Friday, November 22, 2002

PV System architecture Solar cells alone cannot produce usable power. They need to be interconnected with other system components that ultimately serve a specific electrical demand, or ‘load’. PV systems can either be stand-alone, or grid-connected. The main difference between these two basic types of systems is that in the latter case, the PV system produces power in parallel with the electrical utility, and can feed power back into the utility grid if the onsite load does not use all of the PV system’s output. When the sun is shining, the direct current electricity (DC) from the PV modules is converted to alternating current (AC) by the power of an electronic inverter, and then fed directly into the building power distribution system where it supplies electric power. Any excess solar power is exported to the utility power grid and any shortfall is made up with electricity supplied by the grid. During non-sun hours, the building load is supplied by utility power alone. Following are the basic components of a PowerLight grid-connected photovoltaic system, which is illustrated in the diagram above: PV modules A number of photovoltaic cells electrically interconnected and mounted together, usually in a sealed unit of convenient size for shipping, handling and assembling into arrays. The term "module" is often used interchangeably with the term "panel." Mounting technology/equipment Used to mount the PV modules in place. Depending on the application, the PV modules can be mounted on rooftops, in parking structures, covered reservoirs and in open fields. Combiner box Where the electrical wiring from the PV modules is joined together in parallel to combine electrical currents. Inverter Converts the DC output of the PV system into usable AC output that can be fed directly into the building load. Transformer Used to step up or down the voltage emerging from the inverter to match the required voltage of the onsite load or the utility interconnection. Load The amount of electrical demand used in the building at any given time. For a detailed description of how photovoltaics work, please click here. 1:30:21 PM    comment []

How Solar Cells Work Solar cells are converters. They take the energy from sunlight and convert that energy into another form of energy, electricity. Solar cells convert sunlight to electricity without any moving parts, noise, pollution, radiation, or maintenance. The conversion of sunlight into electricity is made possible with the special properties of semiconducting materials. Semi-Conductors Most solar cells are made from silicon, the 14th element. Silicon is a "semi-conductor" or a "semi-metal," and has properties of both a metal and an insulator. Atoms in a metal have loosely bound electrons that easily flow when electrical pressure is applied, whereas atoms in an insulator have tightly bound electrons that cannot flow when electric voltage is applied. Atoms in a semi conducting material bind their electrons tighter than metals, but they may be manipulated to have conductive properties. Solar cells are made by joining two types of semiconducting material: P-type and N-type. P-type semiconductors are manufactured to contain negative ions, and N-type semiconductors to manufactured to contain positive ions. The positive and negative ions within the semiconductor provide the environment necessary for an electrical current to move through a solar cell. Sunlight Converted At the atomic level, light is made of a stream of pure energy particles, called "photons." This pure energy flows from the sun and shines on the solar cell. The photons actually penetrate into the silicon and randomly strike silicon atoms. When a photon strikes a silicon atom, it ionizes the atom, giving all its energy to an outer electron and allowing the outer electron to break free of the atom. The photon disappears from the universe and all its energy is now in the form of electron movement energy. It is the movement of electrons with energy that we call "electric current." Sunlight to Electricity A typical solar cell consists of a glass cover to seal the cell, an anti-reflective layer to maximize incoming sunlight, a front and back contact or electrode, and the semiconductor layers where the electrons begin and complete their voyages. The electric current stimulated by sunlight is collected on the front electrode and travels through a circuit back to the solar cell via the back electrode. 1:29:46 PM    comment []

World energy consumption is expected to increase 57 percent by 2020 and to double or triple by 2050. The U.S. Energy Department expects most of the increase in energy production to 2020 will come from oil, natural gas, and coal, fossil fuels which emit greenhouse gases when burned. If so, carbon emissions are expected to rise to 9.9 billion metric tons by 2020, more than doubling emissions of the past 20 years. Developing countries are projected to pass the industrial countries in total carbon emissions by 2015. 10:48:31 AM    comment []

Future strategies for carbon sequestration include injecting carbon dioxide into the earth or into the ocean, separating the gases from the air and storing them by planting trees, and using chemistry to produce new products from these gases, such as methanol to fuel hybrid cars, and as an energy source for fuel cell cars. 10:48:03 AM    comment []