Solar thermal gets a dopamine boost

Improving the efficiency of solar thermal heating could be improved with the use of polydopamine (PDA) nanoparticles. NCCR Bio-Inspired Materials researchers at the Adolphe Merkle Institute have applied a hybrid nanoparticle system to increase heating efficiency in solar fluid.

When it comes to solar power, photovoltaics usually garner all the attention. But sunlight is actually more frequently used to heat water. Energy is usually captured by a black absorber surface, and the heat generated is transferred to a fluid running through the panel. Two limitations to this system are the efficacy of the absorbing panel, as well as the efficiency of the heat transfer to the solar fluid. One way to overcome this is the so-called direct absorption solar collector (DASC) approach, where absorption takes place directly in the solar fluid. A variety of fluids have been proposed, starting with India ink, which contains soot particles, but also clogs and damages the collector. More efficient nanoparticle-doped fluids have also been trialed, but also fall short because of increased costs, wear and tear on installations, and issues with the disposal of potentially toxic waste. 

NCCR Bio-Inspired Materials Principal Investigator Prof. Barbara Rothen-Rutishauser and her team at the Adolphe Merkle Institute chose to investigate the applicability of polydopamine. PDA is mussel-inspired material, but more importantly, can form black nanoparticles, and has extraordinary heating properties when excited with light. It could therefore be a candidate for a stable, nontoxic, and highly efficient alternative to boost solar heating. Previous research in collaboration with Prof. Vincent Ball from the University in Strasbourg, France, has shown that polydopamine can be easily modified, a characteristic the NCCR researchers used to their advantage. They synthesized hybrid PDA nanoparticles of different sizes with a capping protein, bovine serum albumin (BSA), which was used as a proof-of-concept. The heating ability of the different samples, along with food coloring containing micrometer‐sized soot particles, as well as silver nanoparticles, was measured with lock-in thermography at a fixed wavelength. These results showed that that smaller hybrid nanoparticles provided the best heat transfer, although not bettering the silver nanoparticles, which have been used as solar fluids.

The hybrid nanoparticles also proved themselves to be equal to one of the major challenges facing solar fluid doping: the degradation or aggregation of the dopant in the fluid over time. They were still stable after UV irradiation with an energy equivalent to six-and-a-half years of the sunlight reaching Switzerland, based on data from the Swiss national weather service.  Heating ability itself only dropped slightly.

Taking their investigations further, the researchers built a test flow circuit including a collector. This was exposed to solar radiation in a simulator where the temperature of the collector fluid was monitored. The solar simulation replicates the entire spectrum of the sun’s light, unlike lock-in thermography. Results in this case showed that the hybrids could outperform substantially the silver nanoparticle fluids. An additional benefit was that no hybrid PDA nanoparticle deposits were found in the collector in comparison to the solar fluids doped with silver nanoparticles or food coloring.

Nanoparticle-doped solar fluids face a number of challenges, not least high production costs, a certain instability, and a tendency to degrade collector components. PDA nanoparticles are well-equipped to overcome these hurdles as their main ingredients – polydopamine and BSA – are easy to obtain at a low cost. And PDA is already present in the natural environment and biocompatible at high concentrations, making it unlikely to pose any kind of hazard. Combined with their heating efficiency, these hybrid nanoparticles should be considered for future use in solar thermal applications.

Reference: Hauser, D.; Steinmetz, L.; Balog, S.; Taladriz‐Blanco, P.; Septiadi, D.; Wilts, B. D.; Petri‐Fink, A.; Rothen‐Rutishauser, B. Polydopamine nanoparticle doped nanofluid for solar thermal energy collector efficiency increase, Adv. Sustain. Syst., 2019, 4, 1900101.