STEAM BUFFER FOR A STEAM ENGINE PLANT

Abstract: The invention refers to a steam buffer (4) for utilization in a steam engine plant with a closed steam system and is designed to alternately accumulate and emit steam under high pressure and temperature. A conventional steam accumulator of the type used in old steam engine system is heavy and bulky and awkward in mobile applications. It also contains water and steam with relative high temperature and pressure in a large pressurized vessel, which results in a safety risk at damage as well as a gradual pressure decrease when steam is discharged from the steam buffer. With the steam buffer according to the invention these drawbacks are eliminated by the fact that the heat storing function of the steam buffer is provided by the solid material (21, 22, 23, 24, 25) in the walls of the large number of pressure resistant flow channels (20) with a hydraulic diameter preferably less than 0.5 mm for the steam and the feed water.

Description: Steam Buffer for a Steam engine plant The present invention relates to a steam buffer working in a steam engine plant with a closed steam system and is design to alternately accumulate and emit steam under high pressure and temperature. In a steam engine plant there is a large need for a buffer, because normal steam generation and the utilisation of the steam can occur at times that do not correspond to each other. Such storage has since long time ago been carried out in a so called steam accumulator. This steam accumulator consists of a pressure vessel, which is partly filled with water, which is heated by a boiler or a steam generator which can operate at optional pace. When the steam is supplied to the steam engine by the steam accumulator the pressure tends to decrease. This pressure drop will in turn cause a spontaneous new steam generation from the heated water. By this steam accumulator large power outputs can be obtained, and the power outputs can be obtained inde¬ pendent of an irregular burning in the steam generator. However, this type of steam-accumula¬ tor has several drawbacks because it will become heavy and bulky and the large amount of water and steam at high temperature will constitute a great hazard in case of fractures in the pressure vessel casing. In a steam accumulator the energy is stored in the pressurized water. There is also a pos¬ sibility to store the heat energy in other material. Thus it has since long time ago been consid¬ ered attractive to use such energy storing material which can change between solid and liquid phase ( latent heat). However, when utilizing latent heat there will be problems at phase chang¬ es, as contraction, tensions and chemical exhaustion, which gives rise to mechanical, chemical, heat transfer and function problems. A steam buffer shall, as the name indicates, accomplish a levelling between power input in the shape of the steam arriving from the steam generator and the power output to the steam engine, which will make it possible to use intermittent and stochastic energy sources like solar energy in stationary plants, and above all make it possible to obtain considerably higher peak power outputs for short periods than the power that corresponds to the steam generator capac¬ ity. This will also involve the possibility to let the burner in the steam generator operate at low and constant power even if the steam engine power output is strongly fluctuating. In a steam engine for vehicle applications with strong power output variations, an effec¬ tive steam buffer makes it possibly to design the steam generator only for the highest continu¬ ous power output, which is considerably lower than the highest momentary power output, which will be necessary only for short periods ( as for example at acceleration ). Further, the steam buffer will also constitute an energy storage, which makes it possible to drive the vehicle a certain distance without exhaust gases ( i.e with no firing ). The object of the invention is to accomplish a steam buffer which is small and light and performs high power density and energy density so far not obtained, and also such a design that it will give high safety in case of accidents when it is used together with steam engines in ve¬ hicle applications. This will be obtained by the invention in that the steam buffer is equipped with a high temperature connection for steam and a low temperature connection for feed water and there between a large number of elongated flow channels with a hydraulic diameter smaller than about 0.5 mm for the steam and the feed water between the two connections, and surrounded by pressure resistance walls, which material has a melting point above the highest occurring temperature and constitutes the primary heat storage substance. The invention thus utilizes so called sensible heat, that is, temperature changes in solid material, and the solid material which constitutes the pressure resistance walls of the flow chan¬ nels is mainly responsible for the heat storage capability of the steam buffer. The invention is particularly distinguished by the dimensioning measure that the steam buffer consists of a large number, in reality the maximum possible number, of flow channels with a hydraulic diameter smaller than 0.5 mm. Such small channels will require a high pres¬ sure to feed the steam and water through them. A pressure of at least 100 bar will be required which is a pressure that is appropriate for an effective steam engine e.g. of displacement type. Despite the high pressure the extension strain in the wall material surronding the flow channels will be limited. Since each flow channel by it self has pressure resistance walls there will be no need for a jointly pressure resistance vessel which is exposed for the high pressure on the whole steam buffer diameter. Thus no danger for explosion exists, and, which will be shown below no danger for outflowing steam exists in case of damage to the steam buffer. According to a preferred embodiment of the steam buffer it is designed - and the steam engine too - for a pressure above the critical pressure, preferably 250 bar and a corresponding steam temperature, preferably 500 °C and a hydraulic diameter of 0.2 mm. With these values it is possible to obtained an energy density of 500 kJ kg and a power density of 100 kW/kg for the steam buffer, which can be compared with e.g. a lead battery with only lOOkJ/kg and 100 W kg. According to a further preferred embodiment the flow channels are created by small grains preferably of ceramics material sintered to each other and to the inside of the casing of the steam buffer. The flow channels are formed partly between the grains and partly between the grains and the casing sintered to the grains, which can be thin-walled because it is exposed to small extension strain and mainly has a sealing function , but it constitutes a heat storage function like the other material. The invention will in the following be described in more detail with reference to the at¬ tached drawings, which schematically will show different embodiments of steam buffers ac¬ cording to the invention. Figure 1 shows the layout of the steam engine plant including a steam buffer, figures 2-5 are partial sections, of the steam buffer illustrating different ways to form the flow channel ,figure 6a is a symbolic side view of the steam buffer, figures6b-f show tem¬ perature profiles of the material in the steam buffer at different conditions of charging, and fig¬ ures 7a-d illustrate temperature profiles for both material and steam at the end of the discharge process in the steam buffer at different pressure values and different diameters of the flow chan¬ nels. Figure 1 shows schematically a steam generator 1, which is connected by a steam pipe 2 to a high temperature connection 3 of the steam buffer 4, and to the inlet valve 5 of a multicyl- inder axial piston steam engine 6. From the outlet port of the steam engine 6 a pipe 7 leads to a condenser buffer 8, to which a cooler 9 is connected by the pipes 10, 11 for cooling of the feed water and the steam in the condenser buffer 8. From the condenser buffer leads a pipe 12 to a pump 13 for pumping feed water of high pressure to a low temperature connection 14 which consists of a long heat insulated pipe to the steam buffer 4 via a pipe 15, as well as a pipe 16 to a circulation pump 17, which outlet via a pipe 18 is connected to the steam generator 1. Between the high temperature connection 3 of the steam buffer 4 and the low temperature connection 14 extends a large number of flow channels 20, which is illustrated in figures 2-5. These channels can be formed by a packet of capillary tubes 21, which have ends that are ex¬ tended into the connections 3 and 14 and with the outer surfaces sealingly adhering to each oth¬ er and to the connection 3 and 14. The pipes 21 have circular cross section areas in figure 2, but can even have hexagonal shape like the pipes 22 in figure 3. The flow channels 20 can alterna¬ tively be formed by extrusion of a block 23 of some suitable material in which the flow chan¬ nels are extended. The pipes 21, 22 and the block 23 can consists of metal or ceramics material. A specially preferred design is illustrated in figure 5. Within a thin- walled cylindric casing 24 between the connections 3, 14 are a large number of small grains of ceramics material sintered to each other and to the inside of the casing 24. The flow channels 20 are here formed by the space between the grains 25 and between grains and the inner wall of the casing 24. In all cases are the hydraulic diameter of the flow channels 20 are smaller than 0.5 mm. The steam engine plant will operate in broad outline as follows. The steam generator 1 is designed to generate steam in some discrete power outputs, a high and a low continuous power output level and maybe some intermediate levels depending on required steam generation. When the valve 5 is closed the engine 6 is not getting any steam and all generated steam from the steam generator 1 will flow with the pressure 250 bar and temperature of 500 °C to the steam buffer 4. In the steam buffer the steam will penetrate the flow channels 20, and press away the water inside the flow channels 20. which flows out by the pipe 15 to a buffer vessel 26 which is connected to the pipe and contains a gas cushion against the pressure of which the water is pressed into the vessel. The material 21,22,23,24 or 25 in the steam buffer 4 is heated from the connection 3 with a transverse temperature front, which is moved towards the connec¬ tion 14. When this temperature front has reached to connection 14 the steam buffer is fully charged and the circulation pump 17 is stopped.The plant can remain in this fully charged con¬ dition for a long time period and is equipped with an effective heat-insulation 27 which is hous¬ ing the steam generator 1, the steam buffer 4 with connection 14, the valve 5 and the top of the steam engine 6 and also the belonging pipes, which together constitute a high temperature part, while the rest of the plant constitutes a low temperature part with a temperature of approximate¬ ly 80 °C. Some heat losses will of course be unavoidable, but can be made so small , that they can be compensated by starting the steam generator 1 and let it run only for a couple of minutes with several days interval to restore the intended temperature level. When the valve 5 is opened for driving the steam engine 6 at normal low load the contin¬ uously generated steam from the steam generator will be enough. When the valve 5 is opened for driving of the steam engine 6 at high load for short time periods, for examples at accelera¬ tion when passing another vehicle, the main steam will be supplied from the steam buffer 4, the steam buffer will e.g. give ten times more steam than the steam generator 1 can supply. The steam leaves by the connection 3 and the feed water from the buffer 26 is pressed by its gas cushion into the steam buffer 4 by the connection 14. In the steam buffer 4 is the water vapor¬ ized by the hot surrounding material, and now the above mentioned temperature front is moved slowly in the direction to the connection 3, and when this temperature front reaches the connec¬ tion the steam buffer is fully unloaded and only the steam from the steam generator 1 is avail¬ able. The above mentioned process has been illustrated in the figures 6a-6f. Figure 6a shows the steam buffer 4 with the low temperature connection 14 and the high temperature connection 3. At fully charged condition the temperature in the steam buffer from the one end to the other end is as the curve illustrates in figure 6b, that is approximately 80 °C outside the heat insula¬ tion and 500 °C along the whole steam buffer length. After a long time in fully charged condi¬ tion 1 the temperature distribution along the long pipe in connection 14 will be as figure 6b illustrates. The temperature gradient in connection 14 is responsible for the largest heat leakage from the steam buffer 4, but this leakage can be small, if the pipe 14 is made long. During the discharge the steam flows out via connection 3 and the water flows in via connection 14, and the transverse temperature front T is formed according to figure 6c. The temperature front will move slowly towards the connection 3 with a velocity of propagation which is always lower than the velocity of the fluid of steam and water and is related to the velocity of the flowing fluid as the heat capacity of the fluid is related to the sum of the heat capacity of the fluid and the heat exchanger material. The discharge will take place with unchanged temperature and al¬ most unchanged pressure of the discharged steam until the front T reaches the connection 3 ac¬ cording to figure 6d. If the heat transfer condition is favourable and the flow velocity is not too high ( will be obtained by many flow channels) there will be a very steep rise of the temperature front, which is important in order to obtain high energy density, which is defined as the real power output which is possible to obtain, normalized to the material weight of the steam buffer. The real en¬ ergy discharge will in turn be the energy discharge which can be done with guaranteed quality of the steam, from fully charged steam buffer until that the steam quality can not be kept at the outlet 3. The latter section of time is illustrated in figure 6d. Notably is that during the whole discharge up to the section of time in figure 6d the discharged steam is of the same quality as the steam that charged the steam buffer. When the position in figure 6d is reached, feed water has been flowing in at 14 and has been heated to nominal steam temperature by the heat trans¬ ferred from all the material which has given away its energy content from 500 °C to 80 °C.This has occurred for all the material where the temperature front has passed, and the energy will correspond to the marked section Y in figure 6e. The ratio between Y and the whole section in figure 6b is defined as the ratio of utilization, which for the steam buffer according to the in¬ vention can be 85-95 . With high steam temperatures as 800-900 °C which can come up if the whole steam system is designed in ceramics, it would be possible to obtain an energy den¬ sity of about 1 MJ/kg. At repeated charging the temperature front is moved in the opposite direction as is shown in figure 6f until a new discharge takes place or the steam buffer is again fully charged, as in figure 6b. A condition to obtain high energy density is a rise of the temperature front in the steam buffer that is as steep as possible, and it can be shown that the hydraulic diameter of the chan¬ nels shall be some tenth of millimetre. It can also be shown that high power density, defined as the power per kg which can be withdrawn without large unacceptable pressure losses, requires a high pressure of the steam, a high value on the ratio between the total area of the cross section of the flow channels and the total cross section area of the wall material and the flow channels, a high steam temperature, a low density of the material, which makes ceramics material favour¬ able, and a small hydraulic diameter, that is, the same conditions as for high energy density. The hydraulic diameter and its influence of the steepness of the temperature front is illus¬ trated in figures 7a-7d at different operation modes. Figures 7a,b show the temperature of the steam buffer along its relative length at pressure 250 bar and the steam temperature 500 °C for flow channels with the hydraulic diameter 0.5 and 0.2 mm, respectively. Tg and Ta refers to the temperature curves for wall material and the steam respectively. Figures 7c, d show corre¬ sponding curves at the pressure 100 bar and the steam temperature 450 °C. In both cases it is illustrated that at a change from 0.5 to 0.2 mm hydraulic diameter the temperature steepness will increase dramatically, especially in the case with the higher pressure and temperature vaules. Despite the high pressure and temperature of the steam engine plant there is a very small risk for damage on the surroundings due to explosion and /or outflowing hot steam, especially from the steam buffer, because the steam buffer is not contained in a large pressure resistant vessel as well as the flow channels will only contain a minor amount of hot steam/water .The steam will be generated in the same pace as the feed water flows into the flow channels at dis¬ charge and will only take place if the steam buffer is intact. It can also be equipped with a pipe break valve 30 in the pressurized pipe 15, which is leading the feed water to the steam buffer 4 at discharge. A greater velocity of the feed water than a predetermined value, for example fully open valve 5 (full load ), will rapidly close the valve 30, and the steam generation in the buffer 4 will stop. The invention is of course not restricted to the aboved described steam buffer designs and steam data but can be modified in several ways within the scope of the inventive idea defined by the claims.

Claims: Claims 1. A Steam buffer for utilization in a steam engine plant with a closed steam system and designed to alternately accumulate and emit steam under high pressure and temperature, char¬ acterized in that the steam buffer (4) is equipped with a high temperature connection (3) for steam and a low temperature connection (14) for feed water and therebetween a large number of long flow channels (20) with a hydraulic diameter smaller than about 0.5 mm for the steam and the feed water between the two connections (3,14), and surrounded by pressure resistance walls (21,22,23,24,25 ), the material of which has a melting point above the highest occurring temperature and constitutes the primary heat storage substance. 2. A steam buffer according to claim 1, characterized in that the steam buffer (4) is de¬ signed for a higher pressure than the critical pressure, preferably 250 bar, and a steam temper¬ ature of 500 °C, and a hydraulic diameter of 0.2 mm of the flow channels (20) 3 A steam buffer according to claims 1 or 2 , characterized in that the flow channels (20) are formed by parallel capillary pipes (21,22) which are attached to each other e.g. hard sol¬ dered or sintered together. 4. A steam buffer according to claims 1 or 2 , characterized in that the flow channels ( 20) are formed by extrusion to a block (23). 5. A steam buffer according to claims 1 or 2 , characterized in that the flow channels (20) are formed by sintering fine grains (25) of metallic or ceramic material to each other inside a thin casing (24), to which inside the grains are sintered. 6. A steam buffer according to any of claims 1 - 5 , characterized in that flow channels (20) and intermediate material ( 21,22,23,24, 25) are designed for an energy density of up to 500 kJ/ kg and a power density of 10-100 kW/kg of the total weight for the steam buffer material. 7 A steam buffer according to any of claims 1- 6 , characterized in that a pressurized pipe (15) for feed water connected to the low temperature connection ( 14 ) is equipped with a valve ( 30 ) which is arranged to close when the flow velocity in the pressurized pipe ( 15) exceeds a predetermined value. 8 A steam buffer according to any of claims 1- 7, characterized in that a low temperature connection (14) consists of a long heat insulated pipe.

Inventors: PLATELL OVE, PLATELL, OVE
Applicants: PLATELL OVE, RANOTOR UTVECKLINGS AB, PLATELL, OVE
Application year: 1995
Main classification: F01K3
IPC codes: F01K1, F01K3