This paper presents the design and analysis of a scalable energy harvesting system that uses a solid/liquid phase change material (PCM), namely pentadecane (C15H32), to harvest environmental thermal energy associated with relatively low temperature and small temperature variation. The basic idea is to utilize the volume expansion of the PCM in the phase transition process to do work and generate electricity, which may be particularly suitable for powering sensors and small-scale devices that are designed to carry out long-term missions in an environment that undergoes regular, cyclic temperature variation in space or time. In this paper, we first present the fabrication and testing of a small-scale prototype in the form of a tube system, as well as the development of a thermo-mechanical model that couples the thermodynamics of the involved PCM and fluid materials with the elastic deformation of the structure. Using both the prototype and the model, we show that achieving high performance requires maintaining a high pressure inside the system, which complicates structural design and leads to incomplete melting of PCM. Towards mitigating this critical issue, we propose the idea of using a hydraulic accumulator to regulate the internal pressure. To examine this approach, we add a piston-type hydraulic accumulator to the prototype, and modify the thermo-mechanical model accordingly. We show that the hydraulic accumulator leads to a onefold increase in both thermal efficiency and specific energy output. In particular, the thermal efficiency obtained by the prototype is comparable with state-of-the-art thermoelectric generators when operating between 1 and 20 degrees C. Using the thermo-mechanical model, we also present a parametric study that shows the dependence of the system's performance on a few key design parameters.