circulated though the pipes. The HTF then drives a conventional steam-power process. The solar central receiver or power tower is surrounded by a large array of two-axes tracking mirrors (heliostats), which reflect direct solar radiation onto a fixed receiver located on the top of the tower. Within the receiver, a fluid (water, air, liquid metal and molten salt have been tested) transfers the absorbed solar heat to the power block, where it is used to heat a steam generator. Advanced high-temperature power tower concepts 30 MW •20 10 o O 2 • 6 1 10 12 14 11 11 20 22 24 Tim• ot Day are now under investigation; these heat pressurized air to over 1000°C and feet it into the gas turbines of modern combined cycles. Hybrid heat-power processes Using solar heat as the primary source of energy, the heat-toenergy conversion process can be realized with already proven components !hat are used in conventional fossil fuel (or nuclear) heat-power processes. Besides the reliability of these components, such a process can also be driven by a non-solar or non-renewable source, which results in higher capacity factors of !he components and more full load hours for !he entire power planı. This is as important tor the planı components (especially tor the steam or gas turbine) as tor the grid quality. Sun-belt countries such as Egypt or lndia have a strong need tor base load power plants, so such a hybrid cycle may meet the requirements of !he grid and also allow !he use of renewable energy. The hybrid solar power plan! is thus not dependent on • grid compensation because it compensates within !he same plan!. For countries with a poor grid infrastructure, such a hybrid plan! can be !he first step towards power generation with renewable sources. Hybrid processes do not need expensive thermal storage. The only problem may be !he low exergy of the heat produced in trough-fields, which results in highcost, low-efficiency heat that has to compete with cheap fossil fuel heat. There are (at least) four approaches to the integration of solar heat into heat-to-power processes: Back-up fireci Rankine cycle lntegration with !he heat-recovery steam generators of a combined cycle system (the integrated solar combined cycle) lntegration in the Crayton process (preheating of air) Raising turbine efficiency by using absorption cooling processes. These options can be combined in many ways, so that a high 5 o � ECOGENERATION WORLO solar share can be achieved. Besides !his, other integrations such as feed-water preheating and fuel preheating have been studied. Back-up fired Rankine cycle The back-up fireci Rankine cycle is !he most common, or proven, way to hybridize power processes, though in !he development of solar electric generating systems in California some different approaches have been realized. The limitation to a temperature (because of !he thermal stability of !he heat-transfer fluid) of 350-400°C meant that very poor thermal efficiencies were achieved. The first solution was to constantly raise the steam temperature up to 550°C by using a fossil fuel to drive the heater. This configuration leads to low solar shares, but high capacity factors and higher thermal efficiencies. However, it proved too expensive because of !he limitation in the solar electric generating system contracts to 25% fossil share. lntegration with the heat-recovery steam generator ofa combined cycle system lntegration into modern combined cycles has been studied very carefully recently. The problem of this approach is that !he solar heat has to compete directly with !he very cheap exhaust heat in the heat-recovery steam generator. The besi argument tor the use of solar heat here is the out-sourcing of vaporization, which leads to a higher efficiency of !he steam cycle. A fossil-fired vaporizer, or a bottom-fired heat-recovery steam generator, can !hen substitute the solar heat. lntegration in the Crayton process (preheating of air) The most efficient integration of solar hear is by direct integration into the gas turbine or Crayton process. The REFOS projeci, conducted by DLR (Deutsches Zentrum für Luft - und Raumfahrt), is investigating this form of pressurized air preheating as a means ofsaving fuel. Air at a pressure of 16bar (1.6 MPa) is heated up to 800°C in a closed volumetric receiver (see 'Power tower' above - the mid-term goal is a temperature around 1200°C), which reduces fossil fuel combustion. lf solar heat is not available !he fossil fuel source can fully replace !he solar component. Raising turbine efficiency by using absorption cooling processes Another way to raise gas turbine efficiencies with the help of solar heat is to use a solar-driven absorption cooling process. By using a cooled compressor in the turbine (one or two-level cooling), !he turbine efficiency can be raised from 36% to 48%, giving a 30% saving of fuel. The advantage of !his approach is that low exergetic heat can be used. Solar shares and efficiencies To evaluate a hybrid processes, it is essential to demonstrate the proportion of renewable energy used in such a process. The most
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