ME165-3 Solar and Wind Energy Lecture-8.0 Solar Pond, Solar Cell & Semiconductor 2022-2023 / 2T Prepared By: Engr. Estelito V. Mamuyac 26 January 2023 Solar Ponds • Solar Ponds • A solar pond is a pool of saltwater which acts as a largescale solar thermal energy collector with integral heat storage for supplying thermal energy. • A solar pond can be used for various applications, such as process heating, desalination, refrigeration, drying and solar power generation. Solar Ponds Schematic of a Solar Pond Solar Ponds • How solar pond works: • Solar ponds can be naturally occurring; however, most ponds are man-made. • Once the pond is dug, the pond must be lined with an impermeable lining, preferable one that is insulating. • Then the pond is filled with salty water. • Once the sun hits the pond the water warms and divides into three layers. Solar Ponds • The top layer, known as the surface zone, is composed of primarily freshwater due to the fact that salt typically settles at the bottom of water. • The middle layer is known as the insulation zone. The insulation zone has a higher salt concentration than the surface zone. • Crucial to a solar pond is the bottom layer known as the storage zone. The storage zone is where all the hot water is held, and this is what is converted into electricity. The hot salt water produced is similar in chemical characteristics to a brine. Solar Ponds • In a typical freshwater pond, when the sun penetrates the water the layers that are heated up rise to the top of the pond and release the heat into the atmosphere. • This is how a pond maintains a constant temperature. The oxygen in warm water is greater than cold water. • This causes warm water to rise to the top of the water body and this heat is then released. • However, in a solar pond this process does not happen. Instead, the water that is warmed is unable to rise to the top due to the salt concentration. Solar Ponds • Therefore, the warm water stays at the bottom of a pond and gets hotter and hotter with the more sunlight it receives. • The bottom layer of a solar pond can reach 80-90 oC. • What allows a solar pond to be used as an energy source is that a pipe is placed at the bottom of the pond and draws the warm/ hot water out of the pond by a pump and is circulated through a piping system that utilizes the heat. It is similar, to how radiant heat, or solar hot water heaters use the warm water. Solar Ponds • Once the water has run through the pipe it is deposited back into the pond in the storage zone so this water can be heated again. • This system is a close system so is quite efficient in terms of water retention. Typically, this is how a solar pond is used for heating purposes. • Solar ponds can be used in all climates, as long as, there is plenty of sun. Even when a pond is frozen over, a salient gradient solar pond still produces hot water. • Therefore, they can be used all over the United States and the world. Solar Ponds aerial picture of a solar pond Solar Ponds • Advantages of using solar ponds: • It produces heat or electricity with little to no carbon emissions. ▪ The emission depends on the type of pump used to push water through a turbine or piping. • It is unique in its capability in acting both as collector and storage. • The cost of solar pond per unit area is less than any active collectors available today. • When used for desalinization, no energy is required to produce potable water, instead the clean water is a result of the separation of water according to salt concentration. (1003) How Does a Salt Pond Work? – YouTube https://www.youtube.com/watch?v=SqWjhkXVRzM Solar Cells • Solar Cell • A solar cell (also called a photovoltaic cell) is an electrical device that converts the energy of light directly into electricity by the photovoltaic effect. • It is a form of photoelectric cell (in that its electrical characteristics-e.g. current, voltage, or resistance-- vary when light is incident upon it) which, when exposed to light, can generate and support an electric current without being attached to any external voltage source. Solar Cells Solar Cells • How a solar cell works: The solar cell works in three steps: 1. Photons in sunlight hit the solar panel and are absorbed by semiconducting materials, such as silicon. 2. Electrons (negatively charged) are knocked loose from their atoms, causing an electric potential difference. ▪ Current starts flowing through the material to cancel the potential and this electricity is captured. ▪ Due to the special composition of solar cells, the electrons are only allowed to move in a single direction. 3. An array of solar cells converts solar energy into a usable amount of direct current (DC) electricity. Solar Cells • How do photovoltaic cells work? Solar Cells • How photovoltaic cells are made? • The principal components of a photovoltaic cell are semiconductors made of silicon crystals that have been doped with other elements such as Phosphorus or Boron. • The bottom layer is doped with Boron and is termed as the p-type semiconductor while the upper layer is doped with Phosphorus and is termed as the n-type conductor and the space between them is called the P-N junction. • Once the sun’s light enters the PV cells, its energy is transferred to electrons that are then knocked loose in both semiconductors and they are attracted to the p-type semiconductor however, the electric field at the junction makes this difficult. Solar Cells • When you connect to an external circuit you create a path for the flow of electrons which is actually the current necessary for electricity. • The current generated here is Direct and needs to be converted to usable form and this conversion is done by use of a device referred to as an inverter although this leads to some loss of energy. Solar Cells • Methods of increasing the cell efficiency • The efficiency of a solar cell may be broken down into reflectance efficiency, thermodynamic efficiency, charge carrier separation efficiency and conductive efficiency. • The overall efficiency is the product of each of these individual efficiencies. • Due to the difficulty in measuring these parameters directly, other parameters are measured instead: thermodynamic efficiency, quantum efficiency, integrated quantum efficiency, VOC ratio, and fill factor. Solar Cells • Methods of increasing the cell efficiency (cont’d.) • Reflectance losses are a portion of the quantum efficiency under "external quantum efficiency". • Recombination losses make up a portion of the quantum efficiency, VOC ratio, and fill factor. • Resistive losses are predominantly categorized under fill factor, but also make up minor portions of the quantum efficiency, VOC ratio. Solar Cells • Methods of increasing the cell efficiency (cont’d.) • The fill factor is defined as the ratio of the actual maximum obtainable power to the product of the open circuit voltage and short circuit current. • This is a key parameter in evaluating the performance of solar cells. • Typical commercial solar cells have a fill factor > 0.70. Grade B cells have a fill factor usually between 0.4 to 0.7. • Cells with a high fill factor have a low equivalent series resistance and a high equivalent shunt resistance, so less of the current produced by the cell is dissipated in internal losses. Solar Cells • Methods of increasing the cell efficiency (cont’d.) • Single p-n junction crystalline silicon devices are now approaching the theoretical limiting power efficiency of 33.7%, noted as the Shockley–Queisser limit in 1961. • In the extreme, with an infinite number of layers, the corresponding limit is 86% using concentrated sunlight. Solar Cell System • PV System Related Equipment • Photovoltaic modules can be mounted on the ground or a building roof or can be included as part of the building structure, usually façade. Related equipment includes batteries, charge controllers, inverters, and peak-power trackers. • Batteries. They are required in many PV systems to supply power at night or when the PV system cannot meet the demand. Main types of batteries currently available in the market: ▪ Lead-acid ▪ Nickel-Cadmium ▪ Nickel-Hydride ▪ Lithium Solar Cell System • PV System Related Equipment (cont’d.) • Inverters. Used to convert the direct current into alternating current electricity. The output of the inverter can be single or three phase. • Charge Controllers. Regulate the power from PV modules to prevent the batteries from overcharging. The controller can be a shunt type or series type and also function as a low-battery voltage disconnect to prevent the battery from over-discharge. Solar Cell System • PV System Related Equipment (cont’d.) • Peak-Power Trackers. PV cells have a single operating point where the values of the current (I) and the voltage (V) of the cell result in maximum power output. These values correspond to a particular resistance, which is equal to V/I, as specified by Ohm’s law. A PV cell has an exponential relationship between current & voltage, and there is only one optimum operating power point also called a maximum power point (MPP). MPP changes according to the radiation intesity and the cell temperature. Maximum power point trackers (MPPTs) search for this point, thus, allow the converter circuit to extract the maximum power available from a cell. Solar Cell System - Applications PV Array Load • Direct Coupled PV System • The PV array is connected directly to the load. • Therefore, the load can operate only when there is solar radiation, so such a system has very limited applications. ▪ A typical application of this type of system is for water pumping, i.e., the system operates, as long as sunshine is available, and instead of storing electrical energy, water is usually stored. Solar Cell System - Applications • Direct Coupled PV System https://www.youtube.com/watch?v=Zhdl-ta-OCA Solar Cell System - Applications • Stand-Alone PV System • Used in areas that are not easily accessible or have no access to an electric grid. • It is independent of the electricity grid, with the energy produced normally being stored in batteries. Solar Cell System - Applications • Stand-Alone PV System • The system would consist of a PV module or modules, batteries, and a charge controller. An inverter may also be included in the system to convert the direct current generated by the PV modules to the alternating current form required by normal appliances. • The system can satisfy both DC and AC loads simultaneously. Solar Cell System - Applications • Grid-Connected System • During the day, the electricity generated by the PV system can either be used immediately or be sold to one of the electricity supply companies. • In the evening, when the solar system is unable to provide the electricity required, power can be bought back from the network. • In effect, the grid is acting as an energy storage system, which means the PV system does not need battery storage. https://www.youtube.com/watch?v=Q0B9t8dPGe0 Solar Cell System - Applications • Hybrid-Connected System. More than one type of electricity generator is employed. • The 2nd type of electricity generator can be renewable, such as a wind turbine, or conventional, such as a diesel engine generator or the utility grid. Solar Cell System - Applications • Hybrid-Connected System (cont’d.) • The diesel engine generator can also be a renewable source of electricity when the diesel engine is fed with biofuels. • In this system, both DC and AC loads can be satisfied simultaneously. Solar Cell Systems – Types of Application • Remote Site Electrification • Photovoltaic systems can provide long-term power at sites far from utility grids. • The loads include lighting, small appliances, water pumps, and communication equipment. • The load demand can vary from few watts to tens of kilowatts. • PV systems are preferred to fuel generators, since they do not depend on a fuel supply, and they do avoid maintenance and environmental pollution problems. Solar Cell Systems – Types of Application • Communications • Photovoltaics can provide reliable power for communication systems, especially in remote locations, away from the utility grid. • Examples include communication relay towers, travelers’ information transmitters, cell phone transmitters, radio relay stations, emergency call units, and military communications facilities. • These systems are stand-alone units in which PV-charged batteries provide a stable DC voltage that meets the varying current demand. Solar Cell Systems – Types of Application • Remote Monitoring • Because of their simplicity, reliability, and capacity for unattended operation, photovoltaics are preferred in providing power at remote sites to sensors, data loggers, and associated meteorological monitoring transmitters, irrigation control, and monitoring highway traffic. • The batteries required are often located in the same weatherresistant enclosure as the data acquisition or monitoring equipment. Solar Cell Systems – Types of Application • Water Pumping • Stand-alone photovoltaic systems can meet the need for small to intermediate-size water-pumping applications. • These include irrigation, domestic use, village water supply, and livestock watering. • Advantages of using water pumps powered by photovoltaic systems include low maintenance, ease of installation, and reliability. • Most pumping system do not use batteries but store the pumped water in holding tanks. Solar Cell Systems – Types of Application • Building-Integrated Photovoltaics (BIPV) • BIPV is a special application in which PVs are installed either in the façade or roof of a building and are integral part of the building structure, replacing in each case the particular building component. • To avoid an increase in the thermal load of the building, a gap is created between the PV and the building element, which is behind the PV. In this gap, ambient air is circulated so as to remove the produced heat. • A common example where these systems are installed is what is called zeroenergy houses, where the building is an energy-producing unit that satisfies all its own energy needs. Solar Cell Systems – Types of Application • Charging Vehicle Batteries • Photovoltaic chargers keep the battery at a high state of charge by providing a trickle charging current. • The module can be installed on the roof of a building or car park or on the vehicle itself. • Another important application in this area is the use of PV modules to charge the batteries of electric vehicles. Semi-Conductors • Semiconductor • Sometimes referred to as computer chips or integrated circuits (ICs), contain numerous electrical pathways which are capable of connecting up to a billion transistors and other electronic components. • These transistors store information on the semiconductors, either by holding an electrical charge or by holding little or no charge. Semi-Conductors • Semiconductors are available as either elements or compounds. • Silicon and Germanium are the most common elemental semiconductors. • Compound Semiconductors include InSb, InAs, GaP, GaSb, GaAs, SiC, GaN. Si and Ge both have a crystalline structure called the diamond lattice. That is, each atom has its four nearest neighbors at the corners of a regular tetrahedron with the atom itself being at the center. • The advantage of compound semiconductor is that they provide the device engineer with a wide range of energy gaps and mobilities, so that materials are available with properties that meet specific requirements. • Some of these semiconductors are therefore called wide band gap semiconductors. Semi-Conductors • Types • Intrinsic • An intrinsic semiconductor material is chemically very pure and possesses poor conductivity. • It has equal numbers of negative carriers (electrons) and positive carriers (holes). Semi-Conductors • Extrinsic • An extrinsic semiconductor is an improved intrinsic semiconductor with a small amounts of impurities added by a process, known as doping, which alters the electrical properties of the semiconductor and improves its conductivity. • Introducing impurities into the semiconductor materials (doping process) can control their conductivity. • Doping process produces two groups of semiconductors: • the negative charge conductor (n-type) and • the positive charge conductor (p-type). Semi-Conductors Flowchart – Semi-Conductor Basic Manufacturing Process References & Material Sources Reference • Solar Energy Engineering, Processes and Systems, Soteris A. Kologirou, 2009 Websites • • • • • http://climatelab.org/Solar_Ponds http://en.wikipedia.org/wiki/Solar_cell http://ecovized.com/2011/07/13/what-are-solar-panels/ http://www.about-solarenergy.com/how-does-solar-energy-work/ http://en.wikipedia.org/wiki/Semiconductor_device_fabrication Youtube Videos • • http://www.youtube.com/watch?v=x2zjdtxrisc https://www.youtube.com/watch?v=shHjh9QUB9g
