June 11, 2018 Solar Energy Written by Greentumble Editorial Team
Types of PV systems
Solar photovoltaic (PV) systems are more complex

than they look. This is not only due to the fact that you need to determine the energy demand of your household, but you also need to pick the best mounting systems, suitable photovoltaic panels, inverters, batteries and type of the system.

When you request a solar quote, your installer will first ask you to choose between three main types: grid-tied, off-grid or hybrid systems.

The type of your chosen solar system will affect what components will be needed, how the system will operate and the overall costs of your PV system. Therefore, it is crucial to understand the advantages and disadvantages as well as the reasons for choosing one or another type.

The decision takes into account several variables:

    • utility grid service and reliability

    • importance of the load to feed

    • available policy and revenue schemes

    • desired autonomy

    • balance of system costs

    • size of the system

    • solar radiation

    • type of load to be served

    • the requirements of the customer

Let’s take a look at each type of available PV systems and see how these variables matter for each one of them.

 

Grid-Tied Systems

Grid-tied systems are the most used PV system in industrialized countries.

Grid tied configurations consists on interconnecting the PV module to the grid with no backup. This means that if the grid goes off, the PV system goes off as well.

The reasons for this are related mainly to security constraints and voltage stability of the system [2].

The main components of grid-tied system are:

    • solar panels

    • mounting system

    • wiring, circuit breakers

    • grounding system

    • a grid-tied inverter that will transform the direct current (DC) power from the panels into alternating current (AC) power – used by the most appliances

The main purpose of using grid-tied system is to reduce the consumption of electricity from the grid and save your costs in the long term.

Grid-tied systems typically consist on the following scheme of operation:

  1. Solar panels convert solar radiation into DC electricity
  2. Electricity flows through the DC wiring and goes into a combiner box
  3. The combiner box contains fuses and circuit breakers that protect the modules and the loads from overvoltages or short-circuits
  4. From the combiner box, a higher wire gauge cable goes to the inverter containing all the DC power from the modules
  5. Inverter transforms DC into AC current
  6. AC current is fed into the AC loads of the house
  7. Excess energy is fed into the grid through a utility meter

 
In grid-tied systems are two main configuration options for the connection between the modules and the inverter:
 

Grid-tied with central inverter

The classic configuration of the PV array has a central inverter. The solar array is composed of n strings and m panels, which are connected in parallel and send to the central inverter.

Inverter’s nominal power plate must be big enough to account for the power conversion of all the panels.

This type of system entails an extensive DC wiring to make all the series-parallel connections and then send them to the junction box. This translates in higher electrical losses.

These configurations are also sensitive to partial shading as well as mismatch losses within the installation [1].

Moreover, if your PV array consists of several strings with different orientations, then having the central inverter is not suitable because the inverter would not be able to find the optimum Maximum Power Point (MPP) for two or more strings with different orientation angles (azimuth angles).

Although, some inverters offer the possibility to combine two different orientations with two MPP inputs.
 

Grid-tied with string inverters

Due to the above-mentioned reasons, smaller inverters have been introduced to the market. These inverters are advantageous because mismatch and shading losses are diminished. It is because each string works independently from the other. That means that if the MPP of a string is affected (due to shading or manufacturing mismatch), the other strings will still work at their optimum.

Another advantage of this configuration is that the junction box can be discarded and that the DC wiring losses are smaller because the wiring is shorter (only needs the series connection) [1].

Now, let’s analyze how the variables (mentioned at the beginning of this article) influence the decision of using a grid-tied system or not:
 

Reliability of the grid

Choosing a grid-tied system is closely related to the reliability of your power grid.

For this system, the power grid should be reliable. It works well especially in industrialized countries where the power service has been privatized. In these countries, the Independent System Operator (ISO) charges large amounts of money to the power utility whenever the system fails.

Therefore, if you live in an evolved electricity market country like the US, Australia or the United Kingdom, then you can assume that your system is reliable (this of course does not take into account extreme events).

On the other hand, in countries where the electricity system is not reliable, it is good to consider another type of PV system, as the grid could continuously shut down your PV array.
 

Power load and autonomy

Power load and autonomy is closely related to the previous factor.

If you need to be absolutely dependent upon the power load your solar panels generate, then you should consider the option with the battery backup (even in industrialized countries).

Generally, grid-tied systems are configured to be fed directly to the main panel of the house and there is no way to control which appliances are fed by solar energy and which ones are powered from the grid. If you want to control and backup certain electrical loads, then you need to consider other type of a system.

The type of load that you want to feed in is also important factor to consider. If the load works with DC you don’t need the inverter. However, you will need a charge controller and a battery backup to supply the load in a stable way. In this case, it is better to consider a hybrid system rather than solely grid-tied system.

On the other hand, autonomy represents the number of hours or days when the PV system is independent from the power grid.

In the grid-tied scheme, the autonomy is zero. Therefore, if autonomy is desired, then consider a backup battery bank (hybrid system).
 

Revenues and costs

Besides reducing energy consumption from the grid (and therefore saving on your utility bills), the other main purpose of a grid-tied system is to obtain extra revenues.

This revenue comes from the excess energy injected back to the grid. The excess energy is charged at the electricity rate of your utility distribution network through the Net Metering Scheme.

Net metering means that your electricity is used by the utility company to feed other houses or systems and you get paid for it.

You may also opt for a Value of Solar Tariff (VOS) which acts as an alternative to the net metering scheme. The biggest advantage of VOS is that it does not value your solar energy only by the electricity rate, but also by the benefits that solar adds to the grid. VOS is fixed for at least 20 years in the US, while Net Metering changes daily with the electricity rates.

The overall costs of a PV systems comprise the equipment, permitting and installation.

While permitting costs do not fluctuate much from one system to another, equipment costs and installation certainly change. Grid-tied systems generally present the most economical alternative among PV systems because there is no need of adding a battery backup and charge controllers to supply the batteries.

This represent a big difference in costs compared to off-grid and hybrid systems.

Furthermore, installation costs are smaller as well, as time of connection to the battery bank is not taken into account with this configuration.
 

Size of the system and solar radiation

Grid tied configurations offer the possibility to install more powerful systems. That is why they are used for utility purposes as well.

When adding extra panels to off-grid or hybrid systems you might have to add stronger battery backup. You will not encounter this problem with grid-tied systems.

Besides, the availability of solar radiation also affects the decision about the best solar system type for you.

Particularly for grid-tied the relationship is almost linear. Your panels will produce as much electricity as solar radiation allows. Your system will reduce your energy consumption from the grid on sunny days more.
 
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Hybrid systems (Grid-tied with battery backup)

Hybrid systems are still connected to the grid, but they include one very important feature – according to what we previously talked about, it is sometimes necessary to reliably feed a particular electrical load and the hybrid configuration allows us to do so for a certain amount of time (autonomy).

In order to do so, it is necessary to add a battery bank that allows the system to store energy and supply it in a stable way when needed.

Therefore, the main difference between the grid-tied and the hybrid systems lies in the availability of power during blackouts and the possibility to back up a specified load with a particular autonomy.

In hybrid systems there are two types of loads:

  • critical loads
  • non-essential loads

 
Critical loads are electrical devices that you need to back up in the case that the power grid fails. These loads need to go into a separated sub-panel.

Non-essential loads are electrical devices connected to the main panel that will not be backed up during the grid failure.

Now, there are two available topologies to install a hybrid system: DC-coupled and AC-coupled.
 

DC-coupled

DC-coupled scheme presents the most common and simple configuration of a hybrid system and contains the following components:

    • solar panels

    • charge controller

    • inverter

    • sub-panel load

    • battery bank

    • combiner box

    • wiring

    • grounding and mounting system

The operation of the system varies depending on the time of the day and on the availability of the power grid. Thus, the functioning of the system can be divided into two main modes: Grid-connected and Island mode.
 

AC-Coupled

AC-coupled system consists of the following components:

    • solar panels

    • wires

    • sub-panel

    • main panel

    • battery bank

    • grounding system

    • mounting system

    • grid-tied inverter

    • battery based inverter

    • combiner box [3]

As you can see, the main difference from DC coupled module lies in the number of inverters and the way the system interacts with the critical loads.

In the same way, the system operates according to the availability of the power grid and the time of the day.
 

Pros and cons of hybrid systems

The operation process may seem similar at both hybrid systems, but there are some important differences between them. Keep on reading to learn about advantages and disadvantages of each one of them.

AC Coupled – Advantages

  • Can be used with micro-inverters and power optimizers [4,5]
  • PV array and battery backup can be modified without affecting each other [6]
  • Is easier to install if a previous grid-tied system is connected
  • Higher reliability. If one inverter fails for any reason, the other one can supply the load for a period of time [5]

 
AC- Coupled – Disadvantages

  • Complex design [3]
  • Higher cost due to the presence of two inverters [3]
  • Higher conversion losses due to the DC/AC conversion in the grid-tied inverter and the AC/DC conversion to the battery bank [8]
  • Requires synchronization of the two inverters
  • Battery charge rate is determined by the inverters. If one of them has a lower charge rate, then the batteries would charge slower than with the panels [6]
  • No charge controller also means that battery charge rate could be accelerated – it can cause depreciation in the life span of the battery bank [7]

 
DC-Coupled – Advantages

  • No conversion loss to charge the batteries
  • Installation and design is easier as there is only one inverter
  • Cost is usually lower than AC-coupled [3]
  • Charge rate is only limited by the charge controller as a protection against overvoltages [7]
  • Allows to couple DC loads connected to the charge controller

 
DC-Coupled – Disadvantages

  • Expansion of the battery bank or PV array affects the PV system
  • Requires the use of inverters with transformers, which means less efficient energy conversion inverters
  • Harder to couple batteries to an existing grid-tied system
  • Reliability of the system is lower than at AC-Coupled system as there is only one inverter [6]

 
In any case, for both hybrid systems, the design of the PV array must be able to supply the sub-panel load and charge the battery bank during the day (assuming that the battery bank capacity is empty from the previous day).

Now, let’s analyze how these types of systems interact with the variables that we have mentioned.
 

Reliability of the grid

This is one of the main factors affecting our decision for a hybrid system.

If grid is not reliable, the critical load of your house might not be available when you need it the most. In such case, it is justifiable to install a hybrid system instead of a grid-tied system.

Another important advantage of hybrid system is that your PV system acts independently from the grid in times of environmental disasters when the power grid goes off for prolonged times.
 

Power load and autonomy

With hybrid systems you are able to select the critical load.

You can power your entire house with the PV system, but it is not the most optimal nor economic option. Many appliances are not necessary to use during a blackout (dryers and washing machines for example). The critical load should, therefore, be selected according to efficiency and energy conservation standards [8].

The type of load also affects whether you should opt for a DC-coupled and an AC-coupled system. If you want to include DC loads in the system, the best choice is the DC-coupled option.

Autonomy in this configuration is determined by the number of hours during which you want to be independent from the grid. This is entirely related to the reliability of the distribution network in your city.

In most countries, the autonomy period of 3 to 4 hours is more than enough for a grid to be recovered from a blackout.

Autonomy deeply influences the number and size of batteries needed, practically speaking, it is advisable to get just the necessary number.

If you are new in a country, investigate the reliability of the power network through parameters like Lost of Load Expectation (LOLE) in local utility or Independent System Operators websites, they will give you an idea of the average number of days when the grid goes off throughout the year.

If you want to achieve a partial autonomy, then hybrid systems are for you.
 

Revenues and costs

Revenues are the same as at the grid-tied systems, however, costs of the system are different.

Battery banks are a very expensive part of the system, selection and dimensioning of the batteries will deeply influence the initial costs of the system and they may even require to be changed during the life span of the panels [2].
 

Size of the system and solar radiation

The size of the system has two variable components:

  • the critical loads
  • the non-essential loads

 
If you want to size your system only to provide the critical loads, then your system will be smaller. However, if you want to have your needs covered when the grid fails, you need to add more panels to your system.

The solar radiation in this case is important to consider as the system should be able to supply critical loads and charge the batteries in case of grid failure during the previous night or during the day.

Although, since the typical autonomy period of these designs is not high, solar radiation does not need to be extremely good.
 
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Off-grid systems

Off-grid systems are indeed the most expensive ones. They are only profitable and necessary when no grid is available in the place or when there is the need to feed only small loads (electronics or golf carts for example).

These systems are by definition isolated from the grid and must work on their own for a number of days – typically installed in rural areas, islands or for small lighting purposes.

The components of the system are the same as the DC-coupled hybrid system, with the exception that there is only a single panel to feed and there is a possibility to add a generator or another renewable energy source (for reliability purposes).

The main difference from the hybrid system is that the off-grid PV array must be able to supply all the demand of a single panel with a 100 percent of autonomy (ideally) throughout the year.

Let’s take a look now at the variable considerations for this system.
 

Reliability of the grid

By definition, the off-grid system assumes that there is no grid available in the area and that the system must be able to work on its own, therefore, the reliability of the grid is zero.
 

Power load and autonomy

This is a crucial aspect of the off-grid system.

The importance of the load will determine the autonomy of the system. If the load is to feed a rural or isolated house then the typical autonomy is two to three days.

This means that the system should charge the whole battery bank during a day (design consideration) and that the battery bank must be able to supply all the power load for two to three days.

Some types of loads may be more important, for example communication towers, the autonomy in such case may last up to five days, because the reliability of the load needs to be increased in the face of adverse solar conditions.

The off-grid system also allows you to decide whether you want to feed a DC load or an AC load.
 

Revenues and costs

The only revenue that you can apply for is the Federal Tax Credit where you can receive a rebate of 30 percent and that applies to the installation of any residential PV system.

There is no other revenue as there is no contribution to the grid itself [10].

Regarding costs, off-grid systems are the most expensive systems, so they should only be considered when there is no other choice.
 

Size of the system and solar radiation

The solar system size must be accurately measured, as if there is a lot of excess energy in the system you will lose money because the energy has nowhere to go. If the system is undersized, then solar panels will not completely charge the batteries before they get discharged. This will translate into a loss of load and low reliability of the system. This situation is not only undesirable, but also means terrible design sizing as well.

The solar radiation in this case plays a crucial role.

Off-grid systems cannot be implemented in places with low solar radiation as the reliability of the system would be compromised due to the absence of solar energy in the area.

Remember: Solar radiation changes throughout the year, therefore any storm or unexpected variation in the solar radiation could negatively affect the performance of the PV system.

 
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Conclusion

We have examined all the available system topologies on the solar market. The decision of one or another is directly affected by the variables: reliability of the grid, solar radiation and size of the system, revenues and costs and the power load to feed.

However, there are some basic rules that can be applied and that will help you better estimate the appropriate system for your household:

  • If you have access to the power grid, then the choice of off-grid system is discarded (unless you want to install the system to feed a small load).
  • If you want to back up a particular electrical device but still want to receive the revenues from solar energy, you should opt for a hybrid system.
  • Autonomy is determined by the reliability (or absence) of the power grid.
  • If you don’t know what system configuration between the grid-tied and the hybrid system is better for you, then request a quote from a solar installer for both considerations, the price difference might help you decide whether or not the additional costs of the hybrid system are worth it or not.

 
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References

[1] H. Haberlin. Photovoltaic System Design and Practice. John Wiley and Sons, 2012.
[2] https://greentumble.com/best-ways-to-store-solar-energy/
[3] https://www.homepower.com/articles/solar-electricity/design-installation/ac-coupling-methods
[4] https://www.altestore.com/blog/2016/09/dc-coupling-schneider-electric/
[5] https://www.energystoragenetworks.com/ac-dc-coupling-use-next-storage-project/
[6] https://www.sma-sunny.com/en/advantages-of-ac-coupled-high-voltage-battery-over-alternative-solutions/
[7] https://www.solarpowerworldonline.com/2016/10/advantages-dc-coupled-solar-storage-system/
[8] https://www.energystar.gov/
[9] https://aeesolar.com/aee-solar-design-guide-catalog/
[10] https://www.energy.gov/savings/residential-renewable-energy-tax-credit