Renewable Energy Solar Energy

Solar Electricity Two ways of harvesting solar energy Solar –thermally generated electricity- complex collectors to gather solar radiation to produce temperature high enough to drive steam turbines to produce electric power, Solar Photovoltaic energy- direct conversion of sun’s ray to electricity- single junction silicon solar cell has 19% efficiency

Radiance from black body Where El is the emissive power per unit area; l wavelength (mm); T temperature (Kelvin)

Feed in Tariff – Discuss critically whether wind energy is suitable for Malaysia-( discuss on policy, geographical and technology) How “ feed in tariff” can be exploited as a good business.

Sun Radiation Visible light has a wavelength of between 0.40 to 0.71micrometers (μm). The sun emits only a portion (44%)of its radiation in this range. Solar radiation spans a spectrum from approximately 0.1 to 4.0 micrometers. About 7% of the sun's emission is in 0.1 to 0.4 micrometers wavelength band (UV). About 48%of the sun's radiation falls in the region between 0.71 to 4.0micrometers (near infrared : 0.71 to 1.5 micrometers; far infrared: 1.5 to 4.0 micrometers).

Total power By consider the area under the curve we arrive a formula Where s is the Stefan-Boltzmann constant= 5.67 x 10-8 W/m2 K4 and A is the surface area of black body. For sun, the interior temperature is about 15 million Kelvin .

Photovoltaic (PV) System Solar energy converted directly into electricity by means of solar cell From cell can be formed into module, panel or array

PV cell Made of from pure crystalline silicon and some from GaAs. Four common types of silicon PV cells Single crystal silicon Polycrystal silicon known as multicrystal silicon Ribbon silicon Amorphous

Single –crystal silicon Most photovoltaic cells are single –crystal types. Silicon is purified , melted and crystallized into ingots. Then sliced into thin wafers to make individual cell. The cell is attached to a base, layer of metal sheet to provide electrical contact . The cell is mostly positive charge. The top, slightly negative charge is open to sunlight, a thin grid apply for electrical contact but adequate enough to admit sunlight .

PV structure

Mechanism As sunlight (photon)strikes a photovoltaic cell, photon move into the cell, given extra energy to electrons, dislodge them , leaving empty “holes”. The loose electrons move out of the cell and into external electrical circuit. The electrons from the back move up to fill the holes. One cell produces voltage about 0.5V regardless of surface area but larger surface area produces more current

Polycrystalline cell Polycrystalline cells are manufactured and operate in a similar manner. The difference is that a lower cost silicon is used. This usually results in slightly lower efficiency, but polycrystalline cell manufacturers assert that the cost benefits outweigh the efficiency losses. The surface of polycrystalline cells has a random pattern of crystal borders instead of the solid color of single crystal cells

Ribbon silicon Ribbon-type photovoltaic cells are made by growing a ribbon from the molten silicon instead of an ingot. These cells operate the same as single and polycrystal cells. The anti-reflective coating used on most ribbon silicon cells gives them a prismatic rainbow appearance.

Amorphous or thin silicon The previous three types of silicon used for photovoltaic cells have a distinct crystal structure. Amorphous silicon has no such structure. Amorphous silicon is sometimes abbreviated "aSi" and is also called thin film silicon. Amorphous silicon units are made by depositing very thin layers of vaporized silicon in a vacuum onto a support of glass, plastic, or metal. Because the layers of silicon allow some light to pass through, multiple layers can be deposited. The added layers increase the amount of electricity the photovoltaic cell can produce. Each layer can be "tuned" to accept a particular band of light wavelength.

Con’t The performance of amorphous silicon cells can drop as much as 15% upon initial exposure to sunlight. This drop takes around six weeks. Manufacturers generally publish post-exposure performance data, so if the module has not been exposed to sunlight, its performance will exceed specifications at first. The efficiency of amorphous silicon photovoltaic modules is less than half that of the other three technologies. This technology has the potential of being much less expensive to manufacture than crystalline silicon technology. For this reason, research is currently under way to improve amorphous silicon performance and manufacturing processes

Module Usually contains 36 cells connected in series in order to obtain 12V ( actually more voltage as 36 x 0.5V=18V). Some contains 72 cells in series for 24V. Some may be connected in parallel to increase current. So a combination of series and parallel can increase both voltage and current. By adding balance system components such batteries, charge controller and power conditioning devices form a complete photovoltaic system

Photovoltaic system to supply DC and AC DC supply DC motor AC supply

Module performance A photovoltaic module will produce its maximum current when there is essentially no resistance in the circuit. This would be a short circuit between its positive and negative terminals. No infinity current like batteries or grid. So there is no harm when shorted. This maximum current is called the short circuit current, abbreviated I(sc). When the module is shorted, the voltage in the circuit is zero.

PV performance con’t Conversely, the maximum voltage is produced when there is a break in the circuit. This is called the open circuit voltage, abbreviated V(oc). Under this condition the resistance is infinitely high and there is no current, since the circuit is incomplete.

PV I-V characteristic

PV characteristic Power is zero at V=0 and I=0 points Current almost constant up to the knee of the curve which is around 17V. Thus the maximum power is 17 x 2.5Amp= 42.5W. This is under normal condition. If the there some shaded condition, illuminate unevenly or some cell is malfunction, the performance is reduced and may affect other cells. Also under standard sunlight condition 1000W/m2. Less than this condition reduces the current

Under one-half sun condition reduces the current

Affect of shaded module Shaded reduces the current

Temperature reduces the voltage by 0.04V to 0.1V per 1o C

Charging batteries .PV must supply a higher voltage than a battery . If the battery is deeply discharge then the photovoltaic module can charge the battery with a low voltage, shown as point #1 in the Figure. As the battery reaches a full charge, the module is forced to deliver a higher voltage, shown as point #2. The battery voltage drives module voltage.

PV array For more current the module is connected in parallel. Diode is used to stop a reverse current from battery to PV (which will make battery to drain)

PV array If the module connected in series, then the voltage will increase while the current will be maintained low. If one module fail, then the other module may not able to operate. To avoid this, bypass diode will allow other module to operate.

PV array A combination will increase voltage as well as current. Isolation diode may also use to prevent power from rest of the array to flow into the damage series string one.

Tracking array Arrays that track, or follow the sun across the sky, can follow the sun in one axis or in two . Tracking arrays perform best in areas with very clear climates. This is because following the sun yields significantly greater amounts of energy when the sun's energy is predominantly direct. Direct radiation comes straight from the sun, rather than the entire sky.

Tracking array Tracking sensor usually using PV cell. By detecting shade, the motor will adjust the array so the both cells will not receive shade