1 Measuring Solar and Infrared Radiation

If the observation of the amount of heat the sun sends the earth is among the most important and difficult in astronomical physics, it may also be termed the fundamental problem of meteorology, nearly all whose phenomena would become predictable, if we knew both the original quantity and kind of this heat.

Samuel Pierpont Langley

(1834–1906)

Solar radiation measurements are the basis for understanding Earth’s primary energy source. Humans have studied the sun and applied their understanding to improve their lives since the dawn of history. Buildings have been designed to take advantage of solar radiation for daylighting and heating since the time of Greek city states, if not earlier. Today, solar-generating facilities that are 200–300 MW in size are under construction; 17,000 MW of peak solar-generating capacity were installed in 2010 with the equivalent annual power output of four to five large nuclear or coal power plants. The world is fitfully preparing for the solar age when oil will be depleted and environmentally benign sources of energy will be needed. Today a wide variety of solar technologies are being used, and new and improved solar technologies are being explored in the lab and tested in the field. The sun’s energy is free and available to everyone, and the technologies that turn solar irradiance into useable energy are dropping in price as the solar industry grows and production increases. Given the right financial structure, many solar technologies have found cost-effective markets. For example, a cost-effective project to replace kerosene lamps with photovoltaic (PV) panels and batteries in the Dominican Republic has been successful (Perlin, 1999). Since 1985, the Luz plants in California, with 354 MW of generating capacity, have been producing electricity from solar thermal–electric power facilities. The future for the solar industry is bright as we work to develop sustainable energy sources for the growing world population while reducing deleterious effects of greenhouse gas emissions.

Part of the fundamental infrastructure that supports the growing solar industry includes solar resource assessment and forecasting. As in hydroelectric generation where stream flow data are necessary to design and operate the facility, knowledge of the temporal and spatial behavior of the solar resource is critical to the design and operation of solar electric-generating facilities. With multimillion and even billion dollar solar projects under development, the need for high-quality solar data is imperative. Solar data are needed in the planning and design of the facility as well as for its operation after completion. Testing new technologies to characterize and validate the design improvements requires the highest-quality solar irradiance measurements because a 1–2% increase in efficiency can translate into millions of dollars in cost reductions. Developers and financial institutions are interested in the economic viability of projects and the ability of the facility to service its debt in even the cloudiest years. The better the solar resource information available, the more accurately a project’s performance can be estimated, reducing uncertainty and risk for the investors. Lower risks translate into lower interest rates, which result in lower costs to the ratepayers.

The solar industry is not the only group interested in the solar resource. For decades, farmers have used solar data to evaluate their irrigation requirements. Climate and atmospheric scientists use solar resource data in their studies. Solar forecasts based on weather forecasts from the National Weather Service are being used now, although there is a need for considerable improvement in this field. Architects are using sunlight to reduce energy demands on buildings, and some buildings are being constructed that have a zero net energy balance.

There is a broad community of users of solar data and groups who supply this information. Many of these groups are federal agencies tasked to provide these data. Scientists are running solar measurement networks to study the climate and weather. Industry is prospecting for sites with “bankable” solar resource data to convince the financial community of proposed project viability. Private groups and individuals are making measurements to evaluate the performance of solar systems. Some schools are integrating the solar resource data into their science and engineering curricula. To accommodate the needs of this broad group of interested parties with their diverse requirements, a wide variety of solar instruments is available. The price and quality of these instruments vary widely, with the more expensive equipment usually providing more accurate results. This book provides an extensive survey of the solar instrumentation that is available and attempts to provide general performance characteristics that can help the reader select the appropriate instrument for the task and provide information necessary to assess the accuracy and quality of the data produced.

The main body of this book describes the solar irradiance sensors and how they function while providing a description of the type of solar radiation they are designed to measure. Useful information on solar resource assessment and auxiliary information are contained in the appendices. Chapter 2 describes the nature of solar radiation and provides the terminology and basic equations used in solar resource assessment. A thorough understanding of this background information is needed to fully comprehend the information in the following chapters. Chapter 3 is a history of the development of solar instrumentation. This chapter is useful for understanding the difficulty in obtaining accurate solar radiation measurements, and it shows the considerable improvements that have been made over the years. Some of these instruments are still in use today, and understanding historic data requires knowledge of how these instruments operated and performed. Those who want to use these older data should become familiar with the instruments that provided the data and understand their limitations.

Chapters 4, 5, and 6 cover basic solar radiation measurements and instrumentation. Chapter 4 discusses instruments used to measure direct normal irradiance (DNI) or “beam” irradiance (radiation coming directly from the sun). Astronomical-based calculations tell us exactly where the sun is in the sky, and we can calculate the coordinates and angles associated with the direct solar radiation to a high degree of accuracy. This knowledge plus the fact that most of the solar radiation received comes directly from the sun enhances the need to make sound direct normal irradiance measurements. Besides describing the instruments used to make DNI measurements, Chapter 4 discusses issues concerning the calibration and accuracy of these measurements. Chapter 5 discusses the instruments used to measure the total irradiance on a horizontal surface, referred to in this book as global horizontal irradiance (GHI). Due to their relative ease of use, global irradiance measurements are by far the most common and are useful to a wide audience from farmers to scientists. There are three types of instruments used to measure GHI, and these are characterized and evaluated in Chapter 5. The measurement of diffuse irradiance is covered in Chapter 6. While diffuse measurements use the same type of instruments that are used to measure global irradiance, only a limited number of these instruments are suitable for measuring diffuse irradiance. The reason for selecting certain instruments for measuring diffuse horizontal irradiance (DHI) will be covered in more detail in Chapter 6. In all three chapters, the uncertainties associated with the measurements and the methodology used to determine the uncertainties in the measurements are discussed. Chapter 7 discusses the rotating shadowband radiometer (RSR). This single instrument measures the global and diffuse horizontal irradiance and calculates the corresponding direct normal irradiance. The accuracy, advantages, and limitations of this approach are discussed.

Measurements of solar irradiance on tilted surfaces are discussed in Chapter 8. Most solar collectors are tilted to better intercept the incident solar radiation. Some of the instruments suitable for global or diffuse horizontal measurements are unsuitable for tilted irradiance measurements. The problems associated with tilted irradiance measurements are discussed, and the performance characteristics of sensors when tilted are described. This is an important topic as solar system performance is gauged against tilted surfaces measurements. Measurements of surface albedo (ratio of reflected to incident irradiance) are discussed in Chapter 9. Albedo is an important consideration for models that utilize satellite measurements because the albedo determines the amount and nature of the surface-reflected irradiance. In addition, calculations of irradiance on tilted surfaces also depend on characterizing the surface albedo. The amount of reflected global irradiance is dependent on the albedo of the surface in front of the collector, and albedos can vary widely from 10 to 90% reflectance depending on the surface characteristics.

Measurement of infrared (IR) irradiance is covered in Chapter 10. The earth’s surface and the atmosphere emit IR radiation, and the amount of IR radiation emitted is proportional to the temperature and emissivity of the emitter. Most solar instruments do not detect IR radiation; special instruments are needed to measure the incoming and outgoing IR irradiance.

Instruments that measure net radiation are discussed in Chapter 11. Net radiation is the difference between the incident solar and infrared radiation from the sun and sky minus the solar and infrared irradiance from the ground. When studying the earth’s energy balance, which drives weather and climate, good measurements of the net irradiance are essential. Net radiation is measured many ways, and accurate measurements of net radiation are extremely difficult.

Spectral measurements of solar radiation are cover in Chapter 12. Incident solar radiation is composed of different wavelengths of light, and many natural processes from photosynthesis to human vision use different portions of the solar spectrum. In addition, the performance of solar (photovoltaic, or PV) cells is dependent on the spectral composition of the incident solar radiation as well as the magnitude of the incoming irradiance. Instruments that measure the spectrum of incident solar radiation are expensive and sometimes difficult to maintain. However, good spectral measurements are of great scientific value while also having practical applications. Relatively inexpensive instruments are available to measure selected portions of the spectrum, such as that to which the eye is sensitive for daylighting applications. Instruments that provide high-resolution spectral data over most of the spectrum have considerably higher costs, but deliver a much more useful and flexible product.

Nonradiation meteorological measurements are covered in Chapter 13. Instruments that measure temperature, pressure, relative humidity, wind speed, and wind direction are discussed. These meteorological measurements are important because some parameters affect the performance of certain radiometers; for example, both thermopile and photodiode radiometers are sensitive to the ambient temperature, and a temperature correction can be used to improve the accuracy of measurements by reducing the temperature bias in the measurement. Analysis of photovoltaic panel performance is another use of meteorological measurements. PV panel performance mainly depends on the incident solar radiation, but temperature affects its power production.

Important factors to consider before establishing a quality solar monitoring station are discussed in Chapter 14. Even with the best instruments, a well-designed solar monitoring station is essential to ensure the accuracy, validity, and completeness of the data. Siting to minimize blockage by nearby obstructions and to facilitate maintenance of the station are two key factors. Good record keeping and routine calibration of the station instruments are also essential actions to ensure high-quality solar irradiance data. To get full value from solar measurements, good design, adequate maintenance, and validation of the incoming data are required.

Appendices A to F cover various topics related to modeling solar irradiance, conversion factors, and manufacturers’ information that is useful for solar monitoring. Models used to estimate solar radiation are discussed in Appendix A. These include correlations between various irradiance components and models that are used to estimate irradiance on tilted surfaces. The purpose of Appendix A is to present the most common models and give an idea of how they work. These models are also useful in understanding why certain measurements are made and why they are useful. Appendix B discusses the use of satellite imagery to estimate the large-scale spatial variability of solar radiation. This appendix is an overview of how satellite-based remote sensing models work and their accuracy. Appendix C presents a series of sun path charts for various latitudes. This appendix can serve as a quick reference to the sun path at a given site and illustrates where the sun will be at any given time of the day and year. Appendix D discusses the algorithms used to calculate the position of the sun. Knowing the solar position is always useful when analyzing data, designing a solar monitoring station, or applying solar radiation data. Many different units have been used over the years to describe solar radiation: Appendix E contains a table of useful conversion factors. This table provides information on how to convert from one unit to another. A list of solar instrument manufacturers is given in Appendix F. This list is fairly extensive and includes the major sources of solar instrumentation and provides a basis for searching for available instruments.

A sample calibration report from the National Renewable Energy Laboratory is given in Appendix G. This Broadband Outdoor Radiation Calibration (BORCAL) report illustrates what is included in a comprehensive calibration of a solar sensor. Sunshine duration is discussed briefly in Appendix H. This was one of the first proxies for measurements of solar irradiance, and it is still made in many parts of the world. Although it has been superseded by actual measurements of solar irradiance, it is included for completeness. Finally, Appendix I covers some helpful troubleshooting hints that identify common problems in radiometry and how they might be addressed.

The goal of this instrumentation book is to pass on the experience that the authors have acquired over the years of setting up solar monitoring sites, measuring the solar resource, analyzing the data, and working with a wide variety of users in need of accurate solar resource information. It takes a lot of effort and expense to build a quality solar radiation database. From our collective experience we have learned that proper documentation and archiving of the measured solar irradiance and relevant meteorological data are critically important to the full and appropriate use of the information. Additionally, funding for solar resource assessment comes in waves, and it takes considerable planning, ingenuity, and determination to keep a measurement record intact to produce a long-term quality product that meets the needs of generations to come.

Reference

Anon. 2011. Copper Mountain Solar, the largest photovoltaic solar plant in the U.S., Solar Thermal Magazine. http://www.solarthermalmagazine.com (retrieved 3/4/12).

Neville, A. 2010, December. Top Plant: DeSoto next generation solar energy center, DeSoto County, Florida. Power Magazine, 1. http://www.powermag.com (retrieved 3/4/12).

Perlin, J. 1999. From space to Earth: The story of solar electricity. Cambridge, MA: Harvard University Press