The term “oversizing” in the context of solar energy systems involves sizing up your Solar Array with more power than the maximum operating power of the solar inverter or MPPT controller.  Why would you want to do this? Your PV array will often operate below its rated power because of the time of day or year, weather, climate, dust, pollution, and other factors. During these times, your system could produce less energy than you need. Oversizing can help you meet your daily energy consumption. 

For off-grid systems, an oversized PV array gives your system more autonomy and resilience. For example, on a less sunny day, although each module produces less power, there are more of them, helping your system gather more as a whole than with an array that matches the power rating of the controller. Ideally, the additional panels not only help meet the load and charging needs for the system, but it can allow the battery to stay in a higher state of charge more of the time and recover from a low state of charge faster without a loss of load.  

Depending on the location of the array and how it is mounted, the daily maximum power levels can typically be lower than the STC rated power of the solar array leaving some unused controller capacity. This is when oversizing the PV array can allow the controller to operate at full power and provide more consistent charging power throughout the day.  

Morningstar MPPT Controllers can accommodate an oversized PV array that is well above the Nominal Maximum Output Power rating of the controller. This will not damage the controller. However, the controller will limit output current to 100% of the rated current output and not higher. Please note that Morningstar PWM controllers cannot be oversized and the current rating of the array should never exceed the current rating of the PWM controller. 

Being able to use a larger array can provide more string sizing options. For example, if the array power with 5 solar panels is the same as the MPPT controller’s nominal power rating the only sizing options are 5 strings of 1 or 1 string of 5. This may not even meet the input voltage requirements for the controller. Being able to oversize the array with 6 panels makes it possible to use 3 strings of 2 or 2 strings of 3 and perfectly match the desired input voltage for the array. 

The alternative to oversizing the PV array would be to use a higher power controller or an additional controller at a higher cost and possibly additional wiring. 

Morningstar’s MPPT controllers support oversizing up to 200% over the controller’s Maximum Nominal Output Power rating. However, it is not a general practice to design systems with an array size of more than 25-35% of the power rating of the controller. 

The Morningstar String Calculator provides PV array sizing results for MPPT controllers that include the following oversized PV array information.

MPPT Power Indications

     Moderate Oversizing (~5-30% above Max. Nominal Solar Power) for this configuration.

      Significant Oversizing (>30% above Max. Nominal Solar Power) for this configuration.

Visualizing the difference 

SS-MPPT 15L (240W vs. 200W module) on a Clear Day 

This graph illustrates the output power levels of a SunSaver MPPT controller—comparing a 200W module to a 240W module—operating on a clear & sunny day with a potential peak input array power equal to the STC maximum power (Pmp). The output power is slightly less than the input power due to the controller’s efficiency. For a 200 W module, the max output power of the controller will remain below the max nominal output power rating of the controller. However, the 240 W PV Module exceeds that 200 W limit. With the 240 W Module, although you do lose the excess energy above 200W due to the solar controller capping, throughout the day, you have significant gains compared to the 200W PV module. The 240 W panel gathers more energy than the 200 W in the morning and afternoon when there is less solar energy available. This extra energy is pictured as the space between the 200W and the 240W Curve and below the max charging limit. 

The daily peak array power and maximum output power of the SunSaver controller for a system using these modules will vary greatly on a daily basis. 

Some Drawbacks of Oversizing

With research, you’d find some drawbacks to battery oversizing. Is oversizing right for you? 

• Because you are producing more energy than the system can manage, additional energy is “clipped” and is unused. Solar energy is not as expensive in today’s market, so this is less of a concern. 
• Selecting a charge controller that allows the required amount of oversizing (most tolerate 200%) is integral to an effective system, and selecting the wrong devices could result in a shortened lifespan or lessened efficacy.
• Your system setup and wiring become more complicated.  
• Initial system costs would be higher because of the additional PV Modules. Although PV modules have become cheaper in recent years, this could still be a factor.  

Many of the drawbacks of oversizing have been reduced in today’s market, including the cost of “clipped” energy and PV Modules. If you are in a climate with varying solar energy throughout the year, or excessive heat, oversizing could significantly improve your solar energy system’s performance. Oversizing is becoming a more popular solution because the price of solar panels and solar energy is lower than in the past. Because of the lower cost, you can produce more energy using your solar panels without losing a fortune in excess energy.

Oversizing Applications

The overall cost of the system is an important consideration when designing off-grid PV systems and oversizing the PV array with MPPT controllers can often cut costs significantly without affecting the performance of the system. We receive support requests for help with array sizing for a particular PV module and solar controller. With the option to oversize the array, it often means the customer can use the controller and module they want to use with very good results.

One example of an application where oversizing can make a big difference would be if you want to use a PS-MPPT-40 for a lighting system but the array power needs to be higher than the nominal power rating of the controller. Rather than purchasing an additional controller or finding a larger controller that also includes lighting control–which can be a challenge–oversizing the array with the PS-MPPT-40 is the optimal solution.

For larger systems that require several controllers, using fewer MPPT controllers with larger PV arrays not only saves on the controller cost but also on the cost of wiring and balance of system (BOS) components. In addition, it will reduce the installation time and take up less wall space or space inside an enclosure.  

Having the option to oversize the array provides the opportunity to size the PV array larger than it was originally intended to be or increase the size of the PV array at a later date if needed, without having to purchase and install additional solar controllers. Oversizing the array like this gives your system more consistency and reliability. Consistent and reliable energy is key for a variety of applications. 

Perhaps you use your solar energy system for a security or communications system that you need functioning at all times. Maybe you are designing a system for an off-grid home or rural electrification project that includes generator backup and want to reduce the runtime of the generator. Maybe it is for a warehouse or barn, prioritizing a reduced energy bill or consistent power in a remote location. No matter your target results, one of the primary keys to achieving them is consistent, reliable energy production. No one can control the sun, but with a larger PV array, your solar energy system can do more to maintain your basic energy needs. 

More consistent solar energy throughout the day helps maintain battery health by preventing consistent and deep battery drainage, which can negatively impact the battery lifecycle. Additionally, your MPPT Charge Controller operates at maximum power rating, getting to its full charging potential more often. Another factor that helps with system energy consistency is battery size. Oversizing, with the right battery size for your needs, creates system autonomy, capable of being your sole energy source even with little to no sunlight that day. 

Conclusion

With Morningstar’s MPPT controller oversizing capabilities, you have the flexibility to get more from the solar controller, have more solar array sizing options and save money on solar controllers and BOS costs. The additional energy production from an oversized PV array can provide more consistent energy production with more energy when it is most needed, when there is less solar energy available; on cloudy days, earlier and later in the day, and times of the year when there is less direct sunlight. 

Overall, oversizing should be considered as a viable, cost-cutting solution that can also enhance solar energy system performance and ensure a consistent power supply. 

For more information about PV Array oversizing with MPPT controllers, see Morningstar’s TrakStar MPPT Technology & Maximum Input Power technical document.

The post Oversizing Your PV Array with Morningstar MPPT Controllers appeared first on Morningstar Corporation.

Overcurrent Protection is implemented in software and is intended to protect the controller from exceeding the load current rating. Morningstar controllers continuously measure the load current and cause a software-triggered shutdown of the load terminals if it exceeds the nameplate current rating. An example of this would be a user sequentially turning on small loads until the total load current exceeded the controller’s rating.

Short Circuit Protection is implemented in hardware and is intended to protect against rapid increases in load current. Rapid increases in current can be caused by a sudden short circuit of the load terminals or from a startup surge when switching on a load with high capacitance. This hardware protection circuit is tuned to allow the highest amount of current possible (for as long as possible) without damage to the MOSFETs. Only when the current (or duration thereof) approaches a harmful level does the hardware protection activate to shut down the load terminals. Extensive destructive testing was conducted in order to determine the optimal tuning point for the protection circuit.

A more detailed description of these protections is available for the SunSaver Gen 3 Controller in the Load Overcurrent & Short Circuit Protection in the SunSaver Gen 3 Controller tech doc.  https://www.morningstarcorp.com/wp-content/uploads/technical-doc-load-overcurrent-short-circuit-protection-sunsaver-gen3-en.pdf

The protections for other Morningstar load control equipment have the same type of overcurrent and short circuit protections.

These protections may not protect the controller from damage under all circumstances and our product warranties will not cover systems that have been damaged by over-current. Also, the built-in overcurrent protection is not a substitute for external overcurrent protection for the load circuit. 

Startup surges can have inrush currents that are at least 3 times higher than the continuous current of the load. Pumps and DC-DC converters are examples of the type of loads that may have startup surges. When designing a system, one should determine the max startup surge for the device being powered. Some capacitive loads include a slow start feature which limits the startup surge. Inverters have extremely high DC input surges. Therefore, Morningstar’s DC load control should never be used with inverters.

Many inverters, like our SureSine inverter, and DC-DC converters include LVD load control and should be wired directly from the battery as shown in our Inverter wiring diagram. https://www.morningstarcorp.com/wp-content/uploads/tech-note-diagram-inverter-wiring-en.pdf 

The SureSine inverter and many inverters and DC-DC converters include remote on/off control terminals that can be used for implementing LVD. For example, below is a diagram for SureSine Remote LVD using a Relay Driver.

It is also possible to use external relays which can handle higher load currents and surges. It is important to select the proper relay or it can be damaged. Most electronic supply dealers provide a search tool that includes a filter in order to help identify relays that will meet the voltage and current requirements for your loads. 

The Relay Driver is designed to be used with relays. However, it is also possible to use a load controller to turn on and off external relays that the load controller can’t handle. We have a tech note which shows how to do this. https://www.morningstarcorp.com/wp-content/uploads/tech-note-diagram-tristar-load-control-with-inverter-en.pdf

HVD protection is provided for systems that have loads that may get damaged with higher voltages. Many 12V loads are rated to 15V and 24V loads are rated to 30V. Unlike Solar HVD, Load HVD is not temperature compensated. To avoid the loss of load during charging, the maximum regulation voltage of the controller needs to be lower than the Load HVD setting. The charge settings are temperature compensated so if there is a possibility that the charge voltage will get too high in cold conditions it is possible to set the max regulation voltage charge setting lower than the HVD voltage. This will limit how high the charge voltage can be.

The post Off-Grid PV System Load Control: Electronic Load Protections and Problematic Loads appeared first on Morningstar Corporation.

An important consideration regarding LVD/ LSOCD settings is the self-consumption of the solar charge controller(s), load controller(s) and any other devices in the system that are not disconnected when there is an LVD. After there is an LVD the self-consumption will continue to drain the battery. This is not much of a concern for higher LVD/ LSOCD settings or larger battery banks where there is a relatively low self-consumption. 

If LVD is set too low, it can reduce battery life and the additional self-consumption after LVD will only make it worse. This can occur when the LVD setting is too low and/or the self-consumption is relatively high in comparison to the battery bank. It is especially important that the LVD setting isn’t so low that the battery is over discharged overnight and negatively impacts the health of the battery. There are also circumstances that might lead to loss of solar production for longer periods. Conditions like snow or solar controller failure would be examples of such circumstances. If the battery drains below the minimum operating voltage of the solar controller, it will not be able to recover without voltage being applied onsite and thus likely cause permanent degradation or damage to the battery bank. 

Self-consumption can be a bigger problem with lithium batteries that are set with LVD settings that are too low. Often lithium battery manufacturers provide a minimum LVD setting that can leave the battery at ~ < 2% SOC. These settings can only be used when the self consumption is negligible. When a lithium battery reaches a very low voltage, the BMS will disconnect the battery internally. Depending on the amount of self-consumption, 12.8V nominal LiFePO4 lithium batteries with LVD settings below 12.6V may be at risk of a low voltage BMS disconnect. Therefore, LVD settings of 12.7V (12.8V LiFePO4 batteries) or higher are recommended to prevent this–especially for remote sites or sites that are not being monitored. Not only is this detrimental to the health of the battery, but if it lasts more than ~ 30 days, it may not be able to recover from a BMS low voltage cut-off at all. 

N-Load Control for Prioritized Loads and Multiple Load Circuits 

Some off-grid PV systems include loads that are more critical than other loads. One example is remote communications devices that the user would not like to disconnect during an LVD event so they can continue monitoring the system. This is where prioritized load control can help. By disconnecting the non-critical loads first, the critical loads can continue to operate longer without interruption. This is also referred to as “N-Load Control” or “load shedding”. The critical load will need lower LVD/LSOCD settings and/or a longer Warning/LVD delay setting than the other less critical loads to achieve this.

Besides prioritized load control there are other reasons to provide separate load control for more than one load circuit in the system.

• To provide additional load control in case the sum of all loads exceeds the load control current rating of the controller
• To provide load control for a very large load or a load with a high inrush current independently from other loads.
• To provide scheduled, manual or automated load control functionality for specific load(s)
• To provide High Voltage Disconnect (HVD) protection for voltage sensitive loads but not for other non-sensitive loads.

Morningstar provides several options for implementing load control for these applications. The best solution depends on the application and system configuration.

The Relay Driver (RD-1) includes 4 relay driver channels so it can control two or more separate load control circuits with external relays. This is a great option when the solar controller(s) in the system do not include built-in load control (often with TriStar and TriStar MPPT solar controllers). The RD-1 uses Voltage Threshold settings to implement LVD/LVR. The remaining RD-1 channels can also be used for  other applications for the system, such as automatic generator startup control, cooling, Fault-Alarm notifications etc…. 

An alternative to the Relay Driver would be to use two separate load control devices. On the one hand, the TriStar controller provides data monitoring of the current and Ah for the load circuit that you do not get with the Relay Driver. However, using two or more TriStar controllers for load control will cost more, take up much more space and have higher self consumption than the RD-1 with relays. Also, the high current ratings of the TriStar can be overkill for smaller critical load circuits. Using smaller, lower cost solar controllers with built-in load control without using the solar controller function is another option for 12V and 24V systems. 

If there is more than one controller in the system that has built-in load control, they can be set with different load control settings without having to add additional load control devices. If there is only one solar controller with built-in load control in the system a relay driver, TriStar PWM or a second controller with built-in load control can be set up for prioritized load control. 

The GenStar MPPT is an Integrated Series product. Therefore, is it compatible with the ReadyRelay Integrated Series accessory, making it possible to add load control circuits in addition to the GenStar 30A built-in load control. The ReadyRelay includes two relays each rated for 6A @30Vdc, 500mA @60Vdc or 6A @250Vac. These relays can be used as contact relays for remote switches, or with small loads (12V, 24V or AC loads) or larger loads with external relays. Like the relay driver, the second relay can also be used for other applications in the system at the same time. In addition to Prioritized Load Control settings, the GenStar can also be set up with weekly schedules for ON/OFF load control. For more information see the GenStar MPPT and ReadyRelay manuals on the Morningstar website.

The post Off-Grid PV System Load Control: Equipment Self Consumption and Prioritized Loads appeared first on Morningstar Corporation.

Solar Controllers with built-in Load Control (12 and 24V only)

GenStar MPPT (30A load control rating; Integrated Series product compatible with ReadyRelay

Programmable with LiveView 2.0 or built-in display

Presets: LVD = 11.1-12.85V, LVR = 12.6-13.3V (x2 for 24V, x4 for 48V)

ProStar MPPT (15-30A load circuit ratings) Programmable with MSView or display model

Presets: LVD = 11.5V, LVR = 12.6V (x2 for 24V); Firmware v27 and higher

ProStar (PWM) (15-30A load circuit ratings) Programmable with MSView or display model

Preset: LVD = 11.5V, LVR = 12.6V (x2 for 24V); Firmware v18 and higher

SunSaver MPPT (15A load circuit ratings) Programmable with MSView

Presets: LVD = 11.5V, LVR = 12.6V or 11V, LVR = 12.1V (x2 for 24V)

SunSaver (PWM) (6-20A load circuit ratings) Non-programmable

Preset: LVD = 11.5V, LVR = 12.6V  (x2 for 24V)

SunLight (PWM) (10-20A load circuit ratings) Non-programmable

Preset: LVD = 11.7V, LVR = 12.8V (x2 for 24V)

EcoBoost MPPT (20-30A load circuit ratings) Programmable with display model only

Preset: LVD = 11.5V, LVR = 12.6V  (x2 for 24V)

EcoPulse (PWM) (10-30A load circuit ratings) Programmable with display model only

Preset: LVD = 11.5V, LVR = 12.6V  (x2 for 24V)

SHS (PWM) (6-10A load circuit ratings) Non-programmable

Preset: LVD = 11.5V, LVR = 12.6V  (x2 for 24V)

Other Load Control Equipment 

TriStar PWM in load control mode (45-60A load circuit ratings)

7 Presets: LVD = 11.1-12.3V, LVR = 12.3-13.8V (x2 for 24V, x4 for 48V)

Relay Driver (1-4 separate loads; 750mA circuit rating; or 3rd party relay) Programmable

Presets: LVD = 11.5V, LVR = 12.6V or 11.1V, LVR = 12.2V (12V only) 

ReadyRelay (1-2 separate loads; 6A <30V, 500mA <60V; or external relay) 

Integrated Series accessory compatible with GenStar MPPT 

Programmable with LiveView 2.0 or GenStar built-in display

Presets: LVD = 11.1-12.85V, LVR = 12.6-13.3V (x2 for 24V, x4 for 48V)

SureSine inverters with built-in AC output load control

New SureSine 150-2500 Watts continuous, Programmable with Mobile App

Presets: LVD = LVD = 11.8V, LVR = 12.8V; 11.5V, LVR = 12.6V or 10.5V, LVR = 11.6V

(x2 for 24V, x4 for 48V)

SureSine Classic 300W continuous; 600W 15 minute Surge

Programmable with 3rd party MODBUS software only

Presets: LVD = 11.5V, LVR = 12.6V or LVD = 10.5V, LVR = 11.6V

Load Control Settings 

Programable settings can be modified in numerous ways as indicated above. Adjustments to the GenStar MPPT and ProStar family of products can also be made with the built-in displays. The LVD settings for the SureSine inverter cannot be programmed with MSView but can be adjusted with 3rd party MODBUS software as indicated in this document. https://www.morningstarcorp.com/wp-content/uploads/technical-doc-suresine-lvd-adjustment-en.pdf

The built-in load control settings include a delay before LVD (or LVD Warning time). This is the time delay between when the battery voltage drops below the LVD voltage threshold and when the loads are disconnected. This provides two benefits. For systems with larger loads that may turn on briefly this will keep the loads from being disconnected when there is a momentary drop below the LVD voltage. It will also signal an LVD warning to alert the user that LVD may occur shortly.

The threshold function of the Relay Driver is often used for load control. It uses a low voltage threshold for LVD and a high voltage threshold for LVR. Our Relay Driver Overview and Applications tech doc provides an overview of features and how to use the Relay Driver. https://www.morningstarcorp.com/technical-documents/relay-driver-overview-and-applications/

In addition to LVD settings, the GenStar MPPT also includes Low State of Charge disconnect/ reconnect (LSOCD/ LSOCR) settings. The LSOCD/ LSOCR settings can only be activated if SOC data is available from a ReadyShunt or a ReadyBMS Integrated Series accessory.

The ReadyRelay can be configured for Load control with the same type of LVD and LSOCD settings as the built-in load control settings of the GenStar controller.

The post Off-Grid PV System Load Control: Morningstar Load Control Device Selection and Configuration appeared first on Morningstar Corporation.

Battery voltage can most accurately indicate SOC with an open circuit battery. This is commonly referred to as rest voltage. The battery voltage rises above rest voltage during charging and drops below rest voltage during discharging. The greater the discharge current is, the more the voltage will drop below the rest voltage. In addition, when there is a very low SOC (< ~10%) a discharge current will have even more effect on the battery voltage. The impact of charge and discharge current on voltage is also greater with older batteries that are reaching the end of life. 

Since the battery voltage will be pulled lower with higher discharge rates, the voltage will reach the LVD voltage sooner with larger loads. Conversely, the load would disconnect with a higher SOC when there are smaller loads. 

Off-grid systems typically do not have high discharge rates. This would be for systems with continuous loads such as telecom and systems designed with larger battery banks that have more days of autonomy. The voltage of these systems will not be affected by the load current much and the voltage drop below rest voltage will be minimal. For example, discharge rates of < C/100 with lead-acid batteries will likely lower the voltage by < 0.1V below rest voltage for a 12V battery.

To account for higher load currents that do cause more impactful voltage drops, Morningstar provides a Load Current Compensation (LCC) setting. This is provided in the load control settings of the setup Wizards in MSView.

It is important to not set the LCC too high or it can overcompensate and disconnect the battery at too low a voltage in relation to the SOC. LCC settings are inversely proportional to the Ah rating of the battery bank. Therefore, the LCC setting for larger battery banks should be lower than with smaller battery banks in order not to overcompensate for current. This is because the discharge rate corresponds to higher current levels with higher Ah battery banks. A C/20 discharge rate with a 100Ah battery would be 5A while a C/20 discharge rate of a 500Ah battery would be 25A. 

The LLC is also directly proportional to the nominal voltage of the battery. Higher voltages means higher current compensation of the LVD. We recommend the following calculations to determine the settings for LCC.

Lead Acid Batteries   

LCC @12V = (-0.8 / battery Ah) ohms (V/A)

LCC @24V = (-1.6 / battery Ah) ohms (V/A)

LCC @48V = (-3.2 / Battery Ah) ohms (V/A)

Lithium batteries have a more stable voltage in relation to charge and discharge currents. Therefore, the LCC settings for Lithium batteries should be set considerably lower than for Lead Acid batteries. 

Lithium (LiFePO4) Batteries

LCC @12V = (-0.32 / battery Ah) ohms (V/A)

LCC @24V = (-0.64 / battery Ah) ohms (V/A)

LCC @48V = (-1.28 / Battery Ah) ohms (V/A)

The effect of charge and discharge current on voltage increases with colder battery cell temperatures. Therefore, high discharge currents during cold temperatures will cause an LVD disconnect at a higher SOC than it would in warmer conditions. Unlike charge settings that use temperature compensation to increase the regulation voltages there is no temperature compensation used with load control. 

Since the freezing point of lead-acid batteries increases the lower the SOC is, it is somewhat reassuring to know that the load may disconnect the battery sooner when it is cold and potentially keep the battery from freezing as easily in very cold environments. However, this will only happen when there is a high discharge current so it is still important to set LVD higher for very cold environments.   

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Cycle Life vs Depth of Discharge (DoD)

Battery manufacturers publish graphs for cycle life (aka number of cycles) vs DOD. Cycle life is the number of times that the battery is discharged and charged to the specified DOD before the battery’s Ah capacity drops below its rated capacity by a certain percentage. Below are examples of Cycle Life vs DOD graphs for lead-acid batteries and lithium batteries. Please note that these are examples only and are not representative for particular brands and models for these types of batteries.  

As seen in the graphs above, Cycle Life vs DOD does not have a linear relationship. The higher the specified DOD is for each cycle the less overall energy (Ah, kWh) will be available for the system. Doubling the DOD will provide less than half the number of cycles available. 

Lifetime Energy for a 100Ah Lead-Acid battery based on average DOD

Cycle Life (30% DoD) = 2,050 cycles;   Cycle Life (80% DoD) = 630 cycles

Available Lifetime Energy (30% DOD) = 2,050 cycles X 30Ah = 61.5 kAh 

Available Lifetime Energy (80% DOD) = 630 cycles X 80Ah = 50.4 kAh

Lithium batteries have a much higher cycle life than lead-acid batteries even with higher DOD levels. The cycle life of different types of lead-acid batteries can also vary. Therefore, the battery manufacturer’s cycle life specifications should always be reviewed when batteries are being specified for the system. It should also be noted that higher ambient temperatures and higher charge/discharge rates can reduce the expected cycle life of the battery.

Does this mean you should have higher LVD settings to extend the life of the battery? It depends. First, consider the difference between the Average DOD per Cycle vs. the Maximum DOD. Then consider how much the battery will be discharged on a daily basis (more on cloudy days; less on sunny days). A larger PV array will also be able to maintain a higher SOC and reduce the Average DOD and how often the battery is deeply discharged. 

Though a very low LVD setting corresponds with a high Maximum DOD, the Average DOD per Cycle will be much lower and the battery should rarely, if ever, experience an LVD with the Maximum DOD. Therefore, if the system is sized properly to prevent a loss of load, higher LVD settings should not have much of an impact on the life of the battery.

In summary, the DOD for each discharge/ charge cycle will vary greatly. Using a low LVD setting doesn’t mean the battery will discharge to that level every day. If the system design and performance makes it so the battery is rarely deeply discharged, a lower LVD setting should not have a big impact on the life of the battery bank.The question remains, under what circumstances would higher LVD settings help extend the life of the battery?  

• Frequent deep discharging of the battery due to one of the following
  • Undersized PV array or poor performance of a PV array 
  • Underestimated load usage
  • New loads added to the system
  • Undersized battery bank
  • Irregular load usage (common with residential applications)  
  • Systems that include a generator often have smaller battery banks
• Extended periods with a partial state of charge (PSOC) due to not being able to recover from a very deep discharge. 
• Prevent freezing of the batteries in very cold climates 

Using a lower LVD setting will provide more autonomy for the system. However the PSOC issues that can cause harm to lead-acid batteries may be exacerbated with lower SOC levels. It may be necessary to use higher LVD settings or take other measures as previously discussed to prevent PSOC issues when using lower LVD settings.  

In very cold conditions lead-acid batteries can freeze and cause damage. The freezing point of lead-acid batteries increases the lower the SOC is and the specific gravity of the electrolyte is low. Therefore, a higher LVD setting should always be used in very cold environments to continuously maintain a high enough SOC and prevent freezing. 

The post Off-Grid PV System Load Control: When to disconnect loads and why? appeared first on Morningstar Corporation.

Morningstar controllers and inverters are often used in autonomous off-grid systems; telecom, oil and gas, lighting, etc… Therefore, this article primarily focuses on off-grid PV systems that have predictable load usage over the lifetime of the system. 

There are three basic calculations required for sizing an off-grid PV system.

• Average daily load energy usage (Ah / kWh)
• Energy input from solar and other charging sources (Ah / kWh)
• Energy storage capacity (Ah / kWh)

Estimating any of these components incorrectly can have devastating effects, including frequent loss of loads and/or shortened battery life. There are many off-grid system sizing tools available for this purpose. 

Off grid system sizing starts with the load evaluation. The purpose of this evaluation is to determine the total average daily load usage in Amp hours (Ah’s) or kilowatt hours (kWh’s). Load energy usage is based on the size of each load in the system and the estimate of the average number of hours a day that each load will be operating. Power conversion efficiency of inverters and DC-DC converters if used, self-consumption and other losses also need to be accounted for. It is crucial not to underestimate load energy usage or add loads at a later time without reevaluating the solar PV and battery sizing requirements. 

Typically the solar PV array is sized next. The solar PV array’s daily average energy production (Array Watt-hours/ Amp-hours (Wh/ Ah)) must be able to keep up with the daily load usage throughout the year in addition to being able to fully charge the battery on a regular basis. Note that in order to provide enough energy throughout the year the array must be sized for the month with the lowest solar energy and/or highest load usage. This is referred to as the critical month (typically during the winter or rainy season). 

Estimating solar production is calculated using historical monthly solar irradiation data (kWh/m2 per day) and basic algebraic equations, taking into account array size, tilt and other losses (shading, voltage drop, efficiency losses, etc…). This can be challenging since solar energy production can be very inconsistent depending on the season. In some climates it may be impractical to use solar alone without another source of energy. Adding additional solar panels to an array can improve system uptime and keep the battery fully charged on a more consistent basis. 

Once the PV array size (Watts and number of PV modules required) is determined, it is possible to select the solar controller(s), and then determine the PV array string sizing. Morningstar’s online string sizing calculator is available if needed for this. 

The battery bank is usually the last step in sizing the system. Like the solar array sizing, the Ah/kWh capacity of the battery bank requirements are calculated based on several system variables.

• Battery chemistry
• Battery efficiency
• Average depth of discharge (DOD)
• Maximum DOD
• Average daily DOD can increase the # of cycles of the battery and life expectancy
• High charge and discharge rates and lower temperatures reduces both the efficiency and capacity of the battery
• Days of Autonomy (# of days of operation with no charging: Typically 3-7 days) 
• More Autonomy (battery capacity) will reduce the LOLP for critical loads
• Systems which include a generator can reduce the # of days of autonomy requirements 

Lithium-ion batteries have a significantly higher cycle life than lead acid batteries and therefore can be sized for a higher DOD. Increasing the size of the battery bank provides more autonomy to reduce the LOLP and the average DOD for a longer cycle life. Compromising to avoid the high upfront cost of the battery bank can come at the cost of the life of the battery.  

Loss of Load Probability (LOLP)

Some advanced off-grid PV system sizing software tools include a Loss of Load Probability (LOLP) calculation. The LOLP takes into account not only the average daily solar radiation data for the site, but also historic weather pattern variations in the daily levels of available solar radiation. While the system may be sized adequately based on the monthly daily average solar radiation (kWh/m2 per day), there will be extended periods of cloudy weather when the average solar radiation is much lower than the monthly average. The monthly LOLP is the likelihood that the battery will be discharged to the point that there will be a Low Voltage Disconnect (LVD) in a given month. The annual LOLP is the sum of the 12 monthly LOLP values. 

Please note that the LOLP calculations are less predictable and less accurate than the average solar energy per day calculations. Also, both of these calculations do not take into account potential snow cover of the solar array. In addition, the amount of loss of load for a given year will vary annually along with annual weather anomalies. For example, during a year when there is a major hurricane there could be a significant load outage, but no loss of load the following year when there are no hurricanes.  

Increasing the size of the PV array and/or battery bank will reduce the LOLP. These off-grid PV system sizing calculators can be a very useful tool in evaluating this important aspect of the system design and determining what choices will be the most cost effective in preventing loss of load.

Partial State of Charge (PSOC)

Systems with larger battery banks sized for many days of autonomy can spend extended periods of time with a partial state of charge (PSOC). This is because it could take a long time to fully recharge the battery when there is a very low state of charge (SOC). Not being able to recover from a very low SOC in a reasonable amount of time can cause irreversible degradation of lead-acid batteries. The risk of PSOC is much more significant in climates that have rainy seasons or winters with short days and/or snow.  

Some lead-acid battery manufacturers have indicated that PSOC is one of the most common causes of reduced lifetime for lead-acid batteries with off-grid solar systems. Some types of batteries such as lithium and other energy storage products don’t have issues with PSOC. For most lead-acid batteries, having over two weeks with a PSOC should be avoided. Reducing the number of days of autonomy by installing a smaller battery bank or using a higher LVD setting is one option, but it will likely lead to  more frequent loss of load. Increasing the size of the  solar array is a more optimal solution that will fully charge the battery more consistently and help speed up the charging process to recover from a low SOC more quickly. 

Generators can be used to prevent extended periods with a PSOC and avoid letting the battery reach a low state of charge (SOC) and loss of load. The generator should be sized large enough to recharge the battery bank relatively quickly. Note that it is best to size diesel generators so it will operate at 50-80% capacity for continuous loads. This is because when a diesel generator is under-loaded most of the time it will be less fuel efficient and cause carbon build-up, and over time produce smoke and start choking up. On the other hand, operating generators continuously at full capacity can cause the generator to overheat.   

Grid uninterruptible backup systems (UPS’s)often include solar to keep the battery charged during an outage. Usually the battery is not sized as large as for off grid systems since the grid will keep the batteries from getting discharged most of the time. A generator can also be installed with the PV/ battery backup system for longer outages. 

There is a Solar Controller Integration with AC Rectifiers white paper available on the Morningstar website that provides helpful information about Utility and AC generator applications. https://www.morningstarcorp.com/wp-content/uploads/2020/11/Solar-Controller-Integration-with-AC-Rectifiers-whitepaper.pdf

The post Off-Grid PV System Load Control: System Sizing and PSOC appeared first on Morningstar Corporation.

Effective charging, system management capabilities, and long-term reliability are often the primary aspects that are considered when deciding on what charge controller model and other system components to use in an off-grid PV System. In off-grid systems, the proper management of the load is not focused on as much as charging, but it is a critical aspect of the system. The life of the battery depends on proper load management just as proper charging does, and reliable operation of the site equipment is the number one goal of the system itself. Even with properly sized systems, ongoing load operation can be put at risk if careful considerations are not made. In this series of technical articles, we will be taking an in-depth look at many aspects of load control solutions as well as the benefits the controller can provide to the load in critical remote power applications. We’ll also be covering many features of just what the controller can provide in terms of reliable load operation for these systems.

Off-grid Site Uptime

Reliable load operation – what does it mean exactly?

Off-grid solar, wind, or hydro systems are designed to provide autonomous power in locations without utility power. Sizing the system properly is the key to getting reliable power. However, dealing with occasional load disconnects due to extended periods without adequate sunlight, wind or hydro energy is often unavoidable. The long-term reliability of the system depends on the load controller’s ability to protect the battery from overdischarge. Allowing deeper discharge of the battery can provide more uptime and less frequent loss of loads but can come at the cost of the health of the battery life and thus reduce long-term reliability. When considering load control, system designers and operators need to weigh out the long-term reliability vs. uptime of the system to best meet their needs and budget.  

Loss of Load Probability (LOLP) – What’s your tolerance and how big is your wallet?

Depending on the location it may be possible to significantly increase the size of the PV array and battery bank so that there would be non-stop continuous load operation without ever experiencing an interruption of the loads for the life of the system. We see this often with systems where load usage is small and the array spends much of the time keeping the battery in float. However, it is costly to do this with a larger system. 

In order to reduce the Loss of Load Probability (LOLP), one must account for the seasonal variability of solar production.  One solution is to have a sizable battery bank that sustains the system through the longest periods of low solar production; but this is expensive. Another option is to add a backup generator as a cost effective way to provide uninterrupted power for critical off-grid loads. However, getting fuel to a remote site, and maintaining a generator can make this an untenable solution. 

The most advanced off-grid solar system sizing calculators are able to estimate the monthly LOLP by taking into account historic weather patterns and variations. This calculation can be most helpful for sizing autonomous systems where the load usage can be accurately estimated.

Even systems that have been designed for 0% LOLP can experience unforeseen issues such as partial shading from leaves or overgrowth of trees or plants over time, loose wires, wire or panel damage, stolen wire or panels, equipment failure or the generator running out of fuel. Therefore, load control is always recommended to prevent the possibility of over-discharging and irreversible harm to the batteries.

The post Off-Grid PV System Load Control: Off-Grid Uptime appeared first on Morningstar Corporation.

Adding a Relay driver to your system adds functionality that improves your control over the system. The Relay Driver reads digital MeterBus data from the controller in your system and can respond by managing additional loads, generators, and alarm indicators. 

Controllers compatible with the Relay Driver include: 

• TriStar MPPT 600V Controller
•  SunSaver Duo Controller 
• TriStar MPPT Controller
• SureSine Inverter 
• SunSaver MPPT Controller
• MeterHub
• TriStar Controller 

The Relay Driver can be a standalone device controlling your system with battery voltage, analog input voltages, and ambient temperature. Additionally, suppose your system uses a Tristar Controller. In that case, the RD-1 can read the following digital inputs from MeterBus: Battery voltage, charge/load current, battery temperature, TriStar heatsink temperature, PWM duty cycle, PV/load voltage, and all TriStar alarms or faults.

The Relay Driver has 4 channels that can function as inputs and relay driver outputs, with a max current of 750 mA. 

What can the Relay Driver do? 

The Relay Driver can use a user-determined threshold based on its monitored voltage, current, and temperature capabilities to switch a channel on or off. The channel could be hosting a load that impacts this monitored value. 

Scenario 1: The Relay Driver received data from your controller indicating that the controller’s heatsink temperature is too hot for optimal system functioning. After receiving this information the Relay Driver switches one of its four channels to ON, turning on a fan to cool the system cabinet. 

Scenario 2: You have a lithium battery you want to maintain. To preserve your system’s health, you set up High and Low Voltage Disconnect protocols using your Relay Driver. The Relay Driver monitors the battery voltage. With one channel you disconnect loads that may be damaged by voltages above a certain threshold. With another channel, you disconnect loads when the battery voltage gets too low. 

Scenario 3: You would like to manage your at-home security system. One feature of this system is a motion-activated light that turns on whenever someone enters your property. Using your Relay Driver, you use one channel to receive a voltage input from your motion sensor. Using a second channel, the Relay Driver turns on the light when the motion sensor detects movement. 

The Relay Driver can respond to a Morningstar device alarm by turning a channel on. 

Scenario 1: Your Tristar Controller releases an alarm indicating that the input voltage current limit has been exceeded. Your Relay driver senses this alarm and uses one channel to turn on an audible siren. This way it can catch your attention, and you can remedy the situation without losing system functionality. 

The Relay Driver can use one or multiple channels for multi-wire automatic generator startup control functionality. It does this, by using user-indicated threshold battery voltage and required signal and operating specifications. 

Scenario 1: Your system requires you to start a backup diesel generator to prevent a low-voltage disconnect and recharge the battery. The generator requires warm-up and cool-down periods to run with no loads connected. Your Relay Driver turns on the run signal for your generator to start. After waiting for the generator to warm up, another channel turns on the generator load signal for the battery charger. Before turning off the generator run signal, the Relay Driver will first turn off the charging circuit to allow the generator to cool down before shutting off. 

Scenario 2: You have a large air compressor motor that needs to turn on based on your ambient temperature. You use a three-wire generator to power it. When the ambient temperature reaches the threshold that requires your motor, your Relay Driver using its internal ambient temperature sensor, identifies this. It turns on the generator with a channel output when it gets warm, uses an input channel to sense the generator’s feedback voltage signal, and then uses a 3rd output channel to turn on the load, the large air compressor motor. 

The Relay Driver allows you to cycle power to a load in your system. 

Scenario: Your system has a load that requires a power-cycle after a remote firmware update. Using your Relay Driver, you use one channel for Modbus Control. At any time, you can send a MODBUS Coil Command to turn the device off and back on with the Modbus Command Channel. 

What if you want to cycle power to a router or other communications device? If you turn off power to the device you will lose the Modbus connection and will therefore not be able to turn it back on with a second Modbus coil command. In this case, it is possible to use advanced logic with another channel so it can be switched on and off with a delay after the Modbus control channel is turned on. Contact Morningstar at support@morningstarcorp.com for detailed instructions.   

Overall, the Relay Driver is a flexible product that is great to incorporate into your systems aided by many of Morningstar’s Professional Line Controllers. With a  Meterbus connection, the Relay Driver can provide system control functionality using important data like Battery voltage, charge/load current, device temperatures, faults, and alarms. In addition, the four channels can act as both a relay driver output and as an analog voltage input for further functionality and monitoring capabilities. Finally, with an RS-232 Modbus connection, the Relay Driver can provide remote on/off control for relays and signals. It adds responsiveness, control, and features that clarify your system’s day-to-day operations.

The post Four Things You Might Not Know You Could Do With The Relay Driver appeared first on Morningstar Corporation.