Heating water to its boiling point is an important first step, not only for preparing a cup of tea or bowl of pasta but also for applications such as distillation, sterilization, and power generation. Using solar energy to boil water typically requires a costly array of motorized Sun-tracking mirrors to focus sunlight from a large area onto a small volume of water. Now MIT’s Gang Chen and his colleagues have demonstrated a different approach, using the device in the figure to concentrate not the Sun’s light but its heat.
The main challenge was to reduce the heat lost to conduction, convection, and radiation, which otherwise would prevent any part of the system from getting anywhere near 100 °C. To limit radiative losses, the researchers capture the sunlight with a spectrally selective absorber that emits little in the IR but absorbs strongly at visible wavelengths. They limit conductive and convective losses by sandwiching the absorber between layers of bubble wrap and polystyrene foam. As a result, the heat, once absorbed, has nowhere to escape except through the small slot where the absorber is in contact with ambient-temperature water; local heating of the water readily brings it to a boil. Because the absorber gets sufficiently hot after only a few minutes under direct sunlight, the device can even be used on a partly cloudy day to take advantage of intermittent periods of sun.
The proof-of-principle steam generator was a disk 10 cm in diameter. The researchers are working to implement the process on a larger scale. All the components of the device are widely available and inexpensive—spectrally selective absorbers, for example, are currently used for solar domestic water heating. Chen and colleagues estimate that per unit area, the steam generator could be built for just 1–3% of the cost of an optical concentrating array. (G. Ni et al., Nat. Energy 1, 16126, 2016.)
