# Thread: High power density solar panel

1. I need a small (maybe around 25 cubic inches) solar panel rated at 2 watts or higher for under \$40. Do you know of one like this? I can't seem to find one. It will be used to charge some portable devices.

2.

3. When a solar panel has a current rating, like 100ma, does this mean that it will output 100ma no matter what or that 100ma is only its maximum output current through a given amount of resistance? What happens if you draw more current than it can provide?

4. The manufacturer should specify the load condition Cold Fusion, and the light intensity for the measurement. The maximum current occurs when you short circuit the output but without voltage, no power is delivered. Equally, if it is left open circuit, the cell will generate the maximum voltage but without current, no power delivered again.

The power supplied by the solar cell will reach a maximum for a specific load resistance for a given illumination. Again, the manufacturer should specify these conditions as such information can only promote their product being demonstrated in an optimum setting

5. A voltage source like a solar panel can be approximated by a pure voltage source in series with a resistance. This is called a Thevenin equivalent circuit. If two solar panels have the same voltage rating but different power ratings, the higher rated panel will have lower Thevenin equivalent resistance.

What happens is that as you load the panel down there is a greater voltage drop across the internal resistance. Therefore the voltage at the load decreases and the power output decreases. The higher the internal resistance, the less power can be extracted.

6. Yes, as Harold14370 says, the solar cell has an "internal resistance". If this is constant with load, maximum power transfer will occur when the load resistance equals the internal resistance. Given this, half the power is lost in the internal resistance and half in the load.

If you visit the following site and open the 160 unit you will see a curve showing output voltage versus load and output power versus load for three ambient illuminations.

http://www.oksolar.com/panels/

Interestingly, the maximum output power peak occurs for a load current that is only slightly less than the short circuit current. Also, the "best current" reduces only slightly when the illumination almost halves. I suspect the Thevenin equivalent circuit, based on a linear model, may not be strictly accurate for a potentially non linear solar cell (module). In such a case, it is best to use the actual measured data presented graphically.

The Thevenin (or Norton) equivalents are still useful conceptually though!

I just had a quick look at Wikipedia; some interesting information is there at

http://en.wikipedia.org/wiki/Solar_cell

The equivalent circuit does appear non linear

Maximum-power point

A solar cell may operate over a wide range of voltages (V) and currents (I). By increasing the resistive load on an irradiated cell continuously from zero (a short circuit) to a very high value (an open circuit) one can determine the maximum-power point, the point that maximizes V×I; that is, the load for which the cell can deliver maximum electrical power at that level of irradiation. (The output power is zero in both the short circuit and open circuit extremes).

A high quality, monocrystalline silicon solar cell, at 25 °C cell temperature, may produce 0.60 volts open-circuit (Voc). The cell temperature in full sunlight, even with 25 °C air temperature, will probably be close to 45 °C, reducing the open-circuit voltage to 0.55 volts per cell. The voltage drops modestly, with this type of cell, until the short-circuit current is approached (Isc). Maximum power (with 45 °C cell temperature) is typically produced with 75% to 80% of the open-circuit voltage (0.43 volts in this case) and 90% of the short-circuit current. This output can be up to 70% of the Voc x Isc product. The short-circuit current (Isc) from a cell is nearly proportional to the illumination, while the open-circuit voltage (Voc) may drop only 10% with a 80% drop in illumination. Lower-quality cells have a more rapid drop in voltage with increasing current and could produce only 1/2 Voc at 1/2 Isc. The usable power output could thus drop from 70% of the Voc x Isc product to 50% or even as little as 25%. Vendors who rate their solar cell "power" only as Voc x Isc, without giving load curves, can be seriously distorting their actual performance.

The maximum power point of a photovoltaic varies with incident illumination. For systems large enough to justify the extra expense, a maximum power point tracker tracks the instantaneous power by continually measuring the voltage and current (and hence, power transfer), and uses this information to dynamically adjust the load so the maximum power is always transferred, regardless of the variation in lighting.
I guess that a maximum power transfer system would need a microprocessor or equivalent controller to optimize power transfer with lighting conditions, based on the above (possibly using a PWM (etc) switch-mode power supply approach).

Still I guess the original question on low cost supply remains unresolved

7. ahhh ok, I did not know about those things, thanks for the info.

8. Manufacturers usually quote the maximum output.

The average output (watt-hours) is only about 10% of the peak taken over a whole year. Clearly on average it is dark half the time, also the conditions can be cloudy and the Sun will not be square on to the panel.

Solar panels never pay for themselves.

9. I know

At least were on the edge of manufacturing 50% efficiency and above panels by utilizing a new type of crystal technology.

10. The best way to use a solar cell is to stick it in space where it has 100% sunlight all the time and at higher strength than on earth. Some people suggest microwaving the energy back to earth (with suitable precautions) - it may also be possible to use an earth to space tether cable to transport the electricity down to earth.

11. American companies seem to be claiming much higher efficiencies. I think they must be defining and meauring it in a different way.

Even with much higher efficiencies the poor capacity factor (average to peak ouput) factor means a low average output.

A real system..

http://www.simondawson.com/sjpenv/sjppv1.htm

It must have cost a lot to install and does not produce that much output.

12. As an engineer who has worked on high power radio systems I would say beaming power to earth by microwaves at anything like significant power would be totally impractical.

13. Proposals for such systems are in progress ref Wikipedia

A solar power satellite, or SPS or Powersat, as originally proposed would be a satellite built in high Earth orbit that uses microwave power transmission to beam solar power to a very large antenna on Earth. Advantages of placing the solar collectors in space include the unobstructed view of the Sun, unaffected by the day/night cycle, weather, or seasons[1]. It is a renewable energy source, zero emission after putting the solar cells in orbit, and only generates waste as a product of manufacture and maintenance. However, the costs of construction are very high, and SPS will not be able to compete with conventional sources (at current energy prices) unless at least one of the following conditions is met:
http://en.wikipedia.org/wiki/Solar_power_satellite

Artists impression,

Microwave technology can be reasonably efficient in converting DC to RF and the remote earth load would be constant. Also, suitable fail safe measures would be included to prevent accidental scorching if the satellite drifted off target.

A tether approach might be better but this requires materials and constructions that are as yet unavailable. Microwaves represent a doable approach. Microwave ovens have an efficient magnetron with a highly variable load. A space to earth energy transfer system would have a fixed load and providing that the collecting aperture area of the receiving antenna exceeded the beam dispersion then the main energy loss would be atmospheric absorption. Rectification efficiency from RF to DC shouldn't be problematic given even conventional Schottky diode technology.

14. The proposal I have seen worked at around 2.4 Ghz and requires a 2 km diameter dish in space. The dish needs to be within 1 - 2 cm of a perfect parabola over its whole surface, needs to steered accurately at all times and would be a target fror space debris.

DC-RF-transmission loss-RF-DC-AC is never going to be very efficient.

Even at 4 conversions at 90% you end up with an overall 65%.

4 times 80% you get 41%.

You would need thousands of magnetrons.

The MAJOR problem is that there is no air for cooling. Even the wires required to carry the DC would overheat and would have to be enormous.

You would need thousands of space flights.

And all this for a relatively small amount of power .. 1 - 2 GW.

Each kWh would cost the a fortune.

The wideband hash a space power station would generate could cause significant interference.

I don't know what the maximum power a rectifier can handle at 2.4 Ghz. I suspect it is of the order of a few Watts so you would need hundreds of millions of them.

15. Some people dismiss problems as they are inconvenient; others embrace a challenge.

The MAJOR problem is that there is no air for cooling
Space is close to absolute zero. Correct me if I'm wrong but isn't radiation loss based on a power? On earth, heat exchanges from similar kelvin temperatures object-object, in space the opposite is true. Try staying warm in a vacuum and you might realize how easy it is to get rid of heat in space - alternatively prove me and other people investigating possibilities to be so wrong . If you opened your space suit in space you'd be an ice cold Popsicle stick.

However, to presuppose the need to remove massive thermal overheads suggests an incompetent design approach in the first place. Low frequencies like 2.5 GHz are audio in today's technology arena and plenty of LDMOS devices can be shown to exhibit > 90 % DC to RF conversion given benign VSWR environments and harmonic loading. To those in the know this isn't a big difficulty even from a silicon converter. Given competency in the microwave engineers employed, only 10% of heat might need to be radiated to space. Much more is lost as sunlight going elsewhere anyway.

The wideband hash a space power station would generate could cause significant interference
Well the solution to that is a thing called a band pass filter. Some people suggest 2.45 GHz for non ionizing beams but that would affect the ZigBees and the Bluetooths and the other users but there are other windows to those that want to find better ways for power transport. There are other frequencies and band pass filters aren't rocket science are they.

Perhaps pumblechook, reflect microwave wise on your own posted quote,

The average output (watt-hours) is only about 10% of the peak taken over a whole year. Clearly on average it is dark half the time, also the conditions can be cloudy and the Sun will not be square on to the panel.
and ask if the 10% you yourself say you would get from earth bound approaches is superior to a space based one?

Every problem has a solution to those that seek one. I wouldn't dismiss space to earth energy conveyance so offhandedly. The fact that people give audience to the concept of space to earth energy transfer should suggest that some local job issues and familiarity with tasks on a personal work-job table might not reveal a bigger picture envisioned

16. (Space is close to absolute zero)

No it aint.

It would be with no solar radiation. Like on the illumated side of the Moon any surface gets very hot.

I watched a programme on satellites and cooling is a big problem even for low power electronics.

From a website...

""Electronic components utilized throughout
the various subsystems experience thermal stress resulting from high
temperature effects from the sun, from low temperatures occurring during
eclipse, and from heat dissipated internally by components located aboard
the satellite. Thermal devices, such as radiators, are commonly used to
dissipate excess heat and to protect the electronic equipment from thermal
stress. Radiators used on satellites typically include sheets of a highly
thermally conductive material with a high thermal emissibility
characteristic. To provide maximum heat radiation to space, high power
dissipation components are commonly mounted directly to the radiator
panels.""

17. Yes Pumblechook heat is an issue but a manageable one. If this was a "hard task" we would have no satellites in space.
. Yet we do.

Also, SiC is available now for higher temperature operation than Si. Materials are continually being refined for space applications and also here on earth.

18. There is massive difference between a few hundred watts - few kilowatts AND Gigawatts. And there is massive difference between a tonne or so and millions of tonnes.

A one GigaWatts station would cost an astronomical amount and only supply about 1.5% of the UKs needs. In world terms it would be 0.01% or something.

It is one for science fiction only.

19. I didnt notice this post.

I'm working with the solar company with the highest efficiency(except GaAs).

We manufacture solar cells around the size of 16cu.inches at 3 watts. I am not sure about the price. It may range from 6+ to 7+USD each.

The highest efficieny right now is around 22 to 23% for Silicon type cells. GaAs (we do not manufacture this) type solar cells can reach 30+% efficiency but is very expensive and toxic.

Tests are conducted on each cells to get the Isc, Voc, Rshunt, and others.

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