Solar water heaters that use evacuated tubes are a low-cost source of hot water in China and other parts of the world. The evacuated tubes used in these collectors are double-wall Dewar vessels (essentially vacuum bottles). The inner walls of these tubes have a selective absorber coating on their vacuum side that absorbs visible radiation, but does not reradiate infrared radiation from the hot absorber surface.  Solar collection efficiencies are extremely high and stagnation temperatures (i.e., operation without the tubes delivering heat) can approach 200°C (~400°F).  These tubes are now manufactured in very high volume in China, and they can be purchased in the U.S. in quantity for approximately $50 per square meter of absorber area.

     In October 2015, AIL Research was granted U.S. Patent No. 9,157,659 for an innovative solar thermal collector based on Dewar-type evacuated tubes that converts solar radiation directly into steam at atmospheric pressure.  The basic operation of the collector is shown in the first two figures on the left.  As shown in these figures, a horizontal Dewar-type evacuated tube is partially filled with water. Incident solar radiation is absorbed by the selective coating on the inner tube wall.  With outward IR radiation suppressed by the selective coating and a vacuum blocking heat conduction from the inner tube to ambient, the wall heats to a high temperature. The hot wall then radiates inward heating the water in the bottom of the tube. When the water reaches approximately 100°C (assuming sea level operation), steam evolves from the water’s free surface. The evolution of steam has been observed to be quiescent with no active boiling.

​     (Although not essential to the collector’s operation, its solar conversion efficiency will improve if a thin, porous hydrophilic layer is applied to the water-side of the inner tube wall so that water is wicked onto the hot upper section of the inner tube.  We have measured about a 10% improvement is solar conversion efficiency for a collector which has thin layers of mineral scale on the inner walls of its tubes.)

​     Our steam-generating solar collectors are a low-cost, highly efficient alternative to other non-concentrating thermal collectors.  The conceptual installation in the first figure on the right shows one eight-panel leg of a multi-leg solar array. Each panel has 60 Dewar-type evacuated tubes—30 tubes on each side of a common manifold.  The eight manifolds are circuited in series so that the steam generated by the eight panels flows to a main steam line that also collects steam from the other legs of the array.

​     In daily operation, all tubes are partially filled with water prior to sunrise when the tubes are cold. After sunrise as the tubes are illuminated, the temperature of the stored water increases. With the tubes initially filled with air at ambient pressure, steam is produced when the water reaches approximately 100°C (for an installation near sea level). The evolving steam clears the air from the manifolds and lines, and eventually flows to the remote point-of-use load where it condenses, releasing thermal energy at 100°C. The condensate is stored overnight in an insulated tank and returned to the array before the next sunrise.

​     The eight-panel, 480-tube array shown in the second figure on the right was installed in August 2021 on a farm in Freestone, CA. The storage tanks and fan/coil steam condenser that served as the load are in the figure background. 

​     A critical requirement for array installation is to maintain all panels that have a common water/steam circuit within a common horizontal plane.  This requirement was met by a mounting system in which earth anchors, similar to the one shown in the third figure on the left, are first driven in the ground at six points on the perimeter of each panel.  A laser level is then used to position locating nuts on the threaded rods extending from the heads of the earth anchors.  With the locating nuts all in a common plane, the panels are loaded in position and locked in place on the locating nuts. The 8-panel Freestone array was level to within 3 mm. 

​     The Freestone array began operation on September 17, 2021.  During the first week, adjustments were made to the initial charge of water within the tubes.  (An array’s thermal output will be highest when the initial charge of water is close to the minimum that avoids dry-out before the end of the day.)

​     The Freestone array operated continuously with no problems until October 23, 2021 when a violent storm damaged the control panel.  However, although the storm was described as “historic” in news reports, the array was not damaged and stayed in plane. 

​     The monitored operation of the Freestone array confirmed the high solar conversion efficiency for the steam-generating collector.  The data in third figure on the right compares the thermal energy delivered by the steam-generating collector (as measured by the condensate produced each day) and by a conventional collector that uses evacuated tubes with internal heat pipes (i.e., the Apricus AP-20).  Except for days when the tubes were undercharged (which led to dry-out before the end of the day), the steam-generating collector supplied more thermal energy at a higher end-use temperature (i.e., 212°F vs 204°F) than the comparable SRCC rating for the Apricus collector.

​     The fourth figure on the right shows detailed data for the array performance on September 22, 2021 (the circled data point in third right figure).  As shown in this figure early morning fog significantly reduced solar insolation, and steam production was delayed unit 3.4 hours after sunrise and continued for 7.1 hours. The panel was illuminated for 10.7 hours. (The green data points track the cumulative supplied steam energy; the blue/yellow points track the cumulative solar energy incident on the panels.)