Molten Salt Tower Plants Research Articles

Analysis of solar tower plant performance influenced by atmospheric attenuation at different temporal resolutions related to aerosol optical depth

The optical losses associated with the attenuation of the reflected direct irradiance by the heliostats along the optical path to the receiver may be significant in large solar tower plants. This phenomenon, known asatmospheric attenuation loss, may have a stronger impact at those tower plants where high aerosol loads are expected. Performance models like the System Advisor Model (SAM) and the ray-tracing models (DELSOL, MIRVAL) usually estimate the atmospheric attenuation loss by a polynomial expression which is function of the slant range (the optical path between the heliostat and the receiver). Most of the polynomial models proposed to determine this optical loss use two established extreme attenuating conditions corresponding to a clear or hazy atmosphere. This paper presents a sensitivity study of the impact of time-dependent variability of the atmospheric attenuation in the yield performance of two reference large solar tower plants (one similar to Ivanpah 1 and the other one to Crescent Dunes as examples of direct steam and molten salt tower plants, respectively). Five sites have been selected from the AERONET ground station network to obtain the aerosol loading at different time-scales: annual, monthly and daily. Multiple SAM runs have been performed to simulate the annual yield of each plant and site creating different inputs to the code corresponding to each time-scale condition. The results show a significant impact of the time-scale for modeling the atmospheric attenuation on the annual yield and daily energy output of the plant. Although the annual and monthly means produce some compensation of the impact, the differences in several particular days can be significant. Up to 20% difference in the daily energy output is found when the extinction is modeled asa steady-state polynomial representing the annual mean compared to the case of daily time-dependent variability of the attenuation. The sensitivity results presented here show that for more realistic yield performance calculations in solar tower plants, particularly at desert and arid climates, the modeling of the atmospheric attenuation should be performed in a time-dependent way according to the climatological variability conditions characteristic of the site.

Assessment of Improved Molten Salt Solar Tower Plants

The temperature level increase of molten salt solar tower plants is one important task on the development schedule to increase the system's overall efficiency. In the conventional power plant technology, modern supercritical steam power plants work with life steam temperatures near 620°C. To apply these modern power blocks in molten salt solar tower plants, salt temperatures near 650°C are required. Today's molten salt tower plants reach salt temperatures of 565°C. To follow the positive development tendencies of fossil power plants, in the present work the combination of supercritical steam power plants with solar towers is analyzed. As stated in recent literature (Kolb, Kelly, Singer), the state of the art tubular central receiver concept coupled with increased steam parameters of the power block (300bar/600°C/610°C) shows a marginal potential to decrease LEC. The identification of future concept innovations with significant cost reduction potential in the field of molten salt tower plants, the enhancement of the receiver efficiency as well as the assessment of critical aspects related to their feasibility is the task of this paper.Therefore this work first focuses on the state of the art molten salt tower technology and related improved concepts with molten salt. The selected improvements follow on the one hand the aim to increase the receiver's maximum temperature level to approximately 650°C to be able to feed modern supercritical power blocks, on the other hand the aim to require a low development effort. After the reference concept is characterized, the objective is the systematic comparison of receiver technologies, which show a potential of improved thermal efficiencies.The assessed receiver candidates are tubular and direct absorption receivers, either external or located in a cavity. The analyzed power level is 125 MWel, while the steam process parameters are varied from low temperature level (550°C) and subcritical steam to high temperature level (620°C) and supercritical steam. With this, selected molten salt power plants were specified and then modeled with different sizes of solar fields and different storage capacities and analyzed on an annual basis. The results show, that the assessed concept options close to the state of the art require drastically decreased investment costs to end up with significant LEC reduction and cost-competitiveness. The increase in overall efficiency of these improved concepts is compensated by their higher financial effort. Results of further concept assessments as well as the sensitivity analysis of parameters and costs are described.