Dedicatoria

Solar energy

• The scientific definition for energy would be something like: the capacity of a system to perform work.

• The unit of energy is Joule.

• Power is the energy per time and is expressed in Watts.

• 1 Watt is 1 joule per second.

• To express the electrical energy: kiloWatthours.

• 1 kWh equals the energy of a power of 1 kW being used for 1 hour.

• 1kWh equals 3.6 million Joules.

• By burning fossil fuels roughly 90% of the chemical energy is converted into heat.

• We can convert solar energy into electricity, heat and chemical energy.

• Renewable energy sources are energy sources that are replenished by natural processes at a rate comparable or faster than its rate of consumption by humans.

• Consequently, Hydro-, wind- and solar energy are renewable energy sources.

• Fossil fuels and nuclear energy are not renewables, as their fuels are consumed faster than they are generated in nature.

• 65% of the electricity is coming from fossil fuels, where coal is the dominant contributor.

• Unfortunately, coal produces roughly two times more carbon dioxide per generated unit of energy in reference to gas.

• In 2007, 20200 TWh of electricity was generated world-wide.

• If we consider a 0.5 GW nuclear power plant, it means we need around 5000 nuclear plants around the world if our electricity needs would be fully covered by nuclear power.

Energy conversion

Since the power provided by the sun in Spain is \(180 W/m^2\) and the solar panel has an efficiency of \(20\%\) then only \(20\%\) can be utilized from the provided power. That is \(0.2*180\ = 36 W/m^2\).

Each intermediate conversion accounts for a certain loss. The solar energy is converted to heat, the heat is converted to mechanical energy, the mecahnical energy is converted to electrical energy. To answer this question you have to multiply the efficiencies of the intermediate conversion steps by the power provided by the sun. That is \(0.4*0.45*0.9*180 = 29.16 W\).

Photovoltaics

• The mechanism in which solar light is directly converted into voltage or current is called the photovoltaic effect.

• The light is transmitted into the absorber layer where its energy is absorbed.

• The energy is used to excite charge carriers in the semiconductor material, which are a negatively charged electron, indicated by the red dot and a positively charged hole, indicated by the blue dot.

• These charge carriers diffuse around and need to be separated,which occurs at the depletion region between the n- and p-type doped siliconand the depletion region at the back of the solar cell.

• Then the charge carriers have to be collected at the contacts.

• In this example the contacts are connected with a load, in this case a lamp.

• The electron will move through the load back to the solar cell.

• Both charge carriers recombine at the metal/p-layer interface.

• It shows that the photovoltaic proces is based on three important principles:

• the first is excitation of free mobile charge carriers due to light absorption,

• the second is separation of the charge carriers

• and the third one is collection of the charge carriers at the contacts.

Conversion efficiency

• The conversion efficiency is the fraction of the energy in the solar light, which is converted into electricity.

• The efficiency scales with the energy yield of the solar cell.

• The larger the efficiency, the larger the generated power per area will be.

• This power is expressed in Watts per square meters.

• The cost price per Watt corresponds to the cost price of the energy generated by the solar cell. To compete with other energy sources, you would like that your PV technology overlaps with the steepest lines in this graph.

First generation PV technology

• The first generation PV technology is based on using very pure bulky semiconductor materials, like crystalline silicon.

• Pure materials means less defects and in general solar cells with a relative high efficiency

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