A battery bank like the one shown above is a necessity for an off-grid solar power unit, but can also come in handy with a grid-tie system.
Because power outages are rare, grid-tie photovoltaic systems don't normally include battery backup. However, when an outage does occur, the solar power unit must by law disconnect from the grid until the utility power is restored. This is a safety issue. If the inverter were to continue pumping solar electricity into the grid, utility workers or fire fighters could be electrocuted while handling transmission lines or other equipment. It has happened in the past, and the scenario is known as islanding.
But what if the outage lasts more than a few hours? Is there a way to keep powering the refrigerator and a few other essential loads in your home? Yes, but you'll need to purchase a bimodal inverter, a batter bank and a battery charge controller to get the job done.
A bimodal (aka bipolar) inverter has circuitry to allow it to transfer its current to a secondary load panel in the event of a power outage. At this point, it becomes a standalone inverter. The array continues to generate electricity, which is stored in the battery bank until its needed in the home. Then, when utility power is restored, the bipolar inverter will detect its voltage and transfer current back in the direcdtion of the main panel.
Because this type of PV system can act as a standalone unit, naturally, the entire circuit must be designed as if it were a standalone system. That means that the electricity generated by your array first passes through a charge controller, which keeps a battery bank charged, and then the remaining electricity will move on to the inverter. Depending on the size of your battery bank, you may have lots of stored power in an emergency, or you may have very little. Either way, this configuration will cost you nearly twice the price and be less efficient at kilowatt hour generation.
This flowchart shows a stand-alone, hybrid system which might be used in a remote area with no access to the utility grid. A system with a bimodal inverter would look a lot like this, except a critical loads subpanel would have to be installed. Because batteries are used, the charge controller is needed to regulate the flow of electricity into and out of them. As you can see, this inverter doesn't see the output from the array directly, but only after the electricity passes through the charge controller and/or batteries.
Where batteries are used, a charge controller is included in the system to regulate the flow of electricity and prevent both overcharging and under-discharging. This insures a long life for the batteries and less chance of overheating or other hazards occurring.
There are two basic charge controller types on the market, a PWM controller, which stands for "pulse width modulation", and an MPPT controller, which stands for "maximum power point tracking". PWM controllers have been used with PV systems for a long time and are inexpensive.
The MPPT charge controller is a newer technology that helps regulate the voltage of the array, thus optimizing its power output. (MPPT circuits are described in more detail on the Inverters page.) MPPT controllers allow you to design a higher-voltage array than in the case of most standalone units, which are typically restricted to 48 volts or less. Higher voltages mean less current loss as energy travels downstream. For this reason, they're the best choice for a grid-tied PV system with batttery backup. To learn more about charge controllers, here's an article at altestore.com.
Having a battery bank gives you other options as well. For instance, if your utility company pays a higher price for solar electricity delivered to the grid during peak consumption times (usually late afternoon/early evening), you may be able to earn more kWh credits by programming your charge controller to release the battery charge during those hours.
This video describes a standalone (off-grid) system, which includes a charge controller and battery bank.
In addition to the cost and complexity of the added equipment and wiring, any PV system with batteries means you'll have to spend more time on maintenance. Batteries must be inspected regularly and replaced every few years. They require ventillation in the event of hydrogen venting, and are sensitive to extreme heat or cold. Some batteries require you to add distilled water from time to time.
Many DIY enthusiasts build battery banks out of inexpensive (and sometimes used) golf cart lead-acid batteries. This is an acceptable plan, since traction batteries, as they're known, are well-suited for PV applications. If you prefer a lower maintenance battery, you can purchase the more expensive sealed lead-acid batteries, also known as valve-regulated lead acid (VRLA) batteries. Here, a water-based electrolyte is set in a gel or fiberglass mat, which means you don't have to add water periodically.
To reiterate, batteries can release toxic gases, so local building codes require that they be kept in a well-ventilated, climate-controlled space.
To learn more about different types of batteries used in PV systems, check this product page at WholesaleSolar.com. For a quick lesson in how to wire battery banks in series or parallel, check this guide at WindBluePower.com. Also this blog post offers a rundown of common battery selection and installation mistakes, as well as potential hazards to watch out for.
There was a time when wind turbines were all the rage in renewable energy circles. But after a generous federal tax credit was dropped by Congress, the momentum faded. Another major impediment is the noise common to turbines when they're spinning. Experts recommend at least 100 yards between a your property and the nearest neighbor; otherwise you're bound to get some complaints.
Nevertheless, it's possible to add 2 kilowatts of power to supplement a PV array by installing a wind turbine to go with it. Silent operation products, like the Swift wind turbine from Cascade Engineering, may make it easier in densely populated areas to take advantage of wind energy.
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Ideally, a wind turbine should rise at least 20 feet above any object within a 250-foot radius. Those objects -- buildings, trees, and even the ground -- will cut down wind speed substantially. The average annual wind speed should be at least 4.5 meters per second (10 miles per hour) to make the venture worthwhile. Favorable wind conditions are found on hilltops, in rural areas or the desert, astride a large body of water, or at higher altitudes. In some places, the topography may also work to concentrate air currents. You can purchase an anemometer, which measures wind speed, to see how a turbine might fare on your property. Amazon sells an Ambient Weather Handheld Wind Meter for about $30.
The U.S. Dept. of Energy's wind program website has a good FAQ page which answers basic questions. For more info, read this consumer guide. HomePower.com also provides a set of articles on the subject.
Before photovoltaics took over the market, early residential solar systems generated hot water, not electricity. While not nearly as maintenance-free as as a PV array, the installation of a solar water heating system can still help trim your utility bill by making a huge dent on your monthly natural gas (or propane) bill. Many states offer generous rebates as well, and the federal government extends the same 30% tax credit that if offers for a PV system purchase and instalation . For info and links to learn more about rebates and financing, see Tax Credits and Incentives.
At left, a solar water heating panel alongside a PV array. At right is a diagram of a drainback system. Photo: flgreenteam.com Diagram: energy.gov
There are two basic categories of solar water heating technology to choose from when shopping for the best deal. Active heating systems require a small electric pump to circulate the water, as well as some controls to regulate this activity. This type of solar water heating is known as a drainback system. See diagram above.
Passive water heating operates on the principle that hot water rises naturally. Thus, solar collecting pipes are positioned at a lower elevation than the water storage tank. This is known as a Thermosyphon system. It's best suited for mild climates where water is not likely to freeze. As the name implies, a passive system requires no electrical energy to function, making it less expensive in terms of equipment, but not quite as efficient as a drainback system.
Your roof must likewise be sturdy enough to hold the tank, if you decide to go with a passive system. In most cases, standard roof load limits can accommodate one or more the storage collector(s), as they're known, However, it's a good idea to have your installer add some blocking to the rafters or other structural members, where the collectors are positioned.
One creative way of heating up water on a rooftop that DIY installers have tried is to position the water pipes around the PV modules. The pipes act as a heat sync on hot days, keeping the modules cool (which reduces power losses due to high ambient temperatures). At the same time, the water flowing through the pipes will heat up in a giffy.
For more info on installing a solar water heating system, check out SolarOregon's detailed guide. Arizona Solar Center also provides a well-written primer on the subject (although it's a bit out of date), while Pennsylvania Solar Roofs Partnership offers an illustrated tutorial.
Gas-operated, portable engine generators are a popular home backup power option, regardless of whether you a solar electric system is used. This is especially true in hurrican prone regions and along Tornado Alley. But the price of purchasing a generator is fairly cheap nowadays, so you might consider including one in your PV circuit configuration.
As in the case of choosing a battery bank, you should do an analysis of your ebnergy usage and identify those critical loads in your home that need to be covered in an emergency. Knowing how many amps you'll need to get the job done, or how many batteries to keep charged, will provide you with sufficient data in order to choose the right size generator. The size you pick must also fit the parameters of the charge controller or inverter that will be connected to it.
Generators are also commonly integrated into off-grid solar power systems. Their job is to charge the battery bank whenever the available sunlight is insufficient. Different models use different forms of fuel, so you can choose gasoline, diesel or propane, depending on what's easiest for you to access and store. Fuel prices may also factor into the equation.
Since engine generators produce carbon monoxide, this equipment must be located outdoors or in an extremely well-ventilated space away from dwellings (including any animal lodgings). Because they make a lot of noise, it's also customary to place a generator some distance away from where people sleep or congregate.
For more info on selecting a generator for home use, here's a short tutorial at freesunpower.com.
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