Water pumping is certainly the most common PV-direct application, but there are others that are also a perfect fit.
Water aeration is used to oxygenate or prevent freezing in ponds. Some systems use water pumps and fountains; others use compressed air.
A simple air compressor on shore and diffuser system at the bottom of the pond, powered directly by a PV module, can be very effective at keeping the water from freezing. Rising air bubbles bring warmer water from the bottom up to the surface, and aquatic life in eutrophic ponds with insufficient dissolved oxygen levels can also benefit from aeration.
Sizing a PV-direct compressor/aeration system is very similar to sizing a water-pumping system. Instead of total dynamic head, you’ll be calculating air pressure drop (from pump depth and air tubing size), and the flow per minute will be measured in cubic feet instead of gallons. Instead of a pump curve, you’ll consult a compressor curve. But all the basic system design concepts remain the same, and purpose-built, efficient DC compressors are readily available. Water fountain aeration systems are sized using the same math as in other water-pumping applications.
PV-direct water or air pumps can provide aeration and/or freeze protection.
PV-direct ventilation systems, such as greenhouse fans and attic fans, are also popular, and can be a very logical and cost-effective application—the more intense the sunlight, the more airflow is typically needed in an attic, greenhouse, or other structure. Systems retain the same similarities in design—the blower, controller, and PV array must all be matched to the duct size and pressure losses. Efficient, purpose-built PV-direct DC blowers are easy to find.