Something new under the sun
Photovoltaics - plastics take over
Photovoltaics, the heir to 60s-era, silicon-based space technologies, have long prioritised performance over cost. Since the crisis of 2008 and the rise of Asian manufacturers, the industry has been focussing its efforts on semi-conductive materials that are cheaper to produce and implement. Although their performance, at equal surface measurements, is lower than that of first generation silicon cells, the cost of electricity remains the deciding factor.
In this context, plastics, which have long been relegated to a passive role, have become more active in the conversion of photovoltaic energy. The ethylene vinyl acetate (EVA) used to cover crystalline silicon cells and polyvinylidene fluoride (PVDF) used to protect the underside of the panels were joined by polymethylmethacrylate (PMMA). The latter plays a prominent role, both literally and figuratively, in covering "low concentration" photovoltaic modules. This highly transparent polymer enables light to be focused on tiny cells. As a result, the cells produce more energy on panels containing three times less silicon. Additionally, they are, therefore, much lighter too!
The photovoltaic effect
The photovoltaic effect, i.e. transforming solar energy into electricity, is a physical phenomenon inherent to certain materials: semiconductors which produce electricity when exposed to light.
The basic element of this process, the photovoltaic cell, is made up of two layers of a semiconductive material, such as silicon, each covered with a metallic electrode. The semiconductor's conductivity is improved with phosphorus and boron in order to negatively charge one side by increasing the number of electrons, and reducing the other side's electron count to make it positively charged.
All that's left to be done is to connect the electrodes in order to enable the molecules on each layer, once they have been excited by the light, to recombine their electrons through the circuit, thus producing an electrical current.
Towards 100% organic photovoltaics
"Thin-film" technologies involve dropping a few microns of semiconductive metallic materials on a cheap, flexible or rigid medium. They have paved the way for new applications for certain polymers such as polyimides and fluoropolymers which are the only ones able to withstand the temperatures used in vacuum evaporation and laser processing.
The advent of 100% "organic" photovoltaics is a new step forward. Now, both the medium, and the cells, are made up of two semiconductive polymers. The first type, polythiophene, is used as an electron donor. The other type, fullerene, is the acceptor. They are sandwiched between two thin metallic films which act as electrodes. .
Organic panels are manufactured through coating processes or continuous printing processes. These techniques require fewer materials and have contributed to considerably reducing the cost of the panels. The only drawback is their lower life expectancy which is currently a maximum of five years with a conversion rate of 4 to 6% which is a far cry from the 20 years of energy with a yield of 15 to 20% obtained with good old silicon panels.
Polymers are spreading out
The Swiss Centre of Electronics and Microtechnology is gearing up to sell a type of white siding whose nanostructured polymer surface is able to convert the light captured over the entire surface of a building's envelope. The University of Michigan has just recently developed a transparent photovoltaic glazing made up of organic molecules which only converts the invisible spectrum of light rays.
Power and consumption are two different things
The watt (W) is the unit used to quantify power, energy or heat. In electricity, the watt is the unit of power of a system outputting or absorbing an intensity of 1 ampere at a voltage of 1 volt. • The kilowatt (kW), 1,000 watts, is the unit used for the power of electric or internal combustion engines. • The megawatt (MW), one million watts, is the reference unit for high-power generators such as the Rance tidal power plant whose output is 240 MW. • The gigawatt (GW), one billion watts, is used for nuclear reactors whose average output is 1 GW. • The terawatt (TW), one trillion watts, is used for evaluating power on a global scale.
The kilowatt (kW) and the kilowatt-hour (kWh)
The kilowatt (kW), the unit of power, and the kilowatt-hour (kWh), the unit used to measure the energy consumption of a device using 1,000 watts (1 kW) per hour, should not be confused.
With 1kWh, you can watch 3 to 5 hours of TV, work on a portable computer for a day and a half, run the fridge for a day, cook a chicken in the oven, shave twice with warm water, start a laundry cycle or have lighting for a day and half, etc.
A few figures
• Television: power = 80 to 300 W – annual consumption approx. 88 kWh
• Washing machine: power = 500 W to 3 kW - annual consumption approx. 406 kWh
• One person in a studio apartment: installed power 3 kW - annual consumption approx. 3,500 kWh
• A family of four in a house: installed power = 12 to 15 kW - annual consumption approx. 22,500 kWh (15,000 kWh for heating + 4,000 kWh for hot water + 3,500 kWh for household electricity (lighting, appliances, etc.)
• A town of 2,000 inhabitants: installed power approx. 2 to 3 MW - annual consumption approx. 4.5 GWh