Power Generation from Renewable Energy Sources Fall 2012 Instructor: Xiaodong Chu ： Office Tel.:

Flashbacks of Last Lecture Direct-beam radiation – Beam insolation striking a collector face is translated from the direct-beam radiation, where the incidence angle is used

Flashbacks of Last Lecture Diffuse radiation – How much of that horizontal diffuse radiation strikes a collector?

Flashbacks of Last Lecture Reflected radiation – With I BH and I DH expressed by I B, reflected radiation is

Flashbacks of Last Lecture Total insolation strikes a collector on a clear day Examples 7.9, 7.10 and 7.11 of the textbook: you should master them!

The Solar Resource – Monthly Clear- Sky Insolation The instantaneous insolation equations can be tabulated into daily, monthly and annual values that provide considerable insight into the impact of collector orientation We could compute monthly and annual clear sky insolation on collectors with various azimuth and tilt angles, as well as for one- and two-axis tracking mounts for different latitudes

The Solar Resource – Monthly Clear- Sky Insolation Latitude of Jinan is 36 o 41 ’ North

The Solar Resource – Monthly Clear- Sky Insolation Latitude of Beijing is 39 o 55 ’ North

The Solar Resource – Monthly Clear- Sky Insolation Annual performance is relatively insensitive to wide variations in collector orientation for non-tracking systems

The Solar Resource – Monthly Clear- Sky Insolation It is much more important for a stand-alone PV system to consider monthly distribution of insolation, where batteries or a generator provide back-up power – It is quite important to try to smooth out the month-to-month energy delivered to minimize the size of the back-up system needed in those low-yield months

The Solar Resource – Monthly Clear- Sky Insolation The performance boost caused by both two-axis and one-axis tracking systems is apparent

The Solar Resource – Solar Radiation Measurements Creation of solar energy databases is achieved by using measurement sites while incorporating meteorological data such as cloud mapping taken by satellite, as an important complement to the sparse global network of ground monitoring stations

The Solar Resource – Solar Radiation Measurements There are two principal types of devices used to measure solar radiation – The most widely used instrument, called a pyranometer, measures the total radiation arriving from all directions, including both direct and diffuse components – The other device, called a pyrheliometer, looks at the sun through a narrow collimating tube, so it measures only the direct beam radiation

The Solar Resource – Solar Radiation Measurements Pyranometers and pyrheliometers can be adapted to obtain other useful data Case 1: The ability to sort out the direct from the diffuse is a critical step in the conversion of measured insolation on a horizontal surface into estimates of radiation on tilted collectors. By temporarily affixing a shade ring to block the direct beam, a pyranometer can be used to measure just diffuse radiation. By subtracting the diffuse from the total, the beam portion can then be determined.

The Solar Resource – Solar Radiation Measurements Pyranometers and pyrheliometers can be adapted to obtain other useful data Case 2: It is important to know not only how much radiation the sun provides, but also how much it provides within certain ranges of wavelengths. For example, newspapers now routinely report on the ultraviolet (UV) portion of the spectrum to warn us about skin cancer risks. This sort of data can be obtained by fitting pyranometers or pyrheliometers with filters to allow only certain wavelengths to be measured.

The Solar Resource – Solar Radiation Measurements The most important part of a pyranometer or pyrheliometer is the detector that responds to incoming radiation – The most accurate detectors use a stack of thermocouples, called a thermopile, to measure how much hotter a black surface becomes when exposed to sunlight – The thermopile measures the temperature difference between the black segments, which absorb sunlight, and the reference, to produce a voltage that is proportional to insolation

The Solar Resource – Average Monthly Insolation How to estimate the average insolation that can be expected to strike a collector under real conditions at a particular site? – Site-specific, long-term radiation data are required, which is primarily insolation measured on a horizontal surface – Sort out what portion of the total measured horizontal insolation is diffuse and what portion is direct beam – Convert horizontal beam radiation – Adjust the resulting horizontal diffuse radiation into diffuse and reflected radiation on a collecting surface

The Solar Resource – Average Monthly Insolation Decompose total horizontal insolation into its diffuse and beam components – Define a clearness index where the average daily extraterrestrial insolation on a horizontal surface can be calculated as – Correlate clearness index and the fraction of horizontal insolation that is diffuse have been made

The Solar Resource – Average Monthly Insolation Average beam radiation on a horizontal surface where R B is the beam tilt factor and an average value of it is required Average diffuse and reflected radiation on a tilted collector surface

The Solar Resource – Average Monthly Insolation Once the horizontal insolation has been decomposed into beam and diffuse components, it can be recombined into the insolation striking a collector using the following

The Solar Resource – Solar Radiation Measurements Set up a spreadsheet or map by computation or from pre- computed data available

The Solar Resource – Solar Radiation Measurements Set up a spreadsheet or map by computation or from pre- computed data available

The Solar Resource – Solar Radiation Measurements NASA's Surface Solar Energy Data Set provides monthly average solar radiation data for everywhere on earth at The World Radiation Data Center provides worldwide solar data at The US Renewable Resource Data Center provides US data and maps at