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Power Generation from Renewable Energy Sources. Fall 2012 Instructor: Xiaodong Chu Email ： [email protected] Office Tel.: 81696127. Flashbacks of Last Lecture. The location of the sun at any time of day can be described in terms of its altitude angle and its azimuth angle. - PowerPoint PPT Presentation
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Power Generation from Renewable Energy Sources
Fall 2012Instructor: Xiaodong Chu
Email ： [email protected] Tel.: 81696127
Flashbacks of Last Lecture
• The location of the sun at any time of day can be described in terms of its altitude angle and its azimuth angle
sinsincoscoscossin LHL
cos
sincossin
HS
Flashbacks of Last Lecture
• Solar flux striking a collector will be a combination of direct-beam radiation, diffuse radiation, and reflected radiation
• Estimate the extraterrestrial solar insolation that passes perpendicularly through a surface just outside of the earth’s atmosphere
• Attenuation of incoming radiation is a function of the distance that the beam has to travel through the atmosphere as well as factors such as dust, air pollution, atmospheric water vapor, clouds, and turbidity
)(W/m 365
360cos034.01SC 2
0
nI
kmB AeI
Flashbacks of Last Lecture
• Example 7.8 on page 412 of the textbook: you should master it!
The Solar Resource – Total Clear Sky Insolation on a Collecting Surface
• Direct-beam radiation– Beam insolation striking a collector face is translated from
the direct-beam radiation, where the incidence angle is used
cosBBC II
The Solar Resource – Total Clear Sky Insolation on a Collecting Surface
• Direct-beam radiation– The incidence angle is a function of the collector
orientation, the altitude and azimuth angles of the sun at any particular time
cossinsin)cos(coscos CS
The Solar Resource – Total Clear Sky Insolation on a Collecting Surface
• Direct-beam radiation– For the special case of beam insolation on a horizontal
surface
sin)90cos( oBBBH III
The Solar Resource – Total Clear Sky Insolation on a Collecting Surface
• Diffuse radiation– The diffuse radiation on a collector is much more difficult to estimate
accurately than it is for the beam– Incoming radiation can be scattered from atmospheric particles and
moisture, and it can be reflected by clouds whereas some is reflected from the surface back into the sky and scattered again back to the ground
– The simplest models of diffuse radiation assume it arrives at a site with equal intensity from all directions
The Solar Resource – Total Clear Sky Insolation on a Collecting Surface
• Diffuse radiation– Diffuse radiation on a horizontal surface is proportional to
the direct beam radiation
where C is a sky diffuse factor and can be approximated asBDH CII
)100(365
360sin04.0095.0 nC
The Solar Resource – Total Clear Sky Insolation on a Collecting Surface
• Diffuse radiation– How much of that horizontal diffuse radiation strikes a
collector?
2
cos1
2
cos1BDHDC CIII
The Solar Resource – Total Clear Sky Insolation on a Collecting Surface
• Reflected radiation– The simplest model assumes a large horizontal area in front of the
collector, with a reflectance ρ that is diffuse, and it bounces the reflected radiation in equal intensity in all directions• Ground reflectance range from about 0.8 for fresh snow to about 0.1 for a
bituminous-and-gravel roof, with a typical default value for ordinary ground or grass taken to be about 0.2
The Solar Resource – Total Clear Sky Insolation on a Collecting Surface
• Reflected radiation– The amount reflected can be modeled as the product of
the total horizontal radiation (beam IBH , plus diffuse IDH) times the ground reflectance ρ
2
cos1)( DHBHRC III
The Solar Resource – Total Clear Sky Insolation on a Collecting Surface
• Reflected radiation– With IBH and IDH expressed by IB, reflected radiation is
2
cos1)(sin CII BRC
The Solar Resource – Total Clear Sky Insolation on a Collecting Surface
• Total insolation strikes a collector on a clear day
2
cos1)(sin
2
cos1cossinsin)cos(cos CCAe
IIII
CSkm
RCDCBCC
The Solar Resource – Total Clear Sky Insolation on a Collecting Surface
• Examples 7.9, 7.10 and 7.11 of the textbook: you should master them!
The Solar Resource – Total Clear Sky Insolation on a Collecting Surface
The Solar Resource – Total Clear Sky Insolation on a Collecting Surface
The Solar Resource – Total Clear Sky Insolation on a Collecting Surface
The Solar Resource – Total Clear Sky Insolation on a Collecting Surface
• Collectors are designed to track the movement of the sun across the sky
• Trackers are described as being either two-axis trackers, which track the sun both in azimuth and altitude angles so the collectors are always pointing directly at the sun, or single-axis trackers, which track only one angle or the other
The Solar Resource – Total Clear Sky Insolation on a Collecting Surface
• For two-axis tracking– The collector tilt angle equals the complement of the solar
altitude angle, that is, Σ= 90◦ − β
2
)90cos(1)(
2
)90cos(1
o
o
DHBHRC
BDC
BBC
III
CII
II
The Solar Resource – Total Clear Sky Insolation on a Collecting Surface
• For single-axis tracking– A manually adjustable tilt angle along a north-south axis,
and a tracking mechanism that rotates the collector array from east-to-west
The Solar Resource – Total Clear Sky Insolation on a Collecting Surface
• For single-axis tracking– If a collector mount rotates about its axis at the same rate
as the earth turns, 15◦/h, then the centerline of the collector will always face directly into the sun
– Under these conditions, the incidence angle θ between a normal to the collector and the sun’s rays will be equal to the solar declination δ
The Solar Resource – Total Clear Sky Insolation on a Collecting Surface
• For single-axis tracking– The collector tilt angle is Σ= 90◦ − β + δ
2
)90cos(1)(
2
)90cos(1
cos
o
o
DHBHRC
BDC
BBC
III
CII
II
The Solar Resource – Total Clear Sky Insolation on a Collecting Surface
• Please use Box 7.2 to help you memorize the key points associated with clear sky insolation on a collecting surface
