EE535: Renewable Energy: Systems, Technology & Economics

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EE535 Renewable Energy Systems, Technology
Economics

• Session 4 Solar (1) Solar Radiation

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Solar Radiation
Energy from the sun in the form of ultra-violet,
visible and infra-red electromagnetic radiation
is known as solar radiation

• Annual solar radiation on a horizontal surface at
  the equator is over 2000kWh/m2
• In Northern Europe this falls to about 1000kWh/m2
  (per annum)
• The tilt between the sun and the land reduces the
  intensity of the midday sun

Ultraviolet 0.20 - 0.39µ Visible 0.39 -
0.78µ Near-Infrared 0.78 - 4.00µ Infrared 4.00 -
100.00µ
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Orientation
Z
P
z
d
?

• Flux of solar radiation incident on a surface
  placed at the top of the atmosphere, depends on
  time t, geographical location (latitude f,
  longitude ?, and on the orientation of the surface

Horizon
Equator
E(t, ?, ?) S(t)cos ?(t , ?, ?) S(t) is known
as the solar constant
d is the declination of the sun ? is the hour
angle of the sun ? is the angle between the
incident solar flux and the normal to the surface

The solar constant is the amount of incoming
solar electromagnetic radiation per unit area
that would be incident on a plane perpendicular
to the rays, at a distance of one astronomical
unit (AU) (roughly the mean distance from the
Sun to the Earth).
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Solar radiation spectrum for direct light at both
the top of the Earths atmosphere and at sea
level

• The sun produces light with a distribution
  similar to what would be expected from a 5525 K
  (5250 C) blackbody, which is approximately the
  sun's surface temperature
• Radiation interacts with matter in several ways
• Absorption
• Transmission
• Scattering
• Reflection

http//en.wikipedia.org/wiki/Solar_radiation
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Solar Quantities

• The sun generates approximately 1.1 x 10 E20
  kilowatt-hours every second.
• The earths outer atmosphere intercepts about one
  two-billionth of the energy generated by the sun,
  1.5 x 10 E18 kilowatt-hours per year.
• Because of reflection, scattering, and absorption
  by gases and aerosols in the atmosphere, only 47
  of this, (7 x 10 E17 ) kilowatt-hours, reaches
  the surface of the earth.
• In the earths atmosphere, solar radiation is
  received directly (direct radiation) and by
  diffusion in air, dust, water, etc., contained in
  the atmosphere (diffuse radiation). The sum of
  the two is referred to as global radiation.The
  amount of incident energy per unit area and day
  depends on a number of factors, e.g.
• Latitude
• local climate
• season of the year
• inclination of the collecting surface in the
  direction of the sun.
• TIME AND SITE
• The solar energy varies because of the relative
  motion of the sun. This variations depend on the
  time of day and the season. In general, more
  solar radiation is present during midday than
  during either the early morning or late
  afternoon. At midday, the sun is positioned high
  in the sky and the path of the suns rays through
  the earths atmosphere is shortened.
  Consequently, less solar radiation is scattered
  or absorbed, and more solar radiation reaches the
  earths surface.
• The amounts of solar energy arriving at the
  earths surface vary over the year, from an
  average of less than 0,8 kWh/m2 per day during
  winter in the North of Europe to more than 4
  kWh/m2 per day during summer in this region. The
  difference is decreasing for the regions closer
  to the equator.
• The availability of solar energy varies with
  geographical location of site and is the highest
  in regions closest to the equator.

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Solar Absorption and Reflection

• When a photon is absorbed, its energy is changed
  into a different form electrical or heat
• A fraction of the incoming solar radiation is
  reflected back into space this is known as the
  albedo (a0) of the earth-atmosphere system
• Annual average of a0 is 0.35
• Reflection from clouds 0.2
• Reflection on cloudless atmosphere (particles,
  gases) - 0.1
• Reflection on the earths surface 0.05
• Radiation absorbed by the Earths atmosphere
• A0 E (1-a0)

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Solar Corrections

• Direct normal solar radiation
• is the part of sunlight that comes directly from
  the sun. This would exclude diffuse radiation,
  such as that which would through on a cloudy day.
  Iindication of the clearness of the sky.
• Diffuse sky radiation
• is solar radiation reaching the Earth's surface
  after having been scattered from the direct solar
  beam by molecules or suspensoids in the
  atmosphere.
• It is also called skylight, diffuse skylight, or
  sky radiation and is the reason for changes in
  the colour of the sky.
• Of the total light removed from the direct solar
  beam by scattering in the atmosphere
  (approximately 25 of the incident radiation when
  the sun is high in the sky, depending on the
  amount of dust and haze in the atmosphere), about
  two-thirds ultimately reaches the earth as
  diffuse sky radiation.
• Global Horizontal Radiation
• total solar radiation the sum of direct,
  diffuse, and ground-reflected radiation
• however, because ground reflected radiation is
  usually insignificant compared to direct and
  diffuse, for all practical purposes global
  radiation is said to be the sum of direct and
  diffuse radiation only.

http//rredc.nrel.gov/solar/pubs/shining/page12_fi
g.html
Insolation is a measure of solar radiation
energy received on a given surface area in a
given time. It is commonly expressed as average
irradiance in watts per square meter (W/m2) per
day. In the case of photovoltaics it is commonly
measured as kWh/(kWpy) (kilowatt hours per year
per kilowatt peak rating).
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Direct Normal Radiation

• E- EsctRatO3tGatWatAEtrmCi
• Ra Rayleigh scattering by molecules in the air
• O3 absorption by ozone
• Ga absorption by uniformly mixed gasses (CO2
  O2)
• Wa absorption by water vapour
• Ae extinction by aerosol particles
• Ci extinction by high clouds of cirrus types
• Scattering and absorption are strongly wavelength
  dependent - (consequence?)

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Clouds

• Cloudfree (direct beam insolation) and cloudy
  periods (prevailing diffuse radiation) average to
  a mean irradiance
• For the assessment of solar power plant sites,
  short interval recordings of sunshine, direct and
  diffuse radiation are required
• Clouds can be classified by their optical depth
• 2 gt dci (1) gt 0.2 gt dci (2) gt 0.02 gt dci (3) gt 0
  
• Cloud Free Line Of Sight Probabilities (CFLOS)
  are available (World Atlas)
• indicates for a given time and location to what
  percentage the sky is cloudfree

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European Irradiation
The European Commission's Joint Research Centre,
Institute for Environment and Sustainability
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Quotation from MET.ie

• Ireland normally gets between 1400 and 1700 hours
  of sunshine each year.
• The eastern Sahara Desert, however, which is the
  sunniest place in the world, gets an average of
  4300 hours per year.
• Irish skies are completely covered by cloud for
  well over fifty percent of the time. This is due
  to our geographical position off the northwest of
  Europe, close to the path of Atlantic low
  pressure systems which tend to keep us in humid,
  cloudy airflows for much of the time.
• 1887 was the sunniest summer in the 100 years
  from 1881 to 1980, according to measurements made
  at the Phoenix Park in Dublin. A more recent
  summer, 1980, was the dullest. The difference was
  considerable, with the summer of 1887 being twice
  as sunny as that of 1980.

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Typical Figures

• The intensity of the sunlight that reaches the
  earth varies with time of the day and year,
  location, and the weather conditions. The total
  energy on a daily or annual basis is called
  irradiation and indicates the strength of the
  sunshine. Irradiation is expressed in Wh/m² per
  day or for instance kWh/m² per day.
• To simplify calculations with irradiation data
  solar energy is expressed in equivalents of
  hour's bright sun light. Bright sun light
  corresponds with a power of about 1,000 W/m² so
  one hour of bright sunlight corresponds with an
  amount of energy of 1 kWh/m².
• This is approximately the solar energy when the
  sun shines on a cloudless day in the summer on a
  surface of one square meter perpendicular to the
  sun.
• The optimum orientation and inclination angle
  will vary from site to site
• On-site measurements essential
• Ideally you want the cell oriented at 90 to the
  sun at all times

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Solar Panels

• A solar panel produces electricity even when
  there is no direct sunlight. So even with cloudy
  skies a solar energy system will produce
  electricity (see How does it work). The best
  conditions, however, are bright sunlight and the
  solar panel facing towards the sun. To benefit
  most of the direct sunlight a solar panel has to
  be oriented as best as possible towards the sun.
  For places on the Northern Hemisphere this is
  south, for countries on the Southern Hemisphere
  this is north. 
• In practice, the solar panels should therefore be
  positioned at an angle to the horizontal plane
  (tilted). Near the equator the solar panel should
  be placed slightly tilted (almost horizontal) to
  allow rain to wash away the dust.
• A small deviation of these orientations has not a
  significant influence on the electricity
  production because during the day the sun moves
  along the sky from east to west.
• Panels are often set to latitude tilt, an angle
  equal to the latitude, but performance can be
  improved by adjusting the angle for summer and
  winter.

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Important geometrical parameters, which describe
Earth-Sun relations
Declination Angle
n day of year (days since Jan 1st )
Sun Height
h hour angle L Latitude
The optimal solar device tilt Can be estimated
from
Solar Azimuth
Ref http//www.pvresources.com/en/location.php
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Declination Angle
d
The declination angle, denoted by d, varies
seasonally due to the tilt of the Earth on its
axis of rotation and the rotation of the Earth
around the sun. If the Earth were not tilted on
its axis of rotation, the declination would
always be 0. However, the Earth is tilted by
23.45 and the declination angle varies plus or
minus this amount. Only at the spring and autumn
equinoxes is the declination angle equal to 0.
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Solar Panel Tilt Angle

• The sun moves across the sky from east to west.
  Solar panels are most effective when they are
  positioned facing the sun at a perpendicular
  angle at noon.
• Solar panels are usually placed on a roof or a
  frame and have a fixed position and cannot follow
  the movement of the sun along the sky. Therefore
  they will not face the sun with an optimal (90
  degrees) angle all day. The angle between the
  horizontal plane and the solar panel is called
  the tilt angle. 
• Due to motion of the earth round the sun there
  are also seasonal variations. In the winter the
  sun will not reach the same angle as in summer.
  Ideally, in the summer solar panels should be
  placed somewhat more horizontal, to benefit most
  from the sun high in the sky. However these
  panels will then not be placed optimally for the
  winter sun.
• To achieve the best year round performance solar
  panels should be installed at a fixed angle,
  which lies somewhere between the optimum angle
  for summer and for winter. For each latitude
  there is an optimum tilt angle. Only near to the
  equator the solar panels should be placed
  horizontally.

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Tilt and azimuth angle of photovoltaic modules

• The proper tilt and azimuth angle choice is by
  far more important for photovoltaic systems
  design than solar thermal system design.
• Manual or automatic tilt angle adjustment can
  increase the total light-electricity conversion
  up to 30 and more in locations with high values
  of solar radiation.
• Incidence angle should be as close to 90 as
  possible.
• Shaded locations, including partially shaded, are
  not suitable for photovoltaic module fixation.
• Modules should be south oriented.
• The following general recommendations should be
  considered, if you design a photovoltaic
  systemYearly average maximum output power -
  the photovoltaic modules tilt angle should equal
  local latitude.Maximum output power in winter -
  the photovoltaic modules tilt angle should equal
  local latitude 15 (max 20). Such a tilt
  angle is a good solution in areas, where the
  winter load is greater than the summer load. The
  electricity consumption for lighting is greater
  during winter than summer.Manual photovoltaic
  module tilt angle adjustment - in small systems
  modules should be fixed in a way, which allows
  manual adjustment of the module tilt angle. In
  March the tilt angle should be adjusted to equal
  latitude, in May the tilt angle equals latitude
  minus 10 degrees, in September the tilt angle
  equals latitude and in December the tilt angle
  equals latitude plus 10 degrees. With such an
  adjustment the maximal efficiency could be
  obtained throughout the year. Accurate and
  maximum energy output of larger systems should be
  based on exact calculations, because energy
  output is influenced by different factors, such
  as local climatic conditions (solar radiation
  availability in different seasons, local
  cloudiness or fogginess in winter, temperature
  and so on). You will need a long-term solar
  radiation data for the chosen location.

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Watt Peak

• A solar cell produces electricity when it is
  exposed to light. Depending on the intensity of
  the light (the irradiance in W/m²) a solar cell
  produces more or less electricity bright
  sunlight is preferable to shade and shade is
  better than electric light. To compare solar
  cells and panels it is necessary to know the
  so-called nominal power of such a cell or panel.
  The rated power, expressed in Watt peak or Wp, is
  a measure of how much energy such a solar panel
  can produce under optimal conditions.
• To determine and compare the nominal power of
  solar panels, the output is measured under
  standard test conditions (STC). These conditions
  are - An irradiance of 1,000 W/m² - Solar
  reference spectrum AM 1.5 (this defines the type
  and colour of the light)- Cell temperature of 25
  C (Importantly, the efficiency of a solar panel
  drops when the cell temperature rises).

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Site Analysis

• The choice of a proper location is the first and
  the very essential step in solar system design
  procedure.
• It is critical that the modules are exposed to
  sunlight without shadowing at least from 9 am to
  3 pm therefore, the properties and values of
  solar insolation should be studied. The modules
  have to be fixed with proper tilt angle allowing
  the system efficient operation.
• When planning a solar array installation one of
  the first things you'll need to determine is the
  design month, which is the month with the lowest
  insolation. - this assumes power consumption is
  more or less constant throughout the year. If not
  the case then the design month becomes the month
  with the highest average daily power use. In
  systems tied to an electric grid this isn't as
  important because your utility can pick up the
  slack but when dealing with off the grid systems
  it becomes imperative in order to keep the
  battery charged

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Useful Solar Power

• Solar Thermal direct heating of buildings and
  water
• Solar Photovoltaic direct generation of
  electricity
• Solar Biomass using trees, bacteria, algae,
  corn, soy beans, or oilseed to make energy fuels,
  chemicals, or building materials
• Food feeding plants, humans, and other animals

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Global Averages

• The average annual global radiation impinging on
  a horizontal surface which amounts to approx.
• 1000 kWh/m2 in Central Europe, Central Asia, and
  Canada reach approx.
• 1700 kWh/m2 in the Mediterannian.
• 2200 kWh/m2 in most equatorial regions in
  African, Oriental, and Australian desert areas.
• In general, seasonal and geographical differences
  in irradiation are considerable and must be taken
  into account for all solar energy applications.

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Calculation

• From European Irradiation Data (slide 10),
  Ireland has on average 1000kWhrs / m2 / year of
  sunlight
• 2.7 kWhrs / m2 / day
• 108 watts / m2
• Assume average (total) energy consumption in
  Ireland is 120kWh / day / person (
  http//en.wikipedia.org/wiki/List_of_countries_by_
  energy_consumption_per_capita)

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Calculation

• Population of Ireland 4010000
• Assume panels are 10 efficient
• This works out at 440 1m2 panels per person
• Assume approx 4 times this area per panel needed
  for infrastructure 1777 m2
• Required area for the population of Ireland???
• 7182 km2
• gt area of Mayo (5589km2)
• area of Cork (7459km2)

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Moral of the Story

• Country scale problem needs country scale
  solution
• (economically) Harvestable power limited
• Storage needed due to fluctuating nature (scale?)
• Infrastructure requirements substantial
• Impact of efficiency substantial
• What about seasonal variations?

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