The world's largest new CO2 heat pump in Denmark is supplying two entire cities with heat. What's special about it is, that it uses seawater as its heat source. And other Countries want to catch up. A similar powerplant is planned to go into operation in Flensburg by 2027. It is intended to heat 20 percent of the city and could replace 6,000 individual heat pumps for single-family households. Should countries really rely on seawater heat pumps or are heatpumps working with river water the better choice? And how much is the water temperature changed by this technology?