This paper presents a study of energy requirements of membrane distillation (MD) for different lab-made flat module designs of 40 cm(2) membrane area. The MD runs were carried out in direct contact membrane distillation (DCMD) and vacuum membrane distillation (VMD) mode with a 0.2 mu m polypropylene membrane and in all tests pure water was fed to the system. In DCMD, the effect of the operating temperatures and streams flow rates on the flux, the evaporation efficiency and the energy consumption, was studied, whereas in VMD the parameters analyzed were the feed flow rate, the feed temperature and the vacuum applied at the permeate side. The VMD performed better than the DCMD and the cross-flow module resulted to be the most efficient design for obtaining high fluxes with moderate energy consumptions. The highest flux (56.2 kg/m(2) h) was achieved with the cross-flow module working in VMD at a feed flow rate of 235 L/h, feed temperature of 59.2 degrees C and a permeate pressure of 10 mbar. The lowest values of energy consumption/permeate flow rate ratios obtained were 3.55 kW/(kg h(-1)) (longitudinal-flow membrane module) and 1.1 kW/(kg h(-1)) (cross-flow membrane module) for DCMD and VMD tests, respectively. (C) 2007 Elsevier B.V. All rights reserved.