Solar radiation is a diffuse energy resource. Average solar radiation intensity on earth is around 800 W/m2, varying with location, weather and season. Normally, an apparatus for a solar energy application needs a large area to collect solar radiation. Therefore, to make solar power the main stream energy supply, a breakthrough has to be made in creating an extremely low cost solar collector.
A solar concentrator is widely used for condensing solar radiation and is proved as a promising means for realizing extremely low cost solar collection. Generally there are two types of solar concentrator: one is a Fresnel Lens (FS) based on refraction of light, and the other is a parabolic reflector based on reflection of light. Due to the decisive factor of cost, in most of the large scale solar energy application systems, reflective solar concentrators are adopted to collect and condense solar radiation.
Most reflective solar concentrators are designed into a concave reflector structure which is generally in either a parabola form or a parabolic trough form. Most, and probably near all, of these reflectors have been built of relatively rigid materials such as metals. Usually, rigid reflective components are molded and assembled into a predetermined geometric shape and retained by suitable fastening means. Since this type of apparatus is an open system, to protect against wind and load, the rigid reflective components and fastening parts must be made of materials with certain stiffness, thickness and mass. Conventional solar concentrators for large scale solar systems appear bulky, heavy and expensive. The manufacturing processes for forming precision geometric shapes for rigid parts are proved complicated, slow and expensive. In addition, in the open systems, the reflective surface of the reflector structure is directly exposed to the outdoor condition and lacks protection from dust and other contaminations.
Most solar concentrators are effective only with directly incident solar radiation, so most solar concentrators are driven by a power system and a control system to track the sun and maintain direct solar incidence. When the solar concentrator is heavy, it adds significant loads to the tracking system and makes it complicated and expensive.
Several types of solar concentrators using non-rigid, lightweight and cheap materials were proposed to reduce the weight and to lower the cost. Among them, the most typical is the inflatable reflector structure. This structure employs an inflatable balloon which, when inflated, provides the structural frame to hold the concentrator portion of the apparatus in the parabola or parabolic trough shape in the desired position. In some of this type of structure, the reflector surface which forms the concentrator consists of a metallic coating on the inner surface of the balloon and in some other structures the concentrator portion is formed by a diaphragm type structure which is located within the balloon and has its perimeter attached to the inside surface of the balloon so that when the balloon is inflated the diaphragm is pulled into its desired operating shape.
U.S. Pat. No. 2,977,596 to Justice et al. disclosed such a structure using a diaphragm means to form an operating shape. In this disclosure, an air source is employed to maintain the inner pressure of the balloon. U.S. Pat. No. 4,672,389 to Ulry disclosed an inflatable reflector structure in which the concave reflector surface is constructed of a non-rigid flexible material, which is maintained in position and form by means other than the strength of the material itself. Ulry employs an inflation means in communication with the interior of an envelope, which is essentially a balloon, to provide and maintain fluid at super ambient pressure within the interior of the envelope. In GB 2 104 644, Leroy disclosed an inflatable solar collector structure with an element shaped into a parabolic trough reflector and maintained by inflation means. The inflation means is used to generate an internal pressure of the element that is greater than atmosphere. In U.S. Pat. No. 4,328,792, Shores disclosed a solar heat collector with an element which is a closed structure. However this element is composed of two parts: a parabolic trough reflector and a transparent cover. These parts are conventional components made of rigid materials such as metals.
There are certain difficulties connected with these inflatable structures. One of the more obvious ones is the tendency of the balloon to become distorted due to wind and other factors. This distortion, of course, results in some distortion of the reflecting surface of the concentrator which is an undesirable characteristic. The inflatable structure erected by an internal pressure above the external pressure of the structure needs a support system to stand up, and therefore extra fastening means such as rims are necessary. An inflation means in communication with the interior of the structure must be provided to maintain the internal fluid at super ambient pressure within the interior of the structure. Almost all of inflatable systems are used for short term and temporary purposes. Inflatable systems are mainly used in remote areas as a portable and collapsible apparatus.
For large scale and permanent solar applications, solar collectors made of light-weight and low cost materials with a simple structure, a strong resistance to outdoor conditions, and the capability of being easily manufactured are long expected. Obviously, inflatable systems are not the candidates that can fulfill the mission.
My own pending patent Ser. No. 11/983,971 describes a reflector structure through which moldable, light-weight and low-cost materials, including glass, are used to manufacture a solar concentrator without loss of the quality and performance of conventional solar concentrators made of high quality rigid materials such as metals. This disclosure provides a non-inflatable reflector structure to eliminate the extra fastening means and inflation means of the inflatable structures so as to simplify the concentrator system, but still retain the integrity and performance that is usually associated with flexible inflated structures. This non-inflatable reflector structure avoids the distortion of the reflector surface caused by wind and other factors when in use, and guarantees the accuracy of the geometric shape of the reflector surface formed in the manufacture process. This disclosure provides an enclosed, non-inflatable reflector structure that is a single body of thin walled material that is constructed without any other elements so that it can be easily manufactured and utilized. This disclosure provides a non-inflatable reflector structure that is a closed structure, but which has an opening to connect the interior of the structure to the atmosphere so that the pressure within the interior of the structure is equal to the atmospheric pressure. Consequently the reflector structure is formed by the material strength itself rather than the internal pressure of the structure being above atmospheric pressure.
Although, the non-inflatable reflector structure disclosed in my pending patent Ser. No. 11/983,971 effectively overcomes the disadvantages of close structure reflective concentrators, it still suffers from three drawbacks. (1) Separate front receiver; the concentrator systems need receivers positioned on the focal points of the concentrators to convert the concentrated sunlight into electricity or heat. In most of the conventional configurations of the concentration systems including my pending patent Ser. No. 11/983, 971, the receivers are separated from the concentrators and mounted in front of the concentrators. This receiver and concentrator assembly makes it very difficult to design and install the receiver, as well as connect the receivers to the rest entire system. (2) Receiver support arms and shadowing; separated receivers need extra support arms to be mounted on the focal points. While, the extra support arms not only consume extra materials and make system complicated, but also cast shadow on the concentrator dishes. (3) Separated auxiliary optics for receivers; in most concentrator systems, an auxiliary optics are added to the receivers to accommodate the misalignment of the concentrated sunlight due to the limited precision of tracking systems. Normally, the auxiliary optics such as a lens or a CPC (Compound Parabolic Concentrator) separates from the concentrator. This configuration needs extra component and makes the system complicated.
The objective of the present invention is to provide a close reflector structure, which modifies the structure disclosed in my pending patent Ser. No. 11/983,971, but still retains all the advantages of the structure, to (1) move the receiver of the concentrator system to the back of the concentrator; (2) eliminate the support arms of the receivers to simplify the structure and save materials; (3) integrate the auxiliary optics into concentrator to further simplify the system structure. This disclosure is to incorporate a secondary concentrator and the auxiliary optics for receiver into the first concentrator to make the receiver and concentrator assembly one entity.
U.S. Pre Grant Publication N. 2007/0256726 to David K. Fork et al disclosed a laminated solar concentrating photovoltaic device. In Fork system, although a transparent cover and a convex second reflective surface are introduced, their transparent cover is a flat plate which is not shaped to form a parabola such that the second reflector is formed by coating a reflective layer on the parabola. Moreover, in Fork system, the second reflective surface is a convex surface which converges the light concentrated by the first reflector so that the incident light is continuously concentrated to the receiver. In the optics of Fork system, the concentration ratio and other optical parameters are controlled by the distance between the receiver and the second reflective surface.
U.S. Pat. No. 6,668,820 to Gilbert E. Cohen et al discloses a solar concentrator which includes a second reflective surface, in addition to the same concentrating optics as that of Fork system, it is an open system without transparent cover.
The present invention creates a concentrator which is a sealed transparent chamber that forms a closed structure named as “bulb” structure, where a cavity is formed by the chamber for vacuum and for filling in gases with certain compositions. The portions of the transparent body wall of the chamber are shaped into parabolas which serve as substrates for reflective layers coated on them to form the first reflector and the second reflector. The first reflector and the second reflector are configured in such a way that the focal point of the first reflector is overlapped with that of the second reflector to form a new optics in which the incident light concentrated by the first reflector is reflected by the second reflector to form a concentrated and collimated beam light. In the new optics, the concentration ratio and other optical parameters are controlled by the relative positions and features of the first reflector and the second reflector. The new optics does not involve the relative position of receiver. Overall, the present invention provides a concentrator with all its reflectors shaped on its body wall; and the entity of the concentrator body contains vacuum or gases with certain compositions.