DE102013212805A1 - Use of highly efficient working media for heat engines - Google Patents

Abstract

The invention relates to a heat engine for performing an Organic Rankine Cycle (ORC) comprising an evaporator, a motor, a condenser and a circuit containing a fluid working medium, wherein the working medium has a critical pressure (pc) between 4000 kPa and 6500 kPa, preferably between 4200 kPa and 6300 kPa, the working medium has a critical temperature (Tc) between 450 K and 650 K, preferably between 460 K and 600 K, the working medium has a molecular weight between 50 g / mol and 80 g / mol, preferably between 60 g / mol and 75 g / mol, and the gaseous working medium partially condensed out in adiabatic expansion. The invention also relates to the use of a working medium having a critical pressure (pc) between 4000 kPa and 6500 kPa, preferably between 4200 kPa and 6300 kPa, with a critical temperature (Tc) between 450 K and 650 K, preferably between 460 K and 600 K and with a molecular weight between 50 g / mol and 80 g / mol, preferably between 60 g / mol and 75 g / mol, in a heat engine, wherein the gaseous working fluid partially condenses out in an adiabatic expansion of an Organic Rankine Cycle (ORC).

Description

The invention relates to a heat engine for performing an Organic Rankine Cycle (ORC) comprising an evaporator, a motor, a condenser and a circuit containing a fluid working medium and the use of a working medium for a heat engine.

In the chemical industry, there is a great need to use low-energy waste heat streams, which occur in the temperature range from 80 ° C to 250 ° C.

For the optimization of existing combined site systems and with regard to the improvement of energy efficiency and the reduction of CO ₂ emissions, there is a promising option in the generation of electricity from this currently unused waste heat through the use of combined heat and power (CHP). For this purpose, heat engines are used, as for example from the DE 10 2009 024 436 A1 , of the DE 10 2011 076 157 A1 or the EP 1 016 775 A2 are known. In the latter two heat engines, water or water vapor is used as the working medium. These have the disadvantage that they work at relatively high temperatures.

The problem of the high working temperatures of steam processes has been overcome by the application by the ORC technique, since this technology uses organic fluids as the working medium instead of water vapor.

ORC stands for "Organic Rankine Cycle", which in German means "organic Rankine cycle". An ORC process is a thermodynamic cycle for converting heat into mechanical work using an organic working fluid.

An ORC process is a simple thermodynamic cycle in which the working fluid is vaporized by the supply of heat at a high pressure level and optionally overheated. The superheated steam is expanded in an expander (in particular an engine such as a piston engine or a turbine) while cooling to a low pressure while doing work. The work can be used directly mechanically or is converted into electricity by means of a generator. The steam exiting the expander may still be superheated or may already be relaxed down to the wet steam area, so that there is already a proportionate amount of liquid. The complete liquefaction takes place in the condenser. In this case, the power-generating cycle is operated instead of water with an organic working fluid, which can use the heat generated at a low temperature level thermodynamically efficient.

The working medium used plays a key role here, because the optimal interaction between the working medium and the process configuration decisively determines the efficiency and thus the efficiency of the entire process. The working medium has an influence on the system configuration, for example. By optimally selecting a working medium, the utilization of the heat source and the efficiency of the system can be increased.

Suitable working media for ORC processes are in particular chlorofluorocarbons and hydrocarbons as well as mixtures of fluids (hydrocarbons and water, fluorohydrocarbon mixtures) and organic silicon components. In the existing state-of-the-art technology used in addition to hydrocarbons such as pentane, siloxanes such as octamethyltrisiloxane or fluorinated hydrocarbons such as R134a or R245fa ( Quoilin, S., Lemort, V., Technological and Economical Survey of Organic Rankine Cycle, 5th European Conference on Economics and Management of Energy in Industry, Vilamoura, Portugal, 14.04.-17.04.2009 ). A heat engine using such an ORC technique, for example, from the EP 1 174 590 A2 in which pentane is used as an organic working fluid, ie as a working medium.

The disadvantages of the known working fluids consist on the one hand in the potential hazard to the environment (CFC: harm to the ozone layer and global warming) and in operational safety (hydrocarbons: flammability, explosion protection) on the other hand in thermodynamic limitations due to a lack of optimization of system design and fluid properties ,

For certain steam expansion engines (reciprocating engines) there are still no optimized working fluids that can be used in the temperature range from 80 ° C to 250 ° C.

The most widely-used working media include fluorinated hydrocarbons. An essential advantage of these substances is their physical properties. So these substances are usually not flammable and non-toxic. A disadvantage of such substances is that the boiling point of fluorinated hydrocarbons is usually very low, since these were usually developed as a refrigerant and therefore are only suitable for use in an ORC system at higher temperatures.

Another large group of ORC working media are the hydrocarbons, such as toluene, pentane and isobutane. The hydrocarbons are well known as suitable ORC working media and are used in ORC machines. However, the physical properties must be taken into account when using these media. The disadvantage of these substances, above all, that they are usually flammable and environmentally hazardous. In addition, they usually have a high climate damage.

As a known ORC application, for example, ethanol is currently used in an ORC steam engine from DeVeTec GmbH as the most efficient working medium in a temperature range beginning at about 250 ° C.

However, as industrial waste heat streams are often at a temperature level between 80 ° C and 250 ° C, an ethanol-based ORC process can not be economically operated here.

In the light of this prior art, it is an object of the invention, a working fluid for an Organic Rankine Cycle (ORC) with steam expansion motor in the use of waste heat streams in extended temperature ranges between 80 ° C to 250 ° C, in particular from 80 ° C to 200 ° C. , more preferably from 80 ° C to 150 ° C to provide. This broad temperature range results from the different temperature levels of the waste heat streams. While exhaust gases from biogas, biomass or CMM combustion at high temperatures in the range around 450 ° C exist in industrial environment many low-temperature currents in the range of 100 ° C to 200 ° C, which can no longer be used in many chemical sites, their Potential can still be raised in the context of an ORC cycle. Depending on the application, different working fluids are used.

In addition to suitable thermodynamic properties (including thermal stability, enthalpy of vaporization, vapor pressure and heat capacity), the working medium must meet other conditions such as low toxicity and low environmental impact (eg ozone and climate harmlessness), non-flammable and non-corrosive to the components of the system Be heat engine.

Another object of the present invention is therefore to provide a working medium which can be used with heat engines at low temperatures with high efficiency. At the same time, the environmental compatibility of the working medium should be good, in particular with regard to the harmfulness to the ozone layer and the climate. Furthermore, the working medium should attack the components of such a heat engine as little as possible and corrode. Furthermore, the working medium should be as safe as possible in the application, that is as difficult as possible flammable and not explosive.

Other tasks not explicitly mentioned arise from the overall context of the following description, examples and claims.

These and other non-explicitly stated objects, which, however, are readily derivable or deducible from the contexts discussed hereinabove, are solved by a heat engine having all the features of claim 1 and by a method having all the features of claim 9. Advantageous modifications of the method according to the invention according to claim 1 are provided in the dependent claims 2 to 8 under protection. Likewise, a useful modification of the heat engine according to the invention is provided according to claim 9 in the dependent claims 10 to 15 under protection.

The objects underlying the present invention are achieved by a heat engine for performing an Organic Rankine Cycle (ORC) comprising an evaporator, a motor, a condenser and a circuit containing a fluid working medium, wherein the working fluid has a critical pressure (pc) between 4000 kPa and 6500 kPa, preferably between 4200 kPa and 6300 kPa, the working medium has a critical temperature (Tc) between 450 K and 650 K, preferably between 460 K and 600 K, the working medium has a molecular weight between 50 g / mol and 80 g / mol has, preferably between 60 g / mol and 75 g / mol, and partially condensed out the gaseous working medium in an adiabatic expansion.

It can be provided that condenses during an adiabatic expansion during a cycle of the ORC process 1% to 30% of the mass of the working medium, preferably 10% to 20% of the mass of the working medium condenses out.

In these properties ranges of the working medium, a good operation of the ORC process and the heat engine with high efficiency is possible.

Furthermore, it can be provided according to the invention particularly preferably that the working medium is cyclopentene or at least one alkyl formate or a mixture thereof, preferably a methyl formate and / or ethyl formate.

These substances are particularly well suited as examples of suitable working media as working media for the intended application, as will be shown in detail later.

With a development of the invention it is proposed that the heat engine is an expansion engine, which preferably has a steam expansion engine with a piston as a motor or at least one turbine as having.

The engine can thus be realized in the sense of the present invention both by a piston engine and by a turbine. There are also other types of heat engines used as a motor, as long as they are able to implement the expansion work of the working medium in an exploitable outside the process, mechanical work. So could also be used a rotary engine.

The steam expansion motor with linearly moving piston is particularly preferred according to the invention, since it can be dispensed with by the wet behavior of the working medium on a recuperator and thus the implementation of the ORC process is particularly cost feasible.

The mechanical work delivered by the engine can be used directly mechanically or converted into electricity by means of a generator.

It can also be provided that in the cycle of the heat engine, a pump between the condenser and the evaporator is arranged, with which the fluid working fluid from the condenser to the evaporator is conveyed.

This ensures that the ORC process can start well.

According to a particularly preferred embodiment of the invention can be provided that no recuperator is arranged in the cycle of the heat engine.

The waiver of a recuperator (heat exchanger) is possible by the working media according to the invention. This makes the structure easier and less expensive.

Preferably, it can also be provided that the removal rate of the working medium compared to unalloyed steel at 150 ° C is less than 0.05 mm / a and / or the removal rate of the working medium compared to alloyed steel (1.4571) at 150 ° C less than 0.005 mm / a is.

This can ensure that the heat engine can work long term with the working medium.

Furthermore, it can be provided that the working medium in the temperature range between 70 ° C and 200 ° C with temporal temperature changes no endothermic or exothermic reactions or phase transitions first or second order, preferably not even ten times repetition of a temperature-time profile between 70 ° C and 200 ° C.

Such phase transitions could disrupt the ORC process.

The problems underlying the invention are also solved by the use of a working medium having a critical pressure (pc) between 4000 kPa and 6500 kPa, preferably between 4200 kPa and 6300 kPa, with a critical temperature (Tc) between 450 K and 650 K. between 460 K and 600 K and with a molar mass between 50 g / mol and 80 g / mol, preferably between 60 g / mol and 75 g / mol, in a heat engine, wherein the gaseous working medium partially condenses out with adiabatic expansion within one cycle of the ORC process.

The objects of the invention are preferably achieved by the use of alkyl formates or cyclopentene or mixtures thereof as the working medium in a heat engine.

It may be provided that methyl formate and / or ethyl formate are used as the alkyl formate, preferably methyl formate or ethyl formate being used as the working medium in the heat engine.

The use according to the invention is easy to implement and therefore inexpensive to implement.

As a further criterion, the use of mixtures can be very advantageous for reducing the energy losses during the heat transfer, since the evaporation of the temperature does not keep the temperature constant.

In applications according to the invention, it can preferably be provided that the heat engine is operated with an ORC process. For ORC processes, the substances and classes of substances involved are particularly well suited.

Furthermore, it can be provided that an expansion machine, preferably a steam expansion motor with a piston or at least one turbine, is used as the engine as the heat engine.

Finally, it can also be provided that the heat engine is operated with a heat source in a low temperature range between 80 ° C and 200 ° C, preferably between 80 ° C and 150 ° C.

The intended for use working media are particularly well suited for the low temperature range.

A fundamental finding is that working media that have the appropriate physical properties with respect to the critical pressure, the appropriate boiling temperature, and the appropriate behavior for adiabatic expansion, namely partial condensation, manages an ORC process in one Heat engine to be used with the low-temperature exhaust gas streams for power generation, without causing other adverse effects.

In the context of the present invention, therefore, new, efficient working fluids or working media have been developed for a heat engine.

In order to achieve the goal of efficient use of waste heat in the context of the present invention, an identification and development of working media (or working fluids) for low temperature applications were carried out, which allow both under thermodynamic aspects maximum efficiencies, and provide an optimum under safety and environmental aspects.

• 1. Dampfdruck: • Charakterisierbar durch Temperatur- und Druckbereich des Prozesses (Niederoder Hochtemperatur) • Ableitung der Steigung der Sattdampflinie im T-S-Diagramm aus Δh_LV, c_p, (2 Methoden) (nasses oder trockenes Fluid, Kondensation bei adiabatischer Expansion) • Große Verdampfungsenthalpie (großes Druckverhältnis von oberem zu unterem Prozessdruck) • Ableitungen der optimalen Prozessbedingungen
• 2. Wärmekapazität: • Ableitung der Steigung der Sattdampflinie im T-S-Diagramm aus Δh_LV, c_p, (Investkosten Wärmeübertragungsfläche)
• 3. Thermische und chemische Stabilität: • Hohe thermische und chemische Stabilität (in Kontakt mit Stahl, Schmiermittel, Luft, Wasser)
• 4. Viskosität: • Generelle Anwendbarkeit, Pumpenarbeit, Wärmeübergang (Investkosten Wärmeübertrager)
• 5. Korrosivität: • Geringe Korrosionsneigung
• 6. Kritische Daten: • Kritische Temperatur, kritischer Druck und kritisches Volumen
• 7. Wärmeleitfähigkeit: • Wärmeübergang
• 8. Dichte: • Wärmeübergang • Apparatedimensionierung (Hohe Dampfdichte → geringes spezifisches Volumen → kleine Ströme)
• 9. Molmasse • Tendenziell gilt: Je größer die Moleküle sind, desto höher werden das kritische Volumen und die kritische Temperatur und desto schlechter wird die Hochtemperaturbeständigkeit

In this case, the following substance data or measured variables are of central importance for the suitability of a chemical substance as a working medium, which can be characterized by the variables and relationships to be derived.

• 1. Vapor pressure: • Characterized by temperature and pressure range of the process (low or high temperature) • Derivation of the slope of the saturated steam line in the TS diagram from Δh _LV , c _p , (2 methods) (wet or dry fluid, condensation on adiabatic expansion) • High evaporation enthalpy (high pressure ratio from upper to lower process pressure) • Derivation of optimal process conditions
• 2. Heat capacity: • Derivation of the gradient of the saturated steam line in the TS diagram from Δh _LV , c _p , (investment costs heat transfer area)
• 3. Thermal and chemical stability: • High thermal and chemical stability (in contact with steel, lubricant, air, water)
• 4. Viscosity: • General applicability, pump work, heat transfer (Investkosten Wärmeübertrager)
• 5. Corrosivity: • Low tendency to corrosion
• 6. Critical data: • Critical temperature, critical pressure and critical volume
• 7. Thermal conductivity: • Heat transfer
• 8. Density: • heat transfer • device dimensioning (high vapor density → low specific volume → small currents)
• 9. Molecular Weight • The tendency is that the larger the molecules, the higher the critical volume and temperature, and the worse the high temperature resistance becomes

Where Δh _{LV is} the enthalpy of vaporization at constant volume, c _{p is} the heat capacity at constant pressure, T _c , _{fluid is} the critical temperature of the working medium, T _{process is} the process temperature, T is the temperature and S is the entropy.

One advantage of a heat engine filled with a working medium according to the invention (such as, for example, the piston expansion machine of the company DeVeTec GmbH) consists in the fact that so-called "wet" working fluids are used, which can be expanded into the wet steam area. A recuperation is not required in such a fluid, so that the machine for performing the process can be significantly simplified.

In the following, embodiments of the invention and diagrams of the invention will be explained with reference to eight diagrammatically illustrated figures and diagrams, without, however, limiting the invention. Showing:

1 a simplified schematic representation of an ORC process or a heat engine for implementing a method according to the invention;

2 : an ideal-typical mapping of state changes for wet, dry and isentropic fluids in the ORC process in the temperature-entropy diagram;

3 : a schematic representation of a structure for determining the vapor pressure of suitable working media;

4 : the temperature-time profile for a calorimetric measurement (DSC) for the investigation of suitable working media;

s 5 a vapor pressure-time diagram for determining the thermal stability of 1-propanol at 195 ° C to 180 ° C;

6 a vapor pressure-time diagram of methyl formate at 150 ° C;

7 a vapor pressure-time plot of ethyl formate at 150 ° C; and

8th : Cyclic Differential Thermal Analysis (DSC) Charts of Ethyl Formate.

1 shows in a simplified schematic representation of an ORC process for implementing a method according to the invention or an ORC process with which a heat engine according to the invention operates.

The illustrated ORC process is a simple thermodynamic cycle in which a working fluid is vaporized by the supply of heat at a high pressure level and possibly overheated. The superheated steam is expanded in an engine (eg, turbine or piston engine) while cooling to a low pressure while doing work. The steam leaving the expander may still be superheated or may already be relaxed down to the wet steam area, so that the working medium is already proportionally present as a liquid. The complete liquefaction takes place in the condenser. It is the powered by an organic working fluid instead of water, which can use the heat generated at a low temperature level thermodynamically more efficient.

The central variable is the vapor pressure of the components, which initially enables general classification for the low or high temperature range. Efficient working fluids make it possible to achieve the highest possible pressure ratio of upper to lower process pressure at a given temperature of heat source and heat sink. This requirement can be well demonstrated in a logarithmic plot of vapor pressure versus the negative reciprocal absolute temperature, as in 2 shown. Since the slope of the vapor pressure curve in the Raoult diagram according to the Clausius-Clapeyron equation is just proportional to the enthalpy of vaporization, workpieces with high enthalpy of vaporization promise advantages, due to the greater expected pressure ratio in the expander. Along with the heat capacity, estimation methods continue to exist which allow statements about the type of fluid (wet or dry).

The state changes of the working fluid in the cycle can be mapped in the temperature (T) - entropy (S) diagram. 2 shows the process flow in the TS diagram for different fluid types with the simplification that the fluids are isentropically expanded. The working fluids can be divided into wet (dew line with a negative slope), dry (dew line with a positive slope) and isentropic (vertical tau line) working fluids after the course of the saturation and tau line. When using these different fluid types in the ORC process, the main difference is the state of the steam after it has been released. In the case of wet and isotropic fluids, the steam after the expansion is only slightly or not at all overheated or the fluid is released into the wet steam area so that there are already liquid drops. In the case of dry fluids, there is superheated steam at a higher temperature than the condensation temperature. Depending on the amount of heat in the superheated steam, in turbine applications it may be necessary to use this unused heat to heat the cold fluid after the pressure has been increased to achieve better process efficiencies. At the same time, the process costs can be increased by about 30% by using the additional heat exchanger.

In certain cases, wet fluids are advantageous as ORC media and therefore preferred because they make a recuperator (heat exchanger) dispensable. The other required properties (see above) come into play only after this basic requirement, but are no less important. Thermal and chemical stability, low viscosity, no corrosivity, no toxicity, ease of handling (explosion limits outside operating conditions, no flammability) are among the most important requirements.

In order to operate the ORC process economically, a wet or isentropic behavior is preferred by the potential working media in order to be able to dispense with the use of a recuperator. A medium is called wet fluid if the slope of the tau line in the TS diagram is negative ( 2 ).

This leads to the formation of wet steam in an isentropic release from the dew line. In a vertical course of the dew line is called an isentropic and a positive slope of a dry medium.

In order to evaluate the thermodynamic suitability of new working media in the ORC process, a model of the cycle process was created in the simulation computer program "Aspen Plus", with which it is possible to determine the thermal efficiency depending on the medium used and the temperature of the available To calculate heat source.

• • Wirkungsgrad der Pumpe: 65 %
• • Maximaler Druck 35 bar
• • Wirkungsgrad der Expansionsmaschine 88 %
• • Endbedingungen der Expansion entweder 1,1 bar oder 35 °C
• • Totale Kondensation ohne Unterkühlung

The following boundary conditions apply to the simulation, which result from the devices used by DeVeTec:

• • Pump efficiency: 65%
• • Maximum pressure 35 bar
• • Efficiency of the expansion machine 88%
• • Final conditions of expansion either 1.1 bar or 35 ° C
• • Total condensation without hypothermia

The degree of freedom thus results in the maximum temperature in the evaporator. The simulations were carried out for different temperatures: 100 ° C, 150 ° C, 200 ° C and 250 ° C. The calculated thermal efficiency of the process was evaluated for the different conditions.

η

- Wirkungsgrad

Q_Nutz

- Nutzenergie

Q_zu

- zugeführte Energie

The efficiency is generally defined as:

η

- Efficiency

Q _utility

- useful energy

Q _too

- supplied energy

In the case of the Organic Rankine Cycle (ORC) process, the benefit comes from the performance gained in the expander. The effort is composed of the power of the pump and the heat supplied.

After evaluation of the simulation calculations, a list of the theoretically achievable efficiency can be prepared for the various operating conditions. Ethanol was defined as the reference medium. The particularly suitable working media found in the context of the present invention were compared for different temperatures with the working medium ethanol. Generally, it should be noted that the choice of working fluid depends on the available heat source. Depending on the evaporator temperature, some working fluids are better or worse for use as a working fluid in a heat engine. Table 1: Efficiency at the following max. Operating temperatures 200 ° C 150 ° C 100 ° C methyl formate 22.65 19.82 13.72 2,3-dihydrofuran 20.98 16.50 9.46 tetrahydrofuran 19.42 14,60 7.03 cyclopentenes 20.76 16.78 10.30 ethyl formate 20.46 16.19 9.34 ethanol 18.20 13.02 4.75

Compared to ethanol, lower operating temperatures result in a significant improvement in the efficiency. Further investigations were carried out for the use of the selected, particularly suitable substances. In particular, the stability of the materials at the operating temperature was investigated.

The vapor pressure is the pressure that occurs when, in a closed system, a vapor with the associated liquid phase is in thermodynamic equilibrium. The vapor pressure increases with increasing temperature and depends on the substance or mixture present. If, in an open system, the vapor pressure of a liquid is equal to the ambient pressure, the liquid begins to boil.

Vapor pressure is one of the key material properties for the design and operation of an ORC system. Because of the operating conditions defined for the steam engine, the vapor pressure of a suitable liquid should be below 35 bar.

The determination of the vapor pressure of the working media takes place in a closed and tempered high-pressure autoclave. The liquid is heated up and the pressure is measured at the set temperature. The more accurate the measurement of these two values the better the data of the vapor pressure. To compare the literature values, calculations can be performed with "Aspen Plus". In case of deviations of the data then own measurements of the vapor pressure can be carried out.

The specific heat capacity indicates which amount of heat must be supplied to one kilogram or one mole of a specific substance in order to increase its temperature by one Kelvin.

These substance-specific data are required in particular for the design of the thermal components of an ORC system. The experimental determination of the data is done in a calorimeter. The heat capacity is usually measured by means of DSC (English: Differential Scanning Calorimetry).

Viscosity is a measure of the toughness of a fluid and has an impact on heat transfer and pump performance in an ORC system. For comparison, water at 20 ° C has a viscosity of about 1 mPas, edible oils have a viscosity of about 100 mPas and honey one of about 1000 mPas. The lower the viscosity, the more fluid a liquid is and the faster it can flow under the same conditions. Therefore, suitable ORC working media should have a low viscosity of less than 10 mPas at 20 ° C.

The selected working media all have a fairly low viscosity, which is comparable to the viscosity of water (about 1 mPas at 20 ° C). In the interesting range for an ORC system from about 100 ° C, the viscosities of the preselected working media hardly differ from each other.

One of the other important material properties for the design of a thermodynamic cycle is the density of the liquid and gaseous phase of the working medium.

The density of the working media is required for the design of the circulation pumps.

The conversion from the volume flow to the mass flow takes place with the density of the substances.

The data on the mentioned physical quantities of the various substances can be found in the literature or databases on the examined working media.

The enthalpy of evaporation is the amount of heat needed to transfer a liquid from the liquid to the gaseous state. In the reverse process, in which the gaseous medium is re-liquefied, the heat of condensation is released. Both quantities are of great importance for a thermodynamic cyclic process in which a liquid is constantly evaporated and recondensed.

The evaporation enthalpy can be taken from the literature or, as already the heat capacity can be measured by calorimetric methods (eg by means of DSC).

The vapor pressure is one of the most important physical properties of a working medium. For the design of an ORC system and the validation of the simulation data, an exact knowledge of the vapor pressure curve is required. For their experimental determination, a system has been set up which enables accurate measurement in an absolute pressure range of 0 bar to 100 bar and at temperatures of 20 ° C to 400 ° C. Since there are no available, accurate measuring devices for such a large measuring range, the system was divided into three measuring ranges. Table 2 below summarizes the approval data for the individual autoclaves. Table 2: Design parameters for the vapor pressure measurement apparatus Autoclave 1 Autoclave 2 Autoclave 3 temperature range 20-150 ° C 20-250 ° C 20-350 ° C pressure range 0-2 bar 2-50 bar 50-100 bar volume 100 ml 100 ml 100 ml

To increase the measuring accuracy pressure sensors were used (make Endress & Hauser), which have been calibrated for the corresponding pressure and temperature range. The autoclave was heated by means of an electric heating sleeve. For temperature control, the temperature in the individual autoclave and in the heating jacket was measured by means of precise Ni-Cr temperature sensors and compared with each other. To seal the autoclave special copper bark and a copper paste were used. The apparatus and lines were completely isolated to reduce heat losses and better controllability. The integrated vacuum pump enables measurements in deep vacuum. The vacuum is particularly necessary when changing the fluids for cleaning purposes and to flush the measuring device with nitrogen to avoid ex-atmosphere. To record the readings, an automatic data acquisition with a sampling rate of one second was used for the entire duration of the test. A schematic schematic structure of the measuring device shows 3 ,

Commissioning and calibration of the measuring system was carried out with ethanol and water, whereby ethanol is suitable for the pressure range up to 60 bar. The vapor pressure of water at pressures up to 100 bar measured. The two substances were also used because the data of the substances are well known and can be used for validation purposes of the equipment. It was found that the deviation is below 1% of the absolute values, so that a suitable measuring method is available for the further investigations. Even in the high-pressure area, the measuring system was sufficiently validated with the data from water.

The actual suitability as a working medium depends not only on the maximum achievable efficiency but also essentially on the long-term stability of the substances in use. In a thermal decomposition of the substances undesirable by-products may arise, the z. B. can lead to the reduction of the vapor pressure or corrosion of the materials used in the thermal power plant. In the first pre-selection, the working media were briefly claimed and examined on several criteria. From this four substances could be selected for further tests. In the second test phase, extensive corrosion and material compatibility tests were carried out. In the third test phase endurance tests were carried out. Subsequently, the working media were tested in a heat engine under realistic conditions.

Knowledge about the thermal stability of a substance is generally indispensable. An unchecked fabric may suffer quality degradation and unpredictable hazards during manufacturing, storage and transport due to excessive temperatures. It is important for the working media that are sought to avoid undesirable decomposition products that could endanger the operation of the system.

The following measuring principle was used to determine the thermal stability:
The working media were placed in an autoclave at room temperature and inertized with nitrogen. Subsequently, the temperature of the medium was increased to a maximum operating temperature and maintained over a longer period. The vapor pressure of the fabric was first determined at room temperature and compared with literature values. This was followed by a continuous determination of the vapor pressure as a function of temperature and a permanent measurement at the maximum temperature. After completion of the experiment, the working medium was cooled and analyzed by means of a gas chromatographic measurement.

With the gas chromatograph (GC) the composition of substance mixtures can be determined. As a result, a chromatogram is created on which all substances have a clear assignment. The measurement is performed for an untreated and laboratory tested substance. When decomposition products are formed, they can be clearly determined. By measuring not only the type of by-products can be determined but also their percentage determined.

Another method for determining the thermal stability is differential scanning calorimetry (DSC - Differential Scanning Calometry). This method was used to determine stability in multiple cycles.

In the DSC, two closed crucibles (first crucible with approx. 10 mg sample and second empty crucible as reference) are heated up to a target temperature (in this case up to 200 ° C) at a predetermined heating rate (here 10 Kelvin / minute). Both crucibles are exposed to the same temperature program. The energy intake or decrease is analyzed during heating. Depending on the heat capacity of the sample or exothermic and endothermic processes in the sample, such as melting or evaporation, the energy balance changes compared to the empty sample. After heating, the sample is kept at a constant, maximum temperature. In a thermally stable substance results in no change in energy during this time. Decomposition of the substance is observed by a change in energy uptake or energy decrease.

4 shows the temperature-time profile used for the DSC. In a period of 0-20 minutes, the temperature is increased at a constant rate of heating and absorbed accordingly energy. In the range between 20-50 minutes, the temperature is kept constant. With a stable medium, no energy is absorbed or released. Between 50 - 70 minutes, the sample is cooled again, so that the temperature is reduced with appropriate energy decrease.

To confirm the reproducibility of the measurement, several cycles per medium were stirred. The decomposition products can namely occur only after a longer operating time and several cycles.

Since the selected working media could have a corrosive behavior on the used materials of the ORC engine, extensive investigations of the corrosion behavior were carried out. For this purpose, both metallic and non-metallic (essentially elastomeric materials of the seals) were defined and investigated in connection with the individual media.

For metallic materials, samples were created with defined dimensions. To determine the removal rates, the metal strips were weighed and completely immersed in a high-pressure autoclave in the respective fluid. The autoclaves were sealed, rendered inert and brought to a defined temperature and held at this temperature for a longer time. Thereafter, the metal strips were taken out again, cleaned and weighed for the determination of the removal rate. To determine a possible local corrosion, the individual samples were examined microscopically.

To investigate the corrosion behavior of the working media compared to non-metallic materials, the following investigations were carried out:
Long-term thermal stability is critical to the proper functioning of an ORC system. However, working media can be decomposed by use at high temperatures. Therefore, the stability of a new working medium must be proven. The experiments were carried out in a high pressure autoclave with the aim to capture the highest possible operating temperature of each medium. The test temperature and test pressure were measured. Upon decomposition of the fluid, the vapor pressure also changes. This change can again be observed from the measured values. The decomposition products were analyzed in a gas chromatograph and compared with the starting product.

In 5 the study of the thermal stability of 1-propanol at 195 ° C and 180 ° C is shown. The measured vapor pressure (upper curve) increases with time at the same temperature (lower curve) at different temperatures between 195 ° C and 180 ° C. This means that 1-propanol is not stable at these operating temperatures. The vapor pressure is below 180 ° C too low (less than 20 bar) to still be useful as a working medium in a heat engine can be used. In the experimental setup according to FIG. 6, methyl formate was stored at a temperature (upper curve) of about 150.degree. The vapor pressure (lower curve) remains constant, so that the fluid can be called stable at this temperature.

All potential working media were tested in this way for the operating temperatures of 150 ° C to 200 ° C. A similar behavior as methyl formate is also shown by ethyl formate ( 7 ). At a service temperature of 175 ° C, this fluid readily decomposes over time. An operating temperature of 150 ° C (upper curve in 7 ) it remains stable, that is, the vapor pressure (in 7 lower curve) does not increase.

The working media methyl formate, ethyl formate and cyclopentene, for example, due to these studies are particularly advantageous. In the extended studies, the thermal stability of the preselected working media was tested in a longer trial lasting two months.

The tested working media were stored at an operating temperature of 150 ° C in high pressure autoclave. To determine the thermal stability, all samples after the experiment were analyzed for their decomposition rate by GC analysis. The results of this analysis were summarized in Table 3. The maximum decomposition of the working media methyl formate, ethyl formate and cyclopentene was about 2% and is therefore within a technically acceptable range. Table 3: Purity and degree of decomposition of the working media after a 2-week thermal load. before the trial after the trial Percent decomposition methyl formate 98.62% 97.21% 1.43% ethyl formate 97.28% 95.16% 2.18% cyclopentene 98.51% 98.31% 0.20%

In the following investigations it was to be examined to what extent the working media used have a corrosive behavior compared to the typical materials used in heat engines. The following materials were tested in the corrosion tests: unalloyed steel (P265GH) and alloyed steel (1.4571), including one weld. The materials were used as sheet metal (90 mm × 10 mm × 6 mm). The specimens were weighed in a materials engineering laboratory and light microscopic characterized. The experiment was then carried out in the vapor pressure measurement apparatus already described. After the sample storage, the evaluation was again in the material engineering laboratory. Table 4 shows the results of the first corrosion test. Table 4: Test results and evaluation of the corrosion tests material working medium Removal rate [mm / a] Microscopic findings Unalloyed steel (P265GH) 0.0301 without findings Alloy steel (1.4571) methyl formate 0.0024 without findings Unalloyed steel (P265GH) 0.0384 without findings Alloy steel (1.4571) ethyl formate 0.0034 without findings Unalloyed steel (P265GH) 0.0317 without findings Alloy steel (1.4571) cyclopentene 0.0013 without findings

Although no local corrosion and cracks were detected in the course of the light microscopic examination, a crack test was additionally performed on the material 1.4571 with the aid of the penetration method. Again, no cracks were found. The technical resistance limit of metallic materials lies at a removal rate of ≤ 0.1 mm / anno. In addition, no local corrosion attacks may be detected, as these preclude a technical resistance of the materials. Accordingly, the two classes of materials tested under the experimental conditions mentioned above at 150 ° C compared to the preferred working media methyl formate, ethyl formate and cyclopentenes are classified as technically resistant and therefore the three working media basically suitable.

Cyclic stability experiments carried out with the described DSC method showed no deterioration or alteration of the working media (methyl formate, ethyl formate and cyclopentene). Exemplary are the cyclic DSC curves of ethyl formate in 8th where also methyl formate and cyclopentene showed similar curves. The upper curve again shows the temperature profile used for the DSC measurement. The lower sets of curves represent the measurement results of the DSC measurement. Therefore, there is sufficient long-term stability for all three working media.

By measuring the efficiency of the cyclic process with the selected working media (methyl formate (methyl formate), ethyl formate (ethyl formate) and cyclopentene) and by the above experiments, it was found that fluids are particularly well suited for use in the heat engine when the critical pressure p _{C is} between 4000 kPa and 6500 kPa, in particular between 4200 kPa and 6300 kPa, more preferably between 4700 kPa and 6000 kPa, the fluids have a critical temperature (T _c ) between 450 K and 650 K, preferably between 460 K and 600 K. , more preferably between 475 K and 510 K, and the fluids have a molecular weight between 50 g / mol and 80 g / mol, preferably between 60 g / mol and 75 g / mol. Such fluids can be used with high efficiency even at low temperatures of the exhaust gas to be used or at low temperature of the evaporator. It has been shown to simplify the construction of the heat engine that the use of a recuperator (heat exchanger) can be dispensed with when a "wet" working medium is used. The working medium is referred to as a "wet" working medium when the gaseous working medium partially condenses out in an adiabatic expansion.

These criteria are well met by the preferred working media methyl formate, ethyl formate and cyclopentene. Thus, the critical temperature (T _c ) of methyl formate is 487 K, ethyl formate 508 K and cyclopentene 507 K. The critical pressure p _C of methyl formate is 5998 kPa, that of ethyl formate is 4742 kPa and that of cyclopentene is 4820 kPa. The molecular weight of methyl formate is 60 g / mol, of ethyl formate at 68 g / mol and of cyclopentene at 74 g / mol. All of these three working media partly condense on adiabatic expansion, so that a recuperator in the circulation of the ORC can be dispensed with.

According to the simulation conditions, the efficiency of the working fluids of the invention in a heat engine at an exhaust gas temperature (evaporator temperature) is between 80 ° C and 200 ° C better than known working media for heat engines, such as ethanol used ethanol. These results could be confirmed experimentally for methyl formate in an ORC engine (piston expansion machine) of the company Devetec GmbH. With the working media according to the invention, therefore, an improvement in the efficiency of the heat engine is achieved at temperatures between 80 ° C and 200 ° C, in particular between 80 ° C and 150 ° C.

The features of the invention disclosed in the foregoing description, as well as the claims, figures and embodiments may be essential both individually and in any combination for the realization of the invention in its various embodiments.

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 102009024436 A1 [0003]
• DE 102011076157 A1 [0003]
• EP 1016775 A2 [0003]
• EP 1174590 A2 [0008]

Cited non-patent literature

• Quoilin, S., Lemort, V., Technological and Economical Survey of Organic Rankine Cycle, 5th European Conference on Economics and Management of Energy in Industry, Vilamoura, Portugal, 14.04.-17.04.2009 [0008]

Claims (15)

Heat engine for performing an Organic Rankine Cycle (ORC) comprising an evaporator, a motor, a condenser and a circuit containing a fluid working medium, wherein the working medium has a critical pressure (p _c ) between 4000 kPa and 6500 kPa, preferably between 4200 kPa and 6300 kPa, the working medium has a critical temperature (T _c ) between 450 K and 650 K, preferably between 460 K and 600 K, the working medium has a molecular weight between 50 g / mol and 80 g / mol, preferably between 60 g / mol and 75 g / mol, and the gaseous working medium partially condensed out in adiabatic expansion. Heat engine according to claim 1, characterized in that condenses out in an adiabatic expansion during the Organic Rankine Cycle (ORC) 1% to 30% of the mass of the working medium, preferably 10% to 20% of the mass of the working medium condensed. Heat engine according to claim 1 or 2, characterized in that the working medium is cyclopentene or at least one alkyl formate or a mixture thereof, preferably a methyl formate and / or ethyl formate. Heat engine according to one of the preceding claims, characterized in that the heat engine is an expansion engine, which preferably has a steam expansion engine with a piston as a motor or which has at least one turbine as a motor. Heat engine according to one of the preceding claims, characterized in that in the circuit of the heat engine, a pump between the condenser and the evaporator is arranged, with which the fluid working medium from the condenser to the evaporator is conveyed. Heat engine according to one of the preceding claims, characterized in that no recuperator is arranged in the cycle of the heat engine. Heat engine according to one of the preceding claims, characterized in that the removal rate of the working medium compared to unalloyed steel at 150 ° C is less than 0.05 mm / a and / or the removal rate of the working medium compared to alloy steel (1.4571) at 150 ° C less than 0.005 mm / a. Heat engine according to one of the preceding claims, characterized in that the working medium in the temperature range between 70 ° C and 200 ° C with temporal temperature changes no endothermic or exothermic reactions or phase transitions of the first or second order, preferably not with ten times repetition of a temperature-time Profiles between 70 ° C and 200 ° C. Use of a working medium having a critical pressure (p _c ) between 4000 kPa and 6500 kPa, preferably between 4200 kPa and 6300 kPa, with a critical temperature (T _c ) between 450 K and 650 K, preferably between 460 K and 600 K and with a molar mass between 50 g / mol and 80 g / mol, preferably between 60 g / mol and 75 g / mol, in a heat engine, wherein the gaseous working medium partially condenses out in an adiabatic expansion of an Organic Rankine Cycle (ORC). Use according to claim 9, characterized in that during an adiabatic expansion during the Organic Rankine Cycle (ORC) 1% to 30% of the mass of the working medium is condensed out, preferably 10% to 20% of the mass of the working medium is condensed out. Use of alkyl formates or cyclopentene or mixtures thereof as a working medium in a heat engine. Use according to Claim 11, characterized in that methyl formate and / or formic acid ethyl ester are used as the alkyl formate, preferably methyl formate or ethyl formate being used as the working medium in the heat engine. Use according to one of claims 9 to 12, characterized in that the heat engine is operated with an Organic Rankine Cycle (ORC). Use according to one of claims 9 to 13, characterized in that the heat engine used is an expansion machine, preferably a steam expansion motor with piston or at least one turbine as motor. Use according to one of claims 9 to 14, characterized in that the heat engine is operated with a heat source in a low temperature range between 80 ° C and 200 ° C, preferably between 80 ° C and 150 ° C.

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