spring and fall equinoxes with less accurate focusing of the light at other times during the year. The daily motion of the sun across the sky also introduces errors, greatest at the sunrise and sunset and smallest at noon. Due to these sources of error, seasonally adjusted parabolic troughs are generally designed with a lower concentration acceptance product. Parabolic trough concentrators have a simple geometry, but their concentration is about 1/3 of the theoretical maximum for the same acceptance angle, that is, for the same overall tolerances of the system to all kinds of errors, including those referenced above. The theoretical maximum is better achieved with more elaborate concentrators based on primary-secondary designs using nonimaging optics which may nearly double the concentration of conventional parabolic troughs and are used to improve practical designs such as those with fixed receivers. Heat transfer fluid (usually thermal oil) runs through the tube to absorb the concentrated sunlight. This increases the temperature of the fluid to some 400 °C. The heat transfer fluid is then used to heat steam in a standard turbine generator. The process is economical and, for heating the pipe, thermal efficiency ranges from 60-80%. The overall efficiency from collector to grid, i.e. (Electrical Output Power)/(Total Impinging Solar Power) is about 15%, similar to PV (Photovoltaic Cells) but less than Stirling dish concentrators. Design A parabolic trough is made of a number of solar collector modules (SCM) fixed together to move as one solar collector assembly (SCA). A SCM could have a length up to or more. About a dozen or more of SCM make each SCA up to length. Each SCA is an independently-tracking parabolic trough. A SCM may be made as a single-piece parabolic mirror or assembled with a number of smaller mirrors in parallel rows. Smaller modular mirrors require smaller machines to build the mirror, reducing cost. Cost is also reduced in case of the need of replacing a damaged mirror. Such damage may occur due to being hit by an object during bad weather. In addition, V-type troughs exist which are made from 2 mirrors and placed at an angle towards each other. In 2009, scientists at the National Renewable Energy Laboratory (NREL) and SkyFuel teamed to develop large curved sheets of metal that have the potential to be 30% less expensive than today's best collectors of concentrated solar power by replacing glass-based models with a silver