With all of the attention that the green-movement is getting nowadays, there is a new injection of inspiration and development into the solar industry. In the past ten years, solar-technology has expanded from a fringe technology that only the rich and the super-environmental could afford, to a mainstream consumer market that is giving conventional sources of power legitimate competition. In the past five years alone, several new technologies have emerged that have potential to “release the flood gates” so to speak of a wave of new scientific, technological, and economic breakthroughs.
In order to help people keep track and understand the subtle differences between each technology, I explain a little bit about each technology, and discuss the potential for efficiency, the production costs, and other facets of the way they work.
Black-body Thermal (Hot Water)
Probably the oldest consumer product available involving the use of solar power, the emergence of this technology before the invention of modern insulation and heat-storage materials left many solar early-adopters with bitter, cold, and unfavorable feelings. Often times these water heaters consisted of a black-painted hose or a darkly-painted membrane stretched out across a flat surface such as the roof. The idea behind the construction is that darkly-painted objects heat up quickly when placed out in direct sunlight for extended periods of time. The scientific concept that this technology was founded on is sound, however materials are only recently catching up to the requirements necessary to prevent the heat from escaping and being lost from the system.
It was this heat loss that caused initial problems for customers trying to make use of these hot-water systems. As a result, a stereotype began to appear depicting solar-power as unreliable and problematic. This stereotype and misconception is what spawned the rationale behind the absurd question, “what happens if the Sun doesn’t come up one day?” So many questions and concerns arose about the reliability of the technology that people began to question how reliable the Sun itself was! It has taken nearly thirty years for the consumer basis to shed this misconception and finally begin to open up to the idea of using solar-power as a primary source of energy.
Concentrated Thermal (Electricity)
A modified version of the black-body thermal technology is the use of mirrors to concentrate the sunlight into a single point. This change begets some interesting benefits, such as increased efficiency in terms of both material costs and power output. Sunlight is concentrated into a single point, superheating it to several hundred degrees Celsius. From this point, a thermal-exchange transfers heat from the focal-point to either liquid oil or molten salt (literally table-salt super-heated to a lava-like state). This super-heated liquid then flows to another heat-exchange, typically with water in order to make steam which in turn, powers a steam turbine. With the exception of the source of the heat, the concept is very similar to that of a coal, natural-gas, or nuclear-powered power-plant. Some heat-source super-heats water to create steam, which then powers a steam-turbine to turn a generator, to create electricity.
The only true difference is that in the case of the solar-concentrator tower, fuel is not required to be shipped in or supplied from another location, rather the fuel needed to provide the heat is available on-site and merely needs concentration. One of the reasons that so much sunlight is concentrated into a single point is in order to achieve an extremely high temperature for the focal-point. The use of heat transferring from a hot source to a cold sink for the purpose of generating power output is called a “heat-engine.” One of the facts about heat engines that Solar-Engineers are aware of is that heat-engines become more efficient with a greater temperature-difference. That means that the hotter the heat-source, the more efficient the overall plant.
The efficiencies of each concentration-type plant vary somewhat, however they are the highest of all of the current commercially available options, ranging in the 50% to 60% area. Unfortunately, large, high-efficiency facilities such as this are also expensive to build and have lots of moving parts thus requiring a notable amount of maintenance. In addition, the requirement for direct sunlight makes placement of these facilities a critical factor in their availability. The reason that they are placed in locations like the Arizona desert is because there is little to no cloud-cover year-round. Even small amounts of clouds can cause a massive loss in power and efficiency for the plant, which can cause the cost to keep such a plant running to skyrocket. The combination of these downsides is a major barrier that will prevent this particular form of solar-power from becoming mainstream, with the exception of certain desert regions that may rely more on centralized-generation, such as permanent industrial complexes.
Wind Generators
Wait…what? Wind is actually solar?
Yes.
All wind-generators are indirectly powered by the Sun, because all winds on Earth are indirectly caused by the Sun. The Sun expends about 1300 Watts of power on every square meter of the Earth’s surface that’s facing it at any given time. Since the Earth is about 6300km across, the math works out to about 174,000,000,000,000kW of power striking the earth at any given time. That’s about ten-thousand times the amount of power that the entire human race consumes. That’s also enough power to heat up the oceans and the atmosphere by several degrees Celsius each day. This constant beating down of radiant energy causes the air under the Sun to heat up and expand outward, creating a large “bubble” of higher-pressure air that follows the Sun as the Earth rotates beneath it. Since the land masses on the Earth’s surface absorb and reflect different amounts of light than the oceans, “bubbles” of different densities form across the surface, creating pockets of higher and lower air pressures.
Air naturally behaves as a fluid (because it is a fluid), and will always attempt to smooth itself out to an even density if given the chance. This same effect can be seen when water breaks free from a levee, the surrounding areas eventually even out so that the same amount of water is distributed evenly throughout its new environment (someone’s living room). This effect of re-distribution of fluid is what makes the wind blow. It is literally air moving from a pocket of higher density to a pocket of lower density. As the air moves past the blades on the wind-generator, it passes by each side of the blade at different speeds. This difference in speeds is intentional by design, which is meant to take advantage of the Bernoulli Effect, which exhorts a force on an object that has air traveling past it at different speeds on each side. With the winds generally blowing in a single direction, placing three blades spaced around a central shaft allows all three blades to be forced in the same turning direction. This turning force exhorts a torque on the central shaft of the wind-generator.
This torque is what creates the power from the wind-generator. The central shaft is connected to a gearbox and then to an electrical generator, similar to those in hydro-electric dams. These enormous generators induce a voltage in their output wires when some external force is applied to their central shaft. The methods for electricity conversion may differ, but for all wind-generators, the energy must first be converted to DC before being re-converted back to AC current and sent onto the grid.
Wind speeds can vary, so the blades of the wind-generator will change how fast they rotate over the course of time. This is problematic for applications that need a constant frequency from the output of their generator. As a result, some generators simply install a DC-generator connected to an inverter, which is the device that converts the DC electricity into AC electricity. Unfortunately, DC-generators are far more complicated and therefore expensive than AC-generators, so this solution presents its own set of problems as well. Other generator companies have opted instead to use a variable-AC generator connected to the blades and central shaft, and place a DC-rectifier into the circuit path. This setup creates variable-frequency AC energy from the torque generated by the spinning blades. This AC energy is converted into DC energy by the rectifier, which is then re-converted back to constant-frequency AC energy by the inverter. The added complexity of the additional step in many cases alleviates the excess costs of the DC-generator.
Both types of wind-generators come with their own pros and cons; however they both compare about evenly in terms of cost, reliability, and performance. Both types typically range anywhere from 60% to 70% efficiency, however according to some estimates, only 4% of the energy absorbed from sunlight is effectively converted into usable wind that will run a turbine. Still, the important thing to remember is that despite their notable separation from the solar industry in the economic world, windmills are really just another form of solar-power technology.
Photovoltaic (PV) Silicon Cells
Perhaps the most discernable and strikingly visible of the various solar technologies, Photovoltaic modules are currently enduring a surge into the main stream of the solar industry. With the cost of silicon wafers coming way down in the past few years, a flood of new growth and demand has poured into the industry, spawning new development and new ideas. In fact, silicon wafer cells are starting to become a back-seat contestant compared to thin-film or concentrator PV technologies.
Silicon PV-cells are made of less-refined crystalline silicon than that used in modern micro-chips, however the base material is pretty much the same. Each cell basically consists of a negatively-doped layer of silicon that faces toward the Sun and a positively-doped layer of silicon that faces toward the ground. This connection between the positive and negative sides of the material is called a “P-N Junction.” As the sunlight strikes the exposed surface, electrons are freed from their parent atoms and pushed toward the positive-side of the silicon cell. This effect continues, building a voltage across the cell’s output terminals, adding together in series to form an entire module. Modules are then connected into strings in series, which are then fed into an inverter. The inverter converts the DC voltage generated by the PV cells into AC voltage that is transmitted onto the utility grid.
Most mono-crystalline PV cells range from 15% to 20% efficiency, while poly-crystalline cells tend to range around 10% to 15% efficiency. Poly-crystalline cells are cheaper to make, since their crystalline structure is fractured. They lose a little bit of efficiency due to this fracturing effect, but since they save so much cost, they’re about on-par with mono-crystalline cells in terms of popularity. As of 2011, PV technology is the cheapest and most abundantly available option for consumers interested in harnessing solar power, costing roughly $2.50 per Watt installed (commercial rates). Since the “main stream” portion of the solar-industry is fixated around PV technologies and its various spinoffs, much of the content of this website focuses on these technologies.
Thin-film PV
In the 90’s and early 2000’s there was a major demand for high-quality silicon to be used for micro-electronics and computer chips. To this day, silicon prices are still coming down from the incredible boom that was the “heyday” of the semiconductor industry. An unfortunate side-effect of this demand was the skyrocketing costs of solar-modules, which relied on refined silicon, processed much the same way as a microchip in order to generate their power. Despite the technology and the science being available and ready, other economic markets made acquiring the materials for making solar-modules impossible.
Each solar module is made of large solar cells, which are specially-doped wafers of silicon. A typical module can be made up of ten to thirty solar wafers, depending on the size and construction. However, each wafer could also be used to create thousands of microchips, which can all be sold at hundreds of dollars each. Silicon wafer customers making microchips could literally make around $100,000 revenue from each wafer almost immediately, while solar-manufacturers had customers that would take 20 years to pay off their purchase…the competition was just completely unfair.
Out of this need rose new technology, specifically, a photovoltaic material that did not require large amount of silicon that had to compete with other industries. This material is like a film…more in fact, like a thick ink. This ink can be spread in a thin layer like paint across a pane of glass, have an electrical wire strapped to each side and “presto”… you have a solar module. The material itself is a concoction of elements that are baked together, specifically Copper Indium Gallium Selenide, or CIGS for short. This amazing technology was supposed to change the face of the solar industry as a whole, and it indeed has, but not in the way that it was originally intended to.
From the first step forward, thin-film modules are designed to be “cheap.” From their materials, to their construction, to their performance…”cheap” very accurately describes thin-film technology. From an efficiency standpoint, there is only one technology that is less efficient than thin-films, and that is amorphous-silicon, which is a gelatinous and thus flexible form of PV material. Because it focuses on being able to change shape and conform to other surfaces, amorphous silicon is at best only 5% efficient. That means that 95% of the sunlight striking it will always be lost as heat or reflected elsewhere. Thin-films have only hit efficiency levels of about 5% to 8% as of 2011. Companies claiming to achieve a higher output are merely using marketing tricks to “fluff up” their numbers.
Efficiency concerns aside, a nagging concern of mine regarding thin-film modules is that their frameless construction is very problematic. Over the course of six months dealing with several small arrays that had thin film modules from various companies, I got a really hands-on experience with them, and how incredibly delicate they are. I am not lying when I say this, after three days of sitting in the box they had arrived in, half of the modules collapsed and shattered under their own weight. When a co-worker and I attempted to salvage the remaining modules, we lost another eight in the process of moving them from the box to a cart lined with foam. In transit on the cart, the bump entering and exiting the elevator (to take the remainder downstairs to storage) broke another two modules.
Keep in mind, my co-worker and I are both trained and experienced solar-installers. I worked for three years installing modules like this and other types as a field crewman for an installation company (and I had another 5 years as a PV-engineer on top of that). My co-worker was a PV-technician for two years himself before he met me. Despite all of our combined training and experience, and even dealing with thin films before, these modules crumbled to shards in our hands.
I’m perfectly willing to admit that my anecdote could have been caused by a “bad batch” from the thin-film manufacturer; however I’ve had other incidents like this one. Over the years, I’ve cracked thin-film modules while attaching them to solar frames, I’ve had them crack days later because of thermal shock, and I’ve even seen them shattered by the wind. But shoddy-construction issues with thin-films aside, there’s still another reason why they’re really not all they’re cracked up to be: they’re too expensive.
What? But wasn’t the whole point of thin films to be cheaper than the silicon modules?
Yes, but the math doesn’t quite jive. You see, it comes back to that efficiency value again. Thin-film modules generally cost about half as much as conventional silicon-modules, but they only produce a little over a third as much energy. That means for a solar customer looking to “save money” on modules by switching to thin-film technology, they’ll actually end up having to purchase three times the amount of modules, at half the price. The math works out like this:
Total money = cost of normal modules * reduced discount from thin-films / reduced efficiency of thin-films
(1x) * 0.5 / 0.35 = (1.428x)
In other words, a customer thinking they’d be saving money by switching to thin-films would actually end up spending just under 43% more money than they would by sticking to conventional PV modules. This doesn’t even factor in the additional framing material and wire lengths needed to compensate for this larger, less-efficient array.
While thin-film technology was a great stop-gap technology that helped the solar industry survive throughout the silicon-famine, now that the semiconductor industry has cooled in terms of demand and the prices of silicon have plummeted, it either needs to evolve into a more-efficient alternative, or go the way of the telegraph…
Multi-junction PV
The increased demand for silicon-wafer PV cells spurred development of several new technologies, one of which was the further integration between the solar industry and semiconductor markets. Out of desperation for new means of using silicon wafers in a more cost-efficient manner, it was discovered that more than one PN junction can be sandwiched together in a single PV-cell. Unfortunately, it’s not as simple as taking the same kind of solar cell and merely repeating the exact same layers. Each layer of material must be made of slightly different elements, which creates the effect of each junction between each layer being able to absorb a different frequency of light.
While these elements in themselves may not necessarily be expensive or hard to acquire, the process by which they are deposited onto the PV cell is a very delicate and extremely complicated process, meaning it’s expensive as hell. Thus far, the only applications that multi-junction PV cells have seen are in high-end deep-space satellites (where money is essentially not a concern), and research/experimental testing laboratories, searching for the next major PV-material breakthrough.
If the costs of multi-junction cells can ever drop (and there’s no reason it can’t) to those comparable with modern silicon PV cells, then this technology has the ability to take the new lead in terms of main-stream solar technology. Multi-junction PV cells are currently reaching 40% to 50% efficiencies, and new combinations of light-absorbing materials are still being discovered. One new and interesting spinoff of this technology is the rise of Concentrated-PV, or CPV modules, which employ higher-quality PV-cells, often made from multi-junction cell material.
Concentrator PV
An interesting blend of two very different technologies, concentrated PV employs the use of PV-cells, and even arrange their cells into large rectangular planes that resemble the appearance of conventional PV modules, but focus the light into a smaller, more concentrated point to take advantage of the greater efficiencies brought from concentrated light. Concentrator PV comes in two distinct flavors, notably module-CPV which uses many tiny PV-cells each using their own parabolic reflectors which are arranged into a grid and mounted onto a flat surface is the first type. When choosing this type, a customer purchases several dozen CPV modules and installs them onto one or more tracking frames. Typically several tracking frames are needed, each only able to produce around 3 or 4 kW a on their own. Efficiencies typically range to about 25% to 45%, but the additional cost of the moving parts and large unwieldy moving frame negate any financial benefits gained from efficiency alone.
The second flavor of CPV is the single-cell type. These operate on the same principal as the other CPV flavor, but instead use a single enormous parabolic mirror to concentrate about 10 square-meters of sunlight into a single point about 100 times the intensity of the sun. At this single focal point, one lone PV-cell is placed, typically averaging about 3cm (~ 1 inch) on each side. This single cell produces anywhere from 5kW to 15kW from the concentrated sunlight, and can achieve efficiencies of up to 65% on a bright, sunny day. This is by far the most efficient type of commercially available PV-technology, however the cost of the moving delicate parabolic mirror, combined with the maintenance and cleaning costs make this type of solar difficult to deploy on a mass-market scale. In addition, improper care or misuse of the parabolic reflector could cause problems. Children (and immature adults like myself) sometimes play around with magnifying lenses or large mirrors to focus sunlight and burn…stuff. However each CPV single-cell concentrator can focus enough sunlight to burn through a cinder-block in about 10 minutes. Imagine the childlike mayhem potential if these reflectors became widely available…
Imagine what you could burn through with this thing!
Building-integrated PV (BIPV)
No matter what the group affiliation, whether it is fashion, political preference, or social-status…you name it…every group has its posers. These are the people in the group far more concerned with “looking like they fit in” than actually fitting in. Much like the thin veneer on cheap wooden furniture, building-integrated photovoltaic technology is designed from the ground up to focus on appearance only.
As an engineer and an artist, I should be grateful that a portion of the solar-industry has decided to focus on aesthetics of their work. In practice, the emergence and sudden demand for BIPV in modern buildings has shown me that less-noble motives are really the underlying cause here. The artist portion of who I am always necessitates that form and aesthetics are a part of everything I make, but as an engineer and a professional, I never let aesthetics take the driver’s seat to designing a power-plant. To do so would allow elements of safety, function, and necessity of design to be compromised or removed because they simply “don’t look pretty.” Such a line of reasoning is completely absurd and dangerous if employed by the wrong hands. Think about it…would you be alright with a nuclear power plant being designed for optimal feng-shui? To even consider that someone in such a position might employ such a line of thinking causes me gripping fear. Yet, for some reason, as a consumer basis, we not only have allowed such a situation to spawn in the solar-industry, but we are actually demanding that more development be headed in this direction.
One would think that the reduced efficiency of the BIPV arrays would be reason enough to keep most consumers at bay, but the focus on aesthetics also demands different requirements for materials, in addition to more ornate decorations and embellishments. As a result, BIPV modules can easily reach up to five-times the cost of conventional modules while still achieving only a tiny fraction of the efficiency.
Images of Spanish Tiles that are made from solar-panels, courtesy of AlternativeConsumer.com
I once spoke to a BIPV salesman at length about his product and clientele. This was the first and to-date only person who has ever used the term “luxury solar.” From my experience, the only customers I’ve ever dealt with are people who are pushed so close to the brink by their power company that solar was their only financial choice. Most people I know chose solar either because fossil-fuel alternatives were too expensive or not available. The concept that someone would have enough money lying around to splurge on “luxurious solar power” is in itself an oxymoron. People with money to burn on luxury typically don’t have to worry about menial things such as conserving power or saving a few hundred dollars a year.
My best advice is to just steer clear of BIPV technology and businesses. They are literally all marketing and no product, you will be wasting your money on this technology, and I guarantee you’ll regret it.
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