GETTING THE MOST OUT OF A SOLAR POWER SYSTEM WHEN DOING CHOICE BETWEEN MICRO INVERTER AND STRING INVERTER
To maximize the performance of your solar power system, it is crucial to choose the right type of inverters: microinverters or string inverters.
Although solar panels (modules) are important, the efficiency of a solar power system largely depends on the inverters.
But how do you determine which inverter is suitable for your needs? Whether you are a homeowner or an installer, there are several factors to take into account.
1. SAFETY
Let’s start with safety – arguably, the most important consideration.
Safety at operation
What are the potential risks that homeowners should be aware of when it comes to string systems or microinverter systems?
String systems run on high voltage
When a single module in a string system carries a voltage of about 40–50 V, the voltage is relatively low. However, string inverters are connected in series, and when you connect 20–30 modules in series, the DC voltage of the entire system can rise to as much as 1,000–1,500 V.
This increase in voltage poses a significant danger to people's safety and also poses a risk to your building. In the event of a disconnected wire, the system can initiate a high-voltage arc, creating a fire hazard.
Microinverters isolate modules
In contrast, microinverters are connected in parallel, with each carrying only a voltage of 40 V. This significantly reduces the safety risk in the first place.
Furthermore, module-level monitoring enables you to easily stay informed and detect any potential problems.
Safety at rescue
Are there any differences between the two types of systems in terms of the risk posed to firefighters during a rescue operation if a fire were to break out?
String systems – high voltage after shutdown
A string inverter-equipped system will retain a high voltage even after it has been shut down. As a result, it continues to present a significant danger to users and firefighters. In such situations, firefighters must wait until all the panels have burned out.
Microinverter systems – low voltage after shutdown
However, a system equipped with a microinverter carries a low voltage after shutdown, which means it does not pose any danger to rescue staff.
String Inverter | Micro Inverter | |
In Operation | 40V*30 modules = 1200V Connected in series Fire hazard | 40 V/module Connected in parallel Module-level Monitoring |
In Rescue | High voltage even after shutdown, lead to delayed rescue action | Low voltage after shutdown, an immediate rescue can be counducted |
2. RETURN ON INVESTMENT/PROFIT
When considering the return on your investment, what you should ask is:Â How long is the payback period when doing choice between micro inverter string inverter? This depends on several different factors.
Cost
First, consider the total costs. Not just the upfront costs but also:
Equipment replacement costs during the service life of the whole system; and
Maintenance costs during the service life.
Microinverter system costs
With warranties extending up to 25 years, you won't need to worry about incurring equipment replacement costs throughout the operational life of your microinverter system. Additionally, minimal labor effort is required for maintenance, as a simple module replacement is sufficient.
String system costs
In contrast, issues with modules in a string system lead to significantly increased time and labor expenses. Additionally, any problems with the string inverter require the entire system to be powered off.
The amount of electricity generated by the system
A microinverter system has a similar payback period to a string system, as it produces 5-30% more power. There are three reasons for this:
a) The effect of shading
Firstly, one of the main advantages of microinverters is that they significantly reduce the impact of shading on photovoltaic (PV) cell electrical output. Shading, even on a small part of a module, such as from trees, dense building spacing, bird droppings, snow, dust, or other debris, can cause a drastic decrease in the output of that module.
So, how does shading affect a string system? Here's a helpful analogy to understand it better. Imagine a broken wooden barrel with unequal staves (the strips of wood that make up the barrel). The amount of water the barrel can hold is limited to the height of the shortest stave. Scientists refer to this as the 'law of the minimum', and it applies similarly to a string system. In a string system, the panels function as a single larger panel. If shading causes one or more modules to produce less energy, the entire system's output will decrease to the level of the module with the lowest output.
However, a microinverter system overcomes this issue with its module-level Maximum Power Point Tracking (MPPT) technology. This technology electrically isolates solar panels from each other. Consequently, the underperformance of one module does not impact the operation and performance of the other modules or the system as a whole.
b) Microinverters have greater installation capacity
The space restrictions on your roof may not allow all modules to be placed in the same direction. For instance, you may only have enough space to place three modules facing north and two modules facing west. With string inverters, this mismatch in orientation results in a power loss.
However, thanks to MPPT, you can avoid mismatch losses with microinverters. This means that microinverters enable greater installation capacity.
c) Starting power
A microinverter system has a lower starting power compared to a string system. It starts operating earlier in the morning and continues working later in the evening.
As a result, a microinverter system is approximately 1-1.5% more cost-effective.
Power consumption and the local electricity price
Power consumption will largely depend on your family's energy usage habits, such as the number of appliances you have and your efforts to avoid energy waste.
The price of electricity is determined by local grid operations, which are influenced by factors such as population density, fuel prices (coal, natural gas, oil), energy structure, and potential government subsidies.
Power consumption and the local electricity price are typically fixed factors for families. Therefore, when comparing and contrasting return on investment, the main focus should be on the first two factors: system costs and the amount of electricity generated.
3. SERVICE LIFE
You can generally expect the power output of modules to be stable for 25 years. What about inverters?
Service life of string inverters
String inverters have a shorter service life compared to modules. This means that there is a possibility of a string inverter breaking down while the modules are still functioning. Additionally, string inverters usually come with a warranty period of only 5-10 years.
Service life of microinverters
In contrast, microinverters typically offer a warranty period of up to 25 years. This means that you won't have to worry about replacing them throughout the operational lifespan of your PV system.
4. MAINTENANCE FOR THE WHOLE SYSTEM
When it comes to maintenance for the whole system, there are two questions you should ask:
Is it time and labor-consuming?
Does it have high technical requirements? If so, you’ll have to reach out to technicians every time a problem occurs.
Here, there are sizable differences between string systems and microinverter systems.
String system maintenance
First, the modules.
If you encounter any problems with the modules in a string system, you will require professional operational and maintenance personnel, along with professional tools, to individually check each module. This process will take significantly longer and result in higher labor costs compared to checking issues with modules in a microinverter system. Additionally, the technical requirements are stringent.
Second, the string inverter itself.
A string inverter contains high voltage DC, which poses significant safety risks. Therefore, if you encounter any issues with the string inverter, you must power off the entire system during maintenance.
Microinverter system maintenance
By contrast, if a problem occurs with a microinverter, only the corresponding modules will be affected. In contrast, a problem with a string inverter requires shutting down the entire system. However, with a microinverter fault, the rest of the system can continue to operate. Simply replacing the faulty microinverter will solve the problem, saving time, effort, and money.
Additionally, a microinverter system allows for remote monitoring of power production for each module. This enables accurate identification of any modular failures in real-time, regardless of location.
In summary, a fault in a string system is more costly compared to a fault in a microinverter system. Moreover, repairs for a string system need to be arranged much sooner than for a microinverter system.
For example, if a module breaks down in a microinverter system, only that specific module will be affected. However, in a string system, the failure of one module affects the entire string of modules.
5. FLEXIBLE CAPACITY EXPANSION
If you want to expand a system’s capacity in the future, how easy will that be?
String system expansion
In a string system, the capacity is determined by the module with the poorest performance. This is due to the law of the minimum.
Therefore, in order to ensure optimal performance, each module must be identical. This includes the brand, height, direction, and other factors. If you wish to expand the capacity, you will need to take into account these various factors, such as direction and height.
Microinverter system expansion
By contrast, boosting the capacity of a microinverter system is straightforward. You simply add more modules and microinverters.
String systems with an optimizer
Optimizers can offset some of the disadvantages of string inverters by offering advantages such as module-level MPPT, module-level monitoring, and rapid shutdown.
However, string systems with optimizers still encounter certain issues:
Expanding power station capacity is challenging.
Arc-fault detection is necessary to mitigate fire hazards, leading to increased costs.
Operating the string system at high DC voltage poses safety risks for personnel.
Comments