Patent Application
20100326424
This invention relates to solar-thermal energy systems. In particular, the invention relates to a highly efficient residential solar thermal energy collection, storage, and utilization system having a parabolic trough-type solar concentrator rotatably mountable on a preferably fixed structure, such as a residential rooftop, and a tubular heat collector coaxially positioned to receive concentrated sunlight from the concentrator, with the concentrator and collector shaped and oriented to maximize solar collection efficiency and thermal energy delivery to a heat-powered engine for optimizing mechanical and electrical power generation.
Despite over a century of attempts to make solar power commercially viable, solar energy currently makes up an insignificant proportion of per capita energy supply. This has been due primarily to performance and cost inefficiencies of existing solar energy collectors, concentrators, and interfaces to heat storage media which have prevented widespread adoption and use for commercial and residential applications. For example, the SEGS, Solar Electric Generating System, plants in Southern California represent the state of the art today in deployed CSP, Concentrating Solar Power. Based on the experience with the existing SEGS plants, the cost of electricity from newly constructed plants using currently available technology is approximately 10¢/kWh. This cost is much greater than the cost to generate electricity by burning coal, which is approximately 3¢/kWh.
Various solar energy collectors and concentrators, and interfaces to heat storage media and heat engines are known for use in solar thermal electric energy systems, such as the SEGS plants. A few examples include: U.S. Pat. No. 4,586,334 to Nilsson, and U.S. Pat. No. 6,487,859 to Mehos. The Nilsson patent discloses “ . . . a solar energy power generation system which includes means for collecting and concentrating solar energy; heat storage means; Stirling engine means for producing power”, and “ . . . the means for collecting and concentrating solar energy is a reflective dish; and the heat transfer means includes first and second heat pipes; the heat storage means is preferably a phase change medium . . . ” The Mehos patent discloses: “ . . . sodium heat pipe receivers for dish/Stirling systems”, and cites references demonstrating: “ . . . sodium vapor temperatures up to 790° C.” Additionally, U.S. Pat. No. 4,125,122 discloses a heat pipe receiving energy from a solar concentrator, U.S. Pat. No. 6,700,054B2 describes connecting to a Stirling engine, among other things, and U.S. Pat. No. 4,088,120 describes a parabolic trough with a heat pipe at the focus connected to a heat storage medium. U.S. Pat. No. 787,145 describes an elliptical dish mirror that is oriented to track the sun, with a boiler to produce steam at the focus of the mirror. U.S. Pat. No. 3,982,526 describes a device for turning a solar collector about a polar axis, and U.S. Pat. No. 6,886,339B2 describes a parabolic trough solar concentrator with a sun tracking system. U.S. Pat. No. 4,205,657 describes a parabolic trough solar concentrator with a steam generation system. U.S. Pat. No. 4,108,154 describes a parabolic trough solar collector with a windshield.
One particular limitation of currently available solar collectors/concentrators, however, is their relatively low thermal gathering efficiency, which is the ratio of the thermal heat delivered by the heat collecting element relative to the solar heat incident on the concentrating mirror surface area. Based on recent field measurements, the best available collector's, (such as the UVAC heat collector from Solel or the PTR 70 heat collector from Schott, using an oil based heat transfer fluid heated to 400° C.), achieve a maximum value of only 50% thermal gathering efficiency at a solar incidence of 800 W/m2. At either higher or lower solar irradiance levels, the thermal efficiency is even lower. This efficiency is low primarily because the solar concentration factor for these collectors is relatively low. For example, in the current generation of SEGS plants, the diameter of the absorbing surface in the heat-collecting element is 7 cm, while the width of the parabolic trough aperture is 5.77 m, and the ratio of the concentrator aperture area to collector absorber area, the solar concentration factor, is only 26. Another limitation associated with the relatively low concentration factors of parabolic trough collectors is that the axial length of the collector relative to the concentrator aperture width is quite large. In the DISS case, for example, the length to width ratio is 46.
Another efficiency loss factor that is characteristic of the current state of the art parabolic trough collectors is associated with their horizontal deployment. Averaging over the range of solar incidence angles both through the day and through the year, leads to an average geometrical foreshortening factor of 87%.
Since the efficiency of conversion from solar irradiance to power is known to have a great impact on the cost of electricity, it would be advantageous to provide a highly efficient solar thermal power system for the economical utilization of solar thermal energy in the context of a residential/commercial unit which overcomes the limitations of current solar energy technology for reducing energy costs. And in particular an apparatus and method capable of increasing the solar concentration factor for parabolic trough collectors to beyond about 160 and improving the average geometrical foreshortening factor to greater than about 90%, would be particularly beneficial to substantially raise the thermal gathering efficiency of such solar thermal power plants.
One aspect of the present invention includes a solar thermal power plant comprising: a parabolic trough mirror having a longitudinal focal axis for concentrating sunlight therealong; means for rotating said mirror about a longitudinal rotation axis to follow the sun; and a heat collector comprising an elongated heating tube surrounding a flow channel, said flow channel having an oblong cross-sectional shape characterized by major and minor axes with a largest diameter of the channel along the major axis and a smallest diameter of the channel along the minor axis and with the major axis aligned with a longitudinal plane of symmetry of the parabolic trough mirror, said heating tube coaxially positioned along the focal axis of said mirror to receive concentrated sunlight therefrom so that a working fluid in said heating tube is heated thereby and provided for use through an outlet end of said heating tube.
Another aspect of the present invention includes a solar thermal power plant comprising: a parabolic trough mirror having a longitudinal focal axis for concentrating sunlight therealong; means for rotating said mirror about the rotation axis to follow the sun; and a tubular heat collector comprising an optically transparent thick-walled heating tube having an inner wall surface forming a flow channel and a convex curvilinear outer wall surface for magnifying the dimensions of the flow channel, said inner wall surface coated with a sunlight absorbing material, and said heating tube coaxially positioned along the focal axis to receive concentrated sunlight from said mirror so that a working fluid in the flow channel is heated thereby and provided for use through an outlet end of the heating tube.
And another aspect of the present invention includes a solar thermal power plant comprising: a parabolic trough mirror having a longitudinal focal axis for concentrating sunlight therealong; means for mounting said mirror so that the focal axis is parallel with the earth's rotational axis and said mirror is rotatable about a longitudinal rotation axis thereof; means for rotating said mirror about the rotation axis to follow the sun; and an elongated tubular heat collector forming a flow channel and coaxially positioned along the focal axis to receive concentrated sunlight from said mirror so that a working fluid in the flow channel is heated thereby and provided for use through an outlet end of said heat collector.
And another aspect of the present invention includes a solar thermal power plant comprising: a parabolic trough mirror having a longitudinal focal axis for concentrating sunlight therealong; means for mounting said mirror so that the focal axis is parallel with the earth's rotational axis and said mirror is rotatable about a longitudinal rotation axis thereof; means for rotating said mirror about a longitudinal rotation axis to follow the sun; and a tubular heat collector comprising an optically transparent thick-walled heating tube having an inner wall surface forming a flow channel and a convex curvilinear outer wall surface for magnifying the dimensions of the flow channel, said flow channel having an oblong cross-sectional shape characterized by major and minor axes with a largest diameter of the channel along the major axis and a smallest diameter of the channel along the minor axis and with the major axis aligned with a longitudinal plane of symmetry of the parabolic trough mirror, said inner wall surface coated with a sunlight absorbing material, and said heating tube coaxially positioned along the focal axis to receive concentrated sunlight from said mirror so that a working fluid in the flow channel is heated thereby and provided for use through an outlet end of the heating tube.
Generally, the residential solar thermal power plant of the present invention is largely based on the solar thermal power plant used in the solar thermal aircraft described herein. As such, the residential solar thermal power plant of the present invention has several main components, including a solar concentrating mirror capable of focusing/concentrating sunlight and rotating about a rotation axis, a heat collector/heating tube positioned to absorb the concentrated sunlight, a thermal energy storage reservoir connected to an outlet end of the heat collector, and a heat-powered engine operably connected to the thermal energy storage reservoir, all of which are similar in construction and operation to those previously described for the solar thermal aircraft. The residential solar thermal power plant, however, includes additional efficiency-improving features which are enabled in part by being mountable on a preferably fixed structure, such as the roof of a building, and which together operate to improve the overall efficiency of the power plant.
For example, in one particular embodiment the heating tube of the heat collector has an oblong cross-sectional profile which increases the solar concentration factor, i.e. the ratio of the aperture area of the concentrator mirror to the sunlight absorbing area of the heating tube. In another embodiment, an optically transparent thick-walled heating tube is used so that the outer surface of the heating tube operates to magnify the dimensions of the flow channel formed by an inner surface, to increase the solar concentration factor further still. Furthermore, in still another embodiment, the concentrator mirror and the heat collector are capable of being mounted so that the focal axis of the mirror and the heat collector are aligned parallel with the earth's rotational axis. This minimizes the foreshortening effect of solar incidence for different times of the year to improve solar concentration. Since increased efficiency, with negligible impact on system capital cost, directly increases the power generation rate to lower the cost of the electric power, these efficiency improving features of the residential solar thermal power plant of the present invention independently as well as in combination provide energy/power generation at reduced cost.
Table 1 lists several efficiency factors which are well known (based on the experience with commercially running power plants, such as the SEGS plants in Southern California) to contribute to the overall efficiency of parabolic trough systems. Additionally, Table 1 shows how these efficiency factors are improved by the present invention.
The numerical values in Table 1 for conventional parabolic troughs are taken from the Sargent Lundy report for 2004 parabolic trough technology. The net efficiency advantage of the present invention, i.e. the product of all the individual efficiency factors, is shown in the last row in the table.
Since there is little in the current configuration that incurs additional cost relative to those well known in the SEGS plants, it is possible to estimate the cost of electricity by scaling the conventional SEGS cost by the inverse of the relative efficiency factor from Table 1. Assuming no significant increase in capital costs, the Levelized Electricity Cost (LEC) is estimated to be cut from 10¢/kWh to 6¢/kWh. In the residential application, the economic value of the heating derived from the cooling water feed to the steam engine can be estimated based on the quantity of avoided heating fuel. This economic value is approximately 2¢ per kWh of heating energy. The heating energy derived from cooling the engine is approximately double the power produced by the engine. Reducing the LEC cost by the economic benefit derived from water and space heating leads to a cost for the electric power that is less than 4¢/kWh. Since this cost is much less than the retail price of electric power, approximately 10¢/kWh for a typical customer in Northern California, this shows that residential solar thermal power based on the configuration of the present invention is indeed economically competitive.
The accompanying drawings, which are incorporated into and form a part of the disclosure, are as follows:
a is an enlarged cross-sectional view of the heat collection element and back-reflector enclosed in circle 3a of
Reference numerals used in the following description for the solar thermal aircraft are listed in Table 2.
Turning now to the drawings,
Rotational control of the solar concentrator is provided by a solar tracking device or means including a device or means for determining whether the solar concentrator is optimally aligned with the sun, and a device or means for actuating, e.g. rotating, the solar concentrator mirror into optimal alignment with the sun based on the optimal alignment determination. As used herein and in the claims, “optimal alignment” is that alignment and angle producing the highest concentration of solar flux, i.e. a position “directly facing” the sun. The actuation device or means may comprise, for example, a drive motor 115 (
A preferred method of heliostat operation uses the one center and two outer solar cells in a closed loop feedback stabilization system involving two modes of operation: a sun-searching mode, and a sun-tracking mode, shown in
In the sun-searching mode, photo-diodes associated with the two outer cells 117 and 119 are connected electrically as shown in
It is appreciated that sun-searching mode is required at sunrise once per day, and also each time the aircraft heading becomes very close to the projected direction to the sun, and the heliostat is not sufficiently illuminated to maintain sun-tracking. Additionally, the solar cell sensors are adapted to provide power to directly drive the axial rotation actuator, i.e. DC motor 115, and no external power source is required. In this manner, the mass and complexity required for the heliostat system are greatly reduced.
Once the parabolic trough reflector 110 is aligned to the sun, solar radiance is focused onto the center of a heat collector 120 shown best in
As shown in
The shell 125 of the heat pipe shown in
With the addition of a highly reflective, semi-circular back-reflector 113, shown in
The fabrication methods for the heat collector 120 are well known to those skilled in the art of electronic vacuum tube fabrication. Indeed, the overall structure is similar to a long cylindrical “light bulb”, consisting of a transparent envelope with a central high temperature “filament”, i.e. the heat pipe 129. As is well known in the art, such vacuum vessels can maintain a vacuum of sufficient quality to maintain thermal insulation between the filament and the glass envelope for years. A getter, such as titanium, (not shown) may be deposited on the inside of the heat collector envelope in the section between the solar concentrator region and the thermal battery in order to help maintain the requisite vacuum quality, and yet not degrade the heat collection efficiency.
Since the heat transport mechanism in the heat pipe 129 is predominantly driven by capillary action when sunlit, a gentle bend in the heat collector 120 may be employed for the convenience of coupling the heat collector 120 to the thermal battery 130. Moreover, a bend such as downward sloping bend 158 in
As previously mentioned and further shown in
With respect to the heat storage medium, i.e. thermal battery core 136 contained by the thermal battery container, the utility of LiH as a thermal energy storage medium was previously discussed in the Background, and is due to the very high thermal energy per unit mass characteristic of LiH. However, in order to address the problem of lithium-hydride containment for high temperatures, e.g. 700° C. and above, a small admixture of lithium is utilized in order to prevent a hydrogen explosion. Thus the heat storage medium, i.e. the thermal battery core 136, consists of a mixture of lithium hydride and lithium metal, in equilibrium with various dissociation products 134, including hydrogen gas and liquid phase lithium and lithium hydride. The most significant contribution to the total vapor pressure is the partial pressure of hydrogen. The equilibrium hydrogen pressure is a function of both the temperature and the fraction of Li in a LiH—Li mixture, as is displayed in
The fabrication of the LiH and Li mixture may be achieved by starting with an initially pure quantity of LiH in the thermal battery fabrication process, and after initial hermetic sealing of the LiH in its primary containment shell 137, consisting of a LiH—Li impervious alloy, test the quality of the seal by heating the LiH to just below the melting point. Some possible alloys that are relatively inert to Li are Mo—Z, Mo—Re, and Nb—Zr, as described in “High Temperature Liquid Metal Heat Pipes”, by A. Bricard, T. Claret, P. Lecocq and T. Alleau, in the Proceedings of the 7th International Heat Pipe Conference, (1993), incorporated by reference herein. In addition, very low carbon steel is also inert to Li and LiH. According to the reference: “Compatibility of potential containment materials with molten lithium hydride at 800° C.”, by S. J. Pawel, published in the Journal of Nuclear Materials vol. 207, pp. 136-152, in 1993, also incorporated herein by reference, “Stabilized (Nb and Ti) low carbon (<0.06%) steels are observed to be essentially inert in LiH at 800° C. with stable carbides and no grain growth.” The initial “seal test” heating step causes a significant pressure of hydrogen to build up in the LiH container. If the seal is bad, a relatively high hydrogen pressure will be observed. In contrast, if the container is well sealed, a much lower hydrogen pressure will still be seen outside the container, due only to hydrogen permeation. After a small quantity of hydrogen has been allowed to permeate out of the container, the LiH may be slowly raised (in order to avoid an excessive pressure spike) above the melting point, and sufficient hydrogen removed by permeation to bring the Li metal fraction remaining in the core 136 up to a desirable value.
As an example, by getting to a 2% Li metal mixture, the hydrogen pressure at a working temperature of 1100 K will be just over one atmosphere, as can be read from the plot in
Inner cavities inside the thermal battery 130 provide good thermal contact to both the sodium condenser 128 at the end of the heat pipe 129, as illustrated in
As illustrated in
The power produced by the Stirling engine tends to increase with the mean pressure in the expansion space 151 inside the engine. Thus, venting the engine crankcase, through the crankcase pressure relief valve 149 shown in
Pressurization of the crankcase above the ambient atmospheric pressure is preferably achieved by the action of a crankshaft pump 145 that produces a pumping action as the crankshaft rotates, to self-pressurize the crankcase. The crankshaft pump 145 comprises at least one helical groove on either the crankshaft surface or a journal surrounding the crankshaft. It is appreciated that one or more helical grooves may be utilized in the same direction for greater pumping performance. And a filter 146 prevents particulate contamination in the working fluid from clogging the passageways in the crankshaft pump 145.
In the preferred embodiment, the crankcase pressurizes to a value determined by the pressure drop across the crankshaft pump and the outside atmospheric pressure, for the case that the working fluid is simply ambient air. This pressure drop is in turn determined by the design of the grooves, both in terms of the number of grooves, and the groove shape. The steady state speed of the crankshaft pump is designed to produce a given mean operating pressure inside the crankcase of the engine. A pressure drop of one atmosphere across the crankcase pump, for example, produces an operating pressure that is relatively insensitive to the operating altitude of the aircraft. At an altitude corresponding to 10% of atmospheric pressure, the engine operating pressure would be approximately 50% that corresponding to sea level.
An alternative embodiment is shown in
And
The solar thermal power plant which was previously discussed for solar powered aircraft can also be incorporated for use in residential and commercial ground-based applications, hereinafter referenced collectively as “residential solar-thermal power plants.” When used in such fixed, stationary implementations additional benefits may be realized such as for example cost efficiencies which can make such residential solar thermal power plants economically attractive for domestic consumption. While the following description focuses primarily on fixed structure applications, it is appreciated however that the residential solar thermal power plant of the present invention may also be mounted on other structures which are not necessarily fixed or ground based, such as for example on boats, trains, or other mobile but earth-bound platforms, to realize similar benefits of efficient solar-thermal energy generation.
Reference numerals used in the following description for the residential solar thermal power plant are listed in Table 3.
And
Unlike the solar thermal aircraft, however, these main components of the residential solar thermal power plant are preferably mounted on a fixed structure that is sufficiently exposed to the sun, such as for example a residential rooftop shown in
As illustrated in
It is appreciated that when mounted as such for residential applications, the residential solar thermal power plant is often directly exposed to the elements, e.g. wind, rain, snow, dirt, etc. To protect them from environmental effects, a windshield assembly is preferably provided to surround mirror 210 and tube 226.
In the exemplary embodiment of the residential solar thermal power plant shown in
To achieve proper alignment with the earth's rotational angle, various methods may be utilized. For northern hemisphere locations, one example utilizes a small telescope provided with and held parallel to the mirror/collector assembly to locate the North Star on a clear night, as shown in
An alternative method of achieving correct parallel alignment with the earth's rotational axis uses the latitude coordinate of the mounting location and a compass to determine the direction of due north, as shown in
With the focal axis of the parabolic trough substantially parallel with the earth's rotational axis, the angle between the normal to the collector axis and the direction to the sun will not vary by more than 23.5° over the course of a year, i.e. rays from the sun are never more than 23.5° from normal incidence to the aperture plane of the concentrating mirror. These extreme deviations occur on the summer solstice, the longest day, and on the winter solstice, the shortest day. The paths for a pair of extreme rays from the sun on the summer solstice are illustrated in
The limited length of exposed collector tube and the small degree of foreshortening in the polar aligned case is in contrast to that for the horizontal deployment typical of commercial parabolic trough collectors. The annual average foreshortening factor associated with this incident angle effect is listed in the first row of Table 1, shown in the Summary section. Similarly, the end losses associated with conventional horizontal collectors for solar angles for which the focused sunlight converges at positions along the axis beyond the extent of the collector tube are listed. In the polar orientation of the present invention this loss is avoided by having a heating tube that is slightly longer than the trough itself, as illustrated in
By having the axis of the solar collector inclined at an angle substantially equal to the local latitude, and parallel to the earth's rotation axis, several benefits are obtained over the case with a horizontal collector. As previously mentioned, since the angle of the sun's rays to the axis of the solar collector does not deviate by more than 23.5° from normal incidence over the course of the year, the projected mirror area available for solar collection changes by only +/−4% over the course of the year. This is in contrast to horizontally deployed parabolic troughs, typical of current commercial solar thermal energy power plants such as SEGS, for which the mean incidence angle cosine is significantly less. Accounting for the variation of this angle of incidence throughout the year, the conventional horizontally deployed parabolic troughs have a geometrical efficiency factor of 87.3%, while for the case that the angle of the trough is aligned with the North Star, this geometrical efficiency factor increases to 95.9%. The increase in overall solar collection efficiency with respect to horizontal troughs from this deployment angle alone is thus approximately 9%. Another advantage of inclined orientation: by having the thermal energy storage located at the upper end of the solar collector, the liquid phase of the two-phase working fluid in the heat collector may be very effectively returned from the condenser to the boiler primarily by gravitational action. Such heat collectors are called thermo-siphons, and are well known in the art and are commercially available. Another significant advantage of having the collector axis aligned with the North Star is that rotation of the parabolic reflector may be driven by relatively simple and inexpensive clockwork, with only occasional need for adjustment to either run a bit faster or run a bit slower. The control mechanism needed for such gradual adjustments can be very simple and inexpensive.
As previously mentioned the preferred shape of the concentrator mirror 210 is that of a parabolic trough which is straight in the longitudinal direction and which has a parabolic curve cross-section in the perpendicular plane defining the trough width. Furthermore, the focal length, f, for the parabolic curve is preferably equal to 25% of the full width W of the trough. In other words, the focal ratio, designated by f/# in optics nomenclature, is preferably about f/0.25. At this ratio, the relative size of the absorber (e.g. the outer surface of tube 226 in
The relative efficiency for f/# values differing slightly from the optimal f/0.25 varies as follows. For f/# values between f/0.2 and f/0.3, the relative concentration factor decreases by 2% from the maximum possible at f/0.25, while for f/# values between f/0.16 and f/0.4 the maximum achievable concentration factor decreases by 10%.
The primary component of the heat collector 220 shown in
Preferably, heating tube 226 comprises a hollow type-316 stainless steel tube with a sputter-etched surface. Such surfaces on type-316 stainless steel are known to be resistant to deterioration, and are feasible for use in air at temperatures up to 400° C. The preparation and characteristics of such surfaces are known in the art and described in, for example, “Sputter Etched Metal Solar Selective Absorbing Surfaces for High Temperature Thermal Collectors”, by G. L. Harding and M. R. Lake, published in Solar Energy Materials, vol. 5 (1981), pp. 445-464, hereby incorporated by reference. Solar absorptances for sputter-etched stainless steel are observed to be 93%, with a thermal emittance of only 22%. It is further known that type-316 stainless steel is suitable for use with Sodium, Potassium or high pressure steam as heat transfer fluids.
In any case, the major or long axis of this profile is preferably located within the longitudinal symmetry plane 213 (shown in
It is also important to note the angle of incidence of the concentrated sun rays as they meet the surface of tube 226. Rays 229A and 229B encounter the surface of tube 226 at an incidence angle of 45°. In contrast, for a circular collector tube having the same diameter as the major axis of the oblong shape, the incidence angle for such rays would be 90°. On the other hand, rays 229E and 229F encounter the surface of tube 226 at an incidence angle of 90°, while for the circular tube case, the incidence angle would be 45°. Since the marginal rays can encounter the surface of a minimally sized tube 226 at relatively high angles of incidence, it is important for the absorptance of the surface to remain high, even for such grazing angles. According to the reference by Harding and Lake mentioned in the previous section, the relative solar absorptance for sputter etched type 316-stainless steel is above 90% at an incidence angle of 60°, and is about 80% at an incidence angle of 80°. Because the solar absorptance remains high at very high incidence angles, it is feasible for the major axis of collector tube 226 to be no larger than approximately 0.45% of the width W shown in
It is appreciated that with proper suppression of convective losses, collector assembly heat losses tend to be dominated by thermal radiation from the hot central tube. In turn, the power loss associated with thermal radiation is directly proportional to the area of the radiating surface. By decreasing the area of the radiating surface as described with the oblong profile, the efficiency of the collector is improved by the factor listed in the first row in Table 1 in the Summary section. The magnitude of the thermal power loss does increase with higher temperature. The numerical value in Table 1 is calculated assuming a temperature of 400° C., as is currently used in the SEGS plants.
Since the radiating area of the present tube is so much reduced compared to the conventional art, it is feasible to attain higher heat transfer fluid temperatures than for the conventional parabolic trough solar collectors. This can enable more efficient heat engines to be employed. On the other hand, if the conventional heat transfer fluids are used, such as those in the SEGS plants, and the temperature is limited to 400° C., the efficiency will improve substantially by virtue of the decreased thermal radiation losses.
It is appreciated, however, that while a circular shaped tube is not quite as efficient as the oblong cross-sectional tube, it is not necessary to rotate a circular tube with the collector mirror, and a circular tube can thus be completely stationary with respect to the ground, and this can offer a compensating simplicity of operation.
Although windshield 212 substantially reduces wind generated convective cooling of tube 226, an optional transparent glass envelope may be provided to further protect and provide thermal insulation to tube 226.
An example of the effect of this lens action on the converging sunlight is illustrated in
Additionally, with such a reduced cross-section of the collector tube, the axial length of tube 226 relative to the width of collector 210 may be reduced by more than a factor of 25 relative to conventional parabolic trough geometry, such as that studied in the prior DISS, Direct Steam Generation, experiments, and still maintain equivalent heat transfer. This allows the collector to be much more compact than for conventional parabolic trough collectors, and facilitates the packaging of such systems on typical residential rooftops.
The power plant of the present invention preferably also includes a thermal storage reservoir, such as 230 in
Although single gas phase heat engines, such as the Stirling engine embodiment discussed above in the context of the aircraft embodiment, can take advantage of the very low ambient air temperature at high altitude and can thereby achieve very high thermal efficiency, in the context of the ground-based environment, without such low temperature capability, the familiar steam engine is preferred. This is especially so, considering the advantages of water as the thermal energy medium.
The use of steam to generate power is very well known and very well developed technology, and there is such a myriad of approaches that the optimal configuration will depend strongly on the nature of the desired energy product. At one extreme, it may be that all that is required is a supply of high pressure, high temperature steam for some particular process of interest, and there may be relatively little requirement for power. At another extreme, it may be that it is essentially only electric power that is required, and the reject heat is just a nuisance. In the next section, among these myriad cases, the specific case appropriate for the average power and energy needs of a residential consumer is considered. In this example, in line with the needs of a typical residential energy consumer, comparable quantities of heating energy and electrical energy are needed over the course of a year, but with more heating required during the winter, and more electric power required during the summer. It is therefore important to have flexibility in the conversion of concentrated solar energy into heat or electric power.
As shown in
One of the benefits of thermal energy storage in the residential case is that momentary interruptions in the solar illumination do not cause corresponding upsets in the heat supply to the engine. While the primary role of the thermal energy storage in the solar aircraft application is to enable overnight flight, in the residential application it is not always necessary to store an entire day's worth of heat. In some cases it may be economically advantageous to have only a relatively short storage duration capability. Another benefit of thermal energy storage in the residential case is that the normal noon-time peak in the solar illumination may be distributed over a number of hours in the afternoon, thus allowing a lower maximum electric generation capacity design, and thereby a less expensive heat engine and electric generator. Furthermore, by storing thermal energy, the typical noontime peak in solar energy supply may be better matched to the typical mid-afternoon peak in electric energy demand. At another extreme, for energy self-sufficiency, the thermal energy storage capacity may be made great enough for weeks to months of storage, so that the dependence of solar power on the vagaries of the weather may be virtually eliminated.
Collector loop water valve 231 controls the flow of water from the thermal energy storage into the bottom of heating tube 226, while water pump 235 controls the water pressure in the collector loop and automatic check valve 237 prevents excessive pressure from building up in the collector loop. Similarly, steam valve 238 controls the flow of superheated vapor to steam engine 240, while engine loop water pump 236 determines the pressure within thermal storage reservoir 230.
The transfer of heat to thermal storage reservoir 230 from the solar collector and the transfer of heat from the thermal storage reservoir to the steam engine 240 take place in two independent process flows. The collector flow operates in proportion to the solar heating supply, while the engine flow operates in proportion to the power demand. Regarding the collector flow, during periods when adequate sunlight is available, so that sufficient steam pressure is produced in collector tube 226 by the absorption of concentrated sunlight to force open automatic valve 237, heat from the concentrated sunlight is transferred to the water in tube 226, and then transferred to the top of thermal storage reservoir 230. Conversely, at night, or during periods of obscured sun, valves 237 and 231 are closed. It is appreciated that throughout day and night, concentrator mirror 210 is continuously rotated on its axis so that whenever direct sunlight is available, the alignment of the collector is such that heating of the water in tube 226 will occur. And regarding the engine loop, during periods of demand for power, both valves 238 and 239 are opened and high pressure steam from the top of thermal energy storage reservoir 230 is admitted to steam engine 240, and after expansion, is condensed in radiator 261 and drains as liquid water into water tank 244. In winter, when temperatures are low enough to require space heating, the flow of cooling air past radiator 261 may provide a supplemental supply of warm air for space heating purposes. In contrast, during summer, when temperatures are high enough that further space heating is undesirable, radiator 261 simply rejects heat to the outdoors.
The heating process, in more detail is this: cold pressurized water is forced into the lower end of tube 226 by collector loop circulating pump 235 and heated along the axis of the collector. The upward tilt in the axis of tube 226 enables very high heating rates of the steam compared to horizontal tubes as is known in the art. Under normal operating conditions, as the water is heated by the concentrated sunlight, it reaches boiling temperature at a point indicated by level 232. Between the onset of boiling at level 232 and the onset of superheating at level 234, the steam transitions from very wet to very dry at substantially constant temperature. Above level 234, the steam is superheated, and its temperature increases to the design maximum. Once raised in temperature to the design point, the superheated steam flows to thermal storage reservoir 230, and/or to steam engine 240.
In a “cold start” case, corresponding to the lowest quantity of heat in storage, pressure vessel 241 is almost entirely filled with near room temperature water, with a relatively small vapor space at the top, and water tank 244 is almost empty. In this state, the top of the liquid level 232 is near the top of pressure vessel 241. Very shortly after concentrated sunlight is focused onto tube 226, superheated steam is forced into the top of pressure vessel 241, through automatic valve 237. At the same time, cold water is pumped by pump 235 from the bottom of pressure vessel 241 through valve 231. As this steam is blown against rock pebbles 245 at the top of thermal energy storage reservoir 230, the pebbles begin to heat up. A portion of the incoming steam initially condenses on pebbles 245 and drips down to the water level 232 and begins to heat the water in reservoir 230. Because of the relatively low conductivity of gaseous steam, there is relatively little drop in the gaseous steam temperature, and valve 238 may be opened shortly after sunlight becomes available to provide superheated vapor to steam engine 240. As superheated steam continues to flow into the top of reservoir 230, while liquid water continues to be pumped out of the bottom, the liquid water temperature continues to increase until it reaches the boiling point. Also as superheated steam continues to flow past upper pebbles 245, their temperature also soon exceeds the boiling point of the pressurized water in vessel 241. As the water in the pressure vessel boils, as steam is provided to engine 240, and as water is pumped out the bottom of reservoir 230, liquid level 232 drops, and a larger fraction of the rock pebbles 242 are exposed above water level 232, and they too begin to increase in temperature above the pressurized water boiling point. This process may be allowed to proceed until water level 232 has dropped to the lowest permitted safe level; at which point thermal energy storage reservoir 230 has reached its maximum capacity, and essentially all the pebbles 242 above the saturated water level 234 are at the superheated steam temperature, and most of the water in the system is contained in liquid water tank 244. At this point, further removal of liquid water by pump 235 from reservoir 230 must be made up by water pump 236 pumping condensed water from tank 244 through valve 239 back into the bottom of reservoir 230.
After sunset, or during extended periods without available concentrated sunlight, valves 237 and 231 are closed and the collector loop is no longer operative. In this case, as superheated steam is provided to steam engine 240 through valve 238, makeup water is pumped into the bottom of reservoir 230 by pump 236 through valve 239. As water level 232 rises in reservoir 230, so does the saturated vapor level 234, and heat is transferred from the newly immersed hot rock pebbles 234 to the surrounding water and more steam is generated. This process may continue until the saturated vapor level 234 in reservoir 230 reaches the level of the steam valve 283. At this point, it is typically undesirable to continue to operate the steam engine on the saturated water, but extraction of heat from thermal energy storage reservoir 230 by the heating of water from cold water supply 262 and delivery to residential hot water supply 260 is still desirable, especially in winter for space heating purposes. In the limit that practically all of the heat stored in reservoir 230 is extracted overnight, then the diurnal cycle is complete, and a “cold start” condition is again obtained. It is convenient with this system that the natural time of need for heat is at night, which corresponds to the period of relatively lower mean water temperature in reservoir 230, while the natural time of need for power is during the day, corresponding to the period of relatively higher steam temperature and more efficient electric power generation.
The approximate division of the incoming solar energy may be estimated, based on typical steam engine thermal efficiencies, to be ¼ to ⅓ to power and most of the balance to heating. With such a system, well over 90% of the incident solar energy may be exploited for the combination of heating and power. The division between heat and power with such a system is thus quite well matched to the typical heat vs. power consumption for a typical residential consumer in the South Western United States, and especially so in winter.
After sundown, on cold winter nights when there is a possibility of water in collector tube 226 freezing, it is advantageous to allow dry steam from thermal storage reservoir 230 to flow backwards through the collector tube and flush any liquid water out of tube 226.
While particular operational sequences, materials, temperatures, parameters, and particular embodiments have been described and or illustrated, such are not intended to be limiting. Modifications and changes may become apparent to those skilled in the art, and it is intended that the invention be limited only by the scope of the appended claims.
This application is a continuation-in-part of prior application Ser. No. 10/835,665, filed Apr. 30, 2004, by Charles L. Bennett, and incorporated by reference herein.
The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10835665 | Apr 2004 | US |
| Child | 11543659 | US |
