Toward green buildings : design, development and performance evaluation of a solar-powered absorption cooling system

University of Technology, Sydney. Faculty of Engineering and Information Technology.Conventional HVAC systems rely heavily on energy generated from fossil fuels, which are being rapidly depleted. This together with a growing demand for cost-effective infrastructure and appliances has necessitated new installations and major retrofits in occupied buildings to achieve energy efficiency and environmental sustainability. As well as contributing to negative environmental outcomes, HVAC system usage is having a serious impact on electrical infrastructure. As such, the development of clean energy air conditioning units remains an urgent engineering challenge. Solar HVAC systems, which convert thermal energy into cool air, are known to be an efficient source of heating and cooling. Unlike traditional HVAC systems, solar air conditioning units produce maximum cooling capacity when the sun is fierce; that is, they are most efficient during the hottest part of the day, in stark contrast to traditional air conditioning units, which are less effective as temperatures increase. This study represents a synergetic framework of system identification, design, development and performance evaluation of a newly-configured air conditioning system to target energy efficiency and environmental sustainability in buildings. In this study, we have originally designed and developed a single-effect lithium bromide (LiBr)-water absorption air-conditioning system in which hot water is fully supplied by vacuum solar collectors without using any other energy sources such as gas or electricity. Water-cooled condenser of the chiller is supported by a cross-flow cooling tower. In this system, by using water as the working fluid (refrigerant), one can avoid the use of ozone-depletion chlorofluorocarbons and hydro chlorofluorocarbons. Thermodynamic and heat transfer models for absorption chiller components are described in detail. Using these models, a computer simulation software named ABSYS is developed to design the absorption chiller and drive its optimum operating conditions. Thermodynamic design data for single-effect absorption chiller are presented together with the possible combinations of the operating temperatures and the corresponding concentrations in the absorber and generator. The effect of various operating conditions on the performance and output of the absorption refrigeration system are then evaluated. Another computer code is developed by using TRNSYS to evaluate the transient performance of the entire system. Several field tests are carried out to demonstrate the technical feasibility of the system. The utilisation of the solar energy as the heat input to the generator of the absorption chiller is reported. Since Australia has great solar resources and large air condition demand, this system can be uniquely suited in Australia. However, absorption cooling technology and especially the application of solar energy in this technology is still in its infancy in Australia. The proposed design can be helpful to accelerate a global clean society to achieve its sustainable targets, especially in Australia, which has untapped high potential to become a World’s green country. Work on this thesis, supported partially by The NSW Government through its Environmental Trust, is therefore aimed to design and explore sustainable solar-powered absorption air conditioning system to show the viability of this system in Australia and reduce the energy consumption of an air-conditioned building by using this eco-friendly cooling technology

Modeling and optimal control of an energy-efficient hybrid solar air conditioning system

© 2014 Elsevier B.V. All rights reserved. The paper addresses the modeling and optimal control problem of a new hybrid solar-assisted air conditioning system developed for performance enhancement and energy efficiency improvement. To regulate the mass flow rate of the refrigerant vapor passing through a water storage tank for increasing the refrigerant's sub-cooling process at partial loads, we propose a new discharge bypass line together with an inline solenoid valve, installed after the compressor. In addition, to control the air flow rate, a variable speed drive is coupled with the condenser fan. For the control purpose, a lumped parameter model is first developed to describe the system dynamics in an explicit input-output relationship; then, a linear optimal control scheme is applied for the system's multivariable control. The system has been fully-instrumented to examine its performance under different operation conditions. The system model is then validated by extensive experimental tests. Based on the obtained dynamic model, an optimal controller is designed to minimize a quadratic cost function. Numerical algorithms, implemented in a simulation tool, are then employed to predict the energy performance of the system under transient loads. The experimental results obtained from implementation with PLC demonstrate that the newly-developed system can deliver higher system efficiency owing to amelioration of the refrigeration effect in the direct expansion evaporator and adjustment of its air flow rate. The development is thus promising for improvement of energy efficiency, enhancement of the system performance while fulfilling the cooling demand

Modeling and experimental validation of a solar-assisted direct expansion air conditioning system

Continuous increase in global electricity consumption, environmental hazards of pollution and depletion of fossil fuel resources have brought about a paradigm shift in the development of eco-friendly and energy-efficient technologies. This paper reports on an experimental study to investigate the inherent operational characteristics of a new direct-expansion air conditioning system combined with a vacuum solar collector. Mathematical models of the system components are firstly derived and then validated against experimental results. To investigate the potential of energy savings, the hybrid solar-assisted air-conditioner is installed and extensively equipped with a number of sensors and instrumentation devices, for experimentation and data collection. The influence on the system energy usage of the average water temperature, storage tank size and room set-point temperature are then analyzed. Once the air-conditioned room has achieved its desired temperature, the compressor turns off while the cooling process still continues until the refrigerant pressure no longer maintains the desired temperature. The advantages of the proposed hybrid system rest with the fact that the compressor can remain off in a longer period by heat impartation into the refrigerant via the water storage tank. Results show an average monthly energy saving of about between 25% and 42%. © 2013 Elsevier B.V. All rights reserved

Energy-efficient HVAC systems: Simulation-empirical modelling and gradient optimization

This paper addresses the energy saving problem of air-cooled central cooling plant systems using the model-based gradient projection optimization method. Theoretical-empirical system models including mechanistic relations between components are developed for operating variables of the system. Experimental data are collected to model an actual air-cooled mini chiller equipped with a ducted fan-coil unit of an office building located in hot and dry climate conditions. Both inputs and outputs are known and measured from field monitoring in one summer month. The development and algorithm resulting from the gradient projection, implemented on a transient simulation software package, are incorporated to solve the minimization problem of energy consumption and predict the system's optimal set-points under transient conditions. The chilled water temperature, supply air temperature and refrigerant mass flow rate are calculated based on the cooling load and ambient dry-bulb temperature profiles by using the proposed approach. The integrated simulation tool is validated by using a wide range of experimentally collected data from the chiller in operation. Simulation results are provided to show possibility of significant energy savings and comfort enhancement using the proposed strategy. © 2012 Elsevier B.V

Modelling and optimization of direct expansion air conditioning system for commercial building energy saving

This paper presents a comprehensive refinement of system modeling and optimization study of air-cooled direct expansion (DX) refrigeration systems for commercial buildings to address the energy saving problem. An actual DX rooftop package of a commercial building in the hot and dry climate condition is used for experimentation and data collection. Both inputs and outputs are known and measured from the field monitoring. The optimal supply air temperature and refrigerant flow rate are calculated based on the cooling load and ambient dry-bulb temperature profiles in one typical week in the summer. Optimization is performed by using empirically-based models of the refrigeration system components for energy savings. The results are promising as approximately 9% saving of the average power consumption can be achieved subject to a predetermined comfort constraint on the ambient temperature. The proposed approach will make an attractive contribution to residential and commercial building HVAC applications in moving towards green automation

Design Optimization of the Cooling Coil for HVAC Energy Saving and Comfort Enhancement

This article investigates and recommends design improvements of cooling coil geometry contributes for a central cooling system by using a simulation-optimization approach. First, theoretical-empirical system models including mechanistic relations betwee

Modeling and control of an energy-efficient hybrid solar-assisted air conditioning system

This paper addresses the modeling and control problem of a newly-developed hybrid solar-assisted air conditioning system for improved energy efficiency. A 6kW solar-assisted direct expansion air-conditioner is used for experimentation and data collection. To increase sub-cooling of the refrigerant at partial loads, we propose a new discharge bypass line together with an inline solenoid valve, installed after the compressor to regulate the mass flow rate of the refrigerant vapor passing through a hot water storage tank. Additionally, a variable speed drive is coupled with the condenser fan to control the air flow rate and synchronized with the inline valve closing and opening. For the control purpose, a lumped parameter model is first developed to describe the system dynamics in an explicit input-output relationship; then, a linear optimal control scheme is applied for the system's multivariable control. The system has been fully-instrumented to examine its performance under different operation conditions. Numerical algorithms, implemented in a simulation tool, are then employed to predict the energy performance of the system under transient loads. Results show that up to 14% energy savings can be obtained by the proposed system. © 2013 Australasian Committee for Power Engineering (ACPE)

A new single-effect hot-water absorption chiller air conditioner using solar energy

There has been an increasing interest in the development of energy-efficient and environmentally-sustainable systems using green automation technologies. This study presents the analysis and performance evaluation of a newly-developed solar-powered, hot-water single-effect absorption chiller air conditioning system. The system is, to our knowledge, the first vapor-absorption cooling plant fully powered by a renewable energy source with solar radiation, in which no water storage and auxiliary heat exchanger are used. The 6 kW Li-Br water system has been designed and tested to predict its performance. The influence of chilled water, cooling tower water and solar collector hot water temperatures on the system performance is investigated. Our experimental results demonstrate the technical viability of the proposed system in meeting the air-conditioning demand while addressing directly critical issues of electricity consumption and greenhouse gas emissions. © 2013 Australasian Committee for Power Engineering (ACPE)

A novel solar-assisted air-conditioner system for energy savings with performance enhancement

This paper presents an effective technique to enhance the performance of a newly-developed direct expansion air conditioner when combining with a vacuum solar collector that is installed after the compressor. In this approach, a novel configuration including a by-pass line together with a three way proportional control valve is proposed in discharge line after the compressor in order to control the refrigerant flow rate. In this design, the refrigerant flow rate is controlled as a function of the refrigerant temperature leaving the compressor, the refrigerant temperature leaving the solar storage tank and the ambient dry-bulb temperature. A generalized optimization algorithm is developed using sequential quadratic programming (SQR) along with a proposed empirical model for the objective function. The key challenge is to estimate the optimum refrigerant temperature entering the condenser in the new design. The optimization algorithm is simulated in a transient simulation tool to predict the optimum set-points of refrigerant temperature entering the condenser, and then implemented as a reference for an on-line closed-loop controller. The system under investigation is extensively equipped with a number of instrumentation devices for data logging. The benefits of the new design lie in the fact that the new designed system operates at a higher subcool temperature after the air-cooled condenser which significantly result in increasing the overall system coefficient of performance. © 2012 The Authors

Energy-efficient air-cooled DX air-conditioning systems with liquid pressure amplification

The objective of this study is to explore an optimal strategy on energy consumption for a direct expansion (DX) air-conditioning system by using a refrigerant pump in the liquid line to allow the system to operate at a lower condensing pressure. An existing DX rooftop package of a commercial building located in a hot and dry climate zone is used for data collection. The theoretical-empirical modelling approach is used to obtain system model, from which the proposed strategy is formulated. A numerical algorithm is developed to analyse the system transient performance, using an iterative loop. As a minimum pressure differential is required across the expansion device, liquid pressure amplification (LPA) devices can be used on DX systems that operate with fixed head pressure control. They can be fitted to new or existing systems. Results show that the LPA approach is more effective when the ambient temperature is falling, with electricity saving around 25.3% in average