and analysis. The results show that the variation of the included angle can increase the airflow rate by 7.85%, which corresponds to 5.29% increase in heat transfer. The improvements were found to be due to the reduction of the stagnant flow regions of the heat exchanger coils, and more even flow distribution through the coils. Test data were used to verify the computer model of the four-coil heat exchanger. The same tested fan performance characteristic was used in all of the analyses. The research results are important as air-cooled condensing units can be designed to better performance merely by changing the configuration of the coil arrangements.

A simplified method to evaluate the seasonal energy performance of water chillers

The calculation or certification of the energy consumption of buildings equipped with refrigerating or heating units operating on vapour compression cycles requires an accurate evaluation of their performance at full and part load under different operating conditions. Refrigeration systems simulation models are normally useless because of the large amount of input data required for a full characterization. A simplified numerical method for the performance prediction of vapour compression heat pumps and chillers is here presented, based only on performance data at the nominal rating conditions. The proposed procedure was validated against experimental data of different packaged air-cooled water chillers, operating on scroll compressors. In chillers full load conditions approximately 89.5% and 92.1% of the predicted EER and cooling capacity values respectively are consistent with the measured data within a relative deviation of ±10%. Simulation results are in good agreement with the experimental ones also if the experimental seasonal energy efficiency ratio, SEER on, described in the European standard draft prEN 14825, is considered. The deviations range from −3.2% to +5.1%. The proposed mathematical model appears to be a reliable tool to be implemented into dynamic building-plant energy simulation codes or into building energy certification tools.

Part load efficiency of packaged air-cooled water chillers with inverter driven scroll compressors

In this paper different packaged air-cooled systems, operating on scroll compressors, are experimentally analysed from the point of view of the relation between energy efficiency and actual capacity. Single compressor, double compressors and double compressors with uneven volumetric capacity units are tested. Experimental tests demonstrated that cooling capacity control by means of variation of rotational speed is an suitable solution for improving the part load efficiency of these systems. Step capacity units obtained by splitting the system volumetric capacity between two compressors are also effective solutions. Nevertheless they appear to be more efficient than single inverter driven chillers only for part load conditions lower than 60%. In the analysed cooling capacity range (25–50 kW), tandem compressors chillers with one inverter driven compressor appear the most efficient solution combining rotational speed with step capacity control. Seasonal energy efficiency ratios were obtained with prEN 14825 calculation method confirming reduced energy consumption associated to continuous and step cooling capacity control.