Considerations for Off-Grid PV Systems

2 inverter w. 2 ISM w. 1 T80지상에 장착 된 PV 어레이

2 inverter, 1 T80

There are many reasons for choosing an off-grid PV system to power a remote home or cabin. Some people want to avoid the high cost of extending a utility line, while others like the independence of homemade energy production, as well as having a silent, emission-free energy source with a over 5-year warranty.

Off-grid (or “stand-alone”) PV systems are very different than batteryless grid-tied systems. Without the utility as a supplemental electricity source, a PV system’s sizing is critical. Off-grid systems require their owners’ participation—this means living within the original design’s energy budget, planning for future growth, and having a backup energy source for times of high energy usage or low solar production. All maintenance and equipment servicing is also done on-site, at the homeowner’s expense, and by the homeowner or installer—instead of by a power company.

Loads

Carefully considering the appliances—or loads—in a home is crucial in off-grid system design. The first step is to list the power requirements of every desired appliance and determine the average daily hours each will be used. A load analysis calculates the energy consumed by each appliance, with the ultimate goal of determining the total average daily energy consumed by all loads in the home. This daily consumption value is then used to design a battery bank large enough to store that energy each day and a PV array large enough to produce the energy.

Other considerations include whether the appliances will use alternating current (AC) or direct current (DC). All off-the-shelf appliances that can be plugged into a standard wall outlet are AC. For off-grid homes with full-time occupancy, the benefits of AC appliances typically outweigh the benefits of DC appliances. Conventional appliances are readily available, and they run at higher voltages, so you can use smaller, standard AC wiring in your household. In certain applications, such as a system for a small cabin, an RV, or a boat, the greater efficiency created by eliminating the inverter can justify more expensive, harder-to-source DC appliances. For example, a PV system for a boat may run 12 VDC lights, a radio, a TV, and a refrigerator directly from the battery to avoid the need for an inverter.

Certain loads need special consideration because of their high energy use, including space heaters and coolers, water pumps, refrigerators, water heaters, and cook stoves. For these applications, it is best to first determine if there are non-electric methods of accomplishing the same task, such as drying clothes on a line instead of using an electric clothes dryer. If an electrical appliance is still going to be used, consider ways to reduce the demand for the load, and then buy the most efficient appliance that will serve that need. As an example, correct window placement and properly sized overhangs can help reduce cooling loads, as will high-performance windows and well-insulated walls. After exhausting all non-electrical means of cooling, using evaporative coolers (in arid regions) or low-energy ceiling fans are good options instead of using a compressor-type air conditioner. If an air conditioner is used, consider cooling only a portion of the home.

Off-grid consumers need to be aware of their energy allowance and shop carefully for efficient appliances. Many appliances, having large power draws and standby features, can be large energy users. The U.S. Department of Energy’s Energy Star website (energystar.gov) is a good place to research the most efficient appliances—however, even within Energy Star-rated appliance categories there still are wide variances in energy consumption. For instance, a sample LG Electronics’s 42-inch plasma TV energy consumption is estimated at 140 kWh per year, compared to an equivalently sized LCD model which ranges from 83 to 152 kWh per year. Similarly, comparing refrigerators from Whirlpool demonstrates that a side-by-side model uses about 30% more energy than a refrigerator with the freezer on the top.

PV Modules

When choosing PV modules for your off-grid system, it is important to look at the price, the technology, how they attach to the roof or rack, the voltage and current specifications, the UL listing, and the warranty. The most typical type of module (240 W, aluminum-framed, with quick-connect positive and negative wires) has an output voltage that integrates easily with arrays configured for grid-tied inverters. The off-grid market can also take advantage of these common modules by using a maximum power point tracking (MPPT) charge controller that can step down array voltage to the lower voltage of a battery bank. There are still some modules on the market that are a nominal 12 V (or 24 V), made to directly charge a 12, 24, or 48 V battery bank through a non step-down charge controller, but they are becoming harder to find and are typically more expensive.

Proper array sizing is crucial in off-grid design. It ensures that the loads you need to run will have energy and that the battery can be fully recharged after a period of no sun. To size an array, you’ll need to know the modules’ STC watts, the average daily peak sun-hours in the worst month, and the amount of energy the loads consume. An array needs to produce as much as the average daily loads consume (plus efficiency losses) and be able to recharge the batteries and “catch up” after cloudy periods.  Whenever possible, oversize the array to account for inclement weather. Most designers also specify a generator to accommodate for long stretches of low (or no) sun, which then removes the need to oversize the array further.

PV Mounts

Because off-grid home sites often have more room than city lots, there are usually more locations for array placement beyond a home or garage roof. Ground mounts, pole mounts, shed or barn roofs, and solar trackers are options. A pole- or ground-mount system allows the array to be adjusted seasonally, and this additional energy production can reduce a backup generator’s run time (reducing fuel use and maintenance) during winter. For example, adjusting a 3,000 W array in Seoul, from a fixed latitude tilt to a steeper tilt will gain 0.5 daily peak sun-hours during the winter. That additional energy calculates to be about 1,500 Wh per day (3,000 W x 0.5 hours/day).

For the greatest energy harvest from an array, a solar tracking mount can be used, so long as a clear solar “window”  is present (dawn-to-dusk solar access is ideal). With decreasing module prices, however, the additional cost of the tracker plus the introduction of moving parts to an otherwise non-mechanical system makes this option harder to justify. Often, it’s a better investment to increase the array size to increase the system’s year-round output.

Charge Controllers

A charge controller’s primary function is to prevent the batteries from overcharging. Charge controllers monitor the battery voltage—when the batteries are fully charged, they disconnect the charging source (in this case, the PV array) from the battery until it is next needed. Some smaller controllers also have an additional feature that prevents overdischarging from DC loads.

When choosing a residential-sized charge controller, first evaluate whether MPPT, which helps maximize the energy harvest from the array, is needed. MPPT controllers continually track array output—during shifting temperatures and irradiance levels—to optimize the amount of energy sent to the battery. The additional cost is justifiable in nearly all larger systems, since it yields between 10% and 25% more energy. MPPT charge controllers also have a “voltage step-down,” so they can convert high array voltages (up to 600 VDC) to lower battery voltages (typically 24 or 48 VDC). This allows more modules to be wired in series and the use of smaller-gauge (and less expensive) wire from the modules to the controller. Having a large difference in voltage between the array and the battery decreases a charge controller’s efficiency, but the benefits of being able to place the array farther away from the battery bank, reducing the wire size, and having smaller overcurrent protection devices, can be worth it. Non-MPPT controllers still hold a large market share, but generally make sense only in smaller system applications, such as for boats, lighting, RVs, and small cabins. Additional information needs to be considered in choosing a controller, including monitoring requirements, temperature compensation, voltage and current specifications, and the size of the array-to-battery voltage step-down window.

Inverters

Using AC appliances in an off-grid home requires an inverter to convert the PV array and battery bank’s DC electricity to the AC electricity needed. When choosing an inverter, one should consider options such as metering, programming flexibility, type of waveform, idle power draw, generator backup, surge capability, and overall power needed by the loads.

Sizing an inverter requires adding up all the power needs of the appliances that will be on simultaneously. Some loads with motors, like a refrigerator’s compressor, also require a boost of power to start, called a surge, which is typically two to seven times larger than the appliance’s normal operating power needs—continuous power and surge power need to be considered. Choosing an inverter with a higher power rating may be important if loads increase or there may be future system expansion. Inverters also use some energy in standby mode (waiting for any load to run), so choosing an inverter with a low “idle power draw” is important.

AC appliances require a sine wave that alternates between positive and negative voltages 60 times a second. Inverters take DC current and create this AC sine wave with varying degrees of accuracy. Modified square-wave inverters create a rudimentary waveform that can run most appliances but has trouble with more sophisticated electronics, such as dimmers and computers. A true sine wave will run most equipment, especially motor-based, more efficiently—which translates to more useful energy from the system. Thus, many off-grid homeowners choose true sine-wave inverters, which can run all electronic appliances without a problem.

Most home appliances require 120 V to operate. Other loads, like well pumps or some shop tools, require 240 V. In this case, you’ll need to choose a single inverter that provides 120/240 V, or use two inverters that each produce 120 V but can be connected (or “stacked”) to provide 240 V. Alternatively, a transformer can be used between a 120 V inverter and a 240 V load to step up the voltage when needed.

Off-grid PV systems commonly use a generator for backup. If you’ll be using a generator, you’ll need an inverter with a large battery charger to change the generator’s AC electricity to DC for the batteries, and run any AC loads. Most higher-end inverters offer programming options for interfacing with generators, such as automatic start and stop, pre-set quiet hours, and maximum charge amps. Additionally, metering allows users to see how much charge is going from the generator to the batteries, how much energy is leaving the battery to the loads, and the battery’s state of charge.

Batteries

Battery considerations include the technology type, cost, preferred system voltage, ambient temperature, maintenance requirements, and battery location. Nearly all home battery banks are deep-cycle lead-acid, which can handle being regularly and deeply discharged (up to 80%, but 20% to 50% depth of discharge will significantly increase longevity). For lead-acid, the first decision is whether to use a flooded battery that requires regularly adding distilled water, or a sealed valve-regulated lead-acid (VRLA) battery that does not require watering. Sealed batteries are more expensive and have a shorter life than equally sized flooded batteries, but this trade-off can be worth it in cases where the battery maintenance cannot or will not be done.

Modern off-grid battery banks are typically 24 or 48 V, which allows the use of smaller-gauge wire than 12 V systems, which have higher current for the same power level. Choosing a higher-voltage battery also means wiring more batteries in series to increase the voltage, thereby reducing the number of parallel battery strings required for the same energy available. This, in turn, helps reduce imbalances across the battery bank. If there are 12 V loads that need to be powered, a DC-to-DC converter can be used to supply the right voltage.

It is important to keep flooded batteries out of living spaces and all batteries should be protected from unauthorized access—as they contain caustic chemicals and pose shock and burn risk if not handled properly. Choosing a location with moderate temperatures (77˚F is ideal) is critical for battery longevity. For every 18°F increase in temperature the battery experiences, the number of available cycles drops by half. For example, if a battery is rated at 3,600 cycles at 77˚F (or approximately 10 years, at 1 cycle per day), it would then be expected to last 1,800 cycles, or about five years, if installed in a climate of 95˚F.  At lower temperatures, a battery will gain lifespan, but its available capacity will decrease.

Maintenance requirements for all batteries include keeping the terminals and tops of batteries free from corrosion, dirt, and debris. This helps keep electricity flowing equally through the entire battery bank. The battery should be charged to 100% on a weekly basis (and daily is better)— keeping batteries in a discharged state can decrease their life. The electrolyte level must never expose the lead plates to air. Flooded batteries also need to be equalized—a controlled overcharging that is commonly done every few months. Equalization helps to rebalance cell voltage and improves the health of the battery by mixing up the electrolyte, which can stratify over time. Properly venting explosive hydrogen gases (produced by charging lead-acid batteries) from a battery box to the outdoors is extremely important.  Passive venting can be accomplished by intake air vents installed at the bottom of a battery box combined with higher outlet vents at the top. This allows the lighter hydrogen gases to rise up and out. Active venting by a DC fan can also be used.

Sizing a battery bank starts with load analysis. The battery needs to store the amount of energy needed for the daily loads. If the loads need to work on some days where there is no sun available (but before the generator kicks in), then the battery bank needs to be larger—known as days of autonomy. Beyond that, batteries will yield more cycles when less is drawn out of them on a daily basis. For example, a battery that’s discharged 20% may provide 3,300 cycles; if discharged 80%, it may only provide 675 cycles.

Backup Generators

Backup power sources are usually included in off-grid PV systems for when it’s not sunny. Engine generators, which may run on gasoline, propane, or diesel-compatible fuel, are the most common backup source because they provide power on demand. Generators are used more during the shorter days of the winter and for periods of cloudy weather. They can be set up to auto-start if a compatible inverter/charger is used, though many off-gridders do not recommend it—generators should be checked for fluid and fuel before starting. Generators are also important for equalization charging, as it is difficult to get enough power and energy from a PV array to perform this function.

Generator options include fuel type, size (kW), and fuel storage. Noise, exhaust fumes, and access for maintenance influence generator location. Regular maintenance—checking the oil, changing filters, and tuning—is needed so the generator can be available when needed. A generator typically needs to be sized to both handle charging batteries and run the loads simultaneously. Inefficiencies due to high elevations and temperature, and the limitations of the charging capacity available in the inverter/charger, will also influence sizing.

Meters & Data Monitoring

Meters and data monitoring for an off-grid system are even more critical than for a grid-tied system, since you’ll need to make sure the batteries are reaching 100% full on a regular basis; track the trends of loads versus charging over time; and monitor the battery’s state of charge. Meters and monitoring also help you gain insight on future system needs—for example, if more modules are needed to keep up with household usage and to determine when the batteries need replacing.

Data monitoring can show trends about battery charging and load profiles and to help spot potential problems. Some inverters and controllers can log the daily energy consumption and production data, and the minimum and maximum battery state of charge.

User-Interactivity

Living off-grid requires much more interaction with the energy system than does living with a grid-tied system. In a grid-connected system, if there is no sun or if more energy gets used on one day than another, the grid is a (mostly) reliable backup source. In an off-grid system, the user has to strategize to ensure adequate energy to meet the loads—every day. Users with stand-alone systems have to monitor and adjust their energy consumption, watch weather patterns and time usage accordingly, and make sure there’s fuel for the backup generator. Additionally, troubleshooting is more difficult in this complex system—and the stakes are higher when the utility is not there as a backup.