Patent Number: 051246104
Section: summary

TECHNICAL FIELD The present invention pertains to the generation of electrical energy through the combination of a light source and a photovoltaic cell. In particular, this invention pertains to a long-life, electrical energy source generated by the combination of a radioisotope activated polymer material emitting a low level of light with a photovoltaic cell arranged in intimate optical contact with the light emitting polymer material, the light emitting polymer in the present invention being comprised of a tritiated organic polymer to which an organic phosphor or scintillant is bonded. BACKGROUND ART Various types of energy sources consisting of photocells activated by some type of nuclear radiation are known in the prior art. These devices, somtimes referred to as "nuclear batteries" or "atomic batteries", convert nuclear electromagnetic radiation into electrical energy by one of two methods, single conversion systems or double conversion systems. Single conversion nuclear batteries generate electrical energy by converting the nuclear radiation (i.e. alpha articles, beta particles or gamma radiation) into electrical energy by direct absorption of the nuclear radiation at the p-n junction of a semiconductor material, for example, U.S. Pat. Nos. 3,094,634 and 3,304,445. Double conversion nuclear batteries generate electrical energy by converting the nuclear radiation into electromagnetic radiation, usually by irradiating a phosphorescent material that will generate light in the visible spectrum, and then converting that electromagnetic radiation into electrical energy by absorption of the electromagnetic radiation at the p-n junction of a semiconductor material, usually a typical photovoltaic cell, for example, U.S. Pat. Nos. 3,031,519, 3,053,927, and 3,483,040. While the concept of a nuclear battery is not new, a practical and commercially feasible device of this type has not been possible because of the extreme dangers involved in the handling and use of radioactive materials. Most nuclear batteries of the type known in the prior art have either been unsafe or have required such extensive shielding of the nuclear material used to power the battery that the device is rendered impractical for most applications. The regulatory standards for radiation leakage upon container failure impose additional constraints that limit the applications for such devices. One possible means of overcoming these safety limitations is to limit the amount of radioactive material used in such a device. For example, in a typical smoke detector a small amount of radioactive foil containing one microcurie of radioactive Americium 241 is used to power the detection circuit of the device. In general, regulatory standards allow for small amounts of radioactive material to be used under certain circumstances. For example, with proper shielding and packaging, a device containing 5 curies of radioactive material may be approved by the Nuclear Regulatory Commission for limited commercial activities. These low limits on radioactive material effectively limit the radiation energy, and hence, the electrical energy that may be generated from any such source. Using the amount of radioactivity as measured in curies, the total amount of power available from such an energy source can be calculated. Each curie of radioactive material will produce 3.7.times.10.sup.10 Beqerels (decays)/second. Assuming that the radioactive emission is in the form of a beta particle from the radioisotope tritium having an average 5.6 KeV of energy, the total theoretical power emitted is 32.5 microwatts/curie. Theoretically, if there were a complete conversion of all of the power of this nuclear radiation to electrical energy, the total amount of power available from a small, but safe, amount of radioactive material containing less than 25 curies of tritium would be less than 1 milliwatt. Though the total amount of power generated by such a device over the half life of the tritium radioactive material may be on the order of a hundred watt-hours, until recently relatively few applications could operate with a continuous power supply outputting in the microwatt range. With the advent of CMOS and other low power circuitry, however, applications and uses for this type of long-life, low-watthour power supply are now becoming more practical. Although a variety of self-luminous, low light sources have been available for a long time (e.g. radium and tritium activated phosphors used for creating self-luminous paints for watch dials, etc., U.S. Pat. Nos. 3,033,797, 3,325,420 and 3,342,743), it has generally been regarded that such materials were unsuitable for commercial use for the conversion of light into electricity. The low levels of radioactivity associated with such materials, though generally not harmful or dangerous, do not provide an adequate source of power for the nuclear batteries of the type known in the prior art. In addition to the low light level (50 micro-lamberts or less), such sources may also be characterized by rapid and unpredictable light decay and, in the case of radium-activated light sources, may produce undesirable radiation hazards associated with their decay products. Though the concept of a long-life, electrical energy source activated by a radioactive material is attractive and has many potential applications, none of the prior art devices have been able to create a safe, yet sufficiently powerful, energy source that is commercially feasible. Accordingly, there is a continuing need to develop a safe and practical long-life, electrical energy source powered by a radioactive source. SUMMARY OF THE INVENTION In accordance with the present invention, an electrical energy source is created by the combination of a light emitting polymer material having at least one light emitting surface emitting light energy of a specified frequency bandwidth and a photovoltaic cell having a light collecting surface and a pair of electrical contacts. The light collecting surface of the photovoltaic cell is optically coupled with the light emitting surface of the light emitting polymer material. An open-circuit voltage is generated between the pair of electrical contacts as a result of the absorption of emitted light energy from the light emitting polymer material by the photovoltaic cell. The light emitting polymer comprises a mixture of a polymer labelled with a tritium and an organic compound which emits light energy when subjected to radiation generated by the tritium. The organic compound is at least partly bonded to the polymer and the mixture is translucent at the specified frequency bandwidth of the light energy. Maximum absorption of the emitted light energy is achieved by the intimate optical contact between the light emitting surface and the light collecting surface, by matching the maximum absorption frequency bandwidth of the photovoltaic cell with the specified frequency bandwidth of the emitted light energy from the light emitting polymer material, and by the structural arrangement of the light emitting polymer material itself. To maximize the surface area between the light emitting polymer and the photovoltaic cell, the light emitting surface and the light collecting surface are preferably arranged so that they are generally parallel to and in intimate contact with each other. In addition, the light emitting polymer material and the photovoltaic cell may be arranged to allow the photovoltaic cell to be constructed in manner so as to absorb light energy at more than a single surface. In another embodiment of the present invention, the light emitting polymer material is optically separated from the photovoltaic cell by an optical control means for controlling the amount of light that may pass through the optical control means to be absorbed by the photovoltaic cell. The optical control means may be a liquid crystal display (LCD) or lead lantium zirconium titinate (PZLT) or similar material that is either transparent or opaque, depending upon the voltage or current applied to the material. By controlling the amount of light that may be absorbed by the photovoltaic cell, the optical control means also controls the output of the photovoltaic cell and, hence, operates as either a voltage or current regulator, depending upon the particular circuit that utilizes the electrical energy source of the present invention. The optical control means allows the electrical energy source of the present invention to simulate an alternating current source from a direct current source without the need for electrical circuitry external to the electrical energy source. The present invention provides a novel radioisotope-activated, electrical energy source that exhibits several desirable characteristics. Foremost, the electrical energy source of the present invention is relatively safe and is, thus, viable for general commercial use when the quantities of radioactivity are generally below 100 curies. The low emissivity and high energy density of the preferred embodiment utilizing a tritiated organic polymer to which an organic phosphor or scintillant is bonded enable the electrical energy source to realistically utilize 4.0% or more of the theoretical 3.6 amp-hours of electrical energy that are present in each curie of tritium. In this embodiment, an electrical energy source having 100 curies of tritium is capable of providing 1 microwatt of power at 1 volt and 1 microamp for the entire lifetime of the electrical energy source, approximately 20 years. Because the electrical energy generated by the present invention is dependent upon the rate of emission of photons from the light emitting polymer (which is in turn dependent upon the rate of beta-emissions from the radioisotope used to activate the light emitting polymer), the amount of energy available is constant and determinable. In addition to providing a unique source of electrical energy for CMOS, NMOS and other low power types of electronic circuitry, the ouput stability of the electrical energy source of the present invention makes it ideally suited for applications that require a very constant source of power and ensure that it is not drained of its energy if subjected to a short-circuit. Moreover, the materials and packaging of the present invention can be selected to enable the electrical energy source to operate in a cryogenic environment without significant degradation of the power compared to conventional chemical batteries, because the rate of conversion of the photons by the photovoltaic cell is positively affected by decreasing temperature. Although light emitting polymers have been available for a number of years for various uses (primarily as self-luminescent paints), it is not known to use such light emitting polymers to power electrical energy source The present invention has discovered their usefulness for this purpose and, more importantly, the adaptability of light emitting polymers as compared to other prior art radioisotope vehicles to permit the design of electrical energy sources with greater efficiency and safety than in prior art devices. Accordingly, a primary objective of the present invention is to provide a safe, yet sufficiently powerful, long-life, radioisotope-powered electrical energy source that is commercially feasible. Another objective of the present invention is to provide a long-life source of electrical energy by the combination of a radioisotope-activated, light emitting polymer and a photovoltaic cell. A further objective of the present invention is to provide an electrical energy source wherein the conversion efficiency by a photovoltaic cell of light emitted by a light emitting polymer is maximized. An additional objective of the present invention is to provide an electrical energy source that includes an optical control means for controlling the amount of electrical energy generated by controlling the amount of light that is received by the photovoltaic cell from a light source. A still further objective of the present invention is to provide a long-life, electrical energy source that provides a consistent power output by generating electrical energy at a constant watt-hour rate. These and other objectives of the present invention will become apparent with reference to the drawings, the detailed description of the preferred embodiment and the appended claims.