|A direct-coupled PV system is a type of stand-alone system where the output of a PV module or array is directly connected to a DC load. These systems do not include any power conditioning equipment or electrical energy storage. The only other components are BOS components such as mounting systems and disconnects. Without batteries, direct-coupled systems are appropriate only when energy demand coincides with energy generation (when the sun is shining). PV output varies with insolation, the load must also be capable of operating over a range of power levels. DC motors are the most common loads for direct-coupled systems, including water pumps and ventilation fans. Direct-coupled PV systems are common for pumping potable and agricultural water supplies. In most of these applications, the water is used either as it is pumped or is stored in the reservoirs or tanks. Direct-coupled ventilation fans provide airflow in proportion to irradiance, which is useful for applications such as attic ventilation. Circulation pumps for solar water systems are also ideal applications of direct-coupled PV systems because the need to circulate hot water from a solar collector coincides with PV output. In this sense, direct-coupled PV circulation pumps provide inherent system control, eliminating the need for temperature controls to start and stop the circulation pump.
While direct-coupled systems are the best and simplest form of any PV system in terms of equipment, they are perhaps the most complex to design properly due to the lack of energy storage or system control. Matching the electrical load to the array is a critical part of designing a well performing direct-coupled system. The output of the array changes with temperature and irradiance, the load does not always keep the array at its maximum power. DC-DC converters or MPPT devices can help better utilize the array power output.