The objective of this research is to combine renewable solar thermal energy and seawater membrane distillation desalination systems into green processes. The units of the systems include solar collectors, heat exchangers and membrane distillation modules. In order to assess the economic design point of the process, the Aspen Custom Molder (ACM) was used to build the mathematical model to describe each unit of solar membrane distillation desalination systems. Simulation results show that the optimal total annual costs (TAC) of direct contact (DCMD) and vacuum membrane distillation (VMD) modules are $857,990 and $576,359, respectively. The fundamental differences between DCMD and VMD are driven by temperature and pressure variation on either side of the membrane. A larger temperature difference between the hot and cold sides of the membrane will require an increased heat supply from the solar collector and the heat exchanger. This will increase a dramatic in cost of the solar collector and the heat exchanger. Finally, the operability analyses in the optimal design points were done for DCMD and VMD systems. The VMD systems provided a wider operability range than DCMD.