Chapter 6 Solar Energy Characteristics of Solar Radiation History of Solar Heating Solar Heating Today Domestic Water Heating Passive Solar Space Heating Active Solar Space Heating Thermal Energy Storage

Table 6-1, p. 161

Figure 6. 1: U. S. renewable energy consumption (by source), 2003 Figure 6.1: U.S. renewable energy consumption (by source), 2003. The generation of electricity accounts for about one-half of the renewable resources used. Fig. 6-1, p. 162

p. 163 Solar heated house near Chicago, IL.

Figure 6.2: Spectrum of solar radiation reaching the earth at the top of the atmosphere and at ground level. (The minima in the ground level spectrum are a result of the absorption by water vapor, CO2, O2, N2 and ozone [O3].) About 40% of the solar radiation is in the visible region. Fig. 6-2, p. 164

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Figure 6. 3: Energy balance for the earth Figure 6.3: Energy balance for the earth. The earth receives about 50% of the incident solar radiation: 21% is from direct radiation and 29% is scattered through the clouds. The energy leaving the earth’s surface comes from evaporation and conduction to the atmosphere (33%), and infrared radiation (noted here as terrestrial radiation). Most of the infrared radiation (113%) is absorbed by the atmosphere and reradiated back to the surface (the “greenhouse effect”). In order to have temperature equilibrium at the earth’s surface, the energy input must equal the energy output. For this figure, 50% (incident radiation) = 3% (reflected) + 33% (evaporation) + 14% (net terrestrial radiation: 113% + 6% − 105%). Fig. 6-3, p. 165

Table 6-2, p. 166

Figure 6.4: Motion of the earth around the sun, illustrating the seasons and the tilt of the earth’s axis. Fig. 6-4, p. 166

Figure 6.5: Insolation values for a clear day on a horizontal surface located at 40°N latitude, as a function of the month and the hour of the day. Fig. 6-5, p. 167

f: Solar azimuth q: Solar altitude Fig. 6-6, p. 168 Figure 6.6: Yearly and hourly changes in the sun’s position in the sky for 40°N. Also shown are the solar altitude θ (angle above the horizon) and the solar azimuth φ (angle from true south). f: Solar azimuth q: Solar altitude Fig. 6-6, p. 168

Figure 6.8: Daily clear-day insolation as a function of month and collector orientation. Fig. 6-8, p. 169

Figure 6.9: Mean daily solar radiation (on an annual basis) for radiation incident on a horizontal surface, in units of Btu/ft2/d. Fig. 6-9, p. 169

Table 6-3, p. 170

Heliostats http://www.redrok.com/main.htm Fig. 6-10, p. 171 Figure 6.10: Concentrated sunlight from 1775 mirrors strikes a target at the National Solar Test Facility in Sandia, New Mexico. The sun melted a quarter-inch-thick steel plate in 2 minutes. Fig. 6-10, p. 171

Solar Steam Engine, Paris 1878 Figure 6.11: Solar steam engine, Paris, 1878. Water was heated by the sun at the focus of the concentrating dish (A). The steam produced was used to run a steam engine (B) whose mechanical output ran a printing press. The water was supplied from tank (C). Fig. 6-11, p. 171

A Brief History of Solar Heating The Greek philosopher Socrates wrote, “In houses that look toward the south, the sun penetrates the portico in winter.” Romans advanced the art by covering south facing building openings with glass or mica to hold in the heat of the winter sun. Through calculated use of the sun’s energy, Greeks and Romans offset the need to burn wood that was often in short supply. 1767 H.B. DeSaussure found solar temperatures were high enough to cook. Auguste Mouchout, inventor of the first active solar motor in the 19th century. His solar pot could boil 3 liters of water in 1.5 hours. In 1861, Mouchout developed a steam engine powered entirely by the sun. But its high costs coupled with the falling price of English coal doomed his invention to become a footnote in energy history. European scientists through the 19th century developed large cone-shaped collectors that could boil ammonia to perform work like locomotion and refrigeration.

In the United States, Swedish-born John Ericsson led efforts to harness solar power. He designed the “parabolic trough collector,” a technology which functions more than a hundred years later on the same basic design. Ericsson is best known for having conceived the USS Monitor, the armored ship integral to the U.S. Civil War. Albert Einstein was awarded the 1921 Nobel Prize in physics for his research on the photoelectric effect—a phenomenon central to the generation of electricity through solar cells. Some 50 years prior, William Grylls Adams had discovered that when light was shined upon selenium, the material shed electrons, thereby creating electricity. In 1953, Bell Laboratories (now AT&T labs) scientists Gerald Pearson, Daryl Chapin and Calvin Fuller developed the first silicon solar cell capable of generating a measurable electric current. In 1956, solar photovoltaic (PV) cells were far from economically practical. Electricity from solar cells ran about $300 per watt. (For comparison, current market rates for a watt of solar PV hover around $5.) The “Space Race” of the 1950s and 60s gave modest opportunity for progress in solar, as satellites and crafts used solar paneling for electricity.

October 17, 1973 solar leapt to prominence in energy research October 17, 1973 solar leapt to prominence in energy research. The Arab Oil Embargo showed reliance of Western economy on a cheap and reliable flow of oil. As oil prices nearly doubled overnight, leaders became desperate to find a means of reducing this dependence. In addition to increasing automobile fuel economy standards and diversifying energy sources, the U.S. government invested heavily in the solar electric cell that Bell Laboratories had produced with such promise in 1953. By the 1990s, the reality was that costs of solar energy had dropped as predicted, but costs of fossil fuels had also dropped—solar was competing with a falling baseline. However, huge PV market growth in Japan and Germany from the 1990s to the present has reenergized the solar industry. In 2002 Japan installed 25,000 solar rooftops. Such large PV orders are creating economies of scale, thus steadily lowering costs. The PV market is currently growing at a blistering 30% per year, with the promise of continually decreasing costs. Meanwhile, solar thermal water heating is an increasingly cost-effective means of lowering gas and electricity demand.

Early 20th Century Egyptian Steam Generator Water Purifier

Adams Solar Cooker - 1878 Telkes Oven - 1950 Figure 6.14: Adam’s solar cooking apparatus, India, 1878. Sunlight is reflected to the blackened metal container, containing the food, as shown in the insert. The metal container is enclosed in a glass jar. Telkes Oven - 1950

Extra Resources Solar Collectors at DOE Solar Energy & Power Encyclopedia