Solar energy is the most democratic of renewable energy resources. It is available everywhere on the earth in quantities that vary only modestly. Variations that do occur stem from cloud cover patterns and other lesser influences, including elevation and the prevalence of airborne gases and particles such as humidity, pollution, and dust. Solar radiation that avoids atmospheric scattering and arrives at the earth's surface in an unbroken line from the sun is termed direct, while the scattered radiation that reaches the earth from all parts of the sky is called diffuse. The distinction is important because diffuse radiation cannot be effectively focused; only direct radiation is relevant to the solar technologies that use mirrors and lenses to concentrate the sun's rays. The combined contributions of direct and diffuse result in the total, or global radiation, the quantity of interest for non-concentrating solar technologies such as rooftop solar panels.
Like most of the U.S., the various solar properties have been measured at only a small number of locations in Texas. Information from these limited measurements, coupled with estimates based on weather data and satellite images, provides the basis for the design of solar installations such as rooftop solar water and space heating systems, photovoltaic panels (electricity generated directly from solar cells), solar detoxification devices and large solar electric power plants. Solar radiation information is also important for building design and improved management of agriculture.
Insolation is the total amount of solar radiation that strikes a particular location over a given period of time, typically a single day. Horizontal insolation is the amount received by a horizontal surface such as a lake, field, or office building rooftop. Figure 3 depicts the average daily horizontal insolation for a number of Texas cities. The chart partitions the global insolation (blue plus orange) into its direct (orange) and diffuse (blue) components. The diffuse data, important for daylighting applications, indicate that a skylight in East Texas will provide more useful light than a comparable one located in West Texas. Across the state, global horizontal insolation averages about 5 kWh/m2-day and varies by only 25% from Houston to El Paso. This is significant, as many people assume that only West Texas has the good sunshine necessary to use solar energy; in reality, fixed surface technologies can find application throughout the state.
Solar equipment is frequently installed on structures that are at some angle to the ground. The radiation on such a fixed, tilted surface will be more or less than on a horizontal one depending on the angle of tilt and orientation. A pitched, south-facing roof, for example, will generally receive about 10 to 15% more energy than suggested by Figure 3. Based on these levels of insolation, the average Texas family would need to cover about half of their roof with 10% efficient photovoltaic panels to generate as much electricity as they use. Or similarly, the majority of the family's hot water needs could be met with only a few large solar water heating panels. Many of these small-scale, fixed surface solar technologies intended for residential and "off-grid" use are already common in Texas. These systems, some of which can be directly integrated into buildings, often provide added value to the owner or embody unique characteristics that make them the most cost-effective option available.
Direct Radiation
The National Renewable Energy Laboratory (NREL) has developed estimates for solar radiation to complement the small number of solar measurements that are available. Figures 4 and 5, developed by NREL, show the average direct normal insolation in the conterminous United States and in Texas. Normal insolation refers to the amount that strikes a surface that always faces the sun. West Texas exhibits the highest levels of direct normal insolation in Texas as well as some of the highest levels in the entire nation. Compared to East Texas, West Texas experiences 75% more direct solar radiation.
Direct normal insolation is the quantity of interest for concentrating technologies that track the sun throughout the day and intensify natural sunlight to yield very high temperatures or generate electricity efficiently. Since wholesale energy markets are dictated almost solely on price, solar power plants trying to compete in this arena will need to be located in regions with very good direct radiation. To support prospective developments of this type, improved solar radiation data are needed throughout the Trans-Pecos and along the Rio Grande.
Solar radiation is available throughout the state in sufficient quantity to power distributed solar systems such as solar water heaters and off-grid photovolatic panels. On the other hand, large solar power plants will almost certainly be most cost-effective when sited in areas of West Texas that receive very high levels of direct solar radiation. Solar developments of both types can become major contributors to satisfying the future energy needs of Texas.
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