Solar FAQ (pt1)

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[What is meant by solar energy?] [What types of solar energy are there?]
[How does passive solar work?] [How does thermal solar work?] [How does PV solar work?] [How do the utility interactive PV systems work?]
[How do the stand alone PV systems work?] [Why is a charge controller needed?]
[ How does a charge controller prevent overcharging?] [What is multi stage charging?] [What is float charge?] [What is bulk charge?] [What is PWM?]
[How is PWM different from a shunt regulator?]
[What other functions can a charge controller provide?]
[What are self regulating solar panels?]
[What types of inverters are there?] [What can be run on inverters?]

What is meant by solar energy?
Energy that originates from the sun, is radiated to the earth, and is collected and used.

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What types of solar energy are there?
Technically, most forms of energy on earth were originally derived from the sun. Even the fossil fuels are the products of photosynthesis from when ancient plants used the suns energy to grow, and then became fossilized. There are many uses of solar energy, from small portable solar ovens for cooking to large central utility power stations with thousands of mirrors that focus the suns heat on a huge, white hot receiver powering a multi-megawatt generator. There are three common forms of solar energy. There is passive solar, which intelligently uses things like insulation, window placement, thermal mass and seasonal changes to reduce the costs of heating, cooling and lighting buildings. There is thermal solar, which collects heat radiated from the sun and transports it to useful locations such as swimming pools, hot water heaters, room heaters or for industrial process heat. There is photovoltaic (photo - light, voltaic - electric) solar, which converts sunlight directly into electricity. This electricity can then be used to charge DC storage batteries, in what are called stand alone PV systems, or it can be inverted to AC and used by a house or fed directly into the utility grid. These are called utility interactive PV systems.

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How does passive solar work?
Architects can use many design elements to control ambient light and limit heat gain and loss from a structure, reducing the costs of maintaining a comfortable temperature and lighting level in homes and buildings. These elements are integrated into the design of the building and do not actively transport heat, hence the term passive solar. In the summer, heat gain can be reduced by proper placement of large deciduous trees and using overhanging eaves to shade windows form the high sun. In the winter, solar heat that shines in through windows can be stored in specially designed floors and walls and released through out the night. Sky lights provide natural light indoors during the day to reduce the cost of lighting.

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How does thermal solar work?
Thermal solar systems collect and store heat from the sun. Black thermal collectors are often seen on the roofs in residential neighborhoods, where they provide domestic hot water or help to heat a swimming pool. In these collectors, water circulates through a network of channels, getting warm as it goes. The warm water is then returned to the hot water tank or to the swimming pool. The more times the water passes through the collector, the hotter it gets. A controller monitors the temperature of the water and stops circulation when it is hot enough, or if there is no thermal energy available at the collectors.

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How does PV solar work?
The solar panels are made up of many interconnect solar cells, which are solid state devices similar to a large transistor. These cells are made so that when a photon of light strikes a molecule, an electron is knocked free. The cell has an electrical field that causes the electron to migrate to one side of the cell, and into the interconnection network. The accumulated effect of millions of these interactions is to generate electricity. These panels can then be connected together to provide the desired level of power.

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How do the utility interactive PV systems work?
The solar panels generate DC electricity directly from the sun. Then a piece of equipment called an inverter changes the DC into AC, the kind of power that is in the utility grid and flows out of the wall sockets in a normal home. The solar panels supply the home with the power it requires, and any left over power is fed into the grid, running the meter backwards. When the house requires more power than available from the solar system, it draws from the grid, running the meter forward. So the utility grid is like a large storage system.

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How do the stand alone PV systems work?
A basic stand alone PV system consists of the solar panels, batteries and a charge controller. The photovoltaic solar panels generate DC (direct current) electricity, which is used to charge the storage batteries. A charge controller is needed to keep the batteries from being over charged. The batteries store the energy, and are similar to those used to start cars, but the batteries used in solar systems are designed to store and discharge energy over a longer period and at slower rates than the short, big surge needed to start a car. These storage batteries gradually store the power generated during the day for use at night or during cloudy days. This stored energy can then operate DC appliances such as radios and the lights found in recreational vehicles. An inverter can be added to change the DC into AC so standard appliances such as hair dryers and microwave ovens can be used.

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Why is a charge controller needed?
PV systems are generally designed so that there is enough energy in the winter, but a surplus in the summer. This surplus can lead to overcharging and damage to the batteries, shortening their life and increasing maintenance. The charge controller prevents overcharging and minimizes damage to the batteries.

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How does a charge controller prevent overcharging?
The charge controller monitors the state of charge of the battery (how full it is), and regulates or stops the charging when overcharging begins to occur. The most common way for controllers to determine battery state of charge is by reading battery voltage. The higher the voltage, the higher the state of charge. There are many ways to stop or regulate the charging. A simple method is to just disconnect the solar panel from the battery, with a switch in line between the panel and the battery. This is called a series controller. Another way is to connect solar panel plus to solar panel minus, which just loops the current back to the solar panel instead of to the battery. This is called a shunt controller. A third way is to regulate the output from the solar panel. This means to gradually reduce the amount of current allowed to go to the battery.

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What is multi stage charging?
Multi stage charging is when the battery is charged up to different voltages. For example, the battery can be charged up to 14.8 volts, then the controller drops the voltage down to a float or maintenance charge of 14.1. The higher voltage allows the battery to charge up faster and achieve a higher state of charge than if the charge is terminated at a lower voltage. This higher voltage should can be maintained or overcharging would occur, so the controller must drop the voltage down.

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What is float charge?
Float charge is when the controller holds the battery at a lower charge voltage and trickles a small amount of current into the battery, to just maintain it at full charge.

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What is bulk charge?
Bulk charge is a high amperage charge up to a high voltage.

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What is PWM?
PWM stands for Pulse Width Modulation. This is a constant voltage method of regulating the charge current to a battery. The controller will maintain the battery voltage at one point and gradually decrease the width of the current pulse to reduce the net current. This essentially holds the battery at a float voltage and reduces the current.

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How is PWM different from a shunt regulator?
The shunt controller will typically have two set points, a higher voltage that the battery charges up to where the controller turns off charging and a reconnect voltage where the charging starts up again. An example would be 14.3 for a high set point and 13.5 for the reconnect. This charge scheme allows the battery to charge up to a higher voltage than what can be maintained in a float or constant voltage scheme because it shuts down and lets the battery recover a little before it starts again. The benefit of this method, often called a single step charge mode, is that it pushes the battery voltage up higher, reaching a higher capacity sooner but without holding the battery there where it could overcharge.

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What other functions can a charge controller provide?
The PV charge controller can provide deep discharge protection for the battery by automatically disconnecting loads before the battery is completely discharged. The controller can also include monitoring so that controller status and system parameters such as battery voltage and charge current can be monitored. Controllers can incorporate over current protection in the form of circuit breakers or fuses, and provide a central location for system wiring connections.

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What are self regulating solar panels?
There are solar panels that are called self regulating panels. They do not have a little controller built in, they are just have fewer solar cells so that the current output drops off when the voltage reaches close to the full charge voltage of a battery. This is a nice thing, but the problem with these is that the current does not shut off all the way, so over charging can occur, and in some case the current starts to shut down too soon before the battery is charged.

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What types of inverters are there?
There are stand alone inverters and grid interactive inverters. Among the stand alone inverters, there are square wave inverters, modified square wave inverters and pure sine wave inverters.

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What can be run on inverters?
Inverters ran typical AC appliances like blenders, hair dryers, microwave ovens and computers. Heavy duty inverters can run motors like in washing machines.

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