Design and Construction of a Concentrated Solar Furnace

Abstract

This project is about the design and construction of a concentrated solar furnace. The basic aim of the report is to provide information about the design and working principles of a solar furnace. The project utilized a parabolic concentrator and we designed the furnace in a cylindrical shape with steel for its body and an outer copper lid. To improve the efficiency of the furnace in this project, copper fins were attached inside to enhance heat transfer within the furnace. After manufacturing and assembly, we tested the furnace with and without fins and there was a difference noticed after recordings done at 20 minutes interval. The inner air temperatures recorded after 20 minutes was 55 without fins and 65 with fins. During the testing, the maximum temperature recorded inside the furnace was around 173 (air temperature) and 238 (fin tip temperature); these temperatures are high enough to be used in various domestic applications such as cooking.

Background

Solar energy is undoubtedly the oldest source of energy. It is basically the radiant light and heat from the sun. We can however trace all other forms of energy used on earth back to the sun. Harnessing the solar energy has been the aim of various researches for many years. We believe that resources for fossil based fuels such as oil and coal are limited and soon they will vanish. Many scientists are working to use solar energy for different purposes.

The history of solar energy is as old as mankind and over the past two centuries this energy has become increasingly used either directly to produce electricity or in applications to satisfy different needs of mankind. One of such applications is in the production of solar furnaces which could be used for many purposes. A solar furnace is a structure that uses concentrated solar power to produce high temperatures, usually for industry. Parabolic mirrors or heliostats concentrate light onto a focal point. The temperature at the focal point may reach very high temperatures, and this heat can be used to generate electricity, melt steel, and make hydrogen fuel or nano-materials. A solar furnace uses reflectors to channel and concentrate solar energy thus producing heat. The sizes of solar furnaces could range from small to large depending on the desired heat needs. In the past, around the 7th century, people used magnifying glasses in simple form to generate concentrated light from the sun and use this domestically in making wood to catch fire for cooking. However over the years, modifications and improvements in the field of technology have led to the production of more sophisticated solar furnaces. The largest solar furnace is at Odeillo in the Pyrenees-Orientales in France, opened in 1970.

Objectives

The main aim of this project is to design and construct a working solar furnace. The major objective of the solar furnace design will be to generate heat from solar radiation as a high intensity energy source for high temperature processes. In this project the aim is to produce temperatures high enough to be used in cooking applications whether for domestic or industrial uses. 1.3 Limitations and Scope A viable option for this technology is that which this project will be limited to; producing temperatures high enough to cause combustion of wood (carbonization) and also efficient enough to aid cooking in homes. The constituents of a solar furnace will be well documented as the economic benefits of solar furnaces are huge. Nevertheless one constraint to its production is high initial start-up costs but conversely once started there is only little operational costs needed to continue. By replacing conventional furnaces, like electric arc and blast furnaces, with a solar furnace operating at high temperatures, carbon dioxide (CO2) emissions and energy consumption will be reduced greatly, which will better our society immensely. This project will discuss the topic of solar furnace in details by first giving details of other research work done on the subject- literature review, then enumerating the methodology, design and calculations of this project and then ends with a conclusion and discussion part.

A Brief History Of Solar furnace

A solar furnace is not actually a furnace but only implies an optical system which has solar radiation been received from a collector and concentrates it into a small area. In a case where this highly concentrated radiant energy is channeled into a cavity, high temperatures are obtained due to the heat generated. Actually, it is this cavity that is the furnace and it represents a small part of the entire system, hence it is not out of place, to call solar furnaces: - solar energy concentrators.

The idea of using solar energy to produce high temperatures is not new. In 212 B.C. Archimedes presumably set fire to the Roman fleet by concentrating the sun's rays on the ships by means of several hundred plane mirrors. In the 17th and 18th centuries both mirrors and lenses were used, and in 1772 Lavoisier built a furnace with a collecting lens having a diameter of about 5 feet, in which he almost reached the melting point of platinum (1773◦ C). After the work of Lavoisier, until the beginning of the twentieth century, solar furnaces were completely ignored. However in 1921 Straubel and his collaborators at the Zeiss Company in Germany constructed the first modern reflecting furnace. This was done with a glass parabolic mirror of about 6 feet diameter, and focal length of 2 feet; at the end temperatures above 3000◦C were generated. This paved way for the use of different sizes of parabolic mirrors. Straubel had another collaborator in the person of W. Conn, he built a 10 foot furnace and this was installed at Rockhurst College in Kansas USA. This furnace is made of aluminum alloy sheet and is still operational at Convair in San Diego and is used for high temperature materials studies. More so, searchlight mirrors of around 5 feet diameter are good concentrators and are in operation in numerous laboratories in the United States.

The largest installation of solar furnaces of various sizes is located in Montlouis in the French Pyrenees. Professor FelixTrombe, head of the laboratory for the study of solar energy, has six furnaces in operation and these are made of German parabolic searchlight mirrors. They are 6.5 feet in diameter and also there is one large furnace which is 35 feet in diameter. The large size reflector is made of 3500 smaller plane mirrors, attached to a steal frame which is parabolic in shape. To get a better focusing, each mirror used is bent mechanically and a curvature closes to 4 that of an ideal parabola is attained. Another larger furnace has been produced, this is more than 100 feet in diameter and after design its reflecting components have been tested. The power of this large installation is about 1000kW. Furthermore, Professor Guillemonat adopted W. Conn’s design and built a 27 feet diameter furnace in Algiers New Orleans. The parabola for this furnace was made of 144 panels of electro polished aluminum formed to the required curvature. Other notable works in this field includes that of the old soviet Russia, in which a large solar energy research laboratory was installed near Tachkent; however these were low temperature furnaces and no report has exists about solar furnaces designed especially for high temperatures. Lavoisier’s furnace was not the only one built to concentrate the sun’s energy by means of a lens. Between 1930 and 1932 the California institute of technology (Caltech) built a lens furnace and this was spear-headed by George Ellery Hale and its purpose was to achieve high temperatures for spectroscopic studies.This furnace is now positioned on the roof of the Caltech astrophysics department, and it is currently used for high temperature materials research. Furthermore, anyone conversant with the field of high temperatures will find out that the performances of already existing solar furnaces are not spectacular. It is possible to generate temperatures within the range of 3000◦ C through many different techniques: - these include, induction heating, melting in neutral atmosphere, electrical resistance heating, flames and so on. These techniques however have a limitation because they require a specific type of atmosphere around the specimen under study. Whereas for the solar furnaces, heat source is in form of a cone of radiation energy that can be termed as pure heat and this does not impose any restriction to the kind of atmosphere that surrounds the specimen. Another interesting feature of solar furnaces is that, the temperature obtainable at the focal area is concentrated and generates a high heat flux. Solar furnaces have the unique characteristic of being able to heat a body from inside to outside and this is useful in melting refractory substances which react very rapidly with crucible materials at high temperatures.

The Caltech furnace project makes use of the two outstanding features mentioned above. The first project is an investigation of highly refractory thorium oxide and zirconium oxide which have melting temperatures of 3200◦ C and 2700◦ C respectively. They melt these compositions in air at the focus of the furnace and assume oxidizing atmosphere around the melt. The second project is concerned with ceramic body structures and it is based on mixtures of titanium and zirconium dioxides, in which oxygen is usually lower than it should be. The California institute of technology (Caltech) solar furnace has nineteen lenses; they are each two feet in diameter, arranged hexagonally and pointed towards the sun.