As it is known, heat sources are widely available, particularly at a low/medium temperature, which are now dispersed in the environment, and therefore wasted. De facto, the conversion of the heat supplied by said sources into electric power is, by the nowadays available recovering and converting means and processes, too expensive in relation with the power produced. Therefore, such sources, even though are used in a limited way for professional applications, are scarcely used by the people, and particularly in the domestic environment.
The most common heat sources, which here it is preferentially made reference to, are available both as a by-product of the human activity and in nature, such as for example the heat contained in the waste industrial products or the heat contained in the biomasses if the latter are combusted.
Several applications of the Rankine cycle for recovering thermal power and the consequent production of electric power are known. The preferred embodiment consists of using, as expansion chamber, a turbine. However, such solution has some constraints and disadvantages which are well known to the person skilled in the art, and which are:                high cost of the turbine and of the associated control elements;        necessity of a frequent maintenance with following duties of different type;        maximum efficiency which is only obtained at a precisely determined flow rate of the expanding fluid and at a defined rotation speed; specifically, this is perhaps the greatest limitation of the turbine systems because if the rotation speed is affected by a slight variation with respect to the optimal value, the turbine efficiency drastically drops.        
For the above mentioned reasons, it is absolutely evident that the steam turbines are not very suitable for harnessing medium/low temperature thermal sources and having an extremely variable thermal supply (as indicated in the above exemplified examples) and therefore not very suitable for small-sized plants (having a supplied electric power less than 50 KW, for example).
From documents JP 10252558, JP 10252557 and JP 10259966, some known different technical solutions using the Rankine cycle for different objects are known; however, none of the suggested solutions is particularly advantageous for generating electric power, particularly if the thermal power is supplied under an extremely variable range.
In order to overcome the above described disadvantages, it is known to use alternate or rotative volumetric expanders. Such expanders are capable of operating under relatively modest fluid flow rates without excessively reducing the power and efficiency. Further, volumetric expanders, operating at smaller thermal powers, operate at a number of revolutions (cycles) substantially smaller than the turbines rotation speeds eliminating in this way the risk of damaging the movable parts in case the liquid (drops formed by an incorrect vaporization of the working fluid) flows into the expansion chamber. Further, the above described volumetric expanders have a structural complexity smaller than the one of the turbines, with a consequent reduction of the costs.
Besides a reduced complexity, volumetric expanders are extremely more compact than the turbines, which in turn makes easier their implementation, and assembly.
An example of a volumetric expander used for converting thermal power in electric power by means of low temperature heat sources, is described in the patent application US 2012/0267898 A1 of the Applicant.
Such application describes a Rankine cycle machine comprising a cylinder and an associated piston adapted to alternately move inside said cylinder. To the piston is associated a main shaft which, in turn, is connected to a DC voltage generator formed by a rotor and a stator: the rotor is connected to and actuated by the main shaft. The cylinder is provided with an intake port and a discharge port which the working fluid flows through. For actuating the piston, the machine uses a rotative valve enabling the desired sequence among the steps of introducing, expanding, and discharging the fluid. In order to synchronize such steps to each other, the rotative valve is actuated by a plurality of motion transmission members connected to the main shaft.
Despite the described solutions (volumetric expanders) are, under conditions of low temperature heat sources, enhancing in comparison with the turbines, the above described volumetric expanders are not devoid of disadvantages. Particularly, the Applicant believes the known volumetric expanders, and also the machine described in patent application US 2012/0267898 A1 of the Applicant, are further improvable under different aspects.