You would like to generate clean power.
Equipment Required for Stand-Alone Systems
In addition to purchasing photovoltaic panels, a wind turbine, or a small hydro power system, you will need to invest in some additional equipment (called "balance-of-system") to condition and safely transmit the electricity to the load that will use it.
Diagram of a typical AC, battery-based system.
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, for example, your wind turbine is connected directly to the load. However, if you want to store power for use when your turbine isn't producing electricity, you will want to purchase batteries and a charge controller. Depending on your needs, balance-of-system equipment 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.
Charge Controllers for Stand-Alone Systems
Charge controllers regulate the electricity flowing from the generation source into your battery or load.
Photo credit: Harin Ullal
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 Stand-Alone Systems
Inverters condition electricity so that it matches the requirements of the load.
Photo credit: Trudy Forsyth
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.
The cost of inverters is affected by several factors, including:
- Quality of the electricity it needs to produce
- Voltage of the incoming current
- AC wattage required by your loads
- 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. 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.
Batteries for Stand-Alone Systems
Batteries store electricity for use during times that your system is not producing electricity due to resource unavailability. Batteries are most effective when used in wind and photovoltaic systems (variations in hydro resources can be more seasonal in nature, so batteries may be less useful).
Batteries store the electricity produced by your small renewable energy system for later use.
The "deep-cycle" (generally lead-acid) batteries typically used for small systems last five to ten 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.
Safety Equipment for Stand-Alone Systems
Lightning strikes and other power surges can severely damage your system without proper precautions. Photo credit: Dave Parsons.
Safety features protect stand-alone small renewable energy systems from being damaged or harming people.
Here are the major safety features your system will need:
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.
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 photovoltaic 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.
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.
Meters and Instrumentation for Stand-Alone Systems
Meters and other monitoring equipment help you keep track of how your system is performing.
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.
Operating Your System Off-Grid
Stand-alone systems can be more cost-effective than connecting to the grid in remote locations.
For many people, powering their homes or small businesses using a small renewable energy system that is not connected to the electricity grid—called a stand-alone system—makes economic sense and appeals to their environmental values.
In remote locations, stand-alone systems can be more cost-effective than extending a power line to the electricity grid (the cost of which can range from $15,000 to $50,000 per mile). But these systems are also used by people who live near the grid and wish to obtain independence from the power provider or demonstrate a commitment to non-polluting energy sources.
Successful stand-alone systems generally take advantage of a combination of techniques and technologies to generate reliable power, reduce costs, and minimize inconvenience