Photovoltaics

History of Photovoltaic Power Costs: The cost of generating electricity from solar panels has dropped steadily over the past three decades, from nearly $100 per watt in 1975 to less than $7 per watt today. Rising demand for materials has raised prices in recent years, but costs are expected to resume their downward trend as technology improves and production ramps up to meet demand. (Source: Harvard Business Press via McKinsey "Stat of the Day" http://www.mckinseyquarterly.com/Energy_Resources_Materials/Electric_Pow...)

Roof mounted solar energy bullet points - Submitted by Dave Blau, Oct. 2009

DRAWING: (requested from Dave Nov. 1, 2009) The drawing is pretty simple. 6 panels in series, 600 volt link, to an AC converter box (after a breaker). converter makes 220AC60hz and after another breaker pushes it back onto the line through the power meter, which runs backwards and forwards.

1. Generally the panels are pointed for maximum income not maximum power, because income is higher in the summer. So they are pointed about 5 degrees away from the ecliptic.

2. Make sure there is a way to clean them. Need to be cleaned about every 3 months. Even one cell blocked cuts the efficiency of the entire array. weakest link effect. A cell that has no light (or less light) on it acts as a blocking diode. And you have to get up to the panel, and be safe, while cleaning them.

3. The weakest link effect can be mitigated by small maximum power point converters on smaller groups of cells. This is an opportunity, to make mpp converters+cells for smaller power that are cheap enough to deploy multiple units on one installation. Their output would be combined onto a bus that would then go to the main 60hz conversion.

4. Concentrators only work if they cover the entire panel evenly, since the weakest link effect occurs in this case as well. A small, modular assembly with a simple reflector and its own mppt converter might be a cool product. Then the user could buy them and install them as money permits, and get higher output too.

5. Combined solar/thermal conversion nets out at about 25% conversion efficiency and keeps the solar panel cool, so it operates more efficiently. A company in Berkeley offers a product like this.

6. Putting reflectors on standard cells is not optimum because the metallization on the cell gets too much voltage drop to operate at max efficiency. Ideally a cell would be designed to work with concentrators. Alternatively, instead of simple metallization on the cell, bus bars could be run down the metallization to alleviate this effect. The cells need to be connected to bus bars anyway.

7. The metallization on the cell topside is almost certainly not optimum. The optimum is probably a fractal pattern... just as all points of land are drained of water, and the drainages form a fractal pattern, the drainages getting larger as they aggregate more drainage area.

8. Arrays are getting stolen. Buildings should be designed to not show the panels, and hide them from wind as well.

9. As a practical matter roofs that are not pointed e-w are not optimum for panels, but the decrease in output for incorrectly pointed roofs are not as great as one would think. Numbers like 10% effects seem to be in the neighborhood.

10. Solar cells drop in output about .4% per degree C. Doesn’t sound like much but a paneling full sun can get to almost 100 degrees C, ay 60 degrees over ambient, which is a 24% drop in output. Provision has to be made to keep the cells cool.

11. Multiple connections are made to the cells partly because of current sharing but also in case the cell breaks, the output will not decrease as much. Because of the weak-link effect, one broken cell can mess up a whole panel.

12. An immitance converter type power converter is a very good way to connect to the power lines, with simplicity and reliability. As far as I know there are not immitance-converter type converters on the market right now.

13. Panels are typically made with an eva (ethylene vinyl acetate) sandwich covered with a glass cover. The cost of the cell is approaching 3$ for 5 watts, but the mounting costs a lot of money, between hooking up the cells, sealing them, etc. so in terms of total cost the packaging is a major issue.

14. Solar cells are often made with an antireflection coating, because raw silicon will reflect 30% of the light just from index of refraction effects. So typically the cells are coated with silicon nitride, made to be antireflection at 600 nanometers light wavelength. But once the cell is in the EVA sandwich the NR coating is completely wrong! The coating should be set up to be antireflection when in a sandwich of index 1.5 plastic. This requires a coating that has an index of 2.4 or so. SiN is 2.0. 2.4 index materials could be made from something like titanium dioxide. It may be possible to dope the TiO2 to also conduct some of the photoelectrons. Effects are about 6-12 % efficiency improvement.

15. Simple ecliptic articulation would have some payoff. Maybe the panel mounting has 2 positions, one for winter and one for summer, with a lever that gets changed when doing the fall and spring cleaning.

16. Blowing the exit gasses from a 2 stage swamp cooler over the back or even the front of the cells could improve performance without cost, in principle.

17. it would be nice to have a tool on the website that helps people know what they could get from solar installation. It would require input of the house direction, size, roof tilt, latitude, and so forth.

Biomimicry was used to develop Dye Sensitive Solar Cells: http://hbr.harvardbusiness.org/web/2009/hbr-list/business-of-biomimicry

The DSSC Advantage DSSC materials can collect light at shallow angles, capture dim as well as full sun, and work in scorching temperatures (all of which bedevil traditional photovoltaic solar cells). They can go where silicon panels can’t: on vertical surfaces such as windows, on structures in the shaded lower “canopy” of cities (street lamps, for example), in the sunless cores of buildings, and in desert or tropical locations. In the evolution of solar cells, they are the equivalent of aquatic life’s learning to live on land, opening a vast array of new habitats.

Expanding the Solar-Cell Market Although DSSCs are currently less efficient than photovoltaic cells, they’re 60% cheaper to produce and more versatile—they can be made into flexible films or fibers in a process similar to ink-jet printing. Two companies—Konarka, of Lowell, Massachusetts, and Dyesol, of New South Wales, Australia—stand out for having pioneered advances in DSSC materials and manufacturing technologies that lower costs and improve performance.

One Promising Solution Inexpensive, nontoxic and flexible DSSC solar harvesters can work not only on roofs but also on curved surfaces. The fact they can accept low-angle light allows them to work in shade, or even on indoor surfaces. The implications for this breakthrough are profound. Millions of distributed harvesters could be networked on a neighborhood scale or used alone when power grids fail. Steelmakers are already testing ways of incorporating DSSCs into structural steel. Imagine the Golden Gate Bridge doubling as a power plant.

Light down a wire for solar power http://news.bbc.co.uk/2/hi/science/nature/8341186.stm

Solar power could be produced cheaply in specially designed optical fibres, say researchers.

The work, published in the journal Angewandte Chemie, makes use of nanometre-scale wires built around optical fibres like bristles.

Those wires give the light much more surface area to interact with, leading to higher overall efficiencies.

However, only the ends of the fibres must be exposed - they funnel the light elsewhere for power generation.

Instead of roof-sized panels, small collectors could be used on the roof, with the real machinery of solar power generation tucked away, for example, between a home's walls.

"Using this technology, we can make photovoltaic generators that are foldable, concealed and mobile," said Zhong Lin Wang of the Georgia Institute of Technology in the US.