From left: Brandon Koyanagi, Will Wilson, Gerardo Rojo and Michelle Zdanowski
For their senior undergraduate project, four engineering students at Southern Illinois University Edwardsville developed a heat storage system designed to collect thermal energy from the sun and release it at night.
The Solar Owl combines water-based sensible heat storage in an insulated tank and latent heat storage through use of a hydrated salt phase change material. The PCM is contained in a continuous, multiple-coil vessel within the tank.
The team – Brandon Koyanagi, Will Wilson, Gerardo Rojo and Michelle Zdanowski – designed, analyzed, built and tested a scale prototype of the apparatus.
“At full size,” Wilson says, “the tank would be capable of heating an average American home through the evening and night, fourteen hours, using heat accumulated from solar thermal collectors during a typical St. Louis-area winter day. The Solar Owl reduces the size of the heat storage tank by 40 percent when compared to a sensible-only storage solution.”
The team earned an A on the project and will continue to develop the system in graduate studies. Aside from securing a patent on the design elements and a trademark on the name, the team has no immediate plans to commercialize the system.
Here’s a Q&A with Wilson, who was selected as Outstanding Senior in the university’s Mechanical Engineering Department this year.
Q: Why did the team choose sodium acetate trihydrate as the phase change material?
A: SAT fit our needs in four important ways. (1) SAT's latent heat of fusion is high enough to keep tank size adequately small. (2) SAT, and the necessary additives, are readily availability at low to mid cost. (3) SAT exhibits low toxicity and is environmentally friendly. (4) The melting point of 58 C lends itself well to our application. This melting point is low enough to be completely melted by heat collected from a standard flat-plate solar thermal collector, while also providing a good temperature delta for most hydronic loop appliances.
Q: What is the source of the SAT you're using?
A: We utilized SIUE chemistry lab space to synthesize a custom solution of lab-grade hydrated sodium acetate crystals with a small amount of additional water to slightly lower the temperature needed for complete melting. Carboxymethyl cellulose (CMC) was added to mitigate phase separation and potassium sulfate to minimize supercooling.
Q: SAT typically exhibits supercooling. Is that a problem in your application?
A: As the system is not intended to be a long-term heat store, supercooling was not desirable for our design. To reduce supercooling, we added potassium sulfate to the solution as a nucleating agent.
Q. Unmodified SAT's latent heat of fusion is typically 264–289 joules per gram, with a melt point of 58 C. Does your SAT fit those specs?
A: Our SAT did fit within that range per gram of SAT in solution. Of course, the latent heat of fusion for the solution overall, including the extra water and additives, was somewhat lower.
Q: What triggers the release of the heat?
A: Heat is released as the tank temperature falls below the melting point (plus a correction for minimal supercooling, approximately 5 degrees Celsius).
Q: What heat transfer fluid is used between the solar collectors and the tank?
A: I should note that, since the storage tank was the focus of our project, our prototype simulated solar collectors using an electrically heated water bath with submerged heat transfer coil. The transfer fluid from the “collector” to the tank, and between the tank and the testing radiators, is a solution of propylene glycol. This was chosen for (1) its low environmental toxicity and (2) its sensible heat capacity, which is high enough to help maintain tank temperature, but low enough to facilitate an appropriate heat transfer rate.
Q: What are the key design elements of the container, and what material is it made of?
A: The PCM container is a set of interconnected concentric helical coils constructed from standard PEX tubing. A set of likewise interconnected helical heat transfer coils is interspersed between the PEX coils, allowing proximity of the heat source to melt the PCM during the day, with enough space between coils to allow free convective heat transfer to the surrounding water in the tank. This configuration has the following advantages:
• It holds a relatively large amount of PCM with large surface area for heat transfer to the water in the tank.
• The tubing diameter is small enough to allow for an adequate heat rejection rate from the PCM (given SAT’s low heat conductivity).
• The thermal conductivity of PEX is higher than many plastics, such that it does not present a bottleneck to the release of PCM heat.
As far as our research informs us, our design also stands out in that the PCM solution may be readily drained and filled for system maintenance. According to the data we could locate, an SAT solution experiencing daily phase cycling has a predicted useful life of around 10 years. Allowing easy access to flush the PCM container and refill is therefore critical for long-term maintenance and also for the case where the SAT formulation fails early, becomes contaminated, or requires testing or monitoring.
For more on the project, see: