Balance of system
The balance of system (BOS) encompasses all components of a photovoltaic system other than the photovoltaic panels. This includes wiring, switches, a mounting system, one or many solar inverters, a battery bank and battery charger.
Other optional components include, renewable energy credit revenue-grade meter, maximum power point tracker (MPPT), GPS solar tracker, Energy management software, solar concentrators, solar irradiance sensors, anemometer, or task-specific accessories designed to meet specialized requirements for a system owner. In addition, CPV systems require optical lenses or mirrors and sometimes a cooling system.
In addition, ground-mounted, large photovoltaic power station require equipment and facilities, such as grid connections, office facilities, and concrete. Land is sometimes included as part of the BOS as well.
Balance of Plant
Balance of plant ( BOP ) is a term generally used in the context of power engineering to refer to all the supporting components and auxiliary systems of a power plant needed to deliver the energy, other than the generating unit itself. These may include transformers , inverters , supporting structures etc., depending on the type of plant
Whether you decide to connect your home renewable energy system to the electric grid or not, you will need to invest in some additional equipment (called "balance-of-system") to condition the electricity, safely transmit the electricity to the load that will use it, and/or store the electricity for future use.
With stand-alone systems -- those not connected to the electric grid -- the amount of equipment you will need to buy depends on what you want your system to do. In the simplest systems, the current generated by your system is connected directly to the equipment that it is powering (load). However, if you want to store power for use when your system isn't producing electricity, you will need to purchase batteries and a charge controller.
Depending on your needs, balance-of-system equipment for a stand-alone system could account for half of your total system costs. Your system supplier will be able to tell you exactly what equipment you will need for your situation, but typical balance-of-system equipment for a stand-alone system includes batteries, charge controller, power conditioning equipment, safety equipment, and meters and instrumentation.
A grid-connected system -- one that is connected to the electric grid -- requires balance-of-system equipment that allows you to safely transmit electricity to your loads and to comply with your power provider's grid-connection requirements. You will need power conditioning equipment, safety equipment, and meters and instrumentation.
Batteries for Stand-Alone Systems
Batteries store electricity for use during times that your system is not producing electricity (the resource is not available). Batteries are most effective when used in wind and photovoltaic systems (variations in microhydropower resources can be more seasonal in nature, so batteries may be less useful).
The "deep-cycle" (generally lead-acid) batteries typically used for small systems last 5 to 10 years and reclaim about 80% of the energy channeled into them. In addition, these batteries are designed to provide electricity over long periods, and can repeatedly charge and discharge up to 80% of their capacity. Automotive batteries, which are shallow-cycle (and therefore prone to damage if they discharge more than 20% of their capacity), should not be used.
The cost of deep-cycle batteries depends on the type, capacity, climate conditions under which they will operate, frequency of maintenance, and chemicals used to store and release electricity. Wind or photovoltaic stand-alone system batteries need to be sized to store power sufficient to meet your needs during anticipated periods of cloudy weather or low wind. An inexpensive fossil fuel-powered back-up generator can be used to cover unanticipated or occasional slumps in the renewable resource.
For safety, batteries should be located in a space that is well ventilated and isolated from living areas and electronics, as they contain dangerous chemicals and emit hydrogen and oxygen gas while being charged. In addition, the space should provide protection from temperature extremes. Be sure to locate your batteries in a space that has easy access for maintenance, repair, and replacement. Batteries can be recycled when they wear out. Contact your system supplier for information on sizing your battery pack to meet your specific needs.
Charge Controllers for Stand-Alone Systems
This device regulates rates of flow of electricity from the generation source to the battery and the load. The controller keeps the battery fully charged without over-charging it. When the load is drawing power, the controller allows the charge to flow from the generation source into the battery, the load, or both. When the controller senses that the battery is fully (or nearly fully) charged, it reduces or stops the flow of electricity from the generation source, or diverts it to an auxiliary or "shunt" load (most commonly an electric water heater).
Many controllers will also sense when loads have taken too much energy from batteries and will stop the flow until sufficient charge is restored to the batteries. This last feature can greatly extend the battery's lifetime.
The cost of controllers generally depends on the ampere capacity at which your renewable system will operate and the monitoring features you want.
Power Conditioning Equipment
For both stand-alone and grid-connected systems, you will need power conditioning equipment.
Most electrical appliances and equipment in the United States run on alternating current (AC) electricity. Virtually all the available renewable energy technologies, with the exception of some solar electric units, produce direct current (DC) electricity. To run standard AC appliances, the DC electricity must first be converted to AC electricity using inverters and related power conditioning equipment.
There are four basic elements to power conditioning:
Conversion -- of constant DC power to oscillating AC power
Frequency of the AC cycles -- should be 60 cycles per second
Voltage consistency -- extent to which the output voltage fluctuates
Quality of the AC sine curve -- whether the shape of the AC wave is jagged or smooth.
Simple electric devices, such as hair dryers and light bulbs, can run on fairly low-quality electricity. A consistent voltage and smooth sine curve are more important for sensitive electronic equipment, such as computers, that cannot tolerate much power distortion.
Inverters condition electricity so that it matches the requirements of the load. If you plan to tie your system to the electricity grid, you will need to purchase conditioning equipment that can match the voltage, phase, frequency, and sine wave profile of the electricity produced by your system to that flowing through the grid.
A series of requirements for grid-interactive inverters have been developed by Underwriters Laboratories, a leading safety-testing and certification organization. These requirements, referred to as UL 1741, apply to power-producing stand-alone and grid-connected renewable energy systems. Either you or your installer should contact your power provider to see which models they accept for grid-connection; most simply require a grid-interactive inverter listed by an organization such as Underwriters Laboratories.
These factors affect the cost of inverters:
Application (utility-interconnected, stand-alone, or both)
Quality of the electricity it needs to produce for stand-alone
Voltage of the incoming current
AC wattage required by your loads (for stand-alone systems only)
Power required for the starting surge of some equipment
Additional inverter features such as meters and indicator lights.
When you size your inverter, be sure to plan for any future additional loads you might have. In the case of a grid-tied system in which you want to enlarge your renewable energy system, it is often cheaper to purchase an inverter with a larger input and output rating than you currently need than to replace it with a larger one later.
Safety features protect stand-alone and grid-connected small renewable energy systems from being damaged or harming people during events like lightening events, power surges, or malfunctioning equipment.
Safety disconnects -- Automatic and manual safety disconnects protect the wiring and components of your small renewable energy system from power surges and other equipment malfunctions. They also ensure that your system can be shut down safely for maintenance and repair. In the case of grid-connected systems, safety disconnects ensure that your generating equipment is isolated from the grid, which is important for the safety of people working on the grid transmission and distribution systems.
Grounding equipment -- This equipment provides a well-defined, low-resistance path from your system to the ground to protect your system against current surges from lightening strikes or equipment malfunctions. You will want to ground both your wind turbine or photovoltaics unit itself and your balance-of-system equipment. Be sure to include any exposed metal (such as equipment boxes) that might be touched by you or a service provider.
Surge protection -- These devices also help protect your system in the event that it, or nearby power lines (in the case of grid-connected systems), are struck by lightening.
A local electrician or your installer should be able to provide you with more information on the safety features required for your particular situation. For additional information on safety and electrical installation requirements, consult the National Electric Code NFPA 70.
Meters and Instrumentation
Meters and other instruments allow you to monitor your small renewable energy system's battery voltage, the amount of power you are consuming, and the level at which your batteries are charged, for example.
If you are connecting your system to the electricity grid, you will need meters to keep track of the electricity your system produces and the electricity you use from the grid. Some power providers will allow you to use a single meter to record the excess electricity your system feeds back into the grid (the meter spins forward when you are drawing electricity, and backward when your system is producing it).
Power providers that don't allow such a net metering arrangement require that you install a second meter to measure the electricity your system feeds into the grid.