Various Aspects of Solar Thermal

Dr. K. Srinivasa Reddy

Professor
Indian Institute of Technology – Madras

About Speaker

Dr. K S Reddy received his PhD from IIT Delhi in year 1999. He joined IIT Madras in April, 2003. He is the member of various professional bodies like Indian Society for Heat and Mass Transfer(ISHMT), Indian Society of Technical Education (ISTE), Solar Energy Society of India (SESI), International Centre for Theoretical Physics (ICTP), Indian Chapter etc. He is involved in various research projects like Design Development & Demonstration of High Performance Parabolic Trough based Solar Thermal Power Plant, Development of Solar Collector Field for Solar Thermal Power Plant DST, Development and Integration of Biomass – Concentrated Photovoltaic System for Rural and Urban Energy Bridge, Design Development of Tri generation System for Cooling & Heating Water etc.

Talk Abstract

Energy from the sun is in the form of ultra-violet, visible and infra-red electromagnetic radiation which is known as solar radiation. Earth’s diameter is 12,765 km & Sun’s diameter is 1,390,000 km. The earth revolves around the sun every 365.25 days in an elliptical orbit, with a mean earth-sun distance of 1.496 x 1011 m defined as one astronomical unit (1 AU). The sun produces light with a distribution similar to a 5525 K (5250 °C) blackbody, which is approximately the sun’s surface temperature. The inverse square law states that irradiance is reduced in proportion to the inverse square of the distance from the sun. A Pyranometer measures total global solar irradiance from the whole sky. Diffuse solar irradiance can be measured by adding a shadowing device to a Pyranometer, which blocks the direct component of total irradiance. A Pyrheliometer measures the direct component of solar irradiance, which is important when installing concentrating collectors.

Solar Energy Applications are Water Heating, Drying of Food, Cooking, Distillation, Refrigeration, Green Houses, Power Generation etc. Solar Thermal Technologies are Stationary or Non-Concentrating Collector Concentrating Collector, Flat Plate, Evacuated Tube, Compound, Parabolic, Single Axis Tracking, Two axis Tracking, Linear Fresenel reflector, Parabolic Trough, Parabolic dish, Heliostat or Central Tower, Pond, Refractive Type, Reflective Type, Fresnel Refractor/Lens etc.

Solar water heaters has various advantage like Non-tracking Option -Moving structure, motors, and tracking control systems are eliminated – reduced the complexity of the system Utilization of diffuse Solar Radiation, Costs less than concentrating collectors. Its limitations are Low Efficiency – relatively more panel area for the given load Non-tracking – Cosine effect Applicable only for low and medium range temperatures. In Evacuated Tube Collectors (ETC) Air is removed, or evacuated, from the space between the two tubes to form a vacuum, which eliminates conductive and convective heat loss. The overall loss coefficient is in the range of 2 to 10W/m2.K..

Solar cooker is an insulated container with a multiple or single glass (or other transparent material) cover.Cooker depends on the “greenhouse effect” in which the transparent glazing permits passage of shorter wavelength solar radiation, but is opaque to most of the longer wavelength radiation coming from relatively low temperature heated objects. Mirrors may be used to reflect additional solar radiation into the cooking chamber.

Solar Thermal Power Generation systems are Non-Mirror Systems, Mirror Systems & Air Moving Systems. India has Clear sunny weather of 250 – 300 days a year, Annual global radiation 1600 – 2200 kWh /m2, Equivalent energy potential 2,00,000 MWe. Power Cycles for Solar Energy Conversions are Rankine Cycle using water or organic fluid, Brayton Cycle using helium or air, Hybrid Cycles, Stirling Cycle. The central receiver or power tower is based on concentration of solar radiation by an array of mirrors called heliostats, each reflecting the beam to a central receiver. This is similar to a dish broken into a finite no. of segments and each segment tracked individually. Paraboloidal concentrators are three dimensional concentrators that focuses the radiation to a point unlike two dimensional concentrators which focuses the radiation to a line.

 

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