Scientists at the University of Kassel are conducting very promising tests on a virtual power plant the size of Germany. Kombinkraftwerk combined three wind farms, two biogas plants, several photovoltaic solar plants and a hydro plant at various locations in the country for the demonstration program. All of these facilities are linked to a control center via the Internet. The control center draws power from the different plants, adjusting it to match demand for electricity. The control center can select one or several sources, depending on each facility's ability to generate electricity at a given time. For intermittent sources, this depends on sufficiently strong winds to operate wind turbines or enough sunshine for photovoltaic panels. The biogas plants can fill the gap day or night by storing the methane they produce. Once demand is met, surplus power is directed to the hydro plant, where water is pumped back into the reservoir to serve as an energy reserve.