DE19858712A1 - Process for converting thermal energy into mechanical work, e.g. in a turbine, involves a cyclic thermodynamic process using polysiloxane in the vapor phase as heat transfer medium - Google Patents

Abstract

A polysiloxane or polysiloxane mixture in the vapor phase is used as the heat transfer medium in a method for converting thermal energy into mechanical work by means of a cyclic process.

Description

State of the art

Thermal energy is converted into mechanical work in so-called circular processes. Here, a medium cycles through several physical states. Requirement is further the presence of a temperature difference between two sections of the Process, the "hot" and the "cold side".

Heat is added to the process on the "hot side" and heat on the "cold side" withdrawn. The pressure difference of a medium between "hot" and "cold side" becomes Converting thermal energy into mechanical work. This medium flows performing mechanical work from the "hot" to the "cold side". From here it is transported back to the hot side by mechanical work. The difference between the mechanical work generated and used is available for further use Available.

The use of two physical states of the has proven to be thermodynamically favorable Proven medium. On the hot side of the process, the medium is supplied by heat evaporated and condensed on the cold side by heat extraction.

The medium flows in the gaseous state from the "hot" to the "cold side" and is in the liquid state transported back to the cold side (e.g. Clausius-Rankine process).

The mechanical efficiency, i.e. H. the ratio of the difference of mechanical delivered Power and absorbed mechanical power to the absorbed thermal power, rises and a. with the temperature difference between "hot" and "cold side" of the process.

For historical reasons and because of the great chemical stability at high temperatures, water (H ₂ O) is the primary medium.

Only in processes with low temperatures on the "hot side" (<250 ° C) will organic media used [so-called ORC processes (organic cycles according to Rankin)].

Water has the following negative properties for cycle processes and their implementation:

• - critical point at ∼ 374 ° C
• - high vapor pressure e.g. B. 85 bar at 300 ° C
• - changes from saturated steam to wet steam with adiabatic relaxation
• - has a corrosive effect on metal parts

innovation

For the more economical design of thermodynamic cycle processes also in the temperature range <250 ° C are the most suitable media in a wide range of applications so-called polysiloxanes. The Si-O-Si bond is characteristic of this group of substances.

This group of substances can be divided into linear, cyclic, branched and cross-linked polysiloxanes become.  

Linear polysiloxanes correspond e.g. B. the type R3 Si O [R2 Si O] n Si R3 (linear polydimethylsiloxanes) where R = CH ₃ .

Cyclic polysiloxanes are constructed in a ring-shaped arrangement of siloxane units and have, for. B. the design [R2 Si O] n (cyclic polydimethylsiloxanes).

Some of these substances have favorable properties for use in a thermodynamic cycle:

• - high temperature resistance
• - low vapor pressures at high temperatures
• - low tendency to corrode
• - High environmental compatibility compared to other organic compounds
• - little impact on human health
• - As a rule, the transition from saturated steam to adiabatic expansion into dry steam

In the absence of air, these substances are usually stable up to over 300 ° C.

Especially the ring-shaped [Dn] with n <6 and linear polydimethylsiloxanes [MDnM] with n <5 are good for use in cycle processes because of their vapor pressure-temperature characteristic suitable.

While the CH ₃ -Si bond is very stable up to temperatures above 400 ° C, the compound Si-O usually begins to become unstable at temperatures above 300 ° C. There is a constant polymerization and depolymerization or the conversion of polymers into oligomers and vice versa, whereby a pressure-dependent equilibrium is established. When using cyclic polysiloxanes, an equilibrium of cyclic compounds [Dn] is formed, while when using linear polysiloxanes, a mixture of different chain lengths [MDnM] and cyclic polysiloxanes [Dn] is created by the end pieces [M] present. This process is used for heat transfer media based on linear polysiloxanes. However, the previous application of the heat transfer medium is exclusively in the liquid state for the transfer of heat and using predominantly linear chains [MDnM] with n <4, ie a relatively small number of [M] end pieces per volume unit.

In a thermodynamic circular process is usually on the "cold side" in Working temperature ranges from 20-100 ° C, causing the vapor pressure of the medium here is below the ambient pressure. In order to obtain the greatest possible pressure difference, constantly penetrating gases (air) and decomposition products of the medium on the "cold Side ". So far, the penetrating gases and the decomposition products removed together from the cycle. This can result in no chemical equilibrium adjust and the process is caused by loss of the carrier medium through decomposition above 300 ° C. quickly uneconomical. When the medium (decomposition product) and gases are separated e.g. B. by condensation of the decomposition product and reintroduction on the "hot Side "of the system can establish a balance. The increase in steam pressure plays  no significant role here, since it is due to the constant volume flow to the cold side compensates. This measure allows the temperature range and thus the efficiency a thermodynamic cycle based on organic media or polysiloxanes increase.  

Embodiment

In Fig. 1 a cycle with polysiloxanes is shown. The medium is preheated and then evaporated. This can be done indirectly via a heat transfer medium or directly in the boiler. The medium can also be overheated to achieve a high efficiency in the cyclic process. After relaxing to near the isobars of the medium at the condensation temperature on the cooling side of the process, a large part of the remaining heat is removed from the medium in the regenerator and used to preheat the liquid medium after the circulation pump. This part is crucial for the efficiency of the overall process. After the residual heat has been removed, the medium is condensed and returned to the evaporator by means of an increase in pressure by the circulating pump and the heating described in the regenerator via the preheater, and the cycle is thus closed. In Fig. 3 the process is shown in a temperature / entropy diagram.

The minimum components of the process are:
Evaporator
Expansion machine
capacitor

The following may optionally also be included ( FIG. 2):
regenerator
Preheater
Superheater

Preheaters, evaporators and superheaters can be integrated in a wide variety of combinations in one system section. A separation, as in FIG. 2, is preferably used for the functional illustration. The same applies to the regenerator and capacitor. All components of the system described can be assembled together as a portable unit.

Most organic media are not suitable for processes above temperatures of 250 ° C. she change their chemical and thus physical properties under the permanent influence of Heat. Thermal carrier fluids, which are used in industry for heat transfer, are an exception. Songs have such a low vapor pressure in the lower temperature range that the resulting large volume flows can no longer be managed after the turbine.

The group of polysiloxanes described is characterized by a high level thermal stability. Especially the circular compound octamethylcyclotetrasiloxane (D4) is from high stability or is a decay product of linear or cyclic siloxanes. As the main product at thermal conversion of (D4) results in (D5) and (D3). These substances form at higher concentrations under the influence of heat (D4). (D5) has lower vapor pressures than (D4) and remains thus, until a stable ratio is reached, even without special measures in the system. The loss of (D3) by suction in the vacuum area must be coped with or can be handled by a separate one Condensation and recycling of the decay product on the hot process side can be reduced. Same thing applies to the resulting decay products when using linear fluids. When using M (D0) M (hexamethyldisiloxane) or M (D1) M (octamethyltrisiloxane) has most of the Decomposition products also have a lower vapor pressure than the working fluid, so that the Decomposition products remain in the process despite suction.

The physical properties of (D4) form a good compromise of all requirements for a high one Effectiveness and cost-effective construction of an ORC system for high temperatures.

On the condensation side, depending on the desired cooling temperature z. B. at 100 ° C in a force Heat coupling a pressure of approximately 8.7 kPa can be achieved in the condenser. The condensation pressure plus of the pressure drop caused by the flow resistance of the regenerator forms the outlet pressure Turbine.  

The evaporation pressure of the fluid on the high temperature side minus the losses forms the Inlet pressure. At 300 ° C this would only be approx. 1100 kPa.

This would give the turbine a pressure ratio of approx. 120/1.

The critical point of this fluid is between 300 ° C and 350 °. Should the medium be heated further on the hot process side ( Fig. 4), it can e.g. B. in indirectly heated evaporators may be useful to select the steam pressure near the critical point as the process pressure of the hot side.

If the condensation temperature on the cooling side is reduced, the pressure there also increases less.

When using linear siloxanes, preferably M (D1) M or M (D2) M, the following applies when choosing Process parameters a similar approach.

With M (D1) M the pressure in the condenser at 1000 ° C would be approx. 20 kPa and in the evaporator at 260 ° C approx. 885 kPa. So the pressure ratio at the turbine would be about 40 / l. With pure electricity generation can be due the low vapor pressure at the condensation temperature considerably increase this ratio.

For all proposed fluids, the process pressure on the hot side of the turbine should be close to the Vaporization pressure of the fluids in the evaporator and on the cold side near the vapor pressure in the condenser his. When the steam overheats, a pressure near or below the steam pressure is at the critical Temperature preferred.

The high thermal stability of the fluids also enables direct heating in fuel boilers Print. However, a constant forced circulation of the liquid is necessary to get the maximum permissible Film temperature not to be exceeded. The heating must be dimensioned accordingly. The evaporation takes place preferably by relaxing in a so-called flash tank. This procedure is in use of heat transfer media (e.g. diphyl) common in the vapor phase.

When selecting the proposed cyclic siloxane (D4), it should be noted that the pour point is at approx. 15 ° C lies. This could z. B. lead to problems. In the thermal decomposition of siloxanes arises and a. also (D3), which has a pour point of 65 ° C. Therefore, the additional heating can certain parts of the plant z. B. collection vessel and safety devices may be useful. Also useful If (D4) is used, an addition of (D5) appears until an equilibrium is reached arises in the system anyway.

Claims (5)

1. A method for converting thermal energy into mechanical work by means of a cyclic process and a heat transfer medium, characterized in that the heat transfer medium is a polysiloxane or a mixture of polysiloxanes in the vapor phase. 2. The method according to claim 1, characterized, that the cycle as a thermodynamic cycle is executed. 3. The method according to claim 1 or 2, characterized, that the heat carrier in the vapor phase cyclic polysiloxane or a cyclic polysiloxane Mixture of substances. 4. The method according to claim 3, characterized, that polysiloxanes linear [MDnM] with predominantly n <5 and cyclic [Dn] with predominantly n <6 in pure form or as Mixtures of substances can be used. 5. The method according to any one of the preceding claims, characterized,  that there is a separation of foreign gases in the process and a return and a stay in the thermodynamic Cyclic decomposition products with the aim of achieving chemical equilibrium is made.