This primer will provide the reader with a short, to-the-point source of information concerning photovoltaic (P.V.) solar energy. If you are considering the installation of a P.V. system, or would just like to understand the basics of P.V., reading this publication is a must. For more detailed information, consult the Glossary of Terms at the end of this paper.

A photovoltaic solar panel will convert sunlight directly into electricity There are no moving parts, exhausts, noise, or pollutants involved in the process. It is beyond the scope of this paper to explain the process in detail, but suffice it to say that light striking the front of a solar cell will produce a voltage and current.

A typical solar cell may be a 4 inch diameter wafer, about .01 inches thick. A group of these cells interconnected creates a solar panel. A typical solar panel can produce about 16 volts and 3 amps , or 52 watts, of DC power. Other module specifications are available, and such specifications will vary between manufacturers. Solar panels, in turn, can be interconnected in series or parallel to create a solar-array and any voltage-current combination required.

That's it! That's all you need to know about photovoltaic solar panels to contemplate a photovoltaic system.

But now it is necessary to control and utilize the resulting power obtained from the modules.

Since P.V. panels produce power only when the sun is out, something must be done to provide power when the weather's bad or at night; batteries foot the bill. The panels basically power loads during the day, and the batteries provide load power when the panels cannot. The solar panels must also, of course, recharge the batteries so they(the batteries) can be utilized again.

Generally, lead-acid batteries are used in P.V. systems. While it is perfectly feasible to use a standard car battery in this application, it is wise to consult a knowledgeable source as to which battery is the best to use. There are many batteries, which you normally cannot obtain at a regular store, that are the proper choice for a P.V. system. Factors to consider when choosing a battery are:

1. Initial cost

2. Lifetime and replacement cost

3. Size, both physical and power capacity

4. Type of plates (antimony, pure lead, calcium, nickel-cadmium)

5. Type of application, either deep cycle or shallow cycle

6. Type of electrolyte, either liquid or gelled

If you want to, you can throw your P.V. modules on the ground or just lay them on your roof. However, such a practice isn't recommended for obvious reasons, especially after considering how much money you may spend. Module mounts perform several functions:

1. Provide a sturdy rigid platform for the mounting of PV modules.

2. Anchor the modules so they won't blow away.

3. Position the modules so they will face the sun at the correct angle.

4. Allow for changes in module angle to match seasonal changes in the sun's location.

Generally , a P.V. system is sized so that more energy is produced than is consumed. If left to itself, this excess energy would overcharge the batteries, shortening their life. Charge controllers monitor battery condition and decide what to do with the power available from the P.V. panels. The charge controller function is equivalent to the system's owner watching the battery voltage all day and deciding when to disconnect and reconnect the P.V. panels to the batteries. This relatively boring job can be done automatically and indefinitely by a charge controller.

A charge controller can also prevent too much power from being removed from the batteries by automatically disconnecting loads or issuing visual or audible low-voltage warnings. Without such precautions, batteries can be damaged by deep discharge.

It is important to have some type of feedback to indicate what is happening within the P.V. system. The easiest method for accomplishing this is with meters. Basic things to monitor would be array current and battery voltage. These two measurements would indicate what the array is producing and the condition of the battery, I.E., state of charge.

The end result of all of the previous work is to power loads. Loads can be items such as:

1. Lights, indoor and outdoor

2. Appliances, including stereos, and TVs

3. Water pumps

4. Telecommunication equipment

5. Data monitoring/gathering equipment

Since the P.V. system will be producing DC, only DC-type loads will operate directly from the batteries. If it is necessary to run AC loads, a DC to AC inverter will make the proper conversion. Several things to consider when choosing an inverter are:

1. Input voltage. This must match the system's DC voltage.

2. Output voltage. Generally, 120 VAC is adequate, but 240 VAC is available.

3. Output frequency. Most AC appliances in the U.S. are 60 HZ, but 50 HZ is used extensively in other countries.

4. Output power. The inverter must not only run the anticipated AC loads, it must also have the capability to provide surge power. Surge power is required to start motors, especially motors with high start-torque requirements, such as refrigerators and water pumps.

5. Output waveform. Inverters supply either square-waves or sine-waves for their output. Some AC loads may not operate properly with square-waves.

When utilizing a P.V. system, it is important to realize that one is dealing with a limited amount of energy. It is necessary to create an energy budget, that is, one must insure that over a certain period of time, at least as much energy is produced by the solar panels as is consumed by loads. Such factors as yearly average weather conditions, latitude, and load requirements enter into sizing requirements. It is a good idea to contact an expert to perform sizing calculations when installing a P.V. system.

That's about it for the basics. You now know what a photovoltaic solar system is and what it does. If you are contemplating the installation of a system, it is suggested that you do at least two things:

1. Read some P.V. books

2. Contact an expert (a Dealer, Distributor, or a P.V. Panel Manufacturer)

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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.

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.

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.

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.

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.

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.

What is a blocking diode, and what does it do?
A blocking diode is like a check valve for the solar system. It allows current to go from the panels to the batteries, but prevents current drain from the battery into the solar panels at night.

What would I lose if I didn't have a blocking diode?
The losses through a solar panel at night without a blocking diode amount to about .03 amps per solar panel in parallel. So if you have a 4 panel system and the night is 10 hours long, you would loose 1.2 amp hours. If you generate about 12 amps for 6 hours, this is less than 2%.

Are there ways other than blocking diodes to prevent night time losses?
The charge controller can be designed to automatically disconnect the solar panels at night.

What other losses are of concern?
Voltage drops through the controller and in the system wiring is a concern. For example if the max. power voltage of the solar panel is 16 volts, and you need to charge the battery up to 14.5 volts, if you have a voltage drop of 2 volts through the entire system from panels to battery, you will not get the battery fully charged. Other losses include current consumption of the controller and inverter. These are typically small, but should be considered.

Why does the controller turn off when the battery is not fully charged?
If a voltage drop occurs between the controller and the battery, the controller will see a voltage that is higher than actual battery voltage. This will cause the controller to turn off too soon.

Why does my controller turn on and off really fast?
This is related to voltage drop also. What is happening is that the controller sees the high voltage because of the line loss, but as soon as the charging stops, the error disappears, so the controller turns back on.

How can the voltage drop error be avoided?
The correct size of wire must be used to minimize voltage drop in the wiring, but most importantly, the connections must be good. Connections that are crimped but not soldered often corrode over time, and components like fusses and switches often contribute to voltage drops. Where possible, use a controller that offers remote battery voltage sense for higher current systems.

Can PV controllers be used to regulate AC chargers?

How do I check the output from my solar panels?
With a multi-volt meter, check the open circuit voltage, the voltage reading you get between the panel plus and minus with the panel in the sun (should be about 19 volts for a 12 volt system) and the short circuit current, measure the current when array plus is shorted to array minus. Make sure your meter is rated to handle the current that your panels produce.

What size battery can the PV controllers charge?
PV controllers usually do not need to consider the capacity of the battery, only the current and voltage output from the solar panel.

I have a 50 watt solar panel, what can I expect to run?
Depending upon your location, you can expect about 3 amps peak for an average of 3 to 8 hours per day. This gives you 9-24 amp hours per day. If you have two lights that draw 1 amp each, you could run these lights for 4-12 hours per day, depending upon your location.

I have a digital meter, what is the remote shunt?
The remote shunt is a precision resistor that creates a voltage drop in exact proportion to the amount of current passing through it. If you monitor the voltage drop across the shunt, you can tell how much current is flowing through it.

What can you use a remote shunt for?
The remote shunt can be installed to monitor any current in the system, for example current to an inverter or from a charging source like a wind generator.

I installed a remote shunt to monitor my usage, but when I turn on my lights, the load current displayed goes down. How can that be?
The shunt is probably installed where it is reading net battery current, not just load current. What is being displayed is the charging current minus the load current. This would cause the current reading to go down when a load is turned on.

When a controller fails, what will happen?
It depends on whether the controller fails in the charging mode or non-charging mode. The function of the controller is to prevent overcharging, therefore a good design would have the controller fail most often in the non-charging mode, so overcharge does not occur.

What is temperature compensation?
Temperature compensation is a feature of charge controllers that automatically changes the charging set points based on temperature. A good design will monitor battery temperature via a remote sensor and vary the set points accordingly. An ambient sensor is better than nothing but often the battery is at a different temperature. Monitoring the controller temperature is often a bad idea because the temperature of the controller will vary with amount of charge current, charge mode and location.


How do I select a controller?
How much can I run?
How do I size the system?
What size wire do I use?
How do I select batteries?
How do I select panels?

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Last update: July 1, 2005
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