Patent Publication Number: US-4729226-A

Title: Process for mechanical power generation

Description:
BACKGROUND OF THE INVENTION 
     The excellent gross electrical efficiency of large conventional thermoelectric power plants, as well as the current tendency to further enlarge these plants, is well known. 
     At the same time and for specific applications, especially in the highly industrialized countries, a trend exists towards the construction of small plants with power below 50 MW. Plants of this type, using conventional thermodynamic cycles, generally use renewable energies, municipal wastes or waste heat, and are characterized by their low efficiency. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     Proposed herein is a new thermodynamic cycle, characterized by high design point efficiency and high partial load efficiency, high functional stability, simple construction and relatively low cost. 
     The main applications of this process are in the field of energy sources with temperatures greater than 400° C., utilizing solar energy, municipal wastes, biomass, as well as industrial heat effluents. This process is also appropriate for heat recovery at variable temperatures and below 400° C., for example, diesel engine waste heat. The process is also useful for industrial applications, e.g. total-energy plants and for urban district heating. 
     In order to obtain high efficiencies in relation to the Carnot Cycle, operating at high maximum temperatures and in low-power installations, which conventional cycles fail to achieve, a cycle with the following characteristics was sought: 
     1. Good adaptation of the heat absorption curve of the cycle to the cession curve of external heat sources with high minimum temperature, in order to keep low the exergy losses through heat transmission between the external heat source and the cycle. 
     2. Expansion or expansions in a turbine with optimum thermodynamic conditions in order to be able to use simple turbines (single stage, if possible) with high isoentropic efficiency, both at the design point and at partial load. For this, it is necessary to operate with fluids of high molecular weight, with moderate maximum pressures and with low pressure ratios to permit a high degree of reaction and dry expansion. 
     3. Absence of vacuum in the installation in order to eliminate the energy losses in this context and to be able to condense at the minimum temperature that the cooling medium permits, or even with variable condensing temperatures in accord with the time of the year. 
     A cycle of the foregoing characteristics can not be achieved by operating with a single fluid. From studies carried out, it has been deduced that, working with maximum temperatures of the order of 400° C. (to obtain a high absolute efficiency), it is necessary to use at least three cycles, each with a single fluid, coupled in cascade, in order to achieve the above-mentioned objectives. Each of the three cycles would operate with a different fluid whose boiling point would be adapted to the temperature range assigned to said cycle. Water could not be one of the fluids, since, operating as the intermediate cycle, it could fulfill the first and the third conditions above, but not the second, because of its low molecular weight. 
     This solution has the inconvenience of demanding an extra heat exchanger surface for heat recovery (particularly because the fluids of high molecular weight and dry expansion imply recovering considerable heat at the turbine outlet) and needing three cycles and three turbines, with the consequent operational complexity and effect on the costs. 
     The proposed invention herein relates to substituting for the two single-fluid cycles which would operate in high and intermediate temperature range, a single cycle which operates with a mixture of two immiscible fluids with notably different boiling points, while maintaining the single-fluid cycle which operates in the low temperature range. The reason for keeping this last cycle separate is the unavailability of refrigerant fluids appropriate for use at the low temperature range which have a high molecular weight and can withstand temperatures of the order of 400° C. 
     Compared with the above-described ternary cycle, this binary cycle is less complex to operate, i.e. its operation is similar to that of a conventional cycle having a single fluid, since the secondary cycle of refrigerant fluid may be a standard compact unit, which starts up, operates and stops automatically and independently according to the energy it receives from the primary cycle. 
     Working with a mixture of two fluids offers the advantage that, although the fluids used must have suitable boiling points for the temperature range that each of them covers, the condition of having high molecular weight need not be met separately by each of them, but it is sufficient if it is fulfilled by the mixture which expands in the turbine. In this way, water can be used as the fluid of the lowest boiling point in the mixture, provided that the other fluid has a high molecular weight. This offers the advantage of being able to use steam seals in the turbines without contaminating the working fluid. 
     Compared with the two independent cycles (those that have been substituted for) in the cited ternary cycle, the cycle with the fluid mixture also offers another advantage, which is to reduce the circulating fluid masses and, above all, to drastically reduce the heat exchange surface necessary, not only because it has fewer heat exchanges but also because these take place, in great part, with condensations and vaporizations (eutectic at constant temperature and non-eutectic at variable temperature) instead of with superheated vapor. 
     The basic plan of the proposed cycle is shown in FIG. 1. In particular, the primary cycle comprises: 
     (a) dry expanding of the dry vapor mixture from the maximum working pressure and temperature to the minimum working pressure, to produce an expanded vapor mixture; 
     (b) cooling the expanded vapor mixture and then condensing at variable temperatures part of the substance of the higher boiling point, the heat yielded by said condensation being recovered by the primary cycle (in step (e) below); 
     (c) separating the part of the substance of higher boiling point which has condensed in (b), and pumping the condensed liquid to a point of equivalent temperature in step (e); 
     (d) total condensing the vapor mixture remaining after (c) in a heat exchanger which transfers heat from the primary to the secondary cycle, first at variable temperatures until it reaches the eutectic composition and then at the eutectic temperature corresponding to the minimum working pressure of the primary cycle; 
     (e) recovering the heat ceded in operation (b) for heating the mixture condensed in operation (d) and for partially vaporizing it; 
     (f) absorbing heat by the mixture in two phases obtained from operation (e), the mixture vaporizing totally until reaching the maximum working temperature of the primary cycle, to return to operation (a). 
     Depending on the mass ratio of the two substances in the mixture of the primary cycle, the ratio between the heat recovered in the cycle itself and the heat absorbed from an external heat source varies notably. In this way, the cycle can adapt to the temperature curve of the heat source. 
     The practical execution of the heat recovery and the heat absorption from the external heat source varies depending on the mass ratio to be used. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic of the cycle of this invention. 
     FIG. 2 is a schematic of the cycle of this invention using water and diphenyl oxide. 
     FIG. 3 is a plot of t-ΔH for the cycle of FIG. 2 with a maximum temperature of 400° C. 
    
    
     DETAILED DESCRIPTION OF A PARTICULAR CASE OF THE INVENTION 
     In the following practical example, the primary cycle works with a mixture of water and diphenyl oxide and the secondary cycle with FREON R11. 
     FIG. 2 is an embodiment for recovering energy from sources with a constant or variable temperature whose minimum temperature would be relatively high. 
     The cycle includes two turbines (T-I and T-II), external heat supply equipment, two recuperators (R-I and R-II), a kettle boiler, an R11 vaporizer, a condenser, a phase separator and three pumps (P-I, P-II and P-III). In this case, the two recuperators and the kettle carry out the heat recovery of the primary cycle. The process works in the following manner: 
     Mixtures of liquids, diphenyl oxide and water (point 1) coming from the R11 vaporizer, is pumped (P-I) to the maximum process pressure and is introduced (point 2) into the pipes of the recuperator (R-II). 
     The heated liquid mixture (point 3) is then introduced into the boiler shell. In this, the water vaporizes together with a small proportion of diphenyl oxide, generating a eutectic mixture of vapors (point 4) at the eutectic temperature for the maximum process pressure. The remaining liquid diphenyl oxide is extracted from the bottom of the kettle shell, where it accumulates due to its greater density, and is sent to the diphenyl oxide vessel (point 14). 
     Before entering the recuperator (R-I) pipes (point 5), the eutectic mixture of vapors generated in the kettle (point 4) is mixed with the liquid diphenyl oxide pumped by P-II at the maximum process pressure (Point 18). This liquid diphenyl oxide has been collected in a vessel from various points of the cycle (14, 15, 16 and 17), as can be seen in FIG. 2. 
     In the R-I recuperator pipes, a non-eutectic vaporization of liquid diphenyl oxide takes place. This vaporization is at variable temperature, in such a way that at each point of the transformation, the temperature is the saturation temperature of diphenyl oxide for the partial pressure of this in the non-eutectic mixture of diphenyl oxide vapor and steam, which accompanies the liquid diphenyl oxide through the pipes. At the outlet of the R-I recuperator pipes (point 6) there is still an important amount of liquid diphenyl oxide together with a non-eutectic mixture of diphenyl oxide vapor and steam. 
     This stream, which is in two phases then passes to the external heat supply equipment where, again at variable temperature, the liquid diphenyl oxide is vaporized. At the outlet (point 7), all the liquid diphenyl oxide has vaporized, obtaining a mixture of diphenyl oxide vapor and steam, which is dry and saturated in diphenyl oxide (it could also be superheated). In the case of this example, for a determined maximum temperature, the maximum cycle pressure determines the proportions of diphenyl oxide vapor and steam at point 7, because, as the mixture is saturated in diphenyl oxide, the partial pressure of this must be that of saturation of the diphenyl oxide at the maximum cycle temperature. 
     The vapor mixture generated in the external heat supply equipment enters the turbine (T-I) where it expands to a suitable pressure for the subsequent heat recovery stage. The mixture expands, superheating, due to the strong tendency that the most abundant component (diphenyl oxide) has. Accordingly, the expansion is completely dry. 
     The superheated vapor mixture exhausted from the turbine (point 8) passes to the hot side of the successive heat exchangers of the heat recovery stage, whose cold side has been described above. Firstly, it passes to the R-I recuperator shell where it cools until reaching the dew point of the mixture at the existing pressure. From this point, the condensation of diphenyl oxide begins, at variable temperature, for the same reason as in the case of the vaporization therein. 
     At the R-I recuperator outlet, there exists liquid diphenyl oxide which has condensed and a remaining vapor mixture saturated in diphenyl oxide. The condensed liquid diphenyl oxide (point 15) is drained into the liquid diphenyl oxide vessel. The vapor mixture (point 9) passes to the boiler pipes. 
     In the boiler pipes, the diphenyl oxide continues condensing at variable temperatures. At the outlet, a phase separator collects the liquid diphenyl oxide, which is drained into the liquid diphenyl oxide vessel (point 16). The remaining vapor mixture (point 11), saturated in diphenyl oxide, passes to the R-II shell, where, again at variable temperature, a part of diphenyl oxide condenses, to be extracted at the R-II recuperator outlet (point 17) and carried to the liquid diphenyl oxide vessel. The remaining vapor mixture (point 12), saturated in diphenyl oxide, goes to the refrigerant fluid (R11) vaporizer. 
     In the R11 vaporizer, the vapor mixture condenses in the following manner: firstly, a portion of the diphenyl oxide condenses, until the vapor mixture reaches its eutectic composition at practically the same temperature as that of saturation of the water at the given pressure. Then, diphenyl oxide and water condenses simultaneously, until it becomes the liquid mixture at the beginning of the description of the cycle (point 1). 
     In the secondary cycle, the refrigerant fluid, vaporized in the shell zone of the R11 vaporizer (point 21), passes to the turbine T-II to dry expand, superheating, to the saturating pressure for the fixed condensing temperature (point 22). This pressure is equal or slightly higher than the atmospheric pressure. From there it passes to the final condenser (point 19) to temper and condense and finally it is pumped to the vaporizer by P-III, at the maximum pressure of this cycle (point 20). By using a refrigerant fluid with a greater tendency to superheat in the expansion, or by superheating the refrigerant fluid with the heat absorbed from the primary cycle, the liquid could be preheated by the desuperheating of the vapor exhausted by the turbine, which would augment the efficiency of the secondary cycle.