Steam-Generating Solar Collector

In June 2013, AILR was granted US Patent 8,459,250 (Lowenstein)  for an innovative solar collector that converts solar radiation into atmospheric pressure steam.  As shown in the neighboring figure, the steam-generating solar collector (SGSC) is composed of an array of dewar-type evacuated tubes.  (Dewar-type evacuated tubes are constructed like a vacuum thermos bottle with an inner tube positioned within an outer tube and the space between the two tubes evacuated.)  The collectors are installed horizontally.  At the start of the day before the collectors are illuminated, they are filled approximately half-full of water.  The solar radiation that is absorbed by the inner tube heats the water which then converts to steam.  The large free surface of the water allows the generation of steam to occur quiescently without vigorous boiling.  The steam from each tube is collected in a common manifold and delivered to the end use (which for a solar LDAC would be the regenerator).  The steam condenses releasing heat at 100 C to the end use.  The hot condensate is stored in an insulated tank overnight and returned to the collectors the next morning.

In the summer of 2010, AILR tested a SGSC with 80 tubes under contract to NREL.  Performance of the innovative collector was comparable to conventional evacuated-tube solar collectors.  However, the very simple design for the SGSC should permit a dramatic reduction in first-cost.  Furthermore the horizontal orientation will reduce installation costs since racking will not be needed.  Lastly, the direct supply of steam to the regenerator eliminates the cost and power for a hot-water circulating pump.

The horizontal orientation for the tubes of a SGSC will penalize performance when the sun is low in the sky.  Thus, the SGSC is not a good source of thermal energy for mid latitude and high latitude locations in the winter.  However, the SGSC is an excellent, low-cost source of high-grade thermal energy in climates that require cooling.

Table 1 compares the thermal energy provided at 100 C by the SGSC (SG) and a conventional evacuated-tube (ET) solar tilted from horizontal at an angle equal to the location’s latitude.  The last column in this table is the useful thermal energy provided by the SGSC during the full year for low latitude locations and during the cooling season for mid latitude locations as a percentage of the thermal energy provided by the conventional evacuated-tube collector.  As expected, the horizontal orientation for the SGSC penalizes its performance only slightly in low-latitude locations.  In many mid-latitude locations where summer cooling loads are large,  the SGSC can drive either an absorption chiller or a liquid-desiccant air conditioner. 

Table 1

* Cooling season defined as March through October for all cities except LaCrosse where it is April to September.