26. Disadvantages of Solar Energy
oInefficient and costly equipment
oPart Time
oReliability Depends On Location
oEnvironmental Impact of PV Cell
Production
Editor's Notes
The Sun is 93 million miles away.
A tiny fraction of the Sun’s energy hits the Earth (~a hundredth of a millionth of a percent) is enough to meet all our power needs many times over. In fact, every minute, enough energy arrives at the Earth to meet our whole demands for a year.
We call the energy from the sun, solar energy.
Just the tiny fraction of the Sun’s energy that hits the Earth.
Solar energy is transmitted to the earth in the form of radiant energy.
It is vital to us because it provides the world—directly or indirectly– with almost all of its energy.
In addition to providing the energy that sustains the world, solar energy is stored in fossil fuels and biomass, and is responsible for powering the water cycle and producing wind.
Solar energy is radiation produced by nuclear fusion inside the sun’s core.
It takes millions of years for the energy in the sun’s core to make its way to the solar surface.
It takes 8 minutes to travel 93 million miles to earth. (186,000 miles per second)
The Greenhouse effect traps some of the heat making life on earth possible.
A car is a good example of how solar energy can be collected and put to use.
Because solar energy is radiant energy, no heat from the sun travels to the earth.
The radiant energy turns into thermal energy (heat) when it is absorbed by molecules on Earth.
In a car, the insides of the car(sets, floors, walls, etc.) absorb sunlight and change it to heat.
The glass window lets the light in but does not let the heat out.
Even though the Sun provides enough energy arriving at the Earth to meet our demands for a whole year, this energy is spread all over the surface of the earth, is intermittent, and therefore require a large area to collect a useful amount.
There are four factors that affects how much energy any location on the earth receives:
The amount of energy delivered to any places dependent on time of day, season, latitude, and cloudiness.
Due to these four factors, the average number of peak sun hours per day varies from one area of the country to another.
Solar insolation is the solar radiation incident on an area over time.
Solar insolation is equivalent to solar energy and is usually expressed in kilowatts-hours per square meter
The four technologies employed to make use of solar energy are:
Daylighting- the use of natural sunlight to brighten the building’s interior.
Passive Solar Heating- takes advantage of Sun’s warmth and materials that absorb that warmth during the day/release it at night when heat is needed.
Active Solar Heating- solar collectors concentrate the sun’s power on dark color plates that absorb heat. Air or liquid flows through tubes and warmed by the plates.
Concentrating Solar Thermal- mirrors direct sunlight on one point. Water is turned into steam with this heat. The steam turns a turbine to create electricity.
Photovoltaic(PV)- converts sunlight directly to electricity.
These homes in Montana and California with a passive solar design heats the home in the winter and cools the home in the winter.
Langston High School in Arlington, VA and Caywood Elementary in Kentucky use daylighting to keep energy costs down.
Active solar heating is much like passive solar heating except it takes the power of the sun and amplifies it.
The people living in this house enjoy heated water using a solar thermal system.
Flat plate solar collectors are usually large flat boxes with one or more glass covers. Inside the boxes are dark colored metal plates that absorb heat. Air or liquid, such as water, flows through the tubes and is warmed by heat stored in the plates. These systems are particularly useful for providing hot water to households.
A clean, large-scale solar thermal technology known as concentrating solar power is used in special power plants (Concentrating Solar Power or CSP plants) that use different kinds of mirror configurations to convert the sun's energy into high-temperature heat. The heat energy is then used to generate electricity in a steam generator.
World’s largest solar power tower in Seville, Spain.
Solar power tower consists of a large field of sun tracking mirrors, called heliostats, which focus solar energy on a receiver atop of a centrally located tower. The enormous amount of energy, coming out of the suns rays, concentrated at one point (the tower in the middle) produces temperatures of approx. 550 C TO 1500 C.
The gained thermal energy can be used for heating water or molten salt, which saves the energy for later use. Heated water gets to steam, which is used to move the turbine generator. This way thermal energy is converted into electricity.
This graphic shows how the power tower is used to heat molten salt which is used to heat water to produce steam to turn a turbine which produces electricity.
Molten salt is used to transfer the heat because the heat can be stored and used when the sun is behind the clouds or at night.
Photovoltaic systems convert sunlight directly into electricity, and are potentially one of the most useful of the renewable energy technologies.
Also known as solar cells, PV systems are already an important part of our lives. The simplest systems power many of the small calculators and wrist watches we use everyday.
The photovoltaic cell is the basic building block of a PV system.
Individual cells can vary in sizes from about 1cm to about 10 cm across.
Most cells are made with silicon today.
Silicon must be purified– this is one of the biggest expenses in the production of solar cells.
A PV cell is made from a thin disc of almost pure silicon crystal called silicon wafer.
A small amount of boron is added. The boron gives the crystal structure a positive electrical characteristic. Since this part has a positive characteristic it is referred to as a “P” type silicon and it forms the base of the cell.
A thin layer of silicon crystal is formed over the disc of “P” type silicon. This time a small amount of phosphorous is added to the mixture. The phosphorous mixture creates a negative characteristic and thus is referred to as an “N” type silicon.
When light penetrates to the junction of the “N” and “P” type silicon layers it creates a flow of electrons throughout the crystal structure. This flow of electrons occurs because sunlight is composed of photons, or particles of solar energy. When sunlight strikes a PV cell, some photons are absorbed. When enough sunlight (energy) is absorbed by the material (called a semiconductor), electrons are dislodged from the materials’ atoms.
A crystal structure of silicon contains empty areas which accept the electrons.
As one electron moves to fill a hole, it created another hole.
It is the flow of these electrons that produces electricity.
The conversion efficiency of a PV cell is the proportion of sunlight energy that the cell converts into electrical energy.
This is very important because improving this efficiency is vital to making PV energy competitive with more traditional sources of energy, such as fossil fuels.
The first PV cells were converting light to electricity at 1 to 2 percent efficiency.
Today’s PV devices convert up to 17 percent of the radiant energy that strikes them into electric energy. (40% NREL)
One PV cell only produces 1 or 2 watts of electricity, which isn't enough power for most applications.
To Increase power groups of solar cells are electrically connected and packaged into packaged weather-tight modules and arrays to provide useful output voltages and currents to provide a specific power output.
A PV System typically consists of 3 basic components.
PV cells - Electricity is generated by PV cells, the smallest unit of a PV system,
Modules - PV cells are wired together to form modules which are usually a sealed, or encapsulated, unit of convenient size for handling.
Arrays – Groups of panels make up an array.
Solar PV System Solar cells produce direct current (DC), therefore they are only used for DC equipments. If alternating current (AC) is needed for AC equipments or backup energy is needed, solar photovoltaic systems require other components in addition to solar modules. These components are specially designed to integrate into solar PV system, that is to say they are renewable energy products or energy conservation products and one or more of components may be included depending on the type of application. The components of a solar photovoltaic system are:
Solar Module is the essential component of any solar PV system that converts sunlight directly into DC electricity.
2. Solar Charge Controller regulates voltage and current from solar arrays, charges the battery, prevents battery from overcharging and also performs controlled over discharges.
3.Battery stores current electricity that produces from solar arrays for using when sunlight is not visible, nighttime or other purposes.
4. Inverter is a critical component of any solar PV system that converts DC power output of solar arrays into AC for AC appliances.
5. Lightning protection prevents electrical equipments from damages caused by lightning or induction of high voltage surge. It is required for the large size and critical solar PV systems, which include the efficient grounding.
A PV system produces DC-current.
The DC current goes into an Inverter where it becomes AC current.
An inverter is connected to your home’s or building’s electric circuit and is also your meter.
You use all electricity needed, while all excess electricity goes into the grid.
Electricity that goes into the grid, is purchased from you by your utility company through Net Metering, usually at a retail price.
While this option is not offered by many utility companies, the number is growing. In 2002, there 4,472 customers who participated in net metering programs. In 2007, there were 48,820 customers using the net metering program.
In order to generate large amounts of electricity which can be fed into the electric grid, large number of arrays can be wired together to form an Array Field.
Some utility companies in the U.S. are turning to large PV systems to help meet peak power demand and reduce the need for building new power plants.
Photovoltaic system is ideal for remote applications whether other power sources are impractical or unavailable, such as in the Swiss Alps or on navigational buoys. It is not practical to connect these applications to an electric grid.
