This result is consistent with recent studies that demonstrate saturated steam generation at 100☌ using >1,000× thermal concertation but achieve energy efficiency of ≈ 20%. When h approaches 10 W m −2 K −1 (typical for open systems in the air), the energy efficiency quickly drops to below 20% for T b = 100☌, especially when the input solar flux is lower than 1 kW/m 2 due to atmospheric conditions and cosine loss (see Note S1 for detailed thermal analysis). The efficiency of thermal concentration, i.e., the ratio of the concentrated thermal energy to the total incoming solar energy η t h = Q h e a t / Q s o l a r, strongly depends on the base temperature of the absorber fin T b and the heat loss coefficient h as shown in Figure 1C. However, when compared with the optical approach, the additional heat loss due to (1) the larger absorber surface area and (2) the temperature gradient along with the absorber ( Figure 1A, middle panel) severely deteriorates the thermal concentration performance at elevated temperatures. In addition to enabling passive sterilization, this work promises the development of solar thermal energy systems for saturated steam generation in energy conversion, storage, and transport applications. With almost 2× higher energy efficiency (47%) than those previously reported at 100☌, the device demonstrated successful sterilization in a field test performed in Mumbai, India.

Enabled by an optimized ultra-transparent silica aerogel, the device utilizes an efficient thermal concentration strategy to locally increase the heat flux and temperature obviating the need for active optical concentrators. We demonstrate a passive solar thermal device mostly built from low-cost off-the-shelf components capable of delivering saturated and pressurized steam to drive sterilization cycles even under hazy and partly cloudy weather. However, solar-driven steam generation at such high temperature and pressure requires expensive optical concentrators.

Saturated steam (>121☌ and >205 kPa) is widely used in the medical sterilization process known as autoclaving.

Steam account generator 2019