Nanosolar

Tubular PV?

One of the great things about solar cells manufactured on a flexible substrate, such as Nanosolar’s, is their versatility: they can be assembled into all sorts of forms and shapes of solar panel products, and — by only changing the back-end assembly — panels of different form factors and packages can be introduced rapidly and responsively to meet dynamic market requirements.

This makes it possible to deliver solar panel products of all sorts of widths and length, from tile to multi-sqm size; panels that use a glass/glass package for the cells (fire safe, etc.); panels using a glass/foil package (lighter weight); panels that are rollable altogether by virtue of packaging flexible solar cells with flexible foil encapsulants; etc.

With Nanosolar’s flexible solar cell technology, we can also create panel products of more exotic shapes and forms. For instance, we can create tubular solar panels — by rolling our flexible cell strings into cylinder shape and affixing them inside a protective glass tube.

 Let’s explore what sort of defensible utility there might be behind this product concept, beyond it just being novel (or “cute” per one recent industry analyst).

First, can we commercially produce these solar tubes at Nanosolar, on a technically and legally sound basis? Yes, we can it turns out:

- From a technical perspective, we have determined it’d take us approximately 3.5 months to have our factory’s end-assembly robotics adapted to put a tubular product into production with our standard cells (faster if we have Chinese OEMs assemble the panel with cells provided by us).

- From an intellectual property perspective, a solar tube has long been in the public domain, and the few patents recently filed focus on depositing an absorber material circumferentially around a core material to form a tubular cell – which we believe is an overly complicated approach.

A simpler approach — too simple to patent — for achieving the same end result is to roll a flexible solar cell and laminate into a glass tube. In fact, here we have built it — Nanosolar solar tubes:

Note that these Nanosolar tubes are very low cost. They require one glass tube per solar tube as opposed to two concentrically stacked ones, as in a competitive approach. With tubular glass being rather expensive and dominating the materials cost, using only one glass tube is a big advantage. This approach also avoids the expense and complexity associated with circular thin-film deposition.

Furthermore, laminating a foil to the inside of one glass tube greatly simplifies the sealing of the package. Hermetically sealing a glass tube-in-a-glass tube is a complex task. By using a foil, a much simpler approach is possible.

Most importantly, a standard Nanosolar solar cell, printed in high volume at low cost, can be used for this, and thus we can leverage the breakthrough research and development on printing process nanotechnology and associated manufacturing scalability which make thin-film solar significantly less expensive than conventional high-vacuum technology. The result is a cost reduction of approximately 70% over a tube-in-a-tube product created with circumferential high-vacuum thin-film deposition.

Second, is there a viable business opportunity for this approach?

A tubular panel will always only have about half of its solar cell material in line of sight with the sun. That’s a somewhat obvious but still important first point — as sunlight reaching a solar cell is the primary goal. Cost-efficient solar requires maximizing the area of sun collection relative to the area of solar cell material used. So tubes are 50% less efficient to start.

This is where the intriguing idea comes in to develop a solar panel consisting of an array of solar tubes, spaced regularly apart from each other, and then installed on white rooftops only, relying on reflection from the white roof to illuminate the bottom half of the tube.

Even with ideal bottom-side illumination from reflection, basic geometry tells us that with tubes spaced apart by their diameter, pi/2 more solar material is needed, or 60% more, for the same area of sunlight collection. This raises the cost.

In addition, a significant technical issue with the tubular geometry is internal current back flow in the solar cell. This is a measurable device physics effect that results when the bottom side of the tube is not illuminated as well as the front side. Because of this effect, the tubular cell will not generate any power under low light conditions. As a result, the tubular panel loses a good extent of the low-light advantage that thin-film flat-plate panels generally have and will deliver relatively lower kWh performance over time.

In terms of the kWh energy performance we can expect from each kW of power installed, another issue with tubular panels is that their optimal spacing depends on the site’s exact latitude of installation. The further North one deploys, the more the tubes may shade each other for several hours every day. When installers of (high-cost) silicon panels deploy systems, they can select the tilt angle and spacing on the specific site for their silicon panels. Achieving the same optimized site design would require customized per-site production with a tubular panel. This reduces the advantages associated with low-cost material and design.

Last, relying on reflection from the roof makes the roof an integral part of the PV system. This introduces multiple consequences for how this approach can be served and scaled, including:

- The most common white commercial roofing membrane — TPO — is not particularly designed for strong reflectance properties. For instance, standard (Mule-Hide CRRC) TPO starts out with a reflectance loss above 20% when pristine and completely dirt free (measured according to ASTM C1549) and deteriorates by a further 10% in less than five years as a material under pristine conditions. Add significant additional loss under real-world conditions. For instance, whenever there is a fire in the neighborhood or in the state, black coal particles can settle on the panels and the roof. But whereas these can be washed or wiped off the glass of the panels, the white roofing membrane won’t be quite as white going forward. It’s like wearing a white shirt — requires extra care. So as soon as there is some degree of dirt, the impact on the white surface is disproportionately high. And, this does not mean that the light not lost during reflection actually reaches a tube.

- When a bank is asked to finance an installation – and this has become the primary financing mechanism for solar deployments – will banks need to assess the combination of a one-off roof and the tubular solar panels, and become experts in how optical properties change over the 25 years of a roof’s life? What happens when a white roofing membrane loses some of its reflectance? That’s performance risk that commercial banks may not generally want to put up with. (They may still end up financing solar installations with tubes but only with a kWh performance guarantee, which in turns requires the manufacturer to make substantial cash reserves.)

- And, how many of white rooftops are there on buildings? Here’s the data: Only 2.9%, or approximately 1 out of every 30, of the flat rooftops in the United States are white. While “cool roof” initiatives may gradually grow this percentage over the next 30 years, it is also the case that once solar electricity panels are deployed more widely, building owners may as well opt for the lowest cost roofing option, which generally will not be a white roof.

Given that commercial installations are only one out of the three key market segments in solar (along with residential and utility), this makes the addressable market for a tubular panel be a percent of that of a flat-plate panel.

(This does not include a further discount factor that would need to come from the fact that tubular panels do not perform very well in any region near the equator, where sunlight comes down perpendicularly and thus would turn a 12% efficient tubular cell into a 5% panel. Regionally, this includes some attractive, sun-rich strategic growth markets.)

That said, if within this niche market a strong product advantage exists, perhaps there’s an opportunity for a profitable niche product as part of a portfolio of products, so let’s keep considering this. What could the advantage possibly be?

- Advantage on efficiency or kWh performance? The opposite is actually true. Fundamentally, on an apples-to-apples comparison, a flat-plate panel will always catch more photons than an array of tubes with spacing in between them. That’s simply because the flat-plate panel covers more area directly. So on a Watts-per-rooftop basis, the flat panel will always win over a tubular one on an apples-to-apples comparison. Then there’s additional loss during reflectance, internal shading among the tubes, low-light back currents, etc.

- Isn’t the tube effectively self-tracking the sun and so more efficient? It is true that a properly oriented tube (if one wants to go through the additional process of ensuring this during installation) will always have points on its surface where the light enters perpendicularly. Compared with a silicon solar cell, which without anti-reflective coating is known to have some degree of undesired low-angle reflectance, this can result in relatively fewer photons being reflected and thus a slightly higher conversion count. But that’s why anti-reflective coatings are commonly used and proven.

Compared with a flat thin film cell, which collect low-angles-of-incidence light very efficiently, this potential advantage goes away. So unlike with a real (moving) tracker, whose economics is rooted in dynamically maximizing area of sun collection over area of solar cell material required, there’s no benefit of having a tube. In fact, it’s a disadvantage. Effectively, in a tube, while there’s a few better positioned points on the surface of it, there are many worse ones. As a result, the tube will generally always perform worse than a flat-plate panel.

- Perhaps a case can be made via superb ease of installation in this small market? Considered as an isolated unit, it is in fact the case that an installed tubular panel needs to withstand only about half the uplift force from wind. That’s no surprise because its sun-facing area is only half as large. At the same time, however, this comes at the expense of a five to six times higher drag force (the sideways push) from wind. Tubes are unfortunately rather suboptimal from a wind drag force perspective.

Would one rather have twice the uplift or five times the drag force? It turns out that a proper design can largely avoid uplift as an issue in the first place (e.g. by not allowing wind to get below the panels). As a result, drag tends to be generally a bigger mounting issue for solar on flat rooftops than uplift.

As for the practical impact of these differences, we need an assessment that compares the best practices of deployment techniques for each with each other.

For instance, a strawman comparison between a flat-plate and a tubular panel might argue that the flat-plate panel is installed tilted (e.g. 30 degrees) on a flat rooftop and thus allows wind to attack it from the back and create a lot of uplift force, whereas a tubular panel has less of that wind uplift. The problem with this story is that the best-practice installation for low-cost flat-plate panels is to install them quasi flat (with less than 5 degrees tilt) and tightly tiled (to maximize Watts per rooftop). In fact, the best practice for installing standard thin-film panels on flat commercial rooftops is drop-on lay-up of low-tilt panels in a non-penetrating and non-ballasted way. (For instance, Nanosolar and Sunlink have developed one such panel interconnect & mounting system as part of the U.S. Department of Energy’s Solar America Initiative.) This best practice of flat-plate panel installation is no different than what one can do with a tubular panel (with the lower power density of tubular panels remaining).

Is there any weight advantage? Low weight can simplify installation on highly engineered commercial buildings. A tube-in-a-tube panel is relatively heavy it turns out: it weighs approximately 3.2 pounds per square foot. This is 50% more than a glass/foil flat-plate thin-film panel, which weighs approximately 1.6 lbs/sqft and slightly more than a glass-glass flat-plate thin-film panel, which weighs 3.1 lbs/sqft.

- Any chance tubes have a lower operating temperature relative to flat-plate panels with similar materials? Clearly, if the operating temperature in the sun were lower (the so-called “NOCT”), this would be a benefit as lower operating temperature provides higher energy output and improved reliability. Here’s the reality and here’s how one might construct a misleading story on this: The reality is that a tube illuminated from all sides will actually run relatively hotter than a flat plate illuminated by the sun on its sun-facing side. This is because at the back of the solar cell — in the case of the tube — is stagnant air locked in to the interior of the tube while at the back of a solar cell in a flat-plate panel there (ie. the underside of the panel) is free air flow in any best-practice installation. So if the tube is properly illuminated from all sides, as intended through light reflection from the rooftop, it concentrates more heat in a smaller area and thus will run hotter. Measurements we have performed have confirmed this. Of course, if the rooftop does not really reflect the full amount of light, then that lower intensity of irradiation will greatly lower the operating temperature. (But that’s simply a fact with any black surface and not a particular advantage: the less light shines on it, the cooler it is.) Basically one would have hot cells at the sun-facing side of the tube while cooler cells at the bottom where there’s less light coming in. If one now did a calculation that averaged the cool and hot areas of all cells on a tube, then one would technically arrive at a lower NOCT than typical. But in a sense that’s cheating a bit. Because that lower NOCT doesn’t really apply to the cells that receive the bulk of the sunlight, and cell performance is affected by the temperature of the relevant cell, not a neighboring one. So the net is that tubes are hotter primarily due to the inside of the tube not allowing for air flow.

- Is there any potential benefit from a facilities perspective? Unlike a standard flat-plate panel, a tubular panel does not protect the roofing material from sunlight and extend its lifetime. So the roofing below the panels will have to be replaced more frequently with a tubular panel. That’s a disadvantage of a tubular panel. In terms of repairing roofing, both are identical in terms of roof access, repair or replacement with best-practice mounting systems. As far as new roofing is concerned, standard flat-plate panels give the building owner a broader choice, including using the lowest-cost roofing systems.

At this point, it appears to us that while an intriguing concept — and one we’d be capable of producing at less than a third the cost possible with a two-tube high-vacuum approach — we are still searching for a particular benefit or advantage.

Or lack of disadvantages. Because based on the capability we have at Nanosolar to make a direct, apples-to-apples comparison between a tubular and a standard flat panel package (by either rolling our flexible cells or packaging them flat), we find that tubular panels are worse on most if not all metrics. They:
- Have lower power density per area
- Have no higher energy performance
- Perform worse under low light conditions
- Have the same wind uplift per sun facing product area
- Have five to six times higher wind drag
- Are 50% heavier than a weight-optimized flat thin-film panel
- Are installable in only approximately 3% commercial rooftops vs. standard panels
- Have no less installation labor time or cost
- Make a 20-25-year guarantee of the optical properties of the roof an integral part of the PV system for the first time in the industry, and are thus not bankable as a stand-alone product
- Do not extend the lifetime of the roof like a standard panel does
- Utilize semiconductor material less efficiently
- Have higher manufacturing and packaging cost than standard panels

It is possible our analysis is missing something. So we would like to “open source” our considerations on this product concept. Please email us at info@nanosolar.com if you have insights or comments to the above. We welcome any suggestions, especially from experienced down-stream industry participants.

After all, if there is something that makes business sense about the tubular product concept, we’d surely be the ideal company to manufacture it at lowest cost and bring it to the market in volume!

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