Seite - 101 - in Water, Energy, and Environment - A Primer

of clouds. For the radiationpenetrating theEarth’s atmosphere a notinsignificantamountislostduetoscatteringandabsorptionby 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 to a 1m2 surface, that square metre receives no more than (1367W/m2)× (70%)× (12hours/day)× (365 days/year)=4200kWh of solar energy per year. Since on average the sun actually shines less than 12hours/day at any location, the maximum solar radiation a site can receive is closer to 2600kWh/m2 per year. To put this number into perspective, today the average person on Earth uses about 23,000kWhper year (600quads for 7.6billion people). Clearly, the energy we receive from the Sun is more than enough to meet human needs. Nevertheless, for many years some people have asked whether we can rely on renewable energy as a reliable and practical energy source. A definitive answer to this question was presented in the June 2012 NREL Renewable Electricity Futures Study (45) which concluded that ‘Renewable electricity generation from technologies that are commercially available today, in combination with a more flexible electric system, ismore than adequate to supply 80%of total US electricity generation in 2050whilemeeting electricity demandon anhourly 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 overcomingmany 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 andwater heating, and transportation fuels via chemical conversion of biomassmaterials. This iswhypeople get excited about our renewable energy resources. Renewableenergy 101