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The National Solar Mission launched by the Government of India promotes the deployment of 20,000 GW of solar power by 2022. In this initiative CSP plays a significant role. Solar thermal power generation opportunities in India are huge. SunBorne Energy in partnership with the Ministry of New and Renewable Energy (MNRE), Government of India, is deve...
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... air to solid packed bed thermal storage configuration is proposed to store the thermal energy of the hot air into solid particles/rocks. We have been evaluating few indigenous rocks as thermal storage media readily available in India. The cost of such material is about 3-4 INR/kg. We have proposed a control architecture where the heliostat aiming strategy is driven by a central control system. In such control methodology, the local heliostat control is required to achieve the desired azimuth and elevation positions as directed by the central control system. A receiver numerical model has been developed by coupling Monte Carlo calculations with Computational Fluid Dynamics. A novel concept of capturing solar energy spilled over the receiver aperture has been proposed. The proposed receiver design shows efficiency improvement of about 20% over a conventional billboard receiver design. The experimental investigation for optimizing the porous media has been carried out in a solar simulator. Various samples of porous meshes composed of various high temperature metallic alloys and ceramic materials were evaluated. The size of the porous media specimen used for the experimental investigation is 13cm x 13cm x 4cm. The experimental investigation has been further used to validate the numerical model. The validated receiver numerical model has been used to optimize the design of a 50 kW th prototype receiver, which is currently under experimental evaluation. A 1 MW receiver will be designed based on the testresults of the 50 kW pilot receiver. Figure 2 shows a schematic of the 50 kW th receiver test setup which is currently under investigation. A water cooled heat exchanger is used to simulate the steam turbine conditions. Wind data, direct solar irradiance and ambient temperature are monitored to evaluate the impact of wind on the recirculation of the ambient air in the receiver design. The air and water flow rates are controlled from the central control system. The air flow rate is controlled to achieve the desired air temperature at the receiver outlet using a variable frequency drive controlled blower, whereas the flow rate of water in the heat exchanger controls the air temperature at the receiver inlet. The complete assembly of the receiver test setup on the top of the tower is shown in Figure 3. We have selected a non-pedestal heliostat system having a size of 150m for the pilot plant. Figure 4 shows the heliostat field layout. Figure 5 shows the design flux on the receiver at noon on 21 st March, for Direct Normal Irradiance of 750W/m 2 . The average thermal input to the receiver is about 1.5MW th with a peak flux of 1700 kW/m 2 . It is based on the assumption of having parabolic curving of mirrors to provide the required focal length of the heliostat. A dual-axis PV tracking system was adapted for the design of the heliostat tracking system. The system was modified for heliostat use by increasing the structural rigidity and enhanced tracking accuracy requirements for the solar power tower technology. The heliostat structure is supported by four wheels and a central post. The azimuth orientation is achieved by rotating the heliostat about the central post using one of the four wheels to drive the structure. A helical screw mechanism is used for controlling the elevation of the heliostat. Each 150m 2 heliostat is composed of 40 mirror facets. Each facet is composed of a parabolic curved mirror supported on a metallic frame. Figure 7 shows the photograph of the heliostat installed at the pilot plant. We have developed in-house technology for 2 axis cold bending of the mirror to achieve the desired focal length. The parabolic curvature of the mirror was achieved by mechanically shaping the mirror using gravity sagging. In addition, the mirror canting technology to align the mirrors on the heliostat frame was developed. The mirror canting technology enables the creation of a virtual parabola suitable for focusing the individual mirrors on a single focal point. We have adopted a parabolic off-axis canting method for the heliostat field. The civil construction of the heliostat foundations has been completed. The installation of 5 heliostats has been completed, and the rest of the solar field is currently being commissioned. All individual heliostats are controlled through a central control system. The central system computes the desired orientation of the heliostats, and communicates the same to the heliostats. In addition, heliostat orientation pertaining to any emergency event, night stow positions, routine maintenance, etc. is also communicated through the central control system. The heliostat orientation is achieved through an open loop feedback control with the help of optical encoders and variable frequency drive controlled motors. Limit switches on azimuth and elevation positions are also incorporated for calibration and safety requirements. Figure 6 shows the heliostat control architecture adopted in the pilot plant. In the case of communication failure between the local heliostat and the central control systems, the heliostat is stowed to a safer position. The central control system of the solar field has been developed with the software LabView software from the National Instruments®. The central control system is connected through a RJ45 ethernet local network with all the heliostats. Figure 8 shows a screenshot of the graphical user interface of the central control system developed in LabView. The thermal storage system is a packed bed system with rocks as the sensible storage medium. We have evaluated various rocks locally available in India. The selected thermal storage media properties are compared with different thermal storage materials in Table 1. It shows that the new material has much higher energy density in comparison to molten salt. In addition, the packed bed configuration will eliminate the cost associated with molten salt systems due to its corrosive and molten state requirements. A numerical model of the thermal storage system based on one dimensional unsteady two phase energy equations has been developed. The numerical model has been used to study the behaviour of the thermal storage system for various parameters such as the aspect ratio, rock distribution and various mass flow rates. The charging and discharging completion boundary conditions were optimized on the basis of exit air temperature. Further, parametric studies of the aspect ratio and mean particle diameter have been carried out to obtain the performance of the charging and discharging operations including the temperature distribution across the solid and fluid phases. The performance parameters considered in the analysis are the overall system efficiency and specific energy density for the packed bed design. It was found that by considering the lower values of the exit air temperature as the criterion for the completion of charging the overall system efficiency is higher but specific energy density is lower. Thus, an economic consideration is required for selecting the exit air temperature value for the charging completion criterion. The analysis shows that the optimal value of the mean particle diameter is in the range of 20-50mm. It was also found that the aspect ratio of the thermal storage tank size has minimal impact on the overall efficiency of the system. Figures 9 and 10 show the variation of the overall efficiency and volume per MWh of the thermal storage system with particle diameter, and Figures 11 and 12 show the impact of the aspect ratio on the system. An experimental study of a small model thermal storage system is currently ongoing at the CERC, University of South Florida. The pressure drop of a packed bed thermal energy storage system with irregular shaped solid pellets and tank-to-particle diameter ratio of 10.4 is being investigated. The bed height to diameter ratio is 2. The particle sphericity is calculated and used to compare pressure drop correlations to the measured values in the particle Reynolds number range of 353 Rep 5206. The data from the experiments shall be used to further improve the efficacy of the numerical model. A 32 meter tall steel tower has been installed at the pilot plant as shown in Figure 13. A 2 ton jib crane is also installed at the top of the tower to enable lifting of various receiver components on the tower. The tower has two platforms at 26 meters and 32 meters from the ground ...
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... Towers with integrated vertical thermal storage in cores (elevators, stairs, toilets, etc.) could have efficient thermal stratification, perhaps a novel design component for tall building cores. BITNR tower thermal storage may be with rocks [108,109] and PCM [64] for air systems, and with insulated water tanks with liquid systems; and towers adjacent to small heliostats high temperature collector systems [110] safe high temperature storages are possibilities in tall building cores, atriums, etc. (Figure 6). ...
Building Interior Tubes and Nonimaging Reflectors (BITNR) solar thermal concentrators for mid-temperature applications in snow and dust regions, previously titled BIETR now include non-evacuated tube-receivers. A nonpublic walk-in solar collector space has fixed receiver-tubes with matching involute (rounded W) reflectors forming target-rectangles for augmenting large CPC troughs and relaxed tracking E&W ‘wing’ reflectors with a glass or solid polycarbonate building envelope collector cover. Reported is a BITNR reflector ‘wings’ tracker invention by George Helmholz and proof-of-concept development by university undergraduate engineering students. Discussed are: building types, structure, and construction and reflector materials for global applications. BITNR architectural feasibility studies with air and water-steam systems include: small wood frame structures, two-car garages, house size demonstration, long-span atriums, large concourses, towers, additions, and verification-test facilities. Schematic studies indicate integrated solar thermal mid-temperature technologies with building-size nonimaging CPC reflectors are main organizing factors for building design and site planning. BITNR/BIETR undergraduate student mechanical and structural engineering semester projects at the University of Wisconsin-Platteville are reported. A BITNR R&D project proposal is outlined which includes: a greenhouse test facility, evaluation of test stands, and development of a computer simulation design aid for architects.
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