• Energy Research

Abstract Building energy performance assessment is crucial to ascertain the efficiency of energy use in buildings and is the basis to make any decision for enhancing energy efficiency. In order to assess the energy performance of existing buildings quantitatively, the energy use of the assessed buildings should be quantified first. The quantified energy use will be then used to compare with the assessment criteria to determine the energy performance quantitatively. This paper presents an overall review on the state of the art of the research and applications of quantitative energy performance assessment. A framework is proposed for categorizing the energy quantification methods and performance benchmarking methods for energy performance assessment for existing buildings. Energy quantification methods are classified into three categories, i.e. the calculation-based, measurement-based and hybrid methods, according to the energy data acquisition approaches. Energy performance assessment methods are classified according to the assessment scope and depth of assessment, i.e. whole-building benchmarking method at building level and multi-level assessment method.

A complex Brayton cycle is investigated under maximum economic and maximum ecological conditions. The economic function defined as the power output divided by the total cost, while the ecological function defined as the power output divided by the entropy generation rate, is optimised with respect to the cycle temperatures, reheat and intercooling pressure ratios for a typical set of operating conditions. It is found that there are optimal values of the turbine outlet temperature, intercooling, reheat and cycle pressure ratios at which the cycle attains the maximum values of the economic function, ecological function power output, efficiency and the minimum entropy generation rate.

AbstractIn the subtropical urban area with a high density of buildings, district cooling system is regarded as an efficient alternative to supply cooling to a group of buildings with high efficiency and low cost. Appropriate design can render the district cooling system to achieve good performance. This paper attempts to study the performance of the district cooling system integrated with different energy technologies in a new development area. Performance of district cooling system with thermal storage system is calculated to level the energy consumption and reduce monthly bill. Results are compared with that of hydro pumped storage system and recommendations are given about the design and control the thermal storage system under current local tariff. The performance of district cooling system integrated with the combined cooling, heating and power system is also investigated to make full use of the primary energy. Performance of the system designed based on two ways are studied. Results show that the cogeneration system is beneficial for both the investors and users. Suggestions are given to help improve the performance of the district cooling system in the new development area by coupling with different energy technologies.

Abstract Cleanrooms can be 10–100 times as energy-intensive as typical office buildings. The causes of this are mainly high air change rates and counteraction (i.e. overcooling and reheating) processes in applications. Existing studies have addressed the design and control of outdoor and supply air ventilation systems separately without considering the interaction between them, which causes significant energy waste. This study therefore proposes a coordinated demand-controlled ventilation (CDCV) strategy to achieve energy-efficient operation in multi-zone cleanroom air-conditioning systems, by coordinating operation between outdoor and supply ventilation systems. The Lorenz curve and Gini index are introduced to quantify the demand diversity of multiple zones, and degrees of overcooling and overdrying are used to quantify the mismatch between cooling supply and demand. Cleanrooms in a pharmaceutical factory located in Hong Kong, a humid sub-tropical city, are selected to test and validate the effectiveness of the proposed strategy. Test results show that the optimization of supply air volume can alleviate the mismatch between cooling supply and demand, as well as affect the optimal outdoor air ventilation mode. The proposed strategy can achieve up to 89.6% of reheating and 63.3% of overall energy savings.

Thermal energy storage (TES) systems using phase change material (PCM) have been recognized as one of advanced energy technologies in enhancing energy efficiency and sustainability of buildings. The use of PCMs in buildings provides the potential for a better indoor thermal comfort for occupants due to the reduced indoor temperature fluctuations, and lower global energy consumption due to the load reduction/shifting. A good knowledge on dynamic characteristics and energy performance of buildings using PCMs is essential for building researchers and practitioners to better understand building temperature response characteristics and economic feasibility of using PCMs and take further proper actions to fully utilize PCMs to enhance indoor environmental quality and overall energy efficiency of buildings. This paper presents an overview of the previous research work on dynamic characteristics and energy performance of buildings due to the integration of PCMs. The research work on dynamic characteristics and energy performance of active and passive building applications is reviewed, respectively. Since the particular interest in using PCMs for free cooling and peak load shifting, the specific research efforts on both subjects are reviewed separately. A few useful conclusive remarks and recommendations for future work are presented.

The use of phase change materials (PCM) to enhance the building energy performance has attracted increasing attention of researchers and practitioners over the last few years. Thermodynamic models of building structures using PCMs are essential for analyzing their impacts on building energy performance at different conditions and using different control strategies. There are few PCM models of detailed physics providing good accuracy in simulating thermodynamic behavior of building structures integrated with PCM layers. However, simplified models with acceptable accuracy and good reliability are preferable in many practical applications concerning computation speed and program size particularly when involving large buildings or models are used for online applications. A simplified physical dynamic model of building structures integrated with SSPCM (shaped-stabilized phase change material) is developed and validated in this study. The simplified physical model represents the wall by 3 resistances and 2 capacitances and the PCM layer by 4 resistances and 2 capacitances respectively while the key issue is the parameter identification of the model. The parameters of the simplified model are identified using genetic algorithm (GA) on the basis of the basic physical properties of the wall and PCM layer. Two GA-based preprocessors are developed to identify the optimal parameters (resistances and capacitances) of the model by frequency-domain regression and time-domain regression respectively. Validation results show that the simplified model can represent light walls and median walls integrated with SSPCM with good accuracy.

Abstract Determining the proper trade-off among thermal comfort, Indoor Air Quality (IAQ) and energy use is important for optimal control of air-conditioning systems. The number of optimization variables increases as systems become increasingly complex, as with multi-zone VAV (Variable Air Volume) air-conditioning systems, leading to large-scale mathematics programming challenges and inconveniences in the implementation of conventional centralized optimization strategies. This paper therefore proposes a real-time optimal control strategy adopting a multi-agent based distributed optimization method for multi-zone VAV air-conditioning systems. The proposed strategy consists of three novel schemes. First, a temperature set-point reset scheme adopts a linear rule to correlate the resetting of the temperature set-points in individual zones to simplify the optimization problem while applying proper optimization in individual zones. Second, a multi-objective optimization scheme optimizes the fresh air ratio of the supply air and the temperature set-point in the critical zone by formulating the multi-objective optimization problem. Third, a multi-agent distributed optimization scheme is developed to solve the optimization problem in a distributed manner, facilitating the deployment of local control devices of limited capacity. A TRNSYS-MATLAB co-simulation testbed is constructed to test and validate the proposed strategy. Test results show that the strategy is effective in properly balancing thermal comfort, IAQ and energy use while largely reducing programming challenges. The distributed optimization method can provide almost the same optimal outputs as conventional centralized optimization methods.

Abstract This paper presents a practical case study using the computational fluid dynamics (CFD) technique in retrofitting the large cabinet fans and airflow channels in malt processing air-conditioning systems in order to get uniform airflow field, to increase fan capacity and reduce energy consumption. Site measurements were used to validate the models before they were used. CFD simulation results were used to evaluate and optimise the design and construction of splitter vanes in the airflow channels and scrolls of the fans. Simulation tests showed that the fan capacity (maximum total air flow rate delivered at full fan speed) could be increased by 8.3–20% using different shapes of scroll, and 30–34% by adding splitter vanes further in the airflow channels. After retrofitting the fans and airflow channels, site measurements showed that the fan capacity increased 21–24 and 28–29%, respectively in the two buildings validated. The approach of using the CFD technique in the retrofitting, the simulation results, and the site measurements before and after fan retrofitting are presented and compared in this paper.

An online control strategy is developed to optimize the speed of variable speed condenser cooling water pumps by resetting their pressure set-point, which includes an adaptive and a derivative control strategy. Optimal control is achieved by adjusting the pressure set-point according to the estimated derivative of the total power with respect to pressure. The adaptive strategy identifies the changes of the system parameters essential for the control strategy and updates the control accordingly. The control strategies, the implementation and evaluation of the control strategies on an indirect seawater-cooled chilling system with multiple pumps and chillers are presented.