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ABSTRACT
With the increase in surface temperature of solar cells or panels their efficiency decreases quite dramatically. To overcome the heating of solar cell surface, water immersion cooling technique can be used i.e. it can be submerged in water so as to maintain its surface temperature and provide better efficiency at extreme temperatures. In this study, electrical parameters of solar cell were calculated which showed that the cooling factor plays an important role in the electrical efficiency enhancement.
Solar cell immersed in water was monitored under real climatic conditions, cell surface temperature can be controlled from 31- 39 .C. Electrical performance of cell increases up to great extent. Results are discussed; panel efficiency has increased about 17.8% at water. The study can give support to the Concentrated Photovolatics System by submerging the solar cells in different mediums.
Photovoltaic systems directly convert the solar energy into electrical energy while concentrated solar power systems first convert the solar energy into thermal energy and then further convert it into electrical energy through a thermal engine.
Photovoltaic converters are important converters for the application of renewable energy sources because of the direct conversion of solar energy to electric energy. They have some advantages such as low weight and feasibility of small scales, but they are more expensive compared to other types of energy converters. Therefore, it is important to absorb the maximum solar energy in order to increase the efficiency of the energy converter.
Cooling of the solar cells is a critical issue, especially when designing concentrating photovoltaic (PV) systems. In the present work, the cooling of a photovoltaic panel via Water immersion technique is investigated. The aim of this project is to optimize the efficiency of a solar panel. Experiment is done for polycrystalline silicon panel.
INTRODUCTION
One of the most important form of renewable energy is the solar photovoltaic energy. It has undergone a huge research and development in the recent past and is still developing. A solar cell is a device that directly converts the energy in sunlight to electrical energy through the process of photovoltaics. The first solar cell was built around 1883 by Charles Fritts, who used junctions formed by coating selenium (a semiconductor) with an extremely thin layer of gold. In 2009, a thin film cell sandwiched between two layers of glass was made.
Countries that have high solar irradiation reception, this source proves a highly efficient, economic and environment friendly form of energy source. There are various factors that affect the efficiency of a solar cell. Cell temperature and energy conversion efficiency are some of them. The reason for the low efficiency of solar cells is their low energy conversion efficiency. A solar cell converts a part of incident solar light into electrical energy the rest being wasted as heat. The infrared light portion of the solar spectrum attributes to this heat loss thus increasing the cell temperature.
The sun is the most plentiful energy source for the earth. All wind, fossil fuel, hydro and biomass energy have their origins in sunlight. Solar energy falls on the surface of the earth at a rate of 120 petawatts, (1 petawatt = 1015 watt). This means all the solar energy received from the sun in one days can satisfied the whole world’s demand for more than 20 years.
We are able to calculate the potential for each renewable energy source based on today’s technology. (Figure 1) Future advances in technology will lead to higher potential for each energy source. However, the worldwide demand for energy is expected to keep increasing at 5 percent each year.1Solar energy is the only choice that can satisfy such a huge and steadily increasing demand.
World population is expected to double by the middle of the 21st century (Global Energy, 1998). This will consequently result in a 3-5 fold increase in world economic output by the year 2050, and a 10-15 fold increase by the year 2100. Consequently, Primary energy requirements are expected to increase by approximately three folds by the year 2050 and five folds by the year 2100. This is expected to exert tremendous pressure on primary energy supplier.
Energy has an established positive correlation with economic growth. Providing adequate, affordable and clean energy is a prerequisite for eradicating poverty and improving productivity. The inevitable increase in the use of fossil fuels alongside a country’s economic growth presents associated side effects of threat to the nation’s energy security, as well as environmental degradation through climate change. A feasible alternative to the indiscriminate burning of fossil fuels lies in the accelerated use of renewable energy. In tropical countries, which have sunshine almost throughout the year in most parts, solar energy is one of the most viable options.
Energy from the sun has been used to provide electricity for many years. This form of renewable energy occupies less space compared to the space occupied by hydropower projects. Developing countries can cover all their demands for energy by solar systems with 0.1% of the land area.
1.1. Principle of Operation of Solar Energy
Solar energy is available in abundance in most parts of the world. The amount of solar energy incident on the earth’s surface is approximately1.5 x 1018 kWh/year, which is about 10,000 times the current annual energy consumption of the entire world. The density of power radiated from the sun (referred to as solar energy constant) is 1.373 kW/m2.
Solar cell is a device which converts photons in Solar rays to direct-current (DC) and voltage. The associated technology is called Solar Photovoltaic (SPV). A typical silicon PV cell is a thin wafer consisting of a very thin layer of phosphorous-doped (N-type) silicon on top of a thicker layer of boron-doped (P-type) silicon. An electrical field is created near the top surface of the cell where these two materials are in contact (the P-N junction).
When the sunlight hits the semiconductor surface, an electron springs up and is attracted towards the N-type semiconductor material. This will cause more negatives in the n-type and more positives in the P-type semiconductors, generating a higher flow of electricity. This is known as Photovoltaic effect.
1.2. Governing principles of Solar Energy
1.2.1. Solar Irradiance
The Sun is the fundamental driving force for energy in the Earth's climate system. It is of crucial importance to understand fully the conditions of its arrival at the top of the atmosphere and its transformation through the earth. The amount of solar power available per unit area is known as irradiance. Irradiance is a radiometric term for the power of electromagnetic radiation at a surface, per unit area. It is used when the electromagnetic radiation is incident on the surface.
Irradiance fluctuates according to the weather and the sun’s location in the sky. This location constantly changes through the day due to changes in both the sun’s altitude (or elevation) angle and its azimuth (or compass) angle.
1.2.2. Solar Constant
The solar constant is the amount of incoming solar electromagnetic radiation per unit area, measured on the outer surface of Earth's atmosphere on a plane perpendicular to the rays. The solar constant includes all types of solar radiation, not just the visible light. It is estimated to be roughly 1,366 watts per square meter (W/m²) according to satellite measurements, though this fluctuates by about 6.9 % during a year (from 1,412 W/m² in early January to 1,321 W/m² in early July) due to Earth's varying distance from the Sun. For the entire planet (Earth has a cross section of 127,400,000 km²), the power is (1366 W/m2 x 1.274×1014 m2) 1.740×1017 W, plus or minus 3.5 %. The solar constant does not remain constant over long periods of time. The average value cited, 1,366 W/m², is equivalent to 1.96 calories per minute per square centimeter, or 1.96 langleys (Ly) per minute.
1.2.3. Solar Spectrum
The sun radiates power over a continuous band or spectrum of electromagnetic wavelengths. The power levels of the various wavelengths in the solar spectrum are not the same.
Ultraviolet, Visible and Infrared Radiation
The sun’s total energy is composed of 7% ultraviolet radiation, 47% visible radiation and 46% infrared (heat) radiation. Ultraviolet (UV) radiation causes many materials to degrade and is significantly filtered out by the layer of Ozone in the upper atmosphere.
Photovoltaic cells primarily use visible radiation. The distribution of colours within light is important, because, a photovoltaic cell will produce different amounts of current depending on the various colours reflecting on it.
Infrared radiation contributes to the production of electricity from crystalline silicon and some other materials. In most cases, however, infrared radiation is not as important as the visible portion of the solar spectrum.
1.2.4. Solar Insolation
The results of the earth’s motion and atmospheric effects at various locations have led to essentially two types of solar insolation data. These are daily and hourly.
Solar irradiance is related to power per unit area where as solar insolation is related to radiant energy per unit area. Solar insolation is determined by summing solar irradiance over time, and is usually expressed in units of kWh/m2 /day.
Average Daily Solar Radiation
To provide long-term average daily solar radiation data, an average of daily solar radiation is calculated for each month over a period of typically 30 years. This data is useful both in predicting long-term performance and in analyzing the economics of solar energy systems. The actual average daily solar radiation for a given month may vary significantly from the long-term average for that month.
Peak Sun Hours
The number of peak sun hours per day at a given location is the equivalent number of hours at peak sun conditions (i.e., at 1 kW/m2) that produces the same total insolation as actual sun conditions.
Solar Radiation Data
A solar resource assessment begins with quantifying ground-incident solar radiation in a given area. Solar radiation can reach the earth’s surface either directly from the sun (direct normal irradiance) or indirectly after being scattered by the atmosphere or cloud interference (diffuse horizontal irradiance). The sum of the direct and indirect radiation gives the total solar radiation reaching the earth’s surface (global horizontal irradiance or GHI). The amount of ground-incident radiation that actually reaches a solar array is a function of the type of solar system (fixed-axis PV, tracking PV, CSP) and the orientation of the solar array relative to the sun’s position in the sky. PV systems can use both direct and diffuse radiation as well as ground-reflected radiation in the case of tilted arrays.
1.2.5. Direct and Diffuse Solar Radiation
Sunlight coming from the sun is reduced by about 30% before it reaches the earth due to
• Scattering by atmospheric particles
• Scattering by aerosol, dust particles etc.
• Absorption by atmospheric gases
It is common to consider separately the ‘direct’ (or beam) radiation coming from solar disk and the ‘diffuse’ radiation from elsewhere in the sky with their sum known as ‘global’ radiation.
The component of the radiation coming from all direction in the sky is diffused. When the sun is directly overhead, it has diffuse component of about 10% when skies are clear. Percentage increases with increased Air Mass.
3. OVERVIEW OF RENEWABLE ENERGY TECHNOLOGIES
This section provides an overview and brief description, including fundamentals, of the different renewable energy technologies, wind, solar, bioenergy, hydro, and geothermal energy.
One of the first aspects to consider is the cost of renewable energy technologies.
However, this is not an easy question to answer because, as with many energy technologies, many factors affect cost and different sources of information use different criteria for estimating cost. In many cases, the environmental benefits of renewable energy technologies are difficult to take into account in terms of cost savings through less pollution and less damage to the environment. When trying to calculate the cost of these technologies is often best to take a life cycle cost approach, as these technologies often have high up-front capital costs but very low operation and maintenance costs. And of course, there is usually no fuel cost!
The table clearly shows that the minimum to average generation costs for these technologies vary widely between different technologies, and within the same technology, according to differences in national markets and resource conditions. This means that one technology can be cheaper in one country than in another.
3.1. Solar energy
Solar energy technologies can be loosely divided into two categories: solar thermal
Systems and solar electric or photovoltaic (PV) systems
Photovoltaic (PV) systems
Photovoltaic or PV devices convert sun light directly into electrical energy. The amount of energy that can be produced is directly dependent on the sunshine intensity. Thus, for example, PV devices are capable of producing electricity even in winter and even during cloudy weather albeit at a reduced rate. Natural cycles in the context of PV systems thus have three dimensions. As with many other renewable energy technologies, PV has a seasonal variation in potential electricity production with the peak in summer although in principle PV devices operating along the equator have an almost constant exploitable potential throughout the year. Secondly, electricity production varies on a diurnal basis from dawn to dusk peaking during mid-way. Finally, short-term fluctuation of weather conditions, including clouds and rain fall, impact on the inter hourly amount of electricity that can be harvested.
PV devices use the chemical-electrical interaction between light radiation and a semiconductor to obtain DC electricity. The base material used to make most types of solar cell is silicon (approx. 87 per cent). The main technologies in use today are:
• Mono-crystalline silicon cells are made of silicon wavers cut from one homogenous crystal in which all silicon atoms are arranged in the same direction. These have a conversion efficiency of 12-15 per cent);
• Poly-crystalline silicon cells are poured and are cheaper and simpler to make than mono-crystalline silicon and the efficiency is lower than that of mono crystalline cells (conversion efficiency 11-14 per cent);
• Thin film solar cells are constructed by depositing extremely thin layer of photovoltaic materials on a low-cost backing such as glass, stainless steel or plastic (conversion efficiency 5-12 per cent);
• Multiple junction cells use two or three layers of different materials in order to improve the efficiency of the module by trying to use a wider spectrum of radiation (conversion efficiency 20-30 per cent).
The building block of a PV system is a PV cell. Many PV cells are encapsulated together to form a PV panel or module. A PV array, which is the complete power generating unit, consists of any number of PV modules/panels. Depending on their application, the system will also require major components such as a battery bank and battery controller, DC-AC power inverter, auxiliary energy source etc. Individual PV cells typically have a capacity between 5 and 300 W but systems may have a total installed capacity ranging from 10 W to 100 MW. The very modular nature of PV panels as building blocks to a PV system gives the sizing of systems an important flexibility.
Solar thermal systems
Solar thermal systems use the sun’s power in terms of its thermal or heat energy for heating, drying, evaporation and cooling. Many developing countries have indigenous products such as solar water heaters, solar grain dryers, etc. These are usually local rather than international products, specific to a country or even to a region. The main solar thermal systems employed in developing countries are discussed briefly below.
Solar thermal power plants
Solar thermal engines use complex concentrating solar collectors to produce high temperatures. These temperatures are high enough to produce steam, which can be used to drive steam turbines generating electricity. There is a wide variety of different designs, some use central receivers (where the solar energy is concentrated to a tower) whilst others use parabolic concentrator systems.
Although the first commercial thermal power plants have been in operation in California since the mid-1980s, many of the newer designs are still at the prototype stage being tested in pilot installations in the deserts of the United States and elsewhere. The Global Environment Facility (GEF) has supported the first planning phase of a project that is developing a concentrating solar power plant in Egypt in 2004. There are also projects in India, Mexico and Morocco that have been supported by GEF as part of a strategy to accelerate cost reduction and commercial adoption of high temperature solar thermal energy technology.
Solar water heating
Solar water heating systems (see figure II above) may be used in rural clinics, hospitals or even schools. The principle of the system is to heat water, usually in a special collector and store it in a tank until required. Collectors are designed to collect the heat in the most efficient, but cost effective way, usually into a heat transfer fluid, which then transfers its heat to the water in the storage tank. The two main types of collector are: flat plate and evacuated tube. For example, to heat 100 litres of water through a temperature rise of 40o C with a simple flat plate solar collector requires only approximately 2.5 m2 of collector area but saves approx. 10 kg of wood fuel that would normally be required to heat this quantity of water.
The cheapest technology available and the simplest to install is a thermosiphon system, which uses the natural tendency of heated water to rise and cooler water to fall to perform the heat collection task. As the sun shines on the collector, the water inside the collector flow-tubes is heated. As it heats, this water expands slightly and becomes lighter than the cold water in the solar storage tank mounted above the collector. Gravity then pulls the heavier, cold water down from the tank and into the collector inlet. The cold water pushes the heated water through the collector outlet and into the top of the tank, thus heating the water in the tank.
Solar drying
Solar drying, in the open air, has been used for centuries. Drying may be required to preserve agricultural/food products or as a part of the production process, i.e. timber drying. Solar drying systems are those that use the sun’s energy more efficiently than simple open-air drying.
In general, solar drying is more appropriate when:
• The higher the value per ton of products dried;
• The higher the proportion of the product currently spoiled in the open air;
• The more often the drier will be used.
Solar cookers
Solar cookers can be important because of the increased scarcity of wood fuel and the problems of deforestation in many developing country regions. Solar cookers can also promote cleaner air where there is a problem with indoor cooking.
There are basically two types of solar cooker: oven or stove type. As with conventional cooking stoves, solar stoves apply heat to the bottom of the cooking pot while solar ovens apply a general heat to the enclosed area which contains the cooking pot. However, there are important social issues related to the effective use of solar cookers. There will always be some change of habits required and readiness to change is an important factor that affects the potential impact of this technology.
Solar distillation
Solar distillation is a solar enhanced distillation process to produce potable water from a saline source. It can be used in areas where, for instance, drinking water is in short supply but brackish water, i.e. containing dissolved salts, is available.
In general solar distillation equipment, or stills, is more economically attractive for smaller outputs. Costs increase significantly with increased output, in comparison to other technologies which have considerable economics of scale.
Solar cooling
Several forms of mature technologies are available today for solar-thermally assisted air-conditioning and cooling applications. In particular for centralized systems providing conditioned air and/or chilled water to buildings, all necessary components are commercially available. The great advantage of this solar application, especially in tropical and equatorial countries, is that the daily cooling load profile follows the solar radiation profile (i.e. office buildings).
4. Solar Receiver Technologies
The types of receivers used for collecting solar energy are classified as follows:
4.1. Flat Plate Arrays
Flat plate arrays use both diffused and direct sunlight. They can operate in either fixed orientation or in a sun-tracking mode. For most applications, flat plate arrays are in fixed orientation. However, with the advent of low-cost passive sun-trackers, flat plate tracking arrays are becoming more popular.
4.2. Tracking Arrays
In this case, solar array follows the path of the sun and maximizes the solar radiation incident on the photovoltaic surface. The two most common orientations are:
• One-axis tracking: In this tracking mechanism, the array tracks the sun east to west. It is used mostly with flat-plate systems and occasionally with concentrator systems.
• Two-axis tracking: In this tracking mechanism, the array points directly at the sun at all time. It is used primarily with PV concentrator systems
5. Solar Photovoltaic Technologies
The heart of the solar energy generation system is the Solar cell. It consists of three major elements, namely:
• The semiconductor material which absorbs light and converts it into electron-hole pairs.
• The junction formed within the semiconductor, which separates the photo-generated carriers (electrons and holes)
• The contacts on the front and back of the cell that allow the current to flow to the external circuit.
Two main streams of technologies have been evolved for the manufacture of Solar Cells/Modules namely:
• Flat plate Technology
• Concentrated Technology
The Flat Plate Technology is further classified in two ways namely Crystalline Technology and Thin Film Technology. The Concentrated Photovoltaic Technology has been classified according to the Type of cell and the Optical system.
5.1. Crystalline Technology
Crystalline Silicon (c-Si) was chosen as the first choice for solar cells, since this material formed the foundation for all advances in semiconductor technology. The technology led to development of stable solar cells with efficiency up to 20%.
Two types of crystalline silicon are used in the industry. They are
• Mono crystalline Silicon
• Multi crystalline Silicon
Mono-Crystalline Silicon
Mono-Crystalline Silicon cells are produced by growing high purity, single crystal Si rods and slicing them into thin wafers. Single crystal wafer cells are expensive. They are cut from cylindrical ingots and do not completely cover a square solar module. This results in substantial waste of refined silicon.
The efficiency of mono-crystalline silicon cells remains between 17-18% because of the purity level.
Multi-Crystalline Silicon
Poly-crystalline silicon cells are made from sawing a cast block of silicon first into bars and then wafers. This technology is also known as Multi crystalline technology. Poly-Si cells are less expensive to produce than single crystal silicon cells as the energy intensive process for purification of silicon is not required. They are less efficient than single crystalline cells. The efficiency of poly crystalline silicon cells ranges from13-14%.
5.2. Thin Film Technology
In Thin Film Solar technology, a very thin layer of chosen semiconductor material (ranging from Nano meter level to several micro meters in thickness) is deposited on to either coated glass or stainless steel or a polymer substrate.
Various thin-film technologies are being developed to reduce the amount of light-absorbing materials required to construct the solar cell. This results in reduction of processing cost. However, conversion efficiencies are also lower in these cases (average 7-10%). As the modules are of lesser efficiency for same level of energy requirement, longer collector area is required and consequently more requirement of land. This technology is, therefore, apt where non productive land is available for example deserts of Rajasthan. They have become popular compared to wafer silicon due to lower costs, flexibility, lighter weights, and ease of integration.
Effect of Light Radiation Angle on Photovoltaic Output Power
In a photovoltaic converter, the energy of absorbed photons is converted to electric energy by the solar cell. Therefore, the output electric power depends on the radiation angle of the sun light. The electric characteristics of the solar cells change due to the variation of the generated electrons with light intensity. Fig.1 shows the current-voltage characteristic of a sample solar cell [3]. The generated current by the solar cell has a large variation from light intensity changes. As a result, the output power of the solar cell can changes and the photovoltaic array can generates electric power less than its nominal power.
Due to the motion of the sun, the sun light always radiates on the surface with different angel during the day and it is not perpendicular for a fixed PV-array whereas it can be designed to be always perpendicular for a movable array with full degree of motion.
6. Cells, Panels and Arrays
The smallest unit of a solar panel is the solar cell, also called a photovoltaic, or PV cell; it's the individual PV cell that turns sunlight into electricity. Individual cells arranged in a group are called a "module" or panel; a collection of two or more panels is called an array.
Solar modules use light energy (photons) from the sun to generate electricity through the photovoltaic effect. The majority of modules use water-based crystalline silicon cells or thin-film cells based on cadmium telluride or silicon. The structural (load carrying) member of a module can either be the top layer or the back layer. Cells must also be protected from mechanical damage and moisture. Most solar modules are rigid, but semi-flexible ones are available, based on thin-film cells. These early solar modules were first used in space in 1958.
7. THE HISTORY OF SOLAR
Solar technology isn’t new. Its history spans from the 7th Century B.C. to today. We started out concentrating the sun’s heat with glass and mirrors to light fires. Today, we have everything from solar-powered buildings to solar powered vehicles.
Here you can learn more about the milestones in the historical development of solar technology, century by century, and year by year. You can also glimpse the future.
Solar Works
The basic premise behind solar energy conversion of electricity is in the use of photovoltaic (PV) systems. These systems convert the sunlight into electricity.
Basically, solar cells, the heart of the PV system, are made of semiconductor material. At the time that sunlight hits the materials, the rays are absorbed.
These rays are converted into electricity by the solar cells. If you evaluate the PV system, you will find it can only produce a small amount of power. In order to produce a large amount of power, several solar cells would have to be connected to from panels or modules, as they are also called. In most cases, several PV modules are connected together and installed in a rack to form an array. It takes on average about 10-20 PV modules to provide enough power for a house.
PV arrays can be mounted in two different ways: a fixed angle facing south, or they can be mounted on a tracking device which follows the sun.
COOLING TECHNIQUE:
Photovoltaic panels (PV) get overheated due to excessive solar radiation and high ambient temperatures. Overheating reduces the efficiency of the panels.
Hybrid Photovoltaic/Thermal (PV/T) solar system is one of the most popular methods for cooling the photovoltaic panels nowadays. The hybrid system consists of a solar photovoltaic panels combined with a cooling system. Water is circulated around the PV panels for cooling the solar cells, and the warm water leaving the panels pump back to water tank. Warm water mixed with cool water of tank.
It was found that the solar panel with water cooling generates more energy. However, cooling by spraying water is not an efficient method, since the water will not be sprayed over the whole panel, and therefore, some parts of the PV panels will not be cooled, as well as this method results in a very high water loss.
The cooling system consists of an evaporator section and a condenser section. The input heat from the sun vaporizes the liquid inside the evaporator section and then the vapor passes through the condenser section, and finally, the condenser section is cooled down using water. Hence, the heat pipe can transfer the heat from solar panel to water depending on the system. Using air as a coolant was found to decrease the solar cells temperature by 4.7 °C and increases the solar panel efficiency by 2.6%, while using water as a coolant was found to decrease the solar cells temperature by 8 °C and the panel efficiency by 3%. Therefore, cooling by water was found to be more effective than cooling by air.
11.1. Water cooling
Water cooling methods provide improved performance over the air cooling methods due to the increase in heat carrying capacity of water over air. These methods make use of water, chilled or un chilled, as the working fluid. Unlike in air cooling methods that are limited to traditional natural and forced convections, water cooling has a broader range. Some methods that implement water cooling are natural and force convections, front water cooling, heat pipe and immersion techniques. This section highlights the recent developments in water cooling techniques.
11.2. Natural and forced convection
The effects of panel front water cooling. In this method the cooling fluid directly wets the module’s active surface. In this study the PV module was cooled by a continuous film of water running over the front of the PV module. This was shown to not only decrease temperature but also increase electrical efficiency due to the decreased reflection loss. The water was fed at 24 ‘C at the rate of 2 L/min. The temperature on the back surface was reduced from 48 ‘C to 35.5 ‘C while the temperature between the front and the back surface remained around 7–8 ‘C. This also removes debris spot that could potentially damage the PV module. Moharram et al. tried to address the PV overheating problem with minimum amount of water and energy. This approach makes use of intermittent cooling, where the PV modules are allowed to reach the Maximum Allowable Temperature (MAT) that can yield maximum energy before the cooling begins. The cooling rate model was developed to calculate the time needed to cool the panels to normal operating temperatures. This model is used to minimize cooling time, thereby decreasing the water and energy needed for cooling.