Future Of Solar Cells

What is a solar panel?

A solar electric panel, often called a PV panel is basically a set of treated silicon cells arranged in a series string that produces electric power when exposed to light. There are three common types of solar panels manufactured. Briefly, they are:

• Monocrystalline - made from a single large crystal, cut from ingots. Most efficient, but also the most expensive. Somewhat better in low light conditions (but not as good as some advertising hype would have you believe).

• Polycrystalline - basically, cast blocks of silicon which may contain many small crystals. This is probably the most common type right now. Slightly less efficient than single crystal, but once set into a frame with 35 or so other cells, the actual difference in watts per square foot is not much.

• Amorphous - "thin film", here the silicon is spread directly on large plates, usually of something like stainless steel. Cheaper to produce, but often much less efficient, which means larger panels for the same power. Unisolar is one example.

• Vaporware - this is the 4th type - the one that pops up in the news about every 3 months, proclaiming the next major breakthrough that will make plastic spray on solar cells that will cost around 5 cents a watt, or some similar claim. Well, after almost 30 years in this business, we are still waiting for one of those to actually reach production

Currently, solar energy is used in common applications including:

• Electricity Production (Photovoltaic or PV Technology)
• Water Heating (Solar Thermal or Flat Plate Technology)
  
  

  SOLAR PV
  

  PHOTOVOLTAIC TECHNOLOGY

Photovoltaic (PV) cells utilizes semiconductor technology to convert solar radiation directly into an electric current which can be used immediately or stored for future use. PV cells are often grouped in the form of “modules” to produce arrays which have the capability to produce power for orbiting satellites and other spacecraft. Recently, with the continual decline of manufacturing costs (declining 3% to 5% per year in recent years), uses of PV technology have grown to include home power generation, and grid-connected electricity generation. Installations of PV systems have also been increasing due in large part to comprehensive incentive programs which help reduce the costs of these systems and also allows users to sell excess electricity back to the public grid (feed-in).

SOLAR THERMAL EVACUATED TUBE TECHNOLOGY vs. FLAT PLATE SOLAR COLLECTORS

Flat-Plate solar collectors – durable, weatherproof boxes which contain a dark absorber plate located under a transparent cover – are still the most common type of collector used for water heating in many countries despite being inferior to evacuated tube collectors in many ways.

Evacuated heat pipe tubes are designed such that convection and heat loss are eliminated, where as Flat-plate solar panels contain an air gap between absorber and cover plate which allows heat loss to occur. Further, thermal heat pipe systems are capable of limiting the maximum working temperature, where as Flat-plate systems have no internal method of limiting heat buildup which can cause system failure. Finally, evacuated heat pipe systems are lightweight, easy to install and require minimal maintenance. Flat-plate systems, on the other hand, are difficult to install and maintain, and must be completely replaced should one part of the system stop working.

Solar thermal technology from Solar Panels Plus utilizes evacuated-tube technology (above) to covert solar radiation into usable energy for producing hot water for residential and commercial applications, space heating and even space cooling through the use of absorption chilling technology. Below is a diagram and image of a flat plate collector.

Monocrystalline Silicon Panels

15-18% efficiency

Monocrystalline panels use crystalline silicon produced in large sheets which can be cut to the size of a panel and integrated into the panel as a single large cell. Conducting metal strips are laid over the entire cell to capture electrons in an electrical current.

These panels are more expensive to produce than other crystalline panels but have higher efficiency levels and, as a result, are sometimes more cost-effective in the long run.

Polycrystalline Silicon Panels

12-14% efficiency

Polycrystalline, or multicrystalline, photovoltaics use a series of cells instead of one large cell. These panels are one of the most inexpensive forms of photovoltaics available today, though the costs of sawing and producing wafers can be high. At the same time, they have lower conversion efficiencies than monocrystalline panels.

For this technology, several techniques can be used:

Cast Polysilicon:

In this process, molten silicon is first cast in a large block which, when cooled, is in the form of crystalline silicon and can be sawn across its width to create thin wafers to be used in photovoltaic cells. These cells are then assembled in a panel. Conducting metal strips are then laid over the cells, connecting them to each other and forming a continuous electrical current throughout the panel.

String Ribbon Silicon

String ribbon photovoltaics use a variation on the polycrystalline production process, using the same molten silicon but slowly drawing a thin strip of crystalline silicon out of the molten form. These strips of photovoltaic material are then assembled in a panel with the same metal conductor strips attaching each strip to the electrical current.

This technology saves on costs over standard polycrystalline panels as it eliminates the sawing process for producing wafers. Some string ribbon technologies also have higher efficiency levels than other polycrystalline technologies.

EFG (RWE Schott):

Figure out what type this is

Amorphous Silicon or Thin Film Panels

5-6% efficiency

Thin-film panels are produced very differently from crystalline panels. Instead of molding, drawing or slicing crystalline silicon, the silicon material in these panels has no crystalline structure and can be applied as a film directly on different materials. Variations on this technology use other semiconductor materials like copper indium diselenide (CIS) and cadmium telluride (CdTe). These materials are then connected to the same metal conductor strips used in other technologies, but do not necessarily use the other components typical in photovoltaic panels as they do not require the same level of protection needed for more fragile crystalline cells.

The primary advantages of thin-film panels lie in their low manufacturing costs and versatility. Because amorphous silicon and similar semiconductors do not depend on the long, expensive process of creating silicon crystals, they can be produced much more quickly and efficiently. As they do not need the additional components used in crystalline cells, costs can be reduced further. Because they can be applied in thin layers to different materials, it is also possible to make flexible solar cells.

However, thin-film panels have several significant drawbacks. What they gain in cost savings, they lose in efficiency, resulting in the lowest efficiency of any current photovoltaic technology. Thin-film technologies also depend on silicon with high levels of impurities. This can cause a drop in efficiency within a short period of use.

Thin-film panels have the potential to grow in use, and already figure in some of the most exciting enhanced photovoltaic systems, including high-efficiency multijunction devices and building integrated photovoltaics.

Group III-V Technologies

25% efficiency

These technologies use a variety of materials with very high conversion efficiencies. These materials are categorized as Group III and Group V elements in the Periodic Table. A typical material used in this technology is gallium arsenide, which can be combined with other materials to create semiconductors that can respond to different types of solar energy.

Though these technologies are very effective, their current use is limited due to their costs. They are currently employed in space applications and continue to be researched for new applications.

Enhanced Systems

Building-Integrated Photovoltaics (BIPV)

BIPV technologies are designed to serve the dual purpose of producing electricity and acting as a construction material. There are many forms that this technology can take. One common structure is the integration of a semi-translucent layer of amorphous silicon into glass, which can then be used as window panes that let controlled amounts of light into a building while producing electricity. Another common structure is the use of shingle-sized panel of amorphous silicon as a roofing material.

Currently, BIPV technologies have very low efficiency levels due to their use of amorphous silicon, but present the advantage of replacing other construction materials and offering a wide variety of aesthetic choices for the integration of photovoltaics into buildings.

Concentrator Systems

Concentrator systems are designed to increase the efficiency of solar photovoltaics. These systems cover a standard photovoltaic panel with concentrating optics, or lenses that gather sunlight and increase its intensity in hitting the photovoltaic panel. These systems reduce the amount of photovoltaics needed to produce electricity, and also reduce the amount of space needed for a photovoltaic installation.

Their main disadvantage is that they depend solely on direct light to produce electricity, while stand-alone photovoltaic panels can use both direct and diffuse light. Many regions do not receive enough direct light throughout the year for these systems to be practical. Another disadvantage is the complexity of their construction, which makes these systems more difficult to build and install than photovoltaic panels on their own.

High-Efficiency Multijunction Devices

Multijunction devices receive their name from their use of multiple layers of cells, each layer acting as a junction where certain amounts of solar energy are absorbed. Each layer in a multijunction device is made from a different material with its own receptivity to certain types of solar energy.

In a typical device, the top photovoltaic layer responds to solar waves that travel in short wavelengths and carry the highest energy, absorbing this energy and creating an electrical charge. As other solar waves pass through this layer, they are absorbed and translated into electricity by the lower layers. Typical materials used in this device include gallium arsenide and amorphous silicon.

Though some two-junction devices have successfully been built, these devices are still largely in the research and development stage, with most research focused on three- and four-junction devices.

Types of Solar Electric Systems

Small "Stand-Alone" Systems

The small "Stand-Alone" system is an excellent system for providing electricity economically. These systems are used primarily for RV power, lighting, cabins, backup and portable power systems. The size of the photovoltaic array (number of solar panels) and battery will depend upon individual power requirements. The solar panels charge the battery during daylight hours and the battery supplies power to the inverter as needed. The inverter changes the 12 volt batteries DC power into 120 volt AC power, which is the most useful type of current for most applications. The charge controller terminates the charging when the battery reaches full charge, to keep the batteries from "gassing-out", which prolongs battery longevity.

"Grid-tie" Solar Systems

A "Grid-tie" solar system is useful for homes that are already connected to the utility grid. The advantage of this type of system is the price reduction of utility. The system has to be wired with an inverter that produces pure-sine-wave AC electricity, which is necessary for connecting to the utility grid. Another advantage to this type of system are the tax incentives and rebates available from different state and local agencies. Owning a grid-tied system in California qualifies you for the State Buydown program, drastically reducing the overall system cost. Most of these systems typically do not have the battery storage that allows for power when the utility fails. Grid-tie system can be installed with battery backup power to keep critical loads operating in the event of a power failure.

Complete "Stand-Alone" Solar System

A Complete "Stand-Alone" solar system is useful for complete independence from fossil fuels and electric utility companies. The advantage to this type of system is its ability to provide power away from the utility grid, and to create a measure of self independence. A complete stand-alone home solar system will typically have 2 inverters to supply the AC house current necessary to power large loads such as air conditioners. Having a second inverter helps to insure that power is available when one of the inverters eventually requires servicing. These self contained systems need a sizable battery storage capacity to provide electricity when solar power is unavailable due to prolonged adverse weather conditions. A complete stand-alone solar system will usually require at least 20 solar panels to keep the batteries at a safe and proper state of charge. Typically this type of system is most cost effective when the system is located away from the utility grid.

"Hybrid" - Solar Electric and Generator Combination System

The "Hybrid" - Solar Electric and Generator Combination provides a reliable power source, and produces electricity even when the sun is not providing solar power. These "hybrid" systems have the ability to charge the battery bank and provide electricity when weather conditions are unfavorable for solar power production. An advantage to this type of system is the reduction of solar panels (PV array) necessary to supply power, which makes this system an economical alternative to a larger "Stand-Alone" system. When more power is needed than the solar panels are producing, a gasoline, propane or diesel generator is activated. The generator will provide enough power to overcome the difference between solar power available and the electricity you require. This type of system is used for cabins, remote homes and is a common system used to provide power for small medical facilities in third world countries.

Battery State of Charge Voltage Table

Percent of Full Charge	12 Volt DC System	24 Volt DC System	48 Volts DC System
100%	12.7	25.4	50.8
90%	12.6	25.2	50.4
80%	12.5	25.0	50.0
70%	12.3	24.6	49.2
60%	12.2	24.4	48.8
50%	12.1	24.2	48.4
40%	12.0	24.0	48.0
30%	11.8	23.6	47.2
20%	11.7	23.4	46.8
10%	11.6	23.2	46.4
0%	<11.6	<23.2	<46.4

Article

Investors along Sand Hill Road in Menlo Park are pouring money into solar nanotech startups, hoping that thinking small will translate into big profits.

Both inventors and investors are betting that flexible sheets of tiny solar cells used to harness the sun's strength will ultimately provide a cheaper, more efficient source of energy than the current smorgasbord of alternative and fossil fuels.

Nanosys and Nanosolar in Palo Alto -- along with Konarka in Lowell, Mass. -- say their research will result in thin rolls of highly efficient light-collecting plastics spread across rooftops or built into building materials.

These rolls, the companies say, will be able to provide energy for prices as low as the electricity currently provided by utilities, which averages $1 per watt.

Other uses of nanotechnology foreseen by Konarka, Nanosolar and Nanosys include form-fitting plastic batteries for electronic devices like cell phones and laptops.

While all three companies provide prototypes for large corporate research labs and government agencies, company representatives and investors are reticent to predict when nanotechnology-powered solar systems will be commercially available. Industry watchers, however, say that achieving mass production of these products may take five years or longer.

"We take the long view, although we're not averse to having products very quickly," said Bryan Roberts, general partner at Venrock Associates in Menlo Park, a leading Nanosys investor. "Whenever you're developing a novel technology platform, you're looking at a four- to six-year time frame rather than a three- to four-year time frame."

Major investments

Despite the lack of commercial product availability, Konarka, Nanosolar and Nanosys have collectively raised more than $120 million since 2001, the year all three companies were founded.

Recent investments include $7 million in debt financing for Konarka in June, making its total funding to date $38.5 million. Nanosolar recently announced more financial support in a Series B round of funding that secured $20 million in May. With previous investments of $7.25 million, it has secured a total of $27.25 million.

Both Konarka and Nanosolar have said they plan to use the money for new research and development facilities.

Nanosys, which cited poor market conditions as the reason for withdrawing its IPO in August, has raised $55 million to date. The company's last round of funding in April 2003 secured $30 million.

Venture capitalist excitement for these new technologies reflects growth in the solar energy market as whole, say industry experts.

"The technology is maturing, and the industry is maturing. British Petroleum, Shell and the oil companies are all in this field," said David Wooley, vice president of the nonprofit Energy Foundation in San Francisco, a research group funded by major charitable trusts but not affiliated with utilities or energy producers.

Costs must be reduced

A study released by the Energy Foundation in March suggests that the United States could produce 2,900 new megawatts of solar power by 2010 -- enough to power 500,000 homes -- if the cost is significantly reduced.

Solar energy ranges between $4 and $5 per watt. The report suggests market expansion will require $2 to $2.50. If the price breakthrough occurs, says Wooley, the report's assumed price structure represents a $6.6 billion annual market opportunity.

The Energy Foundation report also says that solar energy could furnish much of the nation's electricity if available residential and commercial rooftops were fully utilized. According to the Energy Foundation, using available rooftop space could provide 710,000 megawatts across the United States, whose current electrical capacity is 950,000 megawatts.

"The market is obviously huge, demand is huge. Besides, (alternative energy) is imperative in the world we live in," said Bill Gurley, a general partner at Benchmark Capital in Menlo Park, an early investor in Nanosolar.

As for recent growth in solar energy, Paula Mints, a senior analyst at the technology research firm of Strategies Unlimited in Mountain View, says that 14,000 photovoltaic megawatts were sold last year, representing 54 percent growth in the industry.

Interest from VC investors

Mints says that VC interest in new energy technologies represents a positive development.

"It's very healthy for the industry. They (venture capitalists) see the growth and the possibilities," she said.

However, Mints also cautions against expecting immediate changes in the way energy is produced. She cites the long development history of conventional solar cells.

"It took 20 to 25 years to commercialize (conventional) photovoltaics," she said.

High production costs are among the reasons solar energy hasn't become a major source of electricity.

The black, glasslike photovoltaic cells that make up most solar panels are usually composed of crystalline silicon, which requires clean-room manufacturing facilities free of dust and airborne microbes.

Silicon is also in short supply and increasingly expensive to produce, so high manufacturing costs are the main reason behind high wattage prices.

Long payback time

As a result, the cost of panel installation typically equals four to five years of expensive energy before production costs are recovered and systems begin paying for themselves.

With nanotechnology, tiny solar cells can be printed onto flexible, very thin light-retaining materials, bypassing the cost of silicon production.

"Silicon is very capital-intensive. You don't need a clean room for plastic power where capital costs are one-tenth of silicon," said Raj Atluru, managing director at the venture capitalist firm of Draper Fisher Jurvetson in Menlo Park, a major investor in Konarka.

Konarka, Nanosys and Nanosolar say their solar technology will reduce the time it will take consumers to recover production and installation costs to a matter of months.

In addition to being able to manufacture photovoltaic cells more quickly through printing, the companies also say that manipulating materials 100,000 times smaller than the width of a human hair will provide more light- collecting capabilities.

Each printed nanostructure solar cell would act as an autonomous solar collector, and sheets of these products would have more surface area to gather light than conventional photovoltaic cells.

The companies also say that the printed rolls of solar cells would be lighter, more resilient and flexible than silicon photovoltaics.

A rooftop opportunity

If the technical hurdles can be cleared, the biggest money will be found atop buildings.

According to Matthew Nordan, vice president of research at New York's Lux Research, "The ultimate prize is rooftop distribution applications," in which residential and commercial buildings would generate most of their own power.

The companies envision mass production of flexible plastics that would conform to the shape and pitch of rooftops or would be imprinted onto building materials like tile and siding.

"Flexibility allows you to develop new form factors. Why not integrate solar cells into, say, a Spanish tile?" said Nanosys spokesman Stephen Empedocles.

It remains unclear, however, who would install nanotechnology-based solar components if they become commercially available.

"There's no channel to the market," said Nordan, who sees a fragmented solar installation market made up of numerous contractors, which makes adoption of any technology difficult.

Nordan also sees obstacles in transmitting solar energy from rooftop collection sites back to electrical grids and other buildings not wired with photovoltaics.

Distribution an obstacle

"The problem is distribution. Nanomaterials could provide a way to transmit energy as well as capture it."

Until the distribution issue is solved, Nordan says, solar energy will not be able to meet its potential of supplying vast amounts of power.

Analysts like Nordan and Mints say that while rooftops are the most attractive areas for investors, nanomaterial solar energy may first be implemented on mobile devices like cell phones and laptop computers.

Contracts from the military

These applications have smaller power requirements than buildings, and military research contracts at Konarka, Nanosys and Nanosolar may pave the way for commercial availability of solar batteries for communications devices.

"Price is no object for the military, and they need power on the go," said Nordan. "Besides, the mobile-phone industry is driven by new features."

All three companies rely upon government contracts in addition to private funding. The Defense Advanced Research Projects Agency has been the most generous. Konarka has a $6 million grant, and Nanosolar has received $10.3 million.

Nanosys' $9.4 million in grants comes from that agency, as well as the Department of Energy and the Navy, among others -- although not all of this research is solar-related.

Industry watchers like Wooley of the Energy Foundation say that some kind of government assistance is necessary to make alternative sources of energy viable.

"The (solar) industry has grown and expanded through incentives. The technology doesn't need government support forever, but it's at a crucial point," he said.

Investors, however, are quick to distinguish between grants and regulations mandating alternative forms of energy.

"The bet was not made with the regulation market," said Gurley of Benchmark Capital.

Problems with government

Atluru of Draper Fisher Jurvetson concurs. "Our view is that government can cause big problems, and it is the entrepreneurs who will make the big changes."

So which way will solar energy go? Atluru said that just as there are different ways to get electricity, the same may hold true for solar energy.

"There's opportunities in traditional silicon photovoltaics, and that's really interesting, and there are companies like Konarka. There's room in the market for all these companies. It's still early days for these startups," he said.

Wooley of the Energy Foundation cautiously agrees.

"What we see is similar to the trajectory for wind energy, where it went from a small-scale industry to a large-scale industry.

"We think the solar industry could see the same growth this decade the wind industry saw in the '90s," he said.

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Shrinking solar, expanding profit

Konarka, Nanosolar and Nanosys say that nanotechnology could make the price of electricity less expensive per watt.

Current cost of solar energy, per watt: $4-$5

Average cost of energy from traditional fossil fuel sources, per watt: $1

Estimated cost of energy from nanotech solar panels, per watt: $2

Total energy-generating capacity of the United States: 950,000 megawatts

Potential total rooftop solar energy capacity in the United States: 710, 000 megawatts

Source: Energy Foundation