Abstract:
The present invention proposes a hybrid methodology and apparatus between photovoltaic (PV) and concentrated photovoltaic (CPV) solar panels to lower the photovoltaic solar energy production cost. In particular, the disclosed methodology addresses a simple quasi-parabolic trough PV (QPTPV) low concentration system with greater tolerance to tracker pointing errors. The quasi-parabolic trough (QPT) reflector is defocused to cover the width of a linear solar cells array, which is reduced from a large rectangular solar cells panel. In summary, the QPTPV system consists of low cost quasi-parabolic reflectors, a compact linear PV cells array and a lower cost relaxed pointing 2-axes tracker. The combination of these low cost technologies disclosed can achieve the lowest cost per kilowatt hour of photovoltaic energy production.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based on and claims benefit of U.S. Provisional Application Ser. No. 61/274,888, filed on Aug. 24, 2009. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention proposes a hybrid methodology and apparatus between photovoltaic (PV) and concentrated photovoltaic (CPV) solar panels to lower the photovoltaic solar energy production cost. In particular, the disclosed methodology addresses a simple quasi-parabolic trough PV (QPTPV) low concentration system with greater tolerance to tracker pointing errors. The quasi-parabolic trough (QPT) reflector is defocused to cover the width of a linear solar cells array, which is reduced from a large rectangular solar cells panel. In summary, the QPTPV system consists of low cost quasi-parabolic reflectors, a compact linear PV cells array and a lower cost relaxed pointing 2-axes or 1-axis tracker. The combination of these low cost technologies disclosed can achieve the lowest cost per kilowatt hour of photovoltaic energy production. 
         [0004]    2. Description of the Prior Art 
         [0005]    Photovoltaic (PV) solar panels are usually constructed with a two-dimensional matrix of solar cells packaged on a rectangular solar panel to accumulate solar energy without concentration and converted into utility electricity. Furthermore, if the solar panel is mounted on a 2-axes tracker facing the sun at all times, it can collect 40% to 50% more solar energy relative to a fixed panel. The PV panels commonly seen on residential roof are fixed silicon cells panels due to rooftop installation limitation. Silicon solar cells are much cheaper than other higher efficiency solar cells such as gallium based multiple-junction cells. The best silicon solar cells efficiency in the market today is better than 20% with laboratory solar cells reaching 25%. However, silicon solar panels without sun concentration require a large amount of silicon cells to cover the entire solar panel. Higher efficiency silicon cells (over 20%) are at higher cost since it is harder to make. Therefore, the rooftop solar panels are dominated by lower efficiency (15%-17%) silicon cells today. At year 2010, the retail silicon panel cost around US $2 per watt with rooftop installation cost around $5 to $6 per watt. If we propose a simplified approach of QPTPV system with 2-axes tracker to reduce the PV production cost to around $1 per watt, it is totally unconscionable by the industry today. This disclosure proposes a systematic methodology and apparatus to achieve exactly that lowest cost goal. 
         [0006]    For concentrated photovoltaic (CPV) systems, the multiple junction cells are much more expensive due to the scarcity of material and complicated processing steps and low yield. The triple junction cell today can achieve twice the efficiency of single junction silicon cell to around 40% or higher. But the price of such cell is predominantly higher than the silicon cells; it requires high solar concentration of hundreds to thousands times (suns) to save cost of solar cell and achieve high efficiency at the same time. Therefore, the triple junction solar cells today are cut into very small cell size around 1 square centimeter or less to save the cost of cell and rely on very high precision concentration optics to achieve high energy conversion efficiency. With the CPV systems, the requirement is that the solar panels must face the sun directly to collect sun rays with very high optical concentration using precision solar tracker. The cost of traditional high precision 2-axes solar tracker, the two stage concentration optics and cooling system together with high cost CPV cells constitute major cost items of CPV system. The installation cost per watt is much higher than PV panel today. 
         [0007]    In PV systems when a solar tracker is used, it must cost less than 40% of solar panels; since the improvement in PV energy output with 2-axes tracker is around 40% to 50% depending on the latitude and fixed panel orientation of installation. With current 2-axes tracker on the market, the price has seldom been lower than 40% of the solar panels cost it carried. Therefore, 2-axes trackers are rarely used for PV panels unless it is mandated by a large solar farm. Therefore, there are more fixed panels or 1-axis trackers used in a large utility farm due to the high cost of 2-axes tracker today. 
         [0008]    The tracker used for CPV system requires high pointing accuracy since the CPV cell is very small at around 1 square cm. Pointing error can easily misdirect the sun focus out of the small cell area. Even though some secondary optics or mirror funnel collector are used to mitigate the effect of pointing error, the solar tracker still requires high pointing accuracy with typical pointing requirement under 0.25 degree. On the contrary, the solar tracker used for flat PV panel does not require high pointing accuracy. The PV solar cells can collect sun energy at slanted angle such as used on rooftop panel. The efficiency is proportional to the cosine of slant angle of sun ray versus the normal line (perpendicular line) to the solar panel. For example, if the slant angle is 5 degree from normal, the efficiency is still 99.6% (i.e. cos 5°) of the perpendicular sun ray. 
         [0009]    In summary, one can conclude that 2-axes tracker pointing requirement can be greatly relaxed for PV system. For example, the controller of PV tracker can use an open loop system with only memorized solar orbit sun position (sun almanac) without a sun sensor. Furthermore, it can use an output power measurement feedback pointing algorithm to achieve maximum output power proposed by this disclosure. The pointing resolution can be relaxed as long as it yields maximum output power. These relaxed pointing requirements can translate into lower cost for the tracker. In a separate patent application, we proposed several apparatus and systems which can dramatically reduce the cost of solar tracker. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    With the above discussed backgrounds, the subject disclosure propose a system and apparatus different from PV or CPV systems of today to achieve the lower cost than any CPV or PV systems cost today. 
         [0011]    In one aspect of the invention, the disclosed QPTPV system uses very low cost quasi-parabolic trough (QPT) reflectors. This disclosure proposes a simple technique to form so called QPT reflector. The QPT reflector does not yield high concentration ratio like a true parabolic trough or a dish parabolic reflector, but it is easily realizable with one piece naturally bended metal or plastic sheet reflector with front coated mirror. On top of the front coated mirror, weather protective coating and optional nano-material coating can be applied to prevent water and dust accumulation. In retail mirror market, a low cost glass mirror is commonly seen at the popular hardware stores. With bended sheet QPT reflector, it uses similar low cost mirror film with metal or composite backing and front dielectric protective coatings to enhance solar reflection. 
         [0012]    In another aspect of the disclosure, the panel matrix consists of (6×12) array of solar cells. If a QPT reflector of the same span concentrates the sun rays onto a (1×12) linear cell array (strip), only one sixth of solar cells are needed. With the same reasoning, if the sun exposure area can be twice the span of the PV panel and concentrated on the same (1×12) linear cell array, only one twelfth of the solar cells are needed. 
         [0013]    In yet another aspect of the disclosure, the low concentration ratio of 6 to 12 suns is at the lower end of achievable parabolic trough concentration. The tracker pointing accuracy can be relaxed by using output power measurement feedback pointing algorithm. This could reduce the 2-axes tracker cost, yet produce maximum energy output desired. Specifically, the pointing accuracy required by CPV system is typically under 0.25°. With relaxed pointing requirements around 1°, the lower cost 1-axis or 2-axes tracker is ideal for QPTPV applications. The relaxed pointing tracker uses a hybrid almanac tracker with output power measurement feedback to control the tracker pointing. 
         [0014]    In addition, as an option to the QPTPV system, it can be doubled up as a water heating system. Normally, the concentrated PV system requires cooling to solar cells to operating temperature to function properly. Excessive cooling requirement is a burden to every CPV system. However, the linear solar panel can take advantage of linear array configuration and turn it into a linear water pipe heating system by using active cooling. By using active cooling, we can further absorb the escaped solar heat and transfer around 60% to 70% into water heating energy. The total absorbed solar energy of combined electricity and heating can reach 75% of sun energy based on the data of best commercial water heating system. 
         [0015]    In CSP (concentrated solar power) application, a long concatenated parabolic trough is used to concentrate solar heat focusing on an insulated pipe to heat up water steam for a turbine electric generator. The parabolic trough today uses a large structure using shaped glass mirror or composite material reflectors. The QPT reflectors can be used in such parabolic trough using larger two halves QPT structure. Using QPT reflectors, the CSP structure will be lighter weight and lower cost for a simpler 1-axis tracker rotation. 
         [0016]    In summary, the key to lower cost per watt of QPTPV system are; 1) Lower the cost of parabolic reflectors with simple QPT mirror reflector sheet, 2) Reduce the number of solar cells in linear array from ⅙ to 1/12 of the equivalent PV panel, 3) Lower the cost of 2-axes or 1-axis tracker with relaxed output power measurement feedback pointing algorithm. In this disclosure, the solar concentration is achieved from 6 to 12 times with low cost QPT reflectors made of bended sheet with mirror coating. The reduced number of linear array solar cells can afford to use more efficient silicon cells exceeding 20% efficiency. Together with low cost 2-axes tracker disclosed in previous patent application, it is entirely possible to achieve the target of US $1 per watt of photovoltaic solar energy production today. 
         [0017]    These and other features, aspects and advantages of the present disclosure will become understood with reference to the following description, appended claims and accompany figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  illustrates a QPT reflector focused on linear strip of solar cells. 
           [0019]      FIG. 2  illustrates end view of sun image reflection on a linear strip focal plane. 
           [0020]      FIGS. 3A-3C  illustrate a large QPTPV system mounted on a 2-axes tracker. 
           [0021]      FIGS. 4A-4C  illustrate a QPTPV system on 2-axes tracker with water heating option. 
           [0022]      FIGS. 5A-5D  illustrate a QPTPV system mounted on a 1-D tracker. 
           [0023]      FIG. 6  illustrates the approach to combine low cost features of PV and CPV to derive lowest cost QPTPV system. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    Spatially relative terms, such as “beneath”, “below”, “lower”, “above”, “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. 
       Quasi-Parabolic Trough Definition and Concentration Ratio 
       [0025]    As shown in  FIG. 1 , the main aspect of the disclosure is to use a quasi-parabolic trough (QPT) reflector  10 . The QPT reflector  10  uses one piece bended reflector sheet  52 . Preferred QPT reflector uses rust proof sheet metal front mirror bended in natural shape. The bending force applied on both sides of the reflector sheet  52  would force the bending curve into natural quasi-parabolic shape. The reflector sheet  52  can also use fiberglass, plastic, acrylic, aluminum or aluminum composite, stainless steel sheet coated with front surface mirror and weather protective coating. Bended rigid glass or acrylic in a heated chamber is also possible. Either front or back mirror can be applied for clear sheet of rigid glass or acrylic reflector. 
         [0026]    With true parabolic reflector, focusing a line on solar cell is not good for the solar cell since the sharp sun image may burn or damage the cell. The concentrated sun ray ideally must spread out evenly on the solar cell surface. This is usually done with non-imaging optics in the industry. Bending a flat sheet by pushing from both edges with specified rules will result in a quasi-parabolic trough (QPT) reflector proposed by this disclosure. When facing perpendicular to the sun, the QPT reflector will not focused the sun rays on a line, but rather on a linear strip on the linear solar cells array  12 . This is similar to the effect of a cylindrical reflector which cannot focus parallel rays on a line. The linear solar panel  11  consists of linear solar cell array  12  and the back radiator array  14 . If the focused strip is not wide enough to cover the solar cells array  12 , intentional defocusing by moving the solar panel  11  closer to (or further from) the reflector sheet  52  would defocus the sun rays to fill up the linear area of solar cell array  12 . This is the essence of this disclosure to achieve low QPT concentration ratio at lower cost than traditional parabolic trough. The back radiator array  14  is contacted directly with solar cell array  12  to dissipate accumulated heat from the cells due to solar concentration. The vertical arms  18 , which hold the linear solar panel  11  in place, are adjustable in height from center line of reflector sheet  52 . By adjusting the height of solar panel  11 , a blurred image strip is yielded which can be varied in size until it spread evenly on the surface of linear solar cells array  12 . Preferred height adjustment is done at installation to yield the maximum output power measurement from the solar cells array  12 . 
         [0027]    The parabolic linear span of reflector  52  along the bending direction is indicated by  26 . The width of solar array  12  is indicated by  16 . The solar concentration ratio (suns) of this system is the ratio of the reflector span width  26  subtracted by the array width  16  and divided by cell array width  16 . The subtraction of linear array span  16  is due to the shading of array  12  on the reflector sheet  52 , which discounts the reflecting area on the trough. 
       Method of Making QPT Reflector and a Numerical Example 
       [0028]    In  FIG. 2 , an example end view of QPT reflector is derived by a true parabolic equation Y=X 2 /2, where X indicates half the parabolic linear span, and Y indicate the bended depth of parabolic curve. The focus of parabolic curve is 0.5 meter above the center of parabolic curve in the example. If the parabolic linear span (2X) is 1.28 meter, the bending depth of parabolic curve below the linear parabolic span will be 0.2048 meter. In a perfect parabolic curve, the sun image will be focused on a line at ½ meter above the center of reflector. 
         [0029]    To make a true parabolic trough with sheet metal or glass matching the parabolic curve is not easy. The true size molding and tooling must follow the parabolic curve exactly in order to bend the sheet metal or mold the flat glass to parabolic curve. The surface of the curve must be truly flat without material blemish. However, this invention discloses a simple low cost process of making a near parabolic trough which is defined as a quasi-parabolic trough (QPT) reflector. The QPT curve may not match the parabolic curve exactly due to material and process imperfection, but it is fairly close to true parabolic curve. Yet it can serve the purpose of low concentration PV as desired. In  FIG. 2 , the reflector sheet  52  is bended naturally with two edge secured by screws  13  between two triangular shaped mounting beams  15 . Preferred embodiment of mounting beam is made from elongated metal strip bended into isosceles triangle cross section. The base isosceles angles shall match the slope of parabolic curve at mounting points with open seam at vertex angle. The purpose of open vertex is to allow bolting of the triangle beam onto tracker main frame with pillow clamp and bolts. The techniques of making a QPT reflector proposed by this disclosure shall follow these three rules closely; 
         [0030]    1) The desired QPT linear span  26  shall be the distance between two mounting beams  15 . The distance shall match the linear span  26  of a true parabolic curve closely. 
         [0031]    2) The width of QPT reflector sheet  52  before bending shall closely match the true parabolic curve length between the parabolic span  26 . 
         [0032]    3) The two sloped edges of mounting beams  15  for the reflector sheet  52  shall match the slope of true parabolic curve closely at the point of contact. The contact points are securing with screws  13 . 
         [0033]    With these three closely matched parameters, i.e. span  26 , a width  52  of the reflector sheet  52 , and the slope of sloped edges of the mounting beam  15 , the bended sheet reflector  52  would most likely become near parabolic shaped. We use the word “closely” to allow tolerance in mechanical processing. The bended reflector sheet is defined as a QPT matching closely to a true parabolic curve. As for imperfection, one can observe that the QPT sheet around the mounting point is linear which does not match true parabolic curvature exactly. 
         [0034]    Alternatively to these three steps, the QPT reflector can be made by using symmetrical two halves of the parabolic curve; namely, the span  26  as well as the length of the reflector sheet  52  can be half of the span and width. Two halves of the symmetrical QPT reflectors will be jointed at the bottom by a center beam such as a reversed T-beam. The slope of sloped edges of the mounting beam  15  will match the parabolic curve for both QPT reflectors, but the bottom joint of two halves will be flat angle (zero degree slope). The alternative QPT technique can be used for larger QPT structure, or whenever flat sheet reflector is not large enough for one piece QPT curve. The larger QPT structure is useful for CSP (concentrated solar power) solar thermo concentrator. The disclosed quasi-parabolic curve forming techniques can achieve a lower cost than the parabolic reflector used by the CSP industry today. 
         [0035]    The QPT bended sheet will focus the sun rays on a narrow strip rather than on a line due to bending curve imperfection. The width of the focused strip can be further widened by defocusing to fill up the linear cells array area. Preferred embodiment is to move the linear solar panel  11  closer to the reflector for defocusing with shorter height of vertical arm  18 . In the numerical example of  FIG. 2 , if the span of reflector width  26  is 128 cm and the solar panel  11  width is 12.8 cm, the QPTPV solar concentration ratio is only 9 with solar panel shading deducted. This is in the lower range of achievable concentration ratio by a parabolic trough. Yet at this low concentration ratio, it only needs 11% of solar cells of equivalent area PV panel. This is one of the keys to lower the cost per watt of the QPTPV system. 
         [0036]    For those skilled in the art, the QPT reflector can be bended in other geometric curve closely with the same three rules of sheet bending technique. For example, one can make a cylindrical trough reflector match closely to a circular arc curve with equivalent linear span, curve length and the slope at the mounting points. The cylindrical trough reflector could perform similarly to a QPT reflector but not as desirable as QPT reflector, since parallel rays reflect from cylindrical trough does not focus on a center line. Nevertheless, cylindrical trough is much easier to form than a parabolic trough. Other curved troughs are also possible, but is not as good as QPT reflector, or a cylindrical trough reflector. In general we shall define these bended troughs formed by the three rules of QPT forming as curved trough PV (CTPV) system imitating true geometric curve closely. Even though QPT reflector is most favored with focused strip, but cylindrical trough may be useful in low concentration application when linear cells array is fairly wide. 
       Embodiment of a QPTPV System on 2-Axes Tracker 
       [0037]      FIG. 3A  illustrates a preferred embodiment of QPTPV system with multiple QPT reflectors mounted on a low cost 2-axes tracker  10 . The 2-axes tracker  10  is disclosed in previous U.S. patent application Ser. No. 12/852,454. The tracker  10  described is for ground installation with a ground post  20  secured on a base  40 , or ground. On top of the ground post  20  is a rotating head  30 . A horizontal beam  50  is secured on top of the rotating head  30 . A two sided rectangular tracker frame  51  is attached and balanced on the horizontal beam  50 . The isosceles cross section mounting beam  15  mounted in parallel on the horizontal beam  50  perpendicularly. The 2-axes tracker  10  in this embodiment uses a motor drive  43  and electro-magnetic rotor  42  attracted to rotating head  30  for azimuth rotation as an example. Various azimuth rotation techniques are disclosed in previous patent application. A linear actuator with jack head  57  shown is used for elevation rotation. Each side of the tracker  10  is holding six quasi-parabolic reflectors  52 . As depicted in  FIG. 3C , each quasi-parabolic reflector  52  is mounted between isosceles triangular cross section beams  15  by pushing reflector sheet  52  into quasi-parabolic shape with securing metal screws  13  at both sides. The ends of beam  15  are attached with L-beams  5  at the edge to make a rectangular tracker frame  51 . The linear solar panels  11  are held in place by vertical arms  18 . Two aligned linear arrays  11  are met in the middle and joined with securing bracket  17  and supported by vertical arm  18  in the middle. The adjustment of height of vertical arm  18  in the middle may not be necessary since it is done at both ends of solar panels at edge L-beam  5 . Therefore, the middle arm  18  can be inserted fittingly in a guiding tube  29  secured on bracket  17 . The guiding tube  29  serves as a guiding rail. The bottom of middle arm  18  is secured on the tracker horizontal beam  50 . The adjustment of the height of vertical arms  18  on both ends are sufficient to move the solar panels  11  up or down. 
         [0038]    The defocusing of QPTPV is done by adjusting the height of vertical arms  18  attached on the edge L-beam  5  with screw nuts  19 . The height adjustment can defocus the sun exposure evenly on the solar cells array  12  to yield maximum wattage output. Besides, defocusing can prolong the life of solar cell by not overheating the center of the cell by sharp sun image. Both height adjustments on arms  18  and mounting of reflector sheet  52  is done at tracker installation to yield maximum output power. This is quite different from traditional PV panel and CPV module installation with fixed factory calibration; which can be distorted while shipping and installation. As an example of on site calibration, if the linear length of reflector  52  is 2 meter with parabolic span of 1.28 m, which will exposed to 2560 watts of solar energy under ideal sun shine. With 20% efficiency of silicon solar cells array  12 , 10% shading loss by linear solar panel and optical pass loss of 8%, the QPTPV system can produce around 424 watts of electric power from each panel. If over 400 watts power is yielded, it is pretty good calibration. 
         [0039]    The QPTPV system depicted in  FIG. 3A  is essentially a low concentration CPV system. As depicted in  FIG. 3B , radiator array  14  at the back of solar panel  11  is needed to dissipate the excessive heat generated by the solar cells array  12 . Optional solar fans  58  can be attached at the back of radiator  14  to speed up the radiation. These solar fans are low cost items seen on commercial solar fan caps. The solar cells normal operating temperature is 46° C. with natural air radiation at the back of PV panel. Lower temperature will yields better performance, but it is hard to do with larger PV solar panel. But concentrated linear solar panel must have radiator to dissipate concentrated heat to achieve normal operating temperature. The radiator array looks similar to the radiator in the back of a window air conditioner. Since the ambient temperature is normally lower then 46° C. in most part of the world, the radiator will radiate excessive heat into the air. Optional solar fans  58  can speed up the radiation and achieve a lower temperature at solar cells. This will produce higher output power from the solar cells array  12  than cells in regular PV panels. In addition, the solar fan  58  on top of solar panel  11  is constrained within the width of solar panel, which does not cause additional shading on the QPT reflector and supply its own power with solar cell at the back. 
         [0040]    For traditional PV panel with 6×12 cell arrays, the silicon cells are usually connected in series to obtain higher voltage output for the inverter. In the example of  FIG. 3A , it is designed with 6 panels of reflectors on each side. If linear cells arrays are connected in series, it is designed to yield similar output voltage as the traditional solar panel with roughly 9 times higher current. The design is to take advantage of commercial inverter designed for the traditional PV panels. However, it is not necessary to have 6 reflector panels on each side. It will be a design option to yield the best desired output power per tracker in a solar farm. Another way to boost the voltage output of linear cells array is to use fractional solar cells, such as half or quarter length cells. By connecting double or quadruple number of half or quarter cells, the output voltage can be doubled or quadrupled. Therefore, the design of linear solar cells array is flexible to yield any desired DC voltage. 
         [0041]    In  FIG. 3A , the reflectors  52  linear orientation is in elevation rotating direction while the reflectors  52  bending is toward azimuth rotation direction. The output power is more sensitive in the parabolic bending direction if miss focused; therefore the output power is more sensitive to azimuth rotation. Using azimuth rotation of 180 degree in 12 hours daylight for example, the rotation of 1 degree takes 4 minutes. We shall use a relaxed half degree resolution in 2 minutes period as an example of tracker activation. The initial pointing of the tracker will follow the sun orbit almanac at the date and time of the year. After initial pointing, the azimuth rotation motor  43  will continue step by step, backward and forward to measure the output power as feedback. If output power rise, the stepping motor  43  will go forward one more step until the power measurement go down, then it will back step to the maximum power point and stay idle. Next, the elevation rotation will be activated with the same procedure as azimuth rotation until maximum power is yielded. The azimuth and elevation activations are done in sequence in a relaxed 2 minutes period. Both azimuth and elevation rotation will stay idle between 2 minutes periodical activations. This is what we called a “relax pointing algorithm” since none of pointing accuracy in degrees or the stepping motor resolution in azimuth or elevation is specified as long as maximum power is yielded. We shall abbreviate the output power measurement feedback pointing algorithm as “maximum power pointing algorithm” hereafter. This will allows us to pick whatever reliable low cost actuators on the market. Using this power measurement feedback algorithm, the tracker does not have to verify the sun almanac unless it lost the pointing. However, the sun almanac can be verified frequently as an insurance check of the tracker pointing. The maximum power pointing algorithm can avoid using more expensive sun sensor as well as high precision actuators. In addition, the output power measurement may already exist in net-metering or feed-in-tariff (FIT) which measuring the power feed into utility grid lines. The maximum power pointing can be better than sun sensor or sun almanac pointing since neither the sun sensor nor the almanac pointing is linked to the output power production. The reflectors and linear solar panels may be misaligned with sun sensor or almanac pointing due to potential misalignment of some QPT reflectors or solar panels, especially after long term usage under adverse weather conditions. The maximum power pointing algorithm is indeed the best measure of the QPTPV system performance, and could lower cost of the tracker as well. 
         [0042]    At this point, those skilled in the art may easily change the QPTPV structure in many ways. For example, the mounting beam does not have to be isosceles triangle as long as the mounting strip angle can match the parabolic curve slope at the point of mounting. In addition, the solar cell used is not limited to silicon cells or thin film cells. Other higher efficiency cells such as dual-junction cells or triple-junction cells can be used with higher concentration ratio. In this case, a larger QPT surface together with a secondary optics may be needed in the linear solar panel to boost the concentration ratio for expensive higher efficiency solar cells. This and other variation does not change the main sprit of described QPTPV system. 
         [0000]    Embodiment of a QPTPV System with Water Heating 
         [0043]    An alternative to passive radiator is using active liquid cooling as depicted in  FIG. 4A . The active cooling system is a liquid (coolant) circulation system like automobile radiator. The cold liquid input plumbing is indicated by piping  33 . Since the cooling system is not at high pressure, nor it is at high temperature, the plumbing can be flexible piping made of synthetic material used in the plumbing industry today. Flexible pipes leading to ground connection are easier for the 2-axes tracker in azimuth and elevation rotation. A flat metal radiator pipe  34  made of copper or aluminum heat conductor in close contact with linear solar cells array  12  is needed for concentrated heat dissipation. Circulation of cold liquid in radiator pipe  34  can cool down the linear solar cells array  12  to proper operating temperature and yield heated coolant output. In  FIG. 4B , the vertical arms  18  supporting the linear solar panel  12  can be doubled up as water pipe connected to the radiator pipe  34 . Preferred flow of cold liquid input pipe is indicated by  18 C and heated water output indicated by  18 H in the middle for easier hot water insulation.  18 H and the hot water pipe  35  flows into cold water tank  70  for heat exchange. With operating temperature of silicon cells at 46° C., it is ideal for household hot water output. This heated coolant pipe can perform heat exchange in a thermo insulated water tank  70  for household use similar to existing solar water heating systems. If higher temperature is needed, it can be further heated up from 46° C. with other source of energy for cold weather or night time use. 
         [0044]    The solar cells array operating temperature can be controlled with liquid coolant flow rate. Higher coolant flow rate may be needed in summer than winter, in high noon than morning and dawn. Lower operating temperature in solar cells array yields higher cell efficiency, and therefore higher output power. The energy produced by combination of PV energy and water heater energy could be balanced and optimized by controlling the coolant flow rate in the circulation system. Furthermore, the coolant used in the circulation can be either a closed loop coolant system with heat exchange at the water tank, or direct water circulation system in non-freezing weather. And the liquid used in freezing weather has to be anti-freeze coolant. 
         [0045]    In the solar visible light spectrum, the shorter wave length energy is partially absorbed by the silicon cell and turned into electricity, longer wavelength near infrared spectrum and the escaped photons from silicon cells are turned into concentrated heat. An efficient thermo transfer system such as the concentrated solar system of  FIG. 4A  can yield high heat transfer efficiency in the 60 to 70% range at moderate temperature of 46°. As an example, if 20% of photons are absorbed by solar cells array  12  and turned into electricity, and the remaining 80% photons can heat up the pipe  34 . With a conservative 60% heat transfer efficiency, a total of 68% solar efficiency is achieved by the combined PV and heat solar energy system. Higher efficiency can be yielded by better insulating the heated area from ambient temperature. 
         [0046]    Another application of the QPT reflectors is used for water heating only system as depicted in  FIG. 4C . The linear solar cells panels are replaced with water heating pipe  32  fixed inside a transparent insulated tubing  36  as seen in some commercial solar water heating apparatus. Since the QPT reflectors are dedicated to concentrate heat into liquid pipe  32 , the insulated heating device can be more efficient. Due to less number of insulated tubing  36  and heating pipe  32 , the solar heating device can be lower cost than populated tubing systems. Besides, the low cost 2-axes tracker can generate more heat from sunrise to sunset on 2-axes tracker compared to a fixed panel solar water heater. The number of QPT reflectors will depend on the volume of hot water needed. In general, it does not require 12 panels as depicted in  FIG. 4A  for household use. Therefore, a smaller 2-axes tracker with two or more reflectors can be constructed for domestic solar water heating devices. 
         [0047]    At this point, those skilled in the art may easily change the QPTPV structure of combined PV and heat system in many ways. For example, the hot water piping  35  as well heat conduction pipe  34  can use best insulation to yield lowest heat dissipation on the way to water tank. Also the input and output piping configuration can be optimized or shortened in many ways to prevent heat loss. This and other variations do not deviate from the essence of QPTPV system which can yield high efficiency of condensed heat transfer under the concentrated linear solar cells array. 
       Embodiment of Long Parabolic Trough on  1 -Axis Tracker 
       [0048]      FIG. 5A  illustrates a QPTPV system with multiple QPT reflectors mounted on a 2-axis tracker  60 . The QPT reflectors  52  are concatenated for a long trough similar to a CSP parabolic trough system. Preferred QPT reflectors  52  are two halves construction surface joined by a T-beam  22  at the bottom. The seams at concatenation joints can use back side securing strips, or glue strips. One piece QPT can be used for smaller trough design. The concatenated reflectors  52  are secured on the mounting beams  21 . One important feature for QPTPV on 1-axis tracker is that there shall be no gap between concatenated QPT reflectors  52 , and there shall be no shading on the reflector surface other than the shading of linear solar panel  11  in the middle. Therefore, the tracker  60  structure and mounting frames  27  is under the QPT reflectors  52  with only two parallel mounting beams  21  shown on both sides of QPT reflectors  52 . The bending of reflectors  52  follows three rules discussed in the QPT forming concept. Parallel mounting beams  21  on the mounting frame  27  are sloped at the screw points  13  matching true parabolic slope. There are multiple screws  13  to secure the bended reflectors  52  on the mounting beams  21 . At both ends and the middle of QPT reflectors of the 1-axis tracker, multiple mounting frames  27  under the reflectors  52  are connected with the mounting beams  21  at both sides. An elongated reversed T-beam  22  is attached to the bottom center of mounting frame  27  to support two halves of QPT reflectors  52 . Preferred embodiment of mounting frame  27  uses bended L-beam in three linear sections to form a reverse trapezoid shaped frame. The bending points can make cuts on L-beam and welded for rigid structure. Strengthened thin steel wires  28  can be used to tie between parallel mounting beams  21  for structure support. Since thin wire can be smaller than the sun angle shining on the reflector, the wire shading on reflector surface may disappear or become insignificant. This effect is commonly seen on electric poles without the shadow of thin electric wire under sunshine. 
         [0049]    The linear solar panels  11  are mounted on multiple vertical arms  18  with linear cells array  12  at the front side facing the reflector  52 . Vertical arms  18  are attached on the middle of T-beam  22  perpendicularly with adjustable height mechanism. At the back of linear solar cells array  12 , a passive radiator array  14  is contacted directly with solar cell array  12  facing the sun direction. Solar fans attached to the back of solar panel  11  can be used for the radiator cooling. Alternative radiation using active liquid or water cooling is also possible. The active cooling system needs a water tank for the storage of heated water. 
         [0050]    The entire tracker frame is secured and balanced on an elongated cylindrical beam  25  of the 1-axis tracker. The 1-axis tracker described herein with its rotating mechanism is filed in previous U.S. patent application Ser. No. 12/816,195. In  FIG. 5B  the mounting frames  27  of the tracker  60  is mounted on the cylindrical beam  25  with a rotating bushing  24  in between and secured with a pillow clamp  35 . The rotating bushing is looped on the cylindrical beam  25  facilitating 1-axis rotation of tracker frame. 51 . The cylindrical beam  25  is supported and fixed on multiple ground posts  20  as depicted in  FIG. 5C . The two halves of reflector  52  are secured at the bottom on the reversed T-beam  22  as depicted in  FIG. 5D . Preferred embodiment of the T-beam  22  uses elongated metal strip bended into T shaped beam. The QPT reflector  52  is joined at the center of reversed T-beam  22  and secured with screws. The reversed T-beam  22  is then attached between the mounting frames  27  and secured on the L-beam of mounting frame  27 . The reversed T-beam  22  is aligned with the horizontal beam  25 ; which is sitting at the center line of gravity of the 2-axis tracker  60 . 
         [0051]    The one axis tracker rotation only takes care of daily sunrise to sunset solar tracking. There will be seasonal solar orbit change from summer to winter; which affect the focusing of the QPT reflectors. The orbit change is very slow motion at 0.26 degrees per day. To take care of this small change in solar orbit, the focal plane height of the linear solar panel  11  needs to be adjusted daily. A linear actuator  38  near the center of the 1-axis tracker is designed to move the vertical position of linear solar panel  11 . More actuators at both ends can be used if the QPT reflector is of very long length such as used for CSP application. Also, it is also desirable to have two actuators at both ends of the tracker for a longer QPTPV system. The jack head  37  of linear actuator  38  is connected with center vertical arm  18  to move up or down the linear solar panel  11 . The remaining vertical arms  18  need tight fit guiding rails for the vertical motion of solar panel  11 . A fitted guiding tube  29  attached on the gaps of solar panels  12  and looped on the vertical arms  18  can serve the purpose of guiding rail. At the same time, tight fitting guiding tube  29  can ensure no lateral motion of the linear solar panel. Preferred embodiment use a cylindrical guiding tube  29  with inner wall coated with bushing material and lubricated with solid lubricant. The linear actuator(s)  38  will be activated a few times daily to adjust for the solar orbit change of 0.26 degree per day. The activation period of linear actuator  38  can be much longer than the 1-axis tracking activation period; however, it must also follows maximum power pointing algorithm. In addition, to further securing the vertical arms  18  on the solar panel  11 , an electromagnetic locking coil looped on the guiding tube  29  can be installed. The electromagnetic locks are activated whenever linear actuator  38  is idle to lock up the vertical arms on the guiding tubes  29 . 
         [0052]    The concept of 1-axis linear QPTPV system can be used for the linear parabolic trough in concentrated solar power (CSP) industry. The CSP may need larger QPT reflector surface, but gaps and structure shadings are allowed in CSP systems. This feature will allow framing structure support in the gaps of QPT reflectors to support the long CSP steam pipe structure for continuous heat up of water steam used in CSP turbine generator. 
         [0053]    Those skilled in the art can easily change the configuration QPT reflectors mounted on different type of 1-axis tracker or structure. For example, the simplified QPT reflectors can be mounted on traditional CSP structure. Also due to the light weight of QPT reflectors on the simplified tracker frame, the center heated tubing can be mounted on separated structure supported at both ends and middle of QPT reflector junctions. The only linkage will be the vertical arms  18  linked to the center heating tube. The vertical arms  18  serve as the radius of rotation around horizontal beam  25 . This will enable a simplified reflector structure with lighter rotating mechanism of the reflector. Also, since the heavier center steam pipe swings from sunrise to sunset with rotating actuator(s), the light weight tracker frame can be slaved to the same actuator(s) rotating around the cylindrical beam  25  with the linkage vertical arms  18  serves as the radius of rotation. These variations do not change the essence of the disclosure with simple QPT reflectors. 
       A System Approach to Achieve Lowest Cost Per Watt 
       [0054]    This disclosure intends to establish a system approach to reach lowest cost per watt target using overall lower combination cost. The system approach is illustrated in the flow diagram of  FIG. 6 . The approach takes good features from the PV and CPV systems and avoids bad features of both systems. This disclosure then creates a new approach to lower the overall combination cost of photovoltaic energy production. This system approach is not considered by the PV solar cell manufacturers today. Instead, each solar cell manufacturer acts independently in the pursuit of higher efficiency and lower cost PV solar cells, not considering the combination system power production cost. In general, the good features of PV systems are relative lower cost of silicon cells and relaxed pointing requirement in 2-axes tracking. Also low cost silicon cells today can operate at low concentration ratio with no loss of efficiency; and potentially improve the efficiency under concentration at lower operating temperature. The bad feature of PV panel is that too many cells are needed to populate the solar panel, which constitutes the majority of solar module cost. On the other hand, the good feature of CPV system is that it needs very few solar cells under high concentration. But the solar cell cost is very high, and the balance of system in concentration optics, high precision solar tracker and high cooling requirement are predominantly high cost. These are the main reasons that CPV solar systems are not as competitive as PV panel systems today. 
         [0055]    The disclosed approach illustrated in  FIG. 6  takes advantage of all the good features of PV and CPV systems and avoid the bad features, and then create a new system approach to create a very low cost QPTPV system, or CTPV system. In summary, the disclosed QPTPV or CTPV system approach uses only a fraction of solar panel PV cells, and takes advantage of relaxed tracker pointing requirement to lower the cost of a 2-axes tracker. In addition, a QPT or CT reflector bended by a flat sheet of font coated mirror is believed to be the lowest cost concentration method today. Furthermore, the heat radiation burden can be turned into a bonus water heating system which can achieve as high as 70% of solar energy utilization. Therefore, the proposed system approach is a combination of multiple low cost QPT or CT mirror reflectors mounted on a low cost 2-axes tracker with compact linear PV cells array. This combination system is the key to lower the cost of PV energy production targeted at US $1 dollar per watt today. Other good features disclosed including simple defocusing technique instead of complicated non-imaging optics to spread out the sun rays evenly on linear solar cells. And using maximum power pointing algorithm can lower the tracker cost and optimize the total energy output at the same time. In the case of water heating only or CSP systems, the output power can be replaced with digital thermostat to measure output liquid temperature as feedback for the tracker pointing. 
         [0056]    Although various aspects of the disclosed approach and apparatus to lower cost of solar photovoltaic energy have been shown and described, modification may occur to those skilled in the art upon reading the specification. The present application includes such modifications and limited only by the scope of the claims.