How Solar Energy Becomes Electricity
With so much sunlight reaching our planet every day, it seems incredible how mankind has not yet found a truly efficient way to harness all the free solar energy and use it for every day’s needs.
A lot of progress has been made in the last 60 years, and researchers are finding more efficient and cost effective ways to produce electricity from solar energy, but much still needs to be done before it can be used by every family.
It is every ecologist’s dream that one day we will completely abandon carbon fuels and nuclear plants, that produce extremely hazardous and contaminating waste, and switch completely to renewable energy sources.
Photovoltaic (PV) cells
PV were first developed in 1939, but were not used until the 1950s. The first photovoltaic silicon solar cell was used to power a transistor radio, in 1954, by the Bell laboratories. Soon after PV cells were used as as power source for satellites.
Technical details of a solar power system
A single PV cell is a thin semiconductor sandwich, with a layer of highly purified silicon. The silicon sandwich presents surplus of electrons on one side and a deficit of electrons on the other side.
When the sandwich is stricken by sunlight, photons from the sunlight knock loose some of the excess electrons that tend to migrate to the deficit side. This creates a voltage difference that in each silicon cell is only 0.5 volt.
Metallic contacts are made on both sides of the wafer. When an external circuit is attached, the electrons prefer to take the long way around trough the metal, rather than struggle through the silicon, generating a complete circuit and current flow. The PV cells act as a pump for electrons, with no storage capacity.
The amount of current is determined by the number of electrons that the solar photons knock off. We can get more electrons by:
- Using bigger cells
- Using more efficient cells
- Exposing our cells to more intense sunlight
To make an efficient solar cell, we try to maximize absorption, minimize reflection and recombination, and thereby maximize conduction.
There are practical limits to size, efficiency, and how much sunlight a cell can tolerate.
Photovoltaic Module - What Is It?
A PV module consists of many PV cells wired in a series to produce a higher voltage. Industry standard are typically modules made of 36 cells. Recently the standard production includes 72-cell modules.
The module is enclosed in tempered glass (or some other transparent material) on the front and a waterproof material on the back.
Photovoltaic Array - What Is It?
A PV array consists of a number of PV modules that have been wired together to deliver higher voltage.
An array can be as small as two modules wired together, or large enough to cover acres.
Advantages of solar energy
- Environment friendly. Solar energy is a “clean” energy resource: it does not involve the emission of greenhouse gases.
- Solar energy is a renewable and inexhaustible resource.
- Because PV systems don’t rely on exposed wires, residential PV systems are more reliable than utilities, especially in severe weather.
- PV modules have no moving parts, so there is no deterioration due to movements.
- PV systems degrade very, very slowly; their lifespan is of several decades. In fact, factory warranty is usually of 25 years.
- No fuel is needed.
- They are maintenance free, only need to be hosed down sometimes.
- PV systems are a great way to power units in remote locations, from homes in isolated areas, to satellites.
Disadvantages of solar energy
- Initial cost for installing a solar power system is quite high.
- Efficiency of solar panels is related to the amount of sun received.
- No energy is produced over the night times, though storage batteries can solve this problem.
- Geographically, it cannot be installed in any place. Solar power system's efficiency is related to the latitude and to the weather of the area.
Efficiency of solar panels
PV modules do not convert 100% of the solar energy that strikes them into electricity. The average is about 12-14% of conversion efficiency. Conversion rates of over 20% have been achieved in laboratories by using experimental cells made with esoteric and rare elements, but those are too expensive to be commercialized.
The first PV systems were very expensive, and had an efficiency of about 6%. The cost of PV has come down enormously, and now they are quite affordable. Today's single-crystal silicon modules have shown an increased efficiency of 17-19%, while multi-junction photovoltaic cells hold the current record exceeding 40%, according to National Renewable Energy Laboratory (NREL).
Modules installed under average US conditions reach energy payback in three to four years. Quicker energy paybacks are expected in the future, as more “solar grade” silicon becomes available.
How a household solar system works
1. Solar panels convert solar energy into direct current, and send it to the inverter.
2. Charge controller feeds the solar energy from the panel to the batteries, without overcharging the batteries.
3. Rechargeable batteries (optional) store solar energy to provide electricity when solar energy produced is not enough: peak usage, night, cloudy days.
4. Power inverter transforms the direct current into alternated current, suitable for household use.
Video on how a household solar system works
Net metering is a special agreement between utilities and their customers that allows small renewable energy systems to connect to the power grid.
When a net metering customer produces more energy than what is needed, the electric meter runs backward generating credits.
Net metering customers are charged only for the “net” power that they consume from the electricity service provider or, if their energy-generating systems make more electricity than is consumed, they may be credited or paid for the excess electricity contributed to the grid.
© 2012 Robie Benve