NASA estimates the sun converts nearly four million tons of matter to energy every second. The portion that reaching Earth’s upper atmosphere equates to about five pounds converted each second, which is equivalent to about 1,367 watts per square meter. The entire earth receives about 174,000 terawatts (TW) of solar radiation at the upper atmosphere. The total energy use of the planet is equal to only 16 TW, or less than one ten-thousandth of what the sun delivers. While these numbers are staggering, it’s clear the sun has always been, and will continue to be, the power source that allows life to exist. As solar energy technology continues to advance, we are on the threshold of converting ever-more of the sun’s energy directly into usable electricity.
Harnessing the sun’s energy to generate electricity is done most often through the use of photovoltaic (PV) cells, which convert light energy into direct current electricity. (Interestingly, Albert Einstein was awarded the Nobel prize in Physics in 1921 for his discovery of this effect, not his more famous discovery of relativity). As sunlight strikes a suitable metallic surface, electrons are dislodged from their atoms and become “photoelectric” electrons that can be collected in a storage battery, or applied directly to a circuit (although present PV cells do not generate enough energy to directly power most useful circuits). Commercial PV systems generally use battery storage which, in turn, is connected to an inverter that converts DC to AC power. Industrial systems that connect to the electric grid use electronics known as a grid tie inverter (GTI). Grid tie inverters convert DC current from the PV panels to AC current that is in phase and at the correct voltage level to be fed back onto the grid.
Solutions for Solar Energy Challenges
The biggest engineering challenges that have stymied wider use of solar energy fall into two areas. First, the sun is an intermittent source of energy. Energy collected during daylight hours needs to be stored for use after dark. In addition, the demand for electricity changes throughout each day and each season, causing imbalances — such as what happens when peak demands are reached— that can cause voltage fluctuations throughout a wide area. A number of engineering solutions to these problems have been developed to store solar-generated electricity that include batteries, molten salts of calcium, sodium and potassium, as well as systems that use compressed air or water to spin a turbine. Each of these deliver energy conversion efficiencies well below 20 percent.
Second, to gain widespread acceptance, especially in the consumer market, the cost and efficiency of solar energy production needs to rival that of the traditional power grid. The cost of electricity from the grid varies by geography, running from around 8-cents per kWh in North Dakota and Washington state to more than 30-cents in Hawaii, with a national average around 13-cents per kWh. Solar energy, today, costs between 12- and 20-cents per kWh, depending upon what credits are available from power companies, whether incentive tax credits are taken, and other factors. However, even though the cost for solar power is approaching that of the traditional grid in some locales, the average household uses about 18 kWh of power each day, which vastly exceeds the output available from most solar systems suitable for a residence. It’s clear that developing more efficient and cost effective solar panels and systems is critical to the universal acceptance of solar energy.
In spite of these limitations, the use of solar energy is growing both in residential and commercial applications. The U.S. Energy Information Administration projects solar energy produced by traditional power companies will increase by 84 percent between 2014 and 2016; however, that total will still remain somewhat below one percent of domestic energy production.