There is a lot of discussion in popular literature regarding renewable energy. In particular, the focus has been on solar energy, and for obvious reasons. The sun is an almost inexhaustible supply. But when you take a closer look at the reality of using solar energy, you will see there are many challenges in making solar energy an economical and viable energy resource when compared to other forms of energy.

Over the last several years, the Department of Energy has undertaken detailed studies on analyzing energy in the United States. But if we talk about solar energy versus other forms, it’s an interesting proposition to consider. As most know, the atmosphere is being adversely affected by fossil fuels. These fossil fuels can come in many different forms: coal, natural gas or oil, to name a few.

These all offend the atmosphere differently, with coal being the worst offender, then oil and natural gas. The problem is, these sources of energy are available abundantly and rather economically. Plus, very little processing is needed, relatively speaking, to use these forms as energy sources. In particular, with the recent discovery of Marcellus Shale deposit, natural gas has become incredibly attractive in bringing that industry back to the United States.

But what about solar energy?

The most interesting form of solar energy is a form of directly converting sunlight into electricity. The other form of solar energy, converting sunlight to heat, is interesting, but the efficiency is not that great. It is electricity that we want to ultimately use. The first solar solar cell made of silicone was invented or demonstrated in 1950, though there were some earlier demonstrations in the 1940s.

The solar cell takes sunlight and turns it into electrons and hulls of positively and negatively charged particles and separates those charges in the cell, which then presents a battery-like output that can be used as energy. It’s important to recognize that this battery-like output exists continuously, as long as the sun is present. So if you’re not using solar energy, the electrons and hulls that you separate recombine with each other, and that energy is lost.

The average energy that is separated for a given solar cell averages one to three volts, depending on the materials used. In order to get a battery plant, one would need to put cells in series to increase the voltage and put them in parallel to increase the current. Typically if the rough order magnitude is a sunny day, if you have one centimeter solar panel, you can harbor 100 milliwatts of energy. It’s easy to calculate how many solar panels would be needed and how much land area would be needed to provide enough energy for a house (which is typically 2000 kilowatts) or a city, which could take hundreds of thousands of kilowatts–even megawatts. The larger land area would have to increase if more power was needed.

The important thing to notice is when these cells are put down, they utilize land that could be used for other resources. In order to provide enough energy for the United States, it would take the area of one full state in the country. In other words, the land area covered would not have any other use except solar energy, virtually trading off land that would have farming or habitation valueto produce energy.

And the trade-offs don’t stop there.

Solar cells require an energy cost to produce them. Those energy costs are derived from power needed for furnaces that melt silicon needed to make the cells, labor costs in creating them and putting them together as a panel, and labor costs in installing the panels. The question is, how much energy is needed to invest to get the solar cell, and how long does that cell have to run to balance and repay the energy invested to create it? In the case of fossil fuels, the earth has already invested the energy in making the fossil fuels. When the fuels burn, they have basically paid for themselves. With solar energy, though, companies would need to ensure that the solar cells live long enough to actually return the energy investment in making them. Otherwise, it cannot be considered a good investment as an energy resource.

The calculation becomes more difficult when we truly consider how efficient solar cells are. The cost of today’s terrestrial cells, both lower cost ones graded at 10 percent and higher costs graded are at 30 percent, are still relatively high. Even with all of the price wars going on, and even with China’s participation, solar panels are still averaging $.75 per watt of panel. That’s a wonderfully low price, but in an industrialized country such as the United States, solar energy would only be practical if a utility creator would buy back any excess energy. In other words, if we are to make solar energy work, the utility grid would have to pay consumers for energy that they do not use. In actuality, the grid would act like one giant battery distributing energy made by all of the smaller energy manufacturers, which would be the homeowners. This arrangement could work, but it would be tremendously challenging in order to keep the electrical grid balanced.

All of these things taken together make solar energy a competitively attractive source, but one that has so many challenges to truly be economical. This is why it is listed as one of the engineering Grand Challenges from a previous post.