SOLAR RACKING AND MOUNTING - BATTERIES AND STORAGE - CHARGE CONTROLLERS - SYSTEM MONITORING

RACKING & MOUNTING

Mounting structures are used to fix solar panels (PV modules) to the roof or to the ground so they aren’t moved by wind or snow. Be sure to consult the solar panel manufacturer’s installation manual when selecting and configuring a mounting system as not all modules are compatible with all mounting methods. If the manufacturer doesn’t explicitly allow for the type of clamp and mounting locations or grounding method used by the mounting system, it may not pass inspection.

Most modules can be fastened via holes in the bottom flange of the frame but this can be awkward and time consuming. Some ground-mounting systems fasten to the bottom flange using specialized clips, enabling installers to perform virtually all of their work underneath the modules. Top clamps, which clamp the module frame to a mounting rail or roof attachment, are most popular today as the clamps can double as spacers and clamp two modules simultaneously, reducing the total number of fasteners required. Regardless of clamp type, it is also important to clamp the module in the right places.

Most PV modules are designed to be clamped at the quarter-points where the mounting holes typically are. This ensures optimal loading on the module frame and provides maximum static and dynamic load capacity. Some manufacturers also allow for mounting on the short ends of the module, which can allow two rows of modules to share a rail. However, mounting on the ends typically reduces the load ratings of the module, which is why most manufacturers don’t allow it.

Early equipment grounding for module frames was accomplished with a bolt or screw with a star washer attached to the grounding wire. Our main supplier later introduced grounding lugs which provided a faster and more secure method for attaching the ground wire and these have since given way to WEEB clips which enabled module grounding through the rail. With the advent of the UL 2703 mounting and grounding standard, many mounting systems, such as SnapNrack, accomplish grounding through the mounting components so that the installer only needs to run a grounding wire to the end of each row.

Note that specialized PV products like frameless or flexible modules typically require their own proprietary mounting and grounding components.

SOLAR BATTERIES AND STORAGE

Batteries

Virtually all power generation systems require some form of energy storage. For grid-tied systems, the utility accepts surplus power and gives it back when needed. A battery bank is required for systems that need to function without the grid, either all of the time or during an outage. In these systems, the solar array or wind turbine charges the batteries whenever they are producing power, and the batteries supply power whenever it is needed.

BATTERY TECHNOLOGIES

The most common battery technology used is lead-acid, in which lead plates are used with a sulfuric acid electrolyte. The electrolyte can be fluid or absorbed in fiberglass mats (AGM), or gelled. AGM and gel batteries are together known as VRLA (Valve Regulated Lead Acid) and are sealed, do not require water addition, and do not emit gases when operated within specifications. Lead-acid batteries are relatively inexpensive and readily available compared to other battery types. New advanced lead acid batteries have carbon additives in the negative plate to prevent sulfation at partial states of charge (PSoC), and are still much less expensive than high technology batteries.

Lithium Ion batteries can handle large charging and load currents. They are also lighter weight and compact for their power and energy capacity. One advantage of Lithium Ion (Li-Ion) batteries is their long life even when cycled heavily, and without needing to be brought to a full state of charge each cycle. This makes them particularly suitable for short to long duration use in self-consumption systems where net metering is unavailable or utility rate structures otherwise disincentivize energy exports during peak solar production hours.

Aqueous hybrid sodium-ion batteries have significant safety and environmental advantages over traditional batteries. They are made from non-toxic materials and have an aqueous electrolyte that is non-flammable. They have the ability to cycle for many years at any state of charge, making them suitable for systems that need to take advantage of charging when available, and do not need to be fully charged like lead batteries. They are ideal for long duration applications such as off-grid systems, or larger capacity self-consumption systems. These batteries are very robust, but are similar in size and weight to lead-acid batteries and must be sized carefully to ensure appropriate current for loads or charging.

STANDBY OR CYCLING BATTERIES

Batteries come in a wide variety of sizes and types, but the most important designation is whether they are made for daily cycle service or standby service. Automobile starting batteries should not be used for renewable energy systems.

Standby power batteries are designed to supply power to loads for occasional use, and are preferred for grid-tied solar systems with battery backup. They are optimized to supply moderate to large amounts of power only during utility power outages, and float at full charge most of the time. They are designed to use a minimal amount of energy to stay fully charged. They are not made for frequent deep discharges and have a limited cycle life but often very long calendar life when kept in float conditions. AGM batteries are most common for standby power applications as they are less expensive, have low self-discharge and require little to no manual maintenance.

Deep cycle batteries, are designed to be repeatedly discharged by as much as 80% of their capacity and are therefore a better choice for off-grid PV systems. Even when designed to withstand deep cycling, most batteries will have a longer life if the cycles are kept shallower. Deep cycle batteries can be either flooded or sealed lead acid variants or, increasingly, newer chemistries like lithium-ion or sodium-ion.

CARING FOR BATTERIES

Maintenance requirements vary by battery chemistry and configuration. Additionally, some maintenance tasks, such as adding water or equalization, require on-site manual operations and/or oversight, while charge regulation, voltage checks and related measurements can be automated via sophisticated charge controllers or battery management systems, which are a de facto requirement for lithium-ion batteries.

Sealed lead-acid batteries, gel cells and AGM (Absorbed Glass Mat), are often referred to as maintenance-free because they don’t require watering or an equalization charge. This makes them well-suited for remote or unattended power systems. However, sealed batteries require accurate regulation to prevent overcharge and over-discharge.

Lead-acid batteries should always be recharged as soon as possible. The positive plates change from lead oxide, when charged, to lead sulfate, when discharged. The longer they remain in the lead sulfate state, the more of the plate remains lead sulfate when the battery is recharged. The portion of the plates that become “sulfated” can no longer store energy. Batteries that are deeply discharged and then only partially charged on a regular basis often fail in less than one year. The new lead-carbon batteries substantially reduce sulfation. Always use temperature compensation when charging batteries to prevent over or under-charging. NOTE: Battery warranties do NOT cover damage due to poor maintenance or loss of capacity from sulfation.

Check the electrolyte level in wet-cell, or “flooded” batteries, at least once every 3 months and top-off each cell with distilled water. Do not add water to discharged batteries! Electrolyte is absorbed when batteries are discharged, so if you add water at this time and then recharge the battery, electrolyte will overflow and create a safety hazard. Keep the tops of your batteries clean and check that cables are tight. Do not tighten or remove cables while charging or soon after charging! Any spark around batteries can cause a hydrogen explosion inside the case and potentially ignite a fire or an even larger explosion if the batteries are not properly vented. Use a hydrometer to check the specific gravity of your flooded lead-acid batteries. If batteries are cycled very deeply and then recharged slowly, the specific gravity reading will be lower because of incomplete mixing of electrolyte. An equalizing charge will help mix the electrolyte.

An “equalization” charge should be performed on flooded batteries whenever cells show a variation of 0.05 or more in specific gravity from each other. This is a long steady overcharge, bringing the battery to a gassing or bubbling state. Do not equalize sealed or gel-type batteries! With proper care, lead-acid batteries will have a long service life and work very well in almost any power system.

Always use extreme caution when handling batteries and electrolyte (sulfuric acid). Wear appropriate personal protective equipment, including electrical- and chemical-resistant gloves with sleeves, goggles, and acid-resistant clothing. “Battery acid” will instantly burn skin and eyes and destroy cotton and wool clothing. Similar precautions apply to other battery types – always read and adhere to manufacturer safety recommendations when handling batteries. For any type of battery, be sure to remove any metal jewelry and avoid shorting the battery terminals.

BATTERY STATE-OF-CHARGE

Battery state-of-charge (SOC) can be measured by an amp-hour meter, voltage, or by specific gravity. Some care and knowledge is required to interpret state-of-charge from voltage or specific gravity readings. We recommend amp-hour meters for all systems with batteries. An amp-hour meter is like a fuel gauge for batteries and provides all the information needed to keep batteries charged. At a glance, the user can see system voltage, current, and battery condition.

Battery voltage will vary for the same state-of-charge depending on whether the battery is being charged or discharged, and what the current is in relation to the size of the battery. The table below shows typical battery voltages at each state-of-charge for various battery conditions in flooded lead-acid batteries. Voltage varies with temperature. While charging, a lower temperature will increase battery voltage. Full-charge voltage on a 12 VDC battery is 0.9 VDC higher at 32 °F than at 70 °F. While discharging, a higher temperature will increase battery voltage. There is little temperature effect while a battery is idle, though higher temperatures will increase the self-discharge rate.

Source: Ralph Heisey of Bogart Engineering.

A hydrometer is very accurate at measuring battery state-of-charge in flooded lead-acid batteries if you measure the electrolyte near the plates. Unfortunately, you can only measure the electrolyte at the top of the battery, which is not always near the plates. When a battery is being charged or discharged, a chemical reaction takes place at the border between the lead plates and the electrolyte. The electrolyte changes from water to sulfuric acid while charging. The acid becomes stronger, increasing the specific gravity, as the battery charges. Near the end of the charging cycle, gas bubbles rising through the acid stir the fluid. It takes several hours for the electrolyte to mix so that you get an accurate reading at the top of the battery. Always try to take readings after the battery has been idle or slowly discharging for some time.

This table shows the battery state-of-charge corresponding to various specific gravities for a battery bank in an ambient temperature of 75 °F. Some batteries will have a different specific gravity density by design so check with the manufacturer.

SOLAR CHARGE CONTROLLERS

Charge Controllers

A charge controller is used to keep the voltage across the battery within acceptable limits. The charge controller automatically tapers, stops, or diverts power when batteries become fully charged. Charge controller capacities range from 4 A to 100 A and multiple charge controllers can be used in parallel for larger systems. Some charge controllers offer additional features including charge status display, data logging, automatic battery equalization charging, generator starting, and even lighting controls.

The simplest charge controllers disconnect the power source when the battery reaches a set voltage, and turn it on when a low voltage set point is reached. Pulse Width Modulated (PWM) charge controllers turn on and off very rapidly, maintaining the batteries at full charge voltage, which results in quicker and more complete battery charging. Maximum Power Point Tracking (MPPT) charge controllers optimize the voltage of the PV array to maximize total power output then convert that to the correct voltage to charge the battery. This process significantly increases the power from a solar array, particularly in low temperatures when battery voltage is significantly below the PV array voltage. Most MPPT charge controllers work with higher array voltages, which can greatly reduce the required wire size between the array and the charge controller. While more expensive than PWM controllers, MPPT charge controllers can boost system performance by up to 30% making them very cost effective.

MPPT CHARGE CONTROLLERS

The table below shows recommended maximum nameplate PV array sizes. The wattages shown can be exceeded by up to 20% without damaging the controller, but some “clipping” of potential peak current may occur under cool, clear conditions at the peak of the day. While exceeding these wattages may reduce power harvest at peak times of the day, the total daily amp-hours delivered to the battery bank will be greater because the larger array will produce more power in less-than-peak conditions such as mornings, afternoons, and in hazy or cloudy weather.

PWM Charge Controllers

It is important to note that PWM charge controllers have limited voltage correction capabilities and should only be used with 36 or 72-cell modules in series or parallel to match the battery voltage.

SOLAR SYSTEM MONITORING

BATTERY METERS

Battery-capacity meters serve as a fuel gauge for a battery bank and are an important part of any battery system, both to ensure usability and to properly maintain the battery bank. Simple battery-capacity meters read the voltage across the battery bank and determine a state of charge accordingly. More sophisticated monitoring systems also use a DC shunt to monitor charge and discharge amp-hours. In both cases, it is important that they be installed and calibrated according to manufacturer’s instructions to ensure accuracy.

AC KILOWATT-HOUR METERS

These meters are like those found near a typical residential or commercial utility-service entrance and can be used to meet financing or incentive-program requirements for PV-system output metering. Be sure to verify which meters are approved by the financing provider or incentive program you are using.

GRID-TIE SYSTEM MONITORING

As grid-tied solar PV systems become more popular, online monitoring is playing an increasingly important role in both residential and commercial systems. Most commercial PPA and residential leasing financiers require revenue-grade monitoring to be coupled with online reporting tools. Many incentive programs, particularly performance-based and renewable energy credit-based ones, also require accurate real-time monitoring and some form of automated reporting. Many commercial and residential customers want something they can point to when bragging about their solar PV system and an online monitoring system with a smart-phone app fits the bill nicely. Savvy installers are also finding that online monitoring enables them to be proactive in managing their brand and often pair a monitoring system with a service agreement that includes periodic cleaning and maintenance of the system. Whatever the motive, a good online monitoring system can help reinforce the value of a solar PV system for years after installation.