We call solar collectors that use our technology Sunplates. Our opaque cover technology solves a pressing market need for solar collectors that are economically priced like mass-produced but failure prone Chinese evacuated tube collectors, with greater durability and lifetime heat output than glazed flat plate collectors. We received U.S. Patent 9,829,216 for our opaque cover technology on November 28, 2017.
Our key insight is that trouble-free operation and total lifetime heat output are more important than efficiency or maximum temperature performance.
Heat output in glazed flat plate collectors falls by as much as 4–5% per year, on average over the collector’s useful life, primarily as a result of damaging overheating and moisture intrusion. Evacuated tube collectors, on the other hand, tend to fail suddenly in 10 years or less, usually as a result of thermal stresses that shatter the glass tubes or crack copper pipes inside the glass tubes.
Sunplates® have no glass or other transparent material for transmitting sunlight into the solar collector interior. Glass is fragile, expensive, heavy, promotes solar collector overheating, and expands and contracts at a different rate than other collector materials, which eventually results in moisture intrusion, internal corrosion, glass fogging and loss of heat output. Instead, Sunplates have a patented opaque cover with special infrared heat transfer properties, which eliminates the need for glass in a solar collector. We believe our opaque cover technology offers several advantages over glazed flat plate collectors and evacuated tube collectors, including:
- significantly greater lifetime heat output
- natural overheat protection with no moving parts or special materials
- moisture intrusion from differences in glass and metal expansion is eliminated
- collector interior is not exposed to ultraviolet radiation
- can be profitably priced 40% lower than glazed flat plate collectors, at about the same price or slightly higher than Chinese evacuated tube collectors
- 50% savings in container shipping costs compared to glazed flat plate collectors because of more efficient shipping form factor
- reduced weight means easier and safer handling
- more rugged and durable
Another innovation of our patented technology is a hollow unibody collector frame wall with rounded corners. Our solar collector frame wall, which can also be licensed for use in conventional glazed flat plate collectors, has several benefits:
- inexpensive to fabricate
- less convective heat loss from cover plate
- less wind uplift force on cover plate during high winds
- less vortex shedding off collector edges during destructive winds
- easier to keep airtight because all mating surfaces of collector enclosure are planar
- can withstand atmospheric pressure if enclosure is evacuated
- hollow unibody frame wall can be injected with foam insulation
- unibody design reduces manufacturing steps and costs
- beautiful design
Greater Lifetime Heat Output
Heat output in glazed flat plate collectors falls by as much as 4-5% per year, primarily as a result of damaging overheating and moisture intrusion. Evacuated tube collectors tend to fail suddenly and completely in 10 years or less, usually as a result of thermal stresses that shatter the glass tubes or crack copper pipes inside the glass tubes.
A Texas A&M University engineering professor who monitored the performance of a commercially manufactured solar water heating system that was installed on his home for 22 years found that the glazed flat plate collectors lost 63% of their performance, or about 4.63% per year. He attributed most of the performance loss to reduced solar energy transmission of the glazing as a result of fogging on the underside of the glass.
While a Sunplate’s performance will be comparable to a glazed flat plate collector for most of the year in a warm climate, consider a glazed flat plate collector delivering a hypothetical average annual efficiency when new of 60% and a Sunplate delivering a hypothetical average annual efficiency of 50%. If the glazed flat plate collector loses an average of 4.63% of its heat output each year and the Sunplate suffers no annual loss in heat output, the Sunplate’s lifetime heat output will surpass the glazed flat plate collector’s lifetime heat output between the ninth and tenth years of service.
Under the thermal efficiency scenario above, the Sunplate will produce 25% more energy than the glazed flat plate collector over 20 years. And even if the Sunplate loses 0.5% of its performance each year, it will still surpass the glazed flat plate collector’s total energy production between the tenth and eleventh years of service… and produce 20% more energy over a 20 year period. It is also reasonable to assume the Sunplate will remain in service longer than the glazed flat plate collector.
This type of comparison is even more dramatic for the glass tubes in Chinese evacuated tube collectors, which typically fail and require replacement within 10 years or less.
In a Sunplate® solar collector, solar energy absorbed by the opaque cover is converted to heat, which is then transferred from the hot interior surface of the opaque cover by infrared emittance to an absorber containing the working fluid, inside the solar collector. Infrared heat transfer to the absorber can be maximized by the interior surface coating and structure of the opaque cover, and by a fully wetted absorber surface, which keeps the absorber as cool as possible.
Dark sun-facing surfaces can get very hot.
A Sunplate® uses well understood principles of infrared heat transfer. It is common knowledge among building science engineers that a dark colored roof will reach a maximum temperature about 50°C (90°F) hotter than the ambient air temperature on a clear sunny day.1 This temperature difference is an equilibrium point, where the heat absorbed by the roof is balanced by heat transfer into the roof structure and the air space below, infrared reradiation to the sky, and convective heat loss at the roof surface.
Dark colored building materials (roof shingles, paints, rubber roof coatings, etc.) have infrared emissivities to the sky of about 0.90. But the exterior surface of a Sunplate® opaque cover is specially coated to lower infrared emissivity to as little as 0.04, so much less of the heat gain is reradiated back to the sky. While lower cover plate emissivity will not necessarily result in cover plate temperatures higher than, say, a black roof surface, the lower emissivity does permit more heat to be transferred to the solar collector interior during the heat balancing process described above.
In field tests of a Sunplate® prototype, we have observed afternoon cover plate temperature increases up to 64°C (115°F) above local ambient air temperature. This level of performance most likely results from mounting the solar collector flush on a dark roof, which reduces convective heat loss because the collector is surrounded by a boundary layer of air that is typically about 4 to 5°C (7 to 9°F) warmer than the local ambient air temperature.2
From a medium temperature water heating perspective, a 50°C (90°F) temperature increase above ambient air temperature is very useful. Keeping in mind the idea that we can deliver energy to heat hot water whenever the Sunplate® cover plate temperature is hotter than the collector hot water storage temperature, groundwater temperature remains at or above about 3°C (37°F) even in very cold climates at 55 to 60° north latitude. At an average midday air temperature of -12°C (10°F), a Sunplate® can deliver useful heat at working fluid temperatures up to about 38°C (100°F). At 32°C (90°F), a Sunplate® can deliver useful heat at working fluid temperatures up to about 82°C (180°F).
Inherent stagnation temperature control
The temperature ranges discussed above easily encompass the normal operating temperatures needed for potable water heating and building space heating. The ingenuity of the Sunplate® is that it achieves acceptable thermal performance in cold weather by isolating the absorber from ambient air temperature, but provides inherent stagnation temperature control during the summer without the need for temperature-sensing materials or moving parts, by simply balancing to a maximum temperature that can never be more than about 64°C (115°F) above the ambient air temperature.
How we achieved cost savings
Replacing conduction with infrared radiation, as a primary mechanism for transferring heat into the working fluid, allows you to do three things:
- eliminate expensive copper, including copper absorber plate fins, because you no longer need to maximize thermal conductivity of the absorber.
- reduce the amount of insulation necessary to keep heat from escaping because hot, heat wasting absorber fins are eliminated, or even eliminate insulation in favor of highly reflective surfaces inside the collector.
- eliminate glass, which is heavy and incurs a variety of indirect costs because it is difficult to handle and maintain, expensive to ship and prone to breakage.
Copper and special solar glass are the two most expensive materials in a medium temperature solar collector. We got rid of both.
We also developed a radically different frame design that permits all four sides to be formed from a single piece of extruded aluminum with beautiful rounded corners, eliminating the assembly and fastener costs of four corner joints. Our hollow continuous side wall frame design can also be used as the basis for both conventional and evacuated glazed flat plate solar collectors.
Sunplate has field-tested prototypes of a standard all-aluminum design to prove the concept and obtain preliminary test data. Sunplate plans to submit models of its solar collectors to the Florida Solar Energy Center (SRCC) and Solar Keymark (Europe) for independent laboratory testing and certification.
Opaque Cover Technology can be used to manufacture proprietary solar collector designs across a full range of thermal performance and price point targets. For example:
- standard design constructed entirely of aluminum / no mass insulation
- cold climate design with mass insulation in the sides and back
- design featuring low-profile integral collector storage
- evacuated glazed flat plate collector design using our continuous hollow side wall, that eliminates the problematic glass-to-metal seal issue
- conventional glazed flat plate solar collector designs using our continuous hollow side wall, offering a more attractive product that is more durable and less prone to degradation of insulation and selective coatings
- Note that the 50°C (90°F) temperature increase is a difference and not an absolute temperature. An absolute temperature of 50°C would be 122°F.
- The flush mounted prototype was mounted parallel to the roof slope with about 25 mm (one inch) of air space separating the roof and the solar collector. We measured air temperature near the collector with instrumentation located about 0.3 meters (one foot) above the roof surface and about one meter (three feet) from the center of the lower edge of the collector.