This is a common question, which Sales team at Cares Renewables comes across in their interactions with customer. Before, we get into differences between Grid-Tied and Off-Grid System, let us try and understand some basic characteristics of Solar Energy.
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Solar is not a constant source of Power
As we are all aware Sun’s position in the Sky keeps changing through out the day.
(Image Courtesy: YouTube)
So Solar Panel’s input during the day will keep changing based on Sun’s Position in the Sky. So, Solar Panel’s Output will also vary based on the same. (Note: Solar Panels output also depends on temperature and a few other factors, which is not discussed here). So, A Solar Power Plant without any complementary or back up source cannot supply constant power to load. This is a bit counter intuitive when you compare it with a Diesel/Gas Power Plant, which is able to supply the peak rated power at any point of time.
To Supply Constant Power to Loads, Solar requires another Source of Power to Complement it
In the above graph power production starts around 6.30 am and stops around 6.00 pm. The power production has peaked around 1:00 pm. The dips in power production can be attributed to clouds. Because of the above characteristics of Solar PV power production, for most of the applications (Solar Pumps and applications with drives is an exception) it is imperative to have a complementary source of power (either grid, battery or some other source) along with solar to meet the power requirement of the load.
Grid-tied Solar Power Plant or Grid Connected Solar Power Plant
Photovoltaic systems that use only utility grid as a complementary source of power are called grid-tied systems. In a grid-tied system whenever there is a shortfall in power production from solar due to external conditions, the grid power will compensate the shortfall. Similarly, if the power production by a solar PV system installed in a facility, is more than that of the consumption in the facility, based on inverter setting the excess power is
Either pumped back to the grid by the inverter as export
Or the inverter will limit the power production from Solar PV so that there is no export of Power
In India ‘net-metering ‘ for grid-tied solar PV system is introduced by many utilities as a part of their Solar Policy. Under a simple net metering scheme, the meter will measure the net consumption at the facility from the grid, and the power exported to grid by the Solar Power Plant. At the end of the month customer is billed for the difference between the import and export. (In case there is excess export it is typically carried over to the next month: depends on utility policy).
With net metering, solar power is not wasted even if the consumption in the facility is less than that of the power produced by the solar PV system.
Pros and Cons of Grid-Tied Power Plant
The grid-tied systems are the most cost effective and the most efficient among the different types of solar PV systems available today. It doesn’t require expensive storage technologies to complement solar power and has high system efficiency of >95% (for a system with good design). We don’t have to worry about sizing the inverter based on peak load at facility, as the grid-tied system will take power from utility grid to handle peak loads such as surges during motor operation. They are ideal for areas where power failure is not frequent during the day. The system maintenance cost is also low for grid-tied system as the design life of Grid-tied Inverters are high (5 to 30 years) and, we don’t use any batteries in the system.
However, the grid-tied system automatically shuts down when there is a utility grid shut down or power failure. So, we won’t be able to use a grid-tied system for back-up power. Moreover, the grid-tied system may not give good output in areas where utility power failure is frequent.
Another key consideration before we go for a grid-tied system is the utilities Solar Policy. You have to check with installers, and get confirmation from utility on net-metering policy before you go for a grid-tied system. In certain regions in India, the Solar Policy do not allow net metering for all tariff categories.
Off-grid Solar Power Plant
The solar PV systems, which are not connected to the utility grid and use a battery bank to complement solar power, are called off-grid Solar PV systems.
In an off-grid system, the PV array charges the battery through charge controller/MPPT (Maximum Power Point Tracker) circuitry, whenever there is sufficient solar radiation to produce power. The battery feeds the inverter. And the inverter in turn feeds the AC load. Hence an off grid PV system can power a facility even during night, provided there is sufficient back up power in the battery. When designing an off-grid system, we have to ensure that the Inverter can handle peak/surge loads, so the design is a bit more complicated compared to grid-tied system.
One variant of off-grid systems we come across in Indian market very often is Solar Power Conditioning Unit (SPCU) with back-up. SPCUs typically charge the Batteries both from Solar and Grid Power, based on state of charge of battery. It has a functionality, which will switch the loads to grid when Battery doesn’t have sufficient charge to power the loads. This functionality comes in handy during rainy days or days where energy consumption at site is more than the consumption system was designed for.
Pros and Cons of off-Grid Power Plant
An off grid PV/Solar Power Conditioning Unit with Backup will be ideal in
(a) Regions where the grid is unreliable due to acute power shortage/environmental conditions.
(b) Rural areas/Islands, where the grid has not reached yet. In such cases, an off grid or hybrid system may make better sense than extending the grid.
The system is also good as back-up source of power in regions where cyclones or acute weather events can cause prolonged power failure. (if designed properly)
However, the system efficiency of the off-grid system is very low (70% to 80%) compared to a grid-tied system. This is because of the presence of low efficiency batteries, and lower efficiency of off-grid Inverters compared to grid-tied ones. Moreover, the no load power consumption of off-grid systems are relatively high compared to grid-tied ones.
The initial cost and the maintenance cost of an off-grid system is also very high compared to a grid-tied system. Off-Grid systems require expensive batteries, which require regular maintenance as well as replacement every 5 to 7 years.
Hybrid Solar Power Plant
A Hybrid solar PV system is one, which uses more than one source of power to complement solar power. The hybrid source may use a diesel generator, a wind turbine, a biogas plant, heat engine, fuel cell or a combination of the different sources along with utility grid to supplement PV power. The most popular type of Hybrid
A Hybrid System is more reliable and cost effective as compared to having a large storage along with PV. Hybrid Power Plant is gaining popularity globally as utilities move away from net-metering. The reduction in cost of Lithium based storage technologies have also helped the cause of Hybrid Power Plants.
Wind Solar Hybrids are currently piloted in certain regions of India. Many utilities don’t support net metering for Solar Hybrid Systems in India. So if you are planning Hybrid Systems with net metering please check and get confirmation from utility provider.
At Cares Renewables, we are focused on delivering quality solar power plants to our customers and partners. Please leave a comment, if you have any queries on this blog. Visit https://www.caresrenewables.com/savegen to view a few of our case studies. #Solar #AdvancedSolarPlant #SaveGen #SolarEnergy #SolarPower