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الطاقة الشمسية..

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    تاريخ التسجيل: Mar 2003
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    الطاقة الشمسية..

    السلام عليكم ورحمة الله وبركاته..

    هذا الموضوع ارسل لي من قبل احد الاصدقاء.. اثناء تقديمي لبحث عن الطاقة الشمسية..

    اتمنى ان ينتفع به الجميع..

    جزى الله الف خير من اعده و من ارسله..

    للجميع التحية و التقدير:)

    والان مع الموضوع..

    Overview of the Photovoltaic Power System

    Introduction

    At present, photovoltaic (PV) electrical power generation is a fairly costly technology. Recent developments have led to increased cell efficiencies at reduced costs. A lot of researches are ongoing targeting to make PV more and more efficient and to reduce its cost to a magnitude that is competitive to the generation of conventional power to enable widespread of this progressive technology to penetrate to the bulk power market.

    In spite of all of this, even at this stage of the technology evolution, there are a lot of applications that photovoltaic is the only viable option. In some cases, it might be the only choice. Most of these applications, however, are small-scale power consumption applications. Larger scale power applications, especially if close to a utility grid, have not up till now been proven to be cost-effective.



    Theory of Photovoltaics

    The photovoltaic phenomenon is the process by which light is converted silently and directly into electricity without elaborate machinery which is usually associated with conventional generation of electricity.

    A photovoltaic (solar) cell is typically made by placing a thin layer of phosphorus-doped silicon in intimate contact with a layer of boron-doped silicon. When light falls on the cell, photons are absorbed and electrons are set free. The excess electrons accumulate in the phosphorus-doped silicon, which is called n-silicon because electrons have negative charge. If one end of a wire is attached to this top layer and the other end is connected to the layer beneath, electrons will leave the upper layer, flow through the wire, and be absorbed by the boron-doped silicon, which is called p-silicon, meaning positive. Figure_1 demonstrates the process by which electricity in a photovoltaic cell is generated.

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    System Constituting Components

    The basic components that any PV system consists of are photovoltaic (solar) array, storage batteries and charge controller or alternatively called charge regulator. If the load operates on AC power, an inverter will, also, be required.

    Now, let’s define the function of each component starting with the building block of a PV module, the PV cell. It is the smallest basic unit that can produce electricity from the sunlight. The PV module is considered the smallest assembly that physically stands alone. The PV module, in turn, consists of multiple PV cells that are electrically interconnected and mounted together, usually in a sealed module convenient for shipping, handling, and assembling into arrays. The solar or photovoltaic array is an interconnected system of photovoltaic modules connected in series and/or parallel combinations to satisfy the load requirement.

    The storage battery is an essential part in the photovoltaic power system. It consists of multiple cells. The function of storage battery is to store the excess electrical power from a PV array in a chemical form by the charging process and deliver it when needed in an electrical form by the discharging process. The various battery types and selection will be discussed later.

    The charge controller or regulator is a device that regulates the electricity from the photovoltaic array to the storage battery. Based on pre-selected voltage set-points, the charge controller connects and disconnects the PV array to assure that charging and discharging processes do not harm the battery.

    If the load operates on AC power, an inverter that converts the DC power from the storage batteries and the PV array into AC power is required. If the load operates on DC, the load is directly fed from the batteries.

    Overall System Description

    When sufficient sunlight falls on the photovoltaic array, it yields DC electrical power. The amount of output power produced is affected by many contributing factors that will be discussed in details When sufficient sunlight falls on the photovoltaic array, it yields DC electrical power. The amount of output power produced is affected by many contributing factors that will be discussed in details later. Because of the intermittent nature of the sunlight, storage battery is required to store the excess power produced during peak sunlight intensity periods (e.g. midday) for the load usage later during the low sunlight intensity periods (e.g. night times) bearing in mind that most loads are of continuous duty. Because the average power from a photovoltaic array fluctuates during the day, some means of regulation to control the charges received by the battery is required to protect battery against overcharge and undercharge conditions. This is usually achieved by a charge controller or often called “charge regulator” which connects and disconnects the power from the photovoltaic array to the battery based upon specific battery voltage settings. The PV array is sized to provide the required power to support the load and to recharge the battery in a reasonable time (say for example 4 days) after a complete battery discharge. The battery is sized to supply the load during consecutive days of autonomy or overcast (days of very low sunlight intensity) in a reasonable time (say for example 7 days).


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    بسم الله الرحمن الرحيم
    تحيه طيبه و بعد .....
    كالعاده سباق ...
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    Design Strategy

    PHOTOVOLTAIC ARRAY: Let’s shed some light on the PV array design: the array type selection, the array sizing, and the climate and geographical location considerations.

    There are many types of PV modules from a manufacturing materials point of view. However, it must be emphasized that the following are the two most commercially-available technologies:

    • Crystalline silicon (c-Si) PV modules
    • Amorphous silicon (a-Si) PV modules (often called thin film)

    The amorphous silicon PV modules are primarily manufactured in two types :

    • Single junction PV modules
    • Multi-junction PV modules

    The basic criteria for selection are the module efficiency that compares how much of the sun energy is converted into electricity and the cost of each PV (solar) module.

    The crystalline silicon has thus far been one of the most efficient PV cells. However, because of the exceptionally high costs resulting from slicing the crystals into thin wafers, from which solar cells are made, it turns out to be an expensive and material-wasteful technology. Although today’s multi-junction amorphous PV modules are not as efficient as crystalline silicon PV modules, they are competitive in the PV market because amorphous thin films can be inexpensively manufactured on low-cost glass or stainless steel substrates. Therefore, in present-day PV market, the multi-junction amorphous silicon is the most economically-feasible type.

    Beside the conventional flat-plate photovoltaic cells where the solar radiation strikes a flat surface, there is also another technology called “concentrator solar cells.” Concentrators use lenses or reflectors to focus sunlight into a smaller-area solar cell with a magnification ratios of 10x to 1000x which result in extensively higher conversion efficiency per unit area. The higher efficiency is necessary to offset the inherently higher costs of the concentrator balance of system.

    Details calculation on how to size a PV array will not be discussed in this paper. However, when designing a photovoltaic system, some basic de-rating factors are often taken into consideration. Dependent on the application, as a common practice, the PV array has to be de-rated by around 20% for aging, another 20% for dirt accumulation, and another 10% or so for future expansion.

    The period needed for the PV array to recharge the battery (battery recharge time) is a major contributing factor in PV array sizing. This is totally application-dependant and is usually dictated by the load requirement and its criticality. Four days of battery recharge time has been a traditionally acceptable figure for many applications.

    The system voltage will determine the number of PV modules connected in series and the required load current will determine the number of PV modules connected in parallel.

    The climate and geographical location play a major role in the design of the PV array. The solar insolation, which is the amount of sunlight radiation in kWh falling on each m2 every day and the effective sun hours in which the PV array can produce useful electrical power, are crucial factors to consider when designing a PV array. To optimize the amount of solar insolation striking the solar array, its orientation is also very essential; it should be directed toward the south in the northern hemisphere, toward the north in the southern hemisphere and should be oriented flat at the equator. The tilt (inclination) angle can either be fixed or adjustable. If it is fixed then as a rule of thumb, the latitude of the location plus 10-20 degrees is the optimum tilt angle. To maximize the available solar insolation and increase the PV array output throughout the year, more advanced designs use sun tracking system which involves motors and a complicated control to keep the PV array facing the sun. This, however, consumes additional power and increases the complexity and the maintenance costs of the system. More practically, manual periodical adjustment every three month is recommended for fixed angles array configuration to optimize the amount of sun insolation falling on the PV array each season.

    For this purpose, sun insolation contour maps are usually prepared based on metrological data collection and sun intensity measurement averaged over many years and expressed in kWh/m2/day. According to the sun insolation maps, most of Saudi Arabian regions receive an average solar radiation of around 5.0 kWh/m2/day.

    STORAGE BATTERIES: The storage batteries have to be especially designed for photovoltaic applications. In this paper, battery type selection, battery sizing and the environment conditions considerations on the design of these batteries will be discussed. Detail design of battery is out of the scope of this paper. IEEE Standards 1013 and 1144 describe in details the methods of sizing batteries for PV applications.


    PV array produces electricity only during the sunny hours of the day. It produces no electricity at night and produces very little electricity during hazy days. Because of this cyclic nature of the PV, most importantly, the battery type has to be suitable for deep discharge applications. Tubular plate Lead-Acid or pocket plate Nickel-Cadmium battery types have demonstrated superior performance for cyclic applications.

    For battery sizing, the design system voltage determines the number of cells needed and the load current profile determines the Ampere-hour (Ah) capacity of the battery. The backup time, which is the estimated time that the battery is capable of supporting the load independent of the PV array during hazy days (often called autonomy), is a crucial factor in battery sizing.

    Also, some design de-rating factors have traditionally been the common practice to be considered in designing both Lead-Acid and Nickel-Cadmium batteries such as aging, temperature compensation, and design margin factor as follows:


    De-rating Factor Lead-Acid Ni-Cad

    Aging Factor 25% 10%
    Temp. Compen. 19% 14%
    Design Factor 10% 10%

    For PV applications, to prevent battery from severe discharge, battery is normally designed for an end of discharge voltage of 1.85V per cell for Lead-Acid and 1.15V per cell for Nickel-Cadmium batteries.

    Generally, the environment is not controlled in PV installations. If temperature is not controlled, then a temperature compensation feature is required as will be discussed later. Because of the uncontrolled environment conditions, a battery enclosure suitable for outdoor application is required. Also, the battery shall be located in a shaded location to reduce the effect of direct sun light on the life and performance of the battery.

    CHARGE CONTROLLER: Charge controller or alternatively known as charge regulator is a device that regulates the charges from the PV array to the battery. They are made in several types. Single stage and multistage charge controller are common types of charge controllers. Multistage charge controller by which the PV array is split into multiple parallel sections called stages has demonstrated better charge regulation as well as being benign to the battery. The charge regulator is normally sized based on the maximum amount of current that the solar array can provide to charge the battery and support the load. This figure is often multiplied by a design margin of around 10%. To maintain the required charging current to the battery, the charge controller (regulator) shall be equipped with a temperature compensation probe that allows the regulator to raise the charging voltage at lower temperatures and reduce the charging voltage at higher temperatures while maintaining a constant charging current to the battery. This probe normally compensates in a rates of -5mV/oC/cell for Lead-Acid and -3mV/oC/cell for Nickel-Cadmium batteries. Moreover, more advanced controller units are typically equipped with a low voltage disconnect (LVD) to disconnect the load when the battery end of discharge voltage is reached. As mentioned earlier, for photovoltaic application, it is recommended to set the low voltage disconnect to 1.85 V/cell for Lead-Acid and 1.15 V/cell for Nickel-Cadmium batteries. As explained earlier also, the low voltage disconnect feature will protect the battery from damage caused by severe discharge.

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    Applications

    As a matter of fact, PV systems are not cost-effective for all applications. If connected to a utility grid or within a close proximity, PV is not the right choice. Feasibility can only be determined by an economical study. Many applications have provided evidence that PV is the right choice.

    PV power systems are selected for a variety of reasons such as cost, reliability, modularity, expandability and friendliness to the environment. When the economic studies demonstrate high cost of producing electricity by conventional techniques the PV power system is often the most cost-effective source of electricity. One example is when the maintenance costs are excessive and unjustifiable as in the case of remote locations like remote offshore platforms. Mainly, if the location is remote from the utility grid or is inaccessible, PV systems are usually the choice. Over the years, PV power systems have exhibited more viability with the lower scale (few kVA’s) projects. If the load power requirement is exceedingly high (in the MVA scales), PV is usually not economically justifiable. Lower-scale power consumption applications such as cathodic protection, telecommunications, SCADA systems, non-electrified offshore platforms, remote homes and irrigation water pumping, are typical applications of PV power systems.


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    Conclusions

    ADVANTAGES: PV has some advantages and disadvantages. If designed correctly, the PV system is very reliable because PV system contains no moving parts. The fact that the traditional PV system involves no moving part has granted it the advantage of low maintenance over the conventional diesel generator power system. Furthermore, PV system consumes no fuel. Besides, because of the long expected life of a PV system (2—25 years) with minimal maintenance, long-term savings are anticipated. Another advantage of the PV systems is that it is very friendly to the environment since it produces no pollution. What is more? PV has an advantage that nothing else can match. It is quiet. This is especially important for people who like to stay away from the noise of the city.

    DISADVANTAGES: PV system, however, has some drawbacks. Although the sun’s energy is free, the PV equipment is not free. High initial investment is likely to be involved. It also requires specialized technicians to maintain both the batteries and the power conditioning equipment such as charge controllers and inverters. Besides, PV array takes up a lot of space. This becomes a serious shortcoming when space limitation is an issue such as the case with the offshore platforms installations.

    In conclusion, despite the disadvantages of the photovoltaic power, there are many applications where the PV is the only choice that is economically justifiable.


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    Mr.Solar Cell
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    السلام عليكم ورحمة الله وبركاته

    جزاك الله خير على هذا الموضوع الممتاز ،

    واحب ان اضيف بعض التطبيقات للخلايا الشمسيه ،ولكن في وقت لاحق ان شاء الله ..

    وتقبل تحياتي

    Mr.Solar Cell

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    السلام عليكم.....

    اهلا وسهلا بك اخي الكريم ANUBIS...

    اشكرك على اطراءك.. متمنياً ان اكون قد افدت...

    تقبل تحياتي وتقديري و اشكر لك تشريفك للموضوع :) :) :)

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    السلام عليكم.....

    اهلا وسهلا بك اخي الكريم.. Mr.Solar Cell..

    وجزاك الله الف خير على دعوتك..

    و في انتظار ما ستتحفنا به :)

    تقبل تحياتي وتقديري ز اشكرك على تشريفك الموضوع :) :) :)


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  10. [10]
    Mr.Solar Cell
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    هلا بك اخي ابو اشواق

    وأحب ان اضيف اضافه بسيطه عن استخدام الطاقة الشمسيه في مضخات المياه.


    مضخات مياه بالطاقة الشمسية

    قديماً كانت تُستخدم طلمبات تعمل بحرق الوقود الاحفوري في موتور يشبه موتور السيارة حيث يحتوي على عدد من السلندرات وهو في الغالب ثنائي الأشواط (2-stroke ) يتحرك داخل كل سلندر مكبس (piston) يندفع هذا المكبس نتيجة تمدد الغازات الناتجة عن حرق الوقود، ونتيجة هذا الاندفاع يحرك المكبس عمود الإدارة (crank) الذي يعمل على إدارة مضخات رفع المياه.
    ولكن كما نعلم أن حرق الوقود ينتج عنه ملوثات كثيرة للبيئة، وكذلك فالوقود غالي الثمن، ومن المنتظر أنه سوف يرتفع أكثر في السنوات المقبلة، وأيضاً كفاءة هذه الماكينة تظل منخفضة لوجود أجزاء متحركة بها تستهلك جزءا كبيرا من طاقتها للتغلب على قوى الاحتكاك (friction) .

    وتعتبر مضخات المياه بالطاقه الشمسيه هي الأفضل لدول العالم الثالث؛ نظراً لسطوع الشمس الدائم على معظم أراضيها، وكذلك فإنها رخيصة السعر ولا تحتاج إلى شراء ونقل ووقود باستمرار.
    وتتكون المضخات الشمسية (solar water pump) من (خلايا شمسية-موتور كهربي-مضخة).
    الخلايا الشمسية هي مصدر الطاقة في هذا الجهاز وهذه الخلايا تستطيع إنتاج فرْق جهد ما بين 12-30 فولت من التيار المستمر(DC) ، وبعد ذلك تستخدم الطاقة الكهربية في إدارة موتور يعمل على تشغيل مضخات رفع المياه والري.
    ويصل العمق الذي يمكن أن يستخدم فيه هذا النوع من الطلمبات إلى 60 مترا تحت سطح الأرض.
    وهنا نطرح سؤالاً هامًا: ماذا يحدث لو غابت الشمس في يوم أو جزء من اليوم؟! كانت إجابة هذا السؤال في أذهان العلماء فأضافوا في هذه الطلمبات نظام تحكم (control system) يحتوي على معالج صغير (microprocessor) يقوم هذا المعالج بمراقبة الطاقة بين الخلايا الشمسية والمضخة، ويستطيع تعديل قيمة الجهد الكهربي في حالة غياب الشمس أوتوماتيكياً للحصول على تشغيل مستمر.
    فمثلاً في حالة الإضاءة الضعيفة ( low light condition) يقوم بزيادة الطاقة الخارجة (output) للحفاظ على استمرار الضخ. وكذلك عن طريق تخزين المياه في خزانات .
    ومن أهم مميزات هذه التكنولوجيا أنها:
    تُستخدم في العديد من المجالات مثل ري الأراضي، وإيجاد المياه لرعاية الماشية، وكذلك توفير الماء للوديان والمنازل البعيدة عن العمران، وتستخدم طاقة نظيفة لا تلوث البيئة.
    والجدير بالذكر أن هذه التكنولوجيا قد استخدمت بالفعل في دولة السنغال في غرب أفريقيا،
    واستخدمت ايضا في المملكة العربية السعودية في سد سدوس بمنطقة الرياض وقد أثبتت نجاحاً كبيراً .


    تحياتي للجميع،،،،،،،،

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