Is an AC or DC Solar Thermal Installation Better?

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Like most questions in the solar thermal arena, the choice of an AC or DC system is subjective, depending on your needs and your system type. Let's start with the unavoidable limitations:

  • Draindown and drainback systems can only use AC power because they need high-head pumps, which are not available on 12-volt DC.
  • Any system that requires a motorized valve can only use AC power.

If your system isn't affected by one of those limitations, you can choose freely between AC and DC supplies. They both have minor advantages and disadvantages.

AC systems

An AC system's pump runs at a constant speed. It is either on or off, with nothing in between. When the pump starts in the morning, it will push solar fluid around the system at full speed, regardless of the weaker morning sun. This can cause heat to transfer from the storage tank to the collectors until they warm up. The same thing can happen in cloudy weather, causing a drop in efficiency.

Then again, an AC system's controller will switch the system off once the high limit is reached or if the tank is hotter than the collectors. This is usually a good thing, unless you run a pressurized system and don't have a controller that works with shunt loads – in that case, your solar fluid can stagnate, overheat and become useless. Get a controller with a heat diversion option if you're installing a pressurized system.

DC systems

Compared to the AC setup, a DC system always runs at the "right" speed: when there is little sun, there is little PV power so the pump runs slower; when the sun is very strong, there is more power and the pump runs faster. It is self-managing and uses one less component, as no controller is required.

On the other hand, at the end of the day the collectors are cooling and the storage tank is hot. Since there's no controller, the pump continues to run and could take heat out of the tank to the collector array, causing a drop in efficiency. Then again, the pump will run slower, so the solar fluid will be in the collectors longer and will get hotter.

DC systems also take care of the stagnation problems as the pump is always on whenever there is enough sunlight to provide power and heat the collectors. The solar fluid never sits still, even if there is little demand for heat, so it has a longer life.

Conclusions

In the end, the advantages and disadvantages of both systems are minimal. Pick the one that suits your needs and system design better. The only significant difference is the inclusion of a controller in an AC system, which is the single least reliable component in any solar thermal setup because it is so vulnerable.