A combination of inexpensive materials collects and concentrates heat from the Sun.
Heating water to its boiling point is an important first step not only for preparing a cup of tea or a bowl of pasta, but for a range of applications fundamental to an industrial society, including distillation, sterilization, and power generation. In a solar economy, one could boil water with an electric heater powered by a photovoltaic cell. But it would be far more efficient to use solar energy to heat the water directly.
That’s manifestly possible. For decades solar steam turbines in wide-open sunny spaces have used arrays of mirrors to concentrate sunlight from a large area onto a small volume of water. But those mirrors are expensive: They must be precisely machined to focus light over several hundred meters, and they must be mounted on motors to track the Sun’s position in the sky. Because the motors require that a powerful source of electricity already be available, optical concentrating arrays aren’t suitable to smaller-scale or off-the-grid applications, such as sterilizing medical instruments in a clinic in the developing world.
Now MIT’s Gang Chen, George Ni, and their colleagues have demonstrated a different approach: concentrating not the Sun’s light but its heat.1 Because their steam generator consists entirely of commonly available materials—a conscious choice on their part—they estimate that per unit area, it could be built for just 1–3% of the cost of an array of motorized mirrors.
The device is sketched in figure 1. It works by absorbing solar energy over a large area but giving it nowhere to escape except through a small slot where the absorber is in contact with a reservoir of ambient-temperature water. If the absorption area is large enough and the contact area is small enough, the water is locally brought to a boil to release steam before the heat can diffuse out into the bulk liquid. The challenge, then, is to keep the absorber from losing too much heat to conduction, convection, and radiation. Normally—and not unfortunately—those losses prevent any object heated by unconcentrated sunlight from getting anywhere near 100 °C.
To limit conductive and convective losses, the researchers insulated the top and bottom of the absorbing layer. For the bottom layer, they used ordinary polystyrene foam, which also kept the device afloat. The choice of top layer was a bit more constrained, because they needed something optically transparent. So they tried bubble wrap. “I was surprised by how well the bubble wrap worked,” said Ni. “Most researchers are using high-performance materials, and here we were, testing out bubble wrap, which wasn’t designed for maximum optical clarity.” Indeed, the bubble wrap transmits only 80% of the light that hits it. But its insulation benefits far outweighed that modest optical inefficiency.
Physics Today: Solar steam generator needs no lenses or mirrors, Johanna L. Miller