Frequently Asked Questions (FAQ) About Solar
Energy that originates from the sun, is radiated to the earth, collected and used.
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.
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.
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.
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.
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.
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.
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.
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.
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, and 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 be maintained or overcharging would occur, so the controller must drop the voltage down.
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.
Bulk charge is a high amperage charge up to a high voltage.
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.
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.
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.
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.
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.
Inverters ran typical AC appliances like blenders, hair dryers, microwave ovens and computers. Heavy duty inverters can run motors like in washing machines.
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.
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%.
The charge controller can be designed to automatically disconnect the solar panels at night.
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.
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.
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.
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.
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.
PV controllers usually do not need to consider the capacity of the battery, only the current and voltage output from the solar panel.
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.
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.
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.
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.
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.
Modules are mounted using the existing holes located on the bottom of the module frame. The holes will accept ¼ inch diameter fasteners. To prevent wire chafing, clearance between the modules and mounting surface should be minimal. When installing modules on a building, use stand off rack methods to discourage water or ice damming.
Module interconnects are made with AWG # 10-16 single conductor or dual conductor cable. The wire or cable should be specified for outdoor use and sunlight resistance; such as USE. Follow the NEC guidelines. Specifying the wire or cable running from the module strings to the combiner box will be similar, ranging from AWG # 10-16. (See wire sizing chart) The wire or cable running from the combiner box to the power conditioning equipment depends on the distance between the two components, and other factors such as the sum of the Isc, temperature and environment. Typically, this wire ranges in size from AWG # 2-10.
Not necessarily. Only terminals 1 and 5 are connected to the active material in the solar module. However, when connecting more than two wires in the module junction box, the jumpers are useful for activating terminals 2, 3, and 4
It is recommended that blocking diodes be used in conjunction with fuses to protect the array, and increase daily energy output where shadowing is an issue. Back to the Top
There are two different types of diodes that can be used; the Schottky diode and the silicon power diode. The advantage of the Schottky type is that there is a lower voltage drop (less power loss) across the diode when compared to silicon power diodes. The advantage of the silicon power diode is that they are more tolerant of transient lighting strikes (available in higher rated voltages) and may be more durable in areas that have a higher of lightning activity.
For single string systems, the diode in the charge controller will be adequate for blocking current flow from the battery into the solar array at night. For multiple strings of modules in parallel, diodes are normally located in a combiner box near the array. Diodes require a heat sink and should be mounted in a combiner box (w/ heat sink) along with the array fuses. Do not put blocking diodes inside of the module junction box.
A combiner box is an electrical box where array fuses and diodes can be mounted. The combiner box includes a negative bus bar and compression terminals where the positive wires from the array strings can be connected to the array fuses and diodes. Individual array strings can be parallel wired in the combiner box as well. The combiner box is also a good place to mount lightning protection devices like SOVs (silicon oxide varisters)
For high voltage systems (>48 voc), Class R fuses can be used up to 250 volts. For high voltage systems between 48 VDC and 333 VDC, Class T fuses can be used. For low voltage systems (<48 v DC) AGC or ATC fuses can be used. However, ATC fuses have not been approved by the NEC for use in homes (Class R or T can also be used).
Fuses can be mounted inside of the module junction box for 12 v DC systems (ex. in a small in-line fuse holder) or they can be mounted inside a combiner box where strings of modules can be brought together and wired in parallel.
Yes, modules can be wired in parallel or in series. Open circuit voltage cannot exceed 600 volts so the maximum number of modules that can be wired in series is 20. Any number of strings of modules can be connected in parallel using the appropriate combiner boxes.
Scratched modules, where the top layer of Tefzel has been cut open and active solar material is exposed to the air, can be repaired with common silicone. A small dab of common clear silicone is applied to the hole or scratch and lightly pushed into the hole. Because the silicone may inhibit sunlight from penetrating the module, do not spread an excess amount of silicone around the hole.
Yes, solar modules will absorb sunlight in any part of the world. As you get further from the equator (latitudes 40 to 60 degrees), there will be fewer hours of overhead sunlight, causing a reduction in the total amount of energy generated daily. Closer to the equator, some areas will have high humidity and a haze may form in the atmosphere. This may have a small effect on the intensity of the sunlight and the amount of amps produced by the solar modules. However, this effect does not have as noticeable an impact as the reduced number of hours of overhead sunlight seen in higher latitudes.
Today’s controllers work well with amorphous-silicon modules and arrays. When designing a utility line-tie system (a system with no batteries) you will need to know the nominal DC input range for the inverter, and the Vmax voltage of your module string must be within that range. Make sure to take into account the voltage drop due to high cell operating temperatures under typical conditions at your site.
If the maximum current (amps) is less than 1.5% of the overall battery capacity (measured in amp hours), a controller may not be necessary. For example, if you are charging a 12 volt 210 amp hour battery with a US-42 (max power current = 2.4 amps), a charge controller may not be necessary. Generally, using a controller in the system is encouraged as modern controllers can regulate the charge rate and optimize the charge on the battery. Additionally, many controllers are equipped with a blocking diode, system indicators or meters, and other features such as a low voltage disconnect for attached loads or automatic equalization charge function.
The system voltage and the maximum amps produced by the solar array under short circuit conditions, determines the size of the controller. For example, a solar module may produce 3.88 amps at maximum power and 4.8 amps when measured under short circuit conditions. If you have a two-module system, you would have a maximum of 9.6 short circuit amps at 12 volts. A 10-amp controller that can be used with a 12-volt system would be appropriate for this system.