This paper presents the analysis of the performance of two different pre-commercial MD desalination modules developed by the Singaporean enterprise Keppel Seghers tested under real conditions intermittently for at least 2400 h during two years, directly coupled with a static collector's solar field. Both modules are based on the air gap membrane distillation (AGMD) configuration with a total membrane surface of 9 m(2) each. One of the modules is a compact single design while the other one consists of three stages connected in series. The performance of both modules (i.e. distillate production and quality, thermal efficiency and recovery ratio) has been characterized as a function of operational parameters. Especial attention was paid to thermal efficiency issues. Aqueous NaCl solutions of 1 and 35 g L-1 concentration were used as feed. Minimum specific thermal energy consumption was in the range of 1805 kWh(t)m(-3) for the compact prototype and around 294 kWh(t) m(-3) for the multi-stage one. Distillate quality was excellent (in the range of 2-5 mu S cm(-1)) and practically not affected by feed flow rate, hot inlet temperature or feed salt concentration. The paper shows the production and distillate's quality as well as the thermal efficiency results as a function of the operational parameters. (C) 2012 Elsevier B.V. All rights reserved.