Chapter IV, Section A, Item 1:  The Solar Energy Asset

The Sun’s output is amazingly uniform, predicted with some basic physics–the Stefan-Boltzman and Wein displacement laws–and verified with the first extra-terrestrial spaceflights in the last century. Solar radiation delivers a constant power of 1368 Watts (W) to every square meter ( m2 ) of the Earth’s circular profile surface area. This “solar constant” is delivered to the day-side top of the Earth’s atmosphere, which is reflective and diffusive. The Earth’s albedo, its reflective property, is such that, on average, 30% of the incident radiation is reflected back to deep space, leaving 958 W/m2 as incoming solar radiation. Spreading the incoming radiation evenly over the entire top of the atmosphere, including the night side, yields an average daily energy budget for the entire Earth’s surface to be used as a benchmark for comparative bookkeeping. The surface area of the circular profile receiving the radiation is one fourth the surface area of the spherical Earth, spreading the incoming radiation to an average 238 W/m2 ultimately reaching the ground. The solar radiation reaching the ground surface is called insolation, varying locally by latitude, season, cloud cover, and time of day. The worldwide, annualized, day-night averaged, albedo-adjusted insolation value is 238 W/m2.

What is the monetary energy value of this average insolation value? The answer depends on the efficiency of converting solar energy into a useful form, roughly 15% for photovoltaic panels producing electricity directly, and 75% for thermal panels producing heat. The efficiency of converting the thermal energy to electricity reduces overall solar conversion efficiency to roughly that of photovoltaics. But thermal systems are much cheaper to build and maintain. Thus 15% efficiency for both options is a good number to use for estimating the energy value of insolation. An 80 square meter solar array with 15% solar conversion efficiency, covering the surface area of a double-wide’s roof, would produce 69 kW-hr per day. The energy value is about 4 times a typical double-wide’s electric consumption, as long as the electricity is not used for heating or A/C, leaving some to spare for producing hydrogen for the car in the drive. Unfortunately the cost of such an array is currently estimated to be $120,000 dollars, most likely equal to the value of the double wide and its quarter acre property. A quote from a solar company in Idaho that markets to “off-the-grid” enthusiasts: “A 240 ft2 [ 22 m2 ] system, including frames, with a solar conversion efficiency of 12% produces 10 kW-hr/day and costs $30,000. The price includes a battery storage system for 100% off-grid use. Average home electrical use is 25 kW-hr, but doubled for those using electric heat / AC.” In the hydrogen economy, the battery storage system would be replaced with a hydrogen electrolysis cell to produce hydrogen during daylight, and a hydrogen fuel cell to produce electricity from the stored hydrogen during the night. Solar is nowhere useable always.

Solar can be useable sometime everywhere. Though local variation in seasonal cloud cover and latitudinal sun angle obviously affect the utility of solar power worldwide, ironically the highest 24-hour insolation anywhere on the planet occurs at a pole in June and December, a value just over 500 W/m2 from 24 hour sunshine with a sun angle of 23.5 degrees off the horizon. Thus economic feasibility locally is a balance of the local conditions and the cost to deploy solar. The National Renewable Energy Laboratory provides maps of monthly and annualized, day-night averaged insolation across the country, factoring in all the local conditions. The global average insolation value, 238 W/m2, is available year-round in the arid Southwest, including western Texas, all of New Mexico and Arizona, and the southern portions of Colorado, Utah, and Nevada.

As part of a hydrogen economy, solar farms have been suggested for the Southwest to produce hydrogen fuel for the nation’s transportation energy. Hydrogen production would require electrical voltage, obtained by tapping solar energy with either photovoltaics or thermal-solar electrical generation at roughly 15% conversion efficiency. The U.S. consumption of gasoline currently stands at 390 million gallons per day, equal to the daily output of 16,660 square kilometers of 15%-efficient solar panels under the average insolation. The area would have to be more than doubled to account for the efficiency of making hydrogen, and the extra space needed for infrastructure and maintenance of the farms. At that, the total farm area would comprise 10% of the State of New Mexico, and would likely cost over a trillion dollars.

Thermal solar power is certainly more economically feasible.
Several thermal solar plants already exist in California and elsewhere, and more are planned. But as in the case of making hydrogen from solar energy, electrical production from thermal solar energy requires water, primarily for cooling steam condensers. Given the water limitations for solar electrical and hydrogen production, the true, untapped potential for solar energy may lie with distributed generation (DG). The DG concept is to have a world-wide-web of interconnected rooftop solar panels. The extra power produced on daytime roof tops would provide power to the nighttime roof tops.

Whether captured or not, the equivalent of 12 gallons of gasoline falls on an 80 square meter rooftop under average insolation conditions. With currently feasible efficiencies, we can capture about two of those gallons (15%) photovoltaically, and 9 of those gallons (75%) thermally. Currently, more money is spent on shingles to protect rooftops from the damage of solar radiation than on solar panels to tap it.

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