DE102010056272A1 - Waste heat utilization system - Google Patents

Abstract

The invention relates to a waste heat recovery system for a waste heat source (10), consisting of a downstream ORC (Organic Rankine Cycle), the waste heat source (10) being in connection with the heating device of the ORC, as well as with a generator (5) coupled expansion machine (4) for steam expansion in the ORC. It is the object of the invention to optimize a combined heat and power plant, a waste heat recovery system consisting of an ORC connected downstream from a waste heat source, with regard to structure and operating behavior. According to the invention, the expansion machine (4) for steam expansion in the ORC is therefore started up by means of the generator (5) operating in motor mode and brought to a minimum starting speed that can be predetermined in a control device.

Description

The invention relates to a waste heat recovery system according to the preamble of claim 1.

An ORC (Organic Rankine Cycle) is a thermodynamic cycle according to Rankine. This means that a working medium passes through different thermodynamic states in order to be finally returned to the liquid initial state. The working medium is brought to a higher pressure level with a pump. Thereafter, the working medium is preheated to the evaporation temperature and then evaporated.

It is thus a steam process in which instead of water, an organic medium is evaporated. The resulting steam drives an expansion machine, such as a turbine, a piston or screw motor, which in turn is coupled to an electrical generator to generate power. After the working machine, the process medium enters a condenser and is cooled down there with the release of heat. Since water evaporates at 100 ° C under atmospheric conditions, heat at a low temperature level, such as industrial waste heat or geothermal heat, often can not be used to generate electricity. However, using organic media with lower boiling temperatures, low-temperature steam can be produced.

Advantageous in the application are ORC plants, for example, in the utilization of biomass in connection with combined heat and power, especially at relatively low power, so if the conventional biomass combustion technology seems relatively expensive. Biomass plants often have a fermenter for biogas production, which usually has to be heated.

Generic waste heat recovery systems are known from the field of combined heat and power and consist of a combined with a downstream ORC CHP, so a combined heat and power plant. From the DE 195 41 521 A1 is a plant to increase the electrical efficiency in the generation of electricity from special gases using internal combustion engines, in which the waste heat of the engine is used in a downstream energy conversion plant for further power generation. However, only the high-temperature heat from the cooling water circuit and from the exhaust gas heat exchanger of the engine is provided for recovery.

Furthermore, from the US 4 901 531 a diesel engine integrated into a Rankine process, wherein one cylinder is used for the expansion according to Rankine and the others work as a diesel engine. From the US 4,334,409 shows an operating according to the Rankine process arrangement in which the working fluid is preheated with a heat exchanger through which the air is guided from the outlet of a compressor of an internal combustion engine.

Combined heat and power plants (CHP) as plants for combined heat and power are well known. These are decentralized, usually powered by internal combustion engines power generation systems with simultaneous waste heat recovery. The discharged during the combustion of the cooling media heat is used as completely as possible for the heating of suitable objects.

In particular, in combined heat and power plants with downstream ORC as a waste heat power plant, machines have prevailed that are based on engines with an exhaust gas turbocharger for charging. This meets the demand for machines with very high electrical efficiencies, which can only be achieved with turbocharging and recooling of the fuel gas mixture heated by the compression. Generally, a cooling of the fuel gas mixture is required because otherwise the filling of the cylinder would be relatively poor. With the cooling, the density of the sucked mixture is larger and it improves the degree of filling. This increases the power output and the mechanical efficiency of the engine.

The engine manufacturers prescribe a cooling water inlet temperature of only approx. 40 to 50 ° C for the mixture cooling so that the mixture can be sufficiently cooled. Since this temperature level is relatively low, the heat extracted from the fuel gas mixture in the previously known combined heat and power plants is released to the environment, for example with a table cooler.

It is also known from the DE 10 2005 048 795 B3 the preheating of the working medium in ORC in two steps in a heating device, namely that the process medium is heated in ORC via two in series a feed pump downstream heat exchanger, the first heat exchanger after the feed pump as a first stage for coupling low-temperature heat and the subsequent heat exchanger as the second Stage is provided for coupling high-temperature heat. Here, the mixture cooling of the internal combustion engine via a circuit with the first heat exchanger after the Feed pump connected, the heat from the cooling of the sucked by the internal combustion engine fuel gas mixture for preheating the process medium in ORC and is coupled as low-temperature heat in the first heat exchanger. A second heating circuit draws heat from engine cooling water and exhaust gas of the internal combustion engine and is connected to the second heat exchanger after the feed pump, the heat from the cooling circuit and the exhaust gas for overheating and evaporation of the process medium in ORC and coupled as high temperature heat in the second heat exchanger after the feed pump becomes.

The invention is therefore based on the object to optimize an existing from a waste heat source downstream ORC waste heat recovery system in terms of structure and performance.

This is achieved with the features of claim 1 according to the invention. Advantageous developments can be found in the dependent claims.

The waste heat recovery system is characterized in that the expansion machine for steam expansion in ORC is approached by means of the generator operating in engine operation and brought to a presettable in a control device minimum starting speed. The minimum starting speed preferably corresponds to about two-thirds of a minimum operating speed. A significant advantage of working in motor operation generator is the low position load in the startup phase, because the expansion engine is not yet acted upon by refrigerant. Otherwise, it could possibly come in the still cool expansion machine for unwanted condensation of small amounts of refrigerant. Their cooling, also by a refrigerant partial flow, but in liquid phase, then works but already.

According to the steam valve is opened at the inlet of the expansion engine for steam expansion in ORC when reaching the minimum start speed and during further opening of the steam valve is another ramp up the speed, so that the generator passes from the engine operation in the normal generator operation. This is advantageous because the expansion machine right from the start on the generator or

initially attached to this as an electric motor and does not need to be synchronized to the mains. When the steam valve is fully open and the minimum operating speed in the control device is reached, a process for speed optimization is released with regard to the current operating situation.

In a further advantageous embodiment of the invention determines a control device for the expansion engine for steam expansion in ORC optimal for a current operating point speed. In this case, in a first step, starting from a minimum speed, a slow up-control under evaluation of the generator power, until in a second step with increasing speed and at the same time falling generator power exceeding an apex is detected. In a third step, the speed is reduced, and in further steps, the sequences of steps two and three are repeated until the speed settles at the point of maximum generator power.

Advantageously, the optimum for a current operating point for the expansion engine for steam expansion in ORC speed in a control device via a map is predetermined.

Thus, in a preferred embodiment of the invention in a map of input and / or output pressure at the expander of an optimal speed associated with the current operating condition and the current input and / or output pressure is measured on the expansion machine, evaluated and in the control device matched with the map, in order to regulate the speed. Alternatively or additionally, the inlet and / or outlet temperature at the expander can be assigned to an optimum speed in a map and for determining the current operating state, the current inlet and / or outlet temperature is measured at the expansion machine, evaluated and in the control device with the Characteristic adjusted to control the speed.

Preferably, the generator integrated with the steam expansion expansion machine in the ORC has a coupled frequency converter for variable speed operation.

In yet another advantageous embodiment, a controlled bypass with at least one throttle valve in the ORC circuit is provided around the expansion machine. This bypass is in the start-up phase, ie at a relatively low temperature of the working medium, initially opened, so that the working medium is passed around the expansion machine to avoid the unwanted ingress of liquid phase residues in the working medium in the expansion machine. As soon as the ORC cycle has reached its desired operating state and this, for example, over a corresponding, specifiable temperature level or other parameters is detected, the bypass is closed and opened a steam engine upstream of the expansion machine.

The invention optimizes the design and operating behavior of a waste heat utilization system consisting of an ORC downstream of a waste heat source. Waste heat sources can be, for example, combined heat and power plants, industrial plants or boiler plants.

The starting phase of the expansion machine is also optimized according to the invention. At the same time maximum reliability and protection against refrigerant condensation is achieved when the coupled with the motor-driven generator run-up of the expansion machine takes place without refrigerant. Because the refrigerant partial flow used for this purpose is conducted via the generator unit on the cooling side, it absorbs the heat produced by losses during the engine operation.

The thermal state of the expander is monitored as well as other constraints. These include as starting conditions, for example, a minimum pressure of the refrigerant in the ORC cycle, switch-on conditions for a magnetic bearing of a turbine rotor and a review of all operationally necessary units.

Thus, according to the invention, a fully automatic and electronic starting process for the waste heat utilization system takes place. Likewise, an automated normal operation with variable, adapted to the current operating situation operating speeds and a shutdown.

The drawing illustrates an embodiment of the invention and shows in a single figure the schematic structure of a waste heat recovery system, consisting of one of these downstream ORC.

The essential components for the ORC are an ORC cycle 1 , a feed pump 2 , an evaporator 3 , an expansion machine 4 for steam expansion, which with a generator 5 coupled, a condenser 6 for recooling via a heat sink 7 as well as the heat exchangers 8th . 9 for preheating the working medium in the ORC circuit 1 ,

The two heat exchangers 8th . 9 are in series of the feed pump 2 downstream. The first heat exchanger is used 8th after the feed pump 2 as a first stage for coupling low-temperature heat and the subsequent heat exchanger 9 as a second stage for coupling high temperature heat from a waste heat source 10 ,

A second heating circuit 11 is with its flow area with the evaporator 3 connected to the ORC, because the temperature level is initially high enough for its direct heating. Thereafter, the second heating circuit opens 11 return side in the second heat exchanger 9 and gives off any remaining heat to the ORC.

A liquid refrigerant partial flow 12 for cooling the expansion machine 4 is branched off and first by the generator 5 guided. Thereafter, the cooling medium flows through the housing of the expansion machine 4 and ensures there is sufficient heat dissipation.

Upon reaching a minimum starting speed is a steam valve 13 at the inlet of the expansion machine 4 open for steam expansion in the ORC and during further opening of the steam valve 13 a further ramp up of the speed, so that the generator 5 from engine operation to normal generator operation.

To the expansion machine 4 around is a regulated bypass 14 with at least one throttle valve 15 intended. This bypass 14 is in the starting phase, so at still relatively low temperature of the working medium, initially open. Thus the working medium becomes the expansion machine 4 guided around. Once the ORC cycle 1 has reached its desired operating state, the throttle valve 15 in the bypass 14 closed and that of the expansion machine 4 upstream steam valve 13 open.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 19541521 A1 [0005]
• US 4901531 [0006]
• US 4334409 [0006]
• DE 102005048795 B3 [0010]

Claims (11)

Waste heat recovery system for a waste heat source ( 10 ), consisting of one of these downstream ORC (Organic Rankine Cycle), wherein the waste heat source ( 10 ) is in communication with the heating device of the ORC, and with one with a generator ( 5 ) coupled expansion machine ( 4 ) for steam expansion in ORC, characterized in that the expansion machine ( 4 ) for steam expansion in ORC by means of the generator operating in engine operation ( 5 ) and brought to a presettable in a control device minimum starting speed. Waste heat recovery system according to claim 1, characterized in that the minimum starting speed corresponds to about two thirds of a minimum operating speed. Waste heat recovery system according to claim 1 or 2, characterized in that upon reaching the minimum starting speed, a steam valve ( 13 ) at the inlet of the expansion machine ( 4 ) is opened for steam expansion in ORC and that during the further opening of the steam valve ( 13 ) a further startup of the speed takes place and the generator ( 5 ) from the engine operation to the normal generator operation. Waste heat recovery system according to one of claims 1 to 3, characterized in that when fully opened steam valve ( 13 ) and reached minimum operating speed in the control device, a process for speed optimization is released. Waste heat recovery system according to one of claims 1 to 4, characterized in that a control device for the expansion machine ( 4 ) for steam expansion in the ORC determines the optimum for a current operating point speed by a slow upshifting under evaluation of the generator power takes place in a first step, starting from a minimum speed, in a second step with increasing speed and at the same time falling generator power exceeding a vertex is detected in a third step, the speed is reduced, and in further steps the steps of steps two and three are repeated until the speed settles at the point of maximum generator power. Waste heat recovery system according to one of claims 1 to 4, characterized in that in a control device for the expansion machine ( 4 ) for steam expansion in ORC for a current operating point, the optimum speed can be specified via a map. Waste heat recovery system according to one of claims 1 to 4 and 6, characterized in that in a map of the input and / or output pressure at the expansion machine ( 4 ) are assigned to an optimal speed and that for determining the current operating state of the current input and / or output pressure at the expansion machine ( 4 ) is measured, evaluated and adjusted in the control device with the map in order to regulate the speed. Waste heat recovery system according to one of claims 1 to 4, 6 and 7, characterized in that in a map, the inlet and / or outlet temperature at the expansion machine ( 4 ) are assigned to an optimal speed and that for determining the current operating state, the current inlet and / or outlet temperature at the expansion machine ( 4 ) is measured, evaluated and adjusted in the control device with the map in order to regulate the speed. Waste heat recovery system according to one of claims 1 to 8, characterized in that the with the expansion machine ( 4 ) for steam expansion in ORC integrated generator ( 5 ) has a coupled frequency converter for a variable-speed operation. Waste heat recovery system according to one of claims 1 to 9, characterized in that around the expansion machine ( 4 ) is a regulated bypass ( 14 ) with at least one throttle valve ( 15 ) in the ORC circuit ( 1 ) is provided. Waste heat recovery system according to one of claims 1 to 10, characterized in that the regulated bypass ( 14 ) around the expansion machine ( 4 ) is initially open in the starting phase and that it is closed when the ORC cycle ( 1 ) has reached a specifiable temperature level.

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