http://hdl.handle.net/10397/15 Search the Channel search http://repository.lib.polyu.edu.hk/jspui/simple-search http://hdl.handle.net/10397/5690 Title: Effective energy simulation and optimal design of side-lit buildings with venetian blinds<br/><br/>Authors: Tian, Cheng<br/><br/>Abstract: Venetian blinds are popularly used in buildings to control the amount of incoming daylight for improving visual comfort and reducing heat gains in air-conditioning systems. Studies have shown that the proper design and operation of window systems could result in significant energy savings in both lighting and cooling. However, there is no convenient computer tool that allows effective and efficient optimization of the envelope of side-lit buildings with blinds now. Three computer tools, Adeline, DOE2 and EnergyPlus widely used for the above-mentioned purpose have been experimentally examined in this study. Results indicate that the two former tools give unacceptable accuracy due to unrealistic assumptions adopted while the last one may generate large errors in certain conditions. Moreover, current computer tools have to conduct hourly energy simulations, which are not necessary for life-cycle energy analysis and optimal design, to provide annual cooling loads. This is not computationally efficient, particularly not suitable for optimal designing a building at initial stage because the impacts of many design variations and optional features have to be evaluated. A methodology is therefore developed for efficient and effective thermal and daylighting simulations and optimal design of buildings with blinds. Based on geometric optics and radiosity method, a mathematical model is developed to reasonably simulate the daylighting behaviors of venetian blinds. Indoor illuminance at any reference point can be directly and efficiently computed. They have been validated with both experiments and simulations with Radiance. Validation results show that indoor illuminances computed by the new models agree well with the measured data, and the accuracy provided by them is equivalent to that of Radiance. The computational efficiency of the new models is much higher than that of Radiance as well as EnergyPlus.; Two new methods are developed for the thermal simulation of buildings. A fast Fourier transform (FFT) method is presented to avoid the root-searching process in the inverse Laplace transform of multilayered walls. Generalized explicit FFT formulae for calculating the discrete Fourier transform (DFT) are developed for the first time. They can largely facilitate the implementation of FFT. The new method also provides a basis for generating the symbolic response factors. Validation simulations show that it can generate the response factors as accurate as the analytical solutions. The second method is for direct estimation of annual or seasonal cooling loads without the need for tedious hourly energy simulations. It is validated by hourly simulation results with DOE2. Then symbolic long-term cooling load can be created by combining the two methods with thermal network analysis. The symbolic long-term cooling load can keep the design parameters of interest as symbols, which is particularly useful for the optimal design and sensitivity analysis. The methodology is applied to an office building in Hong Kong for the optimal design of building envelope. Design variables such as window-to-wall ratio, building orientation, and glazing optical and thermal properties are included in the study. Results show that the selected design values could significantly impact the energy performance of windows, and the optimal design of side-lit buildings could greatly enhance energy savings. The application example also demonstrates that the developed methodology significantly facilitates the optimal building design and sensitivity analysis, and leads to high computational efficiency.<br/><br/>Description: xix, 231 leaves : ill. ; 30 cm.; PolyU Library Call No.: [THS] LG51 .H577P BSE 2012 Tian Sun, 01 Jan 2012 00:00:00 GMT http://hdl.handle.net/10397/5689 Title: Artificial neural network based modelling and control of a direct expansion air conditioning system<br/><br/>Authors: Li, Ning<br/><br/>Abstract: Direct expansion (DX) air conditioning (A/C) systems have been increasingly used over the recent decades in buildings, especially in small to medium scaled buildings. This is because they are more energy efficient and more flexible in installation, but cost less to own and to maintain, as compared to large chilled water based central A/C systems. Conventional DX A/C units equipped with single-speed compressor and fan rely on on-off cycling of compressor to maintain the indoor dry-bulb temperature, leading to either a space overcooling or an uncontrolled equilibrium indoor air humidity, and resulting in a reduced level of thermal comfort for occupants and low energy efficiency. With the development of variable-speed drive technology, it becomes possible for DX A/C units to have the speeds of their compressors and supply fans varied, so as to achieve simultaneous control over both indoor air temperature and relative humidity (RH). On the other hand, artificial neural network (ANN) has been proven to be powerful in modeling the dynamic operating performance of a nonlinear multivariable system, because ANN has a powerful ability in recognizing accurately the inherent relationship between any set of inputs and outputs without requiring a physical model. This ability is essentially independent of the system complexity such as nonlinearity, multiple variables, coupling, with noise and uncertainty. An ANN-based control strategy which could deal with a nonlinear multivariable complex system, such as a DX A/C system, can then be developed. As an intelligent nonlinear dynamic control method, an ANN-based control strategy offers a viable solution to the control over complex systems. This Thesis reports on a study of developing a multi-input multi-output (MIMO) control strategy that can simultaneously control indoor air temperature and humidity by varying speeds of both compressor and supply fan in a DX A/C system, using ANN-based modeling and control approaches. The Thesis starts with reporting the development of a two-in two-out ANN-based steady-state model for an experimental variable speed DX A/C system. The model can be used for simulating the steady-state total output cooling capacity (TCC) and Equipment Sensible Heat Ratio (SHR) of the DX A/C system under different combinations of compressor and supply fan speeds. Extensive experiments were carried out to collect data for ANN training and testing, as well as for validating the ANN-based steady-state model developed. The ANN-based steady-state model has been validated experimentally by comparing the measured results of TCC and SHR using the experimental DX A/C system with the predicted results using the ANN-based steady-state model developed. The ANN-based model developed can be used to predict the steady-state operating performance of the experimental DX A/C system with a higher accuracy.; Secondly, the Thesis presents the development of an ANN-based dynamic model for the experimental DX A/C system, linking the indoor air temperature and humidity controlled by the DX A/C system with the variations of compressor and supply fan speeds. The ANN-based dynamic model has been validated experimentally by comparing the measured results of indoor air dry-bulb and wet-bulb temperatures under different compressor speed and/or supply fan speed using the experimental DX A/C system, with the predicted results using the ANN-based dynamic model developed. The calculated values of average relative error (ARE) and maximum relative error (MRE) when experimentally validating the ANN-based dynamic model developed indicated the high accuracy of the ANN-based dynamic model developed. Thirdly, using the ANN-based dynamic model developed, an ANN-based controller for controlling simultaneously the indoor air temperature and humidity by varying the compressor speed and supply fan speed in a space served by the experimental DX A/C system was developed. This ANN-based controller was designed using the direct inverse control (DIC) strategy. The controllability tests including command following test and disturbance rejection test were carried out using the experimental DX A/C system, and the test results showed that the ANN-based controller developed was able to track the changes in setpoints and to resist the disturbances, with adequate control accuracy and sensitivity. Finally, to further address the problem of limited controllable range for the ANN-based controller, which is common to all controllers developed based on system identification, an ANN-based on-line adaptive controller has been developed and is presented. The ANN-based on-line adaptive controller was able to control indoor air temperature and humidity simultaneously within the entire expected operational range by varying compressor and supply fan speeds. The controllability tests for the controller were carried out using also the experimental DX A/C system. The test results showed that the ANN-based on-line adaptive controller developed was able to control indoor air dry-bulb and wet-bulb temperatures both near and away from the operating condition at which the ANN-based dynamic model in the ANN-based on-line adaptive controller was initially trained, but within the entire range of operating conditions, with a high control accuracy.<br/><br/>Description: xxiii, 195 leaves : ill. ; 30 cm.; PolyU Library Call No.: [THS] LG51 .H577P BSE 2012 LiN Sun, 01 Jan 2012 00:00:00 GMT http://hdl.handle.net/10397/5688 Title: Investigation on an Axial Passive Magnetic Bearing System (APMBS) and its application in building integrated vertical axis wind turbines<br/><br/>Authors: Kumbernuss, Jan<br/><br/>Abstract: Obvious weather changes have been taking place in the world and global warming and greenhouse gas emissions is still a hot topic. However, for many people global warming is of less importance when faced with economic hardships. The link between the economic development and the consumption of fossil fuel of the past is analyzed first in this thesis by showing that a sustainable energy supply is crucial not only for reducing greenhouse gases emissions, but also for the economic development. The negative economic implication of the dependency on crude oil and other fossil fuels is introduced. The instability of the world economy has been caused partially by the crude oil price fluctuations. The only way to create a stable and sustainable economy is to minimize the consumption of fossil fuels, and the money that might otherwise be lost in future financial collapses could be used wisely now to initiate the move away from a petrol-based economy. To facilitate this move, huge investment is needed for the development of a smart utility grid, non-petroleum based transportation and renewable energy-based energy supply economy, as along the lines of the financial bailout packages and economic stimulus packages issued by the American Government after the financial crisis in 2008. The current situation has forced a number of governments to increase research and development investment in the renewable energy sector. As one of the well-known renewable energy resources, wind energy, which attracts a larger part of today's total investments, is now playing an increasingly important role, especially in China. The work developed in this thesis is focusing wind energy utilization in urban areas. The off-shore and on-shore wind farms are well known, but recently a new application for wind turbines has attracted significant interest from architects, engineers and developers, namely the building-integration wind turbine (BIWT). Several prototype BIWT projects have been developed in Hong Kong, mainland China and other countries, and it is estimated that future urban wind turbines can produce a substantial amount of energy if they are integrated into urban buildings. However, the integration of large rotating machines into buildings has some structural effects on the buildings, like noise and vibration transmissions. The purpose of this project was thus to develop a novel Axial Passive Magnetic Bearing System (APMBS) and to investigate its application in Building Integrated Vertical Axis Wind Turbines (BIVAWT) for wind power generation from buildings in urban areas. In order to get a good estimate of the vibrations of a VAWT, the air velocities and the rotation speed of the wind turbine must be known, therefore the air velocities surrounding a building in an urban area were investigated first in this study. A building in Hong Kong was chosen and its air velocities surrounding the building for a one-year period were simulated at the beginning of this research project. The results of the calculations were then used for wind tunnel tests of several Vertical Axis Wind Turbines (VAWTs), which were designed and manufactured on the basis of CFD simulations. Each constructed Savonius-type vertical axis wind turbine (VAWT) was tested with different overlap ratios, shift angles, and the previously found wind speeds. The wind tunnel test results produced the benchmarks of the rotation speeds for the development of the novel axial passive magnetic bearing system, an invention from this project.; An axial passive magnetic bearing system was then invented, which is thought to be best suited for the VAWTs at inner city locations due to its vibration dampening character, low maintenance and low friction. This novel and special Axial Passive Magnetic Bearing System (APMBS) was developed specifically to minimize the transmission of vibrations to buildings. This permanent magnetic bearing is much cheaper and simpler than traditional magnetic bearing systems for achieving highly reliable vertical supporting functions. Many current systems adopt ring magnets to supply magnetic levitation force, but the current size of ring magnets produced is limited because of the difficulty of charging the magnet evenly to produce a uniform magnetic field. This new system consists of small, cuboidal magnets aligned along the rotation path of the bearing. The only problem was that the repulsion force was strong when the stator and the rotor magnets aligned, and weak when they did not align, which caused a higher torque and would induce vibration. This problem was overcome by introducing a unique configuration of the location of the magnets, in conjunction with a thin iron or mild steel sheet (mild steel is the most common form of steel), which was able to unify, strengthen the magnetic field and protect the magnets from aging. Using this method, thinner air-gaps are produced between the rotor and stator, which can increase the stiffness of the bearing. Besides that will the mild steel sheet also distribute the magnetic flux within the iron or mild steel plate more uniformly, which will lead to reduced vibrations. Furthermore, due to the enhanced strength of the magnetic field, cheaper magnets can be used, which makes the bearing desirable for many high performing applications. To optimize the magnetic block arrangement, countless simulations of the magnetic field of the bearing were made and a number of prototypes of different versions of such a bearing were developed from the study. A test rig was constructed for testing the prototypes. The tests found the invented system to be reliable during the wind tunnel test of the VAWT. A simulation using the Finite Element Method (FEM) was carried out to predict the torque of the bearing of any size and loading. This bearing was then tested extensively under different rotation speeds for different air velocities. The torque of the bearing and the vibration transmission form the rotating turbine to the structural frame were recorded and analyzed. The simulation and experimental results demonstrated the advantages of such a bearing. The test results showed that the bearing decoupled the wind turbine from the building. Overall, this new bearing system can lower rotational friction considerably, and minimizes vibration transmission as well. This innovative bearing system should not only be applied to the VAWTs, but also to other rotating devices like flywheels, which can benefit greatly from such a bearing system. The findings of this study have shown that the novel bearing is very well suited for decoupling the buildings from the turbines for renewable power generation in an urban environment. This development has been condensed into a patent application and a large VAWT with this bearing system has been designed and constructed for the Hong Kong Water Services Department (HK WSD) for future on-site tests. Another remarkable finding from the wind tunnel tests of the Savonius wind turbines is that a second performance peak at high Tip Speed Ratios (TSR) of the wind turbines exists, which has been reported only rarely and not been explained in the literature to date. The Savonius turbine has considerable lift properties, but the turbine is commonly considered as a drag driven turbine. The reasons for the existence of this second performance peak are explained in the thesis. The results of the study demonstrated that a wider range of rotation speeds has to be considered during the design of the bearing. For further development of the VAWTs, the concept of a double rotor motor for counter rotating VAWTs was also developed. This motor is based on the structure of a transfer flux machine, which was developed comparatively recently (1989) and has been used commercially in large horizontal axis wind turbines for power production. This new development of the double-rotor motor can be used in the VAWTs to solve the problem of different air velocities at different heights, as well as to eliminate the gear system. This system can be further developed in the future.<br/><br/>Description: xxxviii, 235 leaves : ill. (some col.) ; 30 cm.; PolyU Library Call No.: [THS] LG51 .H577P BSE 2012 Kumbernuss Sun, 01 Jan 2012 00:00:00 GMT http://hdl.handle.net/10397/5687 Title: Experimental and numerical investigation of integration of cooled-ceiling, nocturnal sky radiative cooling and MPCM slurry thermal storage<br/><br/>Authors: Zhang, Shuo<br/><br/>Abstract: The energy consumption of buildings for cooling purposes has increased considerably during the last decades. Especially in hot climate countries, the penetration of conventional air conditioning units is extremely important, having a serious impact on the peak electricity load. In recent years, new type phase change materials (PCM), microencapsulated phase change materials (MPCM) slurry, are investigated to be used as the thermal storage medium for building cooling applications to achieve the purpose of peak load shifting and energy saving. In this study, paraffin was chosen as the core material of MCPM slurry. The effect of supercooling, one of the major problem for PCM application, on the thermal properties of MPCM slurry was experimental investigated. In addition, the study investigated the proper method to prevent supercooling of PCM by using carbon nanotube (CNT) particles as nucleating agent. Finally, the utilization potential of nocturnal sky radiator combined with MPCM slurry as thermal storage medium is simulated and discussed. At the beginning of the thesis, latent heat of fusion, melting and solidification temperatures and supercooling degree of the material of PCM and MPCM slurry were investigated by using differential scanning calorimeter (DSC). An experimental test device with whole hybrid air conditioning system was also set up to measure the dynamic heat transfer performance of hybrid system combined with cooled ceiling system and MPCM slurry TES under the practical operation scheme. The water slurry of microencapsulated n-hexadecane with a melting temperature of 15.7 ℃ was cooled to 5 ℃ and heated to 25 ℃ cyclically in a storage tank of 230 litre. Melting and crystallization behaviours of MPCM slurry running in a thermal storage test system were investigated experimentally. Furthermore, supercooling and its effect on effective latent heat of MPCM slurry with different experimental conditions were also calculated and evaluated. Empirical equations were obtained to describe the relationship between the effective latent heat and the charging temperature, which can be used for determining the medium mass needed for a specified working temperature range.; Afterwards, CNT was used as the nucleating agent for prevention of supercooling. Various surfactants are firstly tested as additives to overcome the rapid aggregation and sedimentation of the nanoparticles in the organic liquid. Stable and homogenous dispersion was finally attained through surface modification of the MWCNT particles with strong acids H₂SO₄ and HNO₃, plus the addition of 1-decanol as a surfactant to hexadecane organic liquid. The average hydrodynamic diameter of MWCNT-1-decanol in hexadecane was measured by dynamic light scattering (DLS) analysis and the morphology of nano-additives was observed by transmission electron microscopy (TEM). The visible aggregation was negligible even after seven days. Thermal analysis of the n-hexadecane with well dispersed MWCNT particles by DSC indicated that the supercooling of n-hexadecane was significantly decreased with the concentration of 0.1% and 0.5% but only slightly with the concentrations over 1.0%. It appears that well dispersed nanoparticles provided stable foreign nuclei of proper size to promote the heterogeneous nucleation process and accelerate crystallization process, thus the supercooling was significantly reduced. The obvious effects of MWCNT particles on the decrease of supercooling of n-hexadecane provide promising way of improving the performance of system energy efficiency in building cooling and heating applications. At the end of the thesis, a mathematical model of the combined system of MPCM slurry, nocturnal sky radiator and cooled ceiling was built. The cooling energy consumption and the effect of energy-free nocturnal radiation application were simulated using the energy simulation code ACCURACY and MATLAB model based on hour-by-hour calculations in five typical cities across China. It can be drawn that MPCM slurry appears to be a good medium for the combined application of passive cooling technology and the nocturnal radiation in air conditioning system due to its relatively high working temperature. The results showed the energy saving potential in Lanzhou and Urumqi can reach 77% and 62% for low-rise buildings, which exhibits strong attractions for building energy conservation and emission reduction. Hong Kong has the weakest effect in the five typical cities under the same operating condition due to the hot and humid climate condition. The present hybrid system is recommended to be used in northern west and central China cities where the weather is dry and the ambient temperature is low at night.<br/><br/>Description: xviii, 190 leaves : ill. ; 30 cm.; PolyU Library Call No.: [THS] LG51 .H577P BSE 2012 Zhang Sun, 01 Jan 2012 00:00:00 GMT