【正文】 native sources of energy can bee a satisfactory substitute for fossil fuels The most satisfactory single alternative would be solar energy One may discover that wind solar biomass etc all piled on top of each other may have to do but their success may turn out to require an effort that started a generation ago The most popularly suggested alternative energy is solar his last suffers because it is intermittent sources of energy and the end product is electricity There is no way to store large amounts of electricity for use when wind and sunshine are not with us Geothermal and tidal energy are insignificant energy sources in total but can be locally important II SOLAR ENERGY The sun is a giant ball of hydrogen and helium gas The enormous heat and pressure in the interior of the sun cause the nuclei of two hydrogen atoms to fuse producing one helium atom in a process called fusion During fusion nuclear energy is converted into thermal heat and radiant energy The adiant energy is emitted form the sun in all directions and some of its reaches Earth Radiant energy is energy that travels in electromagic waves and rays Radiant energy includes visible light x rays infrared rays microwaves gamma rays and others These rays have different amounts of energy depending upon their wavelength The shorter the wavelength the more energy they contain [3] Only a small part of solar energy that the sun radiates into space ever reaches the earth but that is more than enough to supply all our energy needs Every day enough solar energy reaches the earth to supply our nations energy needs for a year It takes the sun energy just a little over eight minutes to travel the 93 miles to the earth Solar energy travels at a speed of 186000 miles per second the speed of light Today people use solar energy to heat buildings and water and to generate electricity [7] III SOLAR WATER HEATER IN POPULAR WORLWIDE Solar hot water systems capture energy from the sun to heat water for homes and businesses thereby displacing the use of natural gas or in some cases electricity with free and limitless solar energy Solar hot water could save some billion thermos of natural gas a year To prevent global warming pollution reduce dependence on imported fuel and ease the price of natural gas [5] An example of solar water heater is shown in figure 1 Those systems uses energy from the sun to heat water reducing the amount of natural gas or in some cases electricity needed for residential or mercial water heating Solar hot water systems are highly efficient up to 87 percent of the suns energy that reaches a given section of roof can be absorbed by the systems collector Liquid in the collector heated by the sun is pumped to one or more storage tanks thereby reducing or eliminating the need for conventional water heater fuel The benefits of using the solar water heater are the followings Heating water with sunshine costs less Once it is bought the water heating costs will be zero A solar water heater cannot give a shock or set fire to the house like electrical or flame based water heating systems Solar water heaters do not make any smoke so they dont make the home smell or get dirty If one usually heats water with gas or electricity from fossil fuels a solar water heater helps reduce the carbon dioxide produced by the household helping in the fight against global Solar hot water heaters have been in use in various parts of the world since their inception While conceptually simple the systems continue to improve as newer technologies enhance performance As environmental political and financial concerns with fossil fuels grow worldwide solar hot water has bee popular in many countries [10] Mediterranean climates are characterized by warm summers and mild winters making them ideal for solar hot water systems The small temperature range throughout the year allows the systems to cover most hot water demands in the winter without overheating in the summer meaning a larger portion of the water heating needs can be provided by the sun Figure 2 demonstrates the total world solar hot water capacity by