Solar Cells

1. Solar Cells • Typically 2 inches in diameter and 1/16 of an inch thick • Produces 0.5 volts, so they are grouped together to produce higher voltages. These groups can then be connected to produce even more output. • In 1883 the first solar cell was built by Charles Fritts. He coated the semiconductor selenium with an extremely thin layer of gold to form the junctions. The device was only around 1% efficient.

2. Generations of Solar cells • First generation • large-area, high quality and single junction devices. • involve high energy and labor inputs which prevent any significant progress in reducing production costs. • They are approaching the theoretical limiting efficiency of 33% • achieve cost parity with fossil fuel energy generation after a payback period of 5-7 years. • Cost is not likely to get lower than $1/W.

3. Generations of Solar cells • Second generation-Thin Film Cells • made by depositing one or more thin layers (thin film) of photovoltaic material on a substrate. • thickness range of such a layer varies from a few nanometers to tens of micrometers. • Involve different methods of deposition: • Chemical Vapor deposition the wafer (substrate) is exposed to one or more volatile precursors, which react and/or decompose on the substrate surface to produce the desired deposit. Frequently, volatile by-products are also produced, which are removed by gas flow through the reaction chamber.

4. Thin Film deposition techniques • Electroplating • electrical current is used to reduce cations (positively charged ions) of a desired material from a solution and coat a conductive object with a thin layer of the material. • Ultrasonic nozzle • spray nozzle that utilizes a high (20 kHz to 50 kHz) frequency vibration to produce a narrow drop size distribution and low velocity spray over the wafer • These cells are low cost, but also low efficiency

5. The Third Generation • Also called advanced thin-film photovoltaic cell • range of novel alternatives to "first generation” and "second generation” cells. • more advanced version of the thin-film cell.

6. Third generation alternatives • non-semiconductor technologies (including polymer cells and biomimetics) • quantum dot technologies • also known as nanocrystals, are a special class semiconductors. which are crystals composed of specific periodic table groups. Size is small, ranging from 2-10 nanometers (10-50 atoms) in diameter. • tandem/multi-junction cells • multijunction device is a stack of individual single-junction cells • hot-carrier cells • Reduce energy losses from the absorption of photons in the lattice • upconversion and downconversion technologies • Put a substance in front of the cell that converts low energy photons to higher energy ones or higher energy photons to lower energy ones that the solar cells can convert to electricity. • solar thermal technologies, such as thermophotonics(TPX) • A TPX system consists of a light-emitting diode (LED) (though other types of emitters are conceivable), a photovoltaic (PV) cell, an optical coupling between the two, and an electronic control circuit. The LED is heated to a temperature higher than the PV temperature by an external heat source. If power is applied to the LED, , an increased number of electron-hole pairs (EHPs) are created.These EHPs can then recombine radiatively so that the LED emits light at a rate higher than the thermal radiation rate ("superthermal" emission). This light is then delivered to the cooler PV cell over the optical coupling and converted to electricity.

8. Efficiency and cost factors • In 2002 average cost per peak watt was $2.90-$4.00. Coal fired plant is $1.00/watt. • Efficiency is not great. • Recall, 77% of the incident sunlight can be used by the cell. • 43% goes into heating the crystal. • Remaining efficiency is temperature dependent • Average efficiency of a silicon solar cell is 14-17% • The second and third generation technologies discussed are designed to increase these efficiency numbers and reduce manufacturing costs

9. Solar Cooling • Consider a refrigeration system with no moving parts. • Heat the coolant (say ammonia gas dissolved in water) and force it via a generator into an evaporator chamber where it expands into a gas and cools. Move it to a condenser and cool it back to a liquid and repeat the process. • These systems actually have existed for a number of years, refrigerators in the 1950s were sold with this technology (gas powered and there was/is a danger of CO emissions). • Energy to heat the coolant and drive it through the system comes from burning fuel or a solar cell to provide electricity to do the heating. • Need what is called a concentrating collector (lens or other system to concentrate more light on the solar cell). • Ideally, you could do this with a flat plate collector system, though you do not obtain as much cooling. • Devices are not widely used, due to the intermittency of sunlight

11. Other renewable energy sources • Hydropower • Wind energy • Ocean Thermal • Biomass • Geothermal • Tidal

12. Hydropower • Well established electrical generation technology, about 100 years old • Known for over 2000 years that the force of moving water on a water wheel could save human labor • 13th century-hammers in iron works in Europe were operated with waterwheels. • 16th century –primary source of industrial power in Europe • In the US, mills were established at sites with reliable water flow and dams were constructed to regulate water flow. • Cave mill here in BG. Several hydro powered mills for corn, flour and sawing in the 19th century existed on this site at different times. • With the advent of electricity, water wheels were used to drive electricity generation. • About 7% of US energy generation is from hydroelectric plants

13. The physics of Hydropower • Gravitational potential energy in the water at a height h above the wheel is converted to kinetic energy of the wheel which drives a turbine and generates electricity. • So each mass element of water, m, falls a distance h and attain a velocity v. So its initial potential energy is mgh, where g is the acceleration due to gravity (9.8m/s2) and the kinetic energy is 1/2mv2. • This tells us the amount of potential energy available to be converted to kinetic energy is 9.8 joules per kilogram of water per meter of height above the wheel. • h is often called the head. • Efficiencies of 80-90% can be achieved. • Power = (Height of Dam (distance the water falls)) x (River Flow) x (Efficiency) / 11.8 where the height is in feet, river flow is in cubic feet per second , efficiency is what you expect and 11.8 converts from feet and seconds to killowatts

14. Plant operation

15. Hydro Turbine

16. Advantages • No Pollution • No waste heat • High efficiency • Plants have decades long lifetimes and low maintenance costs • Good response to changing electricity demands • Damming of rivers can serve other purposes: flood control, irrigation, drinking water supply

17. Limitations • About 50% of the US capacity for Hydro is developed • Limited lifetimes for certain reservoirs-as the fill with silt, they become less useful for water storage. But the dam must be maintained long term, if it fails, communities downstream are in danger from the tremendous volume of silt that would be released. • Loss of free flowing streams due to damming and the loss of the lands flooded by damming a river-environmental impacts • Salmon population in the Nothwest has been impacted • Flood risk due to dam failures • Currently hundreds thousands of people in danger if dam failures occur

18. Fish Ladders • Solution to the salmon problem - have not been very effective

19. Wind power • Not subject to day night cycles • Direct result of solar heating of the Earth’s atmosphere • Use of wind for energy first noticed by sailors the old sailing ships could extract the equivalent of 10,000 hp from the wind! • Windmills were prevalent in Europe in the 19th century • Several million were pumping water in the US in the early 1900s

20. WHAT YOU’RE PROBABLY THINKING OF….

21. Power in a windmill • The power in the wind can be calculated by P/m2 =6.1 X 10-4v3 • This gives the power in kilowatts per meter squared, where the cross sectional area is oriented perpendicular to the wind direction. • This is the total power, of course not all of it can be extracted. According to Betz’s Law, developed in 1919 by German physicist Albert Betz, no turbine can capture more than 59.3 percent of the potential energy in wind. • However, the total amount of economically extractable power available from the wind is considerably more than present human power use from all sources!

23. Extracting the energy: The turbine • The world's first automatically operated wind turbine was built in Cleveland in 1888 by Charles F. Brush. It was 60 feet tall, weighed four tons and had 12kW turbine.

24. Turbine types: • 2 types, based on the direction of the axis that the turbine rotates about. • Horizontal axis wind turbines (HAWT) -the turbine rotates around an axis that is horizontal. • Vertical Axis Wind Turbines (VAWT) –the turbine rotates around a vertical axis

25. HAWT • Horizontal Axis Wind Turbines • main rotor shaft and electrical generator are locate at at the top of a tower, and must be pointed into the wind. • Small turbines are pointed by a simple wind vane, while large turbines generally use a wind sensor coupled with a servo motor. • Most have a gearbox, which turns the slow rotation of the blades into a quicker rotation that is more suitable to drive an electrical generator.

26. HAWT • the turbine is usually pointed upwind of the tower since it creates turbulence behind it. • Turbine blades are made stiff to prevent the blades from being pushed into the tower by high winds. • The blades are placed a considerable distance in front of the tower and are sometimes tilted up a small amount.