Final Report: Develop a Concentrated Solar Power-based Thermal Cooling System via Simulation and Experimental Studies

EPA Grant Number: SU835493
Title: Develop a Concentrated Solar Power-based Thermal Cooling System via Simulation and Experimental Studies
Investigators: Tang, Yan , Atticks, Kendra , Kasper, Kirsten , Compere, Marc , Wood, Nicholas , Boetcher, Sandra , Engblom, William , Judson, Zachary
Institution: Embry - Riddle Aeronautical University
EPA Project Officer: Packard, Benjamin H
Phase: I
Project Period: August 15, 2013 through August 14, 2014
Project Amount: $15,000
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet (2013) RFA Text |  Recipients Lists
Research Category: P3 Challenge Area - Air Quality , Pollution Prevention/Sustainable Development , P3 Awards , Sustainable and Healthy Communities

Objective:

Embry-Riddle’s EPA P3 2013 Phase I project is to design and test a 300W solar cooling system using parabolic solar troughs. The system captures the sun’s heat with a line-focus, high-temperature parabolic trough collector. The concentrated solar power from the collector drives an absorption cooling cycle of a small refrigeration unit. The circulating collector-loop fluid stores the heat in a storage unit for use during periods solar applications are not practical such as poor weather and night-time. A simulation tool has been developed to mimic the time-dependent thermal and hydraulic performance of the parabolic trough based on classic empirical and analytical models. The simulation results are used to guide and support the selection of system components to meet design goals. The primary focus is solar thermal energy storage and heat exchanger design for the absorption cooling cycle.

Our objective is to research methods for replacing or reducing fossil fuel consumption for thermal driven applications with solar thermal power (e.g., solar-assisted air conditioning). This will improve US energy security and reduce both greenhouse gas and the EPA’s criteria emissions. The emphasis to reduce greenhouse gas (GHG) emissions has increased, and will continue to increase, as climate change proliferates as a national and global priority. Both solar photovoltaic (PV) and solar thermal energy represents an abundant, freely available, renewable energy resource. Solar thermal, in particular, is an alternative energy source that can reduce dependence upon fossil fuels for heat driven processes. This directly supports the EPA’s Planet theme by reducing greenhouse gas emissions.

The operating performance of thermally driven systems using solar thermal power relies on thermal energy storage and retrieval. Some drawbacks to solar energy are the daily periodic nature, intermittent cloudy conditions, and seasonal variations. Thermally driven applications typically need power on a schedule different from the incoming solar power, so it is clear that at least short-term energy storage is necessary to address the intermittent aspects. Storage size and efficiency are two major concerns when designing solar thermal storage system to ensure continuous operation. Another important component in such applications is heat exchangers which determine the efficiency of the whole system.

The solar cooling design proof of concept design was demonstrated through the operation of a commercially available absorption refrigerator powered by the storage heat collected from a parabolic trough as a proof of concept. The results will serve as a good foundation to further our research on solar cooling of buildings.

Summary/Accomplishments (Outputs/Outcomes):

The developed solar-powered absorption refrigeration system can be divided to three subsystems including the heat collection loop, the storage unit, and the heat delivery loop. Through the Phase I project, several critical issues of each subsystem were identified and addressed. In the heat collection loop, the performance of the parabolic trough is critical to provide high-quality heat for storage. A simulation tool was developed to characterize the parabolic trough to identify important operational parameters to improve the heat collection performance. The size and efficiency of the thermal storage tank are essential to achieve continuous operation. Experimental studies were conducted using various parameters and requirements to size the storage tank and improve the storage efficiency by applying thermal stratification. Different heat exchanger designs have been evaluated through simulations and experiments to optimize the performance of the absorption refrigerator.

The heat collection loop consists of a parabolic solar collector with an aperture area of 4 m2 and a hydraulic system. A Matlab code was developed to characterize the parabolic solar collector in order to estimate the collected thermal energy and the temperature of the heat transfer fluids (HTF) at the parabolic trough outlet. The simulation results have been validated through experiments on the trough to increase the fidelity of the simulation tool. The code allows for convenient evaluation of various design decisions by inputting volumetric flow rate, system dimensions, solar data, and HTF properties. This resulted in an analysis that could be easily modified to evaluate various locations, conditions, and design scenarios. Sources of error in the analysis primarily relate to unknowns that were estimated due to difficulty in measurement or calculation.

Although the study showed a continuous operation of 24 hours from the thermal storage would require approximately 80 gallons of HTF, a 10 gallon storage tank was used for demonstration purposes and convenience. A major investigation regarding thermal storage was the application of thermal stratification to evaluate its effect on the storage efficiency. Data has been collected through experimental studies, and further analysis is needed to draw a conclusion. It is expected the results will be reported during the EPA P3 Expo.

Three different heat exchanger designs, a straight tube, a coil, and a collar, were tested and compared for the heat delivery loop. The results showed that the collar heat exchanger can most effectively deliver stored heat to power the absorption refrigerator by providing increased surface area contact and increased heat flux into the boiler of the absorption cycle. Simulation studies were conducted to optimize heat exchanger design by choosing a suitable annulus. The results indicated that a small annulus of approximately 0.45-3 mm would be required.

Educational outcomes include both undergraduate and graduate student educations in the Mechanical Engineering Clean Energy Systems track. Students learned the iterative design process, how to focus on customer needs and requirements, conceptual design brainstorming, and the process of selecting a single design for further study. They also learned how detailed design is influenced by the practical considerations of real hardware fabrication and testing. They learned how to write and execute a test plan and then present data and test results. The specific topics researched include thermal system design, hydraulic system design, and power and energy concepts. The P3 has contributed substantially to our Clean Energy Systems track education.

Conclusions:

The senior design team on the Mechanical Engineering Clean Energy Systems track has designed, built, and tested a concentrated solar power-based thermal cooling system along with a system performance simulation tool. The system captures the sun’s heat with a parabolic trough, stores the heat in a 10 gallon storage unit, outfitted with baffles, and drives an absorption cooling cycle continuously for 2.5 hours with the stored heat. The simulation tool is capable of predicting the captured thermal energy, the temperature of the HTF at the collector outlet for given conditions (i.e., the flow rate, system dimensions, solar data, and HTF properties), and sizing the annulus of the collar heat exchanger for the absorption cycle.

Based on the results from the project supported by the EPA P3 Phase I award (SU835301), the system serves as a proof of concept to advance our research on solar thermal air conditioning. Building HVAC represents 0.8% of the entire United States’ energy consumption. This project is ideally suited for migration away from nonrenewable fossil fuel towards sustainable solar. Other applications that can be improved by thermal energy storage for developed nations include electricity generation, water desalination, water purification, and multiple industrial process heating needs. For off-grid or developing nations, solar thermal power with storage would make refrigeration, sterilization in medical clinics, cooking, and pasteurization all more viable approaches. These applications improve quality of life, offset fossil fuel consumption which reduces emissions, and represents an entirely new sector for jobs selling and maintaining thermal storage systems. These directly support the EPA P3’s priorities of People, Planet, and Prosperity (P3) in the developed and developing nations.

Practical issues and technical challenges have been identified in the project for further improvements. These issues and the challenges will be addressed if the project is identified as a viable candidate for the Phase II funding award.

Supplemental Keywords:

solar thermal energy, solar cooling, absorption chiller, solar energy storage, sustainability, net-zero, zero emissions, sustainable energy

P3 Phase II:

Develop a Concentrated Solar Power-based Thermal Cooling System via Simulation and Experimental Studies