FIELD OF THE INVENTION
[0001] The present invention is related to the novel design of combined outer spherical cum inner cylindrical solar furnace, which is used for generating steam, that may be used for various household and industrial applications e.g., solar based cooking, textile treatment, pasteurization, baking, air conditioning, desalination, rural electrification through power generation and in some specific heat treatment processes. This invention refers to the performance improvement of a solar furnace thorough two staged heating of working fluid as ordinary / demineralized water. A device design is unveiled that concentrates the incident solar energy in increasing proportions subsequently at lower and higher temperature solar furnace stages.
BACKGROUND OF THE INVENTION
[0002] In the last decade significant research has been taken place in the vast area of solar energy utilization which is very important for reducing our dependency on non-renewable energy source such as finite fossil fuel reserve. Solar energy has been identified as most promising renewable energy source and significant research has been made in the evolution of solar technology such as photovoltaic solar cell to climate responsive building and solar thermal system. The solar thermal system is unique among all other solar technology as it illustrates one of the assets of the direct use of solar energy without converting in any other form.
Two Stage Solar Furnace
[0003] The solar thermal system has been in use even before started using the solar photovoltaic technology. Solar thermal heater of natural convection type are in use since the beginning of twentieth century and remained continued till about 1940 until natural gas and oil resource were explored. But as on now the demand has increased to a great extent, the resources of oil and natural gas have become limited and hence the demand of solar thermal has increased again in last decade, so is the evolution in solar technology become important. Different types of solar thermal collectors and thermal absorbers have been proposed and evolved by many scientists and research scholars but still there is a huge scope for novel and good performing systems.
[0004] The solar thermal system generally consists of a central receiving system which uses solar collector (a plane mirror or parabolic dish collector or compound parabolic collector) that capture the sunlight and focus it to the energy receiver or heat absorber. The receiver or absorber’s heat is used to heat a fluid or heat transfer medium, which is generally water (ordinary or demineralized (DM)); to generate steam for driving turbine which when coupled with generator generates electricity. But the whole embodiment is not as simple as it seems at first sight. It involves lot of challenges such as designing a collector, optimizing glazing glass thickness, handling/minimizing unavoidable energy losses from high temperature surfaces, safety precaution for life threatening temperature of steam etc. The greatest challenge is to devise a high temperature device that operates on the dilute form of incident solar energy. So a compact self-contained, efficient and sager solar thermal unit which can provide a tangible and efficient energy is desirable. These above aspects and challenges have been kept in mind by the inventor while designing this furnace.
Two Stage Solar Furnace
SUMMARY OF THE INVENTION
[0005] The proposed invention describes a novel conceptual design of a solar furnace which is used to capture maximum amount of incident solar radiation and to transfer it maximally to the working fluid flowing through two stages; operating at low and high temperature. The gradual heating of working fluid in two stages yields better performance of the solar furnace, which is an innovative aspect of this creation.
[0006] The main objectiveof the present invention is to develop a solar furnace which can be used for raising temperature of a working fluid as ordinary water/DM water. The water after heated in the said solar furnace produces steam which can be used for various applications discussed.
[0007] This device receives concentrated solar flux either through compound parabolic collector (CPC) or parabolic dish collectors. These collectors direct and focus solar thermal energy on the main opening of the said furnace which is a combination of spherical stage-I (outer)-cylindrical (inner) stage-II. The spherical enclosure is made up of copper metal which is internally coated with Zinc and it is externally brazed with copper tube. A separate layer of insulting material (Asbestos/Glass wool) is provided on the exterior of the whole embodiment to avoid heat loss to the environment. A tube formed in cylindrical shape is placed at the center of the said spherical furnace and is in line with solar energy entering through the glazing glass.
[0008] The opening of the said furnace of optimum dimension is covered by a glazing glass that suits for admitting incident concentrated solar flux and to prevent the convection as well as
Two Stage Solar Furnace
radiation heat losses from the furnace internal parts. The glazing material used is simple glass having optimum thickness.
[0009] The energy captured inside the spherical enclosure is absorbed primarily by the cylinder formed using helical coils. The energy passed un-captured or un-intercepted though the cylindrical coil shall be absorbed by the interior of copper enclosure which is internally coated with Zinc. The Zinc coating is well-known for its high absorptive coefficient at high temperatures. The tube material used is copper which is conductive, corrosion resistant and can sustain high temperature levels. The working fluid is first passed through the copper tubes which are brazed on the exterior of copper sphere (acting as stage I, lower temperature stage) and then it is directed to flow through the tube formed in cylindrical-helical shaped (acting as stage II, higher temperature stage) the axis of which is in-line with the incoming concentrated flux passed through by the glazing. The internal cylindrical coil which is subjected to higher heat flux is capable to deliver a high temperature to the working fluid. However, the outer spherical coiled stage subjecting little dilute heat flux is used for preheating the working fluid, termed here as low temperature stage. The tube used to form internal cylindrical enclosure is made up of finned copper tube suited for maximum heat absorption. The working fluid can be heated gradually in the stage I up to a low temperature which is later superheated in the stage II of solar furnaceupto 450˚C. The proposed solar furnace is totally self-reliant as it does not require any external power except solar energy.
[0010] The tube used for manufacturing of a high temperature cylindrical stage-II is made up of finned tube for better heat absorption. For spherical shaped stage-I, un-finned (smooth) copper
Two Stage Solar Furnace
tube is wound over the copper spherical shell. The heat transfer medium first passed through stage-I and then it is directed to flow through stage-II. The fluid comes out at the exit of stage-II as high temperature (superheated) steam that can be used for applications discussed.
DESCRIPTION OF THE APPARATUS DRAWINGS
[0011] Fig 1-Fig. 1 demonstrates the top view of the said two stage solar furnace. The element 1 is the outer layer which is made up of insulating material6. Here insulating material used is asbestos (glass wool can also be an alternative). The element 2 is a circular glass cover which is fastened on the opening of the said furnace with the help of frame 3. The concentrated solar thermal energy is focused inside the glass cover2 with the help of compound parabolic collector (CPC). The focused thermal energy is admitted by the glass cover 2 which acts as glazing material. The glass used here provides good transmittance to solar thermal energy but does not allow energy reradiated or lost from the interior of the furnace which is at very high temperature.
[0012] Fig 6- Fig. 6 illustrates the cylindrical shell formed using finned tube 7b. The thermal energy thus entered through glass cover strikes on element 7b which is fined copper tube shaped as a coiled cylinder. The energy thus falling on finned copper tube is absorbed and conducted to the working fluid flowing through the tube 7b.The zoomed section of element 7b, exhibiting fins, is shown in Fig. 7.The cylindrical shell16 is placed in such a way that its axis is in line with the thermal energy enters through glass. Some part of energy left un- intercepted and some reradiated from element 7b reaches to copper shell 8. The copper shell is coated from inside with Zinc for better heat absorption. The energy reaching the inner surface is thus conducted through copper shell 8wall and to the element 7a.
Two Stage Solar Furnace
[0013] Fig 4- Fig. 4 demonstrates low temperature stage I formed using spirally wound copper tube7a at optimum pitch (distance). The energy conducted through copper shell 8is absorbed by the working fluid flowing through element 7a. The copper tube 7ais brazed on the exterior of copper shell8.
[0014] Fig 2- Fig. 2 demonstrates the front view of the said furnace. The working fluid entering at main inlet 13is circulated by pump 12. It enters low temperature stage I at inlet14 and circulates through the spirally wound copper tube 7a. The working fluid thus entered at 14 passed through the copper tube 7a, gains heat energy from the copper shell 8. The outlet9 of element 7a is connected to the inlet of element 7bthrough gate valve 4. As discussed earlier, the element 7b forms a cylindrical shaped high temperature stage exposed to high concentration radiation heat flux. The preheated working fluid, getting superheated while flowing through stage II formed using element 7b,comes out at exit11 as a superheated steam at temperature of 450˚C and pressure of 10 bar. At exit 11 the working fluid having higher energy content can be used for the applications discussed.
[0015] Fig 3-Fig. 5 portrays cross section of an assembly of spherical shaped low temperature stage I, consist of copper shell 8 brazed with a copper tube 7a insulated from outside using Asbestos insulation 6.
[0016] Fig 5-Fig. 5 depicts only the positions of heating elements 7a and 7b.
Two Stage Solar Furnace
[0017] The proposed invention can also be used for applications demanding lower temperature working fluid. Utilizing steam for low temperature purposes, the valve 14 need to be opened before closing the valve 4, which redirects the preheated working fluid to the high temperature stage of the receiver. Closing of valve 4 prevents entry of preheated working fluid to element 10 (which is an inlet to the high temperature stage II). The opening of valve 14 provides direct supply of working fluid at exit5to the appliances working at lower temperatures.
[0018] While providing superheated steam for higher temperature applications, gate valve 4 need to open prior to closing of gate valve 15 which is an exit for use at low/medium energy applications. The opening of gate valve 4 will lead the preheated fluid through the high temperature cylindrical shaped furnace stage 7b and provides superheated steam at higher pressure for utilization at higher energy applications.
[0019] The copper shell8 along with brazed tube on it, is completely isolated from the outer ambience by the provision of highly insulating material. The insulating material used is Asbestos having higher thermal resistance.
DESIGNED SPECIFICATIONS
[0020] The solar furnace has been designed for an electrical power rating of 5 kW. The thermal energy collection targeted to achieve at solar furnace is equal to 19.5 kW (by incorporating energy conversion efficiencies). In order to concentrate 19.5 kW at the said solar furnace the area required for the collection by the compound parabolic collector (CPC) or by parabolic dish collector is 24.41 m2.
Two Stage Solar Furnace
[0021] The designed working fluid inlet and outlet temperatures are 350C (308K) and 4500C (723K), respectively. The steam leaves out of solar furnace at a dryness fraction of 0.85. The system is designed for a pressure of 10 bar and mass flow rate of 0.0046 kg/s to 0.00655 kg/s. The copper tube is used for the fabrication of solar furnace of dimensions 0.5 inch outer diameter and wall thickness of 22 SWG (Standard Wire Gauge). The tubes are to be brazed with the copper shell 8.
[0022] The said invention can be seen as an embodiment that provides high temperature working fluid though utilization of concentrated form of solar energy. The said furnace can be fabricated at reasonable cost.

We claim,
1. A two staged solar furnace,meant for supply of superheated steam. It comprises;
2. A spherical shell, having circular opening covered with glass, wounded and brazedwith a copper tube from outsideact as a low temperature stage I for preheating working fluid.
3. A cylindrical shell formed using finned copper tube inserted through the mouth of spherical shell is acting as a high temperature stage II.