(578g) Study on Ice Storage System Based on Cold Energy Utilization in Lcng Fueling Stations

Authors: 
Zhao, S., Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University
Lin, W., Shanghai Jiao Tong University
Qiu, W., Shanghai Jiao Tong University
As the increasing of the CNG vehicles for the advantages of low cost and environmental-friendly, the amount of the LCNG fueling station is also rapidly increasing. As we all know, the cold energy will be waste when the LNG (Liquefied Natural Gas) gasify into CNG (Compressed Natural Gas). So, it is necessary to design a cold energy recycle system in the LCNG fueling stations, which can achieve reasonable utilization of resources. The discontinuous flow makes it is hard to reuse the cold energy, so we design a heat exchanger and the ice storage facilities in the main system. In the heat exchangers, the LNG gasify into CNG, releasing the cold energy to the intermediate fluid. The ice storage facilities can be used to storage the cold energy into ice, and when we need this cold energy we can melt the ice into cold water for air-conditioning in the LCNG stations. Comparing to the previous ice storage systems, the new ice storage systems use the cold energy from the intermediate fluid at low temperature., so, the intermediate fluid can provide more cold energy for the air-conditions. As a result, the optimal operating point will be different from the normal systems.

There are many restrictions to the type of the intermediate fluid, such as low-freezing, high potential heat value, environmental and safety. The EGS (Ethylene glycol solution), which can meet the conditions perfectly, is one of the most common intermediate fluid. With the increasing of the mass concentration of the EGS, the freezing temperate decrease. As a result, in the LCNG station, we should choose the EGS with high concentration. Considering the viscosity, 50% mass concentration of EGS is suitable for the study.

In this paper, two-dimension model is performed to investigate the dynamic performance of the ice storage. Commercial CFD software ANSYS Fluent 15.0 and meshing software ICEM are used in this study. The model is a casing pipe with length of 1000 mm. The EGS is in the inner pipe with diameter of 20mm and the water is in the outer one with diameter of 100mm. A uniform velocity assumption is applied at the inlet boundary of the EGS. The outlet boundary condition of the EGS is outflow boundary condition. The water is static in a fixed temperature. According to the size of the mesh, we set the time step as 0.1s and the iterations as 30 times. Using the solidification & melting model in ANSYS Fluent 15.0, we can get the ice storage result in 6000 seconds.

Three influence factors are put forward through analyzing the actual situation——water temperature, EGS temperature, and EGS flow velocity. The range of EGS temperature is from 258K to 268K; the water is from 278K to 293K, and the EGS flow velocity range from0.2m/s to 1.5m/s. Based on the control variable method, we change different variables and calculate 11 different models in the Fluent 15.0. Then, we can obtain the temperature and the ice packing factor in the casing pipe.

Analyzing the three different factors, we can draw the conclusion: to some extent, the lower the EGS and the water temperature is, the faster the water freezes. However, the relationship between the EGS flow velocity and the freezing time is not linearity. There is an optimal velocity for the model. Comparing with the three different factors, we discovered that the EGS temperature have the most important influence to the freezing. So when we choose the optimal operating point, we should take it into consideration.

In order to verify the result of the software simulation, we also designed an experiment system. Three key section consist of the system. The first part is the precooling system. In consideration of safety in the lab, LNG is substituted by LN2 (liquid nitrogen). PHE(Plate Heat Exchanger)is used to cooling the EGS from the cold energy of LN2.This heat transfer process simulates the gasification process of LCNG station. The second part is the main part of the system. A visual casing pipe consisted of three different pipe is made in the experiment. The EGS is in the inner pipe made by 304 stainless steel, and the water is in the middle pipe made by. Polymethylmethacrylate The outer pipe is also the Polymethylmethacrylate pipe. A vacuum pump is used in the air between the middle pipe and the outer pipe to keep the vacuum. Through the visual design, we can not only observe the forming of the ice, but also satisfy the demand of thermal insulation of the system. The third part is the water cycle system composed with water chilling unit and flowmeter, which can adjust the initial temperature of the water.

On the one hand, temperature sensors, pressure sensors and flowmeters are used in the experiment to measure and collect the data of the parameters; on the other hand, a camera and image processing techniques are also used to observe the icing phenomenon with time in different working condition.

The influence of subcooled temperature and the gravity can be observed in the experiment. The ice is quite thick under the inner pipe than the ice above the inner pipe. Since the software simulate does not take the subcooled temperature into account when the water solidifies, we can find the subcooled temperature in the different condition in our experiment.

Based on the software simulate and the experiment, one can find out the theory and the influence factors of the icing. What’s more, an optimal operating point can be found to fit for the status of LCNG fueling station, this will become useful advice for the development of cold energy utilization in LCNG station.