A PROCESS FOR CONVERTING THERMAL ENERGY INTO MECHANICAL ENERGY

Abstract: The invention relates to a process for converting thermal energy into mechanical energy by utilizing low grade heat in a hydrophilic liquid. The hydrophilic liquid is mixed with a hydrophobic working medium, which is chemically and physically inert in the presence of the hydrophilic liquid, and the mixture is introduced into an evaporation chamber wherein the working medium is brought to boil, whereby heat is taken up from the hydrophilic liquid by direct heat transfer therefrom. The working medium vapour is expanded in a turbine, and the expanded working medium vapour is condensed.

Description: A process for converting thermal energy into mechanical energy. The present invention relates to a process for converting thermal energy into mechanical energy by utilizing low grade heat in a hydrophilic liquid. The invention is particularly suitable when waste heat in the lower tempera¬ ture range (<100°C) is to be utilized. The maximum performance of thermodynamic engines is usually defined by the so-called Carnot efficiency which sets forth the maximum working efficiency by means of the temperature difference heat source - heat sink and the actual working temperature. At a heat source temperature of 40 - 60°C and a heat sink temperature of 5 - lθ°C about 10 % of the heat flow is obtained as mechanical work. If it is desired to obtain 1 MW as mechanical work in a working machine the heat surface for evaporating working medium shall be passed by 10 MW and the heat surface for condensing the working medium by 9 MW. Since it is important to use as little as possible of the totally available temperature decrease in the heat exchange processes the costs for the heat surfaces tend to preclude the utili¬ zation of waste heat for economic reasons, even in cases where the waste heat is free of charge. If it is desired in a heat-power process to utilize the temperature difference in the seas between warm surface water and cold bottom water said heat exchanger costs will amount to 30 - 50 % of the total investment costs. According to the invention it is possible to replace the heat transfer via such heat exchange surfaces by heat transfer over the interface between two non-miscible liquids. Said interface may be increased without any great costs, e.g. by emulsifying one medium in the other. By a contem¬ poraneous evaporation of one medium this phase will be further divided. Condensation in a jet condenser will in¬ crease the interface by mechanical surface enlargement in a well-known manner. The invention relates to a process for converting thermal energy into mechanical energy by utilizing low grade heat in a hydrophilic liquid, and this process is charac¬ terized in that the hydrophilic liquid is mixed with a hydrophobic working medium, which is chemically and physi¬ cally inert in the presence of the hydrophilic liquid, that the mixture is introduced into an evaporation chamber wherein the working medium is brought to boil, whereby heat is taken up from the hydrophilic liquid by direct heat transfer therefrom, that the working medium vapour is expanded in a turbine, and that the expanded working medium vapour is condensed. A process adapted to utilize the heat in liquids, particularly water, the temperature of which cannot be allowed to sink particularly much, is provided according to the invention. Examples of such liquids are scrubber water, spent cooling water, etc. but also warm surface water in the seas. According to the invention expensive and cumbersome evaporator constructions are avoided. The above mentioned objects are achieved by mixing a hydrophilic liquid, e.g. water, with a hydrophobic working medium, e.g. normal butane, which is chemically and physically inert in the presence of the hydrophilic liquid. The mixture is then introduced into an evaporation chamber where the working medium is brought to boil, whereby heat is taken up from the hydrophilic liquid by means of direct heat transfer there¬ from. In order to utilize the invention efficiently it it advantageous to mix controlled quantities of liquid and working medium at such a pressure that evaporation of working medium is avoided. Because of the controlling mixing an interface of well defined size is obtained between the liquid and the working medium, which results in a constant heat transfer rate. Since there is no solid surface between the two media it is favourable to control the amount of working medium so that the desired temperature decrease is attained. When using scrubber water this often means 5 - 10°C, when using sea water often 1 - 2°C. No problems therewith are implied in the present invention since an increase of the interface for heat transfer is very cheap. According to the invention the heat source, normally water, should be brought into contact with the working medium in such a way that a satisfactorily large heat transfer surface is guaranteed from the beginning at the interface between the two media. The size of this surface must be maintained because the heat transfer between the media and thus the amount of working medium vapour to the turbine is determined by the size of the heat transfer surface. In order to use the turbine in an efficient way it is essential that the amount of working medium vapour is kept as constant as possible. This is achieved by using a static mixer which divides the two supplied flows, liquid and working medium, in smaller and smaller parts, the mixer being operated at such a pressure that evaporation is avoided. Such a mixer may consist of a cylindrical tube in which a number of diametrically arranged plates are posi¬ tioned after one another. The angular position of each plate in the tube varies along the length of the plate such that the angular position at the outlet end is displaced 90° relative to the angular position at the inlet end. since all plates are mounted in the same way the inlet end of one plate will be at right angles to the outlet end of the preceding plate. In this way it is easy to divide the flows so that the number of particles of working medium will be e.g. between 2 15 and 2 50 per ton of working medium. At the later evaporation of the working medium vapour volumes are created such that satisfactory mixing of the media is obtained to guarantee a substantially complete evaporation. It is possible to work with very small temperature differen¬ ces between the two media in the evaporator. If, for in¬ stance, a temperature difference of 1.0°C corresponds to a vapour pressure of 2 - 3 m water column vaporization of butane can not be prevented in a liquid layer lying 1 - 2 m below the liquid surface. According to one embodiment the evaporation vessel may consist of an upright cylinder containing a surface enlarging packing (solid bodies) or perforated plates. Also nozzles may be used. With the simplified invented way of producing working medium vapour of suitable inlet pressure for turbines it is possible to find more applications for the technique according to the invention than has been possible earlier. One example is the case where scrubber water is obtained when flue gas is purified in a closed water system. Said scrubber water is cooled in the evaporator. If there is no use for waste heat said heat source may be utilized for electric power production in accordance with the invention, that is by expanding the working medium vapour in a turbine. The chilled scrubber water is returned to the scrubber where it is re-heated. According to the invention it is also possible to replace the condenser surface of the turbine, where the expanded working medium vapour is condensed, with cooling water in a jet condenser where the water is in direct contact with the expanded working medium vapour. After the direct contact between the working medium and the cooling water the two phases are separated from each other gravimet- rically after which the cooling water is discharged while the liquid working medium is returned for repeated use in the process. As working medium one can of course use a number of substances. The criteria for their suitability are chemical stability against water, reluctance to form hydrates in the actual temperature and pressure ranges, suitable vapour pressure curve in respect of the actual temperature range within which the turbine is to work, low costs and, not the least, harmlessness from a discharge aspect. It has turned out that normal butane, isobutane and other aliphatic hydrocarbons fulfil all these requirements. They are inert in their relation to water, they show no tendency to form hydrates, they are noncorrosive, they have suitable vapour pressure curves in respect of the actual temperature range and they are considered to be unusually harmless in respect of discharge. By using a working medium which boils at a temperature of about 0°C or below it is avoided that the turbine construction becomes unnecessarily expensive due to large vapour volumes and low working pressures. An embodiment of the invention is described below with reference to the accompanying drawing which schemati¬ cally shows a device for carrying out the process according to the invention. Warm water being the waste heat source is conveyed through at line 5 to a mixer 3. The main part of the circu¬ lation working medium (e.g. normal butane) in liquid form is also conveyed to the mixer 3 through a line 6. The pressure in the mixer is adjusted by means of a valve 7 so that evaporation of the working medium is avoided in the mixer. By adjusting the number of mixing stages in the static mixer 3 the total interface for the initial heat transfer may be selected such that the temperature difference between the media will be desirably small. The mixture in the mixer 3 is fed, via the valve 7, into an evaporation vessel 1 provided with perforated plates 2. Evaporation takes place spontaneously downstream the expansion valve 7. The working medium vapour formed is washed with a minor amount of liquid working medium which is fed to the top of the evaporation vessel 1 via a line 8. The working medium vapour is fed to a turbine 10 through a line 9, and the work performed by said vapour in the turbine 10 is converted into electricity in a generator 11. After the pressure reduction in the turbine 10 the working medium vapour is conveyed through a line 12 to a jet condenser 13 wherein cooling water added through a line 15 is atomized by means of nozzles 14. The condenser 13 is vented via a line 17 and a pressure control valve 18. The mixture of liquid working medium and cooling water is conveyed through a line 16 to a separation vessel 19 wherein the media are separated into two layers. The lower layer consists of used cooling water which is dis¬ charged through a line 20, while the upper layer consists of liquid working medium which is returned to the process through a line 21 by means of a pump 22 after heat exchange in a heat exchanger 4 against cooled water 23 coming from the evaporator 1.

Claims: C l a i m s 1. A process for converting thermal energy into mecha¬ nical energy by utilizing low grade heat in a hydrophilic liquid, c h a r a c t e r i z e d in that the hydrophilic liquid is mixed with a hydrophobic working medium, which is chemically and physically inert in the presence of the hydrophilic liquid, that the mixture is introduced into an evaporation chamber wherein the working medium is brought to boil, whereby heat is taken up from the hydrophilic liquid by direct heat transfer therefrom, that the working medium vapour is expanded in a turbine, and that the expanded working medium vapour is condensed. 2. A process according to claim 1, c h a r a c t e - r i z e d in that the condensed working medium is recircu¬ lated for being mixed with hydrophilic liquid. 3. A process according to claim 1, c h a r a c t e ¬ r i z e d in that controlled amounts of hydrophilic liquid and hydrophobic working medium are added to and mixed in a mixing chamber wherein the pressure is maintained at a level such that evaporation of the working medium is avoided, whereby an interface of well defined size is created between said liquid and said working medium so that a constant heat transfer rate is obtained.

Inventors: HELLMAN LARS-GUNNAR, HELLMAN, LARS-GUNNAR
Applicants: HELLMAN LARS GUNNAR, HELLMAN, LARS-GUNNAR
Application year: 1990
Main classification: F01K21
IPC codes: F01K21, F01K25, F22B1