Chapter 1 Introduction

1.1 Introduction

During the 21st century, the lifestyle of many societies has changed and the demand for energy

has increased rapidly and the consumption of energy wills strong increase along the next decades as

a result of the improvement on living conditions in emerging economies like Egypt. Solar energy can

be defined as the energy formed from the Sun’s radiation. This energy comes in two forms, light and

heat. Since solar energy reaches from the sun it is considered a renewable source of energy as nothing

is expended to use this energy [1].

Many renewable energy projects and feasibility studies for the

construction of roads and cities all requires reliable information and data to support decision-making

processes by investors, energy entrepreneurs, and government and non-government organizations. In

order to reduce the lack of information, we have been working in research activities related to solar

assessment to deliver reliable solar irradiation data to support the Egyptian energy sector.

Renewable energy is recognized as a main source for the future, not only for Egypt but also

for the world [2].

The location of Egypt on the surface of the globe gives it a large number of

sunshine hours, which makes the Egyptian atmosphere, has a high potential of solar energy, which is

distinguished in the field of solar energy applications. Egypt is characterized by relatively high

average-daily radiation rates, both global and direct, and a relatively high frequency of clear days[3]

. With the development of solar-based renewable energy technologies, national meteorological

services must face increasing demands for reliable data on solar resource.

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Complete and accurate solar

radiation data for specific regions are also indispensable for a large variety of solar-energy-related

studies [4]. One of the short ways to satisfy that goal is the creation of quality control techniques for

the solar radiation data. Solar energy is also a clean source of energy that does not destructed the

environment with harmful emissions or waste like other source such as nuclear and conventional

energy. Solar energy sources can be situated anywhere since there is sunlight, thus solar energy sites

can be created close to where consumers are located; this could potentially reduce distribution and

transmission costs. solar energy sources availability could also achieve economic and political

independence to help societies [5, 6]. In the past decade, solar energy production has intensely

improved. The world’s capacity of total installed solar power generation capacity has increased from

almost 2 GW in 2002 to more than 100 GW at the end of 2012 [7].

1.2 Fundamentals of surface solar radiation

1.2.1 Solar radiation spectrum

Solar radiation consists of electromagnetic radiation emitted by the Sun in spectral regions

ranging from gamma-rays to radio waves (see Fig.1). It can be divided into two major regions with

respect to the capability of ionizing atoms in radiation-absorbing matter ionizing radiation (X-rays

and gamma-rays ) and nonionizing radiation (ultraviolet radiation, visible light infrared radiation

,microwaves and radio waves) [8].

Fig.1: Solar spectrum ranging from gamma rays to radio waves in Wavelengths

Terrestrial applications of renewable energy employing solar radiation usually depend on photons, or

radiation, referred to as “optical radiation”, with a spectral range of about 300-4000nm. The Sun acts

as a quasi-point source, illuminating the Earth with very nearly parallel rays of radiation. This quasicollimated beam is the extraterrestrial direct beam, or extraterrestrial radiation referred to as ETR [9].

The sun is the unique source of radiant energy for planet earth. It is almost perfectly spherical

and includes magnetic fields with hot plasma interwoven. It has a diameter of about 1,392,000 km,

about 109 times that of Earth, and its mass (about 2?1030kilograms, 330,000 times that of Earth)

accounts for about 99.86% of the total mass of the Solar System. Chemically, the Sun’s mass includes

about one quarter of helium, while the rest is mostly of hydrogen. Less than 2% consists of heavier

elements, including neon, iron, oxygen, carbon, and others. The mean distance of the Sun from the

Earth is about 149.6 million kilometers, however the distance differs as the Earth moves from

perihelion in January to aphelion in July. By this average distance, light moves from the Sun to Earth

in about 8 minutes and 19 seconds[10].

The received solar irradiant is really depends on the known location on the earth such as, on

its geographical position, the time of the day and on the date of the year. That is because of the rotation

of the earth around the sun in a year and its rotation around itself in a day [11].

1.2.2 Components of solar radiation

Knowledge of solar radiation incident on the earth’s surface is important to engineers and

architect for solar energy applications and energy-efficient building designs [12, 13]. The total amount

of quantum energy created by incident photons per unit of area can be defined as solar radiation. Solar

radiation can be expressed in Joules per square meter J/m2 or watt-hours per square

meter1/3600Wh/m2. This value of incident energy on the earth’s surface depends on factors such as

air pollution, cloud cover, and location[14]. Information on global solar radiation received at any site

is not only useful to the locality where the radiation data is collected but also for a wider community.

For example, a study of the world distribution of global solar radiation involves radiation data in

several countries. Also, for the purpose of worldwide marketing, manufacturers and designers of solar

energy equipment need to recognize the mean global solar radiation available in specific and different

regions (World meteorological organization) (WMO) [15] .

As the Extra Terrestrial Radiation (ETR) beam traverses the atmosphere, interaction between

the atmosphere and the photons in the beam. Result in scattering and absorption of solar radiation out

of the beam into random paths in the atmosphere. Solar radiation scattered (mostly at short

wavelengths) produce the diffuse sky radiation, which we will represent by diffuse horizontal

radiation (DHI). The remaining unabsorbed and unscattered radiation, still nearly collimated,

constitute the direct normal beam radiation, responsible for the casting of shadows, which we will

denote as (DNI). The total radiation flux on a horizontal surface in the presence of diffuse and beam

radiation is often called “total” or “global” radiation. We will represent the Global Horizontal

Irradiance on a horizontal surface as (GHI) see figure 2.

Fig.2: The radiation components

The term “global” indicates to the concept that the radiation on a horizontal surface is received

from the entire 2? solid angle of the sky dome (see Fig.3). The difference between GHI at ground

level and its corresponding value at the top of the atmosphere is what has been absorbed or reflected

away by the atmosphere. On average, the Earth reflects about 29% of the incident solar irradiance

back to space [9].

Fig.3: The 2? solid angle of sky dome

The albedo is the ratio of the outgoing irradiation reflected by the surface of an object to the incoming

solar irradiation incident upon it. Snow-covered surfaces have a high albedo; the albedo of soils ranges

from low values of ~0.04 for calm, deep water and overhead sun, to > 0.8 for fresh snow or thick

clouds.

1.2.3 Surface solar radiation measurements

Fig.4: measuring global radiation over horizontal plane

Fig.5: The pyranometer and its use in measuring diffuse solar radiation using

shadow disk or shadow band system

1.2.3.3 Direct solar irradiance

Fig.6: A normal incidence pyrheliometer (NIP) used for measuring the direct component

of solar radiation

GHI = DNI Cos Z + DHI (1)

1.3 Solar surface Irradiation data

In 1960 The Egyptian Meteorological Authority (EMA) established a network of solar

radiation measuring stations In order to support the scientific and economic community of potential

users with an accurate surface solar irradiance data, measuring the global SSI and its diffuse and direct

components, as well as more common surface meteorological variables. Recently, this network

includes about 10 sites.

For example, solar radiation data is required in improving the parameterization of clouds needed in

general circulation models (GCMs)[18] and as a valuable resource for validating the GCMs[19, 20].

1.4 Satellite-derived solar irradiation data

1.5 Egyptian climate outlines

a) In winter, prevailing conditions of middle latitude conflicts where cloud types are normally

impervious to the direct beam and the turbidity of the atmosphere is low. The climate of Lower

Egypt (Northern Egypt) is moderate with some rain showers, mainly over coastal areas. Upper

Egypt is almost rainless with warm sunny day but rather cool nights.

b) Spring is characterized by the passage of small and shallow thermal disturbances, inducing what

is called “Khamsin weather” with the onset of such thermal lows, vertical visibility deteriorates

gradually with increasing dust content in the lower layers of the atmosphere. The average

frequency of these depressions may vary between two and six per month, [3].

c) In summer general climate is hot, dry and rainless, high transparent clouds prevail, when they

exist, and the sky is “dirty” most of the time; thanks to a deep layer of fine dust particles associated

with continental tropical air. The dust content falls markedly when Mediterranean air arrives, with

associated fine weather.

d) In autumn, the atmosphere is moderately transparent in the average. Morning mists and low clouds

form and dissipate after sunrise. The weather regime is very steady. The depressions at this time

are much less vigorous than in spring and are slower in their eastward movement.

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