This book describes the geothermal potentials of some hydrocarbon reservoirs. It explains the design differences of hydrocarbon and geothermal wells and the different stimulation methods used in hydrocarbon and geothermal exploitation. The methods of heat extraction and electric energy generation from abandoned hydrocarbon wells are described as well as mechanisms to increase permeability and porosity if the corresponding natural values are too low. The book discusses the different scenarios how geothermal energy of hydrocarbon reservoirs can be used: (1) In wells with ongoing oil and gas exploitation where co-produced hot water can be used to generate in abandoned oil and gas wells that can become a source for commercial geothermal electricity production and a huge commercial opportunity; (2) in offshore applications helping to solve the problems of disposal of oil platforms, and using them for extraction of geothermal energy; (3) drilling new geothermal wells in promising hydrocarbon reservoirs where there is detailed subsurface data available from the oil and gas drillings. All these options are discussed in the context of geological setting, technical demands and feasibility, scale, costs, etc.sustainable option for clean water production from seawater and from highly mineralized or otherwise unsuitable ground- and surface water. The book provides an overview on possible cost-efficient techniques and application opportunities for different scales and shows why the implementation of these technologies faces numerous technological, economic and policy barriers and gives suggestions how these hurdles can be overcome. Costs of novel treatment units using renewable energy sources are discussed and compared with those of other technologies for clean water production considering external costs, such as environmental and social costs which are caused by using fossil fuel based technologies. Energy efficiency is highlighted since it is of special importance in systems that are to be powered by renewable energy. Moreover applications of water supply systems providing water in emergency condition are discussed.