US20120085341A1

US20120085341A1 - Solar Thermal System

Info

Publication number: US20120085341A1
Application number: US13/219,606
Authority: US
Inventors: Curtis Britland
Original assignee: Individual
Current assignee: Individual
Priority date: 2010-10-07
Filing date: 2011-08-27
Publication date: 2012-04-12

Abstract

A solar thermal power system with associated heat exchanger, variable focus heliostat, and a heliostat stand with a pneumatic piston for shock absorption and positioning is disclosed. The heat exchanger is a solar receiver possessing a beveled bottom substantially perpendicular to the angles of the reflected light from the surrounding heliostats.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/390,862, entitled “Solar Thermal System” filed Oct. 7, 2010, hereby incorporated by reference.

BACKGROUND

1. Technical Field
The system disclosed in this application relates to renewable energy systems. More particularly, this application discloses a solar thermal system and tower mounted solar receiver for converting solar energy into electricity.
2. Background of the Technical Field
Solar thermal systems utilize solar energy in conjunction with heat exchangers to create superheated steam either in the heat exchanger itself or indirectly through the heating of a intermediate such as a molten salt. A solar thermal system is comprised of a tower mounted solar receiver with rows of heliostats which focus reflected sunlight onto the surface of the solar receiver to heat the water or the heat transfer fluid contained therein. A heat transfer fluid with the appropriate properties can be pumped out of the heat exchanger into a storage facility where it acts to store the thermal energy until it can be utilized. The thermal energy collected is used to convert water into steam for the purpose of driving a steam turbine to create electricity.
Currently available solar furnaces require large arrays of heliostats, i.e. mirrors, to focus sufficient light to heat the contents of the power tower. The solar receiver itself is typically a large cylinder housing a heat transfer material through which a heat transfer fluid is pumped through internal conduits.

SUMMARY

Disclosed herein is a solar thermal power system with associated heat exchanger, variable focus heliostat, and a heliostat stand with a pneumatic piston for shock absorption and positioning. The heat exchanger is essentially a solar receiver mounted onto a tower. In one embodiment, the solar receiver possesses a beveled surface at the bottom to create a vessel floor that is substantially perpendicular to the angles of the reflected light from the heliostats. An increase in the amount of incoming light striking the vessel floor perpendicularly, the greater the efficiency of the system. The vessel is sealed and contains a heat transfer fluid that can be either water or a molten salt.
One type of molten salt utilized is a typically mixture of 60 percent sodium nitrate and 40 percent potassium nitrate. In an alternative embodiment, calcium nitrate is included in the salt mixture. In one embodiment, the salt melts at 220° C. and is kept liquid at 290° C. in an insulated storage tank. Solar thermal systems de-couple the collection of solar energy from producing power by storing thermal energy. Electricity can be generated in periods of inclement weather or even at night using the stored thermal energy in the hot salt tank. Tanks are insulated and can store energy for approximately one week.
In various embodiments of the disclosed system, a compressed air piston, i.e. pneumatic piston, is used as a shock absorber on the stand to minimize breakage from wind gusts and for positioning the heliostat. An additional embodiment of a configurable reflective surface of a heliostat panel employs a reflective film and a means to modify the curvature of the surface from flat to parabolic.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an exemplary arrangement of a solar receiver mounted on a tower with associated heliostats.

FIG. 2 a depicts an exploded cut-away view of an embodiment of the solar receiver.

FIGS. 2 b and 2 c depict embodiments of the solar receiver.

FIG. 2 d depicts a cross sectional view of the outer walls of a solar receiver above the base.

FIG. 2 e depicts a cross sectional view of an embodiment of the solar receiver.

FIG. 2 f depicts a cut-away view of an embodiment of the solar receiver.

FIG. 3 depicts an exploded view of an embodiment of the heliostat stand with pneumatic pistons.

FIG. 4 depicts a heliostat with a flexible reflective surface.

FIG. 5 a depicts an exploded cutaway view of an embodiment of the valved heliostat pneumatic piston.

FIG. 5 b depicts an exploded cutaway view of an embodiment of the valved heliostat pneumatic piston.

FIGS. 6 a, 6 b, and 6 c detail a stepwise progression of using the pneumatic pistons to absorb the shock of wind loading on the heliostats.

FIGS. 7 a and 7 b depict a the logic flow of the control system.

DETAILED DESCRIPTION

The disclosed thermal solar system is designed for placement between 37° latitude North and 37° latitude South. An embodiment of the solar thermal system of the present application, as depicted in FIG. 1, includes a plurality of heliostats 200, i.e. heliostat panels 200, individually arranged to reflect and focus solar energy to the base 21 of a solar receiver 100. The solar receiver 100 acts as a heat exchanger and is elevated above a field of heliostats 200 atop a solar receiver tower 50. Each heliostat 200 is of, at a minimum, a reflective surface 110 and a heliostat support 130. The solar receiver 100 is configured with a base 21 that is essentially an inverted hollow frustum of a cone arranged to minimize skewing of the reflected light from a heliostat 200 across the surface area of the base 21. The base 21 is preferably beveled but could also be bulbous. The disclosed base 21 configurations allow for the solar energy reflected from a heliostat 200 to be concentrated across a smaller surface area and allows for a minimization of the size the solar receiver 100.
The solar receiver 100, as depicted in FIGS. 2 a and 2 e, possesses a vessel 20 that functions to contain the solar receiver heat transfer material 27, a heat transfer fluid conduit 26 which functions to permit the heat transfer fluid 30 to travel in a sealed manner into, through, and out of the solar receiver 100 so as to absorb heat from the heat transfer material 27, and a cap 29 which serves to seal and insulate the vessel 20. The vessel 20 is optimally substantially configured of a substantially rigid material having sufficiently high softening and melting points, preferably copper at 1/16 of an inch thickness reinforced by carbon foam, and having a base 21 that, in some embodiments, is substantially an inverted hollow frustum of a cone that tapers down to a substantially central plane beneath the cylinder of the solar receiver 100 so as to leave a vessel port 23 at the bottom. The outer wall 15 of the base 21 is preferably coated with a carbon based solar selective material to improve the absorption of light and subsequent transfer of thermal energy into the vessel 20. In one embodiment, the total surface area of the solar receiver 100 is approximately 2.3 square meters and the solar receiver 100 has an internal volume of approximately 1.6 cubic meters. Optimally, the solar receiver 100 is designed to operate at temperatures of at least 600° C. and at pressures of 250 to 300 bar. Flow through the conduit 26 can be adjusted to assist in control of the solar receiver's 100 temperature, but is optimal at 1.7 m²/s when the operating temperature is 600° C. Higher flow rates require greater residence time within the solar receiver 100 and lower flow rates would require less residence time. Residence time can be controlled by adjusting the pump speed or by increasing the height of the solar receiver 100 and the height/length of an enclosed heat transfer fluid conduit 26. The longer the length of the conduit 26, the longer the residence time within the solar receiver 100.
The vessel 20 possesses a plurality of compartments 22 segregated by compartment walls 35 concentrically arranged about the vessel port 23. The cap 29 is configured so as to receive the compartment walls 35 and seal the individual compartments 22 across the top of the compartment walls 35. Preferably, the compartment walls 35 are constructed from copper, carbon foam, or carbon foam applied to copper. The inner surface 95 of the solar receiver 100 is preferably lined with carbon foam for structural support as well as to facilitate the distribution of heat throughout the solar receiver 100. As shown in FIG. 2 e, the cap 29 also contains a reservoir 18 and boiler relief valve 180 for bleeding off pressure from the vessel as needed.
The solar receiver 100 utilizes a heat transfer fluid conduit 26 configured in an annular serpentine arrangement within the concentrically arranged annular compartments 22 to circulate a heat transfer fluid 30 through the solar receiver 100 and back out through an outlet conduit 27 which passes through the vessel port 23. The sealed compartments 22 contain the serpentine conduit 26 and a heat transfer material 27. The conduit 26 is arranged in connected concentric rows, each row residing within a compartment 22. The heat transfer fluid 30 enters the conduit 26 at the vessel port 23 where it is directed to the outer ring of serpentine conduit 26. Ideally the conduit 26 is fashioned from RA 330 or 321 stainless steel due to its strength and oxidation resistance, but other useful materials known to those skilled in the art may be substituted.
The heat transfer fluid 30 is preferably a molten salt or water. The heat transfer material 25 is preferably carbon foam, cadmium, or zinc. The internal temperature of the solar receiver 100 must stay below the melting point of the copper alloy utilized, e.g. 1075° for C18100. The use of serpentine conduit 26 increases the residence time of the heat transfer fluid 26 in the heat transfer material 27 and solar receiver 100. When a liquid/molten heat transfer material 25 is used within the solar receiver 100, a wicking layer 24, e.g. a stainless steel felt, is used to inhibit pooling of the heat transfer material 25 and to facilitate the migration of a liquid heat transfer material 25 via capillary action. The integrity of the solar receiver 100 is also maintained by the transfer of heat from the solar receiver 100 to the heat transfer fluid 30 circulating through the solar receiver 100 in the conduit 26 and out through the vessel port 23. The conduits 26 are connected through the compartment walls 35 and the heat transfer fluid travels through the conduit 26 from the vessel port 23 to the outer ring of conduit 26 and then circulates progressively inward through the concentrically arranged annular conduit 26 rows before exiting the solar receiver 100 through conduit exiting the vessel port 23.
The connection between each annular conduit 26 row allows the heat transfer fluid 30 inside the conduit to flow from conduit 26 ring to conduit 26 ring, but each compartment that contains a serpentine conduit 26 ring is sealed around the conduit 26 as it passes through the compartment wall 35, preferably by welding, brazing, or application of a seal, e.g. an o-ring, fastened or clamped to the compartment wall, to keep the compartments 22 separated. Because of the vessel's 20 conical shape, a liquid heat transfer material 27 will be prone to flow to the bottom corner of each compartment 22. The wicking layer 24 on the bottom of the compartment will facilitate the distribution of the heat transfer material 27 on the essel's 20 inner surface. As the heat transfer material 27 evaporates from the base 21 of the vessel 20, it keeps the inner vessel 20 surface at an optimal temperature and prevents breaches resulting from overheating and transfers heat to the conduit 26 and its heat transfer fluid 30. The vaporized heat transfer material 27 condenses on the conduit 26 and drips back to the base 21 of the vessel 20.
The heliostats 200 are deliberately spaced to minimize shadowing, reflective blocking, and mechanical blocking. The furthest mirror in the field is placed at approximately 102 m from the base of the approximately 60 m high solar receiver tower 50. The optimal spacing between the rows is 2 m. With this configuration, the minimum incoming and outgoing ray angle at the last row of heliostats approximately 30° from vertical.
The angle of each reflective surface 110 on heliostats 200 in the last row of the heliostat 200 field is between approximately 31° and approximately 29° depending on the altitude angle of the sun. The tilt angle of the reflective surface 110 is substantially 60°+/−2°. The height of the heliostat 200 with a reflective surface 110 tilt angle of approximately 60° is 0.87 meters. This configuration results in approximately 0.03 meters of shadowing on the bottom of each heliostat 200 reflective surface 110 furthest from the solar tower 50. At approximately 100 meters from the tower, the shadowing effect will no longer occur. The annual light losses from shadowing are approximately 1 percent. Adjacent heliostats 200 are spaced 0.25 meters from each other to avoid mechanical blocking. As heliostats 200 get progressively closer to the tower, their angle of inclination increases so as to maintain focus on the base 21 of the vessel 20 of the solar receiver 100.
The energy loss in solar thermal systems due to atmospheric attenuation is estimated to be approximately 6 percent and the loss due to the cosine effect is estimated to be approximately 23.4 percent. The loss of efficiency is further aggravated by typical reflective losses of 6 percent for standard reflective coatings. The total expected efficiency of the solar thermal field of heliostats 200 is estimated to be approximately 67 percent.
Heliostats 200 are commonly designed with flat or slightly curved mirrors 110, i.e. reflective surfaces 110, which are able to direct light at a specified target. These mirrors 110 are typically constructed using aluminum or glass. Common mirror 110 sizes range from 2 to 10 meters square. In order to collect the light from the mirrors 110, the solar receiver 100 needs to have at least as much surface area as a mirror 110. In order to reduce the size of the solar receiver 100, each heliostat 200 needs to be able to focus the light on the base 21 of the vessel 20 of the solar receiver 100. Each heliostat 200 requires a different focal point to accurately concentrate the light on the base 21. Heliostats 200 have not been designed to efficiently focus solar energy on a small target in this way because each reflective surface 110 would need to be uniquely manufactured to with the appropriate focal length.
The reflective panels 110 on the heliostats 200 can be conventional reflective mirrors used in existing solar collection fields, but equipped with shock loaded pistons and motors. Alternatively, the reflective panels 110 can be frames equipped with a solar mirror reflective film such as 3M® Solar Mirror Film 1100™. The foregoing embodiments permit the reflective panels 110 to be repositioned and/or reshaped so as to optimize solar energy reflection to the appropriate focal point.
As depicted in FIG. 4, the use of a solar mirror film as a reflective surface 110 permits the reflective surface to be reconfigured to optimize reflection of solar energy to the base 21 of the vessel 20 by adjustment of the concavity of the reflective surface 110.
The frame of the heliostat 200 is designed to allow it to flex while keeping it secure against high winds. The mirror 110 is mounted along the edges of the reflective surface 110 to a reflective surface frame 120 using anchors 118 that pivot and rubber bushings 114 to allow the mirror 110 to move while being adjusted.
The adjuster bolt 115 is placed at the middle of the mirror 110 and is used to pull the center of the reflective surface 110 back toward the panel frame 112 in order to create a parabolic shape. The adjuster bolt 115 is positioned by a motor 121.
The heliostat drive uses components that are specialized for high accuracy actuation in applications where high mechanical resistance and sudden shock frequently occur. The configuration is designed for low cost manufacturing and minimal maintenance. The actuator uses a bidirectional pneumatic piston 250 that is coupled with a gear box 264 to deliver the power output and accuracy that is necessary in solar applications. The pneumatic piston 250 described herein to drive the solar actuator delivers high power output and acts as a shock absorber to protect the gear box 264 and reflective surface 110 of the heliostat panel 200.
As depicted in FIGS. 5 a and 5 b, a pneumatic piston 250 is utilized for both shock absorption and movement. As shown in FIGS. 6 a, 6 b, and 6 c, the pneumatic piston 250 is pressurized to induce movement as well as to provide the cushioning effect of a shock absorber should a wind gust jar a heliostat panel 140. A centralized compressor pressurizes the pneumatic piston 250 typically to between thirty and ninety psi. The air flowing from the compressor will flow through a manifold to each of the actuator blocks. The manifold will select the actuator that is being manipulated. Air flowing into a block preferably containing between five and twenty actuators will pass through an electro-pneumatic regulator after which it will pass to a cylinder control valve. In one embodiment, as shown in FIG. 5 a, the electro-pneumatic regulator is optimally a four port, two position valve arranged outside the pneumatic piston, which extends or retracts the cylinder. The pneumatic piston 250 will permit some movement of the heliostat panel 140 along a path that is perpendicular to the general plane of the reflective surface 110 so as to relieve some of the stress from wind loading. Alternatively, the heliostat panel 140 can be rotated so as to minimize the surface area of the panel 140 facing the wind. The use of pneumatic pistons 250 to drive movement of the panels increases the longevity of the heliostats and drive components 200.
The two pneumatic pistons 250 are utilized to control the position of their respective rack gears which rotate the heliostat 200 about vertical and horizontal axes. An electronic control system selects appropriate actuators using the manifold and supplies the appropriate airflow and pressure using the electro-pneumatic flow control. The electronic control system monitors and responds to the heliostats 200 position using an encoder that is connected to the gearbox via an encoder mount. This configuration would step the rotation of each heliostat 200. FIGS. 7 a and 7 b depict the logic flow of the control system.
An individual pneumatic piston 250 is typically driven at 35 psi by a compressor. A valved pneumatic piston 250 is depicted in FIG. 5 a. A valveless pneumatic piston 250 is depicted in FIG. 5 b. Each pneumatic piston 250, i.e. actuator 250, is equipped with a motor 251 controlling the flow of air to a four port, two position valve 256 that regulates the piston's 255 rate of movement and controls the direction of movement. The piston barrel 258 is sealed at one end by the bottom cap 253 and at the other end by the top cap 263. The bottom cap possesses an exhaust port 254 integrated into the valve seat 269. A bottom seal 252 provides an airtight seal for the piston barrel 258. A piston seal 270 segregates the piston 255 into airtight compartments below and above the piston 255. Piston guides 257 facilitate the movement of the piston across the piston barrel and keep it aligned. A pressure relief valve 271 is affixed to the inlets 262 in the top cap 263 of the pneumatic piston 250 to prevent pressure fluctuations in the pneumatic system that could be caused by sudden wind loading.
The pressure delivered to each pneumatic piston 250 is dependent upon wind conditions and optimized to lower energy consumption on calm weather days and reduce wear on components. As the piston 250 is actuated, a rack gear 267 that is attached to a piston rod 267 rotates the main drive via the gearbox 264. This translates the linear motion of the piston 250 into rotary motion for the heliostat panel 200. The gearbox 264 is preferably machined from steel, and serves as the mount for the pneumatic piston 250. At 35 PSI with a head diameter of 3 inches and 6:1 gearbox 264, the system will produce approximately 1500 lbs of torque. The pressure, head/bore diameter, and gearbox ratio are configurable to meet different operational and space requirements. The drive gear is monitored using an encoder 266 in electronic communication with a means for controlling the pneumatic piston 250, e.g. a control system, computer with instructions acting as a control system written in a machine readable language in non-volatile memory, so that the position of the heliostat panel 200 is available to the system. If sudden wind loading changes the position of the heliostat panel 200, the encoder 266 will provide feedback to the control system to allow it to correct the position of the heliostat panel 200. The system will track adjustments to each flow valve 256 and use an incremental encoder to determine output performance. Electronic encoders 266 measure the performance output of the pneumatic pistons 250 and are assigned unique addresses, e.g. serial numbers, MAC addresses, or IP addresses for identification. The data and addresses allow tracking software used by the control system to manipulate each pneumatic piston 250 individually to achieve optimal positioning of the individual heliostat panels 200.
The position of the heliostat panel 200 is monitored and controlled by the aforementioned control system through manipulation of an altitude actuator 620 and an azimuth actuator 610, each being a pneumatic piston 250 and associated components. As depicted in FIG. 3, A heliostat panel mounting bracket 630 engages the piston head 268 of the altitude actuator 620 in a fixed arrangement by means of a altitude actuator gearbox mount 625 so as to translate the rotational movement of the piston head 268 into rotational movement of the heliostat panel mounting bracket 630 about a substantially horizontal axis. The altitude actuator gearbox mount 625 is coupled to the azimuth actuator gearbox mount 615 so as to translate rotational movement of the piston head 268 of the azimuth actuator 610 into rotational movement of the heliostat panel 200 about a substantially vertical axis. The azimuth actuator 610 is mounted into a heliostat stand base 650 for support.
A motorized drive mechanism that rotates the adjuster bolt is mounted to the heliostat panel 140. During the calibration of the heliostats 200, the drive motor is connected to a mobile calibration unit, or “MCU”, that is used to power and control the motors. This MCU is equipped with a generator, plugs for the motors, and a computer that monitors the mirror adjustments and wirelessly communicates with the central heliostat 200 control system. When the reflective surface 110 of the heliostat 200 are being adjusted, the central control system for the field will target the boiler one mirror at a time. Once the reflective surface 110 of the heliostat 200 is directing light on the base 21 of the vessel 20, the focal adjuster will begin to alter the focal point for optimum concentration. A fixed camera may be utilized to observe the base 21 of the solar receiver 100 to determine proper alignment of the reflective surface 110 of the heliostat 200. Once the heliostat 200 has been adjusted, the adjuster bolt 115 is locked in place and the drive motor is removed. The adjuster motor 118 can be reattached if the focal point of the heliostat 200 needs further adjustment.

Claims

1. A solar receiver comprising:

a. a vessel for holding a heat transfer material, said vessel being configured as a substantially vertical cylindrical wall with a bottom edge and a top edge affixed to a base substantially configured as an inverted hollow frustum of a cone having an internal surface, an external surface, a base top connected to said bottom edge of said vertical cylindrical wall of said vessel, and a base bottom having a conduit port sealed about a inlet conduit segment and an outlet conduit segment to permit the entry and exit of a heat transfer fluid through a heat transfer fluid conduit;

b. a cap having a bottom face in a sealed arrangement with said top edge of said cylindrical wall; and

c. concentrically arranged compartments within said vessel separated by substantially annular compartment walls, said compartment walls extending from said bottom face of said cap to said internal surface of said base so as to seal each compartment, said compartments having heat transfer fluid conduits arranged as an annular ring in each compartment, and said annular rings connected so as to permit progressive flow from an outermost ring to an innermost ring through conduit ports in said compartment walls.

2. The solar receiver of claim 1, wherein a reservoir contained within said cap is connected to said vessel by a valve which provides pressure relief for said vessel.

3. The solar receiver of claim 1, wherein said annular rings are arranged in a vertically looping serpentine configuration within each said compartment.

4. The solar receiver of claim 1, wherein said base is beveled above said port.

5. The solar receiver of claim 1, wherein said base is bulbous above said port.

6. The solar receiver of claim 1, wherein said vessel and said base are constructed of a copper alloy.

7. The solar receiver of claim 6, wherein said heat transfer fluid conduit is constructed of a material selected from the group consisting of a copper alloy, stainless steel, and galvanized steel.

8. The solar receiver of claim 1, wherein said heat transfer material is selected from the group consisting of carbon foam, cadmium, and zinc.

9. The solar receiver of claim 8, wherein said heat transfer fluid is selected from the group consisting of water and molten salt.

10. The solar receiver of claim 9, wherein said molten salt is a mixture of 60 percent sodium nitrate and 40 percent potassium nitrate.

11. The solar receiver of claim 10, wherein said mixture contains calcium nitrate.

12. The solar receiver of claim 1, wherein at least part of said internal surface of said base is covered by a layer of a wicking material that extends under at least part of said compartment walls from said conduit port to said cylinder wall.

13. The solar receiver of claim 1, wherein said wicking material is a stainless steel felt.

14. A heliostat drive comprising:

a. a plurality of pneumatic pistons, wherein a first said piston engages a heliostat panel support about a horizontal axis and a second pneumatic piston engages a pneumatic piston support affixed to said first piston and arranged along a substantially vertical axis;

b. a heliostat panel support, said heliostat panel having a substantially planar heliostat support for engaging a heliostat panel, engaged with a drive mechanism which is driven by and engages a distal end of a pneumatic piston;

c. means to control said drive mechanism through the actuation of said plurality of pneumatic pistons; said means to control said drive mechanism controlling the flow of air to said plurality of pneumatic pistons so as to cause said pistons to move linearly along a pneumatic piston barrel and convert said linear movement of said piston into rotational movement of a drive mechanism by means of gears driven by a piston rod, said gears engaging a drive shaft at a distal end of each of said plurality of pneumatic pistons so as to provide linear to rotational translation of said linear motion of each said pneumatic piston; and

d. means for coupling said plurality of pneumatic pistons and said heliostat panel support.

15. The heliostat drive of claim 14, wherein said drive is remotely controlled.

16. The heliostat drive of claim 14, wherein said means to control said drive mechanism is a computer having machine readable instructions written in non-volatile memory to control said plurality of pneumatic pistons through the control of air flow to said plurality of pneumatic pistons.

17. A solar thermal system comprising:

b. a cap having a bottom face in a sealed arrangement with said top edge of said cylindrical wall;

c. concentrically arranged compartments within said vessel separated by substantially annular compartment walls, said compartment walls extending from said bottom face of said cap to said internal surface of said base so as to seal each compartment, said compartments having heat transfer fluid conduits arranged as an annular ring in each compartment, and said annular rings connected so as to permit progressive flow from an outermost ring to an innermost ring through conduit ports in said compartment walls;

d. a heliostat drive comprised of a plurality of pneumatic pistons, wherein a first said piston imparts rotation to a heliostat panel about a substantially horizontal axis and a second pneumatic piston imparts rotation to a heliostat panel about a substantially vertical axis; and

f. means to control said heliostat drive through the actuation of said plurality of pneumatic pistons; said means to control said drive mechanism controlling the flow of air to said plurality of pneumatic pistons so as to cause said pistons to move linearly along a pneumatic piston barrel and convert said linear movement of said piston into rotational movement of a drive mechanism by means of gears driven by a piston rod, said gears engaging a drive shaft at a distal end of each of said plurality of pneumatic pistons so as to provide linear to rotational translation of said linear motion of each said pneumatic piston.