Thermodynamic analysis of Photovoltaic Systems

Submitted by

Abhishek Raj Urs K N

1000988334

Dept. of Mechanical Engineering

University of Texas, Arlington

Under the guidance of

Dr. Hyejin Moon

Dept. of Mechanical Engineering

University of Texas, Arlington

Contents

Introduction

Necessity for solar energy Environmental effects Solar Technologies Photovoltaic Solar panels

Solar Characteristics

Solar Spectrum Intensity and Energy Surface orientation Attenuation Diffusion

Characteristics of PV Modules

Measurements Principle of Operation Band Gap and Response rate Performance and Energy loses

Energy and Exergy

List of Figures

Figure 1: Potential for renewable energy sources.

Figure 2: Energy supply sources

Figure 3: Nest on a solar PV panel

Figure 4: Solar Technologies

Figure 5: Example of a PV system

Figure 6: PV efficiency improvements in future

Figure 7: Expenditure of PV around the world

Figure 8: Expectation of PV capacity growth

Figure 9: Growth in recent years

Figure 10: Solar Spectrum

Figure 11: Angle of Incidence

Figure 12: Equivalent circuit of a solar cell.

Figure 13: Band gap and Operation

Figure 14: Energy Loses in PV systems

Figure 15: Energy Loses in PV systems

Necessity for Solar Energy

Solar Energy is renewable source of energy. It's abundant availability throughout the world can be used as a substitute to overcome the diminishing renewable sources. Most of the power generated nowadays are obtained from fossil fuels which are diminishing at a very quick pace. Also, these fossil fuels are responsible for the increasing pollution rapidly by emitting poisonous gases like carbon dioxide, carbon monoxide etc.

Energy obtained from renewable energy sources like solar energy, wind energy, tidal energy etc are clean energy sources. They do not produce harmful gases during the operation, hence protect the environment. In recent years, there has been an increasing demand for clean energy sources, which resulted in rapid growth in solar energy industry in the market. Solar energy when compared to other renewable clean energy sources has a higher potential to overcome the energy needs of the world.

Fig 1: Potential for renewable energy sources.

Solar energy is the primary origins for all other energy sources like tidal energy, fossil fuel, hydro energy, biomass etc. Solar energy falls on the surface of the earth at a rate of 120 petawatts, which is the solar energy received from sun in one day can be used to overcome the world energy requirements for 20 years. Fig 1 shows the potential for some of the renewable energy sources based on today's technology. Future advances in the technology will lead to higher potential for each energy source.

In solar technology department, there are many devices that directly convert solar energy into electrical or thermal energy. Photo-voltaic cells and concentrated solar power systems are some of those devices. For a specific geographic location, a particular solar technology system has to be incorporated in order to achieve maximum efficiency out of the device.

There are a wide range of applications for solar energy. For instance it can used for controlling room temperature, solar desalination, solar propulsion, photochemical applications, electricity generation etc. Solar energy can easily be converted into electrical energy which again has a lot of applications. Electrical energy is a high grade energy, which means it can further be converted to other energy forms easily. Therefore it has attracted a lot of researchers as electrical energy consumption is continuously increasing.

Fig 2: Energy supply sources.

Environmental Aspects

Solar Energy is clean source of renewable energy. During the operation it does not emit any potentially harmful gas like carbon dioxide, hydro-carbons, carbon monoxide etc. Most of the

materials used these operations are made of silicon or other earth metal/nonmetals. They are abundantly available and environmentally safe.

Fig 3: Nest on a Solar PV Panel

In the above figure we can see that birds have located their nest on the PV panels. The above PV station is located in Merced, CA. These PV stations are usually located on wide barren lands with adequate sunlight throughout the year to trap maximum solar energy.

However not all of the solar technologies are toxic free. Few technologies which achieve relatively higher work efficiency and work on this concept involve toxic materials like gallium arsenide or cadmium telluride. These can cause severe harm to the environment if leaked. So care has to be taken to avoid these drawbacks.

Solar Technologies

Currently there are several kinds of solar technologies. They basically differ from the converting efficiency and materials used, but the concept behind most of them remain the same. Each of those particular methods have their own drawbacks and advantages. Certain methods prove to be more efficient only when it is used at a particular location or at specific set of conditions.

The figure below gives us a broad idea about the different solar technologies that are being used for energy production/conversion. Out of these, the most broadly used methods are Photovoltaic solar panels and concentrated solar power systems. However, in the recent years a lot of research has been made in concentrated photovoltaic systems and solar thermoelectricity systems. They are emerging to be more energy converting efficient and claim a significant share of solar energy market.

Fig 4: Solar Technologies

Photovoltaic Solar Panels

Photovoltaic's is a method of generating electrical power by converting solar radiation into direct current using semiconductor materials that exhibit photovoltaic effect.

Photovoltaic effect is defined as the process in which solar energy is converted into electrical energy upon exposure to light. In most of the photovoltaic applications, radiation used is the sunlight and the devices used to convert the radiation are known as solar cells.

Photovoltaic panels are composed of a large number of solar cells which in turn are made of photovoltaic material. Some of the materials used for photovoltaic's are mono-crystalline silicon, poly-crystalline silicon, amorphous silicon, cadmium telluride, copper indium gallium selenide/sulfide.

Fig 5: Example of a PV System

The working principle of a photovoltaic material is based on 'Hertz Effect'. When electro-magnetic radiation of short wavelength fall on the surface of a photovoltaic material, electrons are emitted as a result of absorbing energy from these radiations[Visible Spectrum, UV radiation]. These emitted electrons are known as photo-electrons. These photo-electrons are captured and hence direct electric current is obtained.

Types of PV panels:

Crystalline Silicon Thin Films Semiconductor thin films

The figure below shows us the expectation of photovoltaic's efficiency improvement over time. PV systems have reached a global capacity of 40,000 MW at the end of 2010 which is about 3% of the whole renewable energy capacity.

Fig 6: PV efficiency improvements in future

The following figures summarize the world data on PV technology including distribution, capacity and cost.

Fig 7: Expenditure of PV around the world

Fig 8: Expectation of PV capacity growth

Photovoltaic technologies are most commonly used solar technologies. They will continuously grow and see rapid changes. Each of these technologies have their own pro's and con's. Even despite the rapid development of these technologies, they have many disadvantages that need to be solved.

Solar electricity is still more expensive compared to other small scale alternate energy production. Solar systems do not produce electricity at night and have less efficiency at cloudy conditions. Also the efficiency depends party on the location as well.

Fig 9: Growth of PV capacity in recent years

Solar Characteristics

Solar Spectrum

The Energy in solar irradiation comes in the form of electromagnetic radiation. We know that longer wavelength radiation have lesser energy than shorter ones such as UV light or visible light. The figure below shows us the spectral distribution and also relative weights of individual wavelengths.

Fig 10: Solar Spectrum

Intensity and Energy

Irradiance is defined as the intensity of the solar radiation hitting the surface. It is the sum of contributions of all wavelengths within the spectrum. Power is the total irradiance on a particular area. Also, energy is measure of irradiance incident on a surface over a period of time

Surface Orientation

Amount of energy obtained by the PV cell is directly proportional to the area of the radiation wave-front it intercepts. In order to obtain maximum energy, the radiation should be perpendicular to the collector. This happens only on the equator; hence the collector must be tilted in an angle to receive maximum insolation.

I (θ) =Iₒ Cos (θ)

Fig 11: Angle of Incidence

Attenuation

To express the amount of intensity that is lost through absorption, the clarity index is defined as the ratio between the observed (global) hourly irradiance on earth, Hg, and the hourly radiation Ho just outside the atmosphere. Typical values for k are 0.6-0.8 at clear sky and 0.1-0.3 on cloudy sky.

K = Hg/Ho

Diffusion

Direct beam radiation is one which strikes the surface from one angle only, directly from the sun. Conversely, diffuse light as a result of absorption and scattering, approaches the horizontal surface from almost any angle. Hence it cannot be focused or concentrated.

Hg = H (beam) + H (diffuse)

Kd = H (diffuse)/Hg

Characteristics of PV modules

Cells, Modules, Panels and Arrays

Cells are basic photovoltaic devices that are building blocks of PV modules. These cells can be round or square and are too small to do much work alone. They usually produce about ½ volt. Generally 36 of these cells are connected in series to charge batteries or motors.

Modules are a group of PV cells connected in series or in parallel to obtain more current or volts. They are encapsulated with a protective glass laminate. Encapsulation is the method in which PV cells are protected from the environment with glass laminate.

Panel is a group of modules connected in series or in parallel to obtain much higher current or voltage. Modules are the basic building blocks of these panels. And these panels combined together constitute a complete PV generating unit called as an PV array. These arrays are usually mounted on ground or support on suitable geographic locations to obtain maximum efficiency.

Fig 12: Cell, Module and Array

Measurements

Maximum power the PV module is capable of delivering is defined as Rated Power. The power obtained by the PV module when it is operating on standard radiation conditions is known as peak power. The rate at which the PV cell converts solar energy into electrical energy is known as cell efficiency.

Principle of operation

Working principle of a typical photovoltaic cell can be explained considering light is made up of particles called photons. These photons are packets of energy. When these packets of energy hit the solar panels, the energy is absorbed by the materials which constitute the panels.

As a result of this, electrons are emitted from their atoms because of excitation. These emitted electrons are made to move in single direction inside the panels. Hence a lot of electrons are obtained through array of solar panels. These electrons constitute direct current which is then collected and stored.

The photons which strike the solar panels pass through the panels get reflected back to the atmosphere or get absorbed by the semiconducting material inside the panel. These photons get absorbed only if the energy is more than band gap value of the material.

These absorbed photons, give energy to the electrons in the valence bond. Thus exciting them and moving them to the conduction band, where it is free to move and hence constitutes DC.

Fig 13: Equivalent circuit of a solar cell.

Where, Id= diode current, IL= photo generated current, Ish= shunt current, I =output current.

Fig 14: Band Gap and Operation

Here, p-type and n-type layers join at the p-n junction, electrons and holes diffuse to create the charge free depletion zone. Moreover, the junction creates a slope in the resulting energy bands. Now, when a photon promotes an electron to the conduction band, it can "roll down" through the depletion zone into a lower energy band rather than instantly re-combine with a hole. This is what generates the photo current.

Band Gap and Response Rate

The band gap, which is the minimum energy required to excite an electron out of the valence band varies by material. It is usually expressed in units of "electron Volts" where "one electron" is the elementary charge, e. To convert electron-Volts into wavelengths of incoming light, we have,

λ = ch/eb

Where, c = speed of light, h = Plank’s constant, b = band gap (ev), λ = wavelength (m), e = electron charge.

Response rate measures the ability of the material to convert light into electric current. In reality, the photo effect is not like an on/off switch depending on the energy of the photon. At longer wavelengths, electrons may still flow due to energy from ambient temperature. On the other hand, short wave photons may not be able to be absorbed, as they have too much energy.

ξ = Electric current/irradiance.

Performance and Energy loses

Yearly sum of irradiation (h) that falls on the module is dependent on the geographic location and can be obtained from databases or irradiance map.

Performance ratio is defined as the ratio between actual yield and target yield. Here, target yield is the theoretical annual energy production considering nominal efficiency only. This ratio is also known as quality factor and is used to compare systems.

Quality Factor (PR) = Actual yield/ theoretical yield

E/A = PRh [η (nom)]

Where, h = irradiance, PR = Quality factor, η = nominal module efficiency, Energy delivered by the system.

It is evident that no system in the universe is 100% efficient. There are always loses incurring within the system or from outside, leading to reduced efficiencies. Major energy loses occurring in these systems are, pre- photovoltaic losses, module and thermal loses, and system loses.

Fig 15: Energy loses in PV systems

Fig 16: Energy Loses in PV systems

Energy and Exergy Analysis

Energy is the measure of quantity, dependent on parameters of matter or energy flow only. It is independent of the environmental parameters. It is guided by the first law of thermodynamics for all processes.

Exergy is the measure of both quantity and quality, dependent both on parameters of matter and energy flow and on the environmental parameters. Exergy is guided by the first and second law of thermodynamics for all irreversible processes.