cover. Typically, polypropylene or EPDM rubber or silicone rubber is used as an absorber. Used for pool heating, it can work quite well when the desired output temperature is near the ambient temperature (that is, when it is warm outside). As the ambient temperature gets cooler, these collectors become less effective. Bowl collectors A solar bowl is a type of solar thermal collector that operates similarly to a parabolic dish, but instead of using a tracking parabolic mirror with a fixed receiver, it has a fixed spherical mirror with a tracking receiver. This reduces efficiency but makes it cheaper to build and operate. Designers call it a fixed mirror distributed focus solar power system. The main reason for its development was to eliminate the cost of moving a large mirror to track the sun as with parabolic dish systems. A fixed parabolic mirror creates a variously shaped image of the sun as it moves across the sky. Only when the mirror is pointed directly at the sun does the light focus on one point. That is why parabolic dish systems track the sun. A fixed spherical mirror focuses the light in the same place independent of the sun's position. The light, however, is not directed to one point but is distributed on a line from the surface of the mirror to one half radius (along a line that runs through the sphere center and the sun). As the sun moves across the sky, the aperture of any fixed collector changes. This causes changes in the amount of captured sunlight, producing what is called the sinus effect of power output. Proponents of the solar bowl design claim the reduction in overall power output compared with tracking parabolic mirrors is offset by lower system costs. The sunlight concentrated at the focal line of a spherical reflector is collected using a tracking receiver. This receiver is pivoted around the focal line and is usually counterbalanced. The receiver may consist of pipes carrying fluid for thermal transfer or photovoltaic cells for direct conversion of light to electricity. The solar bowl design resulted from a project of the Electrical Engineering Department of the Texas Technical University, headed by Edwin O'Hair, to develop a 5 MWe power plant. A solar bowl was built for the town of Crosbyton, Texas as a pilot facility. The bowl had a diameter of , tilted at a 15° angle to