• Energy Research

The thermal performance of a novel solar air collector with metal corrugated packing in the buildings of cold regions is studied in this paper. Mathematical models are developed to investigate the thermal performance of the collector and the results are verified by experiments. The hydraulic analysis is conducted experimentally to study the pressure drops of the air flows in the corrugated packing. Effects of the structural and operating parameters, such as the collector width, height, specific surface area and solar radiation intensity, ambient air temperature, air inlet temperature and velocity are studied to optimize the thermal performance of the collector. Comparisons are conducted among the metal corrugated packing solar air collector, the unglazed transpired solar collector, the glazed transpired solar collector and the packed bed solar collector with iron chips. The results indicate that the metal corrugated packing solar air collector is more appropriate to be used in the rural buildings of cold regions for its advantages of large heat transfer area, high heat transfer coefficient and good economic performance.

Heating, ventilation and air-conditioning (HVAC) system is favourable for regulating indoor temperature, relative humidity, airflow pattern and air quality. However, HVAC systems may turn out to be the culprit of microbial contamination in enclosed spaces and deteriorate the environment due to inappropriate design and operation. In the context of COVID-19, significant transformations and new requirements are occurring in HVAC systems. Recently, several updated operational guidelines for HVAC systems have been issued by various institutions to control the airborne transmission and mitigate infection risks in enclosed environments. Challenges and innovations emerge in response to operational variations of HVAC systems. To efficiently prevent the spread of the pandemic and reduce infection risks, it is essential to have an overall understanding of impacts caused by COVID-19 on HVAC systems. Therefore, the objectives of this article are to: (a) provide a comprehensive review of the airborne transmission characteristics of SARS-CoV-2 in enclosed spaces and a theoretical basis for HVAC operation guideline revision; (b) investigate HVAC-related guidelines to clarify the operational variations of HVAC systems during the pandemic; (c) analyse how operational variations of HVAC systems affect energy consumption; and (d) identify the innovations and research trends concerning future HVAC systems. Furthermore, this paper compares the energy consumption of HVAC system operation during the normal times versus pandemic period, based on a case study in China, providing a reference for other countries around the world. Results of this paper offer comprehensive insights into how to keep indoor environments safe while maintaining energy-efficient operation of HVAC systems.

Abstract The growing energy consumption and thermal comfort of an air-conditioned room have attracted increasing public attention. As such, the performance of air-conditioning systems with two air supply modes for a simplified office room was studied by numerical simulation and physical experiment. An experimental investigation was developed in an environmental chamber to validate the simulation model. Numerical investigation was carried out subsequently for an office setting by the validated CFD model. The indoor thermal environment, human thermal comfort and energy utilization efficiency of two air supply modes were analyzed and compared in this study. The results showed that under both air supply modes, thermal stratification occurred in the model room, and the air flow velocity in the occupied zone was low. In addition, the results indicated that under 21 °C supply temperature, the air-conditioning system with bottom-supply mode provided better indoor thermal comfort of the occupied zone, but the energy consumption was higher than the side-supply mode.

Abstract In order to improve the thermal efficiency, reduce the heat losses and achieve high freezing resistance of the solar device for space heating in cold regions, a new type of serpentine compound parabolic concentrator solar collector is presented in this paper, which is a combination of a compound parabolic concentrator solar collector and a flat plate solar collector. A detailed mathematical model for the new collector based on the analysis of heat transfer is developed and then solved by the software tool Matlab. The numerical results are compared with the experimental data and the maximum deviation is 8.07%, which shows a good agreement with each other. The experimental results show that the thermal efficiency of the collector can be as high as 60.5%. The model is used to predict the thermal performance of the new collector. The effects of structure and operating parameters on the thermal performance are mathematically discussed. The numerical and experimental results show that the new collector is more suitable to provide low temperature hot water for space heating in cold regions and the mathematical model will be much helpful in the designing and optimizing of the solar collectors.

This paper deals with the establishment of a grey-box model of plate heat exchangers. First, a second-order state space model of plate heat exchangers is developed. Then, the model is solved by the white-box method and grey-box method, respectively. In the white-box method, the linearization of the state space model is carried out by implicit difference scheme and the overall heat transfer coefficient was calculated with empirical correlation of the Nusselt number. In the grey-box method, a newly developed parameter identification method was established. The simulation results of two outlet temperatures by the grey-box and white-box method, respectively, are compared with the test data. It is shown that the grey-box method is more suitable and accurate for the simulations of outlet temperatures than the white-box method. Thus, it can provide practitioners and heating engineers with tools to design controllers for district heating systems.

Abstract This study was carried out to analyze contrastively two modes of air conditioning with the bottom-supply and the stratum ventilation by combined the numerical simulation with the physical experiment in the laboratory. The changing rules of indoor thermal environment, human body thermal comfort and energy utilization efficiency under two kinds of air supply mode were explored. The mode of the supply air with the better distribution of indoor thermal environment and the better human comfort was gained to meet the requirements of green air conditioning for the human. The studied results showed that, according to the evaluation index of human thermal comfort and the energy utilization efficiency, the mode of bottom-supply air conditioning was better than the mode of stratum ventilation. So it might be suitable to select the mode of bottom-supply in the design of air conditioning engineering for office buildings to meet the requirements of human on indoor comfort air-conditioning.

Abstract The absorption heat pump technology plays an invaluable role in energy structure optimization and sustainable development. In this research, the steam operated double effect water-LiBr absorption heat pump is introduced and the performance is studied at nominal and part load conditions. Mathematical models are developed and verified by experiments, and the results indicate that the maximum deviation is about 13% at heating condition and 9% at cooling condition. An optimization analysis is presented with mathematical models to optimize the absorption heat pump. The optimal combination of multi-factorial internal parameters, which are the eight main heat transfer temperature differences, are obtained from the thermoeconomic optimization analysis. The results will be much helpful in the design and optimization of double effect water-LiBr absorption heat pumps.

The district heating area in China is continuously increasing, which brings an increase in district heating load. In order to solve the shortage of heating power and realize the carbon neutral target, this study proposes two retrofit schemes for district heating system by integrating air source heat pump and water source heat pump, respectively. Mathematical models are established to study the performance of the integrated systems and a bilevel optimization model is proposed to optimize them. The results show that the air source heat pump combined district heating system has better performance compared to the conventional system, which reduces 50% energy consumption, 10.8% carbon emissions and achieves better economy. The dynamic coupling property of the district heating network and power grid are also considered, and the results indicate that the introduction of air source heat pump can effectively improve the efficiency and stability of power grid and reduce the seasonal fluctuation. The potential of large-scale application of air source heat pump combined system in Beijing is evaluated. The results reveal that retrofit scheme of integrating air source heat pumps into district heating system can cover 2930 MW heating load and bring 362 million Chinese Yuan profit by reducing 219,000 tons of carbon emissions and 539,000 tons of standard coal consumption in 2025.