Abstract:
A remotely powered heliostat system includes a plurality of heliostats each having a reflection panel movably disposed on a structural member. At least one motor is disposed between each heliostat and the structural member to position each heliostat. The plurality of heliostats is further divisible into at least two groups of heliostats, each group operable by one of a plurality of radio frequency receivers electrically connected to each motor. Each radio frequency receiver wirelessly receives a heliostat positioning command for the group from a remote transmitter. A processor analyzes solar position and generates the heliostat positioning command. Encoders on each heliostat keep track of the heliostat position. Each motor is connected to a local battery unit to provide electrical power. The system provides local power to operate each heliostat, and a wireless signal to control heliostat position, eliminating dependence on a single source of power for the heliostats.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to heliostats and more specifically to an apparatus and a method to control the operation of one or more heliostats.  
         BACKGROUND OF THE INVENTION  
         [0002]    Heliostats are partially formed of reflective surfaces called facets and are commonly used to collect solar radiation and reflect the solar radiation onto a solar receiver mounted on a tall tower. Heliostat facets are typically arranged such that an individual facet or several facets form a heliostat reflector. A heliostat is formed with at least one reflector supported from a support structure, having at least one motor disposed between the support structure and reflector to position the reflector relative to a solar position. Additional components can also be included with each heliostat. When thousands of heliostats are used to form heliostat fields, solar electrical power can be generated comparable to fossil fuel or nuclear electric generation plants. Heliostat fields are also used in scientific research, to collect/reflect energy from cosmic sources.  
           [0003]    Several disadvantages exist for known heliostats. Large quantities or fields of heliostats require individual cabling between the necessary control facility and each heliostat, often totaling kilometers of cabling for a several thousand heliostat field. Both power and control signal cabling are required. These cables are often buried and difficult to access for maintenance. Initial installation cost as well as maintenance costs are therefore increased by the total amount of cabling. Power and operational signals for each heliostat are commonly provided from a control facility remotely located from the heliostat field. Even a temporary power outage at the control facility can render the entire heliostat field inoperative. The loss of power to an entire field or sector of heliostats can result in a serious thermal threat to the receiver and/or tower as the combined heliostat thermal flux “walks off” the receiver surface and/or impinges on tower structural materials. Even momentary power loss can result in thermal damage to the receiver and/or tower. Back-up electrical sources are therefore often provided in the event of a general power failure, further increasing cost. Heliostats are also susceptible to high wind or weather damage, therefore requiring re-positioning to a safe position during inclement weather. A prolonged loss of electrical power can therefore result in heliostat weather related damage.  
           [0004]    It is therefore desirable to provide a heliostat system capable of powering each heliostat or group of heliostats, independent of each other and independent of the power source required to generate the control signals. It is also desirable to eliminate the individual power and control cabling required between the control facility and each heliostat.  
         SUMMARY OF THE INVENTION  
         [0005]    A locally powered heliostat system includes a plurality of heliostats each having at least one facet operably forming a reflector, the reflector movably disposed on a structural member. At least one motor is disposed between each reflector and the structural member to position each reflector. The plurality of heliostats is further divisible into at least two groups of heliostats, each group operable by one of a plurality of radio frequency receivers electrically connected to each motor (or group of motors). Each radio frequency receiver wirelessly receives a heliostat positioning command for the group from a remote transmitter.  
           [0006]    In one preferred embodiment, a processor which is remotely located from a field of heliostats analyzes a solar position and generates heliostat positioning commands. The heliostat positioning commands are transferred to a radio frequency transmitter. A radio frequency, wireless signal is generated by the transmitter and transmitted to each heliostat to control heliostat position. The receiver located at each heliostat receives the wirelessly transmitted control signal where it is further communicated to each motor. In the event of a loss of control signals from the transmitter, each heliostat or group of heliostats can be directed to a fail-safe position stored in the receiver of each heliostat, or retained in the existing position.  
           [0007]    In another preferred embodiment, each motor is connected to at least one local battery unit to provide electrical power to position the heliostat. The battery unit provides direct current power independent of the power required to operate the processor and transmitter. A photovoltaic cell array is disposed at each heliostat or group of heliostats which generates electrical current to recharge the battery unit. Power conditioning equipment and battery monitoring equipment are also provided to control the recharge process and to monitor battery unit condition, respectively.  
           [0008]    In yet another preferred embodiment, the processor is mounted on the heliostat. In this embodiment, the battery unit also provides direct current power to the processor.  
           [0009]    In still another preferred embodiment, a unique address on a radio frequency signal is generated for each individual heliostat. By controlling each individual heliostat using a unique address signal, individual heliostats or groups of heliostats can be positioned from a remote control facility.  
           [0010]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0012]    [0012]FIG. 1 is a diagrammatic view of a heliostat system according to a preferred embodiment of the present invention;  
         [0013]    [0013]FIG. 2 is a diagrammatic view of the heliostat system according to another embodiment of the present invention, showing individual groups of heliostats in communication with a receiver and tower known in the art;  
         [0014]    [0014]FIG. 3 is a flow chart identifying a method to operate a heliostat system of the present invention; and  
         [0015]    [0015]FIG. 4 is a perspective view of an exemplary heliostat of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]    The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
         [0017]    Referring to FIG. 1, a heliostat system  10  of the present invention includes at least one reflector  12  supported by a structural member  14 . Each structural member  14  is separately positioned and supported by a ground surface or additional support pad (shown in reference to FIG. 4). At least one motor  16  is disposed between the structural member  14  and each reflector  12 . In a preferred embodiment, the motor  16  includes a separate elevation control motor and gear and a separate azimuth control motor and gear (shown in reference to FIG. 4). A radio frequency receiver  18  is also disposed on each structural member  14 . At least one each of reflector  12 , structural member  14 , motor  16  and receiver  18  form an assembly, hereinafter designated as heliostat  19 . A plurality of heliostats  19  are grouped to form a heliostat field.  
         [0018]    In the embodiment shown, at least one battery unit  20  is disposed either in close proximity to or mounted from each structural member  14 . Direct current (DC) power from the battery unit  20  is provided to each of the motors  16  via a motor power line  22 . A photovoltaic cell array  24  is also disposed in close proximity to each structural member  14 , mounted from each structural member  14 , or alternatively, disposed on each of the reflectors  12 . The photovoltaic cell array  24  is selected from systems known in the art to provide the necessary voltage and current for recharging the battery unit  20 . Each battery unit  20  preferably includes at least one lead-acid battery known in the art, or, alternatively, at least one of any type of rechargeable battery. Battery unit  20  voltage is preferably  12  volts DC, to control cost by using existing battery technology, but is selectable to match the voltage requirements of the motors  16  used. The structural member is commonly provided as a tubular column, typically formed of a metal such as steel. The invention is not limited by the shape or material of the structural member.  
         [0019]    Conditioning equipment  26  is disposed between the photovoltaic cell array  24  and each battery unit  20 , and connected to the photovoltaic cell array  24  via a power conditioner supply line  28 . Between the conditioning equipment  26  and the battery unit  20 , a battery recharge line  30  is disposed. The conditioning equipment  26  is known in the art and includes over-voltage protection, over-charging protection, and control of the charging rate. Battery monitoring equipment  32  is connected to the conditioning equipment  26  via a monitoring line  34 . The battery monitoring equipment  32  is known in the art and includes battery voltage measurement, current measurement, battery charge indication, and/or indication of a failed photovoltaic cell array  24 .  
         [0020]    A radio frequency transmitter  36  is remotely positioned from each of the heliostats  19 . The radio frequency transmitter  36  produces a radio frequency wireless signal  38 . The radio frequency wireless signal  38  will be described in further detail with reference to FIG. 2. The frequency of the radio frequency wireless signal  38  is compatible with each of the radio frequency receivers  18 . A signal processor  40  is provided to generate each of the radio frequency wireless signals  38  for transmission by the radio frequency transmitter  36 . A signal transfer line  42  is provided between the signal processor  40  and the radio frequency transmitter  36 . The signal processor  40  is connectable to other computing and data collection equipment, normally positioned at a central processing facility, (not shown), which store data on solar position based on time of day and/or time of year, heliostat global location, etc. This equipment and data are well known and will not be further discussed herein. It is anticipated that the signal processor  40  will generate an updated radio frequency wireless signal  38  at periodic intervals determined by an operator of the system. An exemplary periodic interval is approximately every thirty seconds.  
         [0021]    Referring next to FIG. 2, a heliostat system  44  for another preferred embodiment of the present invention includes a first heliostat group  46  and a second heliostat group  48 . Each of the first and second heliostat groups include at least two heliostats  19 . The first heliostat group  46  also includes a local signal supply  50  which includes one of the signal processors  40  and one of the radio frequency transmitters  36 . Similarly, the second heliostat group  48  also includes a local signal supply  54 . The local signal supply  54  includes one of the signal processors  40  and one of the radio frequency transmitters  36 . The local signal supply  50  transmits a first wireless signal  52  to the first heliostat group  46 . The local signal supply  54  transmits a second wireless signal  56  to the second heliostat group  48 . Each of the first wireless signal  52  and the second wireless signal  56  can further include individual unique frequencies or individual unique addresses on a single frequency for each of the reflectors  12  associated with the first heliostat group  46  and/or the second heliostat group  48 , respectively.  
         [0022]    [0022]FIG. 2 also shows a common solar receiver  58  disposed on a tower  60  as known in the art, for receiving the solar radiation reflected from each of the reflectors  12  of the heliostat system  44 . The solar radiation received at the solar receiver  58  is commonly used to heat a heat transfer fluid provided in an inlet line  62  and discharged in a heated discharge line  64 . The heated fluid (not shown) can thereafter be used to generate steam and further to generate electricity, or depending on the fluid type, directly used to generate electricity.  
         [0023]    It should be obvious that the quantity of reflectors  12  used in each of the first heliostat group  46  and the second heliostat group  48  of the heliostat system  44  can vary and that the number of heliostat groups can vary. At least two reflectors  12  are used in each of the first heliostat group  46  and the second heliostat group  48 , respectively. Each of the reflectors  12  provided in the first heliostat group  46  and the second heliostat group  48  are also commonly or individually locally connected to batteries; photovoltaic cell arrays; conditioning equipment; and battery monitoring equipment, similar to that shown in FIG. 1. This equipment is not shown in FIG. 2 for clarity. It should also be noted that each local signal supply  50  and local signal supply  54  can be located at any distance within radio frequency transmission range of the individual reflectors  12 .  
         [0024]    As best seen in FIG. 3, a method to operate a heliostat system of the present invention is described. In a first step  100 , individual heliostats are arranged into at least two groups of heliostats. At step  102 , a plurality of unique directional control signals is generated by at least one signal processor. In a following step  104 , select ones of the directional control signals are wirelessly transmitted to each heliostat. In a positioning step  106 , each heliostat is positioned using the received directional control signal. At step  108 , all of the heliostats in a heliostat system are globally positioned to a fail-safe position upon loss of the control signals. In a further step  110 , at least one battery powered motor is used to position each heliostat. In a final step  112 , each battery unit powering each motor is recharged from a photovoltaic cell array disposed on each heliostat.  
         [0025]    Referring finally to FIG. 4, an exemplary heliostat  120  includes a first reflector  122  and a second reflector  124 , both supported by a structural member  126 . The structural member  126  is preferably partially inserted in the ground for support. Optionally, the structural member  126  is connectably disposed to a support plate  128 , which is anchored to and transfers heliostat loads to a ground surface. The first reflector  122  and the second reflector  124  are co-rotated by an azimuth motor  130  and an elevation motor  132 . Both the azimuth motor  130  and the elevation motor  132  are supported from the structural member  126 . The first reflector  122  and the second reflector  124  are driven by the azimuth motor  130  to rotate in the rotational direction “A” about the structural member  126  longitudinal axis “B”. Similarly, the first reflector  122  and the second reflector  124  are driven by the elevation motor  132  to rotate in the elevation rotation direction “C”. A solar receiver section  134  (shown in phantom), is located remote from heliostat  120 , and is similar to the solar receiver  58  shown in FIG. 2. A radio frequency receiver  136  is shown mounted to the structural member  126 . A battery unit  138  is connected to the receiver  136  by a battery power cable  140 . The receiver is connected to both the azimuth motor  130  and the elevation motor  132  by a motor power cable  142 . A photovoltaic cell array  144  is mounted in this embodiment to the first reflector  122 . A power conditioner  146  is connected to the battery unit  138  by a power conditioning cable  148 . A battery monitoring system  150  is also supported by the support plate  128  and connected to the battery unit  138  by a monitoring cable  152 .  
         [0026]    In operation, the position of the heliostat  120  is monitored by an encoder (not shown) as known, and the radio frequency control signal  135  is generated similar to first and second wireless signals  52  and  56  respectively, with new position data. The heliostat  120  receives the radio frequency control signal  135  at the radio frequency receiver  136 . The radio frequency receiver closes a current flow path between the battery unit  138  and appropriate one(s) of the azimuth motor  130  and the elevation motor  132  co-rotate the first and second reflectors,  122  and  124 . Either or both of the azimuth motor  130  and the elevation motor  132  can be energized. Individual frequencies or different addresses on the same frequency can be used for the radio frequency control signal  135  to initiate operation of the azimuth motor  130  and the elevation motor  132 . When positioned by either or both the azimuth motor  130  and the elevation motor  132 , light incident along the incident energy path “D” is reflected off both the first reflector  122  and the second reflector  124  along a reflected energy path “E” to the solar receiver section  134  (shown in phantom), which is similar to receiver  58  shown in FIG. 2. Light incident on the photovoltaic cell array  144  generates an electrical current which is conducted by a cable (not shown) to the battery unit  138 , via the power conditioner  146 , to recharge the battery unit  138 .  
         [0027]    A heliostat system of the present invention provides several advantages. By providing local battery power to each heliostat or several heliostats, the kilometers of cabling required to connect to the often several thousand heliostats is eliminated. By wirelessly signaling each heliostat, the processing and transmitting equipment can be located at any distance within radio frequency transmission range of the heliostats. The use of batteries to power the motors of each heliostat provides a low cost, simple system to maintain wherein batteries can be replaced when the individual battery unit fails to accept a recharge. By providing local photovoltaic cell arrays associated with one or several heliostats, the local batteries can be recharged. The use of photovoltaic cell arrays and batteries provides for autonomous operation of the heliostats. By grouping heliostats, the associated transmitter and processor equipment can be positioned local to each group to improve maintenance. Finally, by providing a fail-safe position for each heliostat, each heliostat will reposition to the fail-safe position upon loss of the wireless control signals, thereby reducing the potential to damage the solar receiver, the receiver tower, or the heliostats themselves.  
         [0028]    The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.