Heat engine with adiabatic cycle without internal regeneration

Abstract

Multi-cylinder hot gas system for carrying out a closed heat engine cycle with spatially separated double-acting expansion and compression cylinders, wherein one piston side serves as the active working volume for the cycle and the opposite piston side is used for pushing out or drawing in gas, wherein the expansion and compression cylinders are largely closed during the respective stroke, characterized in that (a) the expansion in the expansion cylinder and the compression in the compression cylinder each take place adiabatically, without isothermal process sections and without internal regeneration, and (b) the heat input takes place exclusively in the high-pressure line upstream of the expansion cylinders.

Description

Field of invention

[0001] The invention relates to heat engines, in particular closed hot gas engines, which are operated in a thermodynamic cycle. It relates to engines in which the expansion and compression of the working gas take place in separate cylinders. The invention is therefore situated in the technical field of closed hot gas engines with external heat input. State of the art:

[0002] Heat engines operating in closed cycles have been known for a long time. Classic examples include Stirling and Ericsson engines, which operate with largely isothermal or isobaric process steps and often employ regenerators for internal heat storage. These engines guide the working gas over heat exchanger surfaces during expansion and compression, resulting in only minor temperature dynamic changes.

[0003] In addition, hot gas engines are known that are based on open or semi-closed cycles, such as variants of the Brayton or Joule cycles. These systems allow for external heat input, but due to their continuous flow, they do not generate pronounced adiabatic temperature changes in the working gas.

[0004] Closed adiabatic expansion machines, such as those used in compressed air or thermal storage systems, are also known. They utilize the significant temperature changes during expansion and compression, but lack a structured method for external heat input within a closed cycle.

[0005] In the prior art, both isothermal hot gas engines with regenerators and adiabatic expansion engines exist. However, a combination of adiabatic compression and expansion processes with external heat input in a closed cycle is not known. Object of the invention:

[0006] In heat engines with isothermal or nearly isothermal state changes—as occur particularly in Stirling-like processes—there is a significant temperature difference in the working gas between the end of compression and the end of expansion. This temperature difference enables the use of regenerators, which store heat and feed it back into the compressed gas.

[0007] The situation is fundamentally different in adiabatic or largely adiabatic processes. Due to the significant temperature changes during expansion and compression, only a small temperature difference remains between the expanded and compressed gas. Internal heat recovery in the sense of a conventional regenerator is therefore of limited use.

[0008] The task therefore arises to provide a heat engine that: • utilizes adiabatic process steps with high pressure and temperature changes, • can be operated efficiently despite small internal temperature differences, • does without a classical regenerator, • can integrate external heat sources with different temperature ranges, and • Optionally, external or system-side heat recovery adapted to the adiabatic temperature profiles is possible

[0009] The aim is to create a machine that utilizes the advantages of adiabatic temperature dynamics without relying on internal regeneration. Structure of the invention

[0010] The invention relates to a heat engine system that implements a closed, continuous cycle process.

[0011] The main element of the system is a reciprocating piston engine with several spatially separated, double-acting expansion and compression cylinders.

[0012] In each of these cylinders, one side of the piston serves as the active working volume for the cycle, while the opposite side is used for exhausting or drawing in gas. The exhaust ports of the expansion cylinders are connected to the inlets of the compression cylinders via a common low-pressure line. Cooling devices can be arranged along the entire length of this low-pressure line to further temper the working gas after it has cooled down following expansion.

[0013] Due to the phase-shifted control of several cylinders, the individual partial flows superimpose to form an essentially continuous flow in the low-pressure line.

[0014] The outlet of the compression cylinders is connected to the inlets of the expansion cylinders via a common high-pressure line. Heat transfer devices are integrated into this high-pressure line to preheat the compressed working gas before it enters the expansion chambers.

[0015] Here too, the phase-shifted generation of the partial flows leads to an almost continuous flow in the high-pressure line.

[0016] A similar basic structure is known from German utility model 20 2022 001806, but without the process flow characteristic of the present invention with adiabatic cylinder strokes and exclusively external heat transfer.

[0017] In an advantageous embodiment, the expansion and compression ratios can be different. The compression ratio can be selected such that, after the external heat input, the pressure reached in the high-pressure line is the same as that required for entry into the expansion cylinders. This allows the cycle to be closed on the pressure side without requiring the expansion and compression strokes to be identical.

[0018] Optionally, the machine can include a heat recovery unit in the low-pressure line, which is not designed as a conventional regenerator but is adapted to the adiabatic temperature profiles. This unit can serve to transfer residual heat from the expansion section to an external, lower-temperature system component located upstream of the heating unit in the high-pressure line. Functioning of the invention:

[0019] The heat engine operates in a closed, continuous cycle in which the working gas is cyclically compressed and expanded.

[0020] The expansion cylinders are mechanically coupled to the compression cylinders, so that the expansion strokes drive the compression strokes. This ensures that the gas discharge from the expansion cylinders into the low-pressure line is synchronized with the gas withdrawal by the compression cylinders. As a result, the gas mass in the low-pressure line remains essentially constant, and the superimposed partial flows form a nearly continuous flow.

[0021] The same applies to the high-pressure line, in which the compressed gas flows of the compression cylinders are provided synchronously with the intake phases of the expansion cylinders.

[0022] The changes of state in the cylinders occur largely adiabatically. This results in the temperature profiles characteristic of the invention, which differ significantly from those of isothermal hot gas engines. The changes of state that do not take place in the cylinders are isochoric, since the working gas is cooled or heated in the lines at a substantially constant volume. These isochoric steps lie entirely outside the cylinders and are realized by the heat exchangers arranged in the high-pressure and low-pressure lines.

[0023] After expansion, the working gas flows via the low-pressure line to the compression side, and after compression, via the high-pressure line to the expansion side. With multiple expansion and compression chambers operating out of phase, a continuous flow through these lines is achieved, resulting in steady-state temperature conditions throughout the entire circuit.

[0024] A key difference compared to isothermal hot gas engines is that heat transfer occurs exclusively outside the cylinders.

[0025] During compression, the working gas is compressed without significant heat exchange, thereby increasing its temperature and pressure. In the subsequent high-pressure line, heat from an external source is supplied to the compressed working gas before it enters the expansion chambers. The amount of heat supplied can be metered. Suitable heat sources include renewable energy storage, industrial waste heat, or other thermal sources. Since the gas is already preheated by compression, a moderate additional heat input is sufficient to reach the temperature level required for expansion.

[0026] The heated working gas then enters the expansion chambers and performs mechanical work there. After expansion, the cooled gas is returned via the low-pressure line and further cooled by the cooling devices located there to improve the efficiency of the subsequent compression.

[0027] The largely adiabatic process results in a comparatively low gas temperature after expansion, allowing an absorption chiller to operate without upstream cooling measures. With isothermal hot gas engines, the gas temperature would be significantly higher, necessitating additional cooling stages.

[0028] The absorption chiller, located in the low-pressure line, stabilizes the temperature level of the recirculated working gas at a substantially constant value. This means that the state of the working gas before compression acts as a thermodynamic fixed point for the cycle. Changes in the operating parameters in the state before expansion (state 1 in Fig. [2]) are therefore not fully propagated to the subsequent states, allowing the compression rate and pressure ratio, in particular, to be freely selected within a wide range without affecting the stability of the cycle.

[0029] The cycle according to the invention is a closed thermal process with adiabatic compression and expansion as well as isochoric heat transfer phases. Heat transfer takes place exclusively via external heat exchangers in the pipes, while the working gas remains completely in the cycle. The process thus differs fundamentally from open combustion processes such as the Otto cycle and from Stirling cycles with isothermal cylinder phases.

[0030] In an advantageous embodiment, a combustion section in the high-pressure line provides the temperature difference between the working gas, already preheated by the compression cylinders, and the inlet temperature required for expansion. Simultaneously, the residual heat remaining in the working gas after expansion can be used to preheat the combustion air, further reducing the fuel requirement of the combustion section. The residual heat remaining after preheating can also be used to supply the absorption chiller, so that the working gas enters the compression cylinders at near freezing point, thus reducing the required compression work accordingly.

[0031] Alternatively, the high-pressure line can also be heated by external heat sources, such as industrial waste heat. The expansion parameters, in particular the compression ratio and outlet temperatures, can be configured so that the absorption chiller can operate within a suitable temperature range without an external energy supply, ensuring that the inlet temperature to the compression cylinders remains close to freezing. Advantages of the invention:

[0032] The invention offers several technical advantages over known heat engines: • High specific work per cycle due to the adiabatic process in the cylinders. High power output is achieved even at comparatively low compression ratios. • Large temperature dynamic changes: As a result of adiabatic pressure changes, the need for external heat transfer is significantly reduced. Since the heat is introduced outside the cylinders and not during the stroke, internal heat flows are minimized and inefficient heat shifts within the system are reduced. • Flexible integration of external heat sources, as heat transfer in the high-pressure line occurs with uniform flow. • Low thermal stress on the cylinders, as the heat is supplied externally. This leads to reduced material stress, lower cooling requirements, and lower maintenance requirements compared to combustion processes such as Otto or Diesel cycles. • High efficiency even at low and medium temperature levels of the external heat source, as the compression already brings the working gas to an increased temperature and pressure level. • After expansion, the working gas is at a comparatively low temperature level, which means that the cooling requirement before compression is low. • This comparatively low temperature level, in conjunction with the continuous flow, enables the use of absorption chillers, which can be operated without external energy input in a drive temperature range typical for the solvents used, so that even with moderate heat sources a large usable temperature gradient (ΔT) is achieved in the overall system. • By stabilizing the temperature level near the freezing point, the state of the working gas before compression acts as a thermodynamic fixed point, so that the compression rate and pressure ratio can be freely chosen within a wide range without affecting the stability of the cycle. • The low gas temperature at the inlet to the compression cylinders results in a low specific compression effort, thereby reducing the compression work required for the cycle process and increasing the overall efficiency of the system. • Modular and structurally simple design that allows for flexible scaling of performance. • High partial load capability, as the power output can be reduced without significant loss of efficiency. Explanations for the illustrations: Fig. Figure 1 shows a schematic representation of a heat engine with several pairs of cylinders, consisting of expansion cylinders [1] and compression cylinders [4], which are coupled to a generator [3] via a common crankshaft [2] as an example. Instead of the crankshaft [2] shown, another mechanical coupling element can also be used, for example, a hypocycloidal drive. The cylinders are designed as double-acting units and are shown in different piston positions. Each working chamber has an inlet and an outlet, each connected to a low-pressure line [5] and a high-pressure line [6], respectively. The individual lines of the cylinders are combined to form a common low-pressure line and a common high-pressure line, which connect the cold and hot sides of the system. An external heat source [7] is coupled to the high-pressure line, which heats the working gas before it enters the expansion cylinders. A cooling device [8] is located in the low-pressure line, which cools the working gas before it enters the compression cylinders [4]. Fig. Figure 2 shows the thermodynamic cycle of the heat engine according to the invention in the pV diagram with states 1 to 4. Starting from state 1, in step 1 → 2 an adiabatic expansion of the working gas takes place in the expansion cylinder, which is characterized by a comparatively steep pressure drop. In this step, the machine performs the useful work

[10] . In the subsequent step 2 → 3, heat

[11] is extracted from the working gas at essentially constant volume in the low-pressure line, thereby cooling it. The transition 3 → 4 represents the adiabatic compression of the working gas in the compression cylinder, which is characterized by a comparatively steep pressure increase. For this, work

[12] is supplied to the working gas. The temperature of the working gas in state 4 can, depending on the operating parameters, be in the range of the temperature of state 2, so that internal regeneration between steps 2 → 3 and 4 → 1 is only of limited use. In step 4 → 1, heat is finally supplied

[13] at essentially constant volume in the high pressure line upstream of the expansion cylinders, thereby restoring the initial state 1.

[0033] Fig. Figure 3 shows an embodiment of the heat engine according to Fig. 1, in which the heat input in the high-pressure line [6] is carried out by a counterflow tube bundle heat exchanger [7]. The in Fig. The heating element shown in Figure 1 is replaced here by a schematic tube bundle structure in which the counterflow between the working gas guided in the high-pressure line and an inflowing hot recirculated air

[17] is shown. The in Fig. 1 The cooling device shown in the low-pressure line [5] is designed in this embodiment as an absorption chiller [8].

[0034] Fig. Figure 4 shows a further embodiment in which the heat input into the high-pressure line [6] is effected by a burner [9]. The fuel gas

[14] is preheated before entering the burner via an upstream heat exchanger

[16] using the residual heat of the expanded gas; the heat exchange is exemplified by a counterflow shell-and-tube heat exchanger. The exhaust gases

[15] then leave the combustion section in counterflow to the compressed working gas. The cooling device or absorption chiller [8] shown in the preceding figures in the low-pressure line [5] remains unchanged and is in Fig. 4 not shown in detail Reference symbol list 1 working cylinder (hot cylinder) 2. Device for mechanical coupling. Here: crankshaft 3 Generator 4 compression cylinders (cold cylinders) 5 Low-pressure line 6 High-pressure line 7 heaters / heat exchangers in high-pressure line 8 Cooling device / absorption chiller 9 burners 10 jobs gained through expansion 11 Waste heat from expansion 12 work supplied for compression 13 Heat supplied to high-pressure line 14 Combustion air (inlet) 15 Exhaust gas 16 heat exchangers for combustion air preheating 17 Heat source (hot circulating air)

Claims

[1] Multi-cylinder hot gas system for carrying out a closed heat engine cycle with spatially separated double-acting expansion and compression cylinders, wherein one piston side serves as the active working volume for the cycle and the opposite piston side is used for pushing out or drawing in gas, wherein the expansion and compression cylinders are largely closed during the respective stroke work, characterized by , that (a) the expansion in the expansion cylinder and the compression in the compression cylinder each take place adiabatically, without isothermal process sections and without internal regeneration, and (b) the heat input takes place exclusively in the high-pressure line upstream of the expansion cylinders. [2] Plant according to any one of the preceding claims, characterized by, that the low-pressure line and the high-pressure line hydraulically connect the phase-shifted flows generated by several cylinders, so that an essentially continuous flow is created in both lines. [3] Plant according to any one of the preceding claims, characterized by , that an optional heat recovery device is arranged in the low-pressure line, which uses the working gas cooled by the expansion to transfer heat to an external medium. [4] Plant according to any one of the preceding claims, characterized by , that an absorption chiller is arranged in the low-pressure line, which is operated without external energy supply and further cools the recirculated working gas.

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