An industrial multi-grade waste heat cascade utilization method based on enthalpy maximum

Abstract

A method for cascade utilization of multi-grade industrial waste heat based on enthalpy maximization includes periodically collecting temperature, flow rate, pressure, and heat pump operating status of the heat source; obtaining real-time load demands from each user; calculating the specific enthalpy of each fluid and the overall efficiency of the system based on the enthalpy maximization objective function; and calculating the losses of each heat exchanger, heat pump, valve, and pipeline; constructing a heat source-load matching combination based on enthalpy matching constraints; and generating the temperature matching degree and enthalpy of the heat source-load combination. The density weighting factor is used to allocate heat source flow based on the optimal matching sequence of enthalpy density weighting factor; for unmatched low-temperature heat sources, the enthalpy economy of heat pump intervention is determined; the optimal control strategy is solved and decoupled into specific instructions and issued, the heat pump and each valve execute the instructions, a new steady-state enthalpy distribution is established, the actual total efficiency is calculated, and compared with the preset total efficiency threshold. If the deviation is not within the preset range, a new round of enthalpy calculation and temperature comparison optimization cycle is started.

Description

Technical Field

[0001] This invention belongs to the field of energy utilization technology, specifically relating to an industrial multi-grade waste heat cascade utilization system and method based on enthalpy maximization. Background Technology

[0002] In modern industrial production processes, numerous technological steps simultaneously generate waste heat or cold of varying grades, ranging from high to low. Traditional waste heat recovery methods generally suffer from problems such as a single recovery path, rudimentary matching logic, and misuse of energy grades. This often results in high-quality, low-use phenomena, such as using high-temperature heat energy for low-temperature demands and directly discharging low-temperature waste heat, leading to significant losses and waste of usable energy.

[0003] The existing technology has the following main drawbacks:

[0004] The lack of a graded matching mechanism often results in high-temperature waste heat being directly used for low-temperature demand, leading to the unnecessary depreciation of high-quality usable energy and low system energy utilization efficiency. A large amount of low-grade waste heat below 50°C is directly emitted, causing energy waste.

[0005] Therefore, there is an urgent need for a method to improve the efficiency of multi-grade waste heat utilization. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a method for the cascade utilization of industrial multi-grade waste heat based on maximizing enthalpy to significantly improve energy utilization efficiency.

[0007] The technical solution adopted to solve the above-mentioned technical problems is: a method for the cascade utilization of multi-grade industrial waste heat based on enthalpy maximization, comprising the following steps:

[0008] Step 1. Periodically collect temperature, flow, and pressure data of all heat sources and the operating status of heat pumps, obtain the real-time load demand temperature of each user side, calculate the specific enthalpy of each fluid and the total efficiency of the system in real time based on the objective function of maximizing enthalpy, and calculate the losses of each heat exchanger, heat pump, valve and pipeline in real time.

[0009] Step 2. Construct a heat source and load matching combination based on enthalpy-voltage matching constraints, wherein the enthalpy-voltage matching constraints include temperature matching constraints, enthalpy-voltage grade constraints, and voltage loss threshold constraints;

[0010] Step 3. Calculate the temperature matching degree for each heat source and load combination according to the following formula. ,

[0011]

[0012] In the formula, The temperature of the i-th heat source is the actual temperature of the working fluid on the heat source side. For the j-th load, the required temperature The ambient reference temperature;

[0013] Step 4. Generate the enthalpy-density weighting factor for the combination of heat source and load according to the following formula. ,

[0014]

[0015] In the formula, Let be the ratio of the i-th heat source. This represents the maximum value of the ratio of all heat sources in the system.

[0016] Step 5. Apply enthalpy-density weighting factors to all heat source and load combinations. Sort the heat source flow rate from smallest to largest to determine the optimal matching sequence and allocate the heat source flow rate sequentially until the load demand is met.

[0017] Step 6. For unmatched low-temperature heat sources, determine the enthalpy-economic viability of heat pump intervention based on the objective function of maximizing enthalpy. If the actual heat pump coefficient of performance... ,in, For condensation temperature, If the evaporation temperature is [value], then start the heat pump and calculate the equivalent heat source parameters and heat pump losses after starting the heat pump. The improved equivalent heat source will be re-submitted for temperature comparison; if the conditions are not met, the heat pump will not be started and the low-temperature heat source will be directly discharged into the low-temperature heat source network.

[0018] Step 7. Solve for the optimal control strategy under the constraints of equipment loss, pipeline pressure limit, temperature limit, and heat pump performance boundary, with the objective function of maximizing the global total enthalpy recovery and minimizing the total loss, and then verify the expected efficiency.

[0019] Step 8. Decouple the optimal control strategy obtained in Step 7 into specific instructions and issue them, sending the evaporation temperature to the heat pump. Condensation temperature The system sends opening or switching status commands to each valve, along with the optimal enthalpy setting command for the compressor frequency, and reconstructs the system's enthalpy flow path topology.

[0020] Step 9. The heat pump and valves execute commands to adjust the system operating status and establish a new steady-state enthalpy distribution;

[0021] Step 10. Calculate the actual overall operating efficiency η ex , a If the deviation is not within the preset range when compared with the preset total efficiency threshold, then repeat steps 1-9 to start a new round of enthalpy calculation and temperature comparison optimization cycle to achieve optimal enthalpy closed-loop adaptive control.

[0022] As a preferred technical solution, the enthalpy maximization objective function for:

[0023]

[0024] In the formula, For the i-th user-side enthalpy, Total losses for equipment, piping, and heat exchangers;

[0025] The specific enthalpy values ​​of each fluid are:

[0026]

[0027] In the formula, For comparison of enthalpy, The specific enthalpy of the working fluid in its working state. The specific enthalpy of the working fluid is the reference state. The specific entropy of the working fluid in its working state. The specific entropy of the working fluid environment is the reference state. The ambient reference temperature;

[0028] The overall efficiency of the system is:

[0029]

[0030] In the formula, For the overall system efficiency, To recover the total enthalpy of the system, The total enthalpy available to the system;

[0031] The heat exchanger loss is:

[0032]

[0033] In the formula, For heat exchanger damage, For the heat exchanger hot side input, For the cold side input of the heat exchanger, For the hot side outflow of the heat exchanger, Cold side outflow from heat exchanger;

[0034] The heat pump loss is:

[0035]

[0036] In the formula, For heat pump losses, The flow rate input to the evaporator side of the heat pump. For the heat pump input power, The outflow rate from the evaporator side of the heat pump;

[0037] The wear and tear on the valves and pipelines is as follows:

[0038]

[0039] In the formula, For valve and pipeline damage, This refers to the flow rate through the valves and pipelines. This refers to the temperature drop through the valves and pipelines. This refers to the time it takes for the flow to pass through the valve and pipeline.

[0040] As a preferred technical solution, in step 2, the temperature alignment constraint is as follows: , Let k be the temperature of the heat source. For the temperature requirement of the j-th user, Minimum heat exchange temperature difference;

[0041] Enthalpy grade constraint is: , Let be the specific enthalpy of the temperature required by the j-th user.

[0042] As a preferred technical solution, the minimum heat transfer temperature difference The temperature is 1 to 3℃.

[0043] As a preferred technical solution, the industrial multi-grade waste heat includes a high-temperature heat source with a temperature ≥80℃, a medium-temperature heat source with a temperature of 50℃~80℃, and a low-temperature heat source with a temperature <50℃.

[0044] The beneficial effects of this invention are as follows:

[0045] This invention improves the overall enthalpy efficiency of the system by optimizing temperature matching, enthalpy, tiered utilization, and global optimization through temperature comparison, avoiding underutilization of high-quality heat and losses from mixed heat exchange. High-temperature waste heat is directly used for high-temperature demands, while medium-temperature waste heat is precisely matched to medium-temperature loads, eliminating unnecessary degradation of energy quality.

[0046] This invention dynamically adjusts the enthalpy matching between the heat pump and the cascade network through temperature ratio selection, increasing the utilization rate of low-grade waste heat enthalpy at ≤50℃ from less than 30% to over 90%. For low-enthalpy density heat sources such as low-temperature wastewater and cooling tower return water, which are difficult to utilize using traditional methods, effective recovery is achieved after the heat pump boosts the heat, significantly expanding the range of recoverable waste heat. Attached Figure Description

[0047] Figure 1 This is a schematic diagram of the structure of the present invention. Detailed Implementation

[0048] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments, but the present invention is not limited to the following embodiments.

[0049] exist Figure 1 In this embodiment, a method for cascade utilization of multi-grade industrial waste heat based on maximizing enthalpy is described. The multi-grade industrial waste heat includes a high-temperature heat source with a temperature ≥80℃, a medium-temperature heat source with a temperature of 50℃~80℃, and a low-temperature heat source with a temperature <50℃. The method includes the following steps:

[0050] Step 1. Periodically collect temperature, flow, and pressure data of all heat sources and the operating status of heat pumps, obtain the real-time load demand temperature of each user side, calculate the specific enthalpy of each fluid and the total efficiency of the system in real time based on the objective function of maximizing enthalpy, and calculate the losses of each heat exchanger, heat pump, valve and pipeline in real time.

[0051] Where, enthalpy maximizes the objective function for:

[0052]

[0053] In the formula, For the i-th user-side enthalpy, Total losses for equipment, piping, and heat exchangers;

[0054] The specific enthalpy values ​​of each fluid are:

[0055]

[0056] In the formula, For comparison of enthalpy, The specific enthalpy of the working fluid in its working state. The specific enthalpy of the working fluid is the reference state. The specific entropy of the working fluid in its working state. The specific entropy of the working fluid environment is the reference state. The ambient reference temperature;

[0057] The overall efficiency of the system is:

[0058]

[0059] In the formula, For the overall system efficiency, To recover the total enthalpy of the system, The total enthalpy available to the system;

[0060] The heat exchanger loss is:

[0061]

[0062] In the formula, For heat exchanger damage, For the heat exchanger hot side input, For the cold side input of the heat exchanger, For the hot side outflow of the heat exchanger, Cold side outflow from heat exchanger;

[0063] The heat pump loss is:

[0064]

[0065] In the formula, For heat pump losses, The flow rate input to the evaporator side of the heat pump. For the heat pump input power, The outflow rate from the evaporator side of the heat pump;

[0066] The wear and tear on the valves and pipelines is as follows:

[0067]

[0068] In the formula, For valve and pipeline damage, This refers to the flow rate through the valves and pipelines. This refers to the temperature drop through the valves and pipelines. This refers to the time it takes for the flow to pass through the valves and pipelines.

[0069] Step 2. Construct a heat source and load matching combination based on enthalpy-voltage matching constraints. Enthalpy-voltage matching constraints include temperature matching constraints, enthalpy-voltage grade constraints, and loss threshold constraints. Among these, the temperature matching constraint is... , Let k be the temperature of the heat source. To minimize the heat exchange temperature difference, The temperature is 1–3℃; the enthalpy grade constraint is: , Let be the specific enthalpy of the temperature required by the j-th user.

[0070] Step 3. Obtain the temperature matching degree of each heat source and load combination according to the following formula. ,

[0071]

[0072] In the formula, The temperature of the i-th heat source is the actual temperature of the working fluid on the heat source side. The ambient reference temperature;

[0073] Step 4. Obtain the enthalpy-density weighting factor for the heat source and load combination according to the following formula. ,

[0074]

[0075] In the formula, Let be the ratio of the i-th heat source. This represents the maximum value of the ratio of all heat sources in the system.

[0076] Step 5. Apply enthalpy-density weighting factors to all heat source and load combinations. Sort the heat source flow rate from smallest to largest to determine the optimal matching sequence and allocate the heat source flow rate sequentially until the load demand is met.

[0077] Step 6. For mismatched low-temperature heat sources, determine the enthalpy-economic feasibility of heat pump intervention based on the enthalpy-maximization calculation system. If the actual heat pump performance coefficient... ,in, For condensation temperature, If the evaporation temperature is [value], then start the heat pump and calculate the equivalent heat source parameters and heat pump losses after starting the heat pump. The improved equivalent heat source will be re-submitted for temperature comparison; if the conditions are not met, the heat pump will not be started and the low-temperature heat source will be directly discharged into the low-temperature heat source network.

[0078] Step 7. Solve for the optimal control strategy under the constraints of equipment loss, pipeline pressure limit, temperature limit, and heat pump performance boundary, with the objective function of maximizing the global total enthalpy recovery and minimizing the total loss, and then verify the expected efficiency.

[0079] Step 8. Decouple the optimal control strategy obtained in Step 7 into specific instructions and issue them, sending the evaporation temperature to the heat pump booster unit. Condensation temperature The system sends opening or switching status commands to each valve, along with the optimal enthalpy setting command for the compressor frequency, and reconstructs the system's enthalpy flow path topology.

[0080] Step 9. The heat pump and valves execute commands to adjust the system operating status and establish a new steady-state enthalpy distribution;

[0081] Step 10. Calculate the actual operating efficiency η ex , a If the deviation is not within the preset range when compared with the preset total efficiency threshold, then repeat steps 1-9 to start a new round of enthalpy calculation and temperature comparison optimization cycle to achieve optimal enthalpy closed-loop adaptive control.

[0082] experiment

[0083] To verify the beneficial effects of the present invention, the inventors used the industrial multi-grade waste heat cascade utilization method based on enthalpy maximization of the present invention to transform a waste heat recovery project in a textile printing and dyeing park.

[0084] 1) Project Overview and Waste Heat Enthalpy Resources

[0085]

[0086] Using the method of this invention, exhaust gas from a heat-setting machine at 180-200°C is introduced into a high-temperature heat exchange network, where it exchanges heat with fresh air to produce hot air at 160-180°C, which is then reused in the heat-setting machine, achieving efficient on-site utilization of high enthalpy. Dyeing wastewater at 70-85°C is introduced into a medium-temperature heat exchange network to heat softened water and prepare 75°C process hot water. Rinsing wastewater at 30-45°C is collected in a low-temperature hot pool, and a heat pump is started simultaneously. The evaporation temperature is set to 35°C and the condensation temperature to 70°C, thus controlling the low-temperature wastewater. The heat pump evaporator extracts low enthalpy and directs 70°C high enthalpy hot water into the medium-temperature network makeup branch. After the heat pump heats the water, the wastewater (approximately 25°C) and the low-temperature drainage at the end of the system flow into the low-temperature heat source network for use in factory heating and domestic hot water preheating, thus further utilizing the waste heat enthalpy. The temperature is optimized by rolling the comparison every 5 minutes, and the heat pump enthalpy is increased to boost the load when the dyeing process is paused. When there is no demand for process hot water at night, all the enthalpy produced by the heat pump is used to heat the domestic hot water storage tank.

[0087] Measurements show that the enthalpy recovery rate of high-temperature waste gas exceeds 90%, the overall enthalpy efficiency of the entire system is about 38% higher than before the upgrade, the enthalpy utilization rate of low-temperature wastewater at 30-45℃ is increased from less than 15% to over 88%, saving about 1.6 million cubic meters of natural gas and about 1,950 tons of standard coal annually, with a static investment payback period of about 2.1 years.

Claims

1. A method for the cascade utilization of multi-grade industrial waste heat based on enthalpy maximization, characterized in that, Includes the following steps: Step 1. Periodically collect temperature, flow, and pressure data of all heat sources and the operating status of heat pumps, obtain the real-time load demand temperature of each user side, calculate the specific enthalpy of each fluid and the total efficiency of the system in real time based on the objective function of maximizing enthalpy, and calculate the losses of each heat exchanger, heat pump, valve and pipeline in real time. Step 2. Construct a heat source and load matching combination based on enthalpy-voltage matching constraints, wherein the enthalpy-voltage matching constraints include temperature matching constraints, enthalpy-voltage grade constraints, and voltage loss threshold constraints; Step 3. Calculate the temperature matching degree for each heat source and load combination according to the following formula. , In the formula, The temperature of the i-th heat source is the actual temperature of the working fluid on the heat source side. For the j-th load, the required temperature The ambient reference temperature; Step 4. Generate the enthalpy-density weighting factor for the combination of heat source and load according to the following formula. , In the formula, Let be the ratio of the i-th heat source. This represents the maximum value of the ratio of all heat sources in the system. Step 5. Apply enthalpy-density weighting factors to all heat source and load combinations. Sort the heat source flow rate from smallest to largest to determine the optimal matching sequence and allocate the heat source flow rate sequentially until the load demand is met. Step 6. For unmatched low-temperature heat sources, determine the enthalpy-economic viability of heat pump intervention based on the objective function of maximizing enthalpy. If the actual heat pump coefficient of performance... ,in, For condensation temperature, If the evaporation temperature is [value], then start the heat pump and calculate the equivalent heat source parameters and heat pump losses after starting the heat pump. The improved equivalent heat source will be re-submitted for temperature comparison; if the conditions are not met, the heat pump will not be started and the low-temperature heat source will be directly discharged into the low-temperature heat source network. Step 7. Solve for the optimal control strategy under the constraints of equipment loss, pipeline pressure limit, temperature limit, and heat pump performance boundary, with the objective function of maximizing the global total enthalpy recovery and minimizing the total loss, and then verify the expected efficiency. Step 8. Decouple the optimal control strategy obtained in Step 7 into specific instructions and issue them, sending the evaporation temperature to the heat pump. Condensation temperature The system sends opening or switching status commands to each valve, along with the optimal enthalpy setting command for the compressor frequency, and reconstructs the system's enthalpy flow path topology. Step 9. The heat pump and valves execute commands to adjust the system operating status and establish a new steady-state enthalpy distribution; Step 10. Calculate the actual overall operating efficiency. If the deviation is not within the preset range when compared with the preset total efficiency threshold, repeat steps 1-9 to start a new round of enthalpy calculation and temperature comparison optimization cycle to achieve optimal enthalpy closed-loop adaptive control.

2. The method for cascade utilization of industrial multi-grade waste heat based on enthalpy maximization according to claim 1, characterized in that, The objective function that maximizes enthalpy for: In the formula, For the i-th user-side enthalpy, Total losses for equipment, piping, and heat exchangers; The specific enthalpy values ​​of each fluid are: In the formula, For comparison of enthalpy, The specific enthalpy of the working fluid in its working state. The specific enthalpy of the working fluid is the reference state. The specific entropy of the working fluid in its working state. The specific entropy of the working fluid environment is the reference state. The ambient reference temperature; The overall efficiency of the system is: In the formula, For the overall system efficiency, To recover the total enthalpy of the system, The total enthalpy available to the system; The heat exchanger loss is: In the formula, For heat exchanger damage, For the heat exchanger hot side input, For the cold side input of the heat exchanger, For the hot side outflow of the heat exchanger, Cold side outflow from heat exchanger; The heat pump loss is: In the formula, For heat pump losses, The flow rate input to the evaporator side of the heat pump. For the heat pump input power, The outflow rate from the evaporator side of the heat pump; The wear and tear on the valves and pipelines is as follows: In the formula, For valve and pipeline damage, This refers to the flow rate through the valves and pipelines. This refers to the temperature drop through the valves and pipelines. This refers to the time it takes for the flow to pass through the valve and pipeline.

3. The method for cascade utilization of industrial multi-grade waste heat based on enthalpy maximization according to claim 1, characterized in that, In step 2, the temperature alignment constraint is as follows: , Let k be the temperature of the heat source. For the temperature requirement of the j-th user, Minimum heat exchange temperature difference; Enthalpy grade constraint is: , Let be the specific enthalpy of the temperature required by the j-th user.

4. The method for cascade utilization of industrial multi-grade waste heat based on enthalpy maximization according to claim 3, characterized in that, The minimum heat exchange temperature difference The temperature is 1 to 3℃.

5. The method for cascade utilization of industrial multi-grade waste heat based on enthalpy maximization according to claim 1, characterized in that, The industrial multi-grade waste heat includes high-temperature heat sources with a temperature ≥80℃, medium-temperature heat sources with a temperature of 50℃~80℃, and low-temperature heat sources with a temperature <50℃.

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