Grantee Research Project Results
Final Report: Enhanced Solar Energy Harvest for Power Generation From Brayton Cycle
EPA Grant Number: SU836032Title: Enhanced Solar Energy Harvest for Power Generation From Brayton Cycle
Investigators: Lee, Hohyun , Zabalegui, Aitor , Olaes, Criselle , Marumoto, Darcy , Valdez, Joe , Barker, Laughlin , Neber, Matthew
Institution: Santa Clara University
EPA Project Officer: Hahn, Intaek
Phase: I
Project Period: August 15, 2011 through August 14, 2012
Project Amount: $15,000
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet (2011) RFA Text | Recipients Lists
Research Category: Pollution Prevention/Sustainable Development , P3 Challenge Area - Air Quality , P3 Awards , Sustainable and Healthy Communities
Objective:
The development of this project directly relates to the transition from fossil fuels to sustainable energy solutions. Moving to sustainable energy sources is not only more secure, but a moral obligation towards protecting the environment. In order to facilitate the development of solar energy, the main objective of this project is to develop a low cost, high efficiency solar receiver for an integrated dish Brayton system.
This type of concentrated solar power (CSP) has the potential to be used as both an electricity producing and combined generation system. Replacing grid-supplied power with a renewable system such as the one in this project can both decrease annual energy expenses and greenhouse emissions from power plants. While the focus of this research is home scale usage, the scalability of the proposed system would make this research applicable to all levels of energy production from home use to utility scale, with special consideration for industrial applications where process heat is used consistently and predictably.
In order for this technology to be used on the residential scale, it must become a cost competitive solution for living in the face of rising utilities costs. A system producing 2.5 kW of electrical power is chosen as the target output, which is sufficient for most average sized singlefamily homes for all appliances excluding cooling and/or heating energy. To meet the heating/cooling requirements, based on energy consumption data, 3.0 kW must be extracted from the waste heat of the system. For spatial concerns, the proposed CSP system will be roughly half the size of an equivalent photovoltaic system. To combat the high cost, fewer and simpler parts should be employed, while increasing the operation temperature and solar concentration for higher efficiency.
The core of the CSP system is the receiver, which absorbs solar energy and converts it to heat. The solar receiver must operate at high temperatures in order to achieve greater conversion efficiency through an increased maximum temperature and higher energy availability for trigeneration. For Phase II the receiver will be developed into a production part, which can be marketed to other CSP applications in addition to its role in this system. Production methods will be explored in conjunction with an industry partner.
Summary/Accomplishments (Outputs/Outcomes):
In order to bring this technology to the residential market, it is estimated that a 2.5 kW dish Brayton power generator will produce enough electricity and useable grade waste heat to sustain an average sized single family home. The electricity produced will be adequate to run household appliances over a 24 hour period, while the waste heat will supply an additional estimated thermal energy of 3 kW by incorporating a tri-generation system. Generating this power from a Brayton cycle generator operating at a maximum temperature of 1500 K allows for cogeneration or tri-generation at reasonable exergetic efficiency where possible combined systems include steam for water purification, hot water or residential heating in cold climates, and process heating for absorption chillers in hot climates. The design must achieve 1500 K air temperatures for a Brayton cycle electric generator to operate at approximately 26% efficiency. This target air temperature is also to reject waste heat at a high enough temperature for the entire system efficiency to reach about 60% to carry the household.
The Brayton cycle was chosen for its open system design, which leads to fewer parts and the ability to use atmospheric air as the single working fluid. The benefits of the Brayton turbine engine are that it is based around a single rotating part, has a high power to weight ratio, and can be reproduced at a relatively low cost. This is in contrast to the Stirling engine that is typically used in some solar receiver dish systems, which has multiple reciprocating parts, a low power to weight ratio, and has expensive production costs. New manufacturing processes allow better product quality and decreased cost when it comes to making the ceramic parts for a Brayton engine. These processes are readily extended to less complex shapes and stationary components that do not require high precision or accuracy.
The receiver is designed so that solar energy is focused into the open end of the cavity with air injected near the open end of the cavity. Air circulates and heats up in a helical duct that goes around the receiver and exits at the rear. With silicon carbide as the material of the solar receiver, the required size of the cavity can be reduced significantly and it does not require an additional absorptive coating. An overall heat transfer coefficient was calculated based on square duct geometry and was used to determine a minimum contact area of .114 m2 for the heat exchange surface. The receiver is designed to operate at 1700 K and heat air to 1500 K to achieve a Brayton cycle efficiency of approximately 26 % with an air exit temperature of 1100 K. The necessary air mass flow for a Brayton system that operates between 1500 K and 1100 K at a pressure ratio of 7.6 is 10 grams/second to produce the required 2.5 kW. The high thermal conductivity of silicon carbide also permits a tube style heat exchanger to be integrated with the proposed cavity absorber, reducing the size and total part count.
The peak efficiency will be a product of the Brayton cycle efficiency and the absorption efficiency. In order to combat the drop in absorber efficiency as temperatures increase, the concentration is increased to improve receiver efficiency. Smaller apertures result in higher concentration and better system efficiency. A silicon carbide cavity with an effective emissivity of 0.99 can be achieved with a cylindrical geometry, whereas most other materials require an aperture that is smaller than the internal diameter of the cavity. Preliminary testing included a furnace test and a parabolic mirror test to determine the response of the receiver and the absorber efficiency. The absorber effective emissivity and efficiency was consistent with predictions.
Conclusions:
The potential of this system to be implemented for residential use at a reasonably low cost is attainable with the proposed high efficiency solar receiver for an integrated dish Brayton system. The system will be designed to produce about 2.5 kW of electrical power and about 3 kW of process heat in order for it to be suitable for an average sized single family home. Research of silicon carbide has shown that it can greatly improve the performance of the solar receiver due to its ability to withstand extremely high temperatures, its high absorption of solar radiation, its high thermal conductivity, and being able to reduce the size and cost of the cavity receiver. With these goals in mind, implementing this system can reduce the reliance on grid power. This will in turn decrease energy expenditures and reduce emissions form power plants.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
Other project views: | All 2 publications | 2 publications in selected types | All 2 journal articles |
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Barker L, Neber M, Lee H. Design of a low-profile two-axis solar tracker. SOLAR ENERGY 2013;97:569-576 |
SU836032 (Final) |
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Neber M, Lee H. Design of a high temperature cavity receiver for residential scale concentrated solar power. ENERGY 2012;47(1):481-487 |
SU836032 (Final) |
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Supplemental Keywords:
Concentrated Solar Power, CSP, Solar Receiver/Absorber, Dish Brayton, CCHP, Tri- Generation, Small-scale, Distributed Power Generation, CogeRelevant 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.