Site Navigation Links : Please click on SITE MAP: Next Page
Solar Energy - Photovoltaic Cells
Just as wind energy is FREE, solar energy is available at any time. Wind has a slight advantage over solar in that it is available for 24 hours each day and solar energy has to be collected during the day. Above the atmosphere a one square meter plate will receive 1.4 kwatts of energy so solar energy really appears to be a powerful source. Down on earth it doesn't look quite so good and in Northern Latitudes we are not best placed for solar energy as the following diagram shows:
In the UK we are roughly on the 100 Watts per square meter level whereas equatorial regions can be 2.5 times as large. Another point which needs to be made regarding photovoltaic cells is the cost. Comparative costs of wind and photovoltaic cells are given below:
Due to the larger scale of production of turbine versus photovoltaic module, the current pricing of wind turbines is approximately 3 euros per watt whereas the photo cell is 25 euros per watt.
(the proton exchange membrane fuel cell (PEMFC) offers attractive projected costings though this industry is in its infancy )
In is necessary here to introduce a bit more Physics as we need another secret of Nature to unfold to understand Photovoltaic cells. The online text www.lightandmatter.com doesn't quite go far enough into electronics to be useful for this section although it does contain a lot of background material.
In general terms any Physics done before1900 is called Classical Physics and that done after is called Modern Physics. Classical Physics has topics such as Statics (Forces on buildings, dams, bridges etc), Dynamics (motion of wind turbines or rockets), Themodynamics (introduced before) and Electromagnetism (all the electricity industry, Radar, and a host of other goodies). In Modern Physics we have Relativity, Quantum Mechanics and Nuclear and Fundamental Particles. The Band Theory of Solids came from Quantum Mechanics in the 1930's and the main idea is based on an analogy. Suppose we have a two story car park and our cars are charge carriers. If the lower part is partially filled then the cars will be able to move around - charge moves around - so this will behave in a similar manner to a conductor. If the bottom park is fully full and the upper park is a long way up then no movement occurs and this behaviour is like an insulator. If the upper park is close to the lower park then there may be a chance of a few cars being jostled upwards and, once there, can move around with ease. One can also see that there is a possibility of movement in the lower park as there is an unoccupied space. With some imaginative driving the space can move as cars successively move into it.
Hence, in a semiconductor we have
(a) cars that can move on the upper park - these are electrons or n-type carriers
(b) spaces that can move on the lower park - we call there "holes" or p-type carriers.
We can go further by adding precise quantities of other elements into a semiconductor and make them either n-type or p-type so producing semiconducors that have closely controlled conductivities and with charge carriersthat may be either holes or electrons. So here we see that a semiconductor is, indeed, a versatile material which has been used to produce countless devices.
Stuart Bowden and Christiana Honsberg ( University of Delaware) have but together a truely remarkable site which is a re-vamp of information originally accessible on a CD rom. The animations are very helpful and each section is filled with quality material www.udel.edu/igert/pvcdrom/CELLOPER/VOLTAGE.HTM --- if you arrive at the University home page you can type photovoltaic cells in the search box; the historical section may open and then click on home page to get a contents page. ( Note as an alternative use Google search with "pn junction and photovoltaic effect")
Other useful web sites www.howstuffworks.com www.play-hookey.com and there is one from the University of Illinois which opens from a Google search with "conductors, insulators and semiconductors" in the text box.
Well, this secret of Nature is mind blowing. On page 8 we mentioned our first secret of Nature, Pythagoras. Every one in the world is aware of this secret. On page 9 the Heat Engine secret was exposed and this was almost too incredible to believe. Yes, little old me can now summon up and army of slaves to transport me from A to B with the ease of switching a switch. ( the analogy with slaves is somewhat over done because for my next journey I demand a new set of slaves!!!) Now the secret of the Band Theory of Solids is yet more astounding. From understanding a semiconductors (usually Silicon) we can build a transistor, from a transistor we move to an Integrated Circuit (IC or silicon chip) and then we rapidly move to the INFORMATION AGE. And it has all happened in these last few decades.
For the case of a solar cell we see that the charge carriers are generated by sunlight - this moves cars to the upper park. But sunlight contains light of different energies ( the spectrum of light) and the low energies haven't sufficient energy to move the cars up. These rays of light are therefore wasted. At some point the light energy is just right and cars move up - we say that this conversion is 100% efficient. Above this threshold the rays will have too much energy, so they lift cars up but the excess energy is now wasted and, again, the conversion is inefficient although this time we have carriers so the rays are not totally wasted.
I hope you can see that the cell cannot be tailored to suit the full spectrum of sunlight and the total efficiency will be much less than 100% which would only be obtained for one special light colour. Modern commercial cells have conversion efficiencies of about 15% although, for specialist cells, it may be as high as 30 % efficient..
And now we look at some practicalities.
In order to find out if this source of energy could be viable in the future for the UK, a small solar array was placed in a south facing location ( the author's home ) in the Greater Manchester area to monitor the electrical output over an extended period of time. The structure is given below:
Each module, made up of 36 cells connected in series, was manufactured by the Ferranti Company, Chadderton and are specified to give an output of 15 Volts, 1 Amp in bright sunlight. The four modules, used in this investigation, failed to meet the full manufacturer's specification and only ONE module ( lower right in the illustration ) gave an acceptable output of 12 Watts, maximum, in full sunlight.
The output from each module was fed into resistive loads comprising of a 20 Ohm resistor in series with a 1 Ohm resistor. The voltage generated over the 1 Ohm resistor was used to monitor current supplied from the module and the direct voltage output from the module was stepped down to a value suitable for interfacing to a computer. All eight voltages were data logged using a Pico ADC 11 interface unit thus requiring eight inputs of the eleven provided by the interface unit. A further input was used to log the outside temperature, purely as a matter of interest.
The yearly data is summarised below
As expected, in the winter months the module gives a very low output but in summer the output is approximately 50 watt-hours per day. Since each module is 1/4 square meter an array of four should give 200 watt-hour.
The daily need of an average home is roughly 5 kwatt-hours per day so about 100 modules would be required and, at a cost of £100 per module the total cost (without installation cost) would be close to £10,000. In addition, the solar electricity would need a large amount of backup for the winter months.
In summary, it is fair to say that this type of renewable supply will be excellent for niche markets such as supplies for isolated marine or communications equipment be not for the domestic market in the UK at this present time.
Sometimes it is necessary to eat one's words.
In 2011 solar (PV) seemed to be an attracive option. Several reasons can be give: carbon reduction is always in one's mind, any invested money was giving a nill return, the Feed In Tariff looked reasonable, solar modules manufactured in China looked to be a sound investment and, as the author has reached an age of 70 plus, the consideration of a 10 year pay-back time suggested a decision should be made sooner rather than later. So, installation took place in April 2011.
It is a 4 kW array and the roof is approximately 18 o W of South. There are 21 modules ( panels) each with a peak generation capacity of 185Watt. The Up-Solar monocrystalline cells are manufactured in China and have all the warrenties that other firms offer. The inverter is a Schueco 3500T model and power is fed to the "mains" via a Generation meter. Data has been recorded manually from this meter as shown:
A rough estimate of the yearly generation total was given by the installer as 3000 kWh and this looks to be a likely figure as the readings are now in exess of 1000 kWh.
An OWL CM160 unit provides historic and current data as a clip-on sensor relays data to a PC by a radio link.
As a final note, it can be stressed that the installation was carried out by United Green Energy
in a very competent manner and there is every hope of success with this microgeneration unit.
| Index Page | Top of Page | Next Page |