Detailed information on Solar Energy

The sun is the source of all life on earth.

Thanks to the sun, there is light, warmth and food on the earth. Many kinds of energy on earth originate from and through solar radiation.

Since the sun doesn’t have the same strength at all places and all times, the earth is warmed up unequally. This causes wind which can be converted into electricity by means of wind turbines.

By evaporation we get precipitation which is carried downstream by rivers. The current of this water can provide the energy needed to generate electricity in a hydro-electric power station.

Also fossil fuels, such as coal, brown coal, oil and natural gas have their beginnings in the sun’s energy, the energy that was needed for the evolution of the microscopic forms of life of animal or vegetable origin.

The reserves of these fossil fuels, however, are limited. We must use them economically. Not only not to mortgage the future of the coming generations, but also for economic and ecological reasons.

Solar energy?

·Solar energy today?

The direct use of solar energy will very probably not be able to replace the use of fossil fuels. The quantity, however, is big enough to contribute to the future supply of energy, also in Belgium.

·The use of solar energy

Belgium gets a total annual amount of solar energy that equals approximately 50 times our yearly primary energy consumption. The bigger part of this solar energy has been used as warmth and daylight since time immemorial but has not been taken into account in the energy consumption. Man benefits from it directly but also indirectly. The sun is the main source of energy for agriculture and stock breeding. We are fed by the sun.

However, if we want to use solar energy for our energy supply, it must first be converted into an employable form of energy, e.g. heat or electricity.

In the former case we speak of “thermic solar energy”, in the latter of “photovoltaic solar energy”.

·Heat from sunlight , thermic solar energy

When the sun shines through the windows, we get free warmth into our house and the stove or heater doesn’t have to heat that much.

Another example is the solar boiler. Here we use a towards the sun directed solar collector: a black panel in which water circulates. The sun heats the water after which it is stored in a storage tank. Then the water may, whether or not warmed up, be used as hot water in the kitchen or the bathroom.

·Electricity from sunlight, photovoltaic solar energy

In our school, we mainly use the photovoltaic system which converts the sunlight directly into electricity by means of solar cells: photovoltaic solar energy or PV (photovoltaic conversion).

At first PV was mainly used in space travel. Now PV is going through quite a fast evolution. Everyone knows the calculators in which the batteries are replaced with solar cells.

There are other applications. Panels with solar cells provide light buoys and emergency telephones with electrical energy. Such appliances are already much bigger than calculators but the consumption of energy is still small.

This is the main feature of present-day applications of photovoltaic solar energy: a small energy consumption at places not connected with the public electricity grid.

Applications:

·In Belgium photovoltaic energy is mainly used at far-off places, e.g. to light summer houses, to drive water pumps in grazing lands and to provide communication and marking and signposting systems with electric current.

·In developing countries, we have a different situation. There, expansion of the grid towards the countryside is mostly too expensive. Solar energy can relatively cheaply provide energy for lighting, telecommunication, refrigeration (e.g. of medicine), water supply.

Solar energy at school:

At school, solar energy is introduced as a subject of the integrated test for the

sixth form of the electricity/electronics department.

The aim is to familiarize the pupils with solar energy because they are the ones who are going to be confronted with the applications on a smaller scale (the range of smaller capacities), and perhaps, later on , also with applications in larger units.

For this purpose, a mains-connected PV-system of approximately 1100 W maximum capacity and an independent PV-system with battery energy storage are at the disposal of the pupils.

Here with you find the working method used during the school year. However, it is not obligatory and subject to alterations.

1st term

1.Gathering of material on non-tradional forms of energy, with special

attention for PV-systems

2.Making a summary of a number of chapters from the reference book “Solar Energy and its Applications”. The chapters are, amongst others, about: Renewable energy; Sunlight, its features and dependability; PV-systems and applications.

Within the summary, cross-references are to be made to the gathered material.

All this has to be assembled in a first dossier, made up according to the BIN-standard (Belgian Institute of Normalization).

2nd term

1.Processing and editing of the results of the mains-connected system.

·study and analysis of the results

·making deductions from the measured results

1.Laboratory measurements of the independent system

·Measurement of a number of physical quantities such as

parallel resistance Rp

serial resistance Rs

open terminal voltage Vot

short circuit current Isc

filling factor VF etc.

·Measurement of the I – V-curve of

othe individual panels

othe shunt of two or more panels

·Determination of the maximum capacity point (MCP)

·Measurement of the I – V-curves during partial shade

·Long term measurements:

oof different panels

oof the shunt of two or more PV-panels

oof different load patterns (day or night applications, etc.)

All this has to be assembled in a second dossier. This dossier has to be presented and defended orally as a probation.

3rd term

·A small PV-unit has to be theoretically designed. Possibilities are:

oa PV-unit for recreational inland navigation

oa PV-unit for the lighting of a terrace

oa PV-system for the supply of energy for garden lighting and pond pump

oa PV-system for autonomous traffic lights with PLC control

o…

·A practical elaboration has to be made of:

oa reference solar cell

oan energy controller

oan air treatment installation supported by solar energy

·This elaboration must contain the following elements:

oThe flowchart drawn according to the standards a motivated choice of materials and parts

othe calculation of the PV-modules, the controller, the batteries

othe price calculation

These personal theoretical and practical elaborations are to be assembled in a third dossier and to be presented and defended before a jury.

Energy from the sunlight

Solar cells convert sunlight in electrical energy. They contain a semiconductor. Over 95% of all solar cells produced all over the world consist of crystalline silicon. The silicon is “doped” with other elements (boron, gallium, phosphorus, arsenic etc.). The upper side of the semiconductor is positively doped with impurities like boron that causes electron gaps. When sunlight falls on the semiconductor material, electrons are activated and start to jump from the negatively doped lower side into the gaps. The flow of electrons between the positively doped layer p(=positive)-conductor and the negatively doped n(=negative)-conductor can be “caught” and led into electrical cables.

With semiconductors, it is possible to convert the energy of the solar radiation into electric energy. Theoretically, each absorbed light quantum (photon) could generate an electron-hole pair. If the energy of the photon surpasses the band gap, then such a generation takes place. The surplus energy is converted into heat. In order to generate more than one electron-hole pair, the photon must provide a multiple of the energy of the band gap.

This generation process is called intrinsic photoelectric effect; in contrast to the extrinsic photoelectric effect where electrons come out of the solid, here, the electrons stay inside the solid as free charge-carriers. Since different semiconductor materials have different band gaps, a material has to be selected that is suitable to the solar radiation spectrum. The maximum photovoltaic efficiency represents the percentage of the solar radiation energy that can be transformed into charge-carrier pairs.

At present, monocrystalline (single-crystal), polycrystalline (semi crystalline) and amorphous silicon (Si) solar cells are being primarily manufactured.

Monocrystalline cells are the most important type, because they have the highest conversion efficiency (>15%), and the base material, which is extremely pure silicon, is already well established in the field of semiconductor production. The manufacturing process for monocrystalline silicon is highly energy-intensive and therefore very expensive. For this reason, in many cases polycrystalline silicon (Poly-Si) is preferred (efficiency 14%). Many houses in Germany have solar panels. The best place for these is on the roof. The solar cells must have a 30-degree angle because so the sun can shine brilliantly on the solar cells.

Solar technology plays an increasing role in the energy supply. The number of solar panels went up by 10% in the last 10 years. In the future the level of efficiency of solar cells will be even higher than today. But in the next years the level of efficiency will not change so much because there is no money for research. That means, it is not profitable to wait.

A solar energy complex is a good supplement to other energies like oil and gas. In Germany you get about half an euro for every kilowatt hour, which you give to your energy supplier. In some towns and cities you get a support if you install solar panels. How much money you get depends on how much energy your panels produce. So there is no risk in investing. But the government should support solar energy more. It should to do more research, for example for the development and for the production of solar cells with a higher level of efficiency.

Solar-thermion constructions

Solar-thermion constructions also called construction for warm water preparation.

Warming up with the help of the sun light of special liquids.

These constructions are in our latitudes more efficient then photovoltaik constructions. Because they do only need the warmth of the sun and also if the sun is darkened by clouds they still do work in contrast to the photovoltaik contractions. 100 per cent of the warm water use is supplied through the radiation of the sun during the summer time.

Function: A warm vehicle medium is being heated up by the sun, gives the warmth from the warmth transmitter into the buffer collector. A circulating pump takes care that only then the medium to the warmth transmitter is being pumped, when the difference in temperature between the temperature in the accumulator and in the collector is about 5 degrees centigrade.

With solarthermie you can produce electric power. There are different kinds of power stations, which function all similar: Because of parabolic formed mirrors the sunlight is being accumulated on an absorb machine (up to 500 degrees centigrade can be reached with accumulating collectors).

Thermooils are being pumped through the accumulating collectors which give the warmth to the water vapor circle in which the water evaporates and through this energizes a turbine, which is connected with a generator, which changes the mechanical energy into electrical energy.

Collectors: There are flat collectors, vacuum tube collectors and concentrating collectors (power station).

Function of the flat- and vacuum tube collectors: The collector collects the sun radiation and transforms it into warmth. The collector is connected through dammed plumbing in which a warmth vehicle medium (air, liquid or gas plus antifreeze) is being pumped.

Flat collectors reach “only” temperatures up to 150 degrees centigrade. In contrary reach vacuum tube collectors up to 300 degrees centigrade.

The main difference between flat- and vacuum tube collectors is the warmth damming and the colloq costs (vacuum tube collectors are more expensive).

Recommended is also the heating up of swimming pools through solarthermie. A accumulator is not necessary because the water in the swimming pool takes care this function. It is being pumped directly out of the pool and is brought into the collector circle where it is heated up and flows back.