Solar Thermal System

Solar thermal systems convert sunlight to heat

Solar thermal system Solar thermal systems are cost effective at low temperatures for water heating or cooking The basic principle of solar thermal utilization is the conversion of short-wave solar radiation into heat If radiation incidences on material a certain part of the radiation is absorbed. A body’s capacity to absorb radiation is called absorbing capacity or absorption α An ideal black body absorbs radiation at every wavelength and therefore has an absorption coefficient equal to one. In solar thermal systems the absorber is normally part of a collector. Other components of the collector are frame, cover and insulation.

Solar collectors Solar collectors transform solar radiation into heat and transfer that heat to a medium (water, fluid or air).

Solar collectors- Types 1.Flat plate 2.Parabolic Trough 3.Parabolic Dish 4.Solar Central Receiver (Solar Power Tower) 5.Lens Concentrators

Flat-plate collectors Flat-plate collectors are the most widely used kind of collectors in the world for domestic water-heating systems and solar space heating/cooling. The first accurate model of flat plate solar collectors was developed by Hottel and Whillier in the 1950's. A typical flat-plate collector consists of an absorber, transparent cover sheets, and an insulated box.

Flat plate Collector The absorber is usually a sheet of high-thermal conductivity metal such as copper or aluminium, with tubes either integral or attached. The radiation is absorbed by absorber plate, which is coated with black absorber paint. The fluid tubes, which are connected to absorber plate, absorb the heat and transferred to the water passing through the tubes and gets heated. The hot water collected from all the tubes flow into a storage tank The insulated box reduces heat loss from the back or the sides of the collector. The cover sheets allow sunlight to pass through the absorber but also insulate the space above the absorber to prevent cool air to flow into this space.

Flat plate collector

Flat-plate collectors Evacuated (or Vacuum) Tubes are provided to reduce heat loss due to conduction and convection. (vacuum is a heat insulator). Different construction types are available: Heat pipes or direct flow All glass tubes With or without concentrator

Cylindrical parabolic concentrating collector In this type of collector, the radiation coming on concentrator is diverted to absorber tube, which is covered with glass tube to avoid re-radiation loss. The heat from the absorber tube is transmitted to the fluid passing and gets heated up. The hot liquid flows out to a storage tank for further application. The direction of concentration is changed with the sun movement by providing single axis tracking. The temperature attained by the collector is C.

Paraboloid concentrating collector This type of collector is provided with two axis tracking and is free to move both on vertical and horizontal axis. The radiation received on the collector is reflected towards the concentrator. The concentrator, which is coated with absorber coating, is heated up with concentrated radiation. High temperature (above C) can be attained with this type of collector.

Cylindrical parabolic concentrating collector

Paraboloid concentrating collector Concentrate solar energy through use of mirrors or lenses. Concentration factor (“number of suns”) may be greater than 10,000.

Parabolic Troughs Parabolic mirror reflects solar energy onto a receiver (e.g. a evacuated tube).

Parabolic trough design These plants use a large field of parabolic trough collectors which track the sun during the day and concentrate the solar radiation on a receiver tube located at the focus of the parabolic shaped mirrors. A heat transfer fluid passes through the receiver and is heated to temperatures required to generate steam and drive a conventional Rankine cycle steam power plant. A parabolic trough is constructed as a long parabolic mirror that is usually coated silver or polished aluminium.

Power Tower design The solar power tower is a system that uses many solar reflectors called heliostats to reflect the energy of the sun to a central tower. The energy of the sun works to heat a fluid like water, air, liquid metals, or molten salts which circulates through the tower. This hot fluid can be used to turn a turbine for power generation.

Parabolic Dish Parabolic dish/engine systems utilize an array of parabolic dish- shaped mirrors to reflect and concentrate the incoming solar radiation directly hitting the array of dishes back onto a single receiver located at the focal point of the dish. Motors are used to continuously track the sun in two axes so that the incoming solar radiation is always hitting the mirrors at the optimal angle to reflect and concentrate the most sunlight to the receiver

Solar Collectors

Paraboloid collectors

Solar collectors LENS CONCENTRATORS PARABOLIC TROUGH

Solar Power Tower 10 MW Power tower in Barstow, California.

The temtemperature that can be achieved in the absorber depends on the concentration factor. Concentration ratio = Aperture area over which light is collected = (Collector dia) 2 Aperture area over which light is concentrated (Receiver dia) 2

Solar Thermal Applications 1.Heating and Cooling of buildings 2. Solar water and air heating 3. Salt production by evaporation of seawater 4. Solar distillation 5. Solar drying of agricultural products 6. Solar cookers 7. Solar water pumping 8. Solar ponds 9.Solar refrigeration 10.Solar green houses 11.Solar thermal power generation 10. Solar furnaces 11. Industrial process heat

Solar Water heating This includes heating water for use in our home for cooking, bathing and cleaning, or for heating swimming pool water or even providing space heating in winter. These systems have been refined and improved and widely used over the years to the point where they pay for themselves in energy savings while making people’s lives more comfortable and lowering their energy bills.

Simplest heating system

Popular applications in the United States include facilities that use a great deal of hot water, such as this army laundry in Massachusetts.

Energy units and their conversion 1 joule=1 N-m 1 kWh=3600kJ 1 calorie=4.182 J 1 BTU =1055 J (British thermal unit) 1 megajoule=278 kWh 1 TOE =11634 kWh ( ton of oil equivalent)