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Solar panels, Grid-Tie / Off-Grid Battery-Based Inverters

Posted by Solar Gear Supply

PV Modules – Solar Panels

Solar photovoltaic modules, often referred to as solar panels, convert light energy into a direct electrical current (DC). As solid-state devices, solar modules have no moving parts and are extremely reliable and durable compared to any other generator technology. While solar modules have become somewhat commoditized in recent years, there are important differences in form, quality, and performance that can impact both installation time and long-term system performance. We offer a selection of high-quality crystalline modules with a variety of features and price points to suit virtually any project.

Output Characteristics

The output power, voltage, and current profile of the solar module will dictate the number of modules needed and what inverters or charge controllers can be used. Small off-grid applications often require 12 VDC output modules to directly charge batteries and/ or operate DC loads. Larger modules with output voltages ranging from 24 to 50 VDC are more commonly used in grid-tie systems where a high DC voltage is required to operate the inverter.

Mechanical Characteristics

Basic mechanical characteristics, such as dimensions, frame profile, and static load rating, as well as grounding and mounting locations will need to be understood when designing your system. Frame and back sheet color may also come into play for residential customers, particularly when they are part of a homeowner’s association. Also be sure you know what type of connector the module output has, if any, since this can impact selection of optimizers, micro-inverters, and cabling.

Grid-Tie Inverters

Often referred to as the “brains” of a renewable energy system, an inverter is an electronic device that converts direct current (DC) from batteries or solar modules into alternating current (AC) at the voltage and frequency required to run electrical loads or feed into the grid.

Grid-tie, or utility intertie, inverters convert DC power from photovoltaic (PV) modules directly into AC power to be fed into the utility grid. Storage batteries are not needed, as any power produced that is not consumed by the owner’s electrical loads is fed into the utility grid to be used elsewhere. Due to the high voltages involved, grid-tie inverters should be installed and serviced only by qualified personnel.

All grid-tie PV systems use the utility grid for energy storage. Whenever the PV array is generating more power than the loads are using, excess energy is fed into the grid, turning the meter backward. When the loads require more power than the PV array can supply, the utility makes up the difference. Known as “net metering,” this arrangement is the most efficient and cost-effective for grid-tied applications since there are no batteries to maintain. However, most grid-tie inverters are required by law to shut down during a utility outage per IEEE 1547, which is incorporated into UL 1741. Battery-based grid interactive inverters (See Battery-Based Inverters) are required for back-up power applications.

Most batteryless grid-tie inverters are called “string” inverters because the PV modules must be wired together in series to obtain a higher input voltage. String Inverters are designed to run at voltages up to 600 VDC in residential systems and up to 1,000 VDC for commercial and industrial systems. String wiring is quick and easy to install, and the higher voltage helps to minimize line losses and required wire size. However, in string wiring, maximum power point tracking (MPPT), along with any monitoring output, is performed at the string or array level.

An important trend to note is that most string inverter manufacturers have introduced Transformerless (aka non-isolated) inverters due to the higher efficiency and lower manufacturing costs of that topology. While transformerless inverters have dominated the European market and are arguably even safer than their isolated counterparts, they do impose special “ungrounded system” requirements according to NEC 690.35. This includes the use of PV-Wire for DC connections, including the module outputs, as well as fusing and switching on both output legs. The term ungrounded should not be confused with the equipment ground, which is still required; it means that neither the positive nor negative outputs are connected to ground. Some jurisdictions will also require special circuit labels noting that both conductors are “hot”.

Module Optimizers

Module Optimizers can be deployed behind each module to provide individual module-level MPPT tracking and monitoring, optimizing the DC output that is connected to a string inverter for very high efficiency. Systems that combine optimizers with low-cost high-efficiency string inverters can simplify system design and maximize safety and energy harvest with minimal impact on cost.

Microinverters

Microinverters are typically mounted behind each solar module. They convert the DC output of each module to AC, replacing the high DC voltages (up to 1,000 VDC) with comparatively lower AC potentials (240 VAC or less) and simplifying system design. The microinverter output connects directly to the breakers in the AC load center using conventional wiring. Since microinverters provide MPPT tracking and monitoring for individual modules, the impact of differences in orientation or shading between modules is reduced. Microinverters are a popular solution for electrical contractors that are new to solar as DC wiring is essentially eliminated.

Three-Phase Inverters

Three-Phase Inverters are used in larger commercial grid-tie systems, and output at 480 VAC, which is more common in larger buildings. Many of these 10 to 50 kW inverters are available with input volt- age ratings of 1,000 VDC. This higher input voltage enables longer module strings, which can improve design flexibility and eliminate external combiners. Traditional Central Inverters are rarely used anymore for systems under several megawatts in scale.

Battery-Based Inverters

A battery-based inverter converts direct current (DC) from batteries into alternating current (AC) at the appropriate voltage and frequency to operate lights, appliances or anything else that normally operates on electricity supplied by the utility grid. All battery-based inverters can be used in off-grid systems and some can also feed power back into the utility grid using net metering, similar to the more common grid-tie inverters. All of these battery-based inverters require a battery bank to function.

Grid-Interactive Inverters for Backup Power Applications

Grid-interactive inverters, also called dual-function or hybrid inverters, can export power to the utility grid, but can also supply backup power to protected loads during a grid outage. These inverters use a battery bank for energy storage, will not operate without batteries, and include an automatic transfer switch that enables them to safely operate off-grid during a blackout.

The grid-interactive inverter is connected to the battery bank (usually 24 or 48 VDC), an AC sub-panel for protected loads, and the building’s utility entrance load center. The battery bank is charged by the PV array connected through a charge controller or through the battery inverter via AC coupling. Under normal conditions, it will export surplus power produced by the PV array. During a grid outage, the inverter will automatically disconnect from the grid and supply AC power to the protected load subpanel by drawing energy from the battery bank and solar array. When the outage is over, the inverter will automatically switch back to grid-tie operation and recharge the batteries.

It is important to note that a significant amount of energy is used to maintain the battery bank. For this reason, systems with battery backup typically provide 5 to 10% less energy (kWh) per kW of PV array than equivalent grid-tie systems that don’t include batteries.

Off-Grid Inverters

Off-grid battery-based inverters convert DC electricity from a battery bank to AC. In this case, the PV array and/or wind generator is used to charge the batteries via a charge controller and only the power demanded by the loads is inverted to AC. Because these systems do not have access to the electrical grid, it is important to properly size the inverter and battery bank.

The nameplate capacity of an inverter is measured by its maximum continuous output in watts. The inverter capacity limits the sum of all AC loads you can operate simultaneously. Most AC appliances list their consumption on a tag located near the power cord and/or in the owner’s manual. You will need to add up the consumption of all the appliances you may need to operate at once – that will represent your minimum inverter size. If your appliances include induction motors, like washers, dryers, dishwashers, furnace electronic controls, and large power tools, be sure to select an inverter with sufficient surge capability to accommodate the higher start-up loads.

Off-grid inverters will output either sine wave or modified sine wave (modified square wave) AC waveforms. Sine wave inverters can closely mimic utility grid power and will run virtually any AC appliance. Sine wave inverters with cleaner waveforms, such as the Exeltech XP line, are often desired for sensitive audio or telecommunications equipment.

Modified sine wave inverters are an economical choice when waveform is not critical. They often have a high surge capacity for motor starting and generally retain good efficiency when partially loaded. Unfortunately, this type of inverter may damage or fail to operate some sensitive appliances, such as rechargeable tools and flashlights, laser printers, copiers, variable speed drives, and any equipment with silicon controlled rectifiers (SCRs). Some audio equipment will have a background buzz when operated with a modified sine wave inverter.

Output Voltage

In the past, most battery-based inverters supplied only 120 VAC 60 Hz single-phase outputs. Now, many of the more popular residential-sized inverters, like the OutBack Radian, Schneider XW, and Magnum MS-PAE inverters, deliver 120/240 VAC power from one inverter. These inverters can also be wired in parallel for greater power output. Pairs of some 120 VAC output inverters like the OutBack FX series and Sunny Island inverters can also be wired in series for 120/240 VAC split-phase, or 120/208 VAC three-phase output.

Inverters that supply 50 Hz power are also available for most product lines. Please contact us with any special requirements you may have.

Interference

Battery-based inverters may interfere with radio and television reception, causing noise on telephones or buzz in audio equipment. Interference can be minimized by using sine wave inverters and by locating the inverter as close to the batteries as practical, twisting together the cables that connect the inverter to the battery, running AC lines separate from other wiring (such as telephone wires), and locating the inverter away from appliances that are susceptible to interference. All inverters can cause interference with AM radio reception.

Wiring Considerations

Battery-based inverters require high current from a battery bank to operate large loads. A 2 kW inverter will draw nearly 200 A from a 12 VDC battery bank. Large cables and good connections are required for safe operation. Use caution when plugging a small inverter into a lighter outlet in a vehicle, as these outlets are usually not robust enough to handle high current for long periods of time. All battery-based inverters require proper fusing between the battery and the inverter.

Pre-wired power systems are available with most battery-based inverters to minimize design and wiring issues. Custom configurations are available for most OutBack FLEXware-based power systems. Please contact us for additional information.


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