Patent Application
20090308433
This invention relates generally to powering of optoelectric chips, and more particularly to use of solar energy for that purpose.
Alternative energy sources are becoming increasingly important in view of dwindling resources of fossil fuels and, more importantly, as a countermeasure against environmental pollution. Among the technologies pursued are wind farms, water turbines and solar energy farms. Water turbines may be considered the ecologically most invasive technology, whereas wind farms and solar plants are generally considered environmentally more friendly and desirable. Both technologies have their own use niches, particularly with respect to the feasibility of their installations, and based on environmental parameters such as the average number of sunny days, or the occurrence and patterns of wind, for example as caused by thermal convection.
Solar energy harvesting can be done in different ways, the most established technology in small scale use being the exposure of a radiator to sunlight for the purpose of heating up water that then can be used for general heating purposes. A more recent implementation is the use of optoelectric converters or optoelectric chips that generate electricity upon exposure to light.
A challenge associate with optoelectric chips for generating electricity is their temperature dependent efficiency derating, or short temperature derating, that is the dependency on low operating temperature. More specifically, exposure to sunlight will necessarily heat up the chip, but increasing temperatures will decrease the conversion efficiency of light to electrical energy. Accordingly, a prerequisite of operation of an efficient optoelectric chip is efficient cooling, to maintain highly efficient optoelectric conversion rates.
One method to collect solar energy is based on the use of reflectors that are focusing, and which concentrate collected light onto a small area occupied by the active die of the optoelectric chip. In that case, thermal management is difficult, because the position of the optoelectric chip in the path of sunlight demands the smallest possible chip size in order to avoid excessive shadow casting and, by extension reduction of the collection area. Small chip size, on the other hand, means a small surface area for heat dissipation into the environment. At the same time, positioning of the optoelectric chip at the focal point of the reflector requires its positioning at the highest point of the apparatus; this preempts the use of orientationally sensitive cooling technologies that rely on convection, as for example heat pipes.
The special idiosyncrasies of solar power create a unique conflict between cooling requirements and availability of cooling area with the additional problem of directional restrictions within the arrangement of components that require a novel solution. Accordingly, there is great need for apparatus and methods that obviate these difficulties and problems.
Most approaches for cooling optoelectric chips, as used for harvesting of solar energy, employ heat pipes or heat pipe related technology. In the latter, a partial vacuum is used to lower the boiling point of water to the desired temperature and to cause the evaporation of distilled water and the associated phase change for chilling of the heat source. Because of the directional sensitivity of this approach and the requirement for a condenser to be at higher elevation than the heat source, this type of cooling has only limited applicability in conjunction with the use with solar energy.
A different approach uses liquid cooling, however, the location of the hottest spot at the highest point of the system precludes the use of convection for moving the fluid; and standard pumps are instead used to move the fluid from the heat source to a radiator where the heat is dissipated into the environment. While liquid cooling is very efficient, the pumps also require use of electrical energy that reduces the net energy production.
The present invention utilizes fiber optics to conduct the collected light from the focus of the reflector to the optoelectric chip. The optoelectric chip is typically positioned at the back side of the primary solar reflector, and in a typical arrangement the chip is either shadowed by the reflector or else is integrated into the backside of the reflector. In either case, the optoelectric chip is typically located at a very low position relative to the rest of the reflector and collector apparatus. Verticality and orientation are crucial factors for desirable heat transfer characteristics, in that it is relatively easy to conduct heat upwards, particularly in designs using phase change such as heat pipes. Therefore, positioning the device to be cooled at the lowest or at a relatively lower point in the combination is advantageous for heat dissipation. This is important for maintaining high efficiency of the optoelectric conversion that degrades as a function of increasing temperature.
One additional beneficiary byproduct is that the optoelectric chip is not exposed to direct sunlight that could heat up the chip or its supporting structure. The light needed for optoelectric energy generation is typically conducted via fiber optics from the apex of the structure (located at the focal point of the re-focusing reflector) to the optoelectric chip.
Advantages of the current invention can be summarized as follows:
a shows details of the
The apparatus of present invention combines fiber optics with a cooling device for optoelectric chips. In the preferred embodiment, the light is focused by one reflector onto a secondary reflector 2 that further focuses the light onto the end 3a of a fiber optics system or conductor 3. The fiber optics then route the light away from the highest point in the system to a lower point, preferably in the shadow of and is alignment with the second reflector 2. See protective tubular walls 50 and 51 defining fluid coolant channels 8 and 10. This type of placement ensures that there is no additional exposure of the photovoltaic chip 4 or its assembly to direct sunlight, and thereby avoids additional heating up of the device. As a result, the optoelectric chip is located at the coolest portion of the entire solar energy apparatus. In addition, the location underneath the reflector 1 allows provision of a large auxiliary cooling apparatus 5 and 6 without incurring the problem of casting shadows on any light-collecting structure.
With respect to the actual arrangement of the fiber optics, several different embodiments are possible. One possibility entails having the fiber optics receiving light directly from the reflector and then bending down to the lower part of the apparatus in a goose-neck fashion to transmit the light towards the optoelectric chip. In this particular embodiment, the fiber optics must be relatively long in order to accommodate the curved route, which results in higher materials cost and lower efficiency with respect to light transmission.
A greatly simplified and preferred configuration employs the fiber optics running in axial direction upward from the center of the parabolic first reflector 1. In this case, the distal face of the fiber optics points upward, that is away from the first reflector 1. In order to receive light, therefore, an additional mirror is provided at 2 to reflect the light back onto the end face 3a of the fiber optics conductor 3. The fiber optics further pass through the center of the first reflector 1 to its back side where the optoelectric chip 4 is typically located. The advantage of this particular arrangement is that the fiber optics are routed the shortest way in a straight line from their light receiving face to the emitting end. Moreover, since the fiber optics extend in axial direction away from the center of the first reflector, they are oriented in parallel with the incoming solar rays and, consequently do not cast any further shadows that would reduce the efficiency of the solar energy-collecting apparatus.
On average, the highest amount of solar energy is collected when the sun it at its apex. The reflector is always tracking the sun using a rotatable platform for the azimuth and a tilting mechanism for adjusting the altitude. Therefore, during peak exposure times, the fiber optics will extend upwards in a substantially vertical direction, which is advantageous for creating buoyancy as a function of thermal gradients and using the buoyancy for fluid movement. This greatly facilitates the implementation of liquid cooling. In this case, the optoelectric chip 4, which absorbs the sunlight conducted by the fiber optics and consequently generates a substantial amount of heat, is positioned at the bottom of the assembly and gives off or transfers heat to the coolant used as at 5a. The coolant absorbs the heat from the optoelectric chip, thereby warming up and, as a consequence develops buoyancy. A channel or path 8 seen in
The backside of the radiator can be equipped with fins 14 for increased surface to dissipate the heat into the environment. The inside of the radiator preferably contains a network of micro-channels that can be formed for example by embedding a mesh 25 (see
Reflector 1 in
