The Promise of Renewable Energy: It Can Do The Job

In my previous blog I talked about the broad range of renewable energy technologies and the fact that most of them are direct or indirect forms of solar energy. I also talked as a physicist (please forgive me – congenital weakness) about the origin of solar energy in that fusion reactor 93 million miles away from earth. Now I’d like to talk about what happens to that solar radiation when it reaches the earth’s atmosphere, and what it promises as an energy resource for the earth’s future.

This is not an obvious discussion. A question raised throughout my years involved with renewable energy has been: Can renewable energy meet human needs for energy or is it something less than that as a practical energy resource? These ‘doubts’ began to be raised in a series of studies sponsored by the U.S. coal industry in the mid 1990’s, at a time when the promise of renewables was beginning to be actively explored and, I believe, the coal industry began to feel threatened as a long-term source of electricity. The studies were refuted which required a lot of work, as is currently true of studies questioning the reality and seriousness of global warming and climate change. Both efforts mirror the long-term battle to educate the public about the serious health effects of smoking.

What happens to the more than 6 million quads of solar energy that annually reach the earth’s atmosphere? While the amount of energy radiated by the sun does vary slightly due to sunspot activity, this variation is negligibly small compared o the energy released by the sun’s basic radiative process. As a result the amount of energy received at the outer boundary of the earth’s atmosphere is called the Solar Constant because it varies so little. This number, averaged over the earth’s orbit around the sun, is 1,367 watts per square meter on a surface perpendicular to the sun’s rays. In fact, the earth’s orbit around the sun is not circular but elliptical, and the ‘Solar Constant’ varies by about three percent during the year. In the northern hemisphere the highest value is in the winter and the lowest in the summer.

About a quarter of the radiation incident on the earth is lost by reflection back into space from the top of the atmosphere and tops of clouds. For the radiation penetrating the earth’s atmosphere a not insignificant amount is lost due to scattering and absorption by air molecules, clouds, dust and aerosols. One must also take into account the earth’s rotation and the resultant day-night (diurnal) cycle. To put a number on all this, if one assumes 30% is lost due to the above factors and the sun shines only 12 hours per day on a one square meter surface, that surface receives no more than (1,367W/m2)x(70%)x(12 hours/day)x(365 days/year) = 4,200 kWh of solar energy per year. Since on average the sun actually shines less than 12 hours/day at any location, the maximum solar radiation a site can receive is closer to 2,600 kWh per square meter per year. To put this number into perspective, the average person on earth uses about 20,000 kWh per year (524 quads in 2010 for 7 billion people).

A definitive and transparent answer to the question ‘How real is renewable energy?’ was given recently in the June 2012 NREL report Renewable Electricity Futures Study (Renewable Electricity Futures Study) which concluded that “Renewable electricity generation from technologies that are commercially available today, in combination with a more flexible electric system, is more than adequate to supply 80% of total U.S. electricity generation in 2050 while meeting electricity demand on an hourly basis in every region of the country.”

This is not a prediction but a statement that renewable electricity can meet our needs if we so choose. It will not happen without overcoming many barriers (need for new transmission lines and storage, technology cost, political opposition) but it is possible if we have the political will to make it so. We must also recognize that renewable resources can be used to supply thermal energy as well as electricity, for space heating and cooling and water heating, and transportation fuels via chemical conversion of biomass materials. This is why I get excited about our renewable energy future!