Solar Energy covers a broad spectrum of energy from the Sun. Light and heat are the primary forms of this energy. They can be used directly or converted to other forms of energy such as electricity. The Sun is also indirectly responsible for other forms of renewable energy such as biomass (photosynthesis), hydroelectricity (evaporation), wind (thermal variation) and waves (from wind). All told, the Sun is responsible for more than 99% of the available renewable energy on Earth.

The total solar energy absorbed by the Earth’s atmosphere, oceans, and land masses is approximately 3850 ZJ (10²¹ Joules). The total worldwide energy consumption in 2004 was 0.471 ZJ. The picture above on the left shows the solar radiation breakdown. The picture on the right shows the average insolation at the Earth’s surface. The black dots on the right-hand picture indicate the total land area required to replace the entire world energy supply with solar cells. So the energy is is there, the question is, how do we make use of it?

For the sake of this article, I will focus on solar power, or the the conversion of solar energy into electricity. There are two primary methods of converting the Sun’s energy to electricity: photovoltaics and concentrators.

Solar Photovoltaic (PV)

For solar photovoltaic (PV), solar cells make use of the photoelectric effect (picture on right) to generate electricity. This effect refers to the process where a material is exposed to electromagnetic radiation causing it to emit electrons. Solar cells have been around since 1883 and were commonly seen a the power sources for satellites and later calculators.

Most modern solar cells are based on silicon or some similar semiconductor material. The cells are often joined into modules with a glass sheet on the top which provides protection for the cells while allowing light to pass through. The amount of electricity produces by a module depends on the materials used and sometimes a lens is used to direct more light to the individual solar cells. Multiple modules are used in conjunction to produce more electricity. Large farms of panels can be installed in open areas with lots of direct sun. For maximum effectiveness, the panels can be mounted on platforms capable of tracking the Sun.

PV modules or panels are becoming more common on commercial and residential buildings. They are typically installed on the roof at an angle to catch the most sunlight possible. The panels produce direct current electricity which is passed through an inverter to create alternating current which can be fed directly to the building’s electrical grid.

Solar Concentrators

The idea behind solar concentrators is to focus the heat from the Sun to drive more traditional means of electrical generators such as steam turbines. This generally means reflecting the light into a concentrated beam. The beam is then used to super heat the working fluid which in turn drives an electric generator. To maintain maximum heat, the concentrators use complex tracking systems. While there are many different implementations of solar concentrators, the most common forms are solar trough, parabolic dish and solar towers (pictured on right). The solar trough makes use of a linear parabolic reflector which concentrates light on a tube of working fluid located at it’s focal line. The parabolic dish focuses on a single point of working fluid, but can track the sun on both axes. A solar tower uses a large array of tracking mirrors to focus light on a central tower containing the working fluid.

Problems with Solar

While solar power has a lot of potential, there are some problems as well. One of the biggest problems for solar is availability,that is to say, the sun is not up 24 hours a day, and the skies are not always clear. There is plenty of solar energy available to produce extra power for use at night, but the technology to store the power is sorely lacking. Transmission could be another issue, where you need to get power from a sunny desert to a cloudy coastal town. The transmission issue exists even for traditional power generation, but is exacerbated by a more limited number of locations to install a solar plant. PV systems can eliminate the transmission issue all together since they can be installed at the end use site. Even then, the storage problem remains and becomes more pronounced if the location does not have adequate sunlight to begin with. Another issue for PV is the availability of the semiconductor material.

I believe solar will be a big part of the future energy equation. It seems almost crazy not to take advantage of the most abundant source of clean energy around. We need to keep improving the conversion technologies as well as push for better storage and transmission solutions. As for which of the solar technologies will win, I think they both have an important role. PV can be used in area with good sunlight to keep homes and businesses off the grid. Good local storage systems can keep a home running through the night. Solar concentrators with improved transmission technology can supply power to areas with insufficient sunlight. Perhaps power storage can be as simple as an underground steam reservoir which is heated in the day and used to power the generators at night or on cloudy days.

The sun is the largest power plant in our galaxy, let’s use it as best we can.