Download
1 / 44
E N D
Solar Energy The Sun emits energy in all directions, all the time at a rate of 64 megawatts per square meter. Most of that energy is sent out to the vastness of space, but a very small fraction of it arrives here on Earth. In fact, the top of the Earth's atmosphere receives around 1367 W per square meter. This is called the solar constant. We can harvest this energy in many ways to meet a substantial portion of our energy needs.
Solar Energy is an energy flow Many different forms of energy ultimately come from the Sun • Solar heating • Photovoltaic • Wind • Biomass • OTEC • Fossil fuels • Other?
How Much Energy is Available? • Solar constant ~ 1367 W/m2. • When averaging over the whole planet, reduce by ½ because ½ of earth is night, reduce by another ½ because earth is curved. (This is still at top of atmosphere.)
Only approximately 49% of the sunlight at the top of the atmosphere actually reaches the surfaces. (We’ll usually assume 50%)
Power Density at the Surface • Average energy at the surface has been reduced by a factor of 8. • Average power over the entire planet is approximately 170 W/m2 . • Obviously, the power density depends on where you are, but this is a global average.
Solar Energy Solar power systems covering the areas defined by the dark disks could provide more than the world's total primary energy demand in 2006 (assuming a conversion efficiency of 8%). That is, all energy currently consumed, including heat, electricity, fossil fuels, etc., would be produced in the form of electricity by solar cells. The colors in the map show the local solar irradiance averaged over three years from 1991 to 1993 (24 hours a day) taking into account the cloud coverage available from weather satellites. http://www.ez2c.de/ml/solar_land_area/
Total Solar Energy in US. • The total solar insolation over the entire US is approximately 5.61019 Btu/year. • We use approximately 941015 Btu/year. • Thus , we get around 600 times more energy from the sun than we use every year.
Solar Electricity: Technique 1 • Use mirrors to focus sunlight onto water. • Boil the water • Use a standard heat engine. The shape of the mirror causes all of the light rays to pass through a single point called the focus. Parabolic Mirror
Seville, Spain 11 MW plant, May 2007
Heliostat • Uses movable mirrors to focus light on a tower. • Similar to parabolic mirror. • Solar One Facility in Barstow, CA generates ~10MW
Solar Chimney Air in a very large greenhouse (2 to 30 km diameter) is heated by the sun and channeled to a tall chimney where there is a wind turbine. Manzanares, Spain
Photovoltaics • Use Semiconductors to directly convert sunlight into electricity. • The theoretical limit is that 77% of sunlight can be used to produce solar electricity.
Semiconductors, Doping, & pn Junctions for silicon p-type: B or Al n-type: P, Ar, or Sb
Basic Solar Cell • Photons impinging on the surface of the solar cell can do the following: • be reflected (not good) • transmit through with no interaction (low energy photons) - (not what we are after) • be absorbed (good) generates heat (oops – bad) • create electron—hole pairs (very, very good) • electron-hole Recombination (bad)
Basic Si Solar Cell • Regardless of size, a typical silicon photovoltaic (PV) cell like the one above exhibits 0.5 – 0.6 volt DC under open-circuit, no-load conditions. The current (and power) output of a PV cell depends on its efficiency and size (surface area), and is proportional the intensity of sunlight striking the surface of the cell. • For example, under peak sunlight conditions a typical commercial PV cell with a surface area of 160 cm2 will produce about 2 watts peak power. If the sunlight intensity were 40 percent of peak, this cell would produce about 0.8 watts.
Efficiency Limits: Some Solutions Surface structuring to reduce reflection loss: for example, construction of the cell surface in a pyramid structure, so that incoming light hits the surface several times. New material: for example, gallium arsenide (GaAs), cadmium telluride (GdTe) or copper indium selenide (CuInSe²). Tandem or stacked cells: in order to be able to use a wide spectrum of radiation, different semiconductor materials, which are suited for different spectral ranges, will be arranged one on top of the other. Concentrator cells: A higher light intensity will be focused on the solar cells by the use of mirror and lens systems. This system tracks the sun, always using direct radiation. MIS Inversion Layer cells: the inner electrical field are not produced by a p-n junction, but by the junction of a thin oxide layer to a semiconductor. Grätzel cells: Electrochemical liquid cells with titanium dioxide as electrolytes and dye to improve light absorption.
Paths to Higher Efficiency • Multi-junction Cells • Concentrators • Nanotechnology Approaches
Junction Choices S. Kurtz, NREL
Multijunction Cells Multi junction cells take greater advantage of the solar spectrum. Each layer is typically made of a III-V semiconductor. The top layer has the highest band gap, only the most energetic light is absorbed, and any lower energy light passes through onto the next layer. Each layer has a lower band gap than the layer above it. The most common multi junction cells currently in production have three layers. These triple junction cells are made of GaInP, GaAs, and Ge, which have band gaps of 1.8 eV, 1.4 eV, and 0.7 eV, respectively. They are often referred to as Gallium Arsenide Triple Junction cells.
Intermediate Band PVs One next generation solar cell is based on nanoscale sized particles called quantum dots.Solutions of 2 to 7 nm CdSe nanocrystals, with increasing diameter from left to right, are shown. The semiconductor band gap increases with decreasing nanocrystal size. (Yu. P. Rakovich, J. F. Donegan, S. A. Filonovich, M. J. M. Gomes, D. V. Talapin, A. L. Rogach, A. Eychmüller. Physica E , 17, 99-100.2003)
