(386b) Study on Cold Energy Utilization System at Lcng Fueling Stations

Authors: 
Xu, S., Shanghai Jiao Tong University
Lin, W., Shanghai Jiao Tong University
Today the world energy market is in great transition and the demand of clean energy such as LNG increases rapidly. According to market prediction, the world demand for natural gas will increase about 2.1% yearly to 2030 and a higher proportion for Asian countries. In China, with the need of the environment protection and support from the government, the amount of natural gas vehicles is rapidly increasing, correspondingly with fueling stations. And it is known that the complete gasification of LNG (Liquefied Natural Gas) converting to CNG (Compressed Natural Gas) can return about 880kJ/kg of cold energy. If the cold energy recycle system is designed for station construction, it will bring not only economic but also environmental benefits. Nevertheless, comparing to the previous utilization of cold energy, key restrictions of utilization for LCNG fueling stations embody on three aspects. Firstly, discontinuous flow makes it hard to reuse the cold energy. It is necessary to convert it to a sustainable form such as reserved by PCM (phase change material). Secondly, CNG needs to be pressured as high as 25MPa where most heat exchangers in the market cannot sustain such pressure. A specific type of heat exchanger with matching flow pipe and measuring apparatus needs to be designed. Thirdly, considering the investment and maintenance cost, cold energy utilization system should be simple and compact. Besides, some other details are required to give sufficient consideration to make the whole system more economical and practical. Rare literature or patent relates to cold energy utilization in LCNG fueling stations. This paper will present an idea to make use of the cold energy and a set of apparatus to verify its feasibility.

Here are brief introduction of solutions to the cold energy utilization presented in this paper. Firstly, a suitable PCM to store the discontinuous cold energy of LCNG should guarantee safety, low toxicity and low flammability. At the same time, itâ??s better to choose a common PCM to match downstream using cold energy apparatus. Thus, ice storage is a good choice satisfying the entire request above. On one hand, ice storage facilities are common in the market which can be bought and applied easily. On the other hand, ice/cold water can be used for air-conditioning in the LCNG stations. Secondly, traditional CNG fueling stations choose the vaporizer which exhausts cold energy directly to air. So a heat exchanger that could withhold high pressure up to 25MPa is quite important for the system. There are many types of heat exchanger such as plate, plate-fin, shell-and tube and so on. At present, according to the literature, only U-pipe and spiral wounded heat exchange can bear ultrahigh pressure. U-pipe ones are generally used in large scale system which needs more space. So spiral wounded heat exchanger with better exchange performance and less scale may be the suitable choice. Thirdly, besides the high pressure flow line and ice storage tank, a single-stage circulation with suitable intermediate refrigerating fluid would achieve the desired effect, which only involves one pump, one tank and pipe lines. Intermediate refrigerating fluid is divided into phase change material and non-phase change material and different material has different properties to affect the design of the size of heat exchanger and pipe diameter. In paper, investment cost, maintenance charge, and the cost payback period are discussed to make the whole system more attractive and practical.

Simulation cannot be avoided to make sure the feasibility of the system. HYSYS is a chemical engineering process dynamic simulation software, which allows designers to use the conceptual design and simplify the production process to complete the project work. Before the simulation, some fundamental parameters should be given combining references with experience. In consideration of safety in the lab, LNG is substituted by LN2 (liquid nitrogen). EGS (ethylene glycol solution) which is chosen as the intermediate fluid should guarantee its fluidity when it is working at low temperature. The condensation point decreases while the mass concentration rises. When mass concentration reaches 70%, the condensation point could be the lowest point at -65â??. Considering the viscosity, 50% mass concentration of EGS is suitable. In addition, the values for the flow rate of LN2, the efficiency of the pump, temperature, pressure and other parameters are explained in paper. By means of optimization by HYSYS, a series of process parameters of cold energy utilization system is given in details.

According to the design of cold energy utilization, a set of experimental apparatus is built to analyze the efficiency of the system. The whole equipment should approach to the real condition as much as possible which means the high pressure and discontinuous cold resource and limited working area. In the high pressure flow line, a made-to-order heat exchanger and pump make LN2(liquid nitrogen) high up to 25MPa. And special venting pipe is designed to give off the N2reducing the pressure and velocity without noise. Measuring transmitters connected to the data collecting computer are arranged in every inlet and outlet of the heat exchange device so that the amount of cold energy could be recorded. The equipment would achieve three different working conditions: ice-filling, ice-melting and ice-filling and melting at the same time. These conditions are based on the real demand in the air conditioning system in LCNG working space, which shows a good connection between cold energy recycle and utilization.

In the end, a discussion of the efficiency comparison between theory simulation and practical application demonstrates the feasibility of the cold energy utilization system and suggestions are made for further optimization and research.

key words: LCNG fueling stations, cold energy, ice storage, phase change material, experimental apparatus efficiency

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