Abstract:
A method to control the temperature of a circulating fluid and thereby control the electrical output of a PV array is provided along with an apparatus for doing so which adds simple mechanical, data measurement and control elements to prior art systems. Given a set amount of sunlight, electrical output from a solar PV array will change if the temperature of the array changes. One can change the temperature of a PV array by circulating a fluid through a loop in thermal contact with the array. Controlling the temperature of this circulating fluid, allows one to control the electrical output from the array.

Description:
BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to the field of controlling photovoltaic solar arrays and more particularly to the control of the temperature of a working fluid circulating through a photovoltaic solar array to enhance control of the power output from the array. 
         [0003]    2. Description of the Prior Art 
         [0004]    There is growing interest worldwide in reliable and predictable low carbon energy sources such as solar photovoltaic (PV), as the cost of solar power falls relative to fossil generated power. Worldwide PV capacity has recently expanded enough that PV solar arrays in the tens of megawatts in size exist, and larger PV solar arrays are being proposed (i.e. utility scale PV plants). Despite the increased interest and decreased cost, many utilities have been slow to adopt or invest in solar PV arrays for power generation. One complaint frequently cited by utilities is “intermittency of sunlight” or a lack of control over output levels for PV arrays relative to fossil generating technologies. Utilities are used to generating more or less power to meet demand. Simply stated a PV array&#39;s power output is primarily dependant on sunlight levels which are hard to predict, let alone control, for any given instant. For such utilities a PV array would provide greater value if they could control its output. 
         [0005]    It is a well established fact that solar PV panels (especially silicon based technologies) suffer from reduced electrical output as their operating temperature increases. It is also common for PV panels to be located in areas with lots of sunlight such that PV panels tend to operate at elevated temperatures. This loss of output can be reversed by lowering the PV panel operating temperature. The prior art teaches numerous methods and arrangements for circulating a working fluid through panels and arrays to reduce the temperature at which they operate, thereby increasing the output from the panels in proportion to the temperature reduction. Frequently this circulating fluid is contained in a loop which may have a reservoir for storing additional working fluid. Many times the heat removed from the PV panel by the working fluid is then used for some additional purpose. 
         [0006]    In U.S. Pat. No. 2,946,945 Regnier et al. teach of a method to improve output from PV solar cells, by circulating a fluid in thermal contact with the PV cells such that the fluid lowers the PV cells operating temperature enabling an increased PV output compared to the output if the fluid were not present. Regnier et al. envisioned using the heat removed from the PV cells by the circulating fluid to heat a battery, improving both the battery&#39;s performance and the PV cells&#39; performance. In U.S. Pat. No. 3,976,508 Mlavsky teaches the use of a tubular solar cell device which may be used with a concentrator and cooled by a fluid circulating inside the tubular cells, and using this heated fluid in turn to provide hot water. There are numerous additional examples of prior art PV panels with and without concentration accessories designed to use a fluid in thermal contact with PV solar cells to both improve the performance of the cells (i.e. increase output by cooling the cells) and utilize the resulting heat removed by the working fluid. 
         [0007]    Unfortunately few such “hybrid” systems have been widely adopted despite the well understood benefits. Perhaps this is because the greater complexity, design, material, and operating costs of maintaining both a PV electrical system and a circulating fluid system exceed the limited extra output generated by such a hybrid system. 
         [0008]    It would be advantageous to have a method and apparatus that provides the user greater control of a PV solar array&#39;s output, especially if the user were a utility. The flexibility that greater control over the PV array&#39;s output might increase the value of PV within the context of managing multiple power generating assets. This would enable much wider use and benefit from our most abundant energy source, the sun. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention relates to a method and apparatus for controlling the power output of a PV solar array. Given a fixed amount of sunlight, one can increase (or decrease) the output from a PV array, panel, or cell by decreasing (or increasing) the operating temperature of the PV array, panel, or cell, via a working fluid in thermal contact with the array, panel, or cell. One can control the operating temperature of a working fluid by adding a cold temperature fluid reservoir (and a hot temperature fluid reservoir) and a heat exchanger or controllable mixing valve, to regulate the working fluid temperature, such that a desired PV array operating temperature is achieved. 
         [0010]    If the amount of sunlight reaching a prior art PV array fluctuates, then the output of the PV array varies in proportion. But if one controls the PV array&#39;s operating temperature, via the control of the working fluid temperature, one can control the output from the solar PV array to offset the variations in sunlight levels. Thus one can produce a consistent PV output given some variation in light levels by modifying the PV array&#39;s operating temperature. Since the operator can select the appropriate working fluid, as well as the hot and cold reservoir temperatures, in principle this setup enables substantial control over the output of the PV array. 
         [0011]    It is an object of the present invention to provide control over the output of a PV array. 
         [0012]    It is another object of the present invention to measure and control the temperature of a working fluid, which is then used to control the temperature of a PV array. 
         [0013]    Finally it is another object of the present invention to increase the range of temperatures over which one may control the temperature of a PV array. 
         [0014]    By adding a cold temperature reservoir and a heat exchanger, or controllable mixing valve, to a system that had a single circulating loop with a fluid at a relatively fixed temperature, one is able to modify (primarily decrease) the operating temperature of the PV array from what it would have otherwise been, boosting the amount of power the PV array can produce at any given light level. And by adding a hot temperature reservoir and a heat exchanger, or controllable mixing valve, one is further able to modify (primarily increase) the operating temperature of the PV array from what it would have otherwise been, reducing the amount of power the PV array can produce at any given light level. 
         [0015]    Intentionally increasing the operating temperature of a PV array (and thus reducing the power output from the array) goes against common sense in the field of solar PV devices, but doing so can be advantageous for two reasons. First this increases the temperature range over which one can control a PV array&#39;s output, which grants one control of a greater fraction of the total output. Secondly, it allows an operator, such as a utility, to manage the PV array output in a way that maximizes the value of all its generating assets in combination, rather than just adding a PV array&#39;s output (whatever it might be) on top of its existing generating profile. 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0016]    Attention is drawn to the following illustrations presented to aid in understanding the present invention. 
           [0017]      FIG. 1  shows a schematic view of a prior art PV array with a circulating fluid loop. 
           [0018]      FIG. 1A  shows a plan view of the upper right corner of a prior art PV array with a circulating fluid loop, showing the fluid loop—dashed lines—behind a PV panel. 
           [0019]      FIG. 1B  shows a side view of a panel in a prior art PV array with a circulating fluid loop, showing the fluid loop in thermal contact with and under PV material in the panel. 
           [0020]      FIG. 1C  shows a plan view of the upper right corner of a prior art PV array built with concentrating optics and with a circulating fluid loop, showing the fluid loop—dashed lines—behind a PV panel. 
           [0021]      FIG. 2  shows a schematic view of an embodiment of the present invention showing a computer for automatic control, a circulating fluid loop with a fluid reservoir, a cold temperature fluid reservoir, a heat transfer device, a PV array, a second heat transfer device, and multiple temperature sensors. 
           [0022]      FIG. 2A  shows a cut away view of a heat transfer device used to control the circulating fluid loop temperature, namely a heat exchanger. 
           [0023]      FIG. 3  shows a schematic view of the preferred embodiment of the present invention showing a computer for automatic control, a circulating fluid loop with a fluid reservoir, a cold temperature fluid reservoir, a hot temperature fluid reservoir, a heat transfer device, a PV array, a second heat transfer device, and multiple temperature sensors. 
           [0024]      FIG. 3A  shows a cut away view of a heat transfer device used to control the circulating fluid loop temperature, namely an adjustable valve that allows the fluid from different reservoirs to mix directly. 
       
    
    
       [0025]    Several drawings and illustrations have been presented to better explain the construction and functioning of embodiments of the present invention. The scope of the present invention is not limited to what is shown in the figures. 
       DESCRIPTION OF THE INVENTION 
       [0026]      FIG. 1  and  FIG. 1A  show a prior art photovoltaic (PV) array with a circulating fluid loop. This is often called a hybrid solar array because it produces both electrical energy and thermal energy (in the form of a fluid heated by the array). A hybrid solar array  20  consists of an array of PV panels  26 , a fluid reservoir  22 , a circulating fluid (not shown) and a circulating fluid conduit  24  in thermal contact with the PV panels  26 .  FIG. 1B  shows how an individual panel in the array might be constructed. A transparent panel cover  28  is located above a PV material  30  which in turn is located above a thermal contact medium  32  which is in thermal contact with both the PV material  30  and the circulating fluid conduit  24 .  FIG. 1C  shows one way in which a concentrating optics  34 , in this case reflectors, may be used to increase the amount of sunlight reaching PV material  30 . These figures are intended to simply show the principle elements present in prior art hybrid solar arrays, there are numerous additional ways to construct hybrid solar arrays. 
         [0027]      FIG. 2  shows a schematic view of the preferred embodiment of the present invention, which may be called a dynamic solar array  40 . From left to right the principle elements of the invention are a controller  42 , a first fluid reservoir  44  containing fluid at a first temperature, a second fluid reservoir  46  containing fluid at a second temperature, a first heat transfer device  50  (upper unit), which delivers a fluid at some intermediate temperature to a circulating fluid loop, in thermal contact with PV array  26 . Heat transfer device  50  can be any device for regulating the temperature of two fluid streams, such as a heat exchanger or fluid mixing valve. The circulating fluid loop while not explicitly labeled in  FIG. 2 , consists of circulating fluid conduit  24  as was shown in the prior art  FIG. 1A ,  FIG. 1B , &amp;  FIG. 1C .  FIG. 2  also shows multiple temperature sensors  52 , and a light level sensor  54  that provide feedback/data to controller  42 . Finally  FIG. 2  shows a second heat transfer device  50  (lower unit) which may be used to recover heat gained by the fluid in traversing PV array  26 , or otherwise condition the fluid to be returned to either first fluid reservoir  44  or second fluid reservoir  46 . 
         [0028]      FIG. 2A  shows an embodiment of heat transfer device  50 , which is essentially a heat exchanger  56 . When controller  42  is able to control both the fluid flow rates, and measure via temperature sensors  52 , (or calculate) the input and output temperature of each fluid, heat exchanger  56  can deliver fluid at a desired temperature between the first fluid temperature and the second fluid temperature. 
         [0029]      FIG. 3  shows a schematic view of an alternate embodiment of the present invention, which may be called a dynamic solar array  40 . From left to right the principle elements of the invention are a controller  42 , a first fluid reservoir  44  containing fluid at a first temperature, a second fluid reservoir  46  containing fluid at a second temperature, a third fluid reservoir  48  containing fluid at a third temperature, a first heat transfer device  50  (upper unit), which delivers a fluid at some intermediate temperature to a circulating fluid loop, in thermal contact with PV array  26 . Heat transfer device  50  can be any device for regulating the temperature of up to three fluid streams, such as a heat exchanger or fluid mixing valve. The circulating fluid loop is (again) not explicitly labeled in  FIG. 3 , but consists of circulating fluid conduit  24  as was shown in the prior art  FIG. 1A ,  FIG. 1B , &amp;  FIG. 1C .  FIG. 3  also shows multiple temperature sensors  52 , and a light level sensor  54  that provide feedback/data to controller  42 . Finally  FIG. 3  shows a second heat transfer device  50  (lower unit) which may be used to recover heat gained by the fluid in traversing PV array  26 , or otherwise condition the fluid to be returned to either first fluid reservoir  44 , or second fluid reservoir  46 , or third fluid reservoir  48 . 
         [0030]      FIG. 3A  shows an embodiment of heat transfer device  50 , which is essentially a fluid mixing valve  58 . When controller  42  is able to control the three fluid flow rates, and measure via temperature sensor(s)  52 , or calculate, the input and output temperature of each fluid, fluid mixing valve  58  can deliver fluid at a desired temperature between the highest and lowest fluid temperatures. 
       Operation of the Invention 
       [0031]    The purpose of this invention is to increase the control that a solar array owner or operator has over the output of a solar array. Although one may not always be able to control how much sunlight a PV array receives, one can control the temperature of the PV array which allows one to change the output. The degree of control will be limited by the temperature difference between the two fluid reservoirs (or the range of hottest and coldest fluid temperature if three reservoirs are used) so one will want a wide temperature range. A PV array operator can increase, decrease or hold steady the PV array output. 
         [0032]    Assuming a steady amount of sunlight falls on the preferred embodiment of this invention, and further assuming controller  42  is circulating the fluid through PV array  26  at a temperature (T.sub.m), in the middle of the available temperature range (T.sub.high-T.sub.low), equal amounts of fluid enter heat transfer device  50  (upper unit) from first fluid reservoir  44  and from second fluid reservoir  46  which by assumption is at the lower temperature (T.sub.low). 
         [0033]    If the operator wants to increase the output from PV array  26 , the operator can set controller  42  to lower the temperature of the circulating fluid. Controller  42  lowers the circulating fluid temperature by increasing the flow of fluid to heat transfer device  50  from second fluid reservoir  46  and/or decreasing the flow of fluid from first fluid reservoir  44 , until the desired circulating fluid temperature (T.sub.d) is achieved, so long as the desired fluid temperature (T.sub.d) is greater than or equal to the second fluid temperature (T.sub.low). In this scenario, multiple temperature sensors  52  provide fluid temperature feedback to controller  42  to help the circulating fluid reach and maintain the desired temperature (T.sub.d). 
         [0034]    If the operator wants to decrease the output from PV array  26 , the operator can set controller  42  to raise the temperature of the circulating fluid. Controller  42  raises the circulating fluid temperature by decreasing the flow of fluid to heat transfer device  50  (upper unit) from second fluid reservoir  46  and/or increasing the flow of fluid from first fluid reservoir  44 , until the desired circulating fluid temperature (T.sub.d) is achieved, so long as the desired fluid temperature (T.sub.d) is less than or equal to the first fluid temperature (T.sub.high). Again multiple temperature sensors  52  provide fluid temperature feedback to controller  42  to help the circulating fluid reach and maintain the desired temperature (T.sub.d). 
         [0035]    Finally we consider sunlight levels that are not steady, but an operator who wishes to generate a consistent amount of power. If light levels decrease which may be measured with light level sensor  54 , the operator can increase output by setting controller  42  to lower the temperature of the circulating fluid, as described above. And if light levels later increase, the operator can decrease output by setting controller  42  to raise the temperature of the circulating fluid, as is described above. 
         [0036]    An alternative embodiment of the invention with third fluid reservoir  48  containing fluid assumed to be at a higher temperature than first fluid reservoir  44  works much the same way, but with three possible fluid flows to heat transfer device  50  for controller  42  to manage, along with a greater range of temperatures that the circulating fluid can achieve. 
         [0037]    In an alternate embodiment, heat transfer device  50  can be constructed from a series of two standard double-flow valves, where the first standard double-flow valve combines two of the fluid flows, and the second standard double-flow valve combines the third fluid flow with this combined fluid flow. 
         [0038]    In any embodiment of this invention the fluid exiting PV array  26  may be at a higher temperature than when it entered PV array  26 . The second heat transfer device  50  (lower unit) can be used to extract this extra heat for some additional purpose, or use the extra heat to condition the circulating fluid for return to one or more fluid reservoirs. 
         [0039]    For any given PV technology it is simple to calculate (or measure) the amount of electrical output change produced by a fixed change in circulating fluid temperature, allowing one to model and predict the amount of control a dynamic solar array will produce. Conversely one can calculate the type of dynamic solar array inputs, reservoir temperatures, flow rates and PV material necessary to produce a given level of output control. 
         [0040]    Several descriptions and illustrations have been presented to aid in understanding the structure and functioning of the present invention. One skilled in the art will realize that numerous changes and variations are possible without departing from the spirit of the invention. Each of these changes and variations is within the scope of the present invention.