Abstract:
Direct solar thermal steam generator with an ultra-compact linear Fresnel reflector field that has “travel and turn” capability by means of independent linear and rotational motion of reflectors. 
     Method of positioning and orienting the reflectors of the traveling field such that the reflected energy of the field is maximized at all times 
     Crescent like rotational support rail of the reflector with its gravitational center in the center of the crescent. The curvature of the reflector is customized for each row of the field 
     Ultra-light, collector-absorber structure, with cable suspended arch-like tubular absorber, wide aperture, and secondary reflector with optimized light entrapment 
     Flow distribution and control method of the large horizontal solar thermal steam generation field.

Description:
TECHNICAL FIELD 
       [0001]    The present application relates generally to solar thermal energy collectors and more particularly relates to direct solar thermal steam generation. 
       BACKGROUND OF THE INVENTION 
       [0002]    The type of solar thermal collectors referred as “Linear Fresnel Reflectors” (LFR) are known and used for their simplicity and cost effectiveness. These are fields of flat or quasi-flat reflector “strips” (long and narrow bands) arranged in parallel rows and oriented to a common collector located at a certain height above the reflector field. The collector is also a strip-like, long and narrow structure, aligned in parallel with the rows of reflectors designed to collect the energy from the reflector field. One collector collects the reflected energy from multiple reflector rows on each of its sides. For discussion purposes the basic unit of the field is defined as two adjacent collectors and the reflectors between them. In theory any reflector can serve either of the two collectors. Multitudes of these basic field units (BFUs)—lined up in parallel with the reflector rows—make up the solar collector field, representing its cyclic linear symmetry. 
         [0003]    The known reflectors have one degree of freedom that is a pivotal, rotational motion along their longitudinal axis. A tracking system rotates the reflectors and follows the Sun&#39;s apparent movement. The orientation of the mirrors is such that the reflected incident sunlight “bounces” to one of the two collectors at the edges of the basic field unit (BFU), thereby each row is “focused” to a collector. Some of the known technologies have mechanical linkages connecting the rows of reflectors into a single tracking array. This concept ensures that the rotation angle of each row in the array is the same and that all mirrors are focused to the same collector. Some technologies prefer a North-South alignment of the rows, while others prefer East-West alignment of the field. To describe the location as well as the orientation of the reflector rows in reference to the collectors, the following terminology is used: Contra-Solar rows are the ones that are on the opposite side of the tracked collector relative to the Sun (on the polar side of the collector in the East-West aligned field or West-Side reflectors during the morning in the North-South aligned field) The Contra-Solar reflectors have a larger “normal” surface area exposed to the sunrays therefore they have higher reflection potential. Pro-Solar rows are the ones on the same side as the Sun relative to the tracked collector (equatorial side of the East-West aligned field or the East-side reflectors during the morning hours and the West reflectors during afternoons for the North-South aligned field). The Pro-Solar rows have typically less exposed normal surface, thus they are less effective. 
         [0004]    The purpose of the collectors is to maximize the absorbed solar radiation by capturing the maximum energy from the reflectors and by minimizing the radiation and convection losses of the collector. Water is circulated through single- or multiple-tube collectors as the heat transfer (or working) fluid. The absorber surfaces of the collectors are in effect, boiler surfaces, since the collected solar heat is directly used for steam generation. The collectors may or may not have secondary reflectors to enhance the collection efficiency. To maintain the low cost of the system, glass vacuum-tube (typical for conventional parabolic through systems) is not used. Instead glass cover is used to protect the collectors from excessive convectional heat losses. The trade-off of such design is the higher convectional heat loss of the absorbers. 
         [0005]    The currently known, direct solar thermal steam generation technologies have the following disadvantages: 
         [0000]    1) Large space requirement or limited reflector surface-to-ground surface ratio. This is typical for systems that are designed to minimize the overlapping-shadowing effect (blocking off either the incident or reflected sunlight) of adjacent reflectors. The distance between the reflector rows and their orientation may be optimized for a specific position of the Sun on the sky that occurs only once (twice for equinox) a year. In order to make the highest use of the reflector surfaces, the rows are spaced with considerable gaps between them. This way the extent of the field required for a given thermal output becomes large. Large field then results in extensive and costly piping and other service infrastructures.
 
2) Limited reflected energy per unit of linear length of the mirror. This is typical for systems that are designed to minimize the area of reflector field. In this case the reflector rows are often spaced evenly, close to each other. These systems have low reflector area utilization because the above described blocking-shadowing effect.
 
3) Limited seasonal energy. This is typical for all known systems, including the floating rotating “Solar-Island” concept. This disadvantage comes from the fixed position of the reflectors in relation to the collectors. This anchored position of the mirrors, even if it is optimized, it is ideal only for a single hour of the year, however for the rest of the year the mirrors would require a different optimized distribution between the collectors.
 
4) Limited collector efficiency. The known collector systems either have high heat losses or poor radiation capturing efficiency. Heat losses are caused by the high surface temperature and high incident radiation flux. The root cause of poor collection efficiency is the inaccuracy of focusing of quasi-flat (slightly curved) mirrors over relatively large distances to the absorber. On one hand the active absorber surface of the collector must be limited (to an optimum value), on the other hand the collector aperture (the opening of the collector) receiving the reflected radiation needs to stay large to be able to capture the somewhat scattered sunlight.
 
5) Limited hydraulic stability, poor turndown ratio and insufficient controllability of the water and steam loop systems. As a consequence of horizontal feedwater and evaporator-tubing, extended over large areas and distances, the known systems have very large pressure losses, poor control over the stability of heat transfer and the quality of steam. They have limited or no freeze protection and are prone to high velocity water-hammer—stemming from plug-type fluid flow.
 
6) High cost and complexity of construction. While the LFR technologies in general and the Compact LFR in particular is the simplest and most cost effective compared to other technologies, its installation cost is still considerable and leaves room for significant improvements.
 
       SUMMARY OF THE INVENTION 
       [0006]    The present application thus describes a “traveling” ultra-compact reflector field, where the reflector rows have a new, additional degree of freedom of horizontal mobility, perpendicular to the longitudinal axis of the rows. The traveling rows have the ability to adjust and optimize their position between two collectors such that the reflected sunlight from the field as a whole is maximized throughout the day and throughout the year. 
         [0007]    The present application further describes the carriage apparatus of the traveling reflectors. This device provides the linear and rotational mobility of the reflector structure as well as the tracking and positioning required for maximizing the reflected energy of the BFU. 
         [0008]    The present application further describes the ultra-light, high-efficiency collector-absorber structure. The assembly has a simple construction, advantageous for manufacturing and field erection. The features of the collector are: wide aperture, optimized curvature of the secondary reflector surface, arch-like absorber, rolling-bead cable suspension of absorber, pre-stressed cable-bridge support structure, light-gauge, bent sheet metal enclosure, and flat-plane glass cover. 
         [0009]    The present application further describes the crescent like support rail of the reflector. The gravitational center line of the reflector structure is in the rotational center of the crescent-rail. The reflector may overhang beyond the extent of the crescent-rail. The curvature of the reflector is customized for each row of the BFU. 
         [0010]    The present application further describes the flow distribution and control method of the steam generation system. Each absorber of a collector comprises of multiple tubes. The field comprises of multitude of absorber grids. Optimal control of the thermodynamic conditions (pressure, temperature, velocity and phase) throughout the entire grid is given. 
         [0011]    The present application provides a description of the optimization method (or algorithm) of positioning and orienting the reflectors of the traveling field such that the reflected energy of the field is maximized at all times. 
         [0012]    These and other features of the present application will become apparent to one of ordinary skill in the art upon review of the following detailed description when taken in conjunction with the several drawings and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  shows a Basic Field Unit (BFU), the reflectors positioned between two collectors. The BFU is shown in three different phases of the Sun&#39;s apparent movement represented in increasing radiation angles a, b and c. The position and orientation of the reflectors are optimized for maximum reflected energy in all three phases. 
           [0014]      FIG. 2  shows the carriage apparatus of the traveling reflectors. It illustrates the operation of the “travel &amp; turn” operation of the tracking mechanism. 
           [0015]      FIGS. 3(   a  and  b ) depict the solar-thermal collector apparatus.  FIG. 3   a  is an overall view and some details of the ultra-light, cable-truss-bridge structure of the collector, while  3   b  provides the details of the absorber and secondary reflector. 
           [0016]      FIG. 4  is the process flow diagram of the solar thermal steam generation system 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Referring now to the drawings, in which like numerals indicate like elements throughout the several views,  FIG. 1  shows a schematic view of Basic Field Unit  100 , comprised of multiple rows of reflectors  102  and  103 , between two adjacent collectors  101  and  104  elevated on columns, located above the reflectors. A multitude of BFUs aligned parallel with the reflector rows, and connected to one common steam generation loop comprises one Steam Generation Module (SGM). Multitude of SGMs comprises the Solar Thermal Steam Generation (STSG) Field. 
         [0018]    Based on optimization strategies, the reflectors may target either of the two collectors on the edges of the BFU. Pending on which side of the targeted collector the reflector is, compared to Sun&#39;s position; there are Pro-Solar  102  and Contra-Solar  103  reflectors. The Pro-Solar ones are on the same side of the collector as the Sun. The Contra-Solar ones are on the opposite side of the collector compared to the Sun. Similarly the collector that is on the “Sun&#39;s side” of the BFU may be referred to as Pro-Solar collector  101 . The collector that is on the opposite side of the Sun may be referred as Contra-Solar collector. Contra Solar reflectors target Pro-Solar collectors and vice-versa. 
         [0019]      FIG. 1  also shows the BFU in three phases of the Sun&#39;s apparent path on the sky represented by radiation angles a, b and c. It presents the “travel &amp; turn” or linear  106 , and rotational  105 , movement requirements of the tracking system of the i th  reflector. The position of reflector “i” in the row for phase “a” is represented with distance from the Contra Solar collector d i   a . The rows of reflectors are continuously optimized for maximum energy by a travel &amp; turn movement. Few or all of the rows may require linear motion (travel) adjustment. For a given arrangements of the BFU, for increasing radiation angles a, b and c, the relation of corresponding distances are: d i   a &lt;d i   b  and d i   b &gt;d i   c    
         [0020]      FIG. 2  shows one embodiment of the traveling solar reflector assembly  200 . It illustrates the junction of two adjacent reflectors  201  and  201   b  in a row of connected reflector structures. The reflective surface is a slightly curved glass mirror adhered to a supportive platform  215 . The truss-bridge type, support structure comprised of longitudinal beam  216 , cross beams  217 , trusses  203  and crescent-like end-pieces  202 . This circular-arch-shaped crescent provides the rotational freedom to the reflector around the center of its symmetry. The rotational axis is co-aligned with the center of gravity  204  of the reflector structure to provide smooth, balanced rotation for the tracking mechanism. The crescent  203  is formed from angle iron. 
         [0021]    The two adjacent reflector structures are connected by (at least) 3 driving pins  218 . The pins attached to one end of the reflector structure freely slide into a sleeve  219  attached to the other end of the adjacent reflector. The loose-fit pin-sleeve connection transfers rotational torque from one structure to the other and allows for longitudinal thermal expansion. The driving pin/sleeve is one component and embodiment of positioning and orienting system of the reflector-row. 
         [0022]    The drive train of the tracking system is mounted on the traveling carriage  206 . A dual function electrical step-motor  207  is the drive of the train. It provides two independent, non coincidental rotational drive through two coaxial (one solid, one hollow) shafts. The rotational tracking movement of the reflector is carried out by a sprocket  208  driven roller chain  212  secured to the circumference of the crescent-piece  202 . The linear tracking movement is accomplished through a sprocket  220  driven roller chain  221  that transmits the rotation to sprocket  209 . This sprocket is on a common shaft with sprocket  209 /a that is engaged to a roller chain  213  secured to the bottom edge of the flat rail  214  mounted on a concrete base. The carriage  206  has four wheels  211  rolling on the flat rail. One carriage assembly provides the support and drive for two connected reflectors. Guiding for the carriage on the rail is provided by the guide-plates  205  secured to the base-plate  206  of the carriage. The bottom notch of the guiding plates—extending under the flat rail—provides the stability and security of the reflector structure in case of strong winds. Part of this wind protection system is the “T” shape guide plate—extending over the top of the two adjacent crescents—securing them to the carriage in case of lift. There are four roller wheels  210  mounted on the base plate of the carriage that provide support and free rotation of the crescents of the reflector structures. 
         [0023]      FIG. 3   a  is an overall side view of the ultra-light, pre-stressed cable supported, truss-bridge structure of the collector  300 . The tension-cable structure  315 —supported with truss-rods  314  provides the required rigidity of the large-span bridge.  FIG. 3   b  shows the locations of cross sections A and B on the enlarged side view of the collector for clarity. It also provides a side view of the flexible joint of the collector  316 . The cable truss bridge structure may provide bottom support with two columns supporting at the flexible joint or top or suspended bridge support where a single column is placed in the middle of the bridge between the flexible joints. 
         [0024]    The details of the absorber and secondary reflector are provided on  FIG. 3   c  on cross sections A and B. The solar absorber is comprised of multitude of pressurized water tubes  301 , freely laid over and supported on suspended guy-wire cables  310 . The supporting portion of the cable is covered with rolling beads  302  of cylindrical or oval shape forming a rolling “pearl necklace” type support for the tube. The tube bundle is loosely tied with a bent, narrow, brace-strip  304  such to prevent the tubes on the edges of the bundle from rolling over the tubes in the middle. In the center of the absorber, a rolling pin  303  divides the tubes into two sections, such that thermal expansion is not prevented by friction or other force of resistance on the side, bottom or any other place. The slight arch shape of the absorber bundle facilitates an evenly distributed radiation flux along the tube surfaces. The cable supporting the tube-bundle is suspended on cable saddles  313  and a compressed cross beam  312 . The rigidity of the collector box is provided by angle-iron bars  307  secured to the sides of the covering box. The angle irons also serve as legs of the collector, resting on the square tube supports  308  secured to the trusses  314 . 
         [0025]    A light gauge, polished aluminum sheet metal is used for the secondary reflector  311 . The bent reflector profile is uniquely shaped to provide optimum ratio of aperture-to-absorber width, as well as to capture and entrap most, if not all reflected energy. The body of the collector is a bent, sheet metal cover-box  309 . The enclosed space between the cover-box and the secondary reflector is filled with thermal insulation  306  (fiberglass or such). The insulation is not shown on Section B for clarity. The radiation aperture (bottom opening of the collector) is covered with clear glass pane  305 . The function and benefits of the glass covering are: Reduction of convective heat losses of the collector, resistance to high temperatures and ultraviolet radiation, high transparency, low cost and simple maintenance. 
         [0026]      FIG. 4  illustrates the flow distribution and control method of the steam generation system. It depicts one Steam Generation Module (SGM)  400  of the solar field. Each absorber  412  of a collector  401  comprises of multiple tubes forming a tube bundle as depicted on  FIG. 3   c . The water or steam tubes are connected in supply  416  and return  417  manifolds such that each absorber has a two-pass coil arrangement within one collector. The flow distribution and control of the thermodynamic properties of the fluid throughout the absorber grids of the SGM is of a key importance for high thermal efficiency of the solar steam generation. 
         [0027]    Depending on supply steam quality requirements, the collectors of the SGM are divided into three functional sections: feedwater heaters  402 ; partial or pre-evaporators  403 , evaporators  404  or evaporator-super-heaters  405 . The tube-bundle configuration of the absorbers in each section is custom designed for the specific steam generation duty assigned for the section. These configuration characteristics may include the total number of the tubes in the bundle, the diameter of the tubes, the ratio of supply  416  and return  417  tubes of the bundle etc. The number of collectors assigned to each of the sections is also predetermined during the design phase pending on the split of the overall heat duty of the SGM. 
         [0028]    High pressure feedwater is supplied to the SGM by feedwater pump(s)  404 . There are two adjacent fields  414  and  415  supplied from a common line that make up SGM  400 . The collectors in the feedwater heater section  402 , are connected in series, such that the total flow (feeding the  414  portion) will pass through each collector. The liquid leaving the feedwater heaters is close to boiling temperature. The flow distribution of the pre-evaporators  403  is parallel—such that the total flow is divided between the collector loops of the section. The self-balancing, reverse-return piping provides even flow distribution. The pre-evaporator section is designed to generate a mix of steam and liquid water with a steam quality between 50% and 70%. The ratio of supply-to-return tubes of the bundle is reduced to maintain flow velocity for mixed phase flow. From the pre-evaporator section the total flow is separated to liquid and saturated steam in the separator vessel  406 . The liquid portion is transferred to the evaporator  404  portion. In case superheated steam is required, this third section is configured as an evaporator-super-heater section  405 . The remaining liquid portion of the flow is fully evaporated and/or superheated. 
         [0029]    The mass flow output of the SGM is controlled by a level control loop  407  modulating the feedwater flow by control valve  410  (or variable pump speed). The quality of the supplied steam  413  is controlled by modulating the flow of the evaporator/super-heater section. The control loop  411  maintains the leaving temperature of the steam slightly above saturation for saturated steam or maintains the desired superheat set-point for superheated steam