Conversion of solar energy to electrical and/or heat energy

This application is a divisional of U.S. application Ser. No. 11/650,739, filed Jan. 8, 2007, which is incorporated herein by reference in its entirety.

Improvements in the conversion of solar energy to electrical and/or heat energy are described. More particularly, a system of mirrors and lenses/prisms for economically collecting solar energy and converting it to electrical and/or heat energy is described.

Solar energy has been a desirable energy source for over thirty years. However, cost has always been an obstacle to its widespread use. The most familiar solar energy systems comprise an array of solar cells that cover enough area, or intercepts enough incident sunlight, to yield the desired amount of electrical power at relatively low conversion efficiencies of ten to fifteen percent (10%-15%). This approach requires large areas of expensive semi-conductor solar cells. To date, these systems have been uncompetitive without cost subsidies of some sort. In general, the prohibitive cost of solar energy systems has been primary due to the cost and the quantities required of the semi-conductor conversion devices called solar cells. There have been several approaches to alleviating the cost issue. One approach is to fabricate thin-film solar cells that use only a minimal amount of semi-conductor material. Unfortunately, this approach generates still lower efficiencies, six to eight percent (6%-8%) and the materials have proven to be problematic. A second approach has been used various optical devices such as fresnel lenses or mirrors to concentrate the solar energy to higher intensity and then convert it using a smaller area of the expensive solar cells. All of these approaches have been, and are still being pursued. None to date have resulted in economical solar energy generation without some sort of financial incentives being offered by the utilities or by government agencies. There is a need for a more economical way of collecting solar energy and converting it into electrical and/or thermal energy.

The solar energy collection system according to the various embodiments comprises a primary mirror and a secondary mirror. The primary mirror has a concave specular surface constructed and positioned to receive solar energy and focus it towards a focal point. The secondary mirror has a convex specular surface constructed and positioned to receive focused solar energy from the primary mirror and refocus it onto an annular receiver.

In an embodiment, the annular receiver includes an annular array of optical elements constructed to focus the solar energy received from the secondary specular surface onto a ring of discrete areas. In the various embodiments, a ring of solar-to-electrical conversion units are positioned on the ring of discrete areas.

In the various embodiments, the concave specular surface of the primary mirror is substantially parabolic. The convex specular surface of the secondary mirror is a hyperbolic surface modified to refocus the solar energy onto the annular receiver.

In the various embodiments, the annular receiver comprises a pattern of lenses/prisms arranged to further concentrate the solar energy and deliver it onto an annular array of photovoltaic cells.

The various embodiments also include methods of making the primary and secondary mirrors. It also relates to a relationship between the secondary mirror and an optical concentrator, and between the optical concentrator and a system of photovoltaic cells. The photovoltaic cells serve a dual function in the system. They absorb the concentrated sunlight and convert a portion of it to electricity and a portion to heat, or thermal energy. Thus, it serves as both an electrical generator and a heat generator. In order to accomplish these two roles efficiently, the photovoltaic cells are fabricated for semiconductor materials with sufficiently wide band gap to maintain the efficient electric conversion at relatively high temperatures. In general, the wider the band gap of the semiconductor material, the less the photovoltaic cells efficiency will be degraded with rising temperature. A tradeoff is required for the application being considered depending on the relative importance of electricity production and heat production.

The various embodiments provide a mirror that is composed of a thin metal body having a curved specular surface, comprising a polymer layer on said metal body surface, a reflective metal layer on the polymer layer, and a thin glass layer on the metal layer. This construction can be used for both the primary mirror and the secondary mirror.

In the various embodiments, the thin-metal body of the mirror is formed from sheet aluminum alloy. A particularly suitable alloy is aluminum alloy 6061 that has been hardened to a T-6 condition. The thin metal body is formed into a desired shape and then is rotated while the polymer layer, the reflective metal layer and the thin glass layer are successively applied to it.

In the various embodiments, the specular surface of the secondary mirror is a convex surface that has been shaped to cause it to reflect and focus light/heat energy received by it onto an annular focus area.

In the various embodiments, a system lends itself well to wide band gap photovoltaic cells having both single and multiple tandem junctions. To date, the cost of photovoltaic cells made from wide band gap materials and in multi-junction configurations has precluded use in terrestrial applications. The concentrator system produces a very high light intensity and allows the use of a small, economical area of photovoltaic cells.

The various embodiments include a unique design of high intensity photovoltaic cells. These cells have unique, long and narrow active areas that are optimum for two reasons. Firstly, the cell pattern corresponds to the illumination pattern provided by the tertiary concentrator lenses. Secondly, the cell pattern provides a very short path length for conducting the photo-generated current off the cells. Photovoltaic cells under light concentration operate very large currents at low voltage. Therefore, any series resistance in the cell would drop the voltage and, in turn, the efficiency of the cells. The current from the high intensity cells is collected and conducted off of the cells by means of a pattern of electrically conducting metal grids overlaying the active areas of the cells. The series resistance in the grids is proportional to the length of the grids. For this reason, the large narrow cell design with its electrical bus bar running parallel to the long dimension of the cells permits the necessary short conductor grids. As will hereinafter be described, the various embodiments include a construction of the photovoltaic cells and the pattern of such cells.

In the various embodiments, a solar energy conversion system converts solar to thermal energy in the form of hot water at useful temperatures while simultaneously converting solar power to electrical power at high efficiency. In the system, the concentrated solar energy is first absorbed by the photovoltaic cells. The photovoltaic cells convert a portion of the absorbed energy to electricity because the photovoltaic cells are made from wide band gap semiconductor materials, they can maintain high efficiency even at elevated temperatures.

In the various embodiments, a sensor and control system consisting of a sun sensor may be provided to supply sun position signals to a microcomputer that processes information and sends control signals to gear motors that drive the concentrators and hold them locked onto the sun to an accuracy of +/−0.1 degrees. The micro computer system further serves to shut the system down at night and position the primary mirrors to face the ground, wake the system up in the morning and acquire the sun, monitor the photovoltaic cell temperatures and drive the concentrators off sun if the cells overheat, monitor wind speed and rotate the concentrator mirrors to face down (edge-on to the wind) if wind speed exceeds a threshold amount.

Other objects, advantages and features will become apparent from the description set forth below, from the drawings, and from the principles that are embodied in the specific structures that are illustrated and described.