Bir soğutucuyu güneş enerjisi ile besleyen sistemin tasarımı

Abstract

Güneş enerjisini silisyum güneş pilleri yardımı ile elektrik akımına dönüştüren ve bir akümülatör grubunda depoladıktan sonra gerilimin genlik ve formunu değiştirerek soğutucunun çalışacağı uygun genlik ve frekansta gerilim üreten bir sistem tasarlanmış ve gerçekleştirilmiştir. Sistem, güneş pilleri, akümülatör, DC/DC çevirici, evirici ve soğutucudan oluşur. Güneş pillerinin uç gerilimleri, üzerlerine düşen güneş ışığına ve akıma bağlı olarak değişmektedir. Bu nedenle gece çalışması da göz önüne alınarak sisteme bir akümülatör grubu eklenmiş böylece çalışma güvenilirliği arttırılmış ve çalışma sürekliliği sağlanmıştır. Eldeki doğru gerilim, yüksek verimli bir anahtar lamali güç kaynağı ile yükseltilip, daha sonra evirilerek soğutucuya uygulanmıştır. Evirici yarı köprü ve sinüs darbe genişlik modülasyonlu olarak seçilmiş, MOSFET 1er in kullanımıyla verimi daha da büyütülmüştür. Soğutucunun bir fazlı asenkron motorunun kalkış anında çektiği büyük yolalma akımı ise, frekans ile yolverme yöntemi kullanılarak küçültülmüştür.

This study tries to build a solar -cooler system which is portable and reliable. There are four main blocks in the system. These are, solar cell array, DC to DC converter, sinus pulse-width modulation inverter and a conventional freezer which contains a single-phase asynchronous motor and a compressor pump. The photovoltaic CPVD technology has been succesfully commercialized since the late 1970' s for use in terrestrial applications. While ongoing research with the high tecnology solar cell remains exciting, the power system designer is beginning to treat the PV module as a commodity. With the exception of nuclear power, the sun is the source of all energy used on Earth. Wind power, tidal energy, fossil fuels, hydro-electric plants all depend on the sun. The sun generates an amount of energy so vast as to be almost inconceivable. Its output is approximately 3.8*10 kilowatts. Less than a billionth of this energy is intercepted by the Earth, and over half of that is reflected back into space or absorbed by our atmosphere. But the energy that does reach the Earth's surface is very substantial: On a sunny day, each square-meter facing the sun receives about one kilowatt. Photovoltaic devices collect solar energy and convert it directly, without moving parts or by produts of any kind, into electricity. Technologically, photovoltaic power systems are capable of providing power for any purpose, large or small, remote or urban. Unlike such power sources as coal or diesel generators, however, the cost of PV power increases almost linearly with capacity. For example, a PV power system capable of supporting a 100 kilowatts load costs almost 100 times as much as a system for a 1 kilowatt load. Thus, economics limits PV's present use primarily to electrical loads smaller than 20 kilowatts and to sites remote from the commercial power grid. The first meaningful use of photovoltaic devices was providing on-board power for spacecraft. These PV power systems cost in the vicinity of S500 per peak watt of capacity. Production of true commercial PV power systems for terrestrial use didn't occur until around 1980, and the industry has expanded rapidly in the past five years. - vi - Thanks to substantial development in materials and manufacturing, commercial systems now typically cost between $10 and $30 per peak watt, depending on the system's scale, complexity and difficulty of installation. The photovoltaic cell, commonly called a solar cell, is the basic unit of a PV power system Such cells utilize photovoltaic conversion of light energy to electrical energy, an effect discovered over a hundred years ago. Photovoltaic conversion occurs in thin layers of many solid materials when light strikes their surface. In essence, electric charges are free from the material's atoms by the light energy; if made to flow thru a circuit this electric current can perform work. Virtually all present commercial solar cells are constructed from silicon, with boron and phosporus diffused into the silicon to form a p-n junction in the cells. CThis is the same technique used to form diode junctions. D When light is absorbed in the silicon it generates hole-electron pairs. Some of this pairs are seperated by the p-n junction, with one side of the junction accumulating and excess of holes C positively chargedl) and the other accumulating electrons C negatively charged}. The result is a potential difference across the junction. If a wire is connected to terminals on each side of the junction, current will flow through it and can be made to operate a load. Many materials exhibit the photovoltaic effect, but only a few are used at present for solar cells. Silicon dominates the field, but specialized cells and experimental devices have used gallium arsenide, cadmium sulfide, cadmium telluride and other materials. Since silicon is the only material in commercial use, the remainder of this study will discuss only silicon cells. Although experimental cells have been made with efficiencies close to %30, all commercial cells are far less efficient. single-crystal Cincludung dendritic webD cells are typically %13 to >S15 efficient; cells sawn from polycrystalline ingots range from /S9 to %13 and thin-film cells are from >S to ÎÛ0 efficient when first deployed. Unlike other silicon cells, the power output of thin-film cells degrades on exposure to sunlight. Degradation is rapid at first, then slows substantially. Electrical output of solar cells and modules is similar to DC sources in some ways but different in many. First, power output is dependent on the intensity of the sunlight striking the cells. A cell's current and power output are directly proportional to its illuminated area and illumination level. Voltage is uneffected by cell - vii - area and is relatively insensitive to illumination level. Above a low illumination treshold, cell voltage stays constant at around 0.5 volts. Temperature affects the performance of photovol tai cs, with increases in temperature increasing current output and decreasing voltage. The decrease in voltage is much greater then the increase in current, so the net effect is decreasing power with rising temperature. There is no inherent limitation to appliying PV technology; PV systems can power anything electrical. Their applications are limited only by PV's near -linear capacity/capital cost relationship, and this limit becomes less and less restrictive as the benefits of PV become better known, PV costs drop and fossil -based energy rates rise. The most common commercial uses of PV today are: Tel ecommuni cati ons -Microwave repeaters. -UHF/VHF radio base stations. -Radi o t el ephones. -Television translators. -Satellite Earth stations. Go ver ment /Mi 1 i t ar y -Portable power. -Security and sensors. -Tar get equi pment s. -Communi cati ons. -Forestry. -Aviation and highway. Railroads -DC and coded track circuits. -Intermediate and repeater signals. -Highway grade crossing warning devices. -Dragging equipment detectors. Oi 1 /Gas -Cathodic production. -Remote telemetry units. -Supervisory control and data acquisition. -Flow metering. - viii - Utilities -Off -grid communications. -System telemetry. -Security. Village Power. -Water pumpi ng. -Lighting. -Refrigeration. -Medi cal el i ni cs. I nstr ument at i on -Data collection. -Weather and flood control -Seismic monitoring. -Security systems. -Markers and beacons. -Navigati on aids. In pr ati cal application circuit, maximum total power coming from four solar array is only about two hundred watts. Because of this limited electrical power the total system efficiency must be as high as possible. The main component of DC to DC converter is a SMPS C Switch Mode Power Supply} integrated circuit SG 3524 which minimizes the number of components. SMPS converters are used to obtain a very well regulated and high efficient DC supplies from an unregulated DC source. CIn this application, DC source is four serial -parallel connected solar array and an accumulator group. D This IC, specially designed for use in fixed frequency switching regulators and other power control applications. This switched mode power supply control circuit can be used to implement single-ended or push-pull switching regulators of either polarity, both transformerless and transformer coupled. DC/DC converter is choosen push-pull type. Switching elements are two pair of N channel MOSFET. By using MOSFETs, switching losses and driving requirements are reduced relative to usage of bipolar junction transistors. Inverter is a half -bridge type sinus pulse width modulated MOSFET inverter. SPWM conducting and cutoff angles calculated by an developed harmonic elimination program and programmed into an ultraviolet erasable programmable read only memory CUVPROM or commonly called EPROMD. Up to 17th. harmonic the harmonic coefficients have been made equal to zero. MOSFETs are protected against inductive high voltage spikes by using RC type - ix - voltage snubber. By the help of SG 3524 integrated circuit, output voltage of inverter is regulated and an over current protection circuit added to the system easi 1 y. The starting over current problem which appears from single phase asyncronous motor solved by using frequency start-up method. Output frequency and voltage of inverter linearly increase from a few volts and hertz up to nominal operating ratings of motor C220 V, SO Hz}. Because of saturation of motor's iron, during start-up the U/f ratio stays constant at 220/50 value.