Micro-cogeneration systems are an efficient way of meeting energy demands in buildings. They achieve the goal of distributed electricity generation, combined with useful heat, with a high global efficiency. Organic Rankine Cycle (ORC) systems are an interesting solution in cases where the heat demand is significantly larger than electricity needs, which is the case of residential and many other buildings. If those systems are fully-driven by renewable energy sources, they will have zero CO2 emissions, contributing to the objectives of nearly Zero Energy/Carbon Buildings. In this paper, a micro-cogeneration ORC system driven by a combination of solar thermal and biomass sources is described. Both sources may be used separately or combined. Its electrical output is equal to 10 kW. The power cycle uses a highly efficient rotary lobe expander. High efficiency vacuum-CPC solar collectors are used, for a target maximum ORC temperature of 145°C. The system is under development and a prototype will be built, in the framework of the REELCOOP project, funded by the EU. A computer model of the overall system is under development, combining EES with TRNSYS. EES is used for the power cycle calculations, and TRNSYS for the solar circuit and overall system. Preliminary results of system simulation under different operating modes are presented, namely electrical efficiency and global efficiency values. The possibility of obtaining useful heat at a temperature around 90°C, through a boiler economiser, is also assessed.