A PV system has many components involved in making it a useful electrical generating device. All these other components other than the PV array, are called the BOS components, standing for Balance Of System components.
The BOS components include all the wiring and connectors, the solar mounting method such as a rack mount, the combiner boxes, inverters, fuses or circuit breakers, shut off boxes, batteries, battery charge controllers, and revenue meters, etc. It really depends on the system itself as to what BOS components may or may not be utilized. Some solar panel arrays can be directly connected to the battery system if the system is designed for that. You may not need an inverter which changes the DC power int AC power for sending into the grid. You may outfit your home with DC appliances, lighting, and any other power using device to eliminate the need for an inverter.
The voltage output of a solar array changes with environmental change such as tempuratures, overcast days, winter snow or a body of water glare. All these variables and more may require the power to be conditioned for use with the electrical loads we want to power. All power generator sources including a solar system face the task of matching electrical energy generated with the demand. Power demand fluctuates greatly over the day or night and this causes the need for storage batteries that can take excess power and store it until it is needed and the solar array is not generating the needed power. This could happen at night time when there is no sun shining.
BATTERIES
Batteries are the primary type of energy storage device used by a solar pv system if it is a system that is off the grid or trying to reduce the grid use to save money. Lead acid batteries are the most common type of battery being used as they are readily available in deep cycle formats. Batteries change electrical energy into chemical energy when receiving a charge and the reverse is true when the battery is supplying electricity. Other kinds of batteries include nickel cadmium, nickel metal hydride, and many more expensive types limited to special applications like outer space vehicles. Typical pv systems employ more than one battery and usually have a battery bank or bunch of batteries arranged in a specific connection path to match their overall voltage to the array voltage. Batteries also help with large start up surges typically created by electrical motor starts.
Batteries arranged in a bank create a stable voltage supply with an amp hour ratings. The amp hour rating gives the owner of the system a number of hours that the system can supply electricity for before the batteries become depleted too far and damaged. Lead acid batteries typically don’t like to be discharged more than 70% of their energy with out losing some life expectancy in the long term. The more you discharge a lead acid battery the more effect on life expectancy reduction occurs. Batteries only have so many charge / discharge cycles they can go through before they are worn out. The less harshly you discharge them the longer the charge / discharge cycle continues.
Sealed lead acid batteries are more common than the vented type lead acid battery because maintaining them is less of a problem.
OTHER STORAGE DEVICES
Batteries are the common way of storing power presently but much research is being done in the use of flywheels, and super capacitors. Flywheels are essentially a large wheel spinning at high speeds usually used with motors, pumps and engines or any other rotating device. The flywheel gathers up energy by spinning it. They resist change in rotational speed and smooth out surges in energy requirements. They can also be used to release large amounts of energy in a short period of time.
Energy is typically stored by using a motor to spin the flywheel to high speeds. The flywheel stays spinning with very little or no friction on high quality bearings or magnetic bearings for a long time. To reverse the situation and get the energy out the flywheel becomes the driver for the motor which is changed into a generator.
Super capacitors are just bigger cousins of the typical small capacitors used in electronic devices. These capacitors use two plates separated by an insulating dielectric to chage up a positive and negative plate. The amount of energy is measured in farads. Using advanced plate and dielectric materials these capacitors are large and store large amounts of energy. They can replace batteries in a lower power requirement application. One advantage to capacitors is the nearly infinite charge / discharge cycles and life expectancy.
Other methods of energy storage are pumped water systems. In this type of system you need to have a large supply of water and of course a tank or holding vessel at a height suitable to store water. Water is pumped up to the higher level reservoir while there is power available from the PV array and then sent through a water turbine generator using gravity as the force that causes the water to move down hill.
Compressed air is another form of storing up pv energy by compressing air via a rotating electric motor and releasing the air again through an air turbine that generates electricity.
By far we rely on batteries at this point in time as the other systems have drawbacks with geography and efficiency losses on energy exchange principles.
INVERTERS
Inverters are the device that changes DC power into AC power either directly from the pv array or from the battery bank. There are a large number of inverters available depending on the pv system design and requirements. Every inverter has a spec sheet that will tell you its parameters and design uses. Inverters are designed for battery use or grid tie use. Inverters that are designed for grid tie use have dc operating ranges that you need to match up to the dc voltage output of your solar array strings. Inverters that are matched well will operate at a higher percentage level than one that is poorly matched. As well as voltage levels the wattage rating is key to having a system that will last much longer. An Inverter is an electrical / electronic device that when taxed to its upper level of wattage has an effect of reducing it’s life expectancy. Many battery type inverters also include a battery charge controller. Future inverters will incorporate smart grid technology which will be in constant communication with the grid. Some solar panel manufacturers are now putting micro inverters directly on each solar panel in the array. The array gives out AC power and is monitored remotely through the internet to show power generation. The advantage to this is shading problems are minimized as only those panels shaded at any given time are limited to their output instead of killing off a whole string of panels. We will get into deeper design with inverters in a later chapter.
CHARGE CONTROLLERS
Almost every pv system that incorporates batteries requires a charge controller to protect the battery from being overcharged and depleted too far. Both voltage and current are controlled being fed into the battery and also monitored and measured to make sure the batteries are not discharged to the point of reducing life expectancy.
COMBINER BOXES
When pv arrays are set up in strings and utilize a common inverter or inverters instead of micro inverters the strings of panels are combined together in a combiner box. Each string of panels is hooked up to achieve the same voltage levels thus being able to be combined with out any issues. The combiner box must also have a fuse that is rated correctly for protection from one string feeding into another if a short circuit occurs. Current and voltage are combined and output to a DC shut off box with larger capacity wire to handle the full current of all the strings coming into the combiner box. The combiner box can be located in different places in the system. Quite often they are installed up on the roof. A large system such as a solar farm may use multiple combiner boxes to combine solar arrays that are set to create the same voltage as each other.
DC SHUT OFF BOX
The dc shut off box is usually located between the combiner box and the inverter. It will have labels on them required by the hydro authority to tag the voltages and current ratings of the circuit breakers or fuses.
SOLAR RACKING
The method of mounting an array on a roof top is chosen by the system designer to match the type of roof. Flat roofs will require a racking method that will optimize the tilt of the panel to the geographic region. For example the solar tilt angle for Southern Ontario Canada is 32 degrees. This angle is variable depending on what type of system design employed. The system may be designed to provide more power in the winter months and thus have a higher tilt angle. For home roof mounts you pretty much have to go with the roof rise and run. The mounting system employed bolts directly to the sub structure through the shingles or waterproofing membrane and needs to have a qualified roofing person to properly install the racking. Racking is generally made of aluminum extrusions and has special grounding clips to ensure a good grounding method. Alternately ground wire jumpers can also be used. The racking holds the panels all in place at the same angle and plane so that the sun hits the array evenly. The racking weight will have to be taken into account by a structural engineer who approves a pv system installation.
GRID TIE METERS
Depending on your location you may need to have a separate revenue meter such is the case for Ontario solar power installations. Other areas generally in the United States you can hook the AC power from the inverter into the opposite side of the regular power meter and spin it backwards. The disadvantage to one meter spinning backwards is it will be limited to the zero or last measured reading, so revenue or electrical savings is not counted at that point.
WIRE
Wire used in the pv system must conform to the hydro authority in your area. The gage of wire for the system must be large enough to accommodate the current being generated. As well the voltage rating of the wire must be larger than the voltage level of the string array. The wire style must be of a weatherproof type approved by CSA in Canada and UL in the United States. As well local authorities may require a certain type of wire. It is best to check with the local jurisdiction first. Wiring a pv system should be done by a qualified electrician.