Temperature distribution and heat losses in molten salts tanks for CSP plants

Abstract

Solar power plants have been deployed in the last 20 years, so the interest in evaluating their performance is growing more and more. In these facilities, thermal energy storage is used to increase dispatchability of power. The two-tank molten salts storage system with "solar salt" (60 wt.% NaNO 3 and 40 wt.% KNO 3 ) is the one commercially used today. To be able to achieve a deep understanding of the two-tank solar storage systems with molten salts, in 2008 a pilot plant was built at the University of Lleida (Spain) and the experimental evaluation of the temperature distribution inside the tanks and their heat losses are presented in this paper.Therefore, this pilot plant is equipped with several temperature sensors inside the tank as well as in the different layers of external insulation. As expected, temperature is lower at the external part of the tank (near the cover, at the bottom and near the walls) and no stratification is seen. It is found that the influencing parameters in the temperature distribution of the salts inside the tank are: insulation, and the existence of different electrical resistances and the orientation and surroundings of the tank. Heat losses were measured and compared both with a simulated 1-D steady state model and previous literature. Measured heat losses were from 61 W/m 2 through the bottom to 80 W/m 2 through the walls (with 73 W/m 2 through the cover).

Analytical approach to ground heat losses for high temperature thermal storage systems

“…Same deviations are obtained for results of q bottom . For the pilot‐scale tank described in subsection 2.3.2.4, correlation results (q bottom = 63.36 W/m 2 ) agree with the experimental results obtained by Prieto et al (q bottom = 61.00 W/m 2 ) with an error of 3.9%.…”

“…The bottom heat losses per unit area results obtained in the present work are validated with the experimental results obtained by Prieto et al in a solar power pilot‐scale plant in Lérida (Spain). They built a solar power pilot plant with a two‐tank solar storage system with molten salts and carried out the experimental evaluation of the temperature distribution inside one of the storage tanks of 1.2 m of internal diameter.…”

“…Mathematical correlations based on steady‐state numerical solutions of the heat transfer beneath a TES tank have been proposed for the estimation of the tank foundation heat losses. The numerical model results were verified with other well‐established benchmark problems with similar BCs and validated with experimental data obtained from pilot plan scale TES tanks containing molten salts . From the validation analysis, it was found an excellent agreement with the experimental results, with an error below 5.5%.…”

“…Results show that numerical simulations can accurately predict the bottom heat losses. The predicted bottom heat losses vary in the range of 59.7 to 64.32 W/m 2 depending on the exterior temperature and type of soil, with deviations below 5.5% in the worst considered scenario (T ext = 10°C and rocky soil) in comparison with the experimental measurement reported by Prieto et al…”

“…Secondly, the results in terms of the soil isotherms are compared with the results reported by Chuangchid and Krarti 8 to gain confidence in the numerical model setup. Finally, numerical results are validated with the experimental results of the bottom heat losses obtained by Prieto et al 19 in a pilot-scale TES tank.…”