How much solar energy available on the Earth?
It’s estimated that the energy from the sun to reach the ground of the earth is about 1kW/m2 at the equator of the Earth. In addition, solar collectors have a lower efficiency because of the low sun angle during evenings and early mornings, and collect none solar energy at night. Considering the earth surface is a sophere, the average available energy reaching the ground at the equator is a little over 300W/m2 (1kW/m2 divided by pi (3.14159265)). Given that the typical efficiency in converting solar energy to useful power is from 15% to 30%, the average available energy reaching the ground at the equator is 50W to 100W/m2. This is the maximum useful solar energy reaching the Earth at the equator.
The maximum useful solar energy reaching the Earth at other places will be less than the value above (50 to 100W/m2), depending on the distance to the equator or, depending on the sun angle (the angle between the incoming sunlight and the Earth’s surface). For example, the maximum useful solar energy reaching the northern U.S. is about 90W/m2 for a 30% converting efficiency, and 45W/m2 for a 15% converting efficiency. Accounting for weather such as the number of cloudy and rainy days, the maximum useful solar energy is further reduced by another 35% on average, to approximately 30W/m2 with a 30% converting efficiency and 15W/m2 with a 15% converting efficiency. This compares to 3,000W, the power a typical U.S. household needs. In other words, a 100m2 solar collecting area is needed for a typical household if all the power the household consumes comes from the solar.
What are the technologies to convert the solar to electricity power?
Several technologies can be used to convert sunlight into electricity: photovoltaics (PV), concentrated solar power (CSP), sterling engine dishes and other technologies.
Solar cells are used to convert energy from the sun into electricity through Photovoltaic effect. Although solar power has grown rapidly only in recent years, the first practical application of PVs was to power satellites and spacecraft back in 50’s. Today due to environmental concerns and government feed-in Tariff in many countries, the major applications of PVs are to connect the power generated by solar cells to grid or grid-tied Building Integrated Photovoltaics (BIPV) like roof-top solar. Other applications include solar-powered batteries for the use in remote dwelling, boats, recreational vehicles, electric cars, emergency equipment, remote sensing and even some toys. The electricity produced by solar cells is direct current. If desired to be connected to power grid, an inverter is required to convert the DC to AC, since the bulk power system operates in AC (Alternative Current).
According to Renewables Global Status Report, Photovoltaic production is the world’s fastest-growing energy technology. It’s also reported that at the end of 2009, the cumulative global PV installations surpassed 21GW. Typical solar PV power stations range from 10-60 MW in the capacity and the capacity of many new solar PV power stations is expected to increase to 150MW or more. The average annual use of solar photovoltaic capacity, i.e. the capacity factor, is typically below 25%. The efficiency of typical practical solar PV panels, i.e. the efficiency of energy conversion, is roughly 12-18%. However, higher efficiency has been achieved by some companies like SunPower (24%) and some research institutions (40%). There is also an energy loss of 4-12% during the DC-AC conversion process due to the heat produced and electric power consumed by the inverter.
Regarding the material for PV panels, thin-film solar cells (CdTe CIGS, amorphous Si, microcrystalline Si) become popular and partially replace crystalline silicon modules.
Usually PVs are costly to install. Even some financial agents offer long term contract such as 25 year contract (or Power Purchase Agreement) to reduce up-from cost, much of the investment in a home-mounted system may be lost due to the change of the owner of the house.
Concentrated Solar Power (CSP)
Concentrated solar power systems are divided into three types: concentrated solar thermal (CST), concentrated photovoltaics (CPV), and concentrating photovoltaics and thermal (CPT).
Concentrated solar thermal (CST) system is basically similar to a conventional power plant but with the heat source being the concentrated light. Concentrating technologies include the parabolic trough, dish stirling, concentrating linear Fresnel reflector, solar chimney and solar power tower. For these concentrating technologies, high temperature is produced in the system, usually at 150-1000 oC.
Concentrated Photovoltaics (CPV) system is basically similar to a PV system but with concentrated sunlight instead of regular sunlight.
Concentrating Photovoltaics and Thermal (CPVT) system produces both electricity and thermal heat in the same module, much like a conventional thermal pant but with heat source being concentrated sunlight. Thermal heat produced can be used for hot tap water or heating purposes for houses and buildings.