SPPRO

AC Coupling vs DC Coupling – Off Grid Solar

Date: October 20, 2012

Off Grid solar DC coupled systems typically use a stand alone (Off grid solar) inverter  and a solar charger (Maximum Power Point or a Switched regulator). Traditionally, these are the most common types of systems and work by converting the charge from the renewable energy source i.e. solar panels to the battery bank and then taken from the battery bank and inverter to supply any electrical load.

Off Grid solar AC coupled systems use a bi-directional (Off Grid solar) inverter-charger and a grid-interactive inverter in the replacement of a solar charger. The bi-directional inverter-charger acts as a mini grid and the grid-interactive inverter converts the renewable energy source to mains electricity i.e. the work traditionally done by the battery bank is by-passed.

They do require that a bi-directional inverter-charger (Off Grid solar inverter-charger) is used rather than a stand alone inverter and the work traditionally done by the solar charger i.e. charging the batteries is performed by the bi-directional inverter-charger. That is they allow any non-used electricity to be converted into DC electrical energy stored in the battery bank.

An Off Grid solar AC Coupled system is slightly more efficient in conversion of renewable energy sources to mains electricity, yet only when the electrical demand occurs during the day i.e. when there is a direct demand for electricity and when renewable soucres are available.

In this scenario, that is when there is a direct use of the energy available, the user can expect the average efficiency of the grid-interactive inverter, approximately 96%. In the scenario of an Off Grid solar DC coupled system, where there is a direct use of the energy available, the user can expect the average efficiency of the inverter minus the conversion from the battery bank, approximately 88%.

Again, however this is only applicable when there is a direct use of the electricity available at the time. This increase of 8% for the same energy source i.e. the same solar array connected to either an Off Grid solar AC coupled or Off Grid solar DC coupled system. An Off Grid solar AC coupled system is favourable to the user when there is a higher demand during day time load i.e. running of air-conditioners, refrigeration, water pumping or any other load occurring during the day time. (The cost for an Off Grid solar AC Coupled system should not ultimately be more than 8% than for the Off grid solar DC system).

Another benefit of an Off Grid solar AC Coupled system is that where there is a long distance between the solar array (or wind turbine etc) and the inverter-charger a smaller gauged wire can be used – as grid-interactive inverters generally have a higher input voltage than solar chargers i.e. Maximum Power Point Trackers. The higher the voltage the less resistance in copper cable, therefore a smaller gauged wire can be used. Hence, there may be additional savings in cabling costs.

If there is not a long distance between the solar array and the inverter-charger and if the demand for electricity does not generally occur during the day then there are certain advantages of Off Grid solar DC coupled systems over an Off Grid solar AC Coupled systems.

In an Off Grid solar DC coupled system, there is a higher efficiency in charging a battery bank. Also there is a higher efficiency at times when there is no renewable energy source available i.e. at night from a solar array. A stand alone inverter generally has a peak efficiency up to 95%, a bi-directional inverter-charger has a peak efficiency around 92%, that is the conversion of electrical energy from the battery to the electrical load.

There is a general acceptance that an Off Grid solar DC coupled system in this instance is approximately 4% more efficient due to these factors.

Another factor in terms what system would be most advantageous is whether the system is designed for future upgrades. As grid-interactive inverters tend to be more flexible in terms of their input values i.e. they usually have a wider voltage input range than solar chargers. System expansion ultimately depends on whether a grid-interactive inverter, or Maximum Power Point Tracker (solar charger) has any contingency to allow for a higher renewable energy source.

Off Grid solar AC coupled systems must have a bi-directional inverter which is greater in size than the grid-interactive inverter, having a higher value feeding from the grid-interactive inverter will cause the bi-directional inverter to ‘back-feed’ hence will cause failure. In an Off Grid solar DC coupled system, due to the renewable energy source feeding directly to the DC side of the system the renewable energy source can have a higher value than the output of the traditional stand alone (Off Grid solar) inverter.

From a users point of view, they generally have to look at their usage of electricity and determine whether they require a significant electrical source during the day i.e. for the use of air-conditioners and refrigeration etc. Where the electrical usage is generally at night i.e. lights, computer, tv, entertainment and kitchen appliances then an Off Grid solar AC coupled system may not necessarily be the most practical option.

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