Solar Energy

Introduction

The most abundant, sustainable source of energy is the Sun, which provides over 150000 TW of power to the Earth; about half of this energy reaches the Earth’s surface while the other half gets reflected to outer space by the atmosphere.

The Earth receives most of its energy from the Sun in the form of solar electromagnetic radiation. Most of the other renewable sources of energy, such as wind energy, wave energy and biofuels depend on the Sun’s energy. Furthermore, some of the nonrenewable energy sources such as fossil fuels were originated by solar energy in the past. The rate at which solar energy reaches a unit area at the Earth is called the solar irradiance or insolation and is measured in W/m2. The integral of the solar irradiance during a period of time is called solar radiation or irradiation and is measured in J/m2. Solar irradiance is also denominated as solar radiation using the same units (W/m2), which is more popular. The total power emitted by the Sun’s surface is about 63 MW/m2. Solar radiation is absorbed by the atmosphere, soil and oceans.

Pyranometers are the sensors used to measure global solar irradiance, that is, the Sun’s energy coming from all directions in the hemisphere above the plane of the instrument. The measurement is of the sum of the direct and the diffuse solar irradiance. To measure the direct normal component of the solar irradiance only, an instrument called a normal incidence pyrheliometer (NIP), or simply pyrheliometer is used. The diffuse irradiance can be measured by modifying a pyranometer using a shadowing device large enough to block the direct irradiance onto this sensor.

Technical Terms

Direct normal irradiance (DNI):  

Solar-beam radiation available from the sola rdisk on a planar surface normal to the Sun as measured by a pyrheliometer with a 5o–5.7o full-angle field of view.

Diffuse horizontal irradiance (DHI): 

Solar radiation from the sky dome, not including DNI, that has been scattered by clouds, aerosols, and other atmospheric constituents available on a horizontal surface, as measured by a shaded pyranometer with a 180o field of view.

Global horizontal irradiance (GHI): 

Total hemispheric down-welling solar radiation on a horizontal surface, as measured by an unshaded pyranometer.

The three solar-irradiance components are related. On any surface, direct plus diffuse irradiance equals global irradiance.

Availability

The National Renewable Energy Laboratory (NREL) developed solar resource maps for nation and states.

Solar Energy Application

Solar energy has been used throughout time, mainly for heating and lighting but also for many other purposes such as refrigeration and primarily for the generation of electricity. Figure 7 shows a diagram of basic solar energy conversion systems, able to convert the solar resource into a useful form of energy.

Electricity Generation

Photovoltaic Plants

The direct generation of electricity from solar energy is based on the photovoltaic effect which refers to the fact that photons of light hitting certain materials will knock electrons into a higher state of energy producing an electrical current.

There are many PV power generation systems for more normal applications such as houses isolated from the grid, pumps for water extraction, electric cars, roadside emergency telephones, remote sensing and cathodic protection of pipelines. When the power generated by the PV cells is transmitted to the grid, an inverter to convert the direct current (DC) to alternating current (AC) is required. PV cells are usually protected by a glass sheet and are connected together to form solar panels. Multiple solar panels or modules must be assembled as arrays when more power production is required.

A grid-connected PV system is shown in Figure 8. 

In PV power systems, maximum power point tracking (MPPT) is essential because it takes fully advantage of the available solar energy. A MPPT is a high efficiency DC to DC converter that presents an optimal electrical load to a solar panel or array and produces a voltage suitable for the load.

Traditional solar inverters perform MPPT for an entire array as a whole. In such systems, the same current, dictated by the inverter, flows though all the panels in the string. However, because different panels have different I –V curves, i.e. different MPP (due to manufacturing tolerance, partial shading, etc.), this architecture means some panels will be performing below their MPP, resulting in a loss of energy. Now, placing peak power point converters into individual panels, allowing each to operate at peak efficiency despite uneven shading, soiling or electrical mismatch, becomes more popular.

Thermosolar Plants/Concentrating Solar Thermal Systems

Concentrating solar thermal (CST) systems use optical devices (usually mirrors) and Sun tracking systems to concentrate a large area of sunlight onto a smaller receiving area. The concentrated solar energy is then used as a heat source for a conventional power plant.

Other Applications

Solar cooling

Solar energy cannot only be used for heating but also for cooling purposes. Solar cooling also has the advantage of requiring little energy storage, because the needs of cold air match solar radiation very well.

There are various ways in which solar energy can be used for cooling. Absorption technology is the most popular and consists of using solar thermal energy to feed thermally driven chillers.

Assessment Method

Measurement of Terrestrial Solar Radiation

Solar radiation data are required for resource assessment, model development, system design, and collector testing. The basic solar radiation measurements are the beam, diffuse, and global radiation components. The expense of radiometric stations and high maintenance make impossible the spatially continuous mapping of solar radiation. Due to the scarcity of real data, the use of representative sites where irradiance data are measured or modeled has been a common practice for engineering calculations. The most commonly used instruments to measure solar radiation today are based on either the thermoelectric or the photoelectric effects.