Translated Abstract
Solar energy is the most abundant, clearest, and wildly accessible renewable energy. High efficient utilization of solar energy can solve the problem of energy shortage and unreasonable structure of energy consumption. However, solar energy has a serious shortcoming such as instablity and discontinuity, which can be solved by efficient thermal energy storage. In order to raise the thermal efficiency of tower solar thermal power plant, the inlet temperature of heat transfer fluid (HTF) should be increased. Thus, a high-temperature thermal energy storage (TES) should be established. High-temperature thermal energy storage can be divided into three classes: sensible heat storage, latent heat storage, and chemical heat storage, and the high-temperature latent heat storage is preferred due to its high thermal storage density and constant temperature.
Due to its high operation temperature, heat transfer and thermal storage characteristics in high-temperature latent heat storage are complex and there is lack of experimental study. Also, the thermal conductivity of phase change material is quite low, and it is thus critical to find a way to enhance the heat transfer in high-temperature latent heat storage. Motivated by these issues, a high-temperature cascaded latent heat storage platform is established, which can be used to test the characteristics of high-temperature latent heat storage. The effects of operation parameters on the latent heat storage are experimentally studied. Also, a mathematical model of latent heat storage is established and the heat transfer characteristics of latent storage is obtained. In order to enhance the heat transfer of latent storage, non-uniform finned tube is proposed, which proves to be effective in enhancing the heat transfer characteristics of latent heat storage. The detailed contents are as follows.
In experimental study:
1) A high-temperature latent heat thermal energy storage experimental platform is established, and uncertainty analysis is performed, which can lay a foundation for experimental study. The high-temperature latent heat thermal energy storage experimental platform consists of 5 parts: high temperature air transport system, testing part, flow and temperature control system, air cooling system and data acquisition system, in which at most 4 testing units can be installed: a cascaded latent heat storage testing can be done by varying phase change materials of different melting point in different testing units. Air is used as heat transfer fluid and inorganic salts are used as the phase change material. Air flow through the inner tube and salts are stored in the shell side. The temperature of air can reach 800 ℃, and the mass flow rate can reach 270 kg·h-1.
2) The effects of operation parameters on charging/discharging characteristics of LHTES are experimentally studied, which can guide the real operation of LHTES. In experimental study, the binary carbonate, 62 mol %Li2CO3-38 mol %K2CO3, is used as the phase change material. The results show that (1) fins can improve the uniformity of PCM temperature, and have a great improvement in latent charging/discharging process, and have a negligible effect on sensible charging/discharging process. Adding fins can raise the average heat-charging power by 29.1-70.4 W, i.e., 5.2-10.4 %, and raise the average heat-discharging power by 33.8- 57.2W, i.e., 9.1-19.4 %; (2) Increasing the inlet temperature of air can increase the average charging power greatly, but has a negligible effect on discharging process. The increase in the average charging power can reach 42.0 %; (3) Increasing the mass flow rate of air, the average heat-charging and discharging power can be greatly increased by 16.1 % and 26.5 %, respectively, and the charging efficiency also improves. Thus, when the high-temperature latent heat storage is operating, fins can be used to enhance the heat transfer characteristics, and increasing the inlet temperature and mass flow rate can be used for further enhancement.
In numerical study:
3) The basic charging characteristics of smooth-tube and fin-tube LHTES is numerically studied, which can lay a foundation for study of enhancement. The results show that due to the effects of natural convection and the nonuniformity of HTF temperature distribution along heat transfer tube, the melting process of PCM is not uniform, which can lead to the decrease of charging power and longer charging time. After the fins are added, the melting uniformity of phase change material improves, the average charging power can increase by 38.1 % at most, the charging time can decrease by 35.0 % at most, the highest temperature of heat transfer tube can decrease by 10.9 K, and the maximum temperature gradient can decrease by 6.0%, which can be beneficial to the long and stable operation of high-temperature thermal energy storage system.
4) The effects of operation conditions and fin parameters on the charging characteristic of LHTES are numerically studied and a nonuniform-fin tube is proposed, which further enhances the charging characteristics of LHTES. The results show that operation conditions and fin parameters should be selected carefully when the LHTES is applied in real situations. There exist optimal fin parameters: fin height H=12 mm, space between fins S=10 mm, and fin thickness δ=2 mm. The nonuniform-fin tube, which improves the uniformity of melting process of phase change material, leads to the reduction of charging time in LHTES by 6.5%.
Translated Keyword
[Experimental Study, High-temperature latent heat thermal energy storage, Nonuniform fins to enhance heat transfer, Numerical study, Solar energy]
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