Grantee Research Project Results
Final Report: Design and Fabrication of a Reduced Cost Heliostat
EPA Grant Number: SU834317Title: Design and Fabrication of a Reduced Cost Heliostat
Investigators: Ostergren, Warren , Gallegos, Anselmo , Luders, Ian , Hebert, Jason , Christian, Joshua , Valdez, Kendra , Berry, Marco , Soas, Saleem
Institution: New Mexico Tech
EPA Project Officer: Page, Angela
Phase: I
Project Period: August 15, 2009 through August 14, 2010
Project Amount: $9,415
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet (2009) RFA Text | Recipients Lists
Research Category: P3 Awards , Sustainable and Healthy Communities , Pollution Prevention/Sustainable Development
Objective:
Rising global concerns about fossil fuel reserves and the harsh environmental consequences of burning them demand the development of clean, renewable energy sources. The most abundant means of renewable energy is provided by the sun. One of the most promising methods of solar power generation is the utilization of heliostat arrays.
A heliostat consists of a large mirror with the mechanisms and circuitry necessary to actuate it, such that it reflects sunlight onto a given target throughout the day. A heliostat array is a collection of heliostats that, in unison, focus sunlight continuously on a central receptor. This central receptor often is called a power tower. In the power tower, the heat energy from the sun is converted into useful energy. In the context of power generation, a heliostat array is usually defined as 50 or more mirrors with a reflective area of at least 100 square meters. Most existing heliostat arrays use the energy of focused sunlight to heat a circulating fluid. The heated fluid is cycled through a conventional power block to generate electricity using the same boiler and steam-turbine technologies employed in conventional fossil fuel and nuclear power plant designs.
Unfortunately, conventional heliostats are too expensive to compete effectively in the global energy market. As of 2006, heliostats cost approximately $126 per square meter of reflective surface area. A conventional heliostat is actuated by a 2-axis servomotor drive system mounted on a concrete pedestal. The motor drive system requires large gear reduction in order to position the mirror with sufficient accuracy and stability. The gear-based drive system, however, is one of the most expensive components of the system. In order for heliostat technology to compete with fossil fuels economically, the cost must be reduced to approximately $80 per square meter of reflective area.
Thus, the goal of the NMT Reduced Cost Heliostat Team is to design and produce a heliostat that is competitive both economically with other types of energy production methods. Over the 1-year project span, the team has been designing and fabricating a second prototype that will improve upon the current liquid-ballast design. Through innovative engineering, the team is bringing heliostat technology closer to being a viable alternative to fossil fuels.
Summary/Accomplishments (Outputs/Outcomes):
After extensive research, calculations, and cost-optimization the final design of the heliostat has integrated substantial changes to the original prototype to several key areas to achieve the goal of reducing the square-meter cost of each unit. These changes were made to the structure, positioning system, and stability.
The structure of the first prototype was one of the areas that underwent vast improvements. The first prototype employed an A-frame structure that was costly in many ways. It was expensive to build, required too much space for the size of the mirror it held and required that it be placed on a flat surface. It was decided to go with a pedestal base instead of the A-frame because it offered several advantages. The first of these is that it occupies much less ground space than the mirror itself, which means that more heliostats can be installed and function in much less space than the previous design. The pedestal can be scaled to hold almost any size heliostat mirror, specifically 150 square meters as is a normal size for reflective surface area in Concentrated Solar Power (CSP) systems. This means that more heliostats can be placed in the same area utilizing the pedestal rather than the A-frame. The second advantage is that the pedestal is very stable. Instead of being placed on the ground, the pedestal is reinforced with a concrete and rebar structure that is drilled into the ground. This is a great improvement over the previous design because it was prone to wobbling under wind and shock loading. The only problem with the pedestal base, as compared to the A-frame, is that it costs more to secure the pedestal system. Also, the pedestal base is expensive to remove because of this, but the reduced cost of the pedestal itself along with its other advantages led us to choose it as the mounting structure for the project. Pedestals also can be inserted into slanted surfaces alleviating the need for extensive surface preparation to maintain a flat surface for the A-frame. This allows the heliostat to be used in many more places that would otherwise not be considered.
The second area of improvement is with the positioning system. The original liquid ballast design was expensive to build and very difficult to control. It consisted solely of liquid ballast tanks that were cactuated using a simple controls system. The hydraulic braking system necessary for the tanks required a large power input, which caused problems with the controls system. The new design utilizes a liquid ballast system supplemented with a power screw. The power screw is from Duff Norton and will be controlling the elevation angle (vertically up and down) of the heliostat and will allow for precision alignment in this position. The liquid-ballast system will be controlling the azimuth angle (horizontal direction) as the heliostat tracks the sun throughout the sky. The power screw incorporation was decided upon in order to increase accuracy while still maintaining cheaper actuation costs than the standard heliostat design. The introduction of the power screw into the liquid-ballast design allowed for more creative and innovative ideas to help improve the three areas of improvement outlined in the goals of this project.
The three areas that were focused on improving were the control system, braking, and the structural stability of the heliostat. The controls were modified from the first prototype in order to maintain a cheap alternative compared to requiring an expensive Data Acquisition (DAQ) system for each heliostat. The introduction of the power screw into the system allowed for a simpler control system to be considered. A micro-controller (MCU) will be used to control the power screw based on the sun’s recorded position, which will be calculated from sun tracking equations provided by Sandia National Laboratories. The liquid-ballast system also will be controlled using the MCU. The MCU will control the flow rate of the pump system required to pump liquid from one of the liquid ballast tanks to the other tank. The MCU also will be controlling the hydraulic cylinders used for braking and stability of the heliostat. The hydraulic cylinders are attached to each other using a two-way solenoid that will allow fluid to be pumped between the cylinders. When the solenoid is closed, it will not allow fluid flow and the cylinders will not expand or contract, which effectively stops the movement in the azimuth direction. When open, fluid flows from one cylinder to the other allowing movement in the azimuth direction. The solenoid can be opened and closed using the MCU to start and stop fluid flow. Having a single MCU on the heliostat will provide a fully automated tracking and braking system while being a cheaper alternative than a DAQ system that was considered in the first prototype. This greatly simplifies the electronics of a single heliostat and makes it more attractive for future development.
The stability of the heliostat has been increased using the improvements made to the structure and positioning system. The pedestal provides shock and wind resistance and the positioning system is very accurate because of the power screw and its gear reduction that allows small changes to be made in the elevation. The ballast system is accurate because the azimuthal positioning is done by pumping water from one side of the heliostat to the other and the flow can be varied and reversed, which means that there is less chance for the system to overshoot its position. Instead of using the drum brakes to brake the system, the heliostat utilizes hydraulic cylinders. The old system was not inherently inaccurate, but it would occupy too much space and required the use of extra components.
Conclusions:
The second prototype is focused on a lower cost alternative to current standard heliostats. The progress made thus far on the second prototype could lead to sustainability focusing on using solar energy instead of fossil fuels. Heliostats hold the potential of becoming a significant contribution to the renewable energy market. By designing new and improved heliostats, this could pave the way for solar technology in the world market on a much larger scale than what is being utilized today.
Through creativity and practicality, the NMT Heliostat team has successfully designed a prototype capable of achieving lower than conventional costs. The design improves heliostat technology by replacing the expensive actuation system with a much less expensive and simpler liquid-ballast and power screw hybrid system. The NMT prototype is a unique alternative to conventional heliostats and with further advancement, could be the building block for an advancing solar energy industry. There is no doubt that with further refinement, the heliostat can be brought down in terms of cost per square meter that will be competitive with fossil fuel and other renewable energy sources for energy generation.
Proposed Phase II Objectives and Strategies:
Upon completion of Phase I of the Reduced Cost Heliostat project, further advances can be made to continue the effort of creating a competitive energy-producing system. To perfect the system, the design will be optimized. This means that the design will continue to be evaluated for any potential areas of improvement to further drive down the cost.
With optimized design in place, the next step in proving the heliostat design is to set up a small heliostat array and create a power generation system. The power generation system will allow the heliostats to be tested and evaluated for accuracy as well as capability of actual energy production. Once in place, investigation into the optimal type of energy producing facility can be conducted. By analyzing the power generation system for any faults or areas of improvement, advancements to the system are possible.
Phase II of the project focuses on the energy producing capability of the heliostat design through set up of an array and some type of power-generating system.
References:
Kolb G, Jones S, Donnelly M, Gorman D, Thomas R, Davenport R, Lumia R. Sandia Report: Heliostat Cost Reduction Study. Albuquerque, NM: Sandia National Laboratories, June 2007, SAND2007-3293.Supplemental Keywords:
Renewable Energy, Solar Thermal Heating, Solar EnergyRelevant Websites:
The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.