13 Principles of Solar Energy Generation

Dr. Nikhil and Dr. Dhanya M.S M.S

1. Introduction

The energy from the sun amounts to 4×1020 MW, of which Earth receives only less than 1 % of the energy. This energy received from the sun can be harnessed directly or indirectly using various technologies for thermal applications as well as for converting into electricity by the means of photovoltaic (PV) systems. Over the years the photovoltaic technology advanced a lot and the efficiency of solar cell has considerably improved. As majority of our energy requirements are in the form of electricity, PV works on the principle of photovoltaic effect. The generation of thermal energy from solar can be realized using various solar reflecting collectors. Most of the technology works on the principle of reflection, radiation and convention or based on the thermosiphon effect.

2. Energy from Solar Spectrum

Sun is a gigantic star, with diameter of 1.4 million kilometer releasing electromagnetic energy of about 3.8 x 1020 MW. The energy from the sunlight extends from 300nm to 3000 nm. Majorly, they are classified as Ultraviolet region (less than 350nm), Visible region (350 nm to 750nm), and Infrared region (more than 750nm). These various components of the sunlight constitute the solar spectrum. The visible (47 %) and infrared (46 %) components of the solar radiation contributes for most of the solar energy (Fig 1). It is important to understand, in general, the spectrum of the sun energy, as the technology used for energy generation and conversion is driven by the inputs received from the respective spectrum of solar irradiance.

The energy from heat and light of solar radiation can be extracted to useful applications and the principle of operation is different depending on the technology. The PV technology convert visible spectrum to electricity and thermal collectors use both infrared and visible spectrum for energy generation.

So the energy generation from solar radiation can be in the form of electrical energy or thermal Energy. The various conversion paths of solar energy is described in the Fig.2

3. Principle of Electricity generation by Solar Photovoltaics

The solar photovoltaic works on the principle of photovoltaic effect. It is the physical and chemical property or phenomenon in which electromotive force is generated in the non-homogeneous materials with the illumination of light of a specific wave length. This effect produces voltage and electric current in a material upon exposure to light. The photovoltaic property is seen in semiconductors when photons or radiant energy falls on surface is capable of converting to electric current.

There are majorly three energy bands in a semiconductor material

filled band

conduction band and

forbidden band

The forbidden band is so wide, that an electron has to acquire sufficient energy to move from filled band to conduction band. This energy is known as band gap energy.

The band gap energy in a silicon semiconductor is near to 1.12eV. The energy bands for other semiconductors like Gallium arsenide is 1.42eV, and Cadmium Telluride is 1.5eV.

The energy gap of 1.12eV indicates that, the photons with energy in the wavelength corresponding to 1.12eV can initiate the electrons to jump and create current flow. Any wavelength, below to that, adds to heat losses in the cell. In a p-n junction as long as the energy from the sun strikes, hole-electron pair is created and continuously recombined. If the recombination is avoided, and electron is forced to flow in one direction will facilitate the current flow in the system. Thus is made possible by a built in electric filed created within the semiconductor by making the semiconductor, impure/extrinsic. An impure semiconductor will have an additional electron or an additional hole in it. Usually, a pentavalent impurity is added to create excess electrons, and a trivalent impurity addition creates an excess hole.

• Calculation of band gap energy

Example: Find out the band gap energy for a semiconductor transparent to light of wavelength 0.87 m?

The solar cell is a p-n junction with large surface area. The n-type material is thin for passing light through it and strike the p-n junction. The electricity is generated inside the depletion zone of the p-n junction. When a photon of light is absorbed by one of the atoms in n-region of silicon, it dislodge an electron from any atom creating a free electron and hole pair. This free electron and hole pair has sufficient energy to jump out of depletion zone. If a wire is connected from the cathode at n-type silicon to an anode of p-type silicon, electrons flow through the wire. The electron is attracted to the positive charge of p-type material and travels through the external load creating flow of electric current. The vicinity of a p-n junction when exposed to light is shown in the Fig.4.

Fig 5. Equivalent circuit for p-n junction solar cell

The intensity of the incident radiation and external load of the cell determines I-V characteristics of a solar cell. The voltage and current generation from the solar cell can be easily calculated from the equivalent circuit.

3.1 Factors affecting the energy generation in a solar PV cell technology

The two main parameters which affect the performance output of a PV cell are temperature and the light (photons) incident on it. The voltage output is driven by the change in the temperature, and the current output is driven by the light received. The increase in the light input, contributes to increase in the current output. However, the junction voltage reduces with the increase in the temperature.

4. Solar Thermal Technology

The solar thermal technology is broadly classified into

Non-concentrated solar thermal technologies Eg: Water heater (Non concentrated)

Solar Dryers (Non Concentrated)

Concentrated solar thermal technologies

Concentrated Collectors use optics to absorb sunlight and concentrate it to a receiver for energy conversion. In general the energy generated from the solar thermal technologies are used for heating application, solar cooking solar drying, process heating, cooling and also electricity generation using solar steam.

• Parabolic trough collectors (Concentrated)
• Scheffler collectors (Concentrated with fixed focus)
• Parabloid collectors (Concentrated with moving focus)
• Linear Fresnel Collectors(Concentrated with fixed focus)
• Fresnel Reflectors (Concentrated with moving focus)
• Sterling Engine.(Concentrated with moving focus) Solar Cookers (Both Concentrated and non concentrated)

5. Principles of Solar Thermal Technology

The heat is an energy form produced by the movement of molecules. The heat transfer occurs between higher and lower temperatures and is proportional to difference in temperatures. The three basic means of heat transfer are conduction, convection and radiation.

Convection and conduction plays important role in the heat transfer mechanism in a solar system. But radiation heat transfer facilitates the bringing of the solar energy to the earth in the form of electromagnetic spectrum.

• Radiation: The heat transfer phenomenon through longer wavelength infrared rays often transmitted across the space as a consequence of thermal agitation of composing molecules. Radiative heat transfer does not require a medium for energy transfer. The radiative flux is often quantified as the emission per unit time per unit surface area (W/m2). The usual way to compare the energy emitted or absorbed by an object is to compare it with a black body. A black body is equally a good emitter and an absorber. The solar collectors like aluminum/glass reflective collectors absorb the sun’s radiative energy from the space. The theoretical abstraction of energy emitted from a black body is defined using Planck’s Law. The total energy between any two wavelengths can be conveniently expressed using Stefan –Boltzmann law of radiation.
• Conduction: The heat transfer happening between two surfaces, which are in direct contact with each other. The receivers in a solar thermal system, absorb the solar energy and conducts the same to another medium for utilization.
• Convection: The heat transfer happening in between two mediums. Like for example, liquid converting to gas using heat gain or gas conversion to liquid by release of heat.

5.1 Working Principle of a solar collector

In a solar collector, the solar energy passes through a glazed glass layer and is absorbed. The solar energy excites the molecules produces heat and gets trapped by the glass layer.

Reflectors/Absorbers: The main types of reflectors used in the solar thermal systems are aluminum or glass reflectors. The reflectivity of these collectors needs to be a high as possible. The glass type reflectors have higher reflectivity and life compared to aluminum type reflectors. In case of water heaters and solar cookers, there are absorbers which are black coated metal sheets.

Receivers: The receivers are used to collect the concentrated energy from the Sun from the reflectors to a common point/area, so as to magnify the energy in it. The ratio of the area of the Reflector to the area of the receiver is often termed as Concentration Ratio (C.R). CR signifies the strength of the systems for higher energy conversion possibilities.

The solar collectors can be classified based on the movement of absorber as

• Fixed Focus System
• Moving Focus system

Fixed Focus System: In a fixed focus system, the energy is focused at the absorber which is fixed at a definite focal axis. The reflectors are rotating so as to focus the sun energy at fixed point continuously throughout the day.

Moving Focus system: In a moving focus system, the energy from the sun is focused at the absorber which keeps moving along with the reflectors. The moving focus is characterized by higher throughput and hence, higher efficiency.

A general understanding of the losses during energy conversion in concentrated solar thermal systems is given below in the Fig 7.

The main losses involved in a concentrated solar system are reflector losses (up to 25%), absorption losses and losses in the receiver. The efficiencies of solar thermal system are between 25% and 30% but however, there are instances when certain dual axis concentrated system has achieved even up to 55 %.

5.2 Principle of solar cooker

For the heat production, it utilizes the basic principles of reflection, concentration and green house effect. A solar box cooker works on the similar concept of green house effect. The incoming short wave radiation passed through transparent glazing material and is trapped inside a box, where in the glazing glass act as a blockade for the re-radiated long wavelength radiation (Fig 8).

5.3 Principle of solar water heaters

The solar collectors or solar panels which are in direct contact with the sun and the heat energy produced is used to heat up the water. The active water heater systems works with the help of a pump which moves water from collector to the tank and passive system works on gravity.

Thermosiphon Effect

The most common process of water heating in the world wide is based on thermo-siphon effect. It is relatively simple and , does not require any external energy. Thermo-siphon works on the basic principle of heat rising.

In an open loop system, the water inlet to the solar collector is through the bottom side and, as the water gets warm it rises to the top. The warm water stays always at the top of the storage tank, which is collected as and when required. A pictorial representation of thermosiphon based water heating system is shown in Fig 9.

In winters, the water is not used as a medium, but an antifreeze solution is used as circulating medium. And the heat is transferred to the water through a closed loop system.

5.5 Principle of solar space heating

The three basic principles used for solar space heating are

Collection of solar radiation by solar collectors and conversion to thermal energy Storage of solar thermal energy in water tanks, rock bins,etc.

Distribution by means of active (pumps) or passive (gravity) methods.

5.6 Principle of solar dryer

A solar air heater/ dryer works on the principle of Green house effect and thermosiphon effect. The air is the medium heated up from the solar energy absorbed by the black surface. The thermal energy absorbed is thus sustained inside a glass envelope. The air thus heated is driven upwards owing to its lower density. The hot air circulation is generally passive, but there are systems which have active systems for hot air circulations.

5.7 Principle of Solar Water Desalination

The basic working principle of solar water desalination is like hydrological cycle, where in water is evaporated and then condensed there by leaving back the contamination. Thus solar desalination cleans the water by vapourization with the help of a solar still, an airtight insulated basin covered with a tilted glass sheet. In a sloping transparent enclosure, the brackish water is collected, the thermal energy from sunlight is trapped inside the vessel, as vessel has a transparent glass cover.

The energy thus, accumulated can raise the water temperature up to 120oC and thus gets evaporated leaving behind the salt/contamination. The humid and warm air thus risen comes in contact with relatively cooler inner surface of transparent cover which causes condensation. The water thus condensed slides down and collected in a trough along the lower end of the basin. This water is conducted out of the enclosure to get the fresh water. The heat is trapped inside the solar still by means of greenhouse effect.

Most of the solar concentrated system can help achieve a temperature in the range of 120oC to 350oC. While single axis tracking system can raise the temperature upto 180oC, dual axis moving focus concentrated system can help achieve up to 350oC.

6. Factors affecting Performance of Solar Thermal System

The main parameters which affect the performance of the solar thermal system are

a) Quality of the reflectors (Reflectivity/Material type)

b) Optical design of the System

c) Mean operating temperature of the system.

d) Location of installation. (Solar Resource of the location).

Conclusion

The energy conversion from sun to useful applications like electricity and thermal applications is very important in the field of renewable energy consumption. The heating and cooling is possible with various solar technologies. The choice of technology can drive the efficiency and the performance of the system. It is always important to understand the basic function of the various solar technologies available for energy conversion, which shall help any learner to contribute further improve the usage and applications of these system in the society.