USE OF HIGHLY EFFICIENT WORKING MEDIA FOR HEAT ENGINES

Abstract

The invention relates to a heat engine for performing an organic Rankine cycle (ORC) which comprises an evaporator, an engine, a condenser and a circuit comprising 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 molar mass between 50 g/mol and 80 g/mol, preferably between 60 g/mol and 75 g/mol, and the gaseous working medium partially condenses out during adiabatic expansion. The invention further 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, having a critical temperature (Tc) between 450 K and 650 K, preferably between 460 K and 600 K, and having 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 during an adiabatic expansion in an organic Rankine cycle (ORC).

Description

The invention relates to a heat engine for performing an organic Rankine cycle (ORC) which comprises an evaporator, an engine, a condenser and a circuit comprising a fluid working medium and to the use of a working medium for a heat engine.

There is a great demand in the chemical industry for using low energy waste heat streams generated at a temperature range from 80° C. to 250° C.

To optimize existing site integration systems, and with a view to improving energy efficiency and reducing CO₂ emissions, one promising option is the conversion of these not as yet utilized waste heat streams into electricity through the use of combined heat and power (CHP). This employs heat engines such as are disclosed in DE 10 2009 024 436 A1, DE 10 2011 076 157 A1 and EP 1 016 775 A2 for example. The latter two heat engines employ water/steam as the working medium. The disadvantage of these is that they operate at relatively high temperatures.

The problem of high operating temperatures of steam processes has been overcome by the use of ORC technology since this technology employs organic fluids rather than steam as the working medium.

ORC stands for organic Rankine cycle “organischer Rankine-Kreisprozess” bedeutet. An ORC process is a thermodynamic cycle for converting heat into mechanical work using an organic working medium.

An ORC process is a simple thermodynamic cycle in which the working medium is evaporated and optionally superheated by supplying heat at a high pressure level. The superheated vapour undergoes expansion cooling to a lower pressure in an expander (in particular an engine such as a piston engine or a turbine) thus performing work. The work may be directly mechanically utilized or is converted into electrical current using a generator. The vapour exiting the expander may still be in the superheated state or may already be decompressed to such an extent that it occupies the wet vapour region so that some of it is already in the liquid state. Complete liquefaction takes place in the condenser. Here, the electricity-generating cycle is operated not with water but with an organic working fluid which can utilize the heat generated at a low temperature level with greater thermodynamic efficiency.

The working medium employed thus has a key role since the optimal interaction between the working medium and the process configuration has a determining influence on the efficacy and thus on the efficiency of the entire process. For example, the working medium influences the plant configuration. Optimal selection of a working medium can enhance the utilization of the heat source and the efficiency of the plant.

Suitable working media for ORC processes include especially (hydro)chlorofluorocarbons and hydrocarbons and also mixtures of fluids (hydrocarbons and water, (hydro)fluorocarbon mixtures) and organic silicon components. The existing industrially realized prior art employs not only hydrocarbons such as pentane, but also siloxanes such as octamethyltrisiloxane or chlorinated hydrocarbons such as R134a or R245fa (Quoilin, S., Lemort, V., Technological and Economical Survey of Organic Rankine Cycle, 5th European Conference Economics and Management of Energy in

Industry, Vilamoura, Portugal, 14.04.-17.04.2009). A heat engine utilizing such ORC technology is disclosed, for example, in EP 1 174 590 A2 where pentane is used as the organic working fluid, i.e. as the working medium.

The disadvantages of the prior art working fluids include possible hazards to the environment (CFCs: harmfulness to the ozone layer and global warming) and to workplace safety (hydrocarbons: flammability, explosion prevention) and also thermodynamic limitations due to insufficient optimization of plant design and fluid properties.

For certain vapour-expansion engines (piston engines) there are no optimized working fluids yet in existence that may be employed in the temperature range from 80° C. to 250° C.

The fluorinated hydrocarbons are some of the most extensively described working media. A substantial advantage of these substances lies in their physical properties. For instance these substances are generally not flammable and nontoxic. The disadvantage of such substances is that the boiling point of fluorinated hydrocarbons is generally very low since said substances were usually developed as coolants and are thus of only limited suitability for use in an ORC system at relatively high use temperatures.

A further large group of ORC working media are hydrocarbons, for example toluene, pentane and isobutane. Hydrocarbons are very well known as suitable ORC working media and are employed in ORC engines. However, when utilizing these media their properties must be taken into account. The main disadvantage of these substances is that they are usually flammable and hazardous to the environment. Said substances generally also have a highly deleterious effect on climate.

As an example of a prior art ORC application, ethanol is currently used in an ORC vapour engine from DeVeTec GmbH as the most efficient working medium in a temperature range starting at about 250° C.

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

In the light of this prior art the problem addressed by the invention is that of providing a working fluid for an organic Rankine cycle (ORC) comprising a vapour-expansion engine using waste heat streams from DeVeTec GmbH in extended temperature ranges between 80° C. to 250° C., in particular from 80° C. to 200° C., particularly preferably from 80° C. to 150° C. This broad temperature range is a result of the different temperature levels of the waste heat streams. While offgases from biogas, biomass or mine gas combustion are present at temperatures in the region of 450° C., the industrial sphere is host to many lower temperature streams in the range from 100° C. to 200° C. which can no longer be utilized in many chemical sites but whose potential can be enhanced via an ORC cycle. Different working fluids are thus utilized depending on the application.

In addition to suitable thermodynamic properties (inter glia thermal stability, enthalpy of vapourization, vapour pressure and heat capacity) the working medium must meet further requirements such as low toxicity and low environmental impacts (for example with regard to innocuousness towards the ozone layer and climate) and must not be flammable nor corrosive towards components of the heat engine.

A further problem addressed by the invention is that of providing a working medium employable with heat engines at low temperatures with a high degree of efficiency. The working medium shall simultaneously exhibit good environmental compatibility, in particular in terms of harmfulness towards the ozone layer and climate. The working medium should further effect as little attack and corrosion as possible on the components of such a heat engine. The working medium shall moreover be as nonhazardous as possible in its application, i.e. should exhibit the lowest possible flammability and present no risk of explosion.

Further problems addressed by the present invention and not mentioned explicitly will become apparent from the overall context of the following description, examples and claims.

These and other problems not explicitly mentioned but readily derivable or discernible from the above context discussed in the introduction hereof are solved by a heat engine having all the features of claim 1 and by a method having all the features of claim 9. Protection for advantageous developments of the inventive method according to claim 1 is sought in subclaims 2 to 8. Protection for an advantageous development of the inventive heat engine according to claim 9 is sought in subclaims 10 to 15.

The problems addressed by the present invention are solved by a heat engine for performing an organic Rankine cycle (ORC) which comprises an evapourator, an engine, a condenser and a circuit comprising 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 molar mass between 50 g/mol and 80 g/mol, preferably between 60 g/mol and 75 g/mol, and the gaseous working medium partially condenses out during adiabatic expansion.

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

These property ranges of the working medium ensure good functioning of the ORC process and the heat engine with a high degree of efficiency.

It may further be provided with particular preference according to the invention that the working medium is cyclopentene or at least one alkyl formate or a mixture thereof, preferably methyl formate and/or ethyl formate.

These substances are particularly suitable as working media for the intended use as is shown in detail hereinbelow.

A development of the invention proposes that the heat engine is an expansion machine which preferably comprises a vapour expansion engine comprising pistons as the engine or which comprises at least one turbine as the engine.

In the context of the present invention the engine may thus be realized either as a piston engine or as a turbine. Other types of heat engines may also be employed as the engine provided they are capable of converting the expansion work of the working medium into mechanical work utilizable outside the process. It is thus also possible to employ a rotary engine.

A vapour expansion engine having reciprocating pistons is particularly preferred in accordance with the invention since the wet behaviour of the working medium makes it possible to eschew a recuperator and the conversion of the ORC process may thus be carried out in particularly cost-effective fashion.

The mechanical work delivered by the engine may be directly mechanically utilized or converted into electrical current using a generator.

It may also be provided that a pump is disposed between the condenser and the evapourator in the circuit of the heat engine, said pump allowing the fluid working medium to be conveyed from the condenser to the evapourator.

This ensures that the ORC process may be readily started up.

A particularly preferred embodiment of the invention may provide that the circuit of the heat engine does not comprise a recuperator.

The eschewal of a recuperator (heat exchanger) is made possible by the working media according to the invention. This makes the heat engine simpler and more cost-effective to set up.

It may also be provided with preference that the erosion rate of the working medium towards unalloyed steel is less than 0.05 mm/a at 150° C. and/or that the erosion rate of the working medium towards alloyed steel (1.4571) is less than 0.005 mm/a at 150° C.

This ensures that long-term operation of the heat engine with the working medium is possible.

It may further be provided that the working medium exhibits no endothermic or exothermic reactions or first or second order phase transitions in the temperature range between 70° C. and 200° C. when subjected to temperature changes over time, preferably not even when subjected to tenfold repetition of a temperature/time profile between 70° C. and 200° C.

Such phase transitions might disrupt the ORC process.

The problems addressed by 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, having a critical temperature (Tc) between 450 K and 650 K, preferably between 460 K and 600 K, and having 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 during adiabatic expansion within a cycle of the ORC process.

The problems addressed by the invention are preferably solved 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 employed as the alkyl formate, preference being given to employing methyl formate or ethyl formate as the working medium in the heat engine.

The process according to the invention is easy to implement and thus cost effective in its realization.

As a further criterion the use of mixtures may be highly advantageous for reducing the energy losses during heat transfer since the evaporation thereof does not occur at constant temperature.

Uses according to the invention may preferably provide that the heat engine is operated with an ORC process. The substances and substance classes at issue are particularly suitable for ORC processes.

It may also be provided that the heat engine employed is an expansion machine, preferably a vapour expansion engine comprising pistons or at least one turbine as the engine.

It may finally 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 working media intended for use are particularly suitable for the low temperature range.

One fundamental finding of the is that working media having suitable physical properties in terms of critical pressure, suitable boiling point and suitable behaviour during adiabatic expansion, namely partial condensation, may be used to carry out an ORC process in a heat engine with which low-temperature offgas streams too may be utilized for conversion into electricity without the occurrence of other deleterious effects.

Accordingly, endeavours in the context of the present invention led to the development of novel, efficient working fluids/working media for a heat engine.

In order to achieve the objective of efficient utilization of waste heat, endeavours in the context of the present invention led to the identification and development of working media (i.e. working fluids) for low temperature applications which not only achieve maximum thermodynamic efficiency but are also optimal from safety and environmental aspects.

Of central importance for the suitability of a chemical substance as a working medium are in particular the following material data/measured parameters which are characterizable by the derivable parameters and relationships that they intimate.

1. Vapour pressure:

• • characterizable by the temperature and pressure range of the process (low- or high-temperature)
  • derivation of the gradient of the saturated vapour line in the T-S diagram from Δh_Lv, C_p, (2 methods) (wet or dry fluid, condensation during adiabatic expansion)
  • large enthalpy of vaporization (large pressure ratio of upper to lower process pressure)
  • derivation of optimal process conditions

2. Heat capacity:

• • derivation of the gradient of the saturated vapour line in the T-S diagram from Δh_Lv, C_p, (heat transfer area capital expenditure costs)

3. Thermal and chemical stability:

• • high thermal and chemical stability (in contact with steel, lubricants, seals, air, water)

4. Viscosity:

• • general applicability, pump work, heat transfer (heat exchanger capital expenditure costs)

5. Corrosivity:

• • low propensity for corrosion

6. Criticality data:

• • critical temperature, critical pressure and critical volume

7. Thermal conductivity:

• • heat transfer

8. Density:

• • heat transfer
  • apparatus dimensioning (high vapour density→low specific volume→small streams)

9. Molar mass:

• • It is a tendency that: the greater the molecules the higher the critical volume of the critical temperature and the poorer the high-temperature resistance

Δ_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 particular advantage of a heat engine filled with a working medium according to the invention (for example the piston expansion engine from DeVeTec GmbH) is that so-called “wet” working fluids, which may be decompressed into the wet vapour region, may be employed. Recuperation is not necessary for such a fluid and the engine for performing the process may therefore be markedly simplified.

Hereinbelow, exemplary embodiments of the invention and diagrams relating to the invention are elucidated by reference to eight schematically represented figures and diagrams without any intention to restrict the invention. Dabei zeigt:

FIG. 1 shows a simplified schematic representation of an ORC process/a heat engine for implementing a process according to the invention;

FIG. 2 shows an ideal-type representation of the changes of state for wet, dry and isentropic fluids in the ORC process in a temperature-entropy diagram;

FIG. 3 shows a schematic representation of a setup for determining the vapour pressure of suitable working media;

FIG. 4 shows the temperature/time profile for a calorimetric measurement (DSC) for analyzing suitable working media;

FIG. 5 shows a vapour pressure/time diagram for determining the thermal stability of 1-propanol at 195° C. to 180° C.;

FIG. 6 shows a vapour pressure/time diagram for methyl formate at 150° C.;

FIG. 7 shows a vapour pressure/time diagram for ethyl formate at 150° C.;

FIG. 8 shows cyclic differential thermal analysis diagrams (DSC curves) for ethyl formate.

FIG. 1 shows a simplified schematic representation of an ORC process for implementing a process according to the invention, i.e. an ORC process, such as is carried out in a heat engine according to the invention.

The ORC process depicted is a simple thermodynamic cycle in which a working medium is evaporated and optionally superheated at a high pressure level by supplying heat. The superheated vapour undergoes expansion cooling to a lower pressure in an engine (for example a turbine or piston engine) thus performing work. The vapour exiting the expander may still be in the superheated state or may already be decompressed to such an extent that it occupies the wet vapour region so that some of the working medium is already in the liquid state. Complete liquefaction takes place in the condenser. Here, the electricity-generating cycle is operated not with water but with an organic working fluid which can utilize the heat generated at a low temperature level with greater thermodynamic efficiency.

A parameter of central importance is the vapour pressure of the components which firstly permits general classification for the low- or high-temperature range. Efficient working fluids make it possible to realize, for a given temperature of the heat source and the heat sink, the greatest possible pressure ratio between the upper and lower process pressure. This requirement may readily be shown in a logarithmic representation of the vapour pressure via the negative reciprocal absolute temperature as is shown in FIG. 2. Since the gradient of the vapour pressure curve in the Raoult diagram is proportional to the enthalpy of vaporization in accordance with the Clausius-Clapeyron equation, working media having large enthalpies of vaporization promise advantages on account of the greater expected pressure ratio in the expander. Together with the heat capacity there are also methods of estimation that allow predictions to be made regarding the fluid type (wet, dry or isentropic).

The changes of state of the working fluid in the cycle may be depicted in the temperature (T) entropy (S) diagram. FIG. 2 shows the advancement of the process for different fluid types in the T-S diagram with the simplification that the fluids are decompressed in isentropic fashion. The working fluids may be categorized according to the path of the saturation line and the dew line into wet (negative gradient dew line), dry (positive gradient dew line) and isentropic (vertical dew line) working fluids. The substantial difference when using these different fluid types in the ORC process lies in the state of the vapour after the decompression. For wet and isentropic fluids the vapour is in the superheated state only to a very limited extent, if at all, after the decompression, i.e. the fluid is decompressed into the wet vapour region so that liquid droplets are already present. In the case of the dry fluids a superheated vapour is present which is at a temperature higher than the condensation temperature. Depending on the proportion of heat in the superheated steam it may be necessary in the case of turbine utilizations to use this unutilized heat for warming the cold fluid after the pressure increase in order to achieve improved efficiencies for the process. Process costs may simultaneously be increased by about 30% due to the use of the additional heat exchanger.

In certain cases using wet fluids as ORC media is advantageous and thus preferable since said fluids make it possible to eschew a recuperator (heat exchanger). The further required properties (see above) only come into play after this fundamental requirement has been met but are then no less important. The most important requirements include thermal and chemical stability, low viscosity, no corrosivity, no toxicity, easy handleability (explosion limits outside operating conditions, no flammability).

In order to operate the ORC process in economic fashion, preference among the potential working media is given to wet/isentropic behaviour in order that a recuperator may be eschewed. A medium is referred to as a wet fluid when the gradient of the dew line in the T-S diagram is negative (FIG. 2). This results in the formation of wet vapour upon isentropic decompression starting from the dew line. When the dew line is vertical the medium is referred to as isentropic and when the gradient is positive the medium is referred to as dry.

In order to evaluate the thermodynamic suitability of new working media in the ORC process a model of the cycle was constructed in the “Aspen Plus” computer simulation program which allows the thermal efficiency to be calculated as a function of the medium employed and the temperature of the available heat source.

The following boundary conditions derived from the apparatuses employed by DeVeTec apply to the simulation:

• • efficiency of the pump: 65%
  • maximum pressure: 35 bar
  • efficiency of the expansion machine: 88%
  • final conditions of the expansion: either 1.1 bar or 35° C.
  • total condensation without supercooling

The maximum temperature in the evaporator is accordingly a degree of freedom. The simulations were performed for various temperatures: 100° C., 150° C., 200° C. and 250° C. The thermal efficiency of the process was evaluated for the various conditions.

The efficiency is generally defined as:

$\eta =\begin{array}{c}{Q}_{\mathrm{useful}}\\ Q_{\mathrm{supplied}}\end{array}$

• η—efficiency
• Q_useful —useful energy
• Q_supplied—supplied energy

In the case of the organic Rankine cycle process (ORC process) the utility is the output of the expansion machine. The input is composed of the power of the pump and the supplied heat.

Evaluation of the simulations makes it possible to compile a list of the theoretically achievable efficiencies 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 with the working medium ethanol for various temperatures. In general terms it should be noted that the choice of working medium is dependent on the heat source available. Depending on the evaporator temperature certain working media are more or less suitable for use as the working medium in a heat engine.

TABLE 1
Efficiency at the following maximum 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
	cyclopentene	20.76	16.78	10.30
	ethyl formate	20.46	16.19	9.34
	ethanol	18.20	13.02	4.75

Compared to ethanol there is a marked improvement in efficiency at lower use temperatures. Further investigations were carried out for the use of the selected particularly preferable substances. In particular, the stability of the substances at the use temperature was analyzed.

The vapour pressure is the pressure established when a vapour is in thermodynamic equilibrium with the associated liquid phase in a sealed system. The vapour pressure increases with increasing temperature and is a function of the substance/mixture present. When the vapour pressure of a liquid is equal to the ambient pressure in an open system the liquid begins to boil.

The vapour pressure is one of the crucial substance properties for the design and operation of an ORC plant. Due to the operating conditions defined for the vapour engine the vapour pressure of a suitable liquid should be below 35 bar.

The vapour pressures of the working media are determined in a sealed and temperature-controlled high-pressure autoclave. This comprises heating the liquid and measuring the pressure at the particular temperature setting. The more accurate the measurement of these two values the better the determined vapour pressure data. Calculations may be performed with “Aspen Plus” for comparison with the literature values. In the case of deviations in the data, in-house measurements of the vapour pressure may then be performed.

Specific heat capacity indicates the amount of heat that needs to be supplied to a kilogram or a mole of a particular substance to raise its temperature by 1 Kelvin.

These substance-specific data are necessary in particular for the design of the heat engineering components of an ORC system. Experimental determination of the data is performed in a calorimeter. Heat capacity is generally measured using DSC (differential scanning calorimetry).

Viscosity is a measure of the resistance of a fluid to deformation and influences heat transfer and pump performance in an ORC system. For comparison at 20° C. water has a viscosity of about one mPas, edible oils have a viscosity of about 100 mPas and honey has a viscosity of about 1000 mPas. The lower the viscosity the more mobile a liquid and the quicker said liquid can flow under constant conditions. Suitable ORC working media should therefore have a low viscosity of less than 10 mPas at 20° C.

The chosen working media all have a rather low viscosity which is comparable to the viscosity of water (about 1 mPas at 20° C.). In the region above about 100° C. which is of interest for an ORC system the viscosities of the preselected working media hardly differ from one another anymore.

One of the further important substance 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 essential to the design of the circulation pumps. Volume flow is converted into mass flow using the density of the substances.

The data for the cited physical parameters of the various substances are obtainable from the literature and/or from databases concerning the working media analyzed.

The enthalpy of vaporization is the amount of heat required to effect the transition of a liquid from the liquid into the gaseous state. The converse process in which the gaseous medium is reliquefied gives off the heat of condensation. Both parameters are of great importance for a thermodynamic cycle in which a liquid is continually evaporated and recondensed.

The enthalpy of vaporization may be obtained from the literature or, similarly to the heat capacity, measured by calorimetric methods (for example by DSC).

The vapour pressure is one of the most important physical substance properties of a working medium. Designing an ORC system and validating the simulation data require accurate knowledge of the vapour pressure curve. Equipment allowing accurate measurement in an absolute pressure range from 0 bar to 100 bar and at temperatures from 20° C. to 400° C. was constructed for the experimental determination of said curve. Since accurate measuring means for such a large measurement range are not available the equipment was divided into three measurement regions. Table 2 which follows summarizes the permissible operation data for the individual autoclaves.

TABLE 2
Design parameters for the vapour pressure measuring 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

Measurement accuracy was enhanced by using pressure sensors (from Endress & Hauser) calibrated for the relevant pressure and temperature range. The autoclaves were heated using an electric heating collar. Temperature control was effected by measuring the temperature in the individual autoclave and in the heating collar using precise Ni-Cr temperature sensors and comparing these temperatures with one another. The autoclaves were sealed using special copper washers and copper paste. The apparatus and the conduits were fully insulated to reduce heat losses and achieve improved controllability. The integrated vacuum pump makes it possible to obtain measurements under high vacuum. The vacuum is also required in particular when changing the fluids for cleaning purposes and for purging the measuring means with nitrogen for avoiding explosive atmospheres. Readings were acquired using an automatic data acquisition means with a sampling rate of one second for the entire duration of the test. A basic schematic construction of the measuring means is shown in FIG. 3.

Startup and calibration of the measuring means was carried out with ethanol and water, ethanol being suitable for the pressure range up to 60 bar. The vapour pressure of water was measured at pressures up to 100 bar. The two substances were also chosen because the data for the substances are well known and may be consulted for validation of the apparatus. It was found that the deviation is below 1% of the absolute value and the method of measurement is therefore suitable for the further investigations. For the high-pressure range too the measuring means was sufficiently validated with the data for water.

The actual suitability as a working medium depends not only on the maximum obtainable efficiency but also to a substantial extent on the long-term stability of the substances when in use. Thermal decomposition of the substances can result in undesired byproducts which can lead, for example, to a reduction in the vapour pressure or to corrosion of the materials employed in the heat engine. In the first screening operation the working media were subjected to short-term stress and analyzed in terms of a plurality of criteria. Four substances were selected therefrom for further tests. The second test phase comprised carrying out extensive corrosion and material compatibility tests. The third test phase comprised carrying out long-term tests.

The working media were subsequently tested in a heat engine under realistic conditions.

Knowledge of the thermal stability of a substance is generally indispensable. An untested substance may suffer a loss of quality and give rise to unforeseeable hazards due to excessive temperatures during production, storage and transport. It is an important feature of the working media sought that no undesired decomposition products are generated during use which could endanger the operation of the plant.

Thermal stability was determined using the following principle of measurement:

The working media were charged into an autoclave at room temperature and inertized with nitrogen. The temperature of the medium was subsequently increased up to a maximum use temperature and sustained for a prolonged period. The vapour pressure of the substance was initially determined at room temperature and compared with literature values. This was followed by continuous determination of the vapour pressure as a function of temperature and long-term measurement at the maximum temperature. On completion of the test the working medium was cooled and analyzed by gas chromatography.

The gas chromatograph (GC) allows the composition of substance mixtures to be determined. This results in a chromatogram in which all substances are unambiguously assigned. The measurement is performed for an untreated laboratory-tested substance. This makes any decomposition products formed unambiguously determinable. The measurement makes it possible to determine not only the type of byproducts but also the percentage fraction thereof.

A further method for determining thermal stability is differential scanning calorimetry (DSC). This method was used to determine stability over a plurality of cycles.

DSC comprises heating two sealed crucibles (first crucible containing about 10 mg of sample and second empty crucible as reference) at a predetermined heating rate (10 Kelvin/minute in this case) up to a target temperature (up to 200° C. in this case). Both crucibles are subjected to the same temperature program. The energy absorption or decrease is analyzed during heating. The energy balance changes in comparison to the empty sample depending on the the heat capacity of the sample or exothermic and endothermic processes in the sample such as melt or vaporization. Once heating is complete the sample is held at a constant maximum temperature. For a thermally stable substance no energy changes occur during this time. Decomposition of the substance is observed via a change in the energy absorption or energy decrease.

FIG. 4 shows the employed temperature/time profile for the DSC. Over the time period from 0-20 minutes the temperature is increased as a constant heating rate and energy is correspondingly absorbed. In the range between 20-50 minutes the temperature is kept constant. For a stable medium no absorption or emission of energy occurs. Between 50-70 minutes the sample is cooled down again and the temperature is reduced with a corresponding energy decrease.

The reproducibility of the measurement was confirmed by carrying out a plurality of cycles per medium. This is because the decomposition products may also arise only after a prolonged operating time and a plurality of cycles.

Since the selected working media could exhibit corrosive behaviour towards the employed materials of the ORC engine, extensive investigations into corrosion behaviour were carried out. To this end, both metallic and nonmetallic (largely elastomeric materials of the seals) materials were defined and investigated in conjunction with the individual media.

For metallic materials samples having defined dimensions were prepared. To determine the erosion rates the metal strips were weighed and completely submerged in the respective fluid in an autoclave. The autoclaves were sealed, inertized and brought to a defined temperature and held at this temperature over a prolonged period. The metal strips were subsequently removed again, cleaned and weighed to determine the erosion rate. To determine any local corrosion the individual samples were examined by microscopy.

To investigate the corrosion behaviour of the working media towards nonmetallic materials the following tests were conducted:

Long-term thermal stability is crucial for trouble-free operation of an ORC system. However, working media may be decomposed by use at high temperatures. The stability of a novel working medium must therefore be established prior to its use. The relevant tests were carried out in a high-pressure autoclave with the objective of determining the maximum use temperature of each medium. The test temperature and test pressure were measured. Decomposition of the fluid also results in a change in vapour pressure. This change may in turn be observed by reference to the measured values. The decomposition products were analyzed in a gas chromatograph and compared with the starting product.

FIG. 5 depicts the investigation of the thermal stability of 1-propanol at 195° C. and 180° C. The measured vapour pressure (upper curve) increases with time at constant temperature (lower curve) at various temperatures between 195° C. and 180° C. This shows that 1-propanol is not stable at these use temperatures. Below 180° C. the vapour pressure becomes too low (less than 20 bar) to be usefully employable as the working medium in a heat engine. In the experimental setup of FIG. 6 methyl formate was stored at a temperature (upper curve) of about 150° C. The vapour pressure (lower curve) remains constant and the fluid may therefore be described as stable at this temperature.

All potential working media were investigated for use temperatures of from 150° C. to 200° C. in this fashion. Ethyl formate also exhibits similar behaviour to methyl formate (FIG. 7). At a use temperature of 175° C. this fluid undergoes slight decomposition over time. At a use temperature of 150° C. (upper curve in FIG. 7) it remains stable, i.e. the vapour pressure (lower curve in FIG. 7) does not increase.

The working media methyl formate, ethyl formate and cyclopentene are particularly advantageous on account of these investigations for example. The extended investigations tested the thermal stability of the preselected working media in a longer test of two months in duration.

The working media tested were stored in high-pressure autoclaves at an operating temperature of 150° C. After the test the decomposition rate of all samples was investigated by GC analysis to determine thermal stability. The results of this analysis are summarized in table 3. The maximum decomposition of the working media methyl formate, ethyl formate and cyclopentene was about 2% and is therefore in an industrially acceptable range.

TABLE 3
Degree of purity and decomposition of the working media
after subjection to thermal stress for 2 weeks.
	before test	after test	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%

The following investigations sought to test the extent to which the working media employed exhibit corrosive behaviour towards the typical materials employed in heat engines. The following materials were tested in the corrosion tests: unalloyed steel (P265GH) and alloyed steel (1.4571) including a weld seam. The materials were employed in the form of sheet-metal (90 mm×10 mm×6 mm). The test specimens were weighed in a materials engineering laboratory and characterized by optical microscopy. The test was then carried out in the abovedescribed apparatus for measuring vapour pressure. Once the samples were removed evaluation was once again performed in the materials engineering laboratory. The results of the first corrosion investigation are shown in table 4.

TABLE 4
Test results and evaluation of the corrosion tests
	working	erosion rate	microscopy
material	medium	[mm/a]	findings
unalloyed steel (P265GH)	methyl formate	0.0301	no findings
alloyed steel (1.4571)		0.0024	no findings
unalloyed steel (P265GH)	ethyl formate	0.0384	no findings
alloyed steel (1.4571)		0.0034	no findings
unalloyed steel (P265GH)	cyclopentene	0.0317	no findings
alloyed steel (1.4571)		0.0013	no findings

Although the optical microscopy evaluation found no local corrosion and cracks a crack test was additionally performed on material 1.4571 using the penetration method. This also found no cracks. The technical stability limit for metallic materials is given by an erosion rate of ≦0.1 mm/annum. There must moreover not be any instances of local corrosive attack since these preclude technical stability of the materials. The two material classes tested must accordingly be ranked as having technical stability towards the preferred working media methyl formate, ethyl formate and cyclopentene under the cited test conditions at 150° C., i.e. the three working media are fundamentally suitable.

Cyclic stability tests carried out with the described DSC method showed no deterioration/change in the working media (methyl formate, ethyl formate and cyclopentene). By way of example FIG. 8 shows the cyclic DSC curves of ethyl formate, with methyl formate and cyclopentene also showing similar curves. The upper curve once again shows the employed temperature profile for the DSC measurement. The lower set of curves represents the results of the DSC measurement. All three working media therefore show sufficient long-term storage.

Measurements of the efficiency of the cycle with the selected working media (methyl formate, ethyl formate and cyclopentene) and the abovementioned tests determined that fluids are particularly suitable 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, particularly 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, particularly preferably between 475 K and 510 K, and the fluids have a molar mass between 50 g/mol and 80 g/mol, preferably between 60 g/mol and 75 g/mol. Such fluids are also usable with a high degree of efficiency at low temperatures of the offgas to be utilized/at a low temperature of the evaporator. It has been found that to simplify the construction of the heat engine the use of a recuperator (heat exchanger) may be eschewed when a “wet” working medium is employed. The working medium is referred to as a “wet” working medium when the gaseous working medium undergoes partial condensation upon 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, that of ethyl formate is 508 K and that of cyclopentene is 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 molar mass of methyl formate is 60 g/mol, that of ethyl formate is 68 g/mol and that of cyclopentene is 74 g/mol. All three of these working media undergo partial condensation upon adiabatic expansion and it is therefore possible to eschew a recuperator in the circuit of the ORC.

For the simulation conditions the efficiency of the inventive working media in a heat engine at an offgas temperature (evaporator temperature) between 80° C. and 200° C. is superior to prior art working media for heat engines, for example ethanol. These results were confirmed by experiment for methyl formate in an ORC engine (piston expansion engine) from Devetec GmbH. The working media according to the invention thus achieve an improvement in the efficiency of the heat engine at temperatures between 80° C. and 200° C., in particular between 80° C. and 150° C.

The features of the invention disclosed in the above description and in the claims, figures and exemplary embodiments may be essential to the realization of the invention in its various embodiments either individually or in any desired combination.

Claims

1. A heat engine for performing an organic Rankine cycle (ORC), the heat engine comprising

an evaporator,

an engine,

a condenser and

a circuit comprising a fluid working medium, wherein

the working medium is methyl formate, and

the heat engine is operated with a heat source at a temperature ranging from 80° C. to 150° C.

2. The heat engine according to claim 1, wherein, during adiabatic expansion during the organic Rankine cycle (ORC), 1% to 30% of the mass of the working medium condenses out.

3. (canceled)

4. The heat engine according to claim 1, wherein the heat engine is an expansion machine.

5. The heat engine according to claim 1, wherein a pump is disposed between the condenser and the evaporator in the circuit of the heat engine, said pump allowing the fluid working medium to be conveyed from the condenser to the evaporator.

6. The heat engine according to claim 1, wherein the circuit of the heat engine does not comprise a recuperator.

7. The heat engine according to claim 1, wherein an erosion rate of the working medium unalloyed steel is less than 0.05 mm/a at 150° C. and/or an erosion rate of the working medium towards alloyed steel (1.4571) is less than 0.005 mm/a at 150° C.

8. The heat engine according to claim 1, wherein the working medium exhibits no endothermic or exothermic reactions or first or second order phase transitions in the temperature range between 70° C. and 200° C. when subjected to temperature changes over time.

9. A process, comprising operating a heat engine with methyl formate as a working medium, wherein the heat engine is operated with a heat source at a temperature ranging from 80° C. to 150° C.

10. The process according to claim 9, wherein, during adiabatic expansion during the organic Rankine cycle (ORC), 1% to 30% of the mass of the working medium is condensed out.

11-12. (canceled)

13. The process according to claim 9, wherein the heat engine is operated with an organic Rankine cycle (ORC).

14. The process according to claim 9, wherein an expansion machine is used as the heat engine.

15. (canceled)