An ultra-high temperature heat pump system and a method of operation

Abstract

The application discloses a super-high temperature heat pump system and an operation method, and belongs to the technical field of high-temperature heat pumps. The super-high temperature heat pump system adopts an open cycle with air as a working medium, and comprises at least two series-connected compressors, corresponding high-temperature molten salt heat exchangers, at least two series-connected expanders, corresponding heat absorbers and a regenerator. The cold side of the high-temperature molten salt heat exchanger is used for connecting a molten salt loop, the hot side of the heat absorber is used for connecting a low-temperature heat source loop, and the cold side of the regenerator is used for receiving ambient air. The method is based on the above system and is operated through the processes of step-by-step heat release to molten salt between compression stages, heat recovery, step-by-step heat absorption from a low-temperature heat source between expansion stages. The application realizes super-high temperature heat supply, has the advantages of high system efficiency and low equipment cost, and is particularly suitable for fields such as high-temperature molten salt energy storage.

Description

Technical Field

[0001] This invention relates to the field of high-temperature heat pump technology, specifically to an ultra-high temperature heat pump system and its operation method. Background Technology

[0002] With the increasing penetration rate of renewable energy (such as wind power and photovoltaics), building large-scale, long-term, and low-cost energy storage systems has become crucial for ensuring energy security and grid stability. Besides electrochemical batteries, the concept of "thermal batteries" based on thermal energy storage (TES) has attracted widespread attention due to their large scale, long lifespan, environmental friendliness, and low cost potential. A typical thermal battery system comprises three stages: "heat charging - heat storage - heat release." When there is a power surplus, electrical energy is converted into heat energy and stored in a heat storage medium; when there is a power shortage, the stored heat energy is converted back into electrical energy through a thermodynamic cycle (such as a steam Rankine cycle or Brayton cycle) or used directly for heating.

[0003] High-temperature molten salts (such as solar salts) are ideal high-temperature thermal storage media due to their high operating temperature, large heat capacity, and relatively low cost, and are widely used in current solar thermal power plants and industrial thermal energy storage systems. However, the development of current thermal battery technology faces a core bottleneck: how to efficiently and cost-effectively convert electrical energy into high-grade thermal energy and store it in molten salt. While direct electric heating (resistance heating) is simple, it essentially converts 100% of high-grade electrical energy into low-grade thermal energy, a conversion method with significant losses, resulting in low overall system round-trip efficiency.

[0004] Theoretically, heat pump technology can achieve a COP (Coefficient of Performance, the ratio of system heat output to input work) greater than 1, meaning that one unit of electricity can transport multiple units of heat, making it a superior "heating" solution. However, existing heat pump technology struggles to meet the specific temperature requirements of thermal batteries for recharging.

[0005] 1. Working fluid and temperature limitations: The operating temperature of conventional heat pumps is generally below 90℃, while some high-temperature heat pumps can reach 150℃; in order to meet the operating conditions at 600℃, the working fluid used needs to maintain chemical stability at extremely high temperatures.

[0006] 2. System efficiency and architecture: The enthalpy rise of a single-stage compression heat pump is limited, which restricts the power of the heat pump under a certain mass flow rate. If multi-stage compression is used, it is equivalent to multi-stage compression in series, which greatly improves the compression enthalpy rise and heat pump power. In addition, multi-stage compression and multi-stage heating can make full use of the thermodynamic properties of the working fluid and improve the COP of the system.

[0007] 3. Low-temperature adaptability: In order to absorb heat from the environment, traditional air source heat pumps require a large air receiver, which is prone to frost in humid areas. Although there is no frost problem in arid areas, the equipment investment and land cost are high, which increases the overall cost of the thermal battery.

[0008] 4. Integration cost: High-efficiency heat pumps usually require the installation of large regenerators and air absorbers, but this investment will significantly increase the initial construction cost of the thermal battery.

[0009] Therefore, developing an ultra-high temperature heat pump capable of efficiently "charging" high-temperature molten salt thermal storage systems has become crucial for promoting the commercial application of "thermal battery" technology. Against this backdrop, this invention proposes an open-type, multi-stage compression heating and expansion heat absorption ultra-high temperature heat pump system designed for charging thermal batteries, aiming to solve the aforementioned bottleneck problems. Summary of the Invention

[0010] The present invention aims to provide an ultra-high temperature heat pump system and its operation method to solve the problem that existing technologies cannot simultaneously achieve ultra-high temperature heating, high energy efficiency and low cost.

[0011] To achieve the above objectives, the present invention provides the following technical solution:

[0012] An ultra-high temperature heat pump system uses air as the working fluid and adopts an open-loop circulation. The system includes: at least two compressors connected in series, and high-temperature molten salt heat exchangers alternately connected thereto; wherein, the outlet of the first compressor is connected to the hot side inlet of the first high-temperature molten salt heat exchanger, the hot side outlet of the first high-temperature molten salt heat exchanger is connected to the inlet of the second compressor, and subsequent compressors and high-temperature molten salt heat exchangers are alternately connected in this sequence.

[0013] At least two expanders connected in series, and receivers alternately connected thereto; wherein the outlet of the first expander is connected to the cold-side inlet of the first receiver, the cold-side outlet of the first receiver is connected to the inlet of the second expander, and subsequent expanders and receivers are alternately connected in this order, and the cold-side outlet of the last receiver is connected to the atmosphere.

[0014] And a regenerator; wherein, the hot-side outlet of the last high-temperature molten salt heat exchanger is connected to the hot-side inlet of the regenerator, and the hot-side outlet of the regenerator is connected to the inlet of the first expander.

[0015] The cold side of the high-temperature molten salt heat exchanger is used to connect to the molten salt circuit, the hot side of the absorber is used to connect to the low-temperature heat source circuit, and the cold side of the regenerator is used to receive ambient air.

[0016] Furthermore, the system also includes an air filter disposed upstream of the regenerator.

[0017] Furthermore, the expander is mechanically coupled to the compressor.

[0018] Furthermore, the multi-stage compressor and the multi-stage expander are arranged on the same or multiple shafts and equipped with electric motors to regulate power balance.

[0019] Furthermore, the compressor is configured to raise the air temperature to 500-600°C; the high-temperature molten salt heat exchanger is configured to cool the air temperature to 290-320°C.

[0020] Furthermore, the heat source in the low-temperature heat source circuit is industrial waste heat, geothermal energy, solar thermal low-temperature fluid, or air.

[0021] Furthermore, it also includes a control system for regulating at least one of the compressor speed, expander speed, and valve opening.

[0022] A method for operating an ultra-high temperature heat pump system includes the following steps:

[0023] Preheating step: Air is introduced into the regenerator for preheating;

[0024] Multi-stage compression and stepped heat release steps: The preheated air is compressed in at least two stages in series, and after each stage of compression, the compressed and heated air is passed into a high-temperature molten salt heat exchanger to exchange heat with the molten salt.

[0025] Regeneration step: The air that has been heated by the last high-temperature molten salt heat exchanger is introduced into the regenerator to recover waste heat.

[0026] Multi-stage expansion and stepped heat absorption steps: The air flowing out of the regenerator undergoes at least two stages of series expansion to do work, and after each stage of expansion, the air is passed into the heat absorber to absorb heat from the low-temperature heat source.

[0027] Exhaust procedure: The air that has undergone the final expansion is discharged into the atmosphere.

[0028] Furthermore, in the multi-stage compression and stepped heat release steps, the air temperature is controlled to rise to 500-600°C in each compression stage, and the air temperature is controlled to drop to 290-320°C after heat exchange with molten salt in each stage.

[0029] This invention, through its innovative architecture of "open air circulation + multi-stage compression heat release + multi-stage expansion heat absorption," produces the following synergistic effects:

[0030] 1. Fundamentally solves the working fluid problem: Air as the working fluid completely avoids the risk of high-temperature decomposition of organic working fluids, enabling the system to operate stably in the ultra-high temperature range of nearly 600°C.

[0031] 2. Improved system efficiency:

[0032] The combination of multi-stage compression and intermediate stage heat release significantly increases the total heat transferred per unit mass of air, reduces the required mass flow rate under the same thermal power, and correspondingly reduces the size of the core equipment.

[0033] The combination of multi-stage expansion and intermediate heat absorption significantly increases the expansion work output and improves the system COP. More importantly, the mechanical coupling between the expander and compressor allows the expansion work to be directly used to drive the compression process, greatly recovering energy. This is one of the key design features for achieving a high COP in the system.

[0034] 3. System simplification and cost optimization: The system completely eliminates the expensive, bulky, and frosty forced convection air absorber found in traditional air source heat pumps. The system draws air directly from the atmosphere and ultimately exhausts it back into the atmosphere, resulting in a streamlined process.

[0035] 4. Optimization of regenerator load and cost control: Since the multi-stage compression and molten salt heat exchanger have effectively improved the total temperature rise, the relative power ratio of the regenerator and the required heat exchange area are reduced, and the cost decreases accordingly.

[0036] 5. Enhanced system reliability and adaptability: The multi-segment structure reduces the operating load and surge risk of single-stage equipment; the control system can be flexibly adjusted to adapt to different conditions.

[0037] This system is the core component of building a large-scale, low-cost "thermal battery" energy storage system. It is especially suitable for ultra-high temperature heating scenarios in areas that are sensitive to equipment investment costs, dry environments, and energy-rich areas, such as efficiently charging "thermal batteries" based on molten salt energy storage. Attached Figure Description

[0038] Figure 1 This is a system flow diagram of the open multi-stage compression heating and expansion heat absorption ultra-high temperature heat pump system (taking three stages as an example) described in this invention. Detailed Implementation

[0039] The present invention will now be described in detail with reference to the accompanying drawings and embodiments, but the scope of protection of the present invention is not limited to the following description.

[0040] In this invention, "ultra-high temperature" refers to a heating temperature capable of reaching above 500°C, typically in the range of 570°C to 600°C. "Low-temperature heat source" refers to any available heat source with a temperature higher than the expander outlet air temperature, which can be used to heat air in the absorber, relative to the high-temperature side of the system releasing heat to the molten salt. This includes, but is not limited to, industrial waste heat, geothermal energy, solar collector fluids, ambient air, and water.

[0041] Example 1: Standard Operating Mode (Taking a Three-Stage Compression and Three-Stage Expansion System as an Example)

[0042] This embodiment demonstrates the complete configuration and efficient operation of the system in a typical scenario where industrial circulating cooling water serves as a low-temperature heat source. The following detailed description uses a three-stage compressor and a three-stage expander as examples, but this should not be construed as limiting the scope of protection of this invention.

[0043] 1. System Composition

[0044] The system mainly includes the core equipment that constitutes the open air Brayton cycle, as well as necessary auxiliary equipment, as described in the reference. Figure 1 :

[0045] Core thermal circulation equipment: air filter 1, regenerator 2;

[0046] The first stage compressor 3, the second stage compressor 4, and the third stage compressor 5 are connected in series, and the first stage high-temperature molten salt heat exchanger 6, the second stage high-temperature molten salt heat exchanger 7, and the third stage high-temperature molten salt heat exchanger 8 are respectively installed after each compressor.

[0047] The first stage expander 9, the second stage expander 10, and the third stage expander 11 are connected in series, and the first stage absorber 12 and the second stage absorber 13 are located after the first stage expander 9 and the second stage expander 10.

[0048] Main auxiliary equipment: exhaust muffler 14 for exhaust noise reduction, located at the outlet of the third stage expander 11;

[0049] The first stage motor 15, the second stage motor 16, and the third stage motor 17, used for starting and power regulation, are coupled to the corresponding compressor and expander shaft systems, respectively.

[0050] External circuit: Molten salt circuit, connecting cold salt tank 18 and hot salt tank 19, and flowing through the cold side of the first high-temperature molten salt heat exchanger 6, the second high-temperature molten salt heat exchanger 7, and the third high-temperature molten salt heat exchanger 8;

[0051] The low-temperature heat source circuit is connected to the hot side of the first heat absorber 12 and the second heat absorber 13.

[0052] The components are connected in sequence through pipelines according to the described process to form an open air circulation.

[0053] 2. Equipment Selection and Key Design Parameters

[0054] Compressor and expander: Centrifugal or axial-flow turbine machinery is selected. The three-stage compressor and three-stage expander are mechanically coupled through three shafts, each equipped with a synchronous motor for starting and power fine-tuning. During normal operation, the expansion work can basically drive the compression work, and the insufficient part is supplemented by the motor.

[0055] Heat exchangers: Molten salt heat exchangers and regenerators use high-temperature and high-pressure resistant printed circuit board heat exchangers, or shell-and-tube, wound-tube, and other types of heat exchangers. Absorbers use plate-fin heat exchangers, or shell-and-tube or plate heat exchangers.

[0056] Key design temperature points: Compressor outlet design temperature is 500℃ to 600℃ (preferably 570℃ to 600℃, e.g., 600℃); molten salt heat exchanger air-side outlet temperature is 290℃ to 320℃, e.g., 315℃; regenerator cold-side outlet (compressor inlet) preheating temperature is 270℃ to 300℃, e.g., 300℃. The specific values ​​for these temperature points can be selected within the stated range based on the target molten salt temperature, material selection, and efficiency optimization.

[0057] The above temperature settings are based on the following: the compressor outlet temperature of 500-600℃ provides the necessary high-grade heat source for ultra-high temperature heating; the molten salt heat exchanger air-side outlet temperature of 290-320℃ ensures that heat can be effectively absorbed by the molten salt, while also providing a high starting point for the next stage of compression; and the regenerator preheats the intake air to 270-300℃, maximizing the recovery of waste heat and significantly reducing compression power consumption. These three temperature points together constitute the thermodynamic basis for the system's efficient and stable operation.

[0058] 3. Operating Status and Performance Data

[0059] The following will describe the operation process of the system in detail:

[0060] Air intake and preheating: The ambient air at 25°C is filtered and enters the cold side of the regenerator 2, where it is heated to about 300°C by the air at about 315°C from the hot side.

[0061] First stage compression and heat release: Air at 300°C enters the first stage compressor 3 and is adiabatically compressed to approximately 0.36 MPa and 600°C. It then enters the first stage high-temperature molten salt heat exchanger 6, where it transfers heat to the molten salt (for example, heating the molten salt from 290°C to 565°C) and is cooled to approximately 315°C.

[0062] Second stage compression and heat release: Air at 315°C enters the second stage compressor 4 and is compressed to approximately 1.2 MPa and 600°C. Heat is released again in the second stage high-temperature molten salt heat exchanger 7 (molten salt is heated from 290°C to 565°C), and the temperature drops to approximately 315°C.

[0063] The third stage of compression and heat release: Air enters the third stage compressor 5 and is compressed to the maximum pressure of approximately 3.9 MPa and 600°C. The final high-temperature heat release is completed in the third stage high-temperature molten salt heat exchanger 8 (the molten salt is heated from 290°C to 565°C), and the temperature drops to approximately 315°C.

[0064] Regeneration: High-pressure air at 315℃ enters the hot side of regenerator 2, transfers heat to the cold side intake air, and then its own temperature drops to about 40℃.

[0065] First stage expansion and heat absorption: The cooled high-pressure air enters the first stage expander 9 to expand and do work, and the pressure and temperature drop significantly. Then it enters the first stage heat absorber 12, where it absorbs heat from a connected low-temperature heat source (such as 40°C industrial circulating cooling water), and the temperature rises back to 25°C.

[0066] Second stage expansion and heat absorption: Air enters the second stage expander 10 and expands twice. Then, it absorbs heat from the low-temperature heat source again in the second stage heat absorber 13, and the temperature rises back to 25°C.

[0067] Third stage expansion and exhaust: The air finally expands in the third stage expander 11, the pressure is close to atmospheric pressure, and the temperature drops before it is discharged into the atmosphere through the silencer 14.

[0068] Molten salt circuit: Molten salt is pumped out from cold salt tank 18 (290℃), flows through the first high-temperature molten salt heat exchanger 6, the second high-temperature molten salt heat exchanger 7, and the third high-temperature molten salt heat exchanger 8, and is finally heated to 565℃ and stored in hot salt tank 19.

[0069] In this state, the system's coefficient of performance (COP) is significantly higher than that of direct electric heating. This operating state indicates that the system described in this invention has successfully achieved its core objective of converting electrical energy into ultra-high temperature molten salt heat energy above 500°C with a high COP, while simultaneously reducing the required mass flow rate and significantly decreasing the size of the main equipment compared to a single-stage system.

[0070] The parameters listed in the table below are exemplary data under a specific design condition, used to illustrate the working principle and potential of the present invention, and are not intended to limit the scope of protection of the present invention. Those skilled in the art will understand that actual operating parameters can be adjusted within the above core process framework according to different molten salt target temperatures, environmental conditions, and equipment selections.

[0071] Node position working medium Temperature (°C) Pressure (MPa) Remark Ambient air intake Air 25 0.101 Regenerator cold side outlet Air 300 0.098 First compressor outlet Air 600 0.36 Air outlet of the first molten salt heat exchanger Air 315 0.355 Second stage compressor outlet Air 600 1.2 Second molten salt heat exchanger air outlet Air 315 1.18 Third stage compressor outlet Air 600 3.90 Third molten salt heat exchanger air outlet Air 315 3.82 First expander inlet (regenerator hot side outlet) Air 40 3.75 First expander outlet (first absorber air side inlet) Air -53 0.95 *Temperature after expansion Second expander inlet (air-side outlet of first absorber) Air 25 0.94 *Heat absorption from 35°C cooling water Second expander outlet (second absorber air side inlet) Air -53 0.3 *Temperature after expansion Third expander inlet (second absorber air-side outlet) Air 25 0.297 *Heat absorption from 35°C cooling water exhaust Air -52 0.102

[0072] Example 2: Simplified Operation Mode Without External Low-Temperature Heat Source

[0073] This embodiment demonstrates a simplified operating configuration of the system in arid regions or specific industrial sites where no suitable low-temperature heat source is available.

[0074] 1. Adjustment of operating logic

[0075] Shut down the low-temperature heat source circuit of the receiver (or short-circuit it). The system process is simplified as follows: after the air is cooled on the hot side of the regenerator, it directly passes through the expander to continuously expand and do work, without passing through the receiver in between, and finally is discharged into the atmosphere through the silencer.

[0076] 2. Performance Impact and Applicable Scenarios

[0077] In this mode, the system degenerates into an open Brayton cycle with regeneration and multi-stage compression. Its coefficient of performance (COP) is lower than the standard mode of Example 1, but the system completely eliminates all external low-temperature heat source loops and receivers, significantly reducing equipment investment, piping system complexity, and operational complexity. This mode is particularly suitable for scenarios that only require ultra-high temperature heating, have relatively relaxed COP requirements, and where obtaining low-temperature heat sources is extremely costly, such as independent heating stations in some remote areas.

[0078] Example 3: Control Method and Variable Operating Conditions

[0079] To further optimize the system's performance under varying operating conditions and its operational reliability, a distributed control system can be preferred, using temperature and pressure sensors to monitor key nodes. This embodiment demonstrates how the control system responds to load and environmental changes. When ambient temperature or load changes, the control system can perform the following operations to maintain stable and efficient system operation:

[0080] Power and flow regulation: Adjust the compressor speed to maintain the overall power balance and design air flow of the system; at the same time, synchronously adjust the speed of the expander or the opening of the inlet guide vanes to match its output with the compressor, thereby stabilizing key parameters, including the cold side outlet temperature of the regenerator, near the design point.

[0081] Surge prevention: By adjusting key valves such as the expander inlet guide vane as described above, ensure that the compressor and expander always operate in a stable operating range and avoid surge.

[0082] Independent adjustment on the heat source side: By adjusting the valves or pump speeds of the molten salt circuit and the low-temperature heat source circuit, the flow rate and final temperature of the molten salt and the low-temperature heat source can be stabilized.

[0083] This control strategy ensures that the system can stably output high-temperature thermal energy at the design temperature under different environmental conditions, and maintain efficient and safe operation.

[0084] It should be noted that the aforementioned operating method steps (preheating, multi-stage compression and stepped heat release, regeneration, multi-stage expansion and stepped heat absorption, and exhaust) and their core parameter control are essentially based on the thermodynamic process of "open air circulation, stepped heat release from the high-temperature molten salt between multi-stage compression stages, and stepped heat absorption from the low-temperature heat source between multi-stage expansion stages." Therefore, even if the specific equipment selection or coupling method in the aforementioned embodiments is not completely adopted, as long as this thermodynamic process is executed, it should be considered to fall within the protection scope of the operating method of this invention.

[0085] The above description is merely a preferred embodiment of the present invention. Based on the core concept of the present invention, those skilled in the art can understand and make various modifications and substitutions. For example, the number of stages of the compressor and expander can be adjusted according to the heat load requirements; different types of heat exchangers can be selected according to the operating conditions; the mechanical coupling can adopt different arrangements such as single-shaft and multi-shaft; and the control system strategy can also be adjusted accordingly. Any equivalent substitution or modification made under the core architecture of "open air circulation, multi-stage compression stage releasing heat to molten salt, and multi-stage expansion stage absorbing heat from low-temperature heat source" proposed in the present invention should be considered to fall within the protection scope of the present invention.

Claims

1. An ultra-high temperature heat pump system, characterized in that, The system, which uses air as the working fluid and employs an open-loop cycle, includes: At least two compressors connected in series, and high-temperature molten salt heat exchangers alternately connected thereto; wherein, the outlet of the first compressor is connected to the hot-side inlet of the first high-temperature molten salt heat exchanger, the hot-side outlet of the first high-temperature molten salt heat exchanger is connected to the inlet of the second compressor, and subsequent compressors and high-temperature molten salt heat exchangers are alternately connected in this order. At least two expanders connected in series, and receivers alternately connected thereto; wherein the outlet of the first expander is connected to the cold-side inlet of the first receiver, the cold-side outlet of the first receiver is connected to the inlet of the second expander, and subsequent expanders and receivers are alternately connected in this order, and the cold-side outlet of the last receiver is connected to the atmosphere. The expander is mechanically coupled to the compressor; multiple compressor sections and multiple expander sections are arranged on the same or multiple shafts and equipped with electric motors to adjust power balance; And a regenerator; wherein, the hot-side outlet of the last high-temperature molten salt heat exchanger is connected to the hot-side inlet of the regenerator, and the hot-side outlet of the regenerator is connected to the inlet of the first expander. The cold side of the high-temperature molten salt heat exchanger is used to connect to the molten salt circuit, the hot side of the absorber is used to connect to the low-temperature heat source circuit, and the cold side of the regenerator is used to receive ambient air.

2. The system according to claim 1, characterized in that, The system also includes an air filter located upstream of the regenerator.

3. The system according to claim 1, characterized in that, The compressor is configured to raise the air temperature to 500-600°C; the high-temperature molten salt heat exchanger is configured to cool the air temperature to 290-320°C.

4. The system according to claim 1, characterized in that, The heat source in the low-temperature heat source circuit is industrial waste heat, geothermal energy, low-temperature fluid from solar thermal collectors, or air.

5. The system according to claim 1, characterized in that, It also includes a control system for regulating at least one of the compressor speed, expander speed, and valve opening.

6. A method for operating an ultra-high temperature heat pump system, characterized in that, The system described in any one of claims 1-5 comprises the following steps: Preheating step: Air is introduced into the regenerator for preheating; Multi-stage compression and stepped heat release steps: The preheated air is compressed in at least two stages in series, and after each stage of compression, the compressed and heated air is passed into a high-temperature molten salt heat exchanger to exchange heat with the molten salt. Regeneration step: The air that has been heated by the last high-temperature molten salt heat exchanger is introduced into the regenerator for heat exchange. Multi-stage expansion and stepped heat absorption steps: The air flowing out of the regenerator undergoes at least two stages of series expansion to do work, and after each stage of expansion, the air is passed into the heat absorber to absorb heat from the low-temperature heat source. Exhaust procedure: The air that has undergone the final expansion is discharged into the atmosphere.

7. The method according to claim 6, characterized in that, In the multi-stage compression and stepped heat release steps, the air temperature is controlled to rise to 500-600℃ in each compression stage, and the air temperature is controlled to drop to 290-320℃ after heat exchange with molten salt in each stage.

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