Why Our Technology is Better
Sunplate’s Opaque Cover Technology™ offers several sustainable competitive advantages over existing solar water heating technologies.
Our technology is highly differentiated from existing solar collector technologies and makes several improvements within an overall framework of simplicity.
While less expensive to manufacture, Sunplates are stronger and more durable than glazed flat plate, evacuated tube, or unglazed plastic solar collectors. Without glass, Sunplates are also lighter, easier to handle and safer to work with on a roof.
Glazed flat plate and evacuated tube solar collectors are mature technologies with future cost reduction breakthroughs unlikely because the costs of materials dominates manufacturing costs. Unglazed plastic collectors, originally developed for swimming pool heating, cannot produce the operating temperatures needed for potable water heating during cold weather.
The major Sunplate improvements over legacy technologies are discussed below:
Inherent Overheat Protection with No Moving Parts
The problem. Overheating in solar collectors damages solar collector gaskets and insulation, breaks down heat transfer fluids, dramatically reduces the lifetime heat output, and can cause catastrophic collector failure due to glass shattering under thermal stress.
Overheating is far more critical to solar collector service life and energy production than chasing maximum thermal efficiency or higher operating temperatures.
Overheating cannot be prevented by undersizing a glazed flat plate or evacuated tube collector because a solar collector’s tendency to overheat when no fluid is circulating through the collector’s absorber during hot weather is a function of the collector’s heat loss characteristics, not its size.
The Sunplate solution. The heat transfer characteristics of our opaque cover permit Sunplates to be engineered for different heat loss profiles depending upon performance requirements, in each case limiting stagnation temperatures to below the maximum recommended temperatures for heat transfer fluids, gaskets, foam insulation, and other collector materials.
The result is that our technology provides natural overheat protection without moving parts or special materials. Solar collectors that use our technology can deliver heat within the needed temperature range of 125–140°F (52–60°C) for home water heating without damaging occurrences of overheating during the summer months.
Our opaque cover solar collectors can be engineered to lose heat at 6 to 12 watts per square meter per degree Celsius of temperature increase, with the particular desired heat loss profile being selected based upon the hottest weather conditions typically experienced in a particular locale climate. For example, at a heat loss of 8 watts per square meter, one our collectors will not reach an internal temperature hotter than 314°F (157°C) when the air temperature is 100°F (38°C) and sunlight intensity is 1,000 watts per square meter.
This is below the 325°F (163°C) maximum recommended operating temperature for Dowfrost® HD, the most popular commercial heat transfer fluid for closed loop solar water heating systems.
By comparison, under the same air temperature and sunlight intensity conditions,
a glazed flat plate collector with a selective absorber coating will reach an internal temperature of 458°F (237°C). A double glass evacuated tube collector will reach an internal stagnation temperature of 741°F (394°C).
A durable but low cost solar collector technology with natural overheat protection and no moving parts presents an opportunity for the first time to offer solar water heating systems that can have oversized capacity, relative to expected daily hot water demand.
This offers exciting possibilities such as, for example:
- solar water heating systems that can be oversized to provide solar fractions in excess of 90% for most climates, even allowing for extended overcast weather, without damaging summer season overheating;
- more dependable electric utility peak load reduction during intermittent overcast weather, especially during cold winter morning peak loads; and
- an effective strategy for an electric utility company to capture its customers’ optimally oriented (south-facing) roof area for utility owned, oversized capacity solar water heating systems, preempting or reducing the roof area available for distributed solar power systems that might otherwise be installed by the electric utility’s competitors.
It is important to understand that maximum stagnation temperatures can be reduced in a glazed flat plate collector by increasing the collector’s heat loss characteristics, by using a non-selective absorber coating and by reducing the thickness of the thermal insulation. However, these measures cannot eliminate the glazed flat plate collector’s inherent design weaknesses and failure points.
Thermal Insulation Isolation Solution
The problem. Most glazed flat plate collectors have some form of fiber or polyurethane foam insulation in the collector interior, to reduce heat loss through the collector frame and back plate. High temperatures cause chemicals in the insulation to vaporize, and cause the insulation material to become brittle and eventually disintegrate into a fine dust. This results in increased collector heat loss and reduced heat output.
Evacuated tube collectors derive their insulation from a vacuum, which eliminates heat loss by conduction and convection. However, these collectors typically fail within 10 years or less as a result of glass breakage due to:
- impacts from wind driven projectiles;
- thermal stress that results from large temperature differences between fused glass surfaces in double wall glass tubes; and
- twisting forces (torque) on the glass tubes caused by movement of mounting hardware holding the glass tubes.
The Sunplate solution. The Sunplate Basic collector derives its insulation value from infrared heat reflection back to the absorber by the interior surfaces of the frame wall and back plate. In Sunplate models containing insulation, a foam insulation can be injected into the hollow unibody frame and encased inside the back plate panel. Our stagnation temperature profile is below the maximum recommended temperatures for polyurethane foam insulation materials, so insulation breakdown should not occur. The results:
- Since any insulation material in a Sunplate is physically isolated from the collector interior air space, chemical outgassing cannot reach the underside of the cover.
- Models containing foam insulation should continue to perform at or near their original heat output for many years longer than glazed flat plate or evacuated tube collectors.
Moisture Intrusion Solution
The problem. Most glazed flat plate collectors have extruded aluminum frames, creating four square corners that must be joined with fasteners (welding would destroy anodizing finishes). And the cover frame is glass, which must be held in place by three dimensional angled cover frame that is inherently difficult to seal. Also, glass has a different coefficient of thermal expansion than aluminum. This means:
- inevitable movement in the corners and the glass cover frame over time, permitting moist air infiltration urged by pressure differences between the ambient air and the collector interior; and
- moisture condenses on the underside of the glass and serves as a base to adhere insulation dust and other particulates into a grimy film, reducing sunlight transmission through the glass and collector heat output.
The Sunplate solution. We invented an inexpensive yet inherently strong hollow unibody frame wall with rounded corners. The frame wall is formed from a single piece of hollow extruded stock and welded at a single point. Also, a Sunplate’s frame wall, cover plate and back plate are all the same material, eliminating dissimilar rates of thermal expansion and contraction and the need for relatively complex angle, difficult to seal frames to hold down a glass cover plate. The results:
- The only two mating surfaces in a Sunplate are easy to seal, two dimensional planar surfaces where the cover plate and back plate each meet the frame wall.
- The unibody frame wall, cover plate and back plate all have the same coefficient of thermal expansion and contraction, substantially reducing or eliminating the potential for moisture intrusion.
Frame Wall Solution
The problem. In high wind zones like Florida, Southeast Asia and coastal India, the glass sheet in a glazed flat plate solar collector must be sufficiently thick to withstand—and must be attached to the solar collector assembly with a frame and fasteners that can withstand—the dynamic air pressures generated by severe windstorms. Consequently, the glass cover plate is usually either 1/8 or 5/32 inches (3.2 or 4 mm) thick, with corresponding weights of about 8 and 10 kilograms per square meter. The use of glass makes flat plate solar collectors difficult to handle and expensive to ship. Also, tempered glass is not impervious to breakage. While tempering produces excellent flat surface impact resistance, the edge strength is poor. A sheet of tempered glass can shatter when lateral compressive force (for example, the force from a solar collector being dropped on its side during handling) drives the head or length of an adjacent glazing frame screw into the edge of the glass.
The Sunplate solution. We believe that the rounded corners of our patented hollow unibody collector frame wall reduce effective wind speed and negative air pressure (wind uplift) across the solar collector cover. Reducing wind speed reduces convective heat loss. Reducing wind uplift means a thinner cover plate can be used to meet a given building code wind load requirement, and the size and strength of mounting hardware and fasteners required to attach the solar collector to a roof or other mounting location surface can be reduced.
Pictured above: the square corner of a typical glazed flat plate collector is shown on the left. The rounded corner of a Sunplate prototype is shown on the right.
Rounded corners reduce dynamic air pressure on the opaque cover during high winds, permitting less cover plate thickness than might otherwise be required to meet a given windload requirement. In addition to material cost savings, an opaque cover with reduced mass will generally attain higher temperatures when exposed to incident solar energy, which is desirable for increasing infrared heat transfer to the absorber. Comparative tests of roof gravel scouring have suggested that aerodynamic corners can double the damage threshold wind speed compared to conventional square corners.
More Efficient Shipping Form Factor
With a one inch (25 mm) side profile height vs. 3-1/2 inches (89 mm) for a typical glazed flat plate collector, twice as many Sunplates can fit into a shipping container as conventional glazed flat plate solar collectors of the same length and width. At current shipping rates, this results in a savings of over $2 million per 100,000 square meters of flat plate solar collector surface area.
An example will illustrate the Sunplate shipping cost advantage. Shipping one 20-foot container between Asia and the Eastern United States costs about $10,000.1 Charges are calculated by volume and not by weight for a full ocean freight container load. However, the ocean container’s net weight capacity is limited by the lesser weight capacity of the tractor trailer rig that will haul the container from the U.S. west coast to its destination. A 53-foot tractor trailer has a capacity of 52,000 pounds and can fit two 20-foot containers, so a 20-foot container is limited to 26,000 pounds by the trucking portion of the journey.
The container’s interior dimensions are 92.6″ wide x 93.9″ tall x 224.8″ in length (2352 x 2385 x 5709 mm). The door opening measures 92.2″ wide x 89.7″ tall (2341 x 2278 mm). The most efficient way to stack solar collectors in the container is upright—with the long dimension of the collector running floor to ceiling—and the collectors’ width oriented parallel to the container’s length. The biggest standard solar collector size that will fit, crated, through the doors is an 84″ (2134 mm) length collector, which is typically 36″ (914 mm) wide and has 21 square feet (1.95 square meters) of gross collector area.
Virtually every glazed flat plate solar collector on the market is about 3-1/2″ (89 mm) thick. Allowing one inch (25 mm) on each side of the width plus an extra half inch (13 mm) of added thickness for crating, the container will hold 115 glazed flat plate solar collectors.
Sunplate solar collectors would have the same length and width, but a model with no mass insulated back plate is only about one inch thick. However, let’s assume that Sunplates with insulated back plates are being shipped. At an increased thickness of 1-3/4″ (44.5 mm) we can still stack two Sunplates in the same space as one glazed flat plate collector, with an extra half inch inch (13 mm) of thickness between each of the collectors for crating. We can comfortably fit 230 crated collectors in the container.
The Sunplates weigh 15,516 pounds plus crating and the glazed flat plates weigh 8,510 pounds plus crating, but the $10,000 shipping bill is the same either way. In this example, the 115 glazed flat plates will cost $86.96 apiece to ship, double the $43.48 apiece for 230 Sunplates with the same length and width.
The total cost savings to ship about 100,000 square meters worth of solar collector surface area (44,850 Sunplates at the 36″ x 84″ collector size) between Asia and the Eastern United States is $1.95 million.
Reduced Cover Plate Weight vs. Tempered Glass
A Sunplate will weigh significantly less than a glazed flat plate solar collector. Most of the weight difference is due to the elimination of a tempered glass cover plate. Reducing weight reduces material cost, lowers shipping costs and makes solar collectors safer and easier to handle.
The heaviest potential opaque cover plate is an aluminum alloy sheet, sufficiently thick to withstand the negative pressure associated with hurricane force winds. The density of aluminum is actually slightly greater than glass, and the modulus of elasticity is about the same. While the theoretical tensile and compressive strengths of aluminum and glass are about the same, glass breakage is statistically unpredictable because surface imperfections and impurities in glass plates cause irregular stress concentrations.
Glass plates are also negatively affected by the duration of a load. A glass plate might successfully sustain a short-term load during testing that exerts more than twice as much pressure as a long-term load that causes failure.
For these reasons, a multiplier factor that adjusts for the statistical probability of glass failure is used to calculate the required thickness of a glass plate in structural engineering applications. A sheet of tempered glass used as solar collector glazing will be about twice as thick as its theoretical strength might suggest is necessary to meet the negative pressure associated with a design windload, or to withstand the compressive atmospheric force on the glazing of an evacuated solar collector.
This is why, for example, a 1/16″ (1.6 mm) thickness aluminum sheet can withstand the same design load for which a 1/8″ (3.2 mm) thickness glass sheet would be required. Given the similar densities of glass and aluminum, the aluminum sheet will weigh about half as much as the glass plate. And it is important to note that we believe we will be able to use aluminum thicknesses that are less than half the corresponding glass thicknesses for a given design load requirement because our cover plates will experience less wind uplift force, due to the rounded corners of our hollow unibody frame wall.
The weight advantage of an opaque cover, compared to tempered glass, may be even more pronounced in future implementations of our technology. For example, we hope to develop and test ultra-light aluminum honeycomb sandwich panels for Sunplate opaque cover plates and back plates. The combined thickness of the aluminum facing sheets on each side of such a honeycomb structure could be substantially thinner and lighter than the 1/16″ (1.6 mm) thickness aluminum sheet in the example above, yet still have strength sufficient to withstand hurricane force winds and other negative pressures, and the compressive atmospheric force on the opaque cover plate and back plate of an evacuated Sunplate solar collector.
40% Lower Cost Than Glazed Flat Plate Collectors
Our cost studies show that manufacturing cost per unit of sun-facing surface area for all-aluminum alloy collectors using our technology is about 40% less than the cost of an average quality glazed flat plate solar collector and about the same as a cheap Chinese evacuated tube collector.
World’s First “Beautiful” Solar Collector
Form follows function. The rounded corners, low side profile and opaque cover of our technology each fulfill important functions. Even so, our design offers elegant simplicity and-we believe it is fair to say-the world’s first beautiful solar collector. This may be a very important competitive advantage in Asia, where researchers report that solar water heaters on residential rooftops are highly visible status symbols that tell neighbors the owner is successful enough to be able to afford advanced water heating.
We believe that our opaque cover technology offers several other advantages over existing technologies, including:
- requires less maintenance;
- less expensive sealing and fastening of the cover plate;
- potential to apply new (replacement) cover plate exterior surfaces in the field without opening up the solar collector interior;
- field removal and replacement of a lighter and less fragile opaque cover is much safer and easier than removal and replacement of a glass cover plate; and
- snow and frost accumulation on cold glass is eliminated because of the higher thermal conductivity of an opaque cover plate.
We believe our opaque cover technology will give both manufacturers and end users lower costs, greater returns over time, and more attractive products than their historical options.
- American Transportation Research Institute and the “Doing Business Project” of The World Bank. The $10,000 shipping cost is taken from a 2011 example, based upon 2010 costs, shipping a single 20-foot container from a Chinese port to California, then 2,019 miles by motor freight from Long Beach, California to Mobile, Alabama. The actual cost in the example was $9,690; $6,238 for export / import costs and the ocean segment, plus $3,452 for trucking costs.