Questions and Answers

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Are the major Sunplate innovations patented?

The Sunplate® opaque cover, continuous hollow side wall, and finless absorber spacing innovations are currently patent pending in the United States, Brazil, China, India, Israel, Malaysia and all 31 month priority Patent Cooperation Treaty countries.. Our founders, John Prutsman and Jeff Prutsman, filed U.S. patent application no. 14/252,765 and Patent Cooperation Treaty (PCT) international application no. PCT/US2014/34058 on 14 April, 2014 (“Solar collector comprising an opaque cover”). Both applications claim priority to U.S. provisional patent application no. 61/811,495 filed on 12 April, 2013.1

Can Sunplate’s innovations be adapted to thermosyphon systems?

Yes, absolutely. Any solar water heating system configuration that employs glazed flat plate or evacuated tube collectors can just as easily employ Sunplate® solar collectors. One of the development projects we are most excited about is an attractive low-profile integral collector storage (ICS) system that combines the collector and hot water storage into a single Sunplate® module.

Why bother introducing a new solar collector technology if the existing technologies are good enough?

One reason is substantial cost savings. Our technology delivers thermal performance comparable to a high quality glazed flat plate collector across a broad range of operating conditions—at about 40% lower manufacturing cost.

Another reason is improved durability. The two individuals who invented our technology—John Prutsman and Jeff Prutsman—have been directly involved with the manufacture, handling, installation, inspection, replacement and repair of thousands of solar collectors over the past 30 years. They know from first-hand experience that most glazed flat plate collectors simply do not deliver their design performance after a few years, for three reasons:

  • Moisture intrusion degrades selective surface coatings, reducing solar absorptance and increasing emissivity.
  • Mass insulation inside the collector—where it is exposed to stagnation temperatures—can essentially disintegrate after as little as five years, increasing the collector’s heat loss.
  • The mixture of moisture, insulation dust and other particulates often forms a grimy film on the underside of the glass cover, reducing solar energy transmission through the cover.

Evacuated tube collectors, the other major medium temperature solar thermal technology, can be notoriously unreliable. The non-glass vacuum seal at the “neck” of the tube is in direct contact with the hottest part of the working fluid tube, so thermal stress at this point is a common cause of failure. Also, when evacuated tubes use self-contained heat pipes, the heat pipes sometimes crack under stagnation temperatures. It is notable that in Germany, a very active solar market with over 20 years of experience with evacuated tube collectors, evacuated tube collectors once had a dominant market share. That has reversed. Glazed flat plate collectors had about 90% market share at the end of 2011, the last year for which data are available.2

Fourth, our innovation of a simplified and inherently stronger—but low cost—frame design that does not require sealing four square corner joints or elaborate (and expensive) glass-to-metal sealing techniques is a potentially major breakthrough for evacuated flat plate collectors.

Our innovations were not undertaken to create the most thermally efficient collector. Our goal was to deliver acceptable thermal efficiency at dramatically reduced cost, while also increasing long-term durability and reliability.

Won’t the opaque cover’s exterior surface selective coating deteriorate in a relatively short time if it is exposed to ambient air and moisture?

No. While many selective coatings are susceptible to wet corrosion, some—PVD sputtered coatings, for example—are quite rugged.3 We plan to test a variety of candidate selective coatings to determine their long-term resistance to wet corrosion and other weather effects.

In any event, our thermal performance simulations conservatively assume an exterior surface absorptance of just 0.92 for the opaque cover, to account for reflectance of a clear hardcoat protective layer over the selective coating.4 That said, we believe we will be able to obtain exterior surface absorptance of 0.95 to 0.96 in production collectors even with a clear hardcoat layer over the selective surface. Energy not transmitted by the clear hardcoat layer and absorbed as heat is still usable by the opaque cover, via conduction.

Won’t the emissivity from the interior surface to the absorber be a lot less than the emissivity from the exterior surface to the sky?

While the exterior surface of the opaque cover is coated to provide a very low emissivity, the interior surface of the opaque cover is coated to provide a very high emissivity. It is true, though, that the emissivity of the interior cover is negatively impacted by a small temperature difference between the opaque cover and the absorber. We increase the interior surface emissivity by:

  • designing the absorber so it has a fully wetted surface, to keep the absorber as cool as possible and thus increase the temperature difference between the opaque cover and the absorber;
  • increasing the flow rate through the absorber, again to cool the absorber; and
  • increasing the effective surface area of emissive surfaces inside the solar collector that are—via conduction—at the same temperature as the opaque cover and have additive view factors relative to the absorber.5

How can a Sunplate® be more efficient than an unglazed plastic solar collector when the fluid inlet temperature and the ambient air temperature are the same? Won’t the Sunplate® lose more energy in the process of transferring heat from the cover plate to the absorber?

Any difference in potential heat removal at maximum efficiency (the y-intercept) is secondary in importance to the unglazed plastic solar collector’s much higher reradiation of infrared energy. Such collectors, which get their color from carbon black in the plastic resin, have emissivities of about 0.88 to 0.90. The emissivity of a Sunplate® opaque cover can be as low as 0.04 to 0.05. So the Sunplate® loses a lot less energy to the sky via infrared reradiation.

You say that your patent pending continuous hollow side wall with rounded corners can also be used for glazed flat collectors. But why would you want to use glass if you say it has so many problems?

First, the operating and durability problems associated with glass (for example, moisture intrusion that degrades mass insulation and selective surface coatings inside a glazed collector) would be either eliminated or substantially reduced by the Sunplate® frame design, which eliminates corner joints and isolates any mass insulation from the collector interior (dust or outgassed vapor from insulation located inside the collector can migrate to the underside of the glazing).

Second, the most important application would be for an evacuated Sunplate® that used glass to achieve higher operating temperatures. Because the maximum temperature attainable by an evacuated Sunplate® is limited by convective heat loss from the opaque cover to the ambient air, it would be advantageous to have a glass evacuated Sunplate® with a conventional finned absorber for design operating temperatures above, say, 82°C (180°F), or for extended periods of operation in very cold weather.

Sunplate® solar collectors sound too good to be true. Do they have any weaknesses?

A Sunplate® experiences more heat loss during windy conditions than a glazed flat plate or evacuate tube solar collector. This happens because convection across the opaque cover’s surface reduces its temperature, which in turn reduces the amount of heat transfer to the absorber. However, the Sunplate® is less affected by windy weather than an unglazed collector, and much less affected by cold weather, because the absorber is isolated from the ambient air temperature. Also, third party research into wind uplift forces on buildings suggests that the rounded corners of our frame design reduce effective wind speed across the collector’s surface.

A Sunplate® solar collector’s thermal performance at 38 to 60°C (110 to 140°F) fluid inlet temperatures is about the same as a glazed flat plate solar collector when the average wind speed is below about 3 m/s (6.7 mph), which is most of the time during the middle of the day in most temperate climates.

Also, an opaque cover comprising an aluminum sheet has extremely low thermal capacitance, so it usually recovers from wind gusts and intermittent cloud cover in less than a minute.

Finally, it’s important to remember that solar collectors are preferably installed to face the equator (for example, facing south in the northern hemisphere). So in most cases, collectors flush-mounted on a sloped roof are shielded from cold winter winds (which are out of the north in the northern hemisphere) by the roof structure. And of course, inexpensive windbreak structures can be employed for large collector arrays. Even including the cost of such structures, a Sunplate® solar collector array will still have a much lower cost per unit of energy delivered than an array of glazed flat plate or evacuated tube solar collectors.

Your thermal performance simulation shows that both Sunplates® and glazed flat plate collectors produce more heat than evacuated tube collectors under most normal operating conditions. But evacuated tube manufacturers claim their collectors produce much higher efficiencies than glazed flat plate collectors. Who is correct?

A glazed flat plate will produce more heat per unit of gross sun-facing solar collector area at the 38 to 60°C (110 to 140°F) temperatures needed for most water heating applications, in most weather. Evacuated tube collectors will typically produce more heat when the average daytime air temperature falls below about 5°C (41°F), and when working fluid temperatures above 70°C (158°F) or so are needed, for some industrial applications and for solar air cooling systems that use absorption chiller technology.

Instantaneous efficiencies at a fluid inlet temperature of 50°C (122°F) are about 57% for glazed flat plate collectors and 41% for evacuated tube collectors, assuming 800 W/m2 solar irradiance, 25°C (77°F) ambient air temperature and 3 m/s (6.7 mph) wind speed.6

Some marketers of evacuated tube collectors publish comparisons showing that evacuated tube solar collectors are more efficient than glazed flat plates when compared on the basis of net aperture area—which for an evacuated tube collector is the sun-facing planar surface area of the evacuated glass tubes, without including the surface area of the heat transfer manifold or the empty air spaces between the glass tubes.

But many of the factors that affect an evacuated tube collectors’s cost effectiveness (for example, the size and cost of the heat transfer manifold and balance of system costs for mounting hardware and circulating lines) are related to gross collector area. Also, when the land or roof area available for a solar collector array is limited—whether by surface area available, shading, or land cost—thermal performance per unit of gross collector area is a primary concern. For these reasons, independent testing laboratories issue thermal performance ratings based upon gross collector area.

What about the superior incident angle performance of evacuated tube collectors?

It is true that evacuated tubes perform better than glazed flat plate collectors when the sun is closer to the horizon, during the early morning and late afternoon hours. The practical problem is that not much solar energy is available when the improved performance occurs. While the small increase in performance might have some meaningful value in a large collector array, improved incident angle performance is not significant for residential systems and smaller collector arrays. And in a large array, the cost of replacing failed evacuated tubes might outweigh any incremental gains from performance at increased incident angles.

Why not just use solar photovoltaic (PV) panels to provide current to an electric water heater?

This is one of the most misunderstood topics in solar energy. Solar PV panels only convert about 15% of the solar energy that strikes the PV cells into direct current (DC) electricity and have a net system efficiency—usable alternating current (AC) after processing the power through an inverter—of about 10%.7 By comparison, a glazed flat plate collector will deliver more than 50% of the solar energy that strikes the solar collector, under identical conditions, as net usable heat after allowing for circulating line and heat transfer losses. So heating water with a solar PV-driven electric water heater would require about five times more sun-facing surface area than solar PV panels, to provide the same net usable energy as a glazed flat plate (or Sunplate®) solar collector.

Also, under apples-to-apples market conditions (the same customer acquisition costs, hourly labor rates for installation, supply chain profit margins, permitting costs, etc.), the solar PV system will be significantly more expensive than the solar water heater.

What about “hybrid” PV/thermal systems that draw heat from underneath the PV panels?

These systems either draw heated air through air ducts, or circulate a heat exchange fluid through pipes underneath the PV panels. The working principle is that because PV panels only capture about 15–18% of the solar energy striking the panels, the other 82–85% can be used to heat water.8 Also, PV panels are more efficient at cooler temperatures, so an additional idea is that the solar thermal portion of the system will remove heat from the PV panels and thus keep them cooler.

Much like using unglazed plastic pool panels to heat potable water, this approach will provide useful heat but has significant practical limitations. For the thermal portion of the system to deliver useful heat, the air or heat exchange fluid from the solar array must be warmer than the hot water storage. If it is, there isn’t going to be much cooling of the PV panels. If it isn’t, there isn’t going to be any heat transfer to the hot water storage.

While hybrid PV/thermal systems are a clever idea, we doubt that the increased energy savings justify the increased system complexity and cost. The solar water heating efficiency of such systems is probably less than the efficiency of roof integrated solar absorber (RISA) systems. RISA systems typically integrate metal plates containing PEX tubing into the roof structure (under the roof shingles and sheathing) and generally only convert about 5–10% of incoming solar energy to usable heat for hot water—without first removing 15–18% of the available tolar energy to a different use. From an economic standpoint, hybrid PV/thermal systems are little more than a marketing gimmic.

Notes

  1. The U.S. utility and PCT international filings met the one year filing deadline for claiming priority to the U.S. provisional application because 12 and 13 April, 2013 were not business days for the receiving office.
  2. Mauthner, Franz and Weiss, Werner. “Solar Heat Worldwide: Markets and Contribution to the Energy Supply 2011” (2013 Edition). International Energy Agency Solar Heating & Cooling Programme. Tables 1 and 2, pp. 9–10.
  3. A specific example, Almeco Solar’s Tinoxenergy blue sputter PVD coatings, incorporate an anti-reflective, clear hardcoat layer of fused quartz that is extremely hard and scratch-resistant. The selective material is a multilayer cermet structure. A diffusion barrier is applied to an adhesive layer between the selective coating and the absorber to prevent metal atoms from entering the absorber layer at high temperatures and changing the optical properties. Tinoxenergy has a published absorptance of 0.95 and emissivity of just 0.04. “Tinoxenergy” is a trademark of Almeco GmbH.
  4. 3M® Ultra Barrier Solar Film is an exemplary transparent barrier material designed to replace glass in flexible thin film solar PV panels. The product has just 3% reflectance, and its moisture barrier performance and weatherability have been subjected to extensive field testing. 3M® is a trademark of 3M.
  5. A view factor technically refers to the percentage of infrared energy leaving an emitting surface that strikes an absorbing surface. As a practical matter, increasing the view factor can accomplished by increasing the surface area of the emitting surface; for example, by creating ridges on the emitting surface. The view factor of the cover plate interior surface is also increased by conductive transverse frames in direct contact with the opaque cover, assuming these secondary surfaces are coated for high emissivity.
  6. Evacuated tube collectors are not affected by wind. Glazed flat plate collectors do not experience any meaningful loss in performance at a 3 m/s wind speed. While instantaneous efficiencies can negatively distort the real world performance of solar collectors over an extended period of time, 50°C (122°F) is a temperature often used to show “apples-to-apples” comparisons of medium temperature performance.
  7. Assuming standard test conditions of 1,000 W/m2 solar irradiance and 25°C (77°F) ambient air temperature.
  8. In reality, the “other 82–85%” of the incoming solar energy is only 70% or so, after taking into account the reflectance and infrared emissivity of the PV panels.