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Photovoltaic vs Thermal Solar Power

Updated on March 12, 2011


If we consider all known energy sources available today, we will soon come to the conclusion that solar energy is the cleanest and most abundant energy of all. It is probably well known that solar radiation comes in form of heat and light, but there is also some high energy electromagnetic (EM) radiation emitted from the sun that we are unable to see. The high EM part is not that dominant. From the point of view of applications, we are primarily interested in the visible light and heat that comes from the sun. This part can be used as an alternative and renewable energy source.

Can you imagine the sheer amount of energy contained in the Solar radiation? It is indeed huge. The average solar radiation hitting the top of our planet per unit of time and unit of area (solar constant) is roughly 1000 watts per square meter. As we said above, this power is spread out across the entire solar spectrum, even though the majority is in the visible (and usable) part.

On a daily basis we all can see the importance of solar irradiation for almost all natural processes on our planet Earth. Solar energy, although plentiful, has hardly been put to any use until recently. But luckily for us, we have now started to discover, or should we say rediscover, the potential benefits of solar energy.

Solar energy can be divided into two categories:light (electromagnetic) and thermal energy.

Photovoltaic Cells

Photo-voltaic (PV) cells use semiconductor technology to convert light directly into electricity. If you wonder what semiconductor technology means, you should just look at your PC's main processor - it represents the most sophisticated semiconductor technology you can imagine. With the solar panels, you can use the produced electricity right away or it can be stored in a battery for later use.

PV panels are now becoming widely used and they can be easily installed on top of our houses, cottages, buildings and other structures - even on vehicles, boat and similar objects. They can be also used as solar charging kits. They can provide clean, renewable energy source that can supplement the needs of your household for energy and thus diminish your dependence on the conventional sources (read the power company).

But the real power of the photo-voltaic cells comes in play in those areas that are far away from the standard power grid, You have probably seen PV modules used for powering emergency telephones, lighting, signage, etc. If we neglect for a moment the obvious intermittent nature of the solar energy, we can say that PV cells can be considered a relatively reliable source of electricity. One big disadvantage of the PV panels is their high cost and relatively low energy conversion rate (only 10-20%) - this part is hopefully to be improved in the near future, after all teams of scientists are putting in a huge effort to improve the conversion rates and diminish the production costs.

Thermal Solar Power

Thermal energy can be used to passively heat buildings. This can be trivially achieved through the use of certain building materials and architectural design, but what's more exciting is to use specially designed heat collectors to directly heat water for domestic use. Very simplified, we can say that the principle of operation of these systems is such that an antifreeze solution flows through collectors and via a heat exchanger, hot water is supplied for domestic use. The heated water is usually stored in insulated tanks until there is need for it.  In many regions, solar water heating is now a feasible supplement, or let us say, a great alternative to electric or gas powered hot water production.

A hot subject of this decade is the problem related with the increased emission of harmful gasses, amongst which carbon dioxide - CO2 holds the most prominent place. CO2 is now believed to contribute to global warming, and environmental issues. Using solar, as well as other forms of renewable energy can reduce our dependence on fossil fuels for energy production and thus, directly reduce the CO2 emissions. An average household can reduce carbon dioxide emissions by as much as 20 percent upon, for example, installation of solar water heaters.

How PV Compares to Thermal Solar Power?

The cost of installing solar heaters similar to that of the PV systems. However there is a difference in the efficiency of both systems. While thermal collectors usually have average surface areas of 50 square feet, the PV cells are about 20 square feet. The thermal heaters are capable of converting over 50 percent of energy into heat, whereas the best PV panels can only approach the 20 percent limit. Considering the larger surface areas, it means that the solar water heaters are capable of delivering 10 times more energy.

However there is a downside to this story. The solar heathers are not always the best choice as all you will get is hot water. If you need electric energy you will need to turn to the PV systems.

Probably the best choice is to combine the two systems. They are both ideal to start to learn about the benefits of using clean, free and renewable energy such is the solar energy.

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