Cities play an essential role in facilitating and supporting the real-world experimentations (for instance in public spaces with real users) of innovative products and services in the field of clean technologies. In this respect, the City of Paris has implemented an experimentation mechanism to help innovative start-ups improving their solutions and robustifying their business models in a multi-stakeholder ecosystem. Nonetheless, a primary investigation demonstrated that the efficiency of these in vivo in situ experimentations have means of being improved. To deal with this issue, the current paper proposes a critical review and a characterization of the existing projects related to clean technology and sustainability themes. A study over 25 experimentation projects results in identifying matches and mismatches between the expectations and the obtained results of tests by innovative start-ups. We have statistically demonstrated that the most important purpose of running experimentation projects by start-ups is to test and build a relevant " stakeholders network " around their businesses. Furthermore, we have assessed the influence of these experimentations on the design of innovative products.