Final Report: Design and Test of a Solar Thermal Energy Storage and DeliveryEPA Grant Number: SU835301
Title: Design and Test of a Solar Thermal Energy Storage and Delivery
Investigators: Compere, Marc , Beckwith, Jenna , Boetcher, Sandra , Camp, Johnathon , Ford, Jessica , Olafs, Bjorg , Pinto, Shavin , Rossi, Alexandria , Spychala, Mark , Tang, Yan , Wong, Yung
Institution: Embry - Riddle Aeronautical University
EPA Project Officer: Hahn, Intaek
Project Period: August 15, 2012 through August 14, 2013
Project Amount: $14,744
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet (2012) RFA Text | Recipients Lists
Research Category: Pollution Prevention/Sustainable Development , P3 Challenge Area - Energy , P3 Challenge Area - Green Infrastructure , P3 Awards , Sustainability
Embry-Riddle’s original EPA P3 2013 entry was a 1kW solar powered Stirling generator. This investigation was completed and found to be infeasible. Within the first 3 months of the Phase I, a Stirling engine was purchased, coupled to an electric generator, and tested using a dish-type concentrating solar collector. The power generated by the Stirling engine was more than an order-of-magnitude below the manufacturer’s claims. Under the supervision of an EPA P3 program manager, Embry-Riddle fulfilled all Phase 1 grant requirements and proceeded to a second technical effort titled Design and Test of a Solar Thermal Energy Storage and Delivery System.
Embry-Riddle’s solar thermal storage research investigates thermal energy storage and retrieval for operating a heat-driven load after sunset. Our objective is to research methods for replacing or reducing fossil fuel consumption with solar thermal power. 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 grows as a national and global priority. Both solar 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.
One drawback to solar energy is 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. We define short term energy storage using a time interval from a few minutes to overnight. The most achievable thermal energy storage goal is to provide heat delivery continuity for typical partly cloudy conditions. This is achievable for most thermally driven processes supplemented by solar thermal collection. The more challenging goal is high quality energy storage for overnight or multiple cloudy days.
Our primary research focus is solar thermal energy storage and retrieval. With this in mind, thermal storage is best demonstrated in the context of a complete working system which also includes solar thermal collection and a thermal load. There were a number of candidate applications considered for the thermal load including solar cooking, solar heating, and solar air conditioning for buildings. We selected an absorption chiller refrigeration unit for the thermal load as a small-scale example of solar powered air conditioning. This choice is to show viability of clean solar thermal power to fuel building HVAC.
Embry-Riddle found solar thermal storage a critical component to making solar thermal power a viable alternative energy. Thermal energy storage design is a complex task with technical complexity in solar collection, heat transfer fluid (HTF) choice, hydraulic system design, and heat delivery to the load. The small scale demonstration unit with absorption chiller refrigerator is an ideal match between thermal supply and demand. The heat- powered refrigerator is a small scale demonstration of solar powered air conditioning. A mathematical model was developed using Matlab/Simulink to characterize tank heat losses and aid pump selection. A two-tank design utilized Canola oil as the heat storage medium and also heat transfer fluid. This allowed the hot tank to remain hot much longer than the single-tank design. However, the drawback was the consuming nature of the hot tank with limited volume being emptied into the cold tank. Insulation was critical for retaining heat in each fluid component. Pump ON/OFF operation was manually implemented and highlighted the need for control system automation. An automatic control system to deliver heat to the thermal load would ideally reverse the pump and continue delivering heat to the thermal load as long as the HTF exceeded the load’s minimum required temperature.
Embry-Riddle’s technical outcomes include a demonstration unit that can store heat overnight in eight gallons of Canola oil HTF. Four high temperature tests were conducted and summarized. The resulting system uses a high temperature pump to deliver stored heat as-needed to an absorption chiller refrigerator’s ammonia boiler. Estimated refrigerator operation was 2 hours 25 minutes from the 8 gallons of HTF at 400F. To operate over 24 hours would require approximately 80 gallons of HTF at 400F, or less using higher temperatures. Multiple avenues of research are clearly identified including improved HTF selection, improved tank baffling and tank temperature monitoring, improved heat delivery methods and heat exchangers, control system design to automate daily outdoor operation.
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, how to brainstorm conceptual designs, and select 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.
Embry-Riddle’s solar thermal energy storage design demonstrates heat storage and delivery to a thermal load for an estimated 2 hours 25 minutes of absorption chiller operation after sunset. The system uses a two-tank design allowing high temperature heat transfer fluid (HTF) to remain at maximum temperature for delivery to the thermal load. Experimental results did not achieve the goal of 24 hour operation but were quantified and clearly presented in the testing section. The thermal load chosen was an absorption chiller device commercially available as a propane or electric powered refrigerator. The absorption chiller device was entirely heat driven making it an ideal candidate for solar thermal power. We present the absorption chiller unit as a small-scale demonstration of solar thermal air conditioning. Building HVAC represents 0.8% of the entire United States’ energy consumption. This portion is ideally suited for migration away from fossil fuel and towards solar. Other applications 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 in the developed and developing nations.
The energy storage contained 8 gallons of Canola oil as both the heat storage medium and also heat transfer fluid. Canola has high thermal density, high smoke point, and is a safe food product rather than a hazardous material, making its purchase, storage, and disposal convenient. The hydraulic circulating pump and fluid circuit can accommodate 650F which is higher than the HTF. The Canola HTF adequately stored heat energy at over 400F and was circulated to the heat exchanger on the absorption chiller unit. Eight gallons of thermal storage fluid is estimated to operate the refrigerator for 2 hours 25 minutes on a single day’s charge. The absorption chiller unit operated equivalently with the heat delivered by the HTF, the electric heating element, or the propane flame. Embry-Riddle’s solar thermal research will continue by using the outdoor concentrating collector field to energize the thermal energy storage unit during the day, then operate the absorption chiller unit after sunset.