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13.03.2014
Humankind has more experience using solar energy than any other form of energy – the resource is well understood, and conversion technologies have long and positive operational track records.
The earliest humans to inhabit the earth recognized and utilized the light and heat energy provided by the sun.
A more sophisticated knowledge of the basic solar characteristics allows for the utilization of solar radiation in a broad assortment of thermal, electrical, photobiological and photochemical processes. The most common applications of solar energy today are to provide heat, electricity and light.
Several utility-scale solar electric power plants have been built in the US and abroad that use concentrating optics to achieve sufficiently high temperatures to produce electricity using conventional steam turbines. Flat-plate photovoltaic devices, solar water heaters, and growing crops utilize both diffuse and direct radiation. Solar radiation is composed of a broad spectrum of wavelengths, from the ultraviolet, through the visible and into the infrared. New research is aimed at producing solar conversion technologies that utilize greater portions of the available spectrum. Solar radiation varies according to a combination of predictable annual and daily cycles, and irregular (though not entirely unpredictable) changes in weather. View Exhibit 3-3: Spectrum Utilization by Full-Spectrum and Multi-Junction Photovoltaic Cells, in Text Format. Solar radiation has a low energy density relative to other conventional energy sources, and for all but the smallest power applications, therefore, requires a relatively large area to collect an appreciable amount of energy. While the construction of large solar power plants is technologically feasible, their size requires that land use issues be considered.
Solar power plants based on photovoltaics and dish-Stirling engine designs, as well as small-scale photovoltaic and solar thermal installations, do not require water. The Texas solar resource generally improves toward the west, and large-scale solar energy power plants are typically located where the resource is best. Aside from its potential to ease challenges associated with grid integration, storage is particularly useful to solar because it enables time-shifting of energy production to peak hours when the value of the energy produced is highest. The storage of grid-scale quantities of electricity as an extended supply is impractical, although progress is being made with high capacity batteries that might provide a bridge of a few minutes that could dampen most adverse effects of solar variability on the grid.
Another pathway for solar energy development is through distributed installations of small-scale systems for producing electricity or hot water, typically on residential, commercial, or industrial building rooftops.
The Texas solar resource is vast and the recoverable energy is many times greater than the state’s total energy demand. Solar measurements in the United States date back to the mid-twentieth century when solar energy information began to be gathered along with meteorological data.
While a variety of instruments have been used to collect solar radiation data, the most common and reliable are broadband thermal-sensing pyranometers and pyrheliometers, typically having measurement uncertainties of less than 5 percent. Because of their relative robustness, better response and lower cost, simple photosensors are currently the most commonly used type of instrument.
Imagery from satellites of cloud cover and ground conditions now permit the estimation of incoming direct and diffuse solar radiation reaching the Earth at any location.
The 1991–2005 NSRDB contains hourly solar radiation (including global, direct, and diffuse) and meteorological data for 1,454 ground stations across the nation. In a project supported by the Texas State Energy Conservation Office (SECO)13 a Texas Solar Radiation Database (TSRDB) was developed using solar data obtained at 15 locations in Texas (Abilene, Austin, Big Spring, Canyon, Corpus Christi, Del Rio, Edinburg, El Paso, Clear Lake, Laredo, Menard, Overton, Pecos, Presidio, and Sanderson) between 1996 and 2002.
There are other measurement networks that emphasize other measurements, but may now or in the future also record solar radiation data, likely with a single photosensor. Direct normal insolation (DNI, that most relevant to concentrating solar plants) is more variable across Texas because cloud cover reduces the direct insolation. Exhibits 3-8 and 3-10 show bar charts for the daily average direct normal solar radiation and direct plus diffuse solar radiation on horizontal surfaces, respectively, for 1991-2005 NSRDB Class I locations in Texas. View Exhibit 3-08: Normal Insolation on a Surface that Tracks the Sun Continuously, in Text Format.
Consistent with the contour maps (Exhibits 3-6 and 3-8), locations further to the west exhibit increasing total horizontal and direct normal insolation levels.
A comparison of direct normal insolation with the total insolation on horizontal surfaces is of interest for low temperature applications. Of course, for applications involving high temperature collection (industrial process heat or solar-thermal power generation) concentrating collectors are superior in areas where direct normal insolation is highest. As with some other renewable energy resources, the intermittent nature of solar radiation may be a barrier to its widespread use. The annual variability in direct normal and global horizontal insolation by year from 1991 to 2005 is shown for several Texas locations in Exhibit 3-10.
The seasonal variability of direct normal and global horizontal insolation is shown in Exhibit 3-11 for several locations across Texas. Global horizontal insolation shows similar seasonal variation to that of direct normal insolation.
The seasonal variation in solar radiation tends to be synchronous with energy demand in Texas because high levels of solar radiation in the summer are a major contributor to heat gain in buildings, increased air conditioning loads, and thus peak electrical demand.
These diurnal and short-term variations in solar energy pose the greatest problem for utilization of the solar resource. Much of the energy use in our society is electrical, which currently is generated at large central power stations.
Solar energy can be used for both central and distributed electrical generation and also for decentralized thermal loads, such as water and space heating. One can consider the potential solar energy contribution to our energy demands in three general categories: 1) central electrical power generation using solar-thermal or direct photovoltaic conversion, 2) distributed thermal or photovoltaic energy production, and 3) small stand-alone electrical applications.
While electrical generation plants are ideally located relatively near load centers, central solar thermal power plants would typically be sited at locations where insolation is best, particularly direct normal insolation. A variety of solar thermal conversion systems have been developed, but the most common in use over the last two decades uses parabolic trough concentrators.
The most recent example of a linear concentrator is the Nevada Solar One project which went online in June 2007 (Exhibit 3-13).
Several other solar thermal power system designs have been tested and operated, though not as extensively as the parabolic trough design. A 10 MW central receiver system was constructed at Kramer Junction in the California desert in the early 1980s and operated as a demonstration project for several years in two design versions, the first being steam generation in the central receiver itself and later with molten salt used as the transfer fluid for a separate steam generator. PV cells convert sunlight directly into electricity by taking advantage of the photoelectric effect. Individual PV cells are typically only a few inches in diameter, but multiple cells can be connected to one another in modules, modules can be connected in arrays, and arrays can be connected in very large systems.
Distributed solar generating systems are sited at the point of use, typically on or near residential or commercial buildings, and serve some or all of the energy needs of the building.
There are many energy applications for which the load is purely thermal, such as water heating, space heating, swimming pool heating, cooking, industrial process heating and drying, and many of these energy needs can be supplied by solar energy. In addition to heating, cooling can be achieved by a number of solar thermal means, one being absorption cooling. Note: This system is part of the Historic Gardens Phase II project, by the San Antonio Development Agency (SADA).
In some cases the customer may have storage (typically batteries) to provide emergency backup for a few hours.
New legislation passed in 2007 is scheduled to be fully implemented for customers in the ERCOT competitive area and for regulated utilities outside ERCOT in 2009. Some of the state’s municipal utilities and rural electric cooperatives, including several of the largest municipal utilities, have voluntarily adopted traditional net metering policies for their customers, though these programs are neither required nor consistently designed.
There are numerous small demand applications for which PV systems are designed to stand alone, without any connection to the electrical distribution system. To meet varying load requirements with the variable power from sunlight, stand-alone systems require some type of storage, typically batteries. The current cost, and cost-effectiveness, of different solar technologies and applications varies widely. Others, such as central solar thermal and thin-film photovoltaic power generation, are rapidly becoming cost-effective at utility scale as their costs decline, efficiencies improve, and the cost of fossil-based electricity continue to increase. Finally, other solar applications such as distributed photovoltaics are expected to be cost-effective within 10 years, but in the meantime can be made cost-effective for customers today through a combination of federal, state, and utility subsidies and policies. The cost to produce energy using solar technologies is not meaningful without reference to the cost to produce energy by other means. The levelized cost of energy from new central solar power stations using solar thermal technologies currently ranges from about 12 to 18 cents per kWh. View Exhibit 3-22: Levelized Cost of Concentrating Solar Power, Historical (1980-2005) and Projected (2006-2025), in Text Format. Solar swimming pool heating, while often considered a luxury, is very economical compared to the alternative of heating with electricity or natural gas.
The levelized cost of energy from photovoltaics currently ranges from about 20 to 35 cents per kWh.
Photovoltaic cost projections are based on increasing penetration of thin-film technology into the building sector. View Exhibit 3-23: Levelized Cost of Photovoltaic Power, Historical (1980-2005) and Projected (2006-2025), in Text Format. The major benefits in making use of solar energy are that the source is renewable, inexhaustible, and generally non-polluting.
The generation of energy from sunlight generally does not contribute to noise, air, or effluent pollution, and does not result in the release of carbon dioxide into the atmosphere. Another benefit of solar energy is that it tends to be synchronous with energy demands, particularly in Texas.
A final benefit of solar is that it can be utilized as a distributed energy source, either electrical or thermal.
The solar energy industry, and in particular the photovoltaics industry, has grown in direct response to federal, state and local tax policies and subsidies.
Industry analysts agree that the federal income tax credit for solar energy has expanded markets for solar products, but note that the limited time period for the credit has created uncertainty in solar industry markets.31 The longer-term extension in 2008 should help provide a more stable environment for solar project development. Texas’ solar resource is vast, accessible, and generally synchronous with energy demand. The main factor limiting utilization of the state’s solar resource at a large scale today is its cost. Expanding the use of renewable energy in Texas can have a significant positive impact on employment. Finally, expanding use of solar energy requires new thinking about the role of customer-sited generation in the electricity marketplace. The Internet offers instant access to a rapidly growing list of solar resource information. The Typical Meteorological Year Data Sets provide hourly values of solar radiation and meteorological elements for U.S.
In comparison to conventional hydrocarbon fuels such as coal or oil in generating electricity, the cost of solar energy is significantly higher.
In our example, a ton of coal on the average produces approximately 6,182 KWH of electric at a cost of about $36 per short ton (2,000 pounds). In comparison to solar energy, the hydrocarbon fuel costs are significantly lower without rebates, tax benefits nor the cost of carbon emissions. Some of the considerations for a solar energy system include the 20-to-30 year lifespan of the system and the hours of available sunlight. In order to compare the solar energy costs to conventional hydrocarbon fuels, we must covert the $8.95 per into KWH. So a $45,000 5KW solar energy system produces about 119,246 KWH of electric over its lifespan meaning the average cost equals $0.38 per KWH. The relatively high solar energy costs in comparison to conventional fuels should improve with utility rebates and government tax incentives. We will explore the some of the advances in thin-film technologies, the declining costs of solar panels, and the improving solar conversion efficiencies that should continue to bring solar energy costs on par with hydrocarbon fuels. I would like kindly ask the cost of solar energy for desalination and and energy generation and the area to be allocated per 0ne MWH solar energy generation. I have a 3k system on my roof and it produces annually between 4,700 and 4,900 kwh of electricity, so your calculations are clearly understating the kwh generation of better systems on well-oriented roofs (my panels are Suntech).
But let’s assume I get 25 years at original efficiency and only another 10 years at 90% efficiency. Note that by putting a solar system on my roof I have LOCKED IN the price per kwh of 4.4 cents per watt for the next 40 years.
As for the stated price of coal per khw in this article, this is meaningless to a homeowner. Oh, and this return is entirely TAX FREE because it represents a savings of a necessary expense and not interest or dividend income from Mr. Shop Windows to the UniverseThe Winter 2010 issue of The Earth Scientist includes a variety of educational resources, ranging from astronomy to glaciers. Previously, we had assumed a simplified model of Earth; one in which our planet had not atmosphere. You probably know that scientists think of our atmosphere as having several distinct layers with specific traits.
Though not universally recognized as a layer of our atmosphere, some scientists consider the exosphere to be the outermost layer of Earth's atmosphere.
Let's now take a look at the electromagnetic radiation, of various wavelengths and energies, from the Sun as it penetrates into Earth's atmosphere. All of the high-energy X-rays are absorbed by our atmosphere well above our heads, which is very fortunate for us indeed! Most of the longer wavelength IR waves, and many of the shorter radio waves, are absorbed by the stratosphere before reaching the ground. Recall how the temperature of the various layers of Earth's atmosphere rises and falls as one moves upward from the ground, in a seemingly haphazard fashion. Most of the solar radiation that reaches the Earth or its lower atmosphere, where albedo from clouds and features on the ground come into play, is thus in the form of visible light. As was demonstrated in the "Infrared - More Than Your Eyes Can See" video earlier, some substances are transparent to certain wavelengths of EM radiation, but largely opaque to others. As infrared radiation from the ground and low-lying clouds works its way up through the atmosphere, much of it is absorbed; by gases in the air and by clouds (which have lots of IR-opaque water). Here (below) are three different diagrams illustrating the flow of visible light into, and infrared light back out of, the lower regions of Earth's atmosphere. Shop Windows to the Universe Science Store!The Fall 2009 issue of The Earth Scientist, which includes articles on student research into building design for earthquakes and a classroom lab on the composition of the Earth’s ancient atmosphere, is available in our online store.
It is vast, environmentally benign and generally synchronous with both daily and seasonal energy demands in Texas.
Shelters evolved to moderate the climate and provide interior lighting, and the sun was used to dry food and heat water. Technologies in these areas, some under development and others available today, represent an opportunity to contribute to the future energy needs of Texas. Today’s solar industry supplies reliable products to provide heat and electricity for residential, commercial, and industrial applications using simple equipment such as flat-plate collectors. Examples of such solar thermal electric technologies are parabolic troughs, central receivers and dish-Stirling systems. To accommodate deep penetration of solar in the nation’s power supply, integration of the resource with either adequate storage capability or other sources of energy to back it up is needed.


The variability characteristic can be in the span of a few minutes (how clouds will affect power production), seasonal (how climate patterns will affect the solar resource), interannual (how the resource will vary year to year), or even decadal (how climate change could affect the resource). Therefore solar panels which track the sun’s path through the sky, or are stationary but tilted to the south, collect more energy than fixed panels mounted horizontally. Global tilt insolation is that received by any flat surface tilted to the south at a tilt angle approximately equal to a site’s latitude, like a sloped residential rooftop.
For horizontal solar equipment and level fields or lakes the pertinent radiation is the global horizontal insolation (GHI). The annual and daily average variation is predictable within certain bounds; hourly variation over the course of a day is more difficult to predict. Small-scale or distributed utilization of the solar resource often mitigates or eliminates the potential impact of some of these considerations by making better use of already-developed sites and by producing power at or very near the point of use.
Typical solar power plant designs, require about 5 acres per megawatt of generating capacity.
However, these concerns may be mitigated to some extent since large solar power plants tend to be located in remote, unpopulated areas, and since small, distributed solar facilities are typically located on rooftops of existing buildings. Solar thermal electric technologies, such as central receiver and parabolic trough designs require a considerable amount of water for cooling. These systems actually reduce water consumption by offsetting energy production from conventional generators which do consume water. As that share grows, solar may present new grid integration challenges similar to those emerging with wind applications. While the solar resource is generally synchronous with demand, especially relative to other renewable resources, Texas electricity demand tends to peak during late afternoons in summer while the solar resource tends to peak in the early afternoon. Other methods, such as pumping water to a higher elevation (potential energy) for later electricity generation, are already in use.
Distributed solar electric (photovoltaic) systems and solar thermal water heaters offer some important advantages. Since the energy crises of the 1970s and 1980s, additional solar monitoring efforts have been undertaken to assist the evaluation of solar energy conversion devices. The pyranometer measures the sum of direct and diffuse radiation, while the pyrheliometer measures only the direct normal component. Such instruments use photovoltaic cells and thus operate over a limited portion of the solar spectrum (300-1120 nm), introducing some uncertainty for broadband applications.
The satellite-based models have been improved and verified against ground-based measurements and may be used to provide solar radiation estimates at any ground location where suitable satellite imagery is available. Earth observation satellites circle the Earth on approximately 90 minute orbits so, with rapid processing of the data, information used in solar energy forecasting need be no older than about 1½ hours.
Ground stations are classified by data quality, with 221 Class I stations (the highest quality), 627 Class II stations, and 596 Class III stations.
These include the Texas Commission on Environmental Quality air quality monitoring,15 Texas Coastal Ocean Observing Network,16 and the Texas Mesonet,17 but the emphasis in each of these is on other meteorological data. TMY3 is based on the 1991 to 2005 NSRDB update and consists of 1020 sites nationwide, including 61 sites in Texas. Since most common solar applications use either concentrating collectors (which collect direct normal insolation, DNI), or tilted flat collectors (which collect global tilt insolation, GTI), it is of interest to have these annual average data as a starting point for a more detailed analysis. In contrast, diffuse insolation (which cannot effectively be used by concentrators, but which can be used by flat plat collectors) is present to some extent throughout markedly varying weather conditions.
The orange bar segments in each chart represent the direct (beam) radiation falling on the surface. Higher levels of direct normal insolation are needed to produce the high temperatures required by these processes, and the scale of these applications is more likely to justify increased initial and ongoing costs of concentrating and tracking systems. The solar resource does vary, but it can be predicted reasonably well over long time periods.
Generally the summer months exhibit the greatest monthly insolation, due to longer days, more direct exposure to the sun due to the tilt of the Earth’s axis, and to generally clearer skies. Local weather conditions have a significant effect on seasonal and short-term solar radiation. Seasonal variation may pose some concern for solar power plants in Texas if solar becomes a significant portion of the state’s energy resource mix, unless technologies for seasonal energy storage to compensate for these seasonal variations become feasible. Note that for this particular week there are significant differences depending on location, and these are due mainly to prevailing weather fronts and patterns. However, unlike the impracticality of long-term storage to ameliorate seasonal variations, storage for diurnal and short-term variations is more likely to be feasible.
But another significant energy demand is for thermal energy, including heating water and living spaces, moderate to higher temperature industrial heating applications, as well as drying of grain crops and wood products.
The distributed generation capability of solar is a major advantage, because energy production at the point of demand reduces the need for transmission and distribution infrastructure.
For central solar power generation the systems may be either solar thermal or photovoltaic. This design uses linear parabolic reflectors (concentrators) to reflect direct solar radiation to a tube carrying a fluid along the focal line.
It has a capacity of 64 MW and is projected to produce approximately 130 million kWh per year. The heart of the design is a parabolic dish reflector, which tracks the sun and concentrates the direct solar radiation to its focal point. Another is the salt-gradient pond, which permits solar radiation to be captured in nearly saturated brine at the bottom of a pond, which then can be recovered to drive a Rankine cycle engine-generator.
This may be accomplished using flat PV panels that are either stationary or tracked to follow the sun, or by using concentrating optics to focus the radiation on a much smaller area, thus reducing the amount and cost of expensive cells. This enables PV cells to be combined in scale to produce large, multi-MW central station power generation facilities. The most common technology in commercial production historically and today uses highly-refined crystalline silicon for its semiconductor layer.
The most difficult thermal applications to achieve are cooking and high-temperature industrial heat applications.
Flat plate thermal collectors consist of a dark absorber panel with incorporated fluid passages housed in an insulated box with a transparent glazing on the front. These come in a number of designs, but in general consist of either a reflector or lens which concentrates solar radiation onto a smaller absorber surface including passages for the transfer fluid. Several different floor plans have had solar water heaters installed by Sun Trapper through grant funding by City Public Service.
For residential applications the panels are usually fixed on a tilted roof facing south (see Exhibit 3-19), while for commercial applications the panels are typically located on flat roofs or mounted on special structures outside the building (see Exhibit 3-20). Conversely, when excess power is produced by the PV system, the excess flows out of the customer’s property and into the utility distribution system. As with solar thermal technologies, solar cooling may be achieved by driving conventional air conditioning systems with PV-generated electricity. Such a policy was in place for Texas customers of vertically integrated utilities until the introduction of competition to the state, at which time net metering was no longer available to customers in the ERCOT competitive area. It replaces traditional net metering with a voluntary program in which utilities or retail electric providers (REPs) do not net a customer’s production against consumption, but instead have the option to buy back excess production at a rate negotiated with customers. Some examples include rural water pumps, traffic signals, emergency call phones, metering and communication equipment in oil-field applications or other remote applications where it would be expensive or impractical to extend a utility distribution line (Exhibit 3-21). The battery capacity is typically designed to provide five to ten days of autonomy so that the systems very rarely fail to meet the load. In general, some solar thermal applications, especially passive applications like daylighting, and active applications like solar water heating, have been cost-effective for many years.
Texas is beginning to see serious interest in development of these projects already, and it is likely that one or more large-scale solar projects will be developed in the state over the next several years. A number of other states and countries have adopted such policies and fostered large domestic markets, industry experience and skilled workforces which they plan to capitalize on when large-scale markets emerge elsewhere. This section focuses on Texas electricity costs, and reports recent retail and wholesale costs of electricity in order to enable meaningful comparison of solar technologies to market costs. Solar energy systems usually generate more electricity during the hottest time of the day, and thus can help to offset the need to add expensive electric generating capacity to satisfy peak demand. Large-scale solar thermal technologies achieved dramatic cost reductions in the 1980s relative to other renewable technologies due to increased efficiencies in parabolic trough, power tower, parabolic dish, and fresnel reflector designs. Department of Energy’s 2005 Multi-Year Program Plan for Solar and based on parabolic rough technologies and a detailed due-diligence study completed in 2002.
Solar water heating (residential, commercial and institutional) is generally cost effective in Texas in comparison to heating with electricity (10 to 15 year payback) but somewhat less cost effective compared to heating with natural gas (15 to 20 year payback).
This cost is mostly a function of the cost of solar photovoltaic modules, though as module costs decrease other factors are likely to become more prominent.
Likely technology improvements include higher efficiencies, increased reliability (which can reduce module prices), improved manufacturing processes, and lower balance of system costs through technology improvements and volume sales.29 Exhibit 3-23 shows the historical and projected levelized costs of energy from photovoltaic power. Department of Energy projects that solar market penetration will increase dramatically in the next 5 to 10 years, once falling prices for solar achieve parity with the costs of conventional generation (see Exhibit 3-24). Additionally, solar energy tends to be synchronous with energy demands, and when deployed as distributed generation can reduce loads and congestion on utility distribution and transmission systems. Producing energy from solar offsets energy produced from other, typically fossil, resources, and therefore reduces emissions that would otherwise be produced from those resources.
In most areas of the state, demand is at its maximum in the summer due to air conditioning when the solar resource is greatest. By producing energy at the point of consumption, distributed generation reduces the need for the transmission and distribution infrastructure and make for a more robust system, with less susceptibility to central systems failures. At the federal level, an important subsidy is a 30 percent federal tax credit (ITC) for solar energy equipment. In Texas, the state provides businesses with both a franchise tax deduction and a franchise tax exemption for solar energy devices. Since 2005, just 9 MW of non-wind renewable generating capacity, in the form of a single landfill gas plant,36 along with several MW of customer-sited solar generation spurred by municipal subsidy programs, has been completed.
Texas’ net metering policies and practices are similarly inconsistent and depend upon the particular retail electric provider, municipal utility or rural electric cooperative to which the distributed generator is interconnected.
While the resource level improves from east to west across the state, it is not highly localized like other renewable energy resources. However, solar costs are declining with the introduction of new technology types and improvements in manufacturing processes.
Research has shown that renewable energy generates more jobs in the construction and manufacturing sectors, per megawatt of installed power capacity, than does fossil fuel generation.44 This conclusion is reflective of the relationship between labor and fuel costs as inputs to energy generation.
The concept of customer choice must be expanded to include not just choice among utilities or retail electric providers, but also the choice to generate or offset some or all of one’s own energy through solar or other on-site renewable distributed generation.
Although you don’t really go on to say how good of an investment solar energy actually is, in the long run. You are not considering the cost of pollution and environmental degradation related to fossil fuels (coal, oil and gas) energy. This allowed us to examine the basic effects of albedo, latitude, and seasons on our planet's overall average temperature. Let's quickly review the structure of the atmosphere, since some aspects of that story are relevant to how and where solar energy gets absorbed.
It extends upward from the ground to an altitude of about 16 km (in the tropics, or 8 km near the poles).
Starting at the top of the thermosphere, this extremely tenuous layer gradually gives way to the vacuum of interplanetary space.
Fortunately for us, all of the high energy X-rays and most UV is filtered out long before it reaches the ground. Recall that the Sun emits a broad range of frequencies, from high-energy X-rays and ultraviolet radiation, through visible light, on on down the spectrum to the lower energy infrared and radio waves. Likewise, most of the UV radiation (especially the highest energy, shortest wavelength regions of the UV spectrum) is blocked by the thermosphere, mesosphere, and stratosphere.
It includes some of the longer wavelength UV frequencies, some of the shorter wavelength IR frequencies, and all of the visble light region of the spectrum.
Recall also that the peak of the Sun's EM emissions are in the visible light region of the spectrum.
So about 30% of the incoming sunlight is reflected back into space by clouds or light areas on Earth's surface, or scattered back out into space by gas molecules in the atmosphere (that scattering is what makes the sky blue, not black!).
Such is the case with the panes of glass in a greenhouse; sunlight readily passes in, providing plants with the energy they need for photosynthesis and warming the inside of the greenhouse, but the IR radiation that the warmed interior emits does not readily pass back out through the glass, for the glass is largely opaque at infrared frequencies.
The Website was developed in part with the support of UCAR and NCAR, where it resided from 2000 - 2010.
Meeting all future Texas energy demands with solar energy is technically possible, but further technology development and cost reductions are required before this immense resource will be able to provide a significant portion of Texas’ energy needs reliably and at an acceptable cost. First, while the solar resource is vast, it is not highly concentrated and, therefore, requires significant surface area to collect an appreciable amount of energy.
Natural sunlight is increasingly utilized in modern building design; day-lighting can be successfully incorporated into almost any structure, even underground buildings, such as the Texas State Capitol Annex. Current commercially-available photovoltaic (PV) solar cells are capable of converting sunlight directly into electricity at 15 to 20 percent efficiency, while cells in research and development environments have achieved greater than 40 percent efficiency.1 Many solar applications are already cost-effective, while costs for others have been continually decreasing.
While the market cost of some of the solar technologies is still relatively high, the desirable characteristics of solar technology - generally synchronous with demand, limited or no emissions and water requirements, and the vast solar resource in Texas — suggest great promise for the near future.
Exhibit 3-1 relates the various solar conversion technologies to the fundamental solar parameters on which they depend.
Utility-scale PV and thermal solar installations often make use of tracking hardware to boost their energy output, though the addition of tracking hardware is usually not cost-effective on smaller installations, such as on the rooftops of residential or commercial buildings. Normal insolation is that received by a tracking surface that always faces the sun, such as a solar collector which tracks the sun’s movement through the sky. More commonly, solar equipment is tilted relative to horizontal (usually tilted toward the equator, e.g. These technologies enable more sunlight to be absorbed and converted into electric current, increasing overall efficiency. Certain events such as major forest fires and, even more significantly volcanic eruptions, can produce unexpected declines in solar irradiance for extended periods of time. For example, a 200 MW thermal trough plant in west Texas would require about 1,000 acres of land. While the quantity of water needed per acre of use is similar to or less than that needed for irrigated agriculture, dependability of the water supply is an important consideration in the sunny, dry areas of the state that are favored for large scale solar power plants.
Intermittent resources such as wind and solar can pose unique problems in transmission planning and in efficient utilization of transmission infrastructure, resulting in higher transmission costs, increased congestion, and even generation curtailments when adequate transmission capacity is not available. Substantial penetration of intermittent energy resources into the Texas electric grid is likely to create additional costs to ensure that adequate operating reserves, demand-response, storage, or other technologies are online and available to respond to short-term fluctuations in energy production.6 Widespread integration of solar resources may compound some of the grid integration challenges already posed by wind in Texas, but may alleviate others through resource diversification.
This means medium-term storage technologies, enabling the delay of energy outflows from solar generators by just a few hours, could be quite valuable economically to solar generators. Some technologies, such as domestic hot water pre-heat systems, have effective storage built-in.
For example, they do not consume water and, to the extent that distributed generation facilities reduce the amount of energy required from traditional power plants, they can reduce the amount of water consumed in the production of electricity. A rotating shadow band (RSB) instrument uses a photosensor with a motorized rotating band that periodically blocks direct sunlight from the sensor.
Within Texas, the 1991-2005 NSRDB has 89 stations, with 15 Class I, 38 Class II, and 36 Class III stations. The TSRDB Internet site14 provides global horizontal, direct normal and diffuse horizontal data for the 15 locations on hourly intervals from 1996 to 2002.


The desert Southwest experiences the highest levels of solar radiation in the United States and far west Texas receives insolation levels within 10-15% of the best in the nation. The additional blue bars in Exhibit 3-9 represent the diffuse radiation falling on the horizontal collecting surface; the sum of the two represents the total solar radiation striking the horizontal surface. Direct normal insolation values also vary over a wider range than do horizontal insolation values. El Paso through Abilene), the direct normal insolation alone is greater than the total horizontal insolation.
The following exhibits depicting resource variability show both direct normal insolation, the most variable component of solar radiation, and global horizontal insolation.
Low and high insolation years typically occur simultaneously for all of Texas, the low years usually a result of persistent rain caused by El Niño events. In the winter months the days are shorter, cloud cover is greater, and the sun is lower in the sky, requiring sunlight to travel a longer path through the through the atmosphere and be scattered by clouds, dust, and pollution before reaching the Earth’s surface. An example is the sharp drop in insolation during late summer in El Paso, when the rainy season occurs in the Desert Southwest. Exhibit 3-12 shows a five-day period in the summer for several locations across the state, a span which includes clear periods and periods with intermittent sunshine. Most large solar thermal power plants, for example, are now designed to accommodate several hours of thermal energy storage. Furthermore, solar energy by its nature is suitable for local generation, producing no air, water, or noise pollution.
The radiation’s energy is absorbed in the fluid which flows to a steam generator and turbine which drive an electric generator. There, the solar energy heats a fluid, which flows to a steam turbine, which in turn drives an electric generator. The tracking and concentrating methods parallel those described in the previous section on solar thermal technologies, and are not addressed here. When photons in sunlight strike the top layer of a PV cell, they provide sufficient energy to knock electrons through the semiconductor to the bottom layer, causing a separation of electric charges on the top and bottom of the solar cell.
It consists of about 70,000 tracking solar panels distributed over 140 acres, is rated at 15 MW (18 MW-DC), and produces about 25 million kWh annually. When used to produce electricity, utility interconnection and net metering policies greatly influence a customer’s ability to install systems and lower their energy bills, respectively.
Since it is generally not cost-effective to transport thermal energy over long distances (more than a mile), these applications are invariably distributed, with energy being collected near the point of demand. There is a wide variety of industrial heat applications requiring temperatures up to and more than 1,000 degrees Fahrenheit and for most of these applications concentrating solar thermal collectors are required. Condensation of vapors provides the same cooling effect as that provided by mechanical cooling systems.
At this point it is not clear how many REPs or utilities will offer a buy back option, how many customers will be served by a REP offering a buy back option, or what the value of buy back offers will be. Inclusion of battery capacity to meet the load during inclement weather period is often more economical than extending a distribution line and incorporating a step-down transformer, or providing fuel and a back-up generator.
Solar thermal generators incorporating some degree of storage are even better able to capture high wholesale prices during periods of peak demand. During the 1990s solar thermal research and development funding levels were lower and cost reductions came largely from improvements in operation and maintenance.
Active solar heating of living space in Texas (except possibly far north Texas), is not generally considered cost effective, because of Texas’ short heating season. Of course, the manufacturing of solar equipment, like equipment for any power system, requires energy inputs and results in some effluent waste. Furthermore, peak demand for electricity typically occurs in the later afternoon when available solar energy is still high.
In addition, Texas has a property tax exemption for the appraised value of a solar or wind-powered energy device for on-site energy production and distribution. In Texas, these programs are exclusively offered by municipal utilities on a voluntary basis. This means solar is useful in central solar power stations in west Texas or in distributed generation applications which reduce the need for transmission from resource-rich areas to load centers. As costs decline, larger projects may leverage economies of scale and become cost-effective within the next few years. Clear and consistent interconnection and net metering policies and processes statewide would further enable solar industry development and foster a cleaner, more diverse energy supply for all Texans. The TSRDB is based on solar radiation measurements taken at 15 locations in Texas (Abilene, Austin, Big Spring, Canyon, Corpus Christi, Del Rio, Edinburg, El Paso, Clear Lake, Laredo, Menard, Overton, Pecos, Presidio, and Sanderson) over a approximately a six year period.
We'll now add the atmosphere into our figuring; this will complicate matters, but will also make our model more realistic. Temperatures once again decline with increasing altitude (as was the case in the troposphere), falling as low as -100° C (-146° F) in the upper mesosphere.
This region is one in which temperatures once again rise with increasing altitude, reaching as high as 2,500°C (4,500°F) in the daytime!
Different wavelengths of this solar radiation behave differently as they enter our atmosphere.
The relatively low energy, long wavelength portion of the UV spectrum that does reach the ground forces us to wear sunglasses and slather ourselves with sunscreen to protect ourselves from sunburn and skin cancer.
The longest wavelength radio waves also fail to penetrate the atmosphere; many are absorbed or reflected by the ionosphere. So, although the atmosphere blocks out much of the range Sun's range of EM emissions at various altitudes, the bulk of the EM radiation from the Sun in the form of visible light does reach at least as far as the troposphere.
Some of that radiation goes upward, escaping into space; but some goes downward, further warming the ground and the lower atmosphere. I think each diagram has its strengths in helping to tell parts of the story; and I'll let you choose which (if any) you think might be most helpful when presenting these ideas to your students. Second, the cost of producing energy in large-scale solar power plants is still high relative to other options. The result is that the sunlight reaching the Earth’s surface is both direct and diffuse. Most subsidy programs encouraging installation of distributed solar specify minimum system design standards, which include standards pertaining to the tilt and orientation of the panels, to ensure that only systems with appropriate surface orientation are eligible to receive subsidy funding. These three solar quantities (GHI, DHI, and DNI) are related by the equation shown at the top of Exhibit 3-2. Some solar processes operate on a limited spectral band, examples being photosynthesis and photovoltaic cells. Exhibit 3-3 illustrates the solar radiation spectrum and compares the spectral responsiveness of different PV cell technologies.
Due to potential transmission constraints, solar project developers will need to evaluate the economic tradeoff of locating where the resource is best versus locating nearer to loads where transmission constraints are less likely. In addition, small-scale solar systems can be sited on existing buildings, eliminating the need for dedicated land to produce energy and reducing, or at least not contributing to, the need for new transmission and distribution facilities.
This single instrument measures both global (beam plus diffuse) and diffuse radiation and from these two quantities direct normal can be computed. Data are available for global horizontal, direct normal, and diffuse solar radiation on a temporal basis. For example, direct normal insolation is about 80 percent higher in El Paso than Houston, but global horizontal insolation is only about 30 percent higher in El Paso than Houston This means that concentrating solar plants, whose performance is driven by normal insolation, have more to gain by locating in far west Texas. In contrast, in almost all other cities the direct normal insolation is lower than the total horizontal insolation. As a general rule, the variability of global horizontal insolation is less than that of direct normal insolation.
This year-to-year variability poses little concern for solar power plants if proper care has been taken to consider the economics and operational effects of low and high solar resource years. In contrast, the eastern half of Texas experiences relatively high insolation during mid- to late-summer.
Research is ongoing to determine the effects of passing clouds on the generation characteristics of large-scale photovoltaic plants. Connecting the bottom layer to the top with a conductor completes an electrical circuit and allows the electrons to flow back to the top, creating an electric current and enabling the cycle to repeat with more sunlight.
SunPower Corporation’s PowerLight subsidiary designed and installed the system (see Exhibit 3-17). For a low temperature application like swimming pool heating, the only thing that is necessary is an absorber panel with integrated fluid passages; however, glazing and insulation are needed to achieve higher temperatures for domestic water heating. Although absorption chillers require electricity for pumping the refrigerant, the amount is very small compared to that consumed by a compressor in a conventional electric air conditioner or refrigerator. In the case of rural water pumps, storage is provided not by batteries by a water storage tank or reservoir, ensuring that water is pumped when sunlight is present but available even when it is not. Future cost reductions are projected to result from improved reflectors and lower-cost heliostat designs, improved solar thermal receivers, heat exchangers and fluid handling technologies, and turbines and generators, as well as from volume manufacturing.26 Exhibit 3-22 shows historical and projected costs of centralized solar thermal power. Solar driven air-conditioning, while it may seem ideal in the sunny and hot Texas climate, is not currently considered economical. At the other end of the equipment’s lifespan, improper disposal of certain photovoltaic technology types which make use of heavy metals, such as cadmium, could result in environmental harm. Some applications, such as water heating, are particularly well matched to solar energy and have a positive effect on a utility’s load factor. Thus far, however, these state policies have not resulted in significant growth in Texas’ solar market. These data are on a resolution about 9 times finer than obtained for the 89 Texas sites in the 1991-2005 NSRDB. So multiply the cost per KWH number for solar some 2-3 times to get more realistic picture. We will just take a basic look at the atmosphere's influence on incoming solar radiation; we won't discuss winds and circulation patterns at this point.
Jet airliners fly in this layer, for it is far less turbulent than the underlying troposphere. This layer is relatively poorly studied, for it is above the reach of most aircraft but below the altitude where satellites orbit. Embedded within the thermosphere are several layers of the ionosphere; regions where ionized gas particles can reflect radio waves, a feature that people used to send messages beyond the line-of-sight range of the horizon before the advent of satellites.
Most radio waves do make it to the ground, along with a narrow "window" of IR, UV, and visible light frequencies. This cycling of the IR radiation's energy through the lower atmosphere warms our planet to a much more comfortable temperature than the frigid range that we calculated for an airless world. This new solar activity stems from the increasing costs and price volatility of fossil fuels, concerns about global climate change, decreasing costs and technology improvements in the solar industry itself, and the combined effect of new federal, state and local subsidies. And third, the solar resource’s intermittency and cyclical nature pose challenges for integrating solar at a large scale into the existing energy infrastructure. On clear days the direct component is high and the diffuse is low, while on overcast days the total radiation is lower and most of it is diffuse. In such cases, both direct normal (DNI) and global horizontal insolation (GHI) data can be used to estimate or model the solar radiation in the plane of interest, with the result referred to as global tilt insolation (GTI). To be useful for adequately assessing the solar resource in a specific location, it is necessary to have long-term and accurate data. RSBs are becoming a standard instrument and data from them are generally designated as Class 2. And conversely, it means that the performance of flat plat collectors, typically used in residential and commercial applications and whose performance is driven by direct plus diffuse insolation in the plane of they collector array, is less dependent on their specific location in the state. Thus, if a low temperature application such as solar water heating is considered, flat plate collectors which collect both the direct and diffuse radiation are not only less expensive, but will perform better in east Texas than concentrating collectors which operate on direct radiation only. Even though these are the sunniest months along the Texas Gulf Coast, the level of direct normal insolation throughout the coastal region is still about 25 percent lower than that experienced in west Texas.
A common example is water heating, which is normally accomplished with electricity or gas, but can also be readily accomplished by solar. Evacuated tube collectors, which house the absorber in an evacuated glass tube, permit even higher temperature collection with flat plate collectors. Solar absorption cooling systems are typically sized to carry the full air conditioning load during sunny periods. Water heating systems typically have their own storage vessel and one of their characteristics is that late in the day in the hottest summer months, when the utility experiences its highest demand, solar water heaters are fully charged during the day and require little if no energy during the late-afternoon peak demand period. The credit originally was set to expire at the end of 2007, but Congress extended it for another year, through December 31, 2008.
Solar at this time is completely lacking any commercial viability for most people and businesses.
The troposphere is warmest near ground level, and cools gradually the higher up in it one goes. The stratosphere extends upward from the top of the troposphere to an altitude of about 50 km.
Our atmosphere is opaque across much of the IR spectrum; take a look again at the image above that shows how much of the incoming IR radiation from the Sun fails to make it to the ground. While the solar resource’s dispersed nature cannot be changed, the cost of utilizing solar energy can be reduced through technological advances, improved manufacturing techniques, and increasing economies of scale.
Only data collected over several years from locations having Class 1 or Class 2 instruments were used as the basis for the latest National Solar Radiation Database described below. Even for locations where the direct normal insolation is higher than total horizontal insolation, flat plate collectors may still be the better choice, especially for smaller-scale systems, because they tend to be less expensive and more reliable.
The summer period is bracketed by May and September, two of the heaviest rainfall months for much of Texas. Because absorption cooling equipment requires input temperatures of approximately 200 to 250 degrees Fahrenheit or greater, concentrating or possibly evacuated tube collectors are needed. Then, in October 2008, Congress extended the credit for an additional 8 years and eliminated the $2,000 cap for residential systems. Many of the atoms and molecules in the thermosphere (and above) have lost electrons, thus becoming electrically charged ions; so the motions of particles in the upper atmosphere are partially influenced by electrical currents and Earth's magnetic field.
On the way in, most of the EM radiation is in the form of visible light, which easily passes through clear air. Exhibit 3-11 indicates that, for most of Texas, these two months have relatively low insolation for most of Texas.
At this point a critical player in the climate drama enters the scene - the Greenhouse Effect!
On the way, back up, however, much of the radiation is in the form of infrared, which is absorbed and thus stopped before it reaches space. It is generally concluded that when this occurs solar energy will become a major contributor to meeting future energy needs in Texas, the nation and the world. Earth's atmosphere, like the panes of glass in the greenhouse, traps much of the infrared radiation, and the heat that it carries, warming our planet. Several different gases play a role in this greenhouse effect; water vapor, carbon dioxide, and methane are amongst the most prominent.



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