Global climate change urges immediate measures to be taken to limit greenhouse gas emission coming from the fossil fuel fired power plants. Solar thermal energy can be involved in different ways in existing power generation plants in order to replace heat produced by fossil fuels. Solar field feed water preheating is mainly discussed in this paper as an option for fast and feasible RES penetration. Rankine regenerative steam cycled power plant has been modelled with Thermoflow software. The plant model incorporates also a field with solar Fresnel collectors that directly heats boiler's feed water. The proposed plant modification yields substantial fossil fuel input reduction. The best results can be obtained when the group of high pressure heaters is replaced and feed water temperature exceeds its original design value. The solar power generation share can reach up to 23% of the power plant capacity in this case, having efficiency higher than 39% for the best solar hour of the year. (C) 2010 Elsevier Ltd. All rights reserved.