By James Stalker
Because of the considerable financial incentives, in the recent years, for solar energy project development efforts from federal, state, and local governments, the solar energy sector has experienced abnormal growth in the United States of America.
However, before choosing a site for development of a utility-scale solar energy project, project developers and investors have to understand the energy production efficiency and the year-to-year variation in that energy production efficiency at that site. With this type of understanding, these stakeholders can confidently determine profitability, financial risks, and operational challenges and opportunities.
In section 2, a brief overview of solar radiation and its various components used in the solar energy industry is given. Section 3 describes a few other approaches in use today. In section 4, details of the SunCloudConfluence approach are provided. Section 5 provides summary and some concluding remarks.
When it comes to ascertaining solar energy potential, the diurnal, seasonal, and year-to-year variation of solar radiation reaching the top of the atmosphere is important and can be obtained accurately from the Sun-Earth positioning. What can not readily be determined is the solar radiation reaching the surface, at any site of interest, since as the radiation passes through the atmosphere it undergoes changes. What make it difficult to determine the solar radiation reaching the surface are clouds. When clouds are present, the solar radiation reaching the surface diminishes significantly, compared to the solar radiation at the top of the atmosphere.
Solar radiation reaching the surface is usually expressed in terms of the total solar radiation impinging on a horizontal surface on the Earth surface. This is called the global horizontal irradiance (GHI). The GHI consists of direct solar radiation and diffuse solar radiation components. The direct solar component is that portion of the solar radiation that is reaching the horizontal surface without being affected by clouds and aerosols. The diffuse radiation component is that portion of the total solar radiation that has undergone multiple interactions with clouds, aerosols, and ground before reaching the horizontal surface. The total and the type of split between the two components of solar radiation are strongly affected by clouds. It is imperative to develop the ability to account for clouds to perform accurate solar energy assessments and that is exactly the capability SunCloudConfluence has to offer.
A fourth kind of solar radiation variable, commonly used in the solar energy assessment efforts, is known as the direct normal irradiance (DNI). It is the direct solar radiation component reaching a panel that is constantly moved, using sun-tracking racks, to keep the Sun always directly perpendicular to the surface of the panel. The DNI is used for concentrated solar power (CSP) systems.
3. Solar Energy Assessment Approaches
3a. Satellite Based Approach
Sensors flown aboard weather satellites are used to collect reflected solar radiation from the earth’s surface, on clear days or that solar radiation reflected from the top of clouds, on cloudy days. In other words, on cloudy days, solar radiation reaching the ground has to be determined in some other way.
Empirical relations are usually developed from the solar radiation reflected from clouds and surface solar radiation measurements made at the site, to account for the effects of clouds. This type of approach requires surface solar radiation measurements to develop reasonably accurate empirical relations between solar radiation at the top of clouds and at the surface. For the long-term solar energy calculations, where long-term surface solar radiation measurements are not available, this type of empirical relations should be relied upon. The statistical nature of these empirical relations assumes all cloud types can be represented by the same empirical relations. This is a rather limiting assumption, even when cloud height is derived from satellites and used to distinguish various cloud types.
Additionally, the horizontal resolution and temporal resolution of satellite observations may be coarser than desirable, especially in certain parts of the world where such observations are not available. Also, records of satellite observations may not go back to fifteen (15) or twenty (20) years for many parts of the world, which is a severe limitation in determining long-term solar energy projections accurately.
All of the aforementioned satellite data limitations are critically important to understand before using any satellite based approaches for the purposes of solar energy assessments.
3b. Site Measurement Approach
As mentioned before, one can choose to collect solar radiation measurements at the site for assessing solar energy potential. There are various instruments (e.g., pyranometers, pyrheliometers, and rotating shadow-band radiometers) to measure total solar radiation and a way to split direct and diffuse solar radiation components. While the site measurement approach is a valid approach, it proves to be expensive to acquire and deploy such instruments. It is also time consuming to make measurements for longer periods of time such as five (5) to ten (10) years so this approach is not practical. Solar radiation information for additional years may be obtained from other sources such as satellites to determine long-term solar energy projections based on the site measurement approach.
4. SunCloudConfluence Approach
The SunCloudConfluence solar energy assessment approach presents a truly viable and comprehensive alternative to either satellite based approaches or surface measurement based approaches. SunCloudConfluence employs a meso-microscale modeling technique to enable cloud processes leading to various types of clouds depending on the environmental conditions. These environmental conditions constantly change at a location leading to different types of clouds or no clouds at that location. Depending on the type of clouds and their internal cloud structures, the loss characteristics of solar radiation passing through clouds may be quite different from one another. These loss characteristics essentially determine the amount of solar radiation reaching the surface within a cloudy environment.
The SunCloudConfluence meso-microscale model uses 1-km horizontal and variable vertical resolution with highest vertical resolution near the surface. Input data to drive this model are obtained from the U.S. National Centers for Environmental Prediction (NCEP) North American Mesoscale (NAM) model and other data sources as appropriate. Using these public domain data and SunCloudConfluence proprietary methods, highly accurate solar radiation information is developed for sites anywhere in the world. SunCloudConfluence can produce yearlong solar radiation datasets in about a month and 10+ years in 3 to 4 months. Robust and accurate long-term solar energy projections can be produced in a matter of months.
SunCloudConfluence can provide solar radiation datasets (total, direct, diffuse, and direct normal) in hourly format or any other temporal resolution customers may require. SunCloudConfluence can produce solar radiation maps, as shown in Figure 1, and/or as profiles, as shown in Figure 2.
5. Summary and Concluding Remarks:
- Three types of approaches (satellite based approaches, site measurement based approaches, and the SunCloudConfluence approach) for solar energy assessments have been described.
- Satellite based approaches rely on simplistic assumptions of cloud structures to build empirical relations between solar radiation observed at the top of clouds and that amount of solar radiation reaching the ground
- Site measurements are limited and expensive so they don’t offer a practical alternative for long-term solar energy projections
- SunCloudConfluence offers accurate and comprehensive solar radiation information for the purposes of long-term solar energy assessments.