Solar Electric Systems – What are Your Options?
Copyright 2010 by Dan Chiras
PV systems fall into three categories: (1) grid-connected, (2) grid-connected with battery backup, and (3) off-grid.
Grid-connected PV Systems
Grid-connected PV systems are the most popular – and least expensive -- solar electric systems on the market today. Grid-connected systems are so named because they are connected directly to the electrical grid – the vast network of electric wires that spans the nation and crisscrosses your neighborhood. These systems are also sometimes referred to as batteryless grid-connected or batteryless utility-tied systems.
Grid-connected system consists of five main components: (1) a PV array, (2) an inverter, (3) the main service panel or breaker box, (4) safety disconnects, and (5) meters.
To understand how a batteryless grid-connected system works, we begin with the PV array. The PV array produces DC electricity. It flows through wires to the inverter, which converts the DC electricity to AC electricity. The inverter is just below the PV array.
The inverter converts the DC electricity to grid-compatible AC – that is, 60 cycles per second, 120-volt (or 240-volt) electricity. Because the inverter produces electricity in sync with the grid, inverters in these systems are often referred to as synchronous inverters.
The 120-volt or 240-volt AC produced by the inverter flows to the main service panel, a.k.a. the breaker box. In the diagram above, the breaker box is immediately to the right of the inverter. From the breaker box, electricity flows to active loads -- that is, to electrical devices that are operating. If the PV system is producing more electricity than is needed to meet these demands -- which is often the case on sunny days -- the excess automatically flows onto the grid.
Surplus electricity travels from the main service panel through the utility’s electric meter, typically mounted on the outside of the house. In the diagram above, it is located just to the right of the breaker box.
Electricity then flows through the wires that connect to the utility lines. From here, it travels along the power lines running by your home or business where it is consumed in neighboring homes and businesses. Once the electricity is fed onto the grid, the utility treats it as if it were theirs. End users pay the utility directly for the electricity you generated.
In most locations, an electric meter monitors the contribution of small-scale producers to the grid. The meter also keeps track of electricity the utility supplies to these homes or businesses when their PV systems aren’t producing enough to meet their demands or when the PV system is not operating, for example, at night.
In addition to the utility electric meter – or meters (some utilities require two or more meters) -- that monitor the flow of electricity to and from the local utility grid, grid-connected solar electric systems also contain two safety disconnects. Safety disconnects are manually operated switches that enable service personnel to disconnect key points in the system to prevent electrical shock when servicing the system.
The first disconnect is located between the solar array and the inverter. This is a DC disconnect. The manual disconnect allows the operator to terminate the flow of DC electricity from the array to the inverter in case the inverter needs to be serviced. These systems also require an AC disconnect switch. This disconnect must be mounted outside the home or business. It must be readily accessible to utility workers and must contain a switch that can be locked in the open position by utility workers so no electricity flows to or from the grid. This disconnect is required so workers can isolate PV systems from the electrical grid and work on electrical lines without fear of shock in your area if, for example, a line goes down in an ice storm.
For many years, lockable AC disconnects were considered critical for the safety of utility personnel. Although utility-company-accessible, lockable, visible AC disconnects are required by many utilities, large California utilities with thousands of solar- and wind-electric systems now online and Colorado’s main electric utility have dropped this requirement. They’ve found that AC disconnects are not needed because grid-compatible (synchronous) inverters automatically shut off when the utility power goes down. Properly installed PV systems will not back feed onto a dead grid. Period.
The Pros and Cons of Grid-connected Systems
Batteryless grid-connected systems represent the majority of all new solar electric and wind-electric systems in the United States. They’re the least expensive of all systems and require the least maintenance, primarily because they contain no batteries. In essence, the electrical grid becomes your battery bank.
The biggest downside of batteryless grid-connected PV systems, is that they are vulnerable to grid failure. That is, when the grid goes down, so does the PV system. A home or business cannot use the output of a batteryless photovoltaic system when the grid is not operational. Even if the Sun is shining, batteryless grid-tied PV systems shut down if the grid experiences a problem – for instance, if a line breaks in an ice storm or lightning strikes a transformer two miles from your home or business, resulting in a power outage. Even though the Sun is shining, you’ll get no power from your system.
If power outages are a recurring problem in your area and you want to avoid service disruptions, you may want to consider installing an uninterruptible power supply (UPS) on critical equipment such as computers or medical equipment. A UPS has a battery pack and an inverter. If the utility power goes out, the UPS will supply power until its battery gets low. Or, you may want to consider installing a standby generator that switches on automatically when grid power goes down.
Or, as discussed next, you may want to consider installing a grid-connected system with battery backup. In this case, batteries provide backup power to a home or business when the grid goes down.
Grid-connected Systems with Battery Backup
Grid-connected systems with battery backup are also known as battery-based utility-tied systems or battery-based grid-connectedsystems . These systems ensure a continuous supply of electricity, despite brownouts and blackouts.
As shown in the accompanying Figure, a grid-connected system with battery backup consists of (1) a PV array, (2) an inverter, (3) safety disconnects, (4) a main service panel, and (5) meters. Although grid-connected systems with battery backup are similar to batteryless grid-connected systems, they differ in several notable ways. One of the most important is the type of inverter. Battery-based grid-connected systems require a special type of inverter – one that can operate in sync with the grid, but also off grid from batteries.
Another more obvious difference is that battery-based grid-connected systems require a battery bank. The third difference is a meter that allows the operator to monitor the flow of electricity into and out of the battery bank. The fourth difference is the charge controller. It regulates battery charging from the PV array.
Batteries for grid-connected systems with battery backup are either flooded lead-acid batteries or, more commonly, low-maintenance sealed lead-acid batteries.
The first point worth noting is that battery banks in grid-connected systems are typically small. That’s because they are typically sized to provide sufficient storage to run a few critical loads for a day or two while the utility company restores electrical service. Critical loads might include a few lights, the refrigerator, a well pump, the blower of a furnace, or the pump in a gas- or oil-fired boiler. It is worth noting, too, that in battery-based grid-tie systems, batteries are called into duty only when the grid goes down. They’re a backup source of power; they’re not there to supply additional power, for example, to run loads that exceed the PV system’s output. When demand exceeds supply, the grid makes up the difference, not the batteries. When the Sun is down, the grid, not the battery bank, becomes a home or business’s power source.
It is also important to point out that battery banks in grid-connected systems are maintained at full charge – day in and day out – to ensure a ready supply of electricity should the grid go down. Keeping batteries fully charged is a high priority of these systems. Maintaining a fully charged battery bank requires a fair amount of electricity over the long haul. This reduces system efficiency. Batteries require a continual input because they self-discharge. That is, they lose electricity when sitting idly by. You’ve seen it happen to flashlight batteries or car batteries sitting idle for months. Because of this, batteries require a continual electrical charge and therefore become a regular load on a renewable energy system.
In the best case, topping off batteries will consume about 5 to 10% of a system’s output. In the worst case — that is, in a system with a low-efficiency, unsophisticated inverter that is used to charge a large or older battery bank -- it may approach 50%.
Battery banks in grid-connected systems don’t require careful monitoring like those in off-grid systems, but it is a very good idea to keep a close eye on them. When an ice storm knocks out power to your home or business, the last thing you want to discover is that your battery bank died on you last year. For this reason, grid-connected systems with battery backup typically include a meter to monitor the total amount of electricity stored in the battery bank. These meters give readings in amp-hours or kilowatt-hours.
Meters in battery-based systems also typically display battery voltage. For experienced renewable energy operators, battery voltage provides a general approximation of the amount of energy in a battery. If, for instance, a battery is not being charged or discharged, the higher the voltage, the more energy it holds. The lower the voltage, the less energy it stores. Because charging a battery bank raises voltage and discharging lowers voltage, you only get an accurate state-of-charge voltage when the batteries have been “at rest” for a couple of hours -- that is, they have not been charged or discharged for a couple hours.
Another component found in grid-connected systems with battery backup is the charge controller, indicated by an arrow in the accompanying figure. The charge controller regulates the flow of electricity into a battery bank – but only when there’s a power outage. Modern charge controllers and inverters often contain a function called maximum power point tracking (MPPT). Maximum power point tracking circuitry optimizes the output of a PV array, thus ensuring the highest possible output at all times.
Most charge controllers on the market also contain a high voltage/low voltage DC conversion function. This feature allows the array to be wired as high as 60 or 72 volts (nominal) and still charge a 12-, 24-, or 48-volt battery bank. This cuts down on the expense of the “home-run” wire – the wire from the array to the balance of system (BOS) components – that is, the rest of the PV system. Higher voltage results in lower current. As a result, smaller wires can be used. It also allows the array to be located further from the BOS.
Pros and Cons of Grid-connected Systems with Battery Backup
Grid-connected systems with battery backup protect homeowners and businesses from power outages. They enable them to continue to run critical loads during outages. Although battery backup may seem like a desirable feature, it does have some drawbacks. One of the biggest downsides is higher cost. They’re about 30% more expensive than batteryless grid-connected systems. . The higher cost, of course, is due to the added components, especially the charge controller and the batteries. Batteries require maintenances and a warm, vented storage room or battery box, resulting in additional cost.
Because grid-connected systems with battery backup are expensive and infrequently required, few people install them. When contemplating a battery-based grid-tie system, you need to ask yourself three questions: (1) How frequently does the grid fail in your area? (2) What critical loads are present and how important is it to keep them running? (3) How do you react when the grid fails?
If the local grid is extremely reliable, you don’t have medical support equipment to run, your computers aren’t needed for business or financial transactions, and you don’t mind using candles on the rare occasions when the grid goes down, why buy, maintain, and replace costly batteries?
In some cases, people are willing to pay for the reliability that a battery bank brings to a grid-connected system.
Off-grid systems are designed for individuals and businesses that want to or must supply all of their needs via solar energy -- or a combination of solar and wind or some other renewable source. As shown in the accompanying Figure, off-grid systems bear a remarkable resemblance to a grid-connected system with battery backup. There are some noteworthy differences, however. The most notable is the lack of grid connection.
As illustrated, electricity flows from the PV array to the charge controller. The charge controller monitors battery voltage and delivers DC electricity to the battery bank. When electricity is needed in a home or business, it is drawn from the battery bank via the inverter. The inverter converts the DC electricity from the battery bank, typically 24 or 48 volts in a standard system, to higher-voltage AC, either 120 or 240 volts, which is required by households and businesses. AC electricity then flows to active circuits in the house via the main service panel.
Off-grid systems system often require a little “assistance,” to make up for shortfalls. Additional electricity can be supplied by a wind turbine, micro hydro turbine, or a gasoline or diesel generator, often referred to as a gen-set. “A gen-set also provides redundancy,” notes National Renewable Energy Laboratory’s wind energy expert Jim Green. Moreover, “if a critical component of a hybrid system goes down temporarily, the gen-set can fill in while repairs are made.”Gen-sets also play a key role in maintaining batteries.
Off-grid systems with gen-sets require battery chargers. They convert the AC electricity produced by the generator into DC electricity that’s then fed into the battery bank. Battery chargers are built into the inverter and operate automatically. When a generator is started and the inverter senses voltage at its input terminals, it then transfers the home loads over to the generator through an internal, automatic transfer switch. It also begins charging the battery from the generator.
Like grid-connected systems with battery backup, an off-grid system requires safety disconnects – to permit safe servicing. DC disconnects, with appropriately rated fuses or breakers, are located between the PV array and the charge controller, between the charge controller and the battery bank, and between the battery and the inverter.
These systems also require charge controllers to regulate battery charging from the PV array. Charge controllers also protect the batteries from overcharging. As is evident by comparing schematics of the three types of systems, off-grid PV systems are the most complex. Moreover, some systems are partially wired for DC – that is, they contain DC circuits. They are supplied directly from the battery bank. DC circuits are used to service lights or DC appliances such as refrigerators or DC well or cistern pumps.
Many people who install them, do so because DC circuits bypass the inverter. Because inverters are not 100% efficient in their conversion of DC to AC, this saves energy. Operating a DC refrigerator, however, over long periods can save a substantial amount of energy.
The problem with this strategy is that DC circuits are low voltage circuits and thus require much larger wiring and special, more expensive plugs and sockets. DC appliances are also harder to find, typically much smaller than those typically used in a home, and less reliable.
If you are thinking about installing an off-grid system in a home or business, your best bet is an AC system – unless your home is extremely small and your needs are few.
Pros and Cons of Off-Grid Systems.
Off-grid systems offer many benefits, including total emancipation from the electric utility. They provide a high degree of energy independence that many people long for. You become your own utility, responsible for all of your energy production. In addition, if designed and operated correctly, they’ll provide energy day in and day out for many years. Off-grid systems also provide freedom from occasional power failures.
Off-grid systems do have some downsides. One of the most significant is that they are the most expensive of the three renewable energy system options. Battery banks, supplemental wind systems, and generators add substantially to the cost – often 60% more. They also require more wiring. In addition, you will need space to house battery banks and generators. Although cost is usually a major downside, there are times when off-grid systems cost the same or less than grid-connected systems -- for example, if a home or business is located more than a few tenths of a mile from the electric grid. Under such circumstances, it can cost more to run electric lines to a home than to install an off-grid system. When installing an off-grid system, remember that you become the local power company and your independence comes at a cost to you. Also, although you may be “independent” from the utility, you will need to buy a gen-set and fuel, both from large corporations. Gen-sets cost money to maintain and operate. You will be dependent on your own ability to repair your power system when something fails.
It also comes at a cost to the environment. Gen-sets produce air and noise pollution. Lead-acid batteries are far from environmentally benign. As noted earlier, although virtually all lead-acid batteries are recycled, battery production is responsible for considerable environmental degradation. Mining and refining the lead, as noted earlier, are fairly damaging. Thanks to NAFTA and the global economy, lead production and battery recycling are being carried out in many poor countries with lax or nonexistent environmental policies. They are responsible for some of the most egregious pollution and health problems facing poorer nations across the globe, according to small wind energy expert Mick Sagrillo. So, think carefully before you go decide to install an off-grid system.
All three solar systems can be designed to include one or more additional renewable energy sources. The result is a hybrid renewable energy system.
Hybrid renewable energy systems are extremely popular among homeowners in rural areas. Solar electricity and wind are a marriage made in heaven in many parts of the world. Why?
In most locations, solar energy and winds vary throughout the year. Solar radiation striking the Earth tends to be highest in the spring, summer, and early fall. Winds tend to be strongest in the late fall, winter, and early spring.
The accompanying graph shows that sunlight is relatively abundant in central Missouri from March through October. It also shows that winds, however, pick up in October and blow through May. Together, solar electricity and wind can provide 100% of a family’s or business’s electrical energy needs.
In areas with a sufficient solar and wind resource, a properly sized hybrid PV/wind system can not only provide 100% of your electricity, it may eliminate the need for a backup generator. Because wind and PVs complement each another, you can install a smaller solar electric array and a smaller wind generator than if either were the sole source of electricity.
Hybrid systems may also make sense for those installing grid-tied system with a buy-sell tariff with a low sell price; a hybrid system reduces the amount of energy that must be bought (in net billing situations). For grid-tied systems with annual net metering or buy-sell with a sell price equal to or higher than the buy price, reducing seasonal variation in production has no advantages. The best strategy is to make as much electricity as you can at the lowest cost. In this case, only one technology makes economic sense, the one with the lowest cost.
If the combined solar and wind resource is not sufficient throughout the year or the system is undersized, a hybrid system will require a backup generator -- a gen-set to supply electricity during periods of low wind and low sunshine. Gen-sets are also used to maintain batteries in peak condition and permit use of a smaller battery bank.
Choosing a PV System
To sum things up, homeowners and businesses have three basic choices when it comes to installing a PV system. If they have access to the electric grid, they can install a batteryless grid-connected system, by far the cheapest and simplest option. Or they can install a grid-connected system with battery backup. If they don’t have access to the grid, they can install an off-grid system. All of these systems can combine two or more sources of electricity, creating hybrid systems.
If you are building a home close to a utility line, a grid-connected system is often a good choice. This system will allow you to use the grid to store excess electricity. Although you may encounter occasional power outages, in most places, these are rare and transient occurrences.
If you are installing a PV system on an existing home or business that is already connected to the grid, it is generally best to stay connected. Use the grid as your battery bank. Grid-connected systems with battery banks are suitable for those who want to stay connected to the grid, but also want to protect themselves from occasional blackouts. They’ll cost more, but they provide peace of mind and security. It’s best to back up only the truly critical loads and make sure they are highly efficient. Doing so will reduce the size of the battery required to meet the loads, reduce the cost of the system, and improve the efficiency of the system. Some loads, like a forced air furnace, can be quite a challenge to back up. Not only is a furnace blower one of the larger loads in a home, it also runs during the time of the year with the least amount of sunlight.
Although more expensive than grid-connected systems, off-grid systems are often the system of choice for customers in remote rural locations. When building a new home in a rural location, grid connection can be pricey – so pricey that an off-grid system makes good sense. Some utility companies, however, pay for for line extension, connection, and metering. Be sure to check, when considering which system you should install. “Utility policies vary considerably when it comes to line extension costs,” notes NREL’s Jim Green. “Sometimes, the utility absorbs much of the cost in the rate base. Others pass most or all of the cost to the new customer.”
To learn more, you may want to check out a copy of my newest book, Power from the Sun.