photovoltaic (solar) cells

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introduction
photovoltaic cells

capturing more solar energy
in a small way, germany starts acting for substitute energy production
large-scale photo-voltaic growth in the usa

concentrators

three-stage pv cells with concentrators achieves over 40% efficiency

less efficient, cheaper photovoltaic technologies

 

 

 

how much energy from the sun do we receive on earth?

the practicality of photovoltaic generation for london
costing for a house

cunning new solar panel design - replaces roof tiles
photovoltaic progress - power generation and storage
technical notes
end notes

site map

advertising disclaimer

introduction

Photovoltaic cell technology is an area that is changing technologically at an extremely high rate. There is an increasing race between the more efficient, but expensive, silicon technology and a variety of cheaper, less efficient and often lighter-weight technologies. Every week hopeful entrepreneurs and scientists attempt to hype another miraculous ‘break-through’. This is further complicated by the rapidly growing field of nanotechnology, which is already being applied to photovoltaics.

I cannot possibly review all these wonders, let alone assess their long-term commercial viability. Therefore, as usual, my aim is to provide you with enough outline to orient you and enable you to start digging around on your own.

Photovoltaic cells

Photovoltiac cells are also known as solar cells. They are a means of harnessing the sun’s energy and converting it into electricity. Photons from the sun are absorbed by semi-conductors, with electrons being knocked along electrical wires by the photons until the current (flow of electrons) reaches a device that can be powered by electricity.

This is known as the photovoltaic effect, and was first noticed in 1839. It works in a similar way to the current flow is handled in computer logic and memory chips.

Solar power from photovoltiac cells has been used since the 1950s, at first for devices where providing other sources of power was a problem, such as with satellites, remote small dwellings and motorway phone boxes. The technology has improved sufficiently that solar cells are now a feasible option for private and business buildings, providing an alternative power source in case of power failures.

The sun is an enormous source of energy, bombarding the Earth each minute with enough energy to supply the Earth’s power needs for a year.

Crystalline silicone-based photovoltaic cells will convert 15% of the sun’s energy to electricity, while newer, cheaper materials such as amorphous silicon and gallium arsenide convert 8% of the sun’s energy, thus being half as efficient as silicon-based cells.

Because the manufacturing costs of photovoltaic cells are still relatively high, solar power is as much as five times more expensive than power derived from fossil fuels. However, photovoltaic technology is changing rapidly.and new research could lead to significant cost reductions within a few years. These advances could be both by making cheaper versions of rigid crystalline silicon cells (which comprise 80% of the solar market), and by creating less expensive flexible photovoltaic technologies that are as reliable and as efficient as crystalline silicon, using, for instance, amorphous silicon and gallium arsenide.

There are new surface coatings that promise to make energy capture far more efficient.

Crystalline silicone comprises approximately 40% the price of photo-voltaic cells, therefore even halving the price of the silicone would improve the cost down from five times to four times that of fossil fuels. You will, therefore, see that there is still a long way to go before photo-voltaic cells will move into serious consideration for major power production.

capturing more solar energy

“Researchers from Lawrence Berkeley National Laboratory, the University of California, and the Massachusetts Institute of Technology have engineered a single material that contains three bandgaps and is capable of capturing more than 50 percent of the sun's energy. The researchers made the material by forcing oxygen into a zinc-manganese-tellurium crystal. The oxygen split the crystal's band gap and formed a third one of its own.

“The material could lead to relatively inexpensive, highly-efficient solar cells that would be much simpler to make than today's high-end multijunction solar cells.”

And in the real world now:

in a small way, germany starts acting for substitute energy production
Scan recommended.

Solar panel park. Image credit: powerlight.com

“PowerLight's three Bavarian solar parks, consisting of 57,600 silicon-and- aluminum panels, will generate 10 megawatts of electricity -- enough to power 9,000 German homes. The amount of electricity produced is much less than power plants fueled by coal or natural gas, but with very low operating costs, the solar project is expected quickly to turn a profit while emitting zero pollution. Schroeder's left-of-center Social Democrat-Green coalition has turned Germany into the world leader in renewable energy since it took office in 1998. Billions of dollars have been spent on wind and solar projects, and Schroeder, in a politically risky move, has sharply increased taxes on petroleum products in an attempt to reduce consumption of conventional fuels.

“The campaign accelerated a year ago when Germany enacted a law forcing electric utility companies -- and, ultimately, all electricity users -- to pay higher rates to businesses or individuals who generate solar or wind energy and feed it back into the grid. With this guarantee of revenue, solar panels have become commonplace on new German houses and huge new windmills are a typical sight in rural areas, especially in the more windy north.”
—
“ A law that has been in effect for a year stipulates that the nation's electric utility companies must buy all wind and solar power generated by residential, commercial and industrial users at a price 10 times higher than the rate that users are charged for the electricity provided by the utilities from coal, nuclear or natural gas plants.

“Enticed by the guarantee of selling electricity at 46 euro cents (about 62 U.S. cents) per kilowatt hour for the next 20 years, as stipulated by the new rule, Berkeley's PowerLight Corp. needed no further prompting. CEO Dinwoodie went to a large investment bank in Frankfurt, Deutsche Structured Finance, and got a $65 million investment..”
—
“Under California utility regulations, by contrast, users are only able to draw their bills down to zero, with no profit possible. As a result, solar installations are small and large commercial facilities like PowerLight's Muhlhausen farm are impossible.”

• More details of the bavaria project [PDF]
  
  
• Powerlight’s website has a large range of useful background information, with various case studies. This includes a simple informative case study presented as a PDF with photos.
  
  
• Rolling photographic examples of residential and business solar projects from a site with much useful data.

large-scale photo-voltaic growth in the usa

Recommended reading.

"But the technology momentum goes only one way. "Eventually solar will become so large that there will be consequences everywhere," they said. This remarkable overthrow of everthing we take for granted in world energy politics may occur within "the better part of a decade". If the hypothesis is broadly correct, solar will slowly squeeze the revenues of petro-rentier regimes in Russia, Venezuela and Saudi Arabia, among others. Many already need oil prices near $100 a barrel to cover their welfare budgets and military spending. They will have to find a new business model, or fade into decline." ... "This will spread as battery storage costs - often a spin-off from electric car ventures - keep dropping. Sanford Bernstein says it may not be long before home energy storage is cheap enough to lure households away from the grid en masse across the world."

related material
Fuel cells and battery-powered vehicles

See also solar pv moving towards undercutting coal.

concentrators

There are various attempts to cut down expensive PV silicon cells by using many different concentrators.

Here is a diagram of a typical example from Japan.

shape silicon differently and reduce costs and silicon usage

silicon spheres provide a less material-greedy method of photovoltaics. Image credit: Clear Venture 21

Little 1mm balls, each in a reflector may give only 12% efficiency, but Clear Venture 21 claim that this conformation uses one fifth the silicon and so manufacture costs should also be one fifth as much as conventional silicon cells, while expending half the usual manufacture energy.

three-stage pv cells with concentrators achieves over 40% efficiency

concentrators
“[...] Spectrolab's concentrator photovoltaic cells generate electricity at a rate that can be more economical than electricity generated from conventional, flat panel photovoltaic systems.”
—
“A significant advantage of concentrator systems is that fewer solar cells are required to achieve a specific power output, thus replacing large areas of semiconductor materials with relatively inexpensive optics that provide optical concentration. The slightly higher cost of multi-junction cells is offset by the use of fewer cells. Due to the higher efficiency of multi-junction cells used in the concentrator modules, only a small fraction of the cell area is required to generate the same power output compared to crystalline silicon or thin-film, flat-plate modules.” [Quoted from boeing.com]

concentrator arrangement

new materials
“These results are particularly encouraging since they were achieved using a new class of metamorphic semiconductor materials, allowing much greater freedom in multijunction cell design for optimal conversion of the solar spectrum," said Dr. Richard R. King, principal investigator of the high efficiency solar cell research and development effort. "The excellent performance of these materials hints at still higher efficiency in future solar cells.” [Quoted from energy-daily.com]

triple-junction cells
“[...] the first layer of Spectrolab's record-breaking triple-junction cell is composed of gallium indium phosphide, which converts short-wavelength portions of the spectrum, such as blue and UV.

“The second layer, made of gallium arsenide, captures the middle part of the spectrum.

“The third germanium layer does a good job with IR light [...]”
—
“[ ..].When the multijunction cells are used in conjunction with a concentrator, their high cost-roughly 20 times that of conventional silicon cells-can be offset by the concentration ratio, Olson explains. The cost of concentration systems is also significant, but Sharps says that the U.S. Department of Energy's goal of bringing the total system cost down to below $2 per watt by 2020 is feasible. At this price, solar energy would be cost-competitive with nuclear fission and wind energy [...]” [Quoted from pubs.acs.org]

40% efficient solar conversion to electricty
“New World Record Achieved in Solar Cell Technology
New Solar Cell Breaks the "40 Percent Efficient" Sunlight-to-Electricity Barrier
WASHINGTON, DC - U.S. Department of Energy (DOE) Assistant Secretary for Energy Efficiency and Renewable Energy Alexander Karsner today announced that with DOE funding, a concentrator solar cell produced by Boeing-Spectrolab has recently achieved a world-record conversion efficiency of 40.7 percent, establishing a new milestone in sunlight-to-electricity performance. This breakthrough may lead to systems with an installation cost of only $3 per watt, producing electricity at a cost of 8-10 cents per kilowatt/hour, making solar electricity a more cost-competitive and integral part of our nation’s energy mix.”
—
The 40.7 percent cell was developed using a unique structure called a multi-junction solar cell. This type of cell achieves a higher efficiency by capturing more of the solar spectrum. In a multi-junction cell, individual cells are made of layers, where each layer captures part of the sunlight passing through the cell. This allows the cell to get more energy from the sun’s light. December 5, 2006” [Quoted from energy.gov]

related material
attempt at another form of concentrator for pv cells

advertising disclaimer

• new photovoltaic efficiency record - even closer to the darpa goal

  “[The] record-breaking combined solar cell efficiency of 42.8 percent from sunlight at standard terrestrial conditions [...] is a significant advance from the current record of 40.7 percent [...] and demonstrates an important milestone on the path to the 50 percent efficiency goal set by the Defense Advanced Research Projects Agency (DARPA).”
  —
  “The highly efficient VHESC solar cell uses a novel lateral optical concentrating system that splits solar light into three different energy bins of high, medium and low, and directs them onto cells of various light sensitive materials to cover the solar spectrum. The system delivers variable concentrations to the different solar cell elements. The concentrator is stationary with a wide acceptance angle optical system that captures large amounts of light and eliminates the need for complicated tracking devices.”
  —
  “Modern solar cell systems rely on the concentration of the sun’s rays, a concept similar to youngsters using magnifying glasses to set scraps of paper on fire. Honsberg said the previous best of 40.7 percent efficiency was achieved with a high concentration device that requires sophisticated tracking optics and features a concentrating lens the size of a table and more than 30 centimeters, or about 1 foot, thick. The UD consortium's devices are potentially far thinner at less than 1 centimeter.”

See also claims that solar pv makes another step forward, with theoretic possibilities up beyond 60% efficiency.

less efficient, cheaper photovoltaic technologies

Standard silicon arrays are approaching 20% efficiency, whereas several less efficient technologies are struggling towards 10% efficiency. However, ‘optimism’ in the field is very high.

These less efficient technologies are often a great deal cheaper than silicon-based solutions. They are also often a lot cheaper and do not require heavy sub-structures. Thus it is hoped that the lower efficiency can be offset by easier usage and lower costs - for example, bonded into normal house tiles, windows or even painted onto structures. As you will see, serious generation may require considerable land use, not something to be taken lightly when blocking out the sun from productive farming or other land use. Even considering such low efficiency for a desert area could generate larger maintenance costs for the greater implied areas. Working life is also a potential economic consideration.

artificial photosynthesis dyes for electricity generation

Basic structure of a dye solar cell (DSC) Image credit: dyesol.com

“[...] new dye-sensitized solar cells (DSSCs) [...] get their pink color from a mixture of red dye and white metal oxide powder in materials that capture light.
—
“ "If you absorb a very broad range of wavelengths, that's going to sacrifice voltage. And if your absorption energy threshold is very high, you can achieve high voltage, but you'll sacrifice current. The idea is to find some balance."

“Silicon-based solar cells have been around since the 1960s. Scientists have been working to develop DSSCs since the 1990s.

“In DSSCs, dye molecules coat tiny metal oxide particles that are packed together into a thin film. The dye molecules capture light energy and release electrons, and the particles act like electrical wires to carry the electrons away to an electrical circuit.”

There is an expectation that these cells can be made in different colours and thus be applied artistically in architecture.

On the difference between natural and artificial photosynthesis.

method mooted as lower cost and scaleable

“[...] a piece of dark polymer foil, as thin a sheet of paper. It is 200 times lighter than the normal glass-based solar materials, which require expensive substrates and roof support. Indeed, it is so light it can be stuck to the sides of buildings.

“Rather than being manufactured laboriously piece by piece, it can be mass-produced in cheap rolls like packaging - in any colour.

“The "tipping point" will arrive when the capital cost of solar power falls below $1 (51p) per watt, roughly the cost of carbon power. We are not there yet. The best options today vary from $3 to $4 per watt - down from $100 in the late 1970s.

“Mr Sethi believes his product will cut the cost to 80 cents per watt within five years, and 50 cents in a decade.

“It is based on a CIGS (CuInGaSe2) semiconductor compound that absorbs light by freeing electrons. This is then embedded on the polymer base. It will be ready commercially in late 2009.

“ "It'll even work on a cold, grey, cloudy day in England, which still produces 25pc to 30pc of the optimal light level. That is enough, if you cover half the roof," he said.”

solar pv moving towards undercutting coal

“Their mission: to deliver cost-efficient solar electricity. The Nanosolar company was founded in 2002 and is working to build the world’s largest solar cell factory in California and the world’s largest panel-assembly factory in Germany. They have successfully created a solar coating that is the most cost-efficient solar energy source ever. Their PowerSheet cells contrast the current solar technology systems by reducing the cost of production from $3 a watt to a mere 30 cents per watt. This makes, for the first time in history, solar power cheaper than burning coal.”

“In San Jose, Nanosolar has built what will soon be the world’s largest solar-panel manufacturing facility. CEO Martin Roscheisen claims that once full production starts early next year, it will create 430 megawatts’ worth of solar cells a year - more than the combined total of every other solar plant in the U.S. The first 100,000 cells will be shipped to Europe, where a consortium will be building a 1.4-megawatt power plant next year.” [Quoted from popsci.com]

430 megawatts is about half a big power station. Keep in mind that photo voltaics [PV] cannot function on a 24 hour basis.

December 2007: The Nanosolar company is now in production on an economic scale, with all the first year’s production already sold.

From a leading PV company, with many interesting links:

• The First Wave started with the introduction of silicon-wafer based solar cells over three decades ago. While ground-breaking, it is visible until today that this technology came out of a market environment with little concern for cost, capital efficiency, and the product cost / performance ratio.

Despite continued incremental improvements, silicon-wafer cells have a built-in disadvantage of fundamentally high materials cost and poor capital efficiency. Because silicon does not absorb light very strongly, silicon wafer cells have to be very thick. And because wafers are fragile, their intricate handling complicates processing all the way up to the panel product.

• The Second Wave came about a decade ago with the arrival of the first commercial "thin-film" solar cells. This established that new solar cells based on a stack of layers 100 times thinner than silicon wafers can make a solar cell that is just as good. However, the first thin-film approaches were handicapped by two issues:

1. The cell's semiconductor was deposited using slow and expensive high-vacuum based processes because it was not known how to employ much simpler and higher-yield printing processes (and how to develop the required semiconductor ink).
   
   
2. The thin films were deposited directly onto glass as a substrate, eliminating the opportunity of o using a conductive substrate directly as electrode (and thus avoiding bottom-electrode deposition cost),
   • achieving a low-cost top electrode of high performance,
   • employing the yield and performance advantages of individual cell matching & sorting,
   • employing high-yield continuous roll-to-roll processing, and
   • developing high-power high-current panels with lower balance-of-system cost.

“Roll-to-roll processing is the manufacturing implementation framework of choice for any product with very low cost required per large areas of deposition. Rolls that are meters wide and miles long can be processed efficiently with very high throughput (and thus minimal capital cost) in equipment with a very small footprint.

“A key advantage of roll-to-roll processing is that after the first few meters of initializing a new roll, the whole process hits a steady state which can then be maintained for the entire rest of the roll, resulting in very uniform deposition process parameters applied to essentially the entire (foil) substrate. This is much better than processing wafers or glass plates, which have to be moved in and out of each process station individually, introducing undesirable start-up and move-out process state variability (and cycle time cost).

“Edge effects are also greatly minimized in roll processing (whereas processing glass plates or wafers requires much work and capital dealing with uniformity issues at the edges of the substrate).”

From another page:
“At Nanosolar, we have taken the highest-performance and most durable photovoltaic thin-film semiconductor, called CIGS (for "Copper Indium Gallium Diselenide"), and innovated on all seven critical areas necessary to reach a breakthrough cost reduction in solar cells, panels, and systems.

“As opposed to using slow and expensive high-vacuum based thin-film deposition processes, we developed a proprietary ink to allow us to use much simpler and higher-yield printing for depositing the solar cell's semiconductor.

“We use a highly conductive yet low-cost foil as a substrate, which allows us to avoid the need to separately deposit an expensive bottom electrode layer (as required for a non-conductive substrate such as glass).

The foil furthermore allows us to

• apply the superior economics of high-yield continuous roll-to-roll processing,
• achieve a lower-cost high-performance top electrode,
• assemble cells by individually matched electrical characteristics, and
• develop high-power high-current panels with lower balance-of-system cost.

how much energy from the sun do we receive on earth?

The generally quoted figures for how much energy from the sun is reaching the Earth (solar flux energy) are:

1. Since 1750, anthropogenic forcing is estimated at a little over 2.5 W/m² at the Earth’s surface.
2. On a clear day, at noon, the sun’s radiation at the surface is about 1,000 W/m². This is then complicated by day and night differences, seasonal changes, cloud cover (see also planetary heat circulation). At any time, sunlight and heat from the sun is about ten thousand times more than the whole world’s present energy consumption.

Using a photovoltaic array, electrical energy is generated locally, on the south-facing roofs of buildings, or set out facing south on land that would be fed back into the national electric grid, thus in a sense using the grid as a battery.

The time of production tends to coincide with the peak usage times, thus allowing a reduction of back-up capacity and so lowering the costs to users and generators. It is very likely that photovoltaics will be market driven by end users.

As the price goes down, it is attractive to increasing parts of the energy-generating market. The capital costs of manufacturing PV arrays and for installation are decreasing rapidly, to a tenth or less in recent decades. At the same time, the efficiencies are also creeping upwards. In addition, upkeep costs are probably quite low.

The solar constant describes the amount of incoming solar radiation that falls on the outer surface of Earth’s atmosphere, in a plane perpendicular to the rays. Currently, the solar constant s = 1.37 kW / m².

In space, solar radiation (insolation) is practically constant.On Earth, it varies with

• the time of day and year - day and night differences and seasonal changes,
• the latitude - and a pitched roof compensates to some degree to a change in latitude,
• weather - particularly cloud cover,
• location - valley and other shading, and which direction the photovoltiac array is pointing (its aspect).

The maximum average insolation value on Earth is between 0.8 and 1.0 kW / m². This is estimated to one quarter of this value when night-time, haze or cloud cover are included. In Germany, for instance, the average annual amount of peak insolation varies between 0.95 and 1.1 kWh / m², depending on the region.

Because of all these factors, advantages will come earlier to those with better conditions, whether climate or location, and business or commercial premises, especially those built with energy efficiency in mind.

The more pressing problems are with mounting, storage and conversion equipment. However, independence from power utility companies and cleaner energy are already driving a growing percentage of the market to install photovoltaics.

the practicality of photovoltaic generation for london

A fairly humorous example suggests covering London in solar panels, and the contribution that this likely to make to the conquest of the universe.

Let’s look at this joke example in closer detail.

1. Greater London is about 600 sq. miles, that is 1,500,000,000 square metres (600 x 2,589,988.11).
   
   
2. Given that a solar flux energy at 1kW per sq m. during daylight and in good conditions, at present achieving 20% efficiency from the photovoltaic arrays is good.
   
   
3. As the sun shines no more than 12 hours a day, that efficiency must be cut, we’ll be generous and claim a 10% overall efficiency. If we took into account claims that cloud and haze also cut generating efficiency by half, we’d have to reduce that efficiency to 5%.
   This does not include any inefficiencies in converting the energy, or inefficiency because the panels do not follow the sun, but let’s ignore those.
   
   
4. So we are now down to 75,000,000 kW per hour (5% x approx. 1,500,000,000 sq m. x 1kW/sq m.).
   
   
5. There are 8,760 hours in a year. This makes it potentially possible to generate 657,000,000,000 kW per year.
   
   
6. Finally, for this all to work, the whole of the Greater London area would have to be covered with a vast sloping photovoltaic array, with council workers going up on top and scrubbing it all to keep it clean. At least it wouldn’t rain on the populace any more. And it would be by far the largest man made structure on Earth!
   

Next, there are all those roads and parks and rails that I do not think most people will welcome being covered in solar panels. And your garden will have to go; you won’t grow anything much without the sunlight.

Then, the efficiency drops rapidly if the photovoltaic arrays are not reasonably clean, or the angle is not optimal.

So, what do you think are reasonable figures for the amount of roof, garden, and so on you would be able to use for energy generation?

For reference, a large power station produces maybe a bit less than 9 billion kW a year. But this reduces by about 10% to allow for for servicing and other down time. The original figure for the Greater London power generation project is the equivalent of roughly 65 big power stations. However, the whole UK only has about 65 such stations equivalent at present. So this still isn’t peanuts (don’t forget we still have the whole of the Greater London area covered at this point).

Photovoltaic energy generation is not useless, but it requires sane numbers. The Sahara might find a useful industry, and then there’s Wales!

Now, costing for one house:

The following costing is purely speculative and, at the moment, all the figures are guesswork. I shall try to obtain and gradually substitute some figures from real installations. In this early stage of development, real prices will be highly variable, according to what the vendors think they can get away with. Doubtless, the quality, value and efficiency of equipment will vary. There are also widespread tax-funded incentives available from a wide variety of government sources. This technology is developing quickly and, therefore, judging when or if to buy will not be easy. Let the buyer beware.

• Consider that buying 50 sq m of photovoltaic arrays costs approximately $6,000. That is, roughly $3,000 each for the array itself, and for the installation of support structure with an invertor to connect the array to the mains and produce electricity for the house.
  
  
• Assume that the claimed cost of photovoltaic generation is $1 per watt, and that the claim is based upon 10% efficiency of the photocells.Thus, each square metre is expected to produce 100Watts.
  
  
• Next, assume an average of 12 hours a day of some sort of sun.
  Therefore, the claimed generating capacity must be reduced by half, for night-time, that is 5% efficiency.
  
  
• Now, we must also consider over-cast conditions and the fact that the PV array is fixed, and so does not follow the sun. Further, the sun may be low in the sky for some months of the year. There are also efficiency losses at the invertor.
  So let’s be generous and assume that all this cuts the efficiency by another 50%, and we are now down to 2.5% efficiency.
  
  
• Say 25 Watts per metre can be generated using photovoltaic panels, that would give 1.25kW per hour (kWh).
  
  
• So there would be generated on average each day, say 15kWh. That is 5475 kW per year.
  
  
• As electricity costs roughly $0.13 a unit (1kWh), this electricity will be valued at $728 per year.
  Thus, the 50 sq.m array will pay back in 8.25 years, if lost interest is ignored or is offset against increased prices.

“SANTA CLARA, California - Demand for glass solar cells will grow at double that for those based on traditional silicon as cheaper price offsets a less-efficient design, the head of Applied Materials' solar unit said Friday.

“Glass-based cells, made by sandwiching ultra-thin layers of materials between two sheets of glass, accounted for only about 10 percent of the 1,800 megawatts of solar capacity installed last year.

“But the technology is winning more converts because cells can be made much more cheaply, on sheets of high-grade glass and using much thinner layers of the costlier materials that give them their energy-generating properties.” [Quoted from planetark.org]

Here is a good place to start looking for prices at the moment. Notice that the current prices quoted are a little under $5/Watt for installed capacity.

“The module cost represents around 50 - 60% of the total installed cost of a Solar Energy System. Therefore the solar module price is the key element in the total price of an installed solar system. All prices are exclusive of sales taxes, which depending on the country or region can add 8-20% to the prices, with generally highest sales tax rates in Europe.

“It should be noted that there was an increase in sales tax rate at the beginning of the year in Germany from 16% to 19%. This change does not effect the index itself, since it is calculated pre-sales taxes.” [Quoted from solarbuzz.com]

related material
energy microgeneration

cunning new solar panel design - replaces roof tiles

Credit for all images in this section: solarcentury.com

Instead of rather inelegant slabs of solar panelling placed on top of and above your current roof, integrate solar tiles with the current roof tiles. There is also a thermal tile option for heating water, but currently they are only available to housing developers, housing associations and registered social landlords, and not for private residential installations, wholesale customers or their installers.

Note that, because each solar tile takes the place of four normal tiles, “this particular solar application works best for 'new builds' or when a homeowner is re-roofing, because each photovoltaic tile replaces four conventional roof tiles and is attached straight to the roof battens”.

Already installed on housing in the UK,

“C21e tiles [52Wp per tile] require just 8 sqm per kWp [kilowatt peak] to generate 800kWh of electricity per year.

“It is expected that each house will generate 2,500kWh of electricity per year, equating to a saving of 1.5 tonnes of CO2. Excess electricity is not wasted but simply exported to the national grid, which pays a small fee back to the householder [...]” [Quoted from 4ecotips.com]

“[...] an average domestic system could cost around £4,000- £9,000 per kWp installed, with most domestic systems usually between 1.5 and 2 kWp so you are looking at between £6,000 at the lower end and £18,000 at the more expensive end. Cost will vary as tiles are more expensive than panels and integrated panels are more than standard.” [Quoted from electricity-guide.org.uk]

Solarcentury is rather coy about their prices. However, recently (to September 2007), 12 C21e tiles cost £3,514, excluding delivery and installation.

photovoltaic progress - power generation and storage

PVs are starting to produce too much electricity in some areas, more than government will pay for. At the same time, battery research is creating what is likely to be the coming revolution in electricity storage.

A real fun read

""California's got a major problem," [...] "The amount of solar that's coming on-stream is just truly remarkable, but it all hits the system between noon and 4pm."

"That does not marry well with peak demand for electricity, which generally comes in the late afternoon and evening, when everyone travels home, turns on the lights, heating or air conditioning, boils the kettle, bungs dinner in the microwave, and so on."

technical notes

some interesting comments on solar cell materials

on indium-gallium-nitride
Known reserves of indium: 13 years at present consumption rate.

[Article dated 2002]
“Photons with less energy than the bandgap slip right through the material. Photons with too much are absorbed, but since each creates just one electron-hole pair, the excess energy is wasted as heat.”
—
“Most solar cells are made from silicon. Cheap, amorphous silicon-based solar cells have efficiencies of less than 10 percent, and the efficiencies of even the most advanced single-crystal silicon cells are limited to about 21 percent.” [Quoted from www.lbl.gov, page 1]

“Two layers of indium gallium nitride, one tuned to a bandgap of 1.7 eV and the other to 1.1 eV, could attain the theoretical 50 percent maximum efficiency for a two-layer multijunction cell. Currently, no materials with these bandgap can be grown together.

“Indium gallium nitride solar cells could be made with more than two layers, perhaps a great many layers with only small differences in their bandgaps, for solar cells approaching the maximum theoretical efficiencies of better than 70 percent.” [Quoted from www.lbl.gov, page 2]

With further links.

 

end notes

1. This applies to the northern hemisphere; in the southern hemisphere the arrays would need to point northwards.
   
   

2. invertor

   designed to change DC current to AC current at, for example, 240 volts

email email_abelard [at] abelard.org

© abelard, 2007, 13 march
v.1.0

all rights reserved

the address for this document is https://www.abelard.org/briefings/photovoltaics_solar_cells.php

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