شبیه سازی عددی از سیستم پیشرفته ذخیره سازی انرژی با استفاده از H2O-LiBr به عنوان سیال کاری، قسمت 2:شبیه سازی سیستم و تجزیه و تحلیل

This paper is the second part of our study on the advanced energy storage system using H2O–LiBr as working fluid. In the first part, the system working principle has been introduced, and the system dynamic models in the operation process have also been developed. Based on the previous research, this paper focuses on the numerical simulation to investigate the system dynamic characteristics and performances when it works to provide combined air-conditioning and hot water supplying for a hotel located near by Yangzi River in China. The system operation conditions were set as follows: the outdoor temperature was between 29 °C and 38 °C, the maximum air-conditioning load was 1450 kW, the total air-conditioning capacity was 19,890 kWh and the 50 °C hot water capacity for showering was 20 tons which needed heat about 721 kWh on a given day. Under these conditions, the system operation characteristics were simulated under the full- and partial-storage strategies. The simulation results predicted the dynamic characteristics and performances of the system, including the temperature and concentration of the working fluid, the mass and energy in the storage tanks, the compressor intake mass or volume flow rate, discharge pressure, compression ratio, power and consumption work, the heat loads of heat exchanger devices in the system and so on. The results also showed that the integrated coefficient of performances (COPint) of the system were 3.09 and 3.26, respectively, under the two storage strategies while the isentropic efficiency of water vapor compressor was 0.6. The simulation results are very helpful for understanding and evaluating the system as well as for system design, operation and control, and device design or selection in detail.

This paper is the second part of our study on the advanced energy storage system using H2O–LiBr as working fluid. The advanced energy storage system is also called the Variable Mass Energy Transformation and Storage (VMETS) system. As shown in Fig. 1, the VMETS system composes of several major components: (I)-solution pump, (II)-solution storage tank, (III)-heat exchanger, (IV)-generator/condenser, (V)-water vapor compressor set, (VI)-moistener, (VII)-auxiliary heater, (VIII)-water storage tank, (IX)-recycle pump, (X)-absorber, (XI)-evaporator, some control valves and throttles. In the first part [1] of our study on the system, it was known that the VMETS system does not directly transform the electric energy in off-peak time into the cold or heat energy, but transforms the electric energy mostly into the chemical potential of the working solution and stored it in the system firstly. And then the potential is transformed into cold or heat energy by absorption refrigeration or heat pump mode when the consumers need the cold or heat energy. As a result, the energy transformation and storage can be carried out at the desirable time to shift electric load efficiently by the VMETS system for cooling, heating or combined cooling and heating. Since the concentration of the working solution in the VMETS cycle varies continuously, the working process of the VMETS system is dynamic. As the first part of our study, the working principle and flow of the VMETS system were introduced in detail, and the dynamic models of the system were fully developed. To investigate the system characteristics and performances under full- and partial-storage strategies, the numerical simulation will be performed in this paper. The simulation results will be very helpful for guiding the actual system and device design. Thus, this paper focuses on the numerical simulation when the VMETS system works to provide combined air-conditioning (AC) and hot water supplying (HWS) for a hotel located near by Yangzi River in China. In subtropical regions such as the South China or the region near by the Yangzi River, the typical weather pattern is hot and humid in summer, whilst the highest outdoor or ambient temperature can reach 38 °C or even higher. Therefore, the AC systems in hotels have to be operated continuously in summer. In order to reduce the energy consumption, we propose a new VMETS system shown in Fig. 2 to replace the common AC and HWS system used in hotels. The new system operates for shifting electric load and storing energy mainly for AC, but it can operate like a heat pump for both cooling and hot water supplying simultaneously when the LiBr concentration of solution in its tank is stronger. Full-size image (25 K)

Through the theoretical research on the working characteristics and performances of the VMETS system, the operation parameters and their variation rules of the system, compressor, storage tank volume, the heat exchange device, etc., can be known. These parameters or data are the bases for system design and control, as well as for the design or selection of the compressor and the heat exchange devices. Moreover, they also provide the basic data for the technical and economic analysis of the system. The conclusions achieved by this research are as follows: (1) The electric energy can be transformed and stored by the system in the means of chemical potential of the working solution, which can be transformed into the cold and heat energy by absorption refrigeration and heat pump modes for later use. However, the electric energy cannot be fully transformed into the solution chemical potential since part of it is transformed into the solution sensible energy. The sensible energy cannot be transformed into the cold energy and hence, it should be reduced as possible during the energy charging process to increase the energy conversion efficiency of the system. (2) The system can supply not only the cold energy for air-conditioning but also hot water for shower. The integrated COPint of the system can exceed 3.0 even though the compressor efficiency is set as 0.6 in this simulation. The available storage density of the system can reach 402.2 (MJ/m3) and 394.0 (MJ/m3), respectively, under full- and partial-storage strategies. (3) Although the electric energy in off-peak time is partially shifted by the VMETS system, the amount of the original solution charged into the system and the compressor power, compressor, heat exchange device and storage tank dimensions can be obviously reduced by applying the partial-storage strategy. (4) The mass flow rates through the control valves, throttles and pumps vary with respect to the AC load, the system operation time and mode and their variations are complex. (5) The intake volume flow rate or size of the water vapor compressor can be reduced significantly by increasing the generation pressure. While the generation pressure is 40 kPa, the average SCDC of the compressor under the full- and partial-storage strategies are 577.1 kJ/m3 and 580.1 kJ/m3, respectively.