WO2020141162A1

WO2020141162A1 - Non-corrosive working medium for thermodynamic cyclic processes, method for recovering exhaust heat, system for carrying out the method, and vehicle

Info

Publication number: WO2020141162A1
Application number: PCT/EP2019/087168
Authority: WO
Inventors: Thomas Maischik; Asmus Carstensen; Artur Semke; André HORN; Thomas Schulenburg; Carolin Lambrecht; Andreas Herr
Original assignee: Volkswagen Ag
Priority date: 2019-01-02
Filing date: 2019-12-30
Publication date: 2020-07-09
Also published as: DE102019100029A1

Abstract

According to the invention, in order to be able to use aluminium or aluminium alloys for components in a thermodynamic cyclic process which has ethanol as the working medium, without the risk of alcoholate corrosion, the working medium consists of ethanol, water and a hygroscopic oil. The invention also relates to a method for recovering exhaust heat, a system for recovering exhaust heat, and a vehicle.

Description

description

Non-corrosive working medium for thermodynamic cycle processes, waste heat recovery process, system for carrying out the process and vehicle

The invention relates to a working medium for thermodynamic cycles

Use of components made of aluminum or aluminum alloys, a method for recovering waste heat with the working medium, a system for carrying out the method and a vehicle.

Waste heat recovery (WHR) systems are used in

different areas. Internal combustion engines represent a preferred area for such measures. These days, they have an efficiency of 30% to 40%. Accordingly, a large proportion of the fuels converted is lost as waste heat if it is not recovered. To recover waste heat, the internal combustion engine can be followed by a thermodynamic cycle in which the exhaust gas and / or a coolant is used as a heat source. This cycle process is usually an ORC (Organic Rankine Cycle), in which a liquid, organic working medium absorbs heat, which is largely evaporated and then expanded, whereby mechanical energy is made available.

Ethanol has established itself as a working medium in a WHR system with an ORC circuit due to its advantageous thermodynamic properties, such as low boiling and freezing points and high achievable efficiencies. With the choice of ethanol as the working medium, however, the influence on aluminum-containing components of the WHR system is disadvantageous, since alcoholate corrosion often occurs with this combination. The consequences are serious

Damage to components of the system for heat recovery until the components are completely destroyed.

Therefore, components made of stainless steel are used almost exclusively as components for the cyclic process, although aluminum would be the more suitable material for such components due to the better heat transfer properties and the lower density. Thermodynamic cycle processes are often described. DE 10 2010 049 916 A1 describes a method and a device for utilizing waste heat from an exhaust gas stream of an internal combustion engine, the internal combustion engine being a

Cycle is connected downstream. Alcohol mixtures or water-alcohol mixtures are used as the working medium, but without specifying the materials of the components for the cycle. DE 10 2017 010 531 A1 discloses a WHR system for a vehicle with an internal combustion engine and a downstream WHR circuit, ethanol being used as the working medium. DE 10 2016 100 916 A1 relates to a vehicle with a thermodynamic cycle that uses a working fluid. DE 10 2014 017 903 A1 describes an arrangement for operating a waste heat utilization device of a vehicle by means of a Rankine cycle. Water, ethanol, a water-ethanol mixture, a silicone oil or a refrigerant are disclosed as the working medium. DE 10 2012 002 768 A1 discloses a heat exchanger module for fluid coupling with at least one cooling circuit. Water, ethanol, carbon dioxide or mixtures of the aforementioned substances are used as the working medium of a Rankine cycle connected with this. US 2012/0037148 A1 describes an element for thermal energy storage with a liquid thermal energy storage material. The element can consist of brass, copper, aluminum, a nickel-iron alloy, bronze, titanium, stainless steel. The storage material is water-free or has a maximum of 1% water. This element can be integrated in a Rankine cycle in which the working medium is water, one or more

Contain alkylene glycols, one or more polyalkylene glycols, one or more oils, one or more refrigerants, one or more alcohols, one or more betaines or a combination thereof.

It is an object of the invention to provide a working medium for a thermodynamic cycle, which enables an increased thermal output compared to the prior art with low costs, good thermodynamic properties and frost resistance.

This task is accomplished by a working medium, a method for waste heat recovery using a thermodynamic cycle and a system for

Waste heat recovery with the features of the independent claims solved.

The explanations below apply equally, even if this is not explicitly described, to the working medium, the method and the system. According to the invention, a working medium which is non-corrosive to components made of aluminum or aluminum alloys and which consists of ethanol and water, preferably distilled water, is provided.

This working medium is suitable for a system for waste heat recovery

Components made of aluminum or aluminum alloys to prevent alcoholate corrosion, especially at a maximum system temperature of 150 ° C - 250 ° C.

According to a preferred embodiment of the invention, the working medium additionally has a hygroscopic oil, preferably a synthetic oil, particularly preferably a PAG oil.

The working medium forms a single-phase mixture with the oil. PAG-ÖI is characterized by a content of polyalkylene glycols (PAG), through which the hygroscopic property of the oil is given. The oil is used to lubricate the components of the system

Heat recovery.

As a result of the composition of the working medium according to the invention, ethanol no longer has a corrosive effect on aluminum and aluminum alloys under the conditions mentioned, which are given for thermodynamic cycle processes in the automotive sector, due to the constituents water and possibly oil added for corrosion protection purposes.

Because of its non-corrosive properties, the working medium according to the invention thus enables the use of components made of aluminum or aluminum alloys in thermodynamic cycle processes, preferably in the Organic Rankine Cycle (ORC). As will be explained further below, the working medium according to the invention is not only non-corrosive, but advantageously also offers repassivation of the aluminum-containing components in the event of damage to the oxide layer of the aluminum-containing material.

Aluminum and aluminum alloys form under the atmospheric influence

Metal surface from a protective oxide layer, for the formation of which the presence of oxygen or water is essential. Already on contact with minor

Oxygen concentrations, for example in the air, form a thin layer

Alumina on the surface of the aluminum. The resulting oxide layer passivates the material and makes it resistant to a certain degree

Corrosion. In the working medium according to the invention, the water represents the oxygen carrier, which leads to a continuous passivation and formation of the oxide layer of the aluminum components used in the system by means of an oxide reaction.

This is essential to the invention, since the system is a closed system in which the working medium and the aluminum-containing components do not come into contact with oxygen and therefore cannot undergo an oxidation reaction with oxygen.

Even if the components are anodized, for example, before assembly, it is

The likelihood that the oxide layer formed will be damaged during assembly and thus offers a contact surface for corrosion of alcoholate is very high, so that the working medium according to the invention can also be used to advantage here.

Local breakdown of the oxide layer when the components are installed can also have other causes. With increasing temperature, the thermal ones are correct

Expansion coefficients of aluminum and the passive layer made of aluminum oxide

and hydroxide no longer match and microcracks develop. Thus, electrochemical stability of the exposed aluminum alloy is no longer guaranteed at this point and corrosion can start.

Thus, by using the working medium according to the invention, it can be ensured that, on the one hand, the aluminum-containing material of the components is not attacked and, on the other hand, if the oxide layer is damaged, it is "repaired".

The working medium preferably has 4% to 10%, preferably 6%, by mass of ethanol, based on the water and ethanol ratio. In this area, on the one hand, corrosion can be reliably avoided and, on the other hand, the advantageous properties of ethanol can be used in the context of a working medium.

Also preferred is a mass fraction of 5% - 10% oil based on the working medium to additionally improve the lubricating properties of the working medium while optimizing the anti-corrosive properties.

A method for waste heat recovery using a thermodynamic cycle, wherein at least a part of the components to form the cycle There is aluminum or aluminum alloys and the above-described working medium is used, is provided according to the invention.

According to a particularly preferred embodiment of the method according to the invention, the content of water in the working medium is dependent on one to be expected

Component temperature of a system for waste heat recovery set.

The water content is preferably 4% - 6% at a maximum component temperature to be expected, which is lower than the maximum system temperature mentioned, from 140 ° C. to 160 ° C., and 6% at a maximum component temperature to be expected. - 8% and at a maximum expected component temperature of 180 ° C - 200 ° C 8% - 10%. The component is preferably a capacitor which, in the system according to the invention, preferably consists of aluminum or an aluminum alloy.

The invention also relates to a system for waste heat recovery which makes use of a thermodynamic cycle and at least part of that for the

Circulation process necessary components made of aluminum or aluminum alloys, the working medium according to the invention described above being used as the working medium.

Aluminum is not usually used as a pure metal, but to achieve the desired material properties, alloy elements are added to the metal, which in turn influence the corrosion resistance of the material.

Common main alloying elements for aluminum alloys are Mg, Si, Mn, Zn, Cu, Fe and Cr.

In the context of the invention, Mg, Si, Mn, Zn, Fe and Cr with the exception of copper are used as alloying elements, preferably Mg, Mn, and Cr are used, since these have a positive influence on the corrosion resistance. Zinc has little influence, silicon has a small negative influence, iron has a negative influence and copper has a very negative influence.

The materials for the components of the device are therefore preferably free of copper components. According to a preferred embodiment of the system, the components made of aluminum or an aluminum alloy are provided with an oxide layer, which additionally improves the corrosion behavior, since only injuries to the oxide layer are to be protected by the working medium according to the invention.

The system has typical components which are known to the person skilled in the art and are necessary for the formation of the cycle. These are at least one fluid pump, at least one evaporator, one expansion machine and one condenser. Furthermore, various other components such as an electric generator, an expansion tank, a tank as well

Pipes, sensors, valves and the like can be provided.

Depending on the water content of the working medium and the selected system parameters (operating pressure, operating temperature), the components carrying the working medium can consist entirely or partially of aluminum or aluminum alloys.

A vehicle is also claimed, having a system for recovering waste heat. For example, the system can convert some of the vehicle's unused exhaust gas energy into mechanically or electrically usable work.

The objects of the invention can in all system layouts of the

Waste heat recovery system including additional components, other interconnections of the components and areas of application (cars, commercial vehicles) can be used.

The new working medium is not corrosive to aluminum and therefore enables the use of aluminum components that come into contact with the working medium.

This affects both the lower cost of the component, the improved one

Thermal conductivity properties and the lower weight compared to components made of stainless steel.

Due to the low density of aluminum of 2.6 kg dm ³ to 2.8 kg dm ³ , which is about a third lower than the density of steel, there are potential energy savings compared to the prior art due to the reduction in mass. Furthermore, due to the good thermal conductivity of 80 W / (m K) to 230 W / (m K), aluminum is more suitable as a material for applications in heat exchangers than steel. The transferred heat output can thus be significantly increased. At the same time, the advantages mentioned are also realized through the use of ethanol mixtures. Further preferred embodiments of the invention result from the other features mentioned in the subclaims.

Unless otherwise stated in the individual case, the various embodiments of the invention mentioned in this application can advantageously be combined with one another.

The invention is explained in more detail below on the basis of experiments. The

Test results are shown in the associated diagrams in FIG. 1 and FIG. 2. a) Corrosion protection by adding water

The influence of temperature and water content on the alcoholate corrosion of aluminum was investigated by means of corrosion tests in a closed container (autoclave). For this purpose, aluminum samples were mixed with ethanol and water in a working medium

different water content in a tube furnace at different temperatures.

The results are shown in Figure 1. The temperature is plotted against the water content in the diagram. A distinction is made between the events "corrosion", marked with "X", and "no corrosion", marked with a "circle".

The respective test temperatures relate to the temperatures at the start of the corrosion.

The area in which initiation of corrosion cannot be reliably ruled out is illustrated in FIG. 1 by a hatched area. No corrosion occurred below this area in the test series. b) Definition of temperature threshold values for the use of

Aluminum components in an ethanol powered waste heat recovery system

Adding water has a positive effect on the resistance of an aluminum material in ethanol, since the water contributes to the repassivation of the surface and thus the

Can prevent or delay corrosion. In order to assess the influence of water, laboratory tests carried out in research are used Temperature thresholds defined for various ethanol-water mixtures. This

Threshold values serve to differentiate between the events "corrosion" and "no corrosion".

This is done on the assumption that temperature is the main factor influencing the occurrence of the corrosion reaction. The investigations show a necessary temperature increase of the heat source for the initiation of alcoholate corrosion with an increasing

Water content in the ethanol mixture. Areas can occur in which the

Corrosion initiation is uncertain (hatched area, Figure 1).

For the unambiguous determination of temperature threshold values, therefore, for each

Test series the limit is set to the maximum temperature at which none

Alcoholate corrosion occurs.

The temperature ranges of the initiation of corrosion and the determination of the

Temperature threshold values according to FIG. 2 are listed in Table 1.

c) Overview of the corrosion tests - definition of the temperature threshold values and definition of a safe range of corrosion resistance

In FIG. 2, test temperatures are plotted against the water content of the ethanol-water mixture between w _H 2o = 0-0.1. The temperatures in the event of “corrosion” relate to the initiation of alcoholate corrosion. If the samples are stable (“no corrosion”), the information relates to the temperatures in the autoclave that are constant over the test period. The determined temperature threshold values are shown in the graphic by a

Regression line with a line equation of

y = 11.9xx100 + 120.1 described.

For the test conditions, alcoholate _corrosion in pure ethanol only occurs from a temperature of Tv _request = _120.1 ° C, with the temperature threshold around 11.9 Kje

Mass percent of the water (mass fraction · 100) increases. However, the temperature for pure ethanol is below component temperatures which are provided according to the invention and which enable an economical process. d) Determination of the ethanol-water ratio when using aluminum components in waste heat recovery systems

According to the invention, depending on the maximum temperature to be expected of the components of the waste heat recovery system, the water content to be preferably set is determined on the basis of the temperature threshold values determined.

Maximum expected component temperature 140 ° C - 160 ° C: water content 4% - 6%,

maximum expected component temperature 160 ° C - 180 ° C: water content 6% - 8% and maximum expected component temperature 180 ° C - 200 ° C: water content 8% - 10%.

Claims

1. Working medium for thermodynamic cycle processes using components made of aluminum or aluminum alloys, characterized in that the

Working medium consists of ethanol and water.

2. Working medium according to claim 1, characterized in that the working medium has a hygroscopic oil.

3. Working medium according to claim 1 or 2, characterized in that the

Working medium has a water content of 4% to 10% based on the mass ratio of water to ethanol.

4. Working medium according to claim 2 or 3, characterized in that the working fluid has an oil content of 5% - 10% based on the ratio of oil to the total mass of the working medium.

5. A method for waste heat recovery using a thermodynamic cycle with a working medium according to one of claims 1 to 4.

6. The method according to claim 5, characterized in that the water content in

Depending on a maximum expected component temperature of components of the circuit, the water content is preferably 4% - 6% at a maximum expected component temperature of 140 ° C - 160 ° C, at a maximum expected component temperature of 160 ° C - 180 ° C 6% - 8% and at a maximum expected component temperature of 180 ° C - 200 ° C 8% - 10%.

7. System for waste heat recovery using a thermodynamic cycle having a working medium according to one of claims 1 to 4, wherein at least some of the components of the system made of aluminum or

Aluminum alloy is made.

8. System according to claim 7, characterized in that the components are free of copper components.

9. System according to claim 7 or 8, characterized in that the components

Have oxide layer.

10. Vehicle having a system for waste heat recovery according to one of claims 7 to 9.