Impact of Heliostat Aspect Ratio on Field Cost of Solar Power Tower Plants | Shiv Nadar University
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Impact of Heliostat Aspect Ratio on Field Cost of Solar Power Tower Plants

Concentrated solar thermal power tower (CSPT) plants with storage is a potential solution to meet the objective of renewable energy on demand. However to make the CSPT plants a mainstay of electricity production, it is very important to reduce the cost of energy from such plant to a level similar to that of a fossil fuel fired plant. In an effort to reduce cost of energy for CSPT, this thesis focuses on reduction of its capital cost. It suggests a way to reduce capital costs by bringing down heliostat field cost, which is around 40-50 % of the total project cost of a CSPT project.

Installed cost of heliostat per unit area is the metric used for measurement of heliostat field cost in a CSPT plant. This is dependent on the heliostat design. Heliostat design is in turn dependent upon the wind loads that they are subjected to. The wind loads on a heliostat are determined by wind speed, its turbulence and the area of the heliostat reflector. Historically heliostats have been designed keeping its reflector nearly square in shape. However it has been found that the aspect ratio (width/height ratio) of the reflector can have a significant influence on the wind loads on the heliostat’s structure and drives and these ought to be investigated for their impact on the unit area cost of a heliostat. Reducing the height of the heliostat reflector reduces the average wind speed on the reflector due to reduced height and thus wide panels would be favourable. On the other hand, to avoid long lever arms and to reach high field densities, square panels are considered better.

In this context, the focus of this thesis is to find out a value for heliostat reflector aspect ratio, which will minimize the installed cost of a heliostat field per unit area.

Mathematical models of wind loads on heliostats of varying reflector area and aspect ratio have been developed. By using Advanced Thermal System’s 148 m2 heliostat’ installed cost breakup as a baseline, optimum values of reflector area and the installed cost/m2 for varying aspect ratios have been derived. Next, the mathematical models of wind loads have been determined through experimental methods too. Heliostat prototypes of various aspect ratios, fitted with strain gauges are deployed at a test site to measure the mechanical stresses on these heliostats in a natural environment.

Further, while the heliostat cost per unit area is a very important parameter to determine optimum cost for a heliostat field, it is important to understand the impact of this change on the efficiency of the heliostat field. This impact of change in aspect ratio of heliostat reflector on the heliostat field’s overall optical efficiency is analysed in this thesis in the following manner.

  • A small laboratory scale experimental investigation of energy being reflected by heliostat models of different aspect ratios placed in a radial staggered pattern around a tower is carried out. The energy reflected by heliostats is collected in the receiver atop the tower is measured and efficiencies of heliostat fields with different aspect ratios is compared.
  • A 50 MW CSPT plant populated with heliostats of different aspect ratios is simulated using SolarPILOT (a simulation tool for CSPT developed by National Renewable Energy Laboratory, USA) software tool. The energy received by the receiver for a typical meteorological year is calculated for various heliostat configurations. The optical efficiency of heliostat fields with varying aspect ratios is analysed. The optical efficiency of each heliostat varies based upon its location and also time of day and the day of the year. This analysis has been carried out on an hourly basis as well as for the whole year.

Finally, an optimum range of the heliostat reflector aspect ratio is recommended based upon the lowest installed cost of heliostat field per unit of energy harvested. This is determined by comparing the installed heliostat field cost and the annual energy output of the field comprising of heliostats of different aspect ratios.

Mechanical Engineering
Student Name: 
Rakesh Singhai
Faculty Advisor: