Ultra-compact, linear, solar-thermal steam generator

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.

Description

TECHNICAL FIELD

The present application relates generally to solar thermal energy collectors and more particularly relates to direct solar thermal steam generation.

BACKGROUND OF THE INVENTION

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.

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'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.

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.

The currently known, direct solar thermal steam generation technologies have the following disadvantages:

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

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.

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.

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.

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.

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.

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.

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

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'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.

FIG. 2 shows the carriage apparatus of the traveling reflectors. It illustrates the operation of the “travel & turn” operation of the tracking mechanism.

FIGS. 3( a and b) depict the solar-thermal collector apparatus. FIG. 3a is an overall view and some details of the ultra-light, cable-truss-bridge structure of the collector, while 3b provides the details of the absorber and secondary reflector.

FIG. 4 is the process flow diagram of the solar thermal steam generation system

DETAILED DESCRIPTION

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.

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'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'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.

FIG. 1 also shows the BFU in three phases of the Sun's apparent path on the sky represented by radiation angles a, b and c. It presents the “travel & 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 & 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<d_i^b and d_i^b>d_i^c

FIG. 2 shows one embodiment of the traveling solar reflector assembly 200. It illustrates the junction of two adjacent reflectors 201 and 201b 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.

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.

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.

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. 3b 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.

The details of the absorber and secondary reflector are provided on FIG. 3c 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.

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.

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. 3c. 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.

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.

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.

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

Claims

1. A solar thermal steam generation system (STSG), comprising: a plurality of solar reflectors lined up and connected in a row, and a plurality of parallel rows located in proximity with the ground, whereas the cross sectional profile of the reflectors is flat (linear) and/or slightly curved, anda plurality of collector rows located above and in parallel with the solar reflectors, such to collect and absorb the solar energy directed to them by the reflectors and directly transfer it to thermal energy of steam flow, whereas one collector is to collect energy from multitude of reflectors.

2. The solar thermal steam generating system of claim 1, wherein the reflectors have two independent positioning degrees of freedom: one rotational along the longitudinal axis, and one linear, parallel with the ground and perpendicular to their longitudinal axis. The combination of rotational and linear movement of the reflectors is used to track the Sun and maintain reflection of incident light to one of the collectors on either side of the reflector as well as to position the reflector in the space between two adjacent collectors such that the total reflected energy of the field is maximized in all positions of the sun during the year.

3. The STSG of claim 1, wherein the ratio of reflectors to collectors is typically between 8 to 24 and the collection angle (between the reflected Sun-rays from the furthest pro- and the furthest contra-solar reflectors) is between 70° and 110°

4. The STSG of claim 1, wherein each of the plurality of collectors comprising of plurality of metal tubes (forming an absorber tube-bundle) containing water in liquid or in vapor form, wherein the tubes are arranged in an ach shape by gravitation, suspended freely on a cable suspension.

5. The cable suspended tube bundle of claim 4, wherein the portion of the cable supporting the tube-bundle is filled with freely rotating hollow “beads” of cylindrical or curved profile. Whereas the arch shape of the tube-bundle is further secured by a pre-formed metal bracelet over and across the top of the tubes to the cable. Whereas further to the support of the tube-bundle, a singularity or plurality of hollow rollers on pins is installed between a singularity or plurality of the tubes supported from the bracelet and the suspension cable.

6. The dual function traveling—rotating reflector support carriage comprising: a base plate mounted on plurality of freely rotating wheels, whereas the support carriage rolls on the wheels on an approximately flat horizontal rail located on the ground and whereas the carriage is guided by guiding-plate or rollers preventing derailing or lifting the carriage from the rail.

7. The reflector of claim 1 wherein the reflector mirror is slightly curved for focus, wherein the curvature of each reflector is customized for its distance from the targeted collector.

8. The reflector of claim 1 is supported by truss-bridge sub-structure with two circular arch-shaped end-pieces also referred as crescent-rails. The crescent rail is formed from angle iron. Whereas the width of the reflector of claim 1 extends over the diameter of the crescent-rail.

9. Carriages of claim 6 support the reflector sub-structure through the crescent-rail end pieces of claim 8. The plurality of reflector-structures of claim 1 is connected chain-like in a row. Wherein one carriage support is shared by two adjacent reflector-structures in the middle of the row. The carriage located on the end of the reflector row supports only one crescent-rail.

10. The tube bundle of claim 5—functioning as absorber of solar energy into water flow—further comprising of: one or plurality of supply tubes enabling the flow of inlet (liquid or vapor) water in the same direction andplurality of return tubes enabling the flow of outlet (liquid or vapor) water in the same direction.Whereas the supply and return tubes of the bundle form a two-pass coil system also referred as absorber coils.

11. A method of steam generation in horizontal tube bundles extended over large areas by capturing the Sun's thermal energy, comprising: Flowing all feedwater through each of the plurality of solar collector absorber coils in series—collectively also referred as feedwater heater section; wherein the inlet water temperature is substantially below the boiling temperature and wherein the outlet water temperature is substantially close but slightly below boiling temperature.Flowing all water leaving the feedwater section into plurality of solar collector absorber coils in parallel—that is to say that the total flow is evenly divided between the number of the coils, wherein the even flow distribution is ensured by a self balancing arrangement known in the art as “reverse-return” system The absorber coils connected in this parallel manner are referred as pre-evaporator section, wherein the inlet water is substantially liquid (has no vapor content) and wherein the outlet water has vapor-phase fraction of approximately from 40% to 80%.Flowing all water leaving the pre-evaporator section into a separator vessel to separate the liquid phase from vapor phase by reduced speed and by gravity.Flowing the liquid portion of the total flow, leaving the separator vessel into plurality of solar collector absorber coils in parallel—also referred as evaporator or evaporator-superheater.Wherein the inlet water has substantially no vapor content and is at boiling temperature and wherein the outlet water is fully evaporated and is slightly or substantially above boiling temperature.

12. The method of steam generation of claim 11 wherein the steam flow output of the STSG is controlled by a level control loop, modulating the feedwater flow either by control valve or other means and wherein the quality of the supply steam leaving the STSG is controlled by modulating the liquid water inlet flow to the evaporator/superheater section, and wherein the control loop maintains the temperature of the outlet steam at the desired set-point.

13. The carriage of claim 6 further comprises of plurality of roller-wheels mounted on the base plate, whereas the rollers symmetrically support the circular-arch crescent rails of claim 8 providing stability and free rotational-pivotal movement (degree of freedom).

14. The carriage of claim 6 further comprises of 2 guiding plates of “T” shape secured to the base plate, whereas the horizontal plate portions extends over the horizontal arm of the angle iron of the rotating crescent-rail of claim 8, preventing derailing or lifting the reflectors from the carriage.

15. The carriage of claim 6 further comprises of dual function drive train, whereas the source of driving motion is an electric step-motor mounted on the base plate of the carriage. Whereas the motor has two independent shafts engaged one at a time and whereas one sprocket is mounted on each of the shafts and whereas one sprocket is smaller than the other.

16. The dual function drive train of claim 15. further comprises of pivotal rotation drive, whereas the smaller sprocket of claim 15. is engaged with a roller chain mounted on the rim of the Crescent-Rail of claim 8 and whereas in the rotational motion mode the step motor of claim 15 engages the shaft with the smaller sprocket and turns the Crescent Rail rolling on the symmetrical rollers of claim 13 through the roller chain on the crescent.

17. The dual function drive train of claim 15. further comprises of linear motion drive, whereas the larger sprocket of claim 15. is engaged—through a set of two transmission sprockets and a transmission roller chain—with a roller chain mounted on the rim of the flat horizontal rail of claim 6.

18. A method of positioning the reflectors by “turn and travel” for maximum reflected energy for the diurnal procession and analemma (Sun's apparent movement on the sky) comprising of: Method of “W sandwiching” that is the orientation of reflectors located between two adjacent collectors, whereas a pro-solar reflector may be sandwiched between two counter-solar reflectors (on the counter-solar portion of the field, next to the median reflector) and a counter-solar reflector may be sandwiched between two pro-solar reflectors (on the pro-solar portion of the field, next to the median reflector)—for the purpose of minimizing optical interference (shadowing and blocking the incident or reflected sunlight).Method of sizing the distance between two adjacent collectors that is based on the position of the Sun at noon on the day of the Equinox, whereas the shadowing/blocking effect is zero and the unutilized reflector gap (space between reflectors that incident sunlight is not reflected but available otherwise) is also zero and whereas the described configuration would form the reflectors in a “W” shape in and around the median reflector located between two collectors.Method for summer, when the space available for positioning the reflectors is larger than the one required for zero shadowing/blocking, whereas the first priority of the method is to eliminate the “W” shadowing from the middle section of the field between two adjacent collectors as much as possible, and whereas the second priority of the method is to cluster the pro-solar reflectors as close to contra-solar collectors—and conversely cluster the contra-solar reflectors as close to pro-solar collectors—as possible, while maintaining zero shadowing/blocking, whereas the middle section between the collectors may be left for concentration of reflector-gaps.Method for winter when the space available for positioning the reflectors is smaller than the one required for zero shadowing/blocking; whereas the first priority of the method is to orient an additional pro-solar reflector, in the next available spot (sandwiched) in the contra-solar portion of the field; whereas the second priority of the method is to sacrifice shadowing/blocking surfaces of reflectors in reverse order of effectiveness (sacrificing pro-solar reflector surfaces staring from the furthest); wherein the sacrificing means positioning the reflectors as close to each other as required and possible and losing some effective reflecting surfaces.