Solar Cells Solar Thermal Energy
As the name indicates, solar cells, technically known as photovoltaic cells, depend directly upon the sun to provide energy. They convert about 10% to 20% of the solar irradiation that reaches our planet directly into electrical energy by the means of semiconducting materials. (See here for an explanation of how this is done). Through the use of sun concentrators (i.e., specially shaped mirrors or lenses) which focus the incoming light onto a smaller "conversion" surface, the amount of semiconducting material can be significantly reduced to anywhere between 250X to 1000X. Sun concentrator also boost the cells absorption efficiency to 20% to 25%. So while the overall size of the solar cell panel will be about the same, great savings are made on the more "expensive" resources.
At optimal times, the earth receives about 1000 W/m2 (92.9 W/ft2) of solar irradiation from the sun, but of course factors, such as seasons, storms and cloudiness, etc come to considerably lower this amount. The information from the data map below of annual mean solar irradiation and the various locations of the Holy Cities, make the average solar irradiation level of the HC’s to be about 153 W/m2 (14.2 W/ft2). Using an overall system efficiency of 23.50% an HC will be able to have on average about 1.2 billion m2 (12.6 billion ft2) = 289,763 acres (750,000 hectares). This represents about 44% of the total HC area, but since these solar units will be out of the way (and also out of sight) as they will be mounted (and concealed) on rooftops, they will not have an environmental impact.
The following table show the location distribution of an HC’s space that can be used for solar cell panels.
HC Solar Energy Potential
Solar Thermal energy units also make use of the sun for energy but unlike Photovoltaic Solar Cells, this energy is produced by using heat rather than light. Incoming sun rays are concentrated in a collector onto focal point where it is used to heat a working fluid (air or water). (See this webpage for more details). Depending on the magnitude of the concentration (called "Suns"), various elevated temperatures in the thousands of degrees (oC and/or oF) can be reached. This heat is then either used directly for heating purposes (e.g., a pool, a house, a furnace) or to generate electricity through the use of steam turbines.
With a solar thermal system, with a extra energy storage system, needing about 10 acres per MW (see in this site), the following estimates can be made.
How Solar Panels Work (with Animated Infographics)
Solar Cell Making Process
Full Spectrum Solar Cells March 30, 2016
Solar Thermal Energy