Enabling Concentrated Solar Thermal Systems in Desert Areas: The Challenges and Innovations
Desert areas, such as Rajasthan in India, receive reasonable direct normal irradiation (DNI) on account of being located at the equatorial belt. Concentrated solar thermal (CST) system, for example heliostat based, can be utilized for harnessing solar energy for applications, such as electricity, process heat and cooling. However, it must be emphasized that adaptation of such CST concepts to local conditions, such as the arid desert in Rajasthan, is necessary for long-term reliable operation.
Water-cooled Radiation Calorimeter
In heliostat based CST system, radiation is concentrated on, for example, an open volumetric air receiver (OVAR). For on-field evaluation of OVAR and its control, measurement of concentrated solar irradiance (CSI) is necessary. Generally, high heat flux gauges or optical techniques are being used to estimate CSI. Watercooled radiation calorimeter (RC) is being developed to measure CSI. However, there is an emphasis on the need for proper calibration. In view of such observations, requirements and reliability, a RC is developed and evaluated up to 400 suns (1sun= 1kW/ m2 ). The developed potable device is shown in Figure 1. It depicts the working principle and the fabricated RC with Copper.
Optical and lumped parameter based heat transfer analyses of RC are performed. A semi-analytical validated approach is developed for calibration of the developed RC. Coatings are developed and evaluated for reflecting and absorbing surfaces of RC. The design prevents dust deposition on reflecting and absorbing surfaces. The performed experiments and analyses demonstrated that the RC body (Cu) temperature is ~55°C for a heating input equivalent to 400 suns of flux concentration with water mass-flowrate of ~2.2 g/s (~8l/h). Thus, it can be inferred that the developed RC can be operated, even at a higher concentration level, for evaluating OVAR based system.
Field-design: Current Practice and Adaptation to Desert Condition
An optimized staggered heliostat field layout with minimal land usage using radial stager pattern was designed with each individual heliostat mirror area of 6.25 m2. The field layout in Figure 2 (a) shows total required area of 2,200 m2 to generate 125 kWth for 600 W/m2, which can operate for 6–7 hours during a day having a tower height of 20 m. The layout is, generally, based on optical analysis considering, blockage, shading, spillage, etc. However, in desert area, gust wind speed is quite high, say 47 km/h in Rajasthan. In such a situation, dust will deposit on the mirror surface and will lead to reduced reflectivity of heliostats and therefore, the overall system efficiency. Moreover, the wake behind heliostat as a result of strong wind will induce vibration and damage the mirror surface or any coating that may be applied (Figure 2b). In view of these, the detailed analysis and experiment demonstrating such strong windrelated effect is presented in scientific research papers. The investigation clearly reveals the presence of wakeaffected region behind a heliostat extending up to 3–4 times the size of heliostat. A distance based parameter will be recommended for wakerelated effect to design heliostat field layout.
Solar Convective Furnace
Materials processing operations, primarily melting and heat treatment of metals are energy intensive processes, requiring furnaces operating at high temperatures. Aluminium is used globally for applications ranging from household to industry. In view of such observations and to reduce the direct energy requirement, a concept of a solar convective furnace (SCF) has been developed and is being evaluated at IIT Jodhpur as shown in Figure 3, based on discussions with Aditya Birla Science and Technology Centre (ABSTC), Mumbai. Here, the obtained hot air from OVAR is used, after storage, for heat treatment of metal. An experimental solar air tower simulator (SATS) facility is developed as in Figure 3 (c) for evaluation SCF system. In this setup, all the required sub-systems are manufactured and assembled locally. This clearly demonstrates that such a concept is viable and can be made available indigenously in future.
Dr Laltu Chandra, Dr Ambesh Dixit, and Dr V Narayanan, Centre for Solar Energy Technologies, Indian Institute of Technology Jodhpur, Jodhpur, Rajasthan, India. Email: chandra@iitj.ac.in; dr.laltu.chandra@iitj.ac.in.