A big increase in the number of solar thermal cooling installations and research efforts could be seen over the last years worldwide. Especially the producers of solar thermal collectors and systems have been looking for thermal chillers in the small capacity range to provide air conditioning for one or two family houses. Furthermore, many developments aim to increase the efficiency of the system and to decrease the specific costs of the produced refrigeration capacity. The growth in the use of solar thermal cooling systems amounted about 860% from 52 units in 2004 to 450 units in 2009 [1]. This tendency is expected to be continuously in the next years. The practical examinations on solar thermally driven absorption machines with refrigeration capacity of 15, 10 and 5 kW have shown that this technology has a good chance to be standardized and to replace partly the conventional one. These systems can save more primary energy at high fraction of solar thermally driving by suitable control and regulation of the system. The investing costs still higher as the conventional one, however, the operating costs are less than the conventional one. The Coefficient of Performance (COP) depends on the kind of the system, work temperatures and conditions as well as the refrigeration capacity of the systems. It lies between 0.4 and 1.2. In the framework of the research on this field, we built, tested and measured two prototypes. After measuring the first prototype, the chillers were redesigned to reduce internal heat losses and make the heat and mass transfer over the surfaces of heat exchangers more effective. Thus, many investigations have been done on some types of heat exchangers for optimization of heat and mass transfer in the system. In this contribution we will show some investigation results on solar thermally driven system in small capacity range. Moreover, we will illustrate an experimental setup for investigation of heat transfer by shell tube heat exchanger consisting of two kinds of tubes. The first one has a smooth outer surface and the other one has a ribbed outer surface. The aim of these investigations is to optimize the whole system. (C) 2012 Elsevier Ltd. All rights reserved.