Heat engine plant with modular high-temperature solid-state heat storage as counterflow regenerator

Abstract

A heat engine system for converting thermal energy into mechanical energy by means of a closed cycle process with a working gas, essentially consisting of a heat engine, solid heat storage units, cooling and heating devices, piping and a heat source, characterized in that a modularly constructed high-temperature solid heat storage unit is designed as a regenerator according to the counterflow heat exchange principle and is an integral part of the working gas cycle of the heat engine.

Description

Field of invention

[0001] The invention relates to heat engines, in particular hot gas engines, that operate in a closed cycle. Hot working gas is directed to the expansion side of the engine, cools down there, and flows back for reheating – a typical principle in Stirling engines. To increase efficiency, the heat from the returning gas can be used to preheat the fresh gas, provided that temperature differences and process configuration permit this.

[0002] In the context of the energy transition, such machines serve to reconvert stored renewable energy or industrial waste heat into electricity. The heat is absorbed in thermal storage units and converted into electricity as needed. However, existing systems are inefficient – ​​limited by their configuration, where heat extracted can only be converted into electricity up to a minimum temperature. Therefore, thermal storage units have mostly been used only for process heat recovery.

[0003] The present invention combines a heat storage system and a heat engine in an integrated unit. The storage system simultaneously functions as a counterflow heat exchanger, thereby storing residual heat from the Stirling process. This "power-to-heat-to-power" tandem enables the storage, smoothing, and reconversion of excess energy from volatile sources as well as industrial waste heat – and can replace conventional storage solutions such as large batteries or backup power plants. State of the art:

[0004] The thermal storage systems under consideration are high-temperature solid-state heat storage devices that store heat either as sensible energy through temperature increase or as latent energy through phase change (solid to liquid). Materials such as concrete, steel, metal granules, or mineral aggregates are used for sensible heat storage. These enable storage temperatures from 600°C (concrete) to over 1000°C (special granules, sand). None of the known solutions are integrated into a heat engine cycle without further heat extraction to a secondary medium.

[0005] Latent storage materials such as salts or metals - e.g. aluminium with a melting point of 660°C - are disregarded, as the invention is based on sensible heat storage in the high-temperature range.

[0006] Conventional storage systems are charged with hot fluid and discharged by reversing the flow direction. Alternatively, heating is achieved through concentrated solar radiation or electric heating elements.

[0007] In contrast, the modular storage system, according to German utility model 20 2023 000 696, enables continuous heat utilization. Separate pipe runs for charging and discharging allow for simultaneous heat absorption and release. This creates a temperature-graded counterflow principle across several modules – comparable to shell-and-tube heat exchangers.

[0008] Current systems typically transfer the heat generated during discharge to a secondary circuit via a heat exchanger, which then drives, for example, a turbine. However, this technology requires complex equipment. The waste heat from such processes can, in practice, no longer be used in the cycle.

[0009] The overall efficiency of conventional reconversion to electricity is therefore low: Thermal storage systems with maximum temperatures of approximately 600°C can usually only be used effectively up to a discharge temperature of around 350°C, which allows for electrical efficiencies of 20-40%. Downstream heat engines themselves only achieve 25-40%, resulting in a maximum overall electrical efficiency of approximately 15%. Object of the invention:

[0010] The object of the invention is the recovery and storage of volatile energy or industrial waste heat for continuous reconversion to electricity during power shortages. This should also be possible with irregular, cyclical, or temporarily low heat levels and can be carried out with a time delay if required – for example, during periods of low wind, nighttime breaks, or shutdowns.

[0011] The heat storage system must be able to bridge several days, be more cost-effective than conventional storage technologies, and have a lower system complexity.

[0012] The heat engine will be operated with the same working gas and in the same circuit as the storage system. This eliminates the need for additional equipment or secondary media between the two system components. The system's efficiency is expected to surpass that of existing regenerative power solutions by utilizing a wider temperature range of the stored heat.

[0013] In addition, heat should be able to be extracted for other purposes, independent of the reconversion of electricity. Solution to the problems • The heat engine is based on a multi-cylinder hot gas engine system based on the Stirling principle with a shared counterflow regenerator in a closed working gas circuit. It has a hot side for expansion and a cold side for compression (according to German utility model 20 2022 001 806). • Unlike conventional Stirling engines, this machine operates with a continuous flow of working gas and a constant flow direction. Separate piping systems exist for the flow from the cold to the hot part of the system ("HP" = high pressure) and back ("LP" = low pressure). • The existing tube bundle regenerator is replaced by a modular thermal storage unit (according to German utility model 20 2023 000 696) through which two independent flow systems for hot (LP) and cold working gas (HP) run. In counterflow operation, a temperature profile comparable to that of conventional heat exchangers is achieved. • The thermally separated modules of the storage system create a cascade-like temperature gradient and simultaneously perform the functions of heat storage and heat exchange. • The heat engine is fully integrated into the same working gas circuit as the thermal storage unit and is supplied with heat directly from it. The design of the storage unit allows operation over extended periods even without an external heat supply. • The modules can be used individually or in combination for charging and discharging – in series, parallel, or targeted configurations. This allows for flexible control of the heat flow depending on operational requirements. • The charging and discharging cycles of the thermal storage modules can be shortened as required, e.g. by diverting the working gas before the last module, so that the hottest modules are thermally protected. • Storage capacity and operating time can be scaled by installing multiple thermal storage units in parallel. For example: ten storage units with six modules each, where only one is actively operated at any given time. The others serve as a thermal reserve. Structure of the invention (Fig. X):

[0014] The system essentially consists of the following components: • Multi-cylinder hot gas engine based on the Stirling principle: Expansion cylinders on the hot side are arranged linearly opposite the compression cylinders on the cold side. • Crankshaft, which transfers the kinetic energy from the expansion cylinders to the compression cylinders. Alternatively, a cycloidal gear can be used for each pair of cylinders. • Generator, which is also driven via the crankshaft. • Modular counterflow thermal storage unit as a shared regenerator for all cylinders of the hot gas engine. The thermally separated modules each contain: ◯ A thermally insulated housing to accommodate the storage mass ◯ at least one structured flow system for loading, ◯ two independent conduction systems of the Stirling cycle: ▪ ND line: from the expansion to the compression cylinders, ▪ HD line: from the compression cylinders to the expansion cylinders. ◯ Optionally, a fourth structured flow system for the separate discharge of low-temperature heat • Thermal storage mass consisting of bulk material or a mixture of bulk materials. The serpentine pipe systems are evenly distributed, interpenetrate each other, and contribute significantly to the storage capacity and the achievement of short response times. • Cooler, to lower the temperature of the working gas to be compressed. • Cooling and heating devices for thermal stabilization of the cylinders of the hot gas engine. • Pipelines and shut-off devices between the individual components • Electronic control, including for the targeted activation of individual storage modules. Functioning of the invention:

[0015] The storage modules are charged via a separate piping system that operates independently of the working gas circuit. Various external systems can serve as heat sources, such as burners, industrial waste heat, or surplus electrical energy from renewable sources.

[0016] Simultaneously, a hermetically sealed quantity of working gas circulates continuously in the closed high-pressure (HP) and low-pressure (LP) circuits of the system. As it flows through the thermal storage regenerator in the HP line, the gas absorbs the previously introduced heat, successively passing through storage modules with increasing temperatures and being heated step by step.

[0017] The gas flow can be introduced into any module and selectively discharged from a downstream module once the required temperature is reached. The number of modules actively flowing through can vary depending on the system's operating state to enable demand-based thermal utilization and flexible power adjustment. In this way, the gas reaches the expansion side of the hot gas engine at high temperature and high pressure.

[0018] There, the conversion of thermal energy into mechanical work takes place in the expansion cylinders. The crankshaft transfers this to the generator and simultaneously drives the compression on the cold side.

[0019] After expansion, the working gas still has a considerable temperature and flows back through the low-pressure line and the thermal storage unit, where it cools down over several modules. The gas flow can also be directed into a freely selectable module. A downstream cooler further reduces the temperature before the gas is compressed on the cold side of the system.

[0020] It then travels through the high-pressure pipeline back into the thermal storage modules, where it is reheated according to the respective module temperatures. In this way, it reaches the expansion side again at high pressure and temperature.

[0021] The useful work produced is the difference between the energy released during expansion and the energy required for compression. The lower the gas temperature during compression, the lower the energy expenditure.

[0022] High efficiency is achieved through: • a large temperature gradient between the expansion and compression sides, • the recovery of residual heat after compression and its storage. • and low system-related losses (heat conduction, friction, radiation).

[0023] This results in three possible operating states: • Charging in standby mode: The heat storage units / regenerators are charged with heat while the heat engine is not active. The supplied energy is stored in the modules; the temperature rises gradually over time. • Simultaneous operation: While the heat engine is running, the storage modules are being charged. The module temperatures are dynamically regulated depending on the heat balance between supply and discharge. • Reconversion of stored heat into electricity: Without an external heat supply, the machine is powered solely by the stored energy. As soon as one storage module is discharged, the system automatically switches to the next storage unit.

[0024] Advantages of the invention: • System integration instead of individual units: Thermal storage and heat engine are structurally combined into a single unit. The classic tube bundle regenerator is no longer needed. • Single working gas: The entire process uses only one working medium – without phase changes or coupling with other media. This eliminates the need for pumps, fans, compressors, condensers, or evaporators. The system is significantly simplified in terms of equipment. • Unlimited scalability: The modular architecture allows for flexible expansion of storage capacity - tailored to desired operating time, power-to-energy output or redundancy requirements. • Selective module use: The gas flow can be selectively introduced into individual modules and discharged from downstream modules - adapted to temperature level and operating condition. • Base load capability through continuous operation: Unlike cyclically operating storage systems, this system allows continuous operation of the heat engine as long as sufficient temperature is present in at least one module. • Efficient counterflow heat exchange: The thermal storage unit takes over the regenerator function for optimal heat recovery. Low-temperature heat is also efficiently integrated for preheating. • Recovering residual heat instead of heat loss: Unlike conventional turbine systems, where the exhaust gas heat can hardly be utilized, in this system the residual heat of the expanded working gas is completely recovered via the thermal storage unit. The heat remains in the cycle – this increases the overall efficiency and minimizes energy losses. Explanations for the illustrations: Fig. Figure 1a shows the previously known integration of a modular thermal storage unit

[10] , for example consisting of six modules, in a circuit

[20] with a wind turbine

[24] , which charges it by means of an air heater

[23] . Fig. Figure 1b shows the previous recirculating air cycle

[21] with a heat engine

[22] , as used in the prior art. Here, the heat present in the thermal storage unit is used to generate electricity

[25] . Fig. Figure 2 schematically shows the thermal storage unit with six exemplary modules and the course of the hot gas flow in the low-pressure line

[11] during serial charging. This results in a cascade-like temperature profile from module 1 to module 6. The high-pressure line and the external charging line are not shown in this illustration. Fig. Figure 3 shows the construction of a previously known hot gas engine with expansion cylinders [1], compression cylinders [2], generator [3], heater [6], cooler [5] and crankshaft [7]. In the regenerator [4], the low pressure

[11] and high pressure

[12] flows meet and exchange heat. Fig.Figure 4 shows the inventive setup of the complete system, consisting of a hot gas engine

[22] and a thermal storage tank

[10] with nine modules as an example, and the associated piping. The thermal storage tank replaces the regenerator and the previously existing heater. It is supplied with fluid from the low-pressure

[11] and high-pressure

[12] lines of the cycle and, in the illustrated example, is charged via the charging circuit

[20] using an external energy source (wind turbine)

[24] . Reference symbol list 1 working cylinder (hot cylinder) 2 compression cylinders (cold cylinders) 3 Generator 4 Regenerator 5 coolers 6 heaters 7 Crankshaft 10 thermal storage tanks 11 ND line 12 HD lines 20 Loading line 21 Recirculation duct 22 Heat engine complete. 23 air heaters 24 Energy source / Wind turbine

Claims

[1] Heat engine plant for converting thermal energy into mechanical energy by means of a closed cycle process with a working gas, essentially consisting of a heat engine, solid heat storage units, cooling and heating devices, piping and a heat source, characterized by , that a modularly constructed high-temperature solid heat storage system is designed as a regenerator according to the counterflow heat exchange principle and is an integral part of the working gas circuit of the heat engine. [2] Heat engine plant according to claim 1, characterized by , that the heat input and output takes place in selectable modules of the thermal storage unit designed as a regenerator, wherein the number of actively flowed modules is variable and the gas flow can be introduced into a selectable subset of the modules and discharged from a downstream subset. [3] Heat engine plant according to one of the preceding claims, characterized by that the plant comprises several independent regenerator / thermal storage systems, with multiple regenerator / thermal storage units installed in parallel and able to be selectively switched on for operation in order to flexibly scale the storage capacity and operating time. [4] Heat engine plant according to any one of claims 1 to 3, characterized by , that the modular regenerator / thermal storage units are each thermally separated from each other and have different temperature levels, thereby creating a cascade-like temperature gradient. [5] Heat engine plant according to one of the preceding claims, characterized bythat the plant is designed for continuous operation of the heat engine, whereby the heat supply either occurs simultaneously with the reconversion to electricity or is ensured independently by previously stored thermal energy in the modular regenerator / thermal storage units.

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