In December 2013, after only nine months of construction, the Gonghe PV solar park was commissioned and connected to the power grid via the nearby Longyangxia hydropower plant on the Yellow River. This marks the first commercial operation of a large-scale solar PV–hydro hybrid system.

The hydropower station was originally designed and commissioned in 1992 as the first load-peaking and frequency regulating power plant for the north-western power grid. It employs quick–response turbines, which smooths the output curve of the PV power, caused by natural fluctuations in sunlight due to cloud cover and time of day.

By smoothing the power curve, the hybrid-connection allows for the exploitation of an intermittent energy source to provide good-quality, safe and reliable power to the grid.

The Longyangxia solar–hybrid power station is located in the arid north-west of China, in an area with vast solar resources. The reservoir supports a 1,280 MW power station, with four 320 MW turbines.

Since 1999, a unified policy for the regulation of the Yellow River was adopted, and the Longyangxia reservoir is one of the main elements of this as one of the first cascaded projects on the main reach of the upper part of the Yellow River, it controls downstream releases to reservoirs lower on the river, providing those projects with a stable and reliable supply of water.

The Longyangxia PV plant has a capacity of 320 MW and covers a 9 km2 area. It is connected directly to one of the turbine units by a 330 kV transmission line. As one of the largest solar PV stations in the world, without the balancing power of the Longyangxia hydro turbine, this could pose a serious problem for the stability of the grid.

While the use of small amounts of intermittent power has little effect on grid operations, larger penetration of variable power can affect the grid’s ability to operate as required.

Already one of the largest PV power plants in the world, the construction of phase 2 of the solar power plant, which will bring the capacity to 530 MW"

Developing a joint hydro/PV operation control system, effectively allowing the PV plant to act as Longyangxia’s fifth turbine, allows for almost immediate compensation between hydropower and PV generation. In essence, the active power output of the PV plant is adjusted using the hydropower turbine to achieve a smooth and stable output curve.

As a whole, this increases the efficiency of both plants, and requires less spinning reserve capacity in the grid itself, further increasing the efficiency of the grid.

The addition of the solar park also increases the operation efficiency of the hydropower plant. Qinghai province is dry, and water is a scarce resource, so the Longyangxia reservoir only releases water with caution. With the addition of the solar project, the hydropower station has been able to increase its annual capacity utilisation and economic efficiency.

Already one of the largest PV power plants in the world, the construction of phase 2 of the solar power plant, which will bring the capacity to 530 MW, was commenced in August 2014, and is scheduled to come online in late 2015.

Large-scale centralised PV power is still in its infancy, and the Longyangxia coupling of PV and hydropower is the first of its kind and provides a valuable example for future hybrid systems linking variable renewables and hydropower.

This case study is featured in the 2015 Hydropower Status Report, which provides detailed analysis of recent global trends in hydropower development. You can download the full report here.