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How PV Cells Work ?

A typical silicon PV cell is composed of a thin wafer consisting of an ultra-thin layer of phosphorus-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, called the P-N junction. When sunlight strikes the surface of a PV cell, this electrical field provides momentum and direction to light-stimulated electrons, resulting in a flow of current when the solar cell is connected to an electrical load

 

 Diagram of photovoltaic cell.

Regardless of size, a typical silicon PV cell produces about 0.5 – 0.6 volt DC under open-circuit, no-load conditions. The current (and power) output of a PV cell depends on its efficiency and size (surface area), and is proportional the intensity of sunlight striking the surface of the cell. For example, under peak sunlight conditions a typical commercial PV cell with a surface area of 160 cm^2 (~25 in^2) will produce about 2 watts peak power. If the sunlight intensity were 40 percent of peak, this cell would produce about 0.8 watts.

PV Cells, Modules, & Arrays

Photovoltaic cells are connected electrically in series and/or parallel circuits to produce higher voltages, currents and power levels. Photovoltaic modules consist of PV cell circuits sealed in an environmentally protective laminate, and are the fundamental building block of PV systems. Photovoltaic panels include one or more PV modules assembled as a pre-wired, field-installable unit. A photovoltaic array is the complete power-generating unit, consisting of any number of PV modules and panels. The performance of PV modules and arrays are generally rated according to their maximum DC power output (watts) under Standard Test Conditions (STC). Standard Test Conditions are defined by a module (cell) operating temperature of 25o C (77 F), and incident solar irradiance level of 1000 W/m2 and under Air Mass 1.5 spectral distribution. Since these conditions are not always typical of how PV modules and arrays operate in the field, actual performance is usually 85 to 90 percent of the STC rating. Today’s photovoltaic modules are extremely safe and reliable products, with minimal failure rates and projected service lifetimes of 20 to 30 years. Most major manufacturers offer warranties of twenty or more years for maintaining a high percentage of initial rated power output. When selecting PV modules, look for the product listing (UL), qualification testing and warranty information in the module manufacturer’s specifications.

How a PV System Works?

Simply put, PV systems are like any other electrical power generating systems, just the equipment used is different than that used for conventional electromechanical generating systems. However, the principles of operation and interfacing with other electrical systems remain the same, and are guided by a well-established body of electrical codes and standards. Although a PV array produces power when exposed to sunlight, a number of other components are required to properly conduct, control, convert, distribute, and store the energy produced by the array. Depending on the functional and operational requirements of the system, the specific components required, and may include major components such as a DC-AC power inverter, battery bank,system and battery controller, auxiliary energy sources andsometimes the specified electrical load (appliances). In addition, an assortment of balance of system (BOS) hardware, including wiring, over current, surge protection and disconnect devices, and other power processing equipment. The Figure above show a basic diagram of a photovoltaic system and the relationship of individual components. Why Are Batteries Used in Some PV Systems? Batteries are often used in PV systems for the purpose of storing energy produced by the PV array during the day, and to supply it to electrical loads as needed (during the night and periods of cloudy weather). Other reasons batteries are used in PV systems are to operate the PV array near its maximum power point, to power electrical loads at stable voltages, and to supply surge currents to electrical loads and inverters. In most cases, a battery charge controller is used in these systems to protect the battery from overcharge and over discharge How Are Photovoltaic Systems Classified? Photovoltaic power systems are generally classified according to their functional and operational requirements, their component configurations, and how the equipment is connected to other power sources and electrical loads. The two principle classifications are grid-connected or utility-interactive systems and stand-alone systems. Photovoltaic systems can be designed to provide DC and/or AC power service, can operate interconnected with or independent of the utility grid, and can be connected with other energy sources and energy storage systems.1.7.1 Grid-Connected (Utility-Interactive) PV Systems. Grid-connected or utility-interactive PV systems are designed to operate in parallel with and interconnected with the electric utility grid. The primary component in grid-connected PV systems is the inverter, or power conditioning unit (PCU). The PCU converts the DC power produced by the PV array into AC power consistent with the voltage and power quality requirements of the utility grid, and automatically stops supplying power to the grid when the utility grid is not energized. A bi-directional interface is made between the PV system AC output circuits and the electric utility network, typically at an on-site distribution panel or service entrance. This allows the AC power produced by the PV system to either supply on-site electrical loads, or to back feed the grid when the PV system output is greater than the on-site load demand. At night and during other periods when the electrical loads are greater than the PV system output, the balance of power required by the loads is received from the electric utility This safety feature is required in all grid-connected PV systems, and ensures that the PV system will not continue to operate and feed back onto the utility grid when the grid is down for service or repair. Diagram of grid-connected photovoltaic system Stand-Alone Photovoltaic Systems Stand-alone PV systems are designed to operate independent of the electric utility grid, and are generally designed and sized to supply certain DC and/or AC electrical loads. These types of systems may be powered by a PV array only, or may use wind, an engine-generator or utility power as an auxiliary power source in what is called a PV-hybrid system. The simplest type of stand-alone PV system is a direct-coupled system, where the DC output of a PV module or array is directly connected to a DC load (Figure 5). Since there is no electrical energy storage (batteries) in direct-coupled systems, the load only operates during sunlight hours, making these designs suitable for common applications such as ventilation fans, water pumps, and small circulation pumps for solar thermal water heating systems. Matching the impedance of the electrical load to the maximum power output of the PV array is a critical part of designing well-performing direct-coupled system. For certain loads such as positive-displacement water pumps, a type of electronic DC-DC converter, called a maximum power point tracker (MPPT) is used between the array and load to help better utilize the available array maximum power output.  Direct-coupled PV system. In many stand-alone PV systems, batteries are used for energy storage. Figure 6 shows a diagram of a typical stand-alone PV system powering DC and AC loads. Figure 7 shows how a typical PV hybrid system might be configured.  Diagram of stand-alone PV system with battery storage powering DC and AC loads.  Diagram of photovoltaic hybrid system. How PV Cells Are Made? The process of fabricating conventional single- and polycrystalline silicon PV cells begins very pure semiconductor-grade polysilicon - a material processed from quartz and used extensively throughout the electronics industry.  The polysilicon is then heated to melting temperature, and trace amounts of boron are added to the melt to create a P-type semiconductor material. Next, an ingot, or block of silicon is formed, commonly using one of two methods:  1) by growing a pure crystalline silicon ingot from a seed crystal drawn from the molten polysilicon or  2) by casting the molten polysilicon in a block, creating a polycrystalline silicon material.  Individual wafers are then sliced from the ingots using wire saws and then subjected to a surface etching process. After the wafers are cleaned, they are placed in a phosphorus diffusion furnace, creating a thin N-type semiconductor layer around the entire outer surface of the cell.  Next, an anti-reflective coating is applied to the top surface of the cell, and electrical contacts are imprinted on the top (negative) surface of the cell.  An aluminized conductive material is deposited on the back (positive) surface of each cell, restoring the P-type properties of the back surface by displacing the diffused phosphorus layer.  Each cell is then electrically tested, sorted based on current output, and electrically connected to other cells to form cell circuits for assembly in PV modules.

PV Technology

Photovoltaics (PV) or solar cells as they are often referred to, are semiconductor devices that convert sunlight into direct current (DC) electricity.   Groups of PV cells are electrically configured into modules and arrays, which can be used to charge batteries, operate motors, and to power any number of electrical loads.   With the appropriate power conversion equipment, PV systems can produce alternating current (AC) compatible with any conventional appliances, and operate in parallel with and interconnected to the utility grid.

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Residential

What is solar energy?

Many, if not most people, when they hear the term 'solar energy' or 'solar power', will think of 'PV' or Photo-Voltaic panels – those shiny things on house roofs that make electricity. But PV is just one piece of the Solar Energy 'pie'. The other slice is solar 'thermal' energy – this is where we use the sun's abundant (and free!) radiated heat to warm things up, then use that heat elsewhere – for example in a solar hot water system. Of course, indirectly the sun provides us with wind energy too – if you're interested in this, please see www.SolarEnergyWind.com for more information.

The solar energy world is a complex one, and this website will help you navigate some of the most interesting areas, some of the history, current products you can get right now, and where future research is taking us as a global community.

What solar options should I consider next?

Well, if you want to get into the whole history of solar energy, for many of you, you just want to know what are the basics of solar systems, is your home is a good property for solar power, and how much it all costs. Read on for a brief education in solar technology!

From solar powered hot water heaters, to solar assisted Air Conditioning systems, to solar attic fans and full photo-voltaic solar electric systems, it is covered here. In addition, if you decide you want to talk with a local expert and get a no-obligation price, ask your Installer  – they can tell you about your own local utility & government incentives, plus much more.

Solar Electricity

A bunch of shiny panels on the roof of a house or commercial building - this is what most people think of when they hear the term solar-energy, solar-power, or solar electricity. These panels are actually called photo-voltaic panels (often referred to a 'PV'), and are made up of a bunch of photo-voltaic 'cells'. Each cell may be a few inches by a few inches square, and is actually a very thin sliver of a special crystal which is capable of converting sunlight into electricity.

In a solar photo-voltaic panel the sun's radiation is converted by a chemical reaction in the individual cells into direct-current electricity (or DC current – just like the charge from a regular household battery). The dozens of cells in a panel are connected by wires to a couple of cables that run off the back of the panel to a common channel on the roof. Here the cables from all the other panels join up and are connected into a product called an 'Inverter' – this is a gizmo that converts the DC current generated by the panels into AC (alternating-current) that can be used in your house.

Now the generated electricity can be primarily used in one of two ways, either grid-tied or off-grid.

Solar Grid-Tied Systems or 'Net Metering'

This is the most common application. Your utility company will typically swap your electric meter for a different one, often digital today, which has the ability to 'spin' backwards. When your PV system is generating more electricity than you need (for example during the day you are at work and the sun is beating down on your house which needs very little electricity), you will get paid by the utility company. Then when you come home in the evening & turn on lights, TV etc. and the sun isn’t shining on the panels, you pay the utility company. The idea is that you end up with a 'net' amount owed to the utility – or sometimes they may owe you!

Off-Grid Solar Systems or Battery-Backed Solar Systems

If you want to get rid of your electricity provider completely, you can go 'off-grid'. This means you'll get a system designed that's big enough to run your entire house's electrical needs, and then have that system backed by a bunch of large batteries. You can also take part of your house off the grid – if you live in an area where there are frequent storms and power-outages, you may want to have a system that powers some lights, ceiling fans & the fridge – It's all possible with a battery-backed off-grid system.

Sizing a Solar Electric System

Let's say your average electric bill is $300 per month, and you want to get rid of it completely, but a solar electric system big enough to do this would take double the roof space that you have. In this example you can put in half the power – basically getting rid of $150 per month.

Is my home solar-appropriate?

This question is typically best answered by your local solar specialist who will take a detailed look at your roof pitch and direction, shade from trees, shade from near-by buildings etc. Essentially you need to consider how many panels will fit onto your roof and face south, south east or south west. There are ways to mount solar panels on tilted racks so even north-facing roofs may be viable for solar power.

How much will a solar electric system cost me?

Typically systems are priced on a per-watt basis. These figures can vary widely based on your location, the local, State & Federal incentives available, utility company rebates, whether you're paying cash or financing, and so on. Broadly the starting price from an installer may be in the Rs.50 - 130 per watt,(it varies)  and you'll probably be looking at a 5 to 10 kilowatt system for a typical house – so if you want a system to get rid of your electric bill completely, think in terms of tens of thousands of Rupees initially – though this figure can be reduced dramatically with rebates and incentives if applicable.

Is solar power a good investment - where's the planet heading with energy use?

Without a doubt, fossil fuels have a limited quantity, whether we're talking a few decades or a few hundred years, the inevitable dead-end of burning oil, coal & gas is there. Only eons of time can make more of that stuff, so we as a species have no alternative but to find an alternative! Since the sun will likely be around a lot longer than we can even begin to imagine, then converting its energy into forms we can use seems like the most logical long-term plan.