Cascaded phase change heat storage type solar drying system and control method

Abstract

The invention discloses a cascade phase change heat storage type solar drying system and a control method, and belongs to the technical field of renewable energy source development and utilization and grain drying, the cascade phase change heat storage type solar drying system comprises a phase change heat storage system and a solar drying system, and the phase change heat storage system comprises a flat plate collector and a cascade phase change heat storage device; the solar drying system comprises a drying box and a vacuum tube heat collector, and the drying box is connected with the heat exchanger. The phase change heat storage module is added in the solar drying box system, so that the drying system obtains the capability of continuously performing drying work in cloudy and rainy days or at night, then the model prediction control unit is used for accurately predicting and controlling the heat storage and drying process, and the heat storage efficiency is improved while the operation stability of the drying system is enhanced; the MPC collects, analyzes and detects dried material data, summarizes the optimal drying time of different materials and controls the drying process of the materials, the controllability of the drying process is enhanced, and the quality of the dried materials is improved.

Description

Technical Field

[0001] The present invention relates to the technical fields of renewable energy development and utilization and grain drying technology, and particularly relates to a cascaded phase change thermal energy storage solar drying system and a control method thereof. Background Art

[0002] As a clean and abundant energy source, solar energy has a wide range of application fields. Compared with other clean energies, solar thermal utilization has higher energy conversion efficiency and lower cost, which is of great significance for achieving the dual carbon goal. Solar drying is one of the applications of solar thermal utilization and has been applied to the drying of various agricultural crops. Solar drying can not only reduce the consumption of traditional energy and carbon emissions, but also improve the lifespan and quality of agricultural products at a lower cost. Therefore, solar drying technology is of great significance for energy conservation and emission reduction and agricultural development. From the perspective of the drying conditions of agricultural crops, most agricultural crops require a drying temperature between 45-75 °C, which conforms to the law of solar energy utilization.

[0003] The utilization of solar energy is restricted by time and space. Traditional solar drying systems rely on direct solar heating and are greatly affected by weather and seasons, resulting in unstable drying efficiency. Phase change thermal energy storage technology utilizes the high energy density and stable heat output characteristics of phase change materials to effectively solve the intermittent problem of solar energy and achieve efficient energy storage and stable heat supply.

[0004] Model Predictive Control (MPC) is an advanced control strategy based on the system model. Its core idea is to use the dynamic model of the system to predict the system behavior in the future for a period of time and achieve the desired control goal by online optimizing the control input. MPC realizes the precise control and optimized management of complex systems by real-time monitoring the system state, predicting future behavior, and adjusting the control input according to the optimization results.

[0005] Existing solar drying devices cannot achieve precise control of the drying degree of agricultural crops and timely regulation of the drying process, and there are problems such as insufficient drying or excessive drying resulting in too high temperature damaging the materials. The present invention realizes all-weather drying operation of the system and rapid response precise control of the drying process by adding a cascaded phase change thermal energy storage device and an MPC control unit to the solar drying box system. Summary of the Invention

[0006] In order to solve the above deficiencies in the prior art, the purpose of the present invention is to provide a cascaded phase change thermal energy storage solar drying system and a control method thereof, which realize all-weather drying operation of the system and rapid response precise control of the drying process by adding a cascaded phase change thermal energy storage device and an MPC control unit to the solar drying box system.

[0007] The technical solution adopted by the present invention to solve its technical problems is as follows:

[0008] A cascaded phase change heat storage solar drying system is provided, including:

[0009] A phase change heat storage system, which includes a flat plate collector and a cascaded phase change heat storage device;

[0010] A heat exchanger, which is connected to the cascaded phase change heat storage device, and the flat plate collector - cascaded phase change heat storage device - heat exchanger - circulation pump - flat plate collector are connected in series in sequence to form a heat exchange fluid circulation pipeline;

[0011] A first confluence valve located between the flat plate collector and the cascaded phase change heat storage device, and a first diversion valve located between the circulation pump and the flat plate collector are provided in the heat exchange circulation pipeline. The first confluence valve is connected to the first diversion valve, separating the heat exchange circulation pipeline into a closed - loop heat exchange pipeline composed of the cascaded phase change heat storage device, the heat exchanger and the circulation pump;

[0012] A solar drying system, which includes a drying box and a vacuum tube collector, and the drying box is connected to the heat exchanger; the heat exchanger - vacuum tube collector - fan are connected in sequence to form a drying air pipeline;

[0013] A second confluence valve located between the heat exchanger and the drying box, and a second diversion valve located between the heat exchanger and the vacuum tube collector are provided in the drying air pipeline. The second confluence valve is connected to the second diversion valve, separating the drying air pipeline into a solar drying pipeline composed of the fan, the vacuum tube collector and the drying box;

[0014] A control module, which is used to regulate the heat storage and heat release processes of the phase change heat storage system and the drying process of the solar drying system according to the light intensity and drying demand.

[0015] Furthermore, the flat plate collector absorbs solar heat and stores it in the phase change material of the cascaded phase change heat storage device. The phase change materials in the cascaded phase change heat storage device are arranged in series along the flow direction of heat transfer, and the phase change materials are arranged in descending order of melting point.

[0016] A control method for a cascaded phase change heat storage solar drying system. The control module includes a model predictive control unit, which monitors the light intensity, the temperatures of each stage of phase change materials, the temperature and humidity of the drying box, and the moisture content of the material in real time through sensors, and performs rolling optimization and feedback correction based on a heat storage - drying combined model and embedded constraint conditions to generate an optimal control instruction;

[0017] The model predictive control unit predicts the heat load demand and drying process in the future period, optimizes the charging / discharging rate of the phase change heat storage system and the drying parameters of the solar drying system, and dynamically adjusts the operating states of the circulation pump, fan, shunt valve and confluence valve; the circulation pump is a variable frequency pump, which is used to adjust the total flow rate of the heat exchange fluid and change the heat storage / discharge rate;

[0018] When the outlet temperature of the vacuum tube collector is lower than 40 °C, switch to the heat storage auxiliary mode and supplement heat through the heat exchanger;

[0019] When the light intensity is lower than 300 W / m 2 , switch the phase change heat storage system to the heat release mode and adjust the pipeline circulation to reduce heat loss;

[0020] The construction of the heat storage-drying combined model includes:

[0021] The phase change heat storage subsystem model, based on the thermodynamic equation of the three-stage phase change material;

[0022] The drying subsystem model, combined with the non-linear equation of heat and mass transfer;

[0023] The coupling term model, connecting the energy transfer of the heat storage and drying subsystems through the heat medium flow.

[0024] Furthermore, use the lumped parameter equation method to list the phase change heat storage subsystem model:

[0025]

[0026] Among them, is the mass flow rate of the heat medium, c h is the specific heat capacity of the heat medium, U is the inter-stage heat transfer coefficient, A is the heat transfer area, T pcm is the temperature of the phase change material, T in,i is the inlet temperature of the i-th stage, T out,i is the outlet temperature of the i-th stage, C p,i is the equivalent specific heat capacity of the i-th stage PCM, m pcm,i is the mass of the i-th stage PCM.

[0027] Specifically, in the cascaded phase change heat storage device, the inter-stage heat transfer of the three-stage phase change material module is connected in series through the heat medium flow, and the heat storage / discharge process of each stage of PCM is modeled by the equivalent heat capacity method and satisfies the following thermodynamic model:

[0028]

[0029] Among them, is the heat flux density input by the heat medium, is the system heat loss, is the equivalent heat capacity, C pis the equivalent specific heat capacity of PCM, ρ is the material density, and f is the liquid fraction;

[0030] The liquid fraction model uses an error function approximation:

[0031]

[0032] where T m is the phase change center temperature and ΔT is the phase change temperature range;

[0033] Establish the inter-stage heat transfer equation:

[0034]

[0035] where T h is the heat medium temperature.

[0036] Furthermore, the drying subsystem model is:

[0037]

[0038] where k is the mass transfer coefficient, which is related to the drying conditions, and the formula is h m is the mass transfer coefficient, A is the material surface area, ρ is the material density, V is the material volume, M eq is the equilibrium moisture content, which is related to the temperature and humidity of the current drying oven. The empirical formula is M eq = aT dry + bRH + c, where a, b, and c are experimental fitting coefficients and RH is the relative humidity.

[0039] Furthermore, the drying subsystem model combines the non-linear characteristics of heat and mass transfer in the drying oven to establish a heat transfer - mass transfer coupling equation:

[0040]

[0041] M is the material moisture content, v air is the drying air velocity, T in is the system inlet temperature, and RH amb is the environmental relative humidity;

[0042] The combined coupling term heat medium serves as an energy carrier to connect the heat storage and drying systems:

[0043]

[0044] T in,dry is the drying system inlet temperature, which is determined by the temperature of the phase change material at the last stage of the heat storage system, and is the real-time heat load of the drying system;

[0045] The system state space equation is:

[0046]

[0047] Among them,

[0048] state variable

[0049] control input u = [valve opening, pump speed, compensation heating power] T ;

[0050] The disturbance d is the environmental temperature and heat source fluctuation.

[0051] Furthermore, the continuous model is discretized using the Euler method and future prediction is carried out:

[0052] x(k + 1) = x(k) + Δt·f(x(k), u(k))

[0053] The discrete time step Δt = 10 s, {x(k + 1|k), …, x(k + Np|k)}, and the prediction horizon Np = 20 steps (200 seconds);

[0054] The model predictive control objective function of the phase change heat storage system:

[0055]

[0056] Tracking term α: Ensure that the temperature of the three - stage phase change material tracks the set value;

[0057] Control cost term β: Minimize the valve regulation and pump energy consumption;

[0058] Energy storage optimization term γ: Maintain the energy state of the heat storage system at 80% to cope with sudden demands.

[0059] Furthermore, the constraint objectives and constraints are:

[0060]

[0061] 0 ≤ valve opening ≤ 100%

[0062] 40°C ≤ T dry ≤ 80°C

[0063] By restricting the valve opening, ensure that the heat medium flow rate is within a certain range, and optimize the phase change heat storage process according to the feedback of the PCM temperature sensor data; during the auxiliary heating process of the cascaded phase change heat storage system, ensure that the inlet air temperature is within the optimal drying temperature of the material, so as to achieve the purpose of heat distribution and drying process control.

[0064] Furthermore, for rolling optimization, the Kalman filter is used to fuse the sensor data to estimate the current state Based on the model to predict the future Np The system states of the steps x(k+1|k),…,x(k+N p |k), the sequential quadratic programming is used to solve the constrained optimization problem, and the optimal control sequence u * (k),…,u * (k+M) is obtained. Only the first-step control input u*(k) is implemented, and re-sampling and repeated optimization are performed in the next cycle.

[0065] Furthermore, the model predictive control unit updates the optimization instruction every 5 minutes and corrects the model parameters according to the difference between the actual output and the predicted value.

[0066] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0067] 1. For the cascaded phase change heat storage solar drying system according to the example of the present invention, the phase change heat storage system can provide auxiliary heating when solar energy is insufficient. Combined with the precise control of MPC, the temperature in the drying chamber can be maintained within the set range, avoiding the influence of temperature fluctuations on the drying effect, and enabling the drying system to obtain the ability to continuously carry out drying work on rainy days or at night, enhancing the stability of the drying system operation;

[0068] 2. For the cascaded phase change heat storage solar drying system according to the example of the present invention, the high-grade heat energy stored in the high-temperature stage PCM in the cascaded phase change heat storage system can cope with peak heat demands and can flexibly meet the different drying degree requirements of dried materials;

[0069] 3. For the cascaded phase change heat storage solar drying system according to the example of the present invention, according to the detected light heat absorbed by the local collector and the humidity and temperature of the materials in the drying chamber, the controller is used to call each subsystem and control the start-stop and opening degrees of the shunt valve, confluence valve, circulation pump and fan, ensuring the maximum storage efficiency of the heat energy of the heat storage system and the precise control of the drying degree of the materials, and improving the energy utilization rate of the system;

[0070] 4. For the cascaded phase change heat storage solar drying system according to the example of the present invention, combined with the system model and real-time data, MPC can timely detect abnormal situations in the system, such as sensor failures, pipeline leaks, etc., and give early warnings and corresponding treatments to ensure the safe and stable operation of the system;

[0071] 5. The cascaded phase change thermal energy storage solar drying system of the present invention example can be monitored and controlled through the MPC control unit. It can quickly respond to the data fed back by the sensors, collect and analyze the detection data, summarize the optimal drying time of different materials and apply it, providing an accurately controllable solution for the drying process of high-value materials (such as medicines, high-end foods, etc.). The intelligent regulation also improves the comprehensive performance of the complex thermal engineering system, reduces the dependence on manual labor, and reduces the human resource input, which can save a large amount of labor costs in the long-term operation and facilitate the large-scale use and market promotion of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] Other features, objects, and advantages of the present application will become more apparent by reading the detailed description of the non-limiting embodiments with reference to the following drawings:

[0073] Figure 1 It is a schematic structural diagram of the present invention;

[0074] Figure 2 It is a flowchart of the control method.

[0075] In the figure: 1, flat plate collector; 2, cascaded phase change thermal energy storage device; 3, circulation pump; 4, drying box; 5, vacuum tube collector; 6, fan; 7, heat exchanger; 8, first confluence valve; 9, first diversion valve; 10, second confluence valve; 11, second diversion valve. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0076] The present application will be further described in detail below with reference to the drawings and embodiments. It can be understood that the specific embodiments described herein are only used to explain the related invention and do not limit the invention. Additionally, it should be noted that for the sake of description, only the parts related to the invention are shown in the drawings.

[0077] It should be noted that, without conflict, the embodiments in the present application and the features in the embodiments can be combined with each other. The present application will be described in detail below with reference to the drawings and embodiments.

[0078] Such as Figure 1As shown in the figure, this embodiment provides a cascaded phase change thermal energy storage solar drying system, including a phase change thermal energy storage system, a heat exchanger 7, a solar drying system and a control module. The phase change thermal energy storage system includes a flat plate collector 1 and a cascaded phase change thermal energy storage device 2. The flat plate collector 1 absorbs solar heat, and the cascaded phase change thermal energy storage device 2 can store the absorbed heat in the phase change material PCM. The heat exchanger 7 is connected to the cascaded phase change thermal energy storage device 2. The flat plate collector 1 - cascaded phase change thermal energy storage device 2 - heat exchanger 7 - circulation pump 3 - flat plate collector 1 are connected in series in sequence to form a heat exchange fluid circulation pipeline. The solar drying system includes a drying box 4 and a vacuum tube collector 5. The drying box 4 is connected to the heat exchanger 7. The heat exchanger 7 - vacuum tube collector 5 - fan 6 are connected in sequence to form a drying air pipeline.

[0079] In this embodiment, a first confluence valve 8 located between the flat plate collector 1 and the cascaded phase change thermal energy storage device 2, and a first diversion valve 9 located between the circulation pump 3 and the flat plate collector 1 are provided in the heat exchange circulation pipeline. The first confluence valve 8 is connected to the first diversion valve 9, separating the heat exchange circulation pipeline into a closed circulation heat exchange pipeline composed of the cascaded phase change thermal energy storage device 2, the heat exchanger 7 and the circulation pump 3. The phase change thermal energy storage system has two working modes according to the different pipelines switched by the first confluence valve 8 and the first diversion valve 9. The closed circulation heat exchange pipeline isolates the flat plate collector 1 to reduce the heat loss of the flat plate collector 1 caused by meteorological factors.

[0080] The flat plate collector 1 absorbs solar heat and stores it in the phase change material of the cascaded phase change thermal energy storage device 2. The phase change materials in the cascaded phase change thermal energy storage device 2 are arranged in series along the flow direction of heat transfer. The phase change materials are arranged in descending order of melting point, providing stable heat conduction conditions for the subsequent low melting point phase change materials and shortening the complete melting time of the phase change materials. The phase change thermal energy storage system conducts system water circulation through the circulation pump 3. In this embodiment, the phase change materials are divided into three levels, namely high-temperature level PCM, medium-temperature level PCM and low-temperature level PCM.

[0081] When the sunlight is sufficient, the phase change thermal energy storage system is in the heat storage mode. The flat plate collector 1 absorbs solar energy, heats the heat exchange fluid, which flows through the high-temperature level PCM, medium-temperature level PCM and low-temperature level PCM in sequence, storing the heat in stages. The frequency conversion circulation pump 3 adjusts the flow rate of the heat transfer fluid. If the drying system has a drying demand, the heat exchange fluid is transported through the heat exchanger 7 to the drying box 4 through the diversion valve.

[0082] When the sunlight is insufficient, the phase change thermal energy storage system is in the heat release mode. The confluence valve switches the pipeline, and the phase change thermal energy storage system releases heat to heat the drying air through the heat exchanger 7.

[0083] The main heat source of the system is the solar collector, and there is no other auxiliary heat source. When the light intensity is weak, there is heat loss from the heat machine to the external environment. Therefore, there are control methods for the phase change heat storage system under different lighting conditions. When the light intensity is weak, the first shunt valve 9 and the first confluence valve 8 are controlled to stop the water circulation in the pipeline on the side of the flat plate collector 1 to reduce heat loss.

[0084] Specifically, it is judged whether the light intensity is less than 300 W / m 2 , if so, the first shunt valve 9 and the first confluence valve 8 are controlled to make the heat exchange fluid circulation pipeline of the phase change heat storage system avoid the flat plate collector 1.

[0085] A second confluence valve 10 located between the heat exchanger 7 and the drying oven 4 and a second shunt valve 11 located between the heat exchanger 7 and the vacuum tube collector 5 are provided in the dry air pipeline. The second confluence valve 10 and the second shunt valve 11 are connected to divide the dry air pipeline into a solar drying pipeline composed of the blower 6, the vacuum tube collector 5 and the drying oven 4.

[0086] The solar drying system has two working modes:

[0087] In the solar direct supply mode, the vacuum tube collector 5 heats the air, and the blower 6 sends the hot air into the drying oven 4;

[0088] In the heat storage auxiliary mode, when the outlet temperature of the vacuum tube collector 5 is insufficient, the air absorbs the heat of the heat storage system through the heat exchanger 7 to achieve continuous drying;

[0089] After the vacuum tube collector 5 absorbs solar heat, it blows air through the blower 6 to provide dry air for the drying oven 4 to dry the materials. The second shunt valve 11 and the second confluence valve 10 are used for pipeline switching of the dry air in the solar drying system. There are control methods for the solar drying system under different meteorological and drying demand conditions.

[0090] Specifically, it is judged whether the outlet temperature of the vacuum tube collector 5 is less than 40 °C. If so, the second shunt valve 11 and the second confluence valve 10 are controlled to make the solar drying system obtain supplementary heat through the heat exchanger 7.

[0091] As Figure 2 shown, a model predictive control unit is added to control the drying process of the materials and the heat storage / discharge process of the phase change heat storage system.

[0092] Sensor data acquisition, collect data such as light intensity, PCM temperature at all levels, temperature / humidity of the drying oven, moisture content of the material, etc., and transmit them to the model predictive control unit. Use Kalman filtering for state estimation, and predict future light intensity and drying demand. Based on the combined model of heat storage and drying, predict the system behavior in the next period. According to the objective function and constraints, solve the optimal control input, such as valve opening, pump speed, and fan speed, and send the optimal control instructions to the actuators, namely valves, variable frequency pumps, and fans. Adjust the model parameters according to the difference between the actual output and the predicted value, and perform the control of the next cycle.

[0093] Specifically, sensor data acquisition serves as the input signal of the control system, including the solar irradiance monitored in real time by a light intensity sensor, the temperature data of high-temperature, medium-temperature, and low-temperature PCM monitored by cascade phase change heat storage material temperature sensors, and the multi-region distributed temperature / humidity sensors and material moisture content sensors in the drying oven to feedback the drying degree of the material in real time. Convert the physical signal into a digital signal and upload it to the model predictive control unit. The model predictive control unit predicts the heat load demand and drying process in the future period, and optimizes the charging / discharging rate of the phase change heat storage system and the drying parameters of the solar drying system, and dynamically adjusts the operating states of the circulating pump, fan, shunt valve, and confluence valve; the circulating pump is a variable frequency pump, which is used to adjust the total flow rate of the heat exchange fluid and change the heat storage / release speed.

[0094] When the temperature at the outlet of the vacuum tube collector is lower than 40 °C, switch to the heat storage auxiliary mode and supplement heat through the heat exchanger;

[0095] When the light intensity is lower than 300 W / m 2 Switch the phase change heat storage system to the heat release mode and adjust the pipeline circulation to reduce heat loss.

[0096] The construction of the combined heat storage-drying model includes:

[0097] Phase change heat storage subsystem model, based on the thermodynamic equation of three-stage phase change materials;

[0098] Drying subsystem model, combined with the non-linear equation of heat and mass transfer;

[0099] Coupling term model, connecting the energy transfer of heat storage and drying subsystems through the flow of heat medium.

[0100] Use the lumped parameter equation method to list the phase change heat storage subsystem model:

[0101]

[0102] Among them, is the mass flow rate of the heat medium, c h is the specific heat capacity of the heat medium, U is the inter-stage heat transfer coefficient, A is the heat transfer area, Tpcm is the phase change material temperature, T in,i is the inlet temperature of the i-th stage, T out,i is the outlet temperature of the i-th stage, C p,i is the equivalent specific heat capacity of the PCM at the i-th stage, m pcm,i is the mass of the PCM at the i-th stage.

[0103] In the cascaded phase change heat storage device, the heat transfer between the three-stage phase change material modules is in series through the flow of the heat medium. The heat storage / release process of each stage of PCM is modeled by the equivalent heat capacity method and satisfies the following thermodynamic model:

[0104]

[0105] where is the heat flux density input by the heat medium, is the system heat loss, is the equivalent heat capacity, C p is the equivalent specific heat capacity of the PCM, ρ is the material density, and f is the liquid fraction.

[0106] The liquid fraction model is approximated by the error function:

[0107]

[0108] where, T m is the phase change center temperature, and ΔT is the phase change temperature range.

[0109] Establish the heat transfer equation between stages:

[0110]

[0111] where, T h is the heat medium temperature.

[0112] Establish a combined heat storage-drying model. The combined model unifies the dynamic processes of the heat storage system and the drying system in the same state space framework, and realizes the physical coupling between subsystems through the flow of the heat medium and energy transfer, including:

[0113] Heat storage subsystem model, establishing a hierarchical energy conservation equation based on three-stage phase change materials:

[0114]

[0115] T pcm1-3 is the temperature of the three-stage phase change material, is the heat medium flow rate, T amb is the ambient temperature, f i is the liquid fraction of the phase change state of the phase change material.

[0116] In this embodiment, simplify the mass transfer equation for the drying subsystem model:

[0117]

[0118] Among them, k is the mass transfer coefficient, which is related to the drying conditions, and the formula is h m is the mass transfer coefficient, A is the surface area of the material, ρ is the density of the material, V is the volume of the material, M eq is the equilibrium moisture content, which is related to the temperature and humidity of the current drying oven. The empirical formula M eq = aT dry + bRH + c, where a, b, and c are experimental fitting coefficients, and RH is the relative humidity.

[0119] The drying subsystem model combines the nonlinear characteristics of heat and mass transfer in the drying oven to establish a heat transfer - mass transfer coupling equation:

[0120]

[0121] M is the moisture content of the material, v air is the drying air velocity, T in is the system inlet temperature, RH amb is the environmental relative humidity.

[0122] The combined coupling term heat medium serves as an energy carrier to connect the heat storage and drying systems:

[0123]

[0124] T in,dry is the drying system inlet temperature, that is, determined by the temperature of the phase change material at the end of the heat storage system, is the real - time heat load of the drying system, g represents the functional relationship, indicating that the drying system inlet temperature is a function jointly determined by the temperature of the phase change material at the end of the heat storage system and the mass flow rate of the heat medium.

[0125] The system state - space equation is:

[0126]

[0127] Among them,

[0128] State variables

[0129] Control input u = [valve opening, pump speed, compensation heating power] T ;

[0130] The disturbance d is the environmental temperature and heat source fluctuation.

[0131] Use the Euler method to discretize the continuous model and make future predictions:

[0132] x(k + 1) = x(k) + Δt·f(x(k), u(k))

[0133] The discrete time step Δt = 10 s, {x(k + 1|k), …, x(k + Np|k)}, and the prediction horizon Np = 20 steps (200 seconds).

[0134] The model predictive control objective function of the phase change heat storage system is as follows:

[0135]

[0136] Tracking term α: Ensure that the temperature of the three - stage phase change material tracks the set value;

[0137] Control cost term β: Minimize the valve regulation and pump energy consumption;

[0138] Energy storage optimization term γ: Maintain the energy state of the heat storage system at 80% to cope with sudden demands.

[0139] The constraint objectives and constraints are as follows:

[0140]

[0141] 0 ≤ valve opening ≤ 100%

[0142] 40°C ≤ T dry ≤ 80°C

[0143] By restricting the valve opening, ensure that the heat medium flow rate is within a certain range, and optimize the phase change heat storage process according to the feedback of the PCM temperature sensor data; during the auxiliary heating process of the cascaded phase change heat storage system, ensure that the inlet air temperature is within the optimal drying temperature of the material to achieve the purpose of heat distribution and drying process control.

[0144] The rolling optimization uses a Kalman filter to fuse sensor data and estimate the current state Based on the model, predict the system states x(k + 1|k), …, x(k + N p |k) in the next N steps, and use sequential quadratic programming to solve the constrained optimization problem to obtain the optimal control sequence u p (k), …, u * (k + M), and only implement the first - step control input u*(k), and re - sample and repeat the optimization in the next cycle. * (k + M), and only implement the first - step control input u*(k), and re - sample and repeat the optimization in the next cycle.

[0145] In the feedback correction and model update of the MPC control unit, calculate the SOC deviation every 5 minutes, update the equivalent heat capacity of the PCM, and after drying, correct the mass transfer coefficient according to the final moisture content error.

[0146] Compared with the traditional independent heat storage modeling and drying model, this embodiment realizes the dynamic coupling of heat storage and drying at the system model level, establishes the phase change process of the three-stage PCM for heat storage optimization research, optimizes the heat medium distribution strategy by predicting the liquid phase rate of each stage of PCM, and jointly solves the optimal heat storage process and drying process by taking multiple state variables such as temperature, humidity, and moisture content. Based on the cascade phase change heat storage type solar drying system and control method, the system can switch corresponding working modes under different meteorological conditions by coupling the cascade phase change heat storage subsystem, and stably and highly quality dry the materials through the MPC control unit. This drying method can meet the drying needs of most materials, especially high-value materials that require precise control of the drying degree, such as medicines, high-end foods, etc.

[0147] Those skilled in the art should understand that the scope of the invention involved in this application is not limited to the technical solutions formed by the specific combination of the above technical features, and should also cover other technical solutions formed by any combination of the above technical features or their equivalent features without departing from the inventive concept. For example, the technical solutions formed by mutually replacing the above features with the technical features (but not limited to) disclosed in this application that have similar functions.

Claims

1. A cascade phase change heat storage solar drying system, characterized in that: include: A phase change heat storage system, comprising a flat plate collector (1) and a cascade phase change heat storage device (2); A heat exchanger (7) connected to the cascade phase change heat storage device (2), wherein the flat plate heat collector (1) - cascade phase change heat storage device (2) - heat exchanger (7) - circulation pump (3) - flat plate heat collector (1) are sequentially connected in series to form a heat exchange fluid circulation pipeline; The heat exchange circulation pipeline is provided with a first converging valve (8) located between the flat plate heat collector (1) and the cascade phase change heat storage device (2), and a first diverting valve (9) located between the circulation pump (3) and the flat plate heat collector (1); the first converging valve (8) is connected to the first diverting valve (9) to divide the heat exchange circulation pipeline into a closed circulation heat exchange pipeline consisting of the cascade phase change heat storage device (2), the heat exchanger (7) and the circulation pump (3); A solar drying system comprises a drying box (4) and a vacuum tube collector (5), wherein the drying box (4) is connected to a heat exchanger (7); the heat exchanger (7) - the vacuum tube collector (5) - the fan (6) are connected in sequence to form a drying air pipeline; The drying air pipeline is provided with a second merging valve (10) located between the heat exchanger (7) and the drying box (4), and a second diverting valve (11) located between the heat exchanger (7) and the vacuum tube collector (5); the second merging valve (10) is connected to the second diverting valve (11) to divide the drying air pipeline into a solar drying pipeline consisting of the fan (6), the vacuum tube collector (5) and the drying box (4); The control module is used to regulate the heat storage and heat release process of the phase change heat storage system and the drying process of the solar drying system according to the light intensity and drying requirements.

2. A cascade phase change thermal storage solar drying system according to claim 1, characterized in that: The flat-plate collector (1) absorbs solar heat and stores it in the phase change material of the cascade phase change heat storage device (2). The phase change materials in the cascade phase change heat storage device (2) are arranged in series along the flow direction of the heat transfer direction, and the phase change materials are arranged from high to low according to melting points.

3. The control method of the cascade phase change thermal storage solar drying system according to any one of claims 1 to 2, characterized in that: The control module includes a model prediction control unit, which monitors the light intensity, the temperature of each phase change material, the temperature and humidity of the drying box and the moisture content of the material in real time through sensors, performs rolling optimization and feedback correction based on the heat storage-drying joint model and embeds constraints to generate optimal control instructions; The model prediction control unit predicts the heat load demand and drying process in the future period, optimizes the charging / discharging rate of the phase change heat storage system and the drying parameters of the solar drying system, and dynamically adjusts the operating status of the circulation pump, the fan, the diverter valve and the converging valve; the circulation pump is a variable frequency pump, which is used to adjust the total flow rate of the heat exchange fluid and change the storage / release rate of heat; When the outlet temperature of the vacuum tube collector is lower than 40°C, it switches to the heat storage auxiliary mode and supplements heat through the heat exchanger; When the light intensity is lower than 300W / m 2 When the heat is released, the phase change heat storage system is switched to the heat release mode, and the pipeline circulation is adjusted to reduce heat loss; The construction of the heat storage-drying joint model includes: Phase change heat storage subsystem model, based on the thermodynamic equations of three-level phase change materials; Drying subsystem model, incorporating nonlinear equations for heat and mass transfer; The coupled model connects the energy transfer of the heat storage and drying subsystems through the flow of heat medium.

4. The control method according to claim 3, characterized in that: The phase change heat storage subsystem model is listed using the lumped parameter equation method: in, is the heat medium mass flow rate, c h is the specific heat capacity of the heat medium, U is the interstage heat transfer coefficient, A is the heat exchange area, T pcm is the phase change material temperature, T in,i is the inlet temperature of the i-th stage, T out,i is the outlet temperature of the i-th stage, C p,i is the equivalent specific heat capacity of the i-th PCM, m pcm,i is the quality of the i-th level PCM.

5. The control method according to claim 4, characterized in that: The drying subsystem model is: Among them, k is the mass transfer coefficient, which is related to the drying conditions and the formula is h m is the mass transfer coefficient, A is the surface area of ​​the material, ρ is the density of the material, V is the volume of the material, M eq To balance the moisture content, it is related to the current temperature and humidity of the drying oven. The empirical formula M eq =aT dry +bRH+c, where a, b, c are experimental fitting coefficients and RH is relative humidity.

6. The control method according to claim 5, characterized in that: The drying subsystem model combines the nonlinear characteristics of heat and mass transfer in the drying box to establish the heat transfer-mass transfer coupling equation: M is the moisture content of the material, v air is the drying air flow rate, T in is the system inlet temperature, RH amb is the relative humidity of the environment; The combined coupling term heat medium is used as an energy carrier to connect the heat storage and drying systems: T in,dry is the inlet temperature of the drying system, which is determined by the temperature of the final phase change material of the heat storage system. is the real-time heat load of the drying system, and g represents the functional relationship, indicating that the inlet temperature of the drying system is a function determined by the temperature of the final phase change material of the heat storage system and the mass flow rate of the heat medium; The state space equation of the system is: in, State variables Control input u = [valve opening, pump speed, compensation heating power] T ; The disturbance d is the fluctuation of ambient temperature and heat source.

7. The control method according to claim 6, characterized in that: Use Euler's method to discretize the continuous model and make future forecasts: x(k+1)=x(k)+Δt·f(x(k),u(k)) Discrete time step Δt = 10s, {x(k+1|k),…,x(k+Np|k)}, prediction time domain Np = 20 steps (200 seconds); Phase change heat storage system model predictive control objective function: Tracking item α: ensures that the temperature of the three-stage phase change material tracks the set value; Control cost term β: Minimize valve adjustment and pump energy consumption; Energy storage optimization item γ: Maintain the energy state of the thermal storage system at 80% to cope with sudden demand.

8. The control method according to claim 7, characterized in that: The constraint objectives and constraints are: 0≤Valve opening≤100% 40℃≤T dry ≤80℃ By limiting the valve opening, the heat medium flow is ensured to be within a certain range, and the phase change heat storage process is optimized according to the PCM temperature sensor data feedback; in the process of calling the cascade phase change heat storage system for auxiliary heating, the inlet air temperature is ensured to be within the optimal drying temperature of the material, so as to achieve the purpose of heat distribution and drying process control.

9. The control method according to claim 8, characterized in that: Rolling optimization uses Kalman filter to fuse sensor data and estimate the current state Predicting the future N based on the model p The system state of the step x(k+1|k),…,x(k+N p |k), sequential quadratic programming is used to solve the constrained optimization problem and obtain the optimal control sequence u * (k),…,u * (k+M), only the first step of control input u*(k) is implemented, and the next cycle is resampled and the optimization is repeated.

10. The control method according to claim 3, characterized in that: The model prediction control unit updates the optimization instruction every 5 minutes and corrects the model parameters according to the difference between the actual output and the predicted value.

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