Patent Application
20140013750
The present invention relates to a waste-heat recovery system.
An ORC (Organic-Rankine Cycle) is a thermodynamic cyclical process according to Rankine. This means that a working medium runs through various thermodynamic states in order to be transferred back into the initial liquid state again at the end. In the process, the working medium is brought to a higher pressure level with the aid of a pump. Then, the working medium is preheated to evaporation temperature and subsequently evaporated.
Thus, it is an evaporation process, in which an organic medium rather than water is evaporated. The created steam drives an expansion machine, e.g., a turbine, a piston or propeller motor, which in turn is coupled to an electric generator in order to generate power. Downstream from the working machine, the process medium enters a condenser where it is cooled down again through heat dissipation. Since water evaporates at 100° C. under atmospheric conditions, it is frequently impossible to use heat having a low temperature level, e.g., industrial waste heat or earth heat, to generate power. However, if organic media having lower boiling point temperatures are used, then it is possible to generate low-temperature steam.
The use of ORC systems is also advantageous when exploiting biomass in connection with the combined generation of power and heat, for example, especially at relatively low power outputs, i.e., when the conventional biomass combustion technology seems relatively expensive. Biomass plants often have a fermenting device for the production of biogas, which normally has to be heated.
Generic waste-heat recovery systems are known from the cogeneration of electricity and heat and composed of a combined heat and power plant linked to a downstream ORC. A system for increasing the electrical efficiency in the power generation from special gases by combustion engines is known from the German Published Patent Application No. 195 41 521; here, the waste heat of the motor is utilized for the further energy generation in a post-connected energy-conversion system. However, only the high-temperature heat from the cooling-water circuit and from the exhaust-gas heat exchanger of the engine is provided for exploitation.
In addition, a diesel power unit integrated into a Rankine process is known from the U.S. Pat. No. 4,901,531, in which one cylinder is used for the expansion according to Rankine, and the other cylinders operate as diesel engine. U.S. Pat. No. 4,334,409 describes a system operating according to the Rankine process, in which the working fluid is preheated by a heat exchanger, through which the air from the outlet of a compressor of a machine having internal combustion is routed.
Block thermal power plants (BHKW) as plants for the cogeneration of electricity and heat are generally known. These are decentralized power generation plants, often driven by combustion engines, featuring a simultaneous utilization of the waste heat. As far as possible, the heat withdrawn via the cooling media is used for heating suitable objects.
In particular in the case of plants for the cogeneration of electricity and heat having a post-connected ORC as waste heat power system, machines that are based on engines having an exhaust-gas turbocharger for charging have come to dominate. That satisfies the demand for machines having very high electrical efficiencies, which are achievable only with turbocharging and recooling of the combustion-gas mixture heated by the condensation. Cooling of the combustion-gas mixture is generally required because the charge of the cylinder would otherwise be relatively poor. The cooling increases the density of the aspirated mixture, and the volumetric efficiency is improved. The output yield and the mechanical efficiency of the engine increase as a result.
Engine manufacturers stipulate a cooling-water intake temperature of only approximately 40 to 50° C. for the mixture cooling so that the mixture can be cooled sufficiently. Since this temperature level is relatively low, the heat extracted from the combustion-gas mixture in the currently known systems for the combined generation of power and heat is dissipated to the environment, e.g., using a table-type cooler.
In addition, the preheating of the working medium in the ORC in two steps in a heating device is known from German Patent No. 10 2005 048 795, i.e., that the process medium in the ORC is heated by two heat exchangers connected in series downstream from a feeding pump; the first heat exchanger downstream from the feeding pump is provided as a first stage for the incoupling of low-temperature heat, and the following heat exchanger is provided as a second stage for the incoupling of high-temperature heat. Via a circulation system, the mixture cooling of the combustion engine is connected to the first heat exchanger downstream from the feeding pump, and the heat from the cooling of the combustion-gas mixture aspirated by the combustion engine is used to preheat the process medium in the
ORC and coupled into the first heat exchanger as low-temperature heat. A second heating circuit obtains heat from the engine cooling water and the exhaust gas of the internal combustion machine and is connected to the second heat exchanger downstream from the feeding pump, the heat from the cooling circuit and the exhaust gas being used to overheat and evaporate the process medium in the ORC and input into the second heat exchanger downstream from the feeding pump in the form of high temperature heat.
Therefore, the present invention is based on the objective of optimizing the design and operating behavior of a waste-heat recovery system made up of an ORC post-connected to a waste-heat source.
A characteristic feature of the waste-heat recovery system is that the expansion machine is started up by the generator which is operating as motor for the steam expansion in the ORC, and is brought to a minimum starting engine speed able to be specified by a control device. The minimum starting engine speed preferably amounts to approximately two thirds of a minimum operating engine speed. An advantage achieved by the generator operating in motor mode is the low positional load in the startup phase, because no coolant has yet been acting on the expansion machine. Otherwise, undesired condensation of small quantities of coolant could perhaps occur in the still cool expansion machine. Its cooling, likewise by a partial coolant flow but in liquid phase, is already taking place by then, however.
According to the present invention, a steam valve at the intake of the expansion machine is opened for steam expansion in the ORC once a minimum starting engine speed has been reached, and during the further opening of the steam valve, a further run-up of the engine speed takes place, so that the generator transitions from motor-actuated operation to normal generator operation. This is advantageous because the expansion machine is linked to the generator right from the start, or is initially linked to it as electric motor, and need not be synchronized to the electrical network. When the steam valve is open completely and the minimum operating engine speed has been reached in the control device, a process for optimizing the engine speed with regard to the current operating situation will be enabled.
In another advantageous development of the present invention, a control device then ascertains the optimal engine speed for steam generation in the ORC for the expansion machine at a current operating point. To do so, starting from a minimum engine speed, a slow rampup takes place in a first step while analyzing the generator output, until it is detected in a second step that a zenith is exceeded with rising engine speed and a simultaneously dropping generator output. In a third step, the engine speed is reduced, and in further steps, the sequences of the steps two and three are repeated until the engine speed stabilizes at the point of the maximum generator output.
The optimum engine speed for steam expansion in the ORC for a current operating point is able to be specified via a characteristics map in a control device.
In one preferred development of the present invention, for example, an optimal engine speed is assigned to the input and/or output pressure at the expansion machine in a characteristic map; to determine the current operating state, the current input and/or output pressure is measured at the expansion machine, analyzed and adjusted with the aid of the characteristic map in the control device, in order to thereby adjust the engine speed. As an alternative or in addition, the input and/or output temperature at the expansion machine may be assigned to an optimum engine speed in a characteristic map, and the current input and/or output temperature is measured at the expansion machine, analyzed and adjusted in the control device with the characteristic map so as to determine the current operating state, to then adjust the engine speed in this way.
Preferably, the generator integrated with the expansion machine for steam generation in the ORC includes a coupled frequency converter for an operation at variable engine speeds.
In still another advantageous development, a controlled bypass having at least one throttle valve is provided around the expansion machine in the ORC circuit. In the startup phase, i.e., at a still relatively low temperature of the working medium, this bypass is open initially, so that the working medium is routed around the expansion machine in order to avoid that liquid phase residue in the working medium makes its way into the expansion machine. As soon as the ORC circuit has reached its setpoint operating state and this is detected via a corresponding specifiable temperature level or other parameters, for example, the bypass is closed and a steam valve upstream from the expansion machine is opened.
Using the present invention, the design and the operating behavior of a waste-heat recovery plant composed of an ORC connected downstream from a waste-heat source is optimized. Waste-heat sources may be, for example, combined heat and power plants, industrial plants or boiler plants.
According to the present invention, the starting phase of the expansion machine is optimized as well. At the same time, maximum operating safety and protection from coolant condensation are achieved if the run-up of the expansion machine, which is linked to the motor-operated generator, takes place without a coolant application. Since the partial coolant flow used for this purpose on the coolant side is routed via the generator unit, it absorbs the heat produced there by losses during motor-actuated operation.
The thermal condition of the expansion machine is monitored in the same way as other marginal conditions. A minimum pressure of the coolant in the ORC circuit, switch-on conditions for a magnetic bearing of a turbine rotor and a check of all power units required for the operation, for example, are among such starting conditions.
A fully automatic and electronic runup process for the waste-heat recovery system thus takes place according to the present invention, as does an automated standard operation at variable operating engine speeds adapted to the prevailing operating situation, as well as a running-down operation.
The Figure represents an exemplary embodiment of the present invention and shows the schematic structure of a waste-heat recovery plant made up of an ORC post-connected thereto.
The components that are used in the operation of the ORC are an ORC circulation system 1, a feeding pump 2, an evaporator 3, an expansion machine 4 for steam expansion, which is coupled to a generator 5, a condenser 6 for recooling via a heat sink 7, and heat exchangers 8, 9 for preheating the working medium in ORC circulation system 1.
The two heat exchangers 8, 9 are connected in series downstream from feeding pump 2. First heat exchanger 8 downstream from feeding pump 2 is used as the first stage for the incoupling of low-temperature heat, and following heat exchanger 9 is used as a second stage for incoupling of the high-temperature heat from a waste-heat source 10.
A second heating circuit 11, via its supply region, is connected to evaporator 3 of the ORC, because the temperature level initially is not high enough for its direct heating. After that, second heating circuit 11 discharges into second heat exchanger 9 on the return side, where it releases still existing residual heat to the ORC.
A liquid partial coolant flow 12 for cooling the expansion machine 4 is rerouted and first guided through generator 5. Then, the coolant medium flows through the housing of expansion machine 4 and provides adequate heat dissipation there.
Once a minimum starting engine speed has been reached, a steam valve 13 at the intake of expansion machine 4 is opened for steam expansion in the ORC, and during the further opening of steam valve 13, a further run-up of the engine speed takes place, so that generator 5 transitions from motor-actuated operation to normal generator operation.
A controlled bypass 14 having at least one throttle valve is provided around expansion machine 4. This bypass 14 is initially open in the start-up phase, i.e., at a still relatively low temperature of the working medium. The working medium is routed around expansion machine 4 in this way. As soon as ORC circuit 1 has reached its setpoint operating state, throttle valve 15 in bypass 14 is closed, and steam valve 13 upstream from expansion machine 4 is opened.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2010 056 272.6 | Dec 2010 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2011/073602 | 12/21/2011 | WO | 00 | 10/3/2013 |
