Transmission technologies will play a key role in any system employing widespread renewable resources for a common supply. Current transmission capacities between EU countries and to adjacent regions are entirely inadequate for transferring the quantities of electricity required for a complete renewable electricity supply. For example, the northern German grid would already be overloaded in the near future, if current plans for a massive realization of offshore wind farms would be realised without grid enforcements [IGW 01] [NDN 01] [BDH+ 03]. Capacity expansion should thus take into account the prospect of transmission over thousands of kilometers using the particularly appropriate high-voltage DC (HVDC) grid technology (s. also [ABB 01]).

The following treatment of transmission costs and losses assumes a HVDC capacity of about 5 GW. For the purpose of analysis, the city of Kassel near the geographical centre of Germany has been selected as the terminal point of the HVDC line. Costs of 60 €/kW for each of the converter stations at both ends of the line as well as 70 €/(kW 1000 km) for (double bipol) HVDC overhead transmission lines and 700 €/(kW 1000 km) for ocean cable have been assumed (s. also [Hau 99]). The relative transmission losses at full load are 4%/1000 km in the lines and 0.6% at each converter. The losses are greatly dependent on electrical loading and have been treated accordingly. The life expectancy has been conservatively estimated at 25 years for cost calculation purposes (more than 100 years lifetime is realistic for overhead lines [Wan 03]). An interest rate of 5% has been assumed, and the annual operating costs have been set at 1% of the initial investment
costs. With transmission line lengths assumed to require extended distances due to the inevitable geographic limitations of direct routes, a rated transmission capacity equal to the rated power of the wind and solar generators is employed. (The rated power of the transmission lines is about 50% below the thermal transmission limit, which is worth to be mentioned since it involves an inherent technical immunity against faults.) The same specific cost figures for the converters and the transmission lines as well as the interest rate and the computational lifetime are used for the individual scenario calculations.