Photovoltaic photo-thermal hydrogen production and energy storage system based on nanofluid and phase change material

Abstract

The invention discloses a photovoltaic photo-thermal hydrogen production and energy storage system based on nanofluid and a phase change material, and relates to the technical field of photovoltaic photo-thermal hydrogen production and energy storage, the system comprises a photovoltaic photo-thermal assembly, a water tank, a solar heat collector and a PEM electrolytic bath; the photovoltaic photo-thermal assembly comprises a photovoltaic cell, a first thermoelectric generator, a nanofluid channel and a composite phase change material which are arranged from top to bottom. The solar heat collector comprises a second thermoelectric generator, a water fluid channel and a composite phase change material which are arranged from top to bottom; mixed nanofluid in the nanofluid channel enters a heat exchange coil in the water tank for heat exchange; water in the water tank firstly enters a water fluid channel of the solar heat collector to be preheated and then enters the PEM electrolytic bath; the mixed nanofluid is composed of BN, MWCNT nanoparticles and pure water; the composite phase change material is composed of BN, MWCNT nanoparticles and paraffin. Efficient output of heat energy, electric energy and hydrogen energy can be achieved.

Description

TECHNICAL FIELD

[0001] The application relates to the field of photovoltaic photo-thermal hydrogen production and energy storage, and in particular to a photovoltaic photo-thermal hydrogen production and energy storage system based on nanofluids and phase change materials. BACKGROUND

[0002] Photovoltaic power generation is intermittent and unstable, and large-scale grid-connected operation has safety risks and causes the phenomenon of "abandoned electricity". Hydrogen energy is clean, zero-carbon emission and high in combustion heat value, and is regarded as one of the most promising energy storage carriers in the future. Using photovoltaic energy to produce hydrogen by electrolysis of water can solve the problem of renewable energy consumption and storage, and realize green and carbon-free hydrogen production, which is a very promising route for hydrogen energy development.

[0003] Compared with other hydrogen production methods such as alkaline electrolysis and solid oxide electrolysis, proton exchange membrane (PEM) electrolysis has the advantages of fast dynamic response, high current density and low working temperature. Therefore, the integration of photovoltaic and PEM electrolysis has considerable development potential.

[0004] The efficiency of photovoltaic modules plays a crucial role in increasing hydrogen production. Photovoltaic modules have high sensitivity to temperature, and the higher the temperature, the lower the electrical efficiency of photovoltaic modules. Photovoltaic / thermal (PV / T) technology integrates photovoltaic modules with heat collection modules to achieve efficient power output and heat collection. The heat generated by photovoltaic modules is usually absorbed by air, water or refrigerant, which helps to maintain a lower working temperature of photovoltaic cells, thereby improving the photoelectric conversion efficiency. The heat absorbed by the working fluid is recovered by the heat collection device, thereby realizing the reuse of thermal energy. Therefore, the integration of PV / T technology and PEM electrolysis technology provides a promising method for maximizing the use of solar energy.

[0005] In view of the above problems, the application provides a photovoltaic photo-thermal hydrogen production and energy storage system based on nanofluids and phase change materials. A mixed nanofluid based on multi-walled carbon nanotubes (MWCNT) and boron nitride (BN) and a composite phase change material are synthesized for the first time. These new materials are combined with a PV / T-PEM coupled hydrogen production system, and through intelligent terminal power regulation, efficient production of electricity, heat and green hydrogen can be realized. In addition, the application also provides a heat management and magnetization hydrogen production scheme for the PEM electrolysis tank, aiming to further improve the energy utilization rate and safety of the system. SUMMARY

[0006] In order to overcome the defects in the prior art, the application provides a photovoltaic photo-thermal hydrogen production and energy storage system based on nanofluids and phase change materials, which can realize efficient production of heat, electricity and hydrogen energy.

[0007] To achieve the above purpose, the application adopts the following technical scheme, comprising: A photovoltaic photo-thermal hydrogen production and energy storage system based on nanofluid and phase change material, comprising a photovoltaic photo-thermal component, a water tank, a solar collector and a PEM electrolyzer; The photovoltaic photo-thermal component comprises, from top to bottom, a photovoltaic cell, a first thermoelectric generator, a nanofluid channel and a composite phase change material; The solar collector comprises, from top to bottom, a second thermoelectric generator, a water fluid channel and a composite phase change material; The mixed nanofluid in the nanofluid channel of the photovoltaic photo-thermal component enters the heat exchange coil in the water tank to heat the water in the water tank; The water in the water tank first enters the water fluid channel of the solar collector to be heated, and then enters the PEM electrolyzer to electrolyze water to produce hydrogen; The mixed nanofluid is used for heat exchange and is composed of BN nanoparticles, MWCNT nanoparticles and pure water; The composite phase change material is used for heat storage and is composed of BN nanoparticles, MWCNT nanoparticles and paraffin.

[0008] Preferably, in the mixed nanofluid, the mass ratio of BN nanoparticles to MWCNT nanoparticles is 1:1, sodium dodecyl benzene sulfonate is added as a surfactant, the use ratio of the surfactant is one-tenth of the mass sum of the BN nanoparticles and the MWCNT nanoparticles, and the mass sum of the BN nanoparticles and the MWCNT nanoparticles accounts for 0.5wt% of the total mass of the mixed nanofluid.

[0009] Preferably, in the composite phase change material, the mass ratio of BN nanoparticles to MWCNT nanoparticles is 1:1, oleic acid is added as a dispersant, the use ratio of the dispersant is one-fifth of the mass sum of the BN nanoparticles and the MWCNT nanoparticles, and the mass sum of the BN nanoparticles and the MWCNT nanoparticles accounts for 2wt% of the total mass of the composite phase change material.

[0010] Preferably, the top of the PEM electrolyzer is provided with a concentrator for concentrating solar radiation energy on the upper part of the PEM electrolyzer; the upper part of the PEM electrolyzer comprises, from top to bottom, a selective absorption coating, a composite phase change material, a permanent magnet and an end plate; the lower part of the PEM electrolyzer comprises, from top to bottom, an end plate, a permanent magnet, a composite phase change material and an insulation layer; the middle part of the PEM electrolyzer is composed of a plurality of electrolyzer single pieces connected in series; the side edge of the PEM electrolyzer comprises, from inside to outside, a composite phase change material and an insulation layer; The selective absorption coating is used to convert solar radiation energy into heat energy to increase the operating temperature of the PEM electrolyzer; The upper and lower permanent magnets of the PEM electrolytic cell are used to generate a magnetic field, and the Lorentz force of the magnetic field provides energy for breaking the hydrogen-oxygen bond of water to reduce the overpotential in the process of electrolyzing water to produce hydrogen; The composite phase change material is used to collect and store the heat of the PEM electrolytic cell, and when the temperature of the PEM electrolytic cell is lower than the phase change temperature, the composite phase change material supplies heat to the PEM electrolytic cell to avoid cold start of the PEM electrolytic cell; The PEM electrolytic cell is further provided with a temperature sensor for monitoring the internal temperature of the PEM electrolytic cell, and when the internal temperature is higher than a set temperature t1, the ventilation opening and the air outlet are opened for air cooling and heat dissipation, and when the internal temperature is lower than a set temperature t2, the ventilation opening and the air outlet are closed, t1>t2.

[0011] Preferably, the power matching between the photovoltaic cell in the photovoltaic photothermal assembly and the PEM electrolytic cell is realized by increasing / decreasing the number of series connection of the electrolytic cell single piece in the PEM electrolytic cell, and the calculation formula is as follows: |A-B|=UxC+D; Wherein, A is the voltage output value of the theoretical maximum power point of the photovoltaic cell at the current moment; B is the coupling voltage value of the photovoltaic cell and the PEM electrolytic cell; U is the operating voltage of a single electrolytic cell single piece; D is the remainder, D<U; C is an integer, when D≤U / 2, then the number of series connection of C electrolytic cell single pieces is increased / decreased, when D>U / 2, then the number of series connection of C+1 electrolytic cell single pieces is increased / decreased; wherein, if A is greater than B, the number of series connection of electrolytic cell single pieces is increased, and if A is less than B, the number of series connection of electrolytic cell single pieces is decreased.

[0012] Preferably, the specific structure of the photovoltaic photothermal assembly is as follows: The first thermoelectric generator is bonded below the photovoltaic cell by heat-conducting silicone; the nanofluid channel is bonded below the first thermoelectric generator by heat-conducting silicone; the upper part of the first thermoelectric generator is the heating end, which absorbs the heat of the photovoltaic cell; the lower part of the first thermoelectric generator is the cooling end, and the mixed nanofluid in the nanofluid channel cools the first thermoelectric generator; the first thermoelectric generator generates electricity through the temperature difference between the upper and lower ends; The composite phase change material is bonded below the nanofluid channel by heat-conducting silicone, which is used for further recovery of the heat of the photovoltaic photothermal assembly, and plays a role of heat preservation and anti-freezing for the photovoltaic photothermal assembly; the composite phase change material is packaged with an aluminum foil bag or an aluminum alloy; All components are assembled by the frame, and the frame has a heat preservation function for the internal structure.

[0013] Preferably, the specific structure of the solar heat collector is as follows: The surface of the second thermoelectric generator is provided with a selective absorption coating for converting solar radiation energy into heat energy; the water flow channel is bonded below the second thermoelectric generator by heat-conducting silica gel; the upper part of the second thermoelectric generator serves as a heating end for absorbing the heat of solar radiation energy; the lower part of the second thermoelectric generator serves as a cooling end for cooling the second thermoelectric generator by water in the water flow channel; the second thermoelectric generator generates electricity through the temperature difference between the upper and lower ends. The upper part of the second thermoelectric generator is provided with a glass cover plate, and an air gap exists between the glass cover plate and the second thermoelectric generator, for reducing the environmental heat loss from the inside to the outside of the solar collector. The composite phase change material is bonded below the water flow channel by heat-conducting silica gel, for further recovering the heat of the solar collector, playing a role of heat preservation and anti-freezing for the solar collector; the composite phase change material is packaged with an aluminum foil bag or an aluminum alloy. All components are assembled by the frame, which has a heat preservation function for the internal structure.

[0014] Preferably, a first gas-liquid separator and a second gas-liquid separator are arranged at the outlet of the PEM electrolytic cell, respectively for separating oxygen and hydrogen; the separated water is recovered into the water tank; the separated oxygen and hydrogen are respectively subjected to heat exchange with the water in the water tank through a first heat exchanger and a second heat exchanger; the heated water is returned to the water tank again; the heat-exchanged oxygen and hydrogen are respectively stored in an oxygen storage tank and a hydrogen storage tank.

[0015] Preferably, the water tank is provided with a temperature sensor, a heater, a radiator, and a liquid level sensor. When the liquid level sensor monitors that the water level in the water tank is lower than a set water level x1, the water tank is replenished until the water level reaches a set water level x2, and the replenishment is stopped, x1 < x2. When the temperature sensor monitors that the temperature in the water tank is higher than a set temperature t3, the radiator is turned on for cooling until the temperature reaches a set temperature t4, and the cooling is stopped; when the temperature sensor monitors that the temperature in the water tank is lower than a set temperature t5, the heater is turned on for heating until the temperature reaches a set temperature t6, and the heating is stopped, t3 > t4 > t6 > t5.

[0016] Preferably, the system operates as follows: When the light intensity is sufficient, a part of the electrical energy output by the photovoltaic cell is converted into alternating current by the inverter to supply the user end, and the other part is input into the PEM electrolytic cell to electrolyze water to produce hydrogen and store it in the hydrogen storage tank; the electrical energy generated by the two thermoelectric generators is used to maintain the operation of the system electrical appliances, and if there is excess electrical energy, it is converted into alternating current by the inverter to supply the user end.

[0017] When the light intensity is insufficient, the electrolysis of water to produce hydrogen is stopped, and the electrical energy output by the photovoltaic cell is entirely converted into alternating current by the inverter to supply the user end; in addition, the hydrogen in the hydrogen storage tank is introduced into the hydrogen fuel cell to generate electrical energy, which is converted into alternating current by the inverter to supply the user end; at the same time, the electrical energy generated by the two thermoelectric generators is used to maintain the operation of the system electrical appliances, and if the electrical energy generated by the two thermoelectric generators is insufficient to maintain the operation of the system electrical appliances, the electrical energy generated by the hydrogen fuel cell is supplied to the system electrical appliances.

[0018] The advantages of the present application are: (1) The present application provides a photovoltaic photothermal hydrogen production and energy storage system based on nanofluid and phase change material, which first synthesizes a mixed nanofluid and a composite phase change material based on multi-walled carbon nanotubes (MWCNT) and boron nitride (BN). These new materials are combined with a PV / T-PEM coupled energy storage system, and through the power regulation of an intelligent terminal, high-efficiency production of electrical power, thermal energy and green hydrogen can be achieved.

[0019] (2) The present application also provides a thermal management and magnetization hydrogen production scheme for a PEM electrolyzer, aiming to further improve the energy utilization rate and safety of the system.

[0020] (3) The BN-MWCNT / H2O mixed nanofluid synthesized by the present application has good thermal conductivity, and the thermal conductivity of 0.5wt% and 2wt% mixed nanofluid is increased by 8.06% and 9.08% relative to the thermal conductivity of water. In addition, the mixed nanofluid has good stability, and after standing for one month, the absolute value of Zeta overpotential of all concentrations of fluid is greater than 30mV.

[0021] (4) The composite phase change material composed of BN nanoparticles, MWCNT nanoparticles and paraffin synthesized by the present application has good thermal conductivity, and the addition of BN and MWCNT nanoparticles does not change the thermal stability and latent heat value of paraffin. After optimization and comparison, the addition amount of nanoparticles in the composite phase change material is selected as 2wt%, and the thermal conductivity is increased by 100.9% relative to paraffin.

[0022] (5) The present application realizes the synergistic thermal management of mixed nanofluid and composite phase change material on photovoltaic photothermal components. Relative to ordinary photovoltaic components, the average temperature of the photovoltaic photothermal components of the present application can be reduced by more than 20℃. Therefore, using photovoltaic photothermal components for PEM electrolysis of water to produce hydrogen can achieve high-efficiency output of thermal energy, electrical energy and hydrogen energy, thereby improving the comprehensive utilization rate of solar energy. Relative to ordinary photovoltaic components, the hydrogen production efficiency of photovoltaic photothermal components is improved by 11.38%.

[0023] (6) The PEM electrolytic cell of the present application releases heat when it is at low temperature, avoiding the risk of low-temperature operation. When the PEM electrolytic cell is at high temperature, the intelligent terminal opens the air vents and air outlets for air cooling. The above measures improve the safety performance of the PEM electrolytic cell during electrolysis. The concentrator (radiation plate) and the selective absorption coating are arranged to allow the PEM electrolytic cell to absorb the heat of solar energy, further increasing the operating temperature of the electrolytic cell and increasing its electrolysis efficiency. In addition, two permanent magnets are installed in the PEM electrolytic cell, and the magnetic field formed by the two permanent magnets can provide energy to break the hydrogen-oxygen bond of water, reduce the overpotential in the process of PEM electrolytic water hydrogen production, and thus realize efficient electrolytic water hydrogen production.

[0024] (7) The photovoltaic-photothermal assembly and the internal temperature difference generator of the solar collector of the present application can convert low-grade heat energy into high-grade electric energy, improving the quality of system energy. In addition, the solar collector heats the water flow at the inlet of the PEM electrolytic cell, also improving the operating performance of the PEM electrolytic cell. Compared with using electric energy to heat water, the present system reduces energy loss.

[0025] (8) The water tank of the present application recovers the residual heat contained in the hydrogen and oxygen produced by the PEM electrolytic cell, improving the heat energy utilization rate of the system.

[0026] (9) The water tank of the present application has the function of intelligently regulating the liquid level and temperature. When the liquid level is lower than 30%, water replenishment operation will be performed, and when the water temperature is lower than 20℃ or higher than 90℃, thermal regulation work will be performed. This function improves the degree of automation and safety performance of the system.

[0027] (10) The energy storage system of the present application has multiple working modes: when the sunlight is sufficient, part of the electric energy output by the system is output to the user end and the system electric appliance end for use, and part of the electric energy is used for electrolytic water hydrogen production and energy storage; when the sunlight is insufficient, the stored hydrogen is introduced into the hydrogen fuel cell to generate electricity to make up for the lack of electric energy when the power output by the system is lower than the power used by the user end and the system electric appliance end. Therefore, the present system realizes the complementation of solar energy and hydrogen energy, improving the flexibility of energy utilization of the system. The system is particularly suitable for operating as an independent energy system and has important engineering value. In addition, the heat energy stored in the water tank not only provides efficient electrolytic water hydrogen production for the PEM electrolytic cell, but also provides hot water for users, enriching the form of energy utilization. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 is a test result curve graph of the mixed nanofluid.

[0029] Figure 2 is a test result curve graph of the composite phase change material.

[0030] Figure 3 Fig. 1 is a structural schematic diagram of a photovoltaic-thermal (PV / T) component.

[0031] Figure 4 Fig. 2 is a structural schematic diagram of a solar collector.

[0032] Figure 5 Fig. 3 is a structural schematic diagram of a PEM electrolyzer.

[0033] Figure 6 Fig. 4 is a structural schematic diagram of a photovoltaic-thermal hydrogen production and energy storage system according to the present application.

[0034] Figure 7 Fig. 5 is a schematic diagram of the operation logic of a photovoltaic-thermal hydrogen production and energy storage system according to the present application.

[0035] Figure 8 Fig. 6 is a schematic diagram of the coupling of the working curves of a photovoltaic cell and a PEM electrolyzer.

[0036] BRIEF DESCRIPTION OF DRAWINGS 1-photovoltaic-thermal component, 2-first water pump, 3-heat-insulated water tank, 4-solar collector, 5-second water pump, 6-PEM electrolyzer, 7-first gas-liquid separator, 8-second gas-liquid separator, 9-first heat exchanger, 10-second heat exchanger, 11-oxygen storage tank, 12-hydrogen storage tank, 13-third water pump, 14-water supplement port, 15-second temperature sensor, 16-heater, 17-radiator, 18-liquid level sensor. DETAILED DESCRIPTION

[0037] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of protection of the present application.

[0038] Example 1, synthesis of mixed nanofluid The main goal of nanofluid research is to achieve higher thermal conductivity than the base fluid. The mixed nanofluid BN-MWCNT / H2O is composed of BN nanoparticles, MWCNT nanoparticles and pure water. The BN-MWCNT / H2O mixed nanofluid is synthesized using a two-step method. First, the nanoparticles are weighed using an analytical balance and added to pure water, and the mass ratio of BN to MWCNT nanoparticles is fixed at 1:1. The nanoparticles are uniformly dispersed in water using a magnetic stirrer for 30 minutes. Subsequently, sodium dodecyl benzene sulfonate (SDBS) surfactant is added to the mixed nanofluid, which is then ultrasonically dispersed for 90 minutes. The surfactant SDBS is used in a proportion of one-tenth of the mass sum of BN nanoparticles and MWCNT nanoparticles. During ultrasonic treatment, the instrument should not be overheated to ensure complete dispersion. According to this procedure, mixed nanofluids with mass concentrations of 0.1wt%, 0.5wt%, 1wt% and 2wt% are prepared, where the mass concentration refers to the proportion of the mass sum of BN and MWCNT nanoparticles to the total mass of the mixed nanofluid.

[0039] Figure 1 In (a), the thermal conductivity of the mixed nanofluid at different mass concentrations is shown, and in (b), the Zeta overpotential of the mixed nanofluid at different mass concentrations is shown. From (a) in Figure 1 It is observed from (a) in that when the mass concentration of the mixed nanofluid exceeds 0.5wt%, the growth trend of the thermal conductivity gradually flattens. The thermal conductivities of the 0.5wt% and 2wt% mixed nanofluids are 8.06% and 9.08% higher than that of water, respectively. From (b) in Figure 1 It is observed from (b) in that when the mixed nanofluid is left to stand for 30 days, the absolute value of the Zeta overpotential of the mixed nanofluid at all mass concentrations is greater than 30mV, which indicates that the configured mixed nanofluid has theoretical stability; in addition, it can be found that the absolute value of the Zeta overpotential of the mixed nanofluid decreases continuously with increasing mass concentration, because the increase in mass concentration promotes the agglomeration of nanoparticles, causing the solution to be unstable. Based on the above data, the preferred mass concentration of the mixed nanofluid is 0.5wt%.

[0040] Example 2, synthesis of composite phase change material Paraffin wax has high thermal stability and latent heat value, but has low thermal conductivity, which limits its large-scale promotion. Therefore, the synthesis of composite phase change material has great significance to improve the thermal conductivity of paraffin wax. The composite phase change material is composed of BN nanoparticles, MWCNT nanoparticles and paraffin wax. The melting point range of paraffin wax is selected as 28-32℃. The composite phase change material is prepared by two-step method. Paraffin wax is melted in a constant temperature water bath at 60℃, then MWCNT and BN nanoparticles are added. The mass ratio of the two kinds of nanoparticles is 1:1. Then add oleic acid as a dispersant to improve the uniformity of the distribution of nanoparticles in paraffin wax. The use ratio of oleic acid is one-fifth of the mass sum of BN and MWCNT nanoparticles. The composite phase change material is magnetically stirred at 60℃ for 60 minutes, and then ultrasonic oscillation is performed for 90 minutes. Magnetic stirring and ultrasonic oscillation are used to achieve uniform nanoparticle dispersion. According to this procedure, composite phase change materials (PCMs) with mass concentrations of 1wt%, 2wt%, 3wt% and 4wt% are prepared. The mass concentration refers to the ratio of the mass sum of BN and MWCNT nanoparticles to the total mass of the composite phase change material.

[0041] Figure 2 In the figures, (a) shows the change of thermal conductivity of the composite phase change material under different mass concentrations, (b) shows the DSC (differential scanning calorimetry) test results of the composite phase change material with a mass concentration of 2wt% and paraffin wax, and (c) shows the TGA (thermogravimetric analysis) test results of the composite phase change material with a mass concentration of 2wt% and paraffin wax.

[0042] It is observed from (a) in Figure 2 that the thermal conductivity of the composite phase change material increases with the increase of mass concentration. When the addition amount of BN and MWCNT nanoparticles reaches 2wt%, the thermal conductivity is improved by 100.9% compared with paraffin wax. However, the thermal conductivity of 4wt% only increases by 12.06% compared with that of 2wt%. At this time, further increasing the addition amount of nanoparticles will slow down the increase of thermal conductivity. Considering the additional cost of further adding nanoparticles, the preferred mass concentration of the composite phase change material is 2wt%.

[0043] From the DSC and TGA comparison curves of 2wt% composite phase change material and paraffin wax given in (b) and (c) in Figure 2 , it can be known that the latent heat values of paraffin wax and composite phase change material are 265.8kJ / kg and 256.3kJ / kg, respectively. The addition of nanoparticles has little effect on the latent heat value, and does not change the energy storage properties of paraffin wax. In addition, it can also be seen that the prepared composite phase change material has good thermal stability, and starts to decompose in small amounts only when the temperature exceeds 100℃.

[0044] Example 3, structure of photovoltaic-thermal (PV / T) component As shown in Figure 3As shown, the photovoltaic thermal component includes photovoltaic cells, thermal conductive silicone, a first thermoelectric generator, thermal conductive silicone, a nanofluid channel, thermal conductive silicone, and a composite phase change material arranged in sequence from top to bottom.

[0045] The first thermoelectric generator is tightly bonded to the bottom of the photovoltaic cell using thermally conductive silicone. The nanofluid channel is tightly bonded to the bottom of the first thermoelectric generator using thermally conductive silicone. By absorbing heat from the photovoltaic cell, the top of the first thermoelectric generator serves as the heating end. The mixed nanofluid in the nanofluid channel cools the first thermoelectric generator, while the bottom of the first thermoelectric generator serves as the cooling end. The first thermoelectric generator generates electricity by utilizing the temperature difference between its top and bottom ends.

[0046] The composite phase change material is tightly bonded to the bottom of the nanofluid channel via thermally conductive silicone. The composite phase change material further recovers heat from the photovoltaic thermal module. In low sunlight conditions or during winter, the composite phase change material provides insulation and freeze protection for the photovoltaic thermal module. Furthermore, the composite phase change material helps improve temperature uniformity across the photovoltaic cell. The composite phase change material is encapsulated in aluminum foil or aluminum alloy.

[0047] Finally, all components are assembled through a frame, which has the function of insulating the internal structure. This photovoltaic thermal module realizes the synergistic thermal management of photovoltaic cells by hybrid nanofluids and composite phase change materials.

[0048] The mixed nanofluid and composite phase change material are prepared according to Examples 1 and 2, respectively.

[0049] According to actual experimental tests, compared to ordinary photovoltaic modules, the photovoltaic thermal module of this embodiment can reduce the average temperature throughout the day by more than 20°C, and the efficiency of solar hydrogen production can be increased by 11.38%. Therefore, the photovoltaic thermal module of this embodiment performs well and improves the hydrogen production efficiency of the system.

[0050] Example 4: Structural composition of solar thermal collector like Figure 4 As shown, the solar thermal collector includes a glass cover, an air gap, a second thermoelectric generator, thermal conductive silica gel, a water fluid channel, thermal conductive silica gel, and a composite phase change material, which are arranged in sequence from top to bottom.

[0051] The surface of the second thermoelectric generator is coated with a selective absorption coating to enhance its absorption of solar radiation. By absorbing the heat from solar radiation, the upper portion of the second thermoelectric generator serves as the heating end. A water channel is tightly bonded to the lower portion of the second thermoelectric generator via thermally conductive silicone. The water in the channel cools the second thermoelectric generator, with the lower portion serving as the cooling end. The second thermoelectric generator generates electricity by utilizing the temperature difference between its upper and lower ends.

[0052] The second thermoelectric generator is provided with a glass cover plate, and an air gap exists between the glass cover plate and the second thermoelectric generator. The air gap is used to reduce the heat loss from the inside of the solar collector to the outside environment.

[0053] The composite phase change material is bonded to the lower part of the water flow channel through the heat-conducting silica gel, and is used to further recover the heat of the solar collector. In weak sunlight or winter, the composite phase change material plays a role of heat preservation and anti-freezing for the solar collector. The composite phase change material is packaged with an aluminum foil bag or an aluminum alloy.

[0054] Finally, all the components are assembled through the frame, and the frame has a heat preservation function for the internal structure.

[0055] The composite phase change material is prepared according to Example 2.

[0056] Example 5, structure of the PEM electrolyzer As shown in Figure 5 The PEM electrolyzer is composed of a condenser, a selective absorption coating, an electrolyzer single piece, a heat preservation layer, a composite phase change material, a permanent magnet, an acrylic end plate, a ventilation port, an air outlet, and a valve.

[0057] The top of the PEM electrolyzer is provided with a condenser for concentrating solar radiation on the upper part of the PEM electrolyzer. The upper part of the PEM electrolyzer includes, from top to bottom, a selective absorption coating, a composite phase change material, a permanent magnet, and an end plate. The lower part of the PEM electrolyzer includes, from top to bottom, an end plate, a permanent magnet, a composite phase change material, and a heat preservation layer. The middle part of the PEM electrolyzer is composed of a plurality of electrolyzer single pieces connected in series. The side edges of the PEM electrolyzer include, from inside to outside, a composite phase change material and a heat preservation layer.

[0058] The selective absorption coating is used to convert solar radiation into heat energy, and the PEM electrolyzer absorbs the heat of solar energy to increase the operating temperature of the PEM electrolyzer, so that the PEM electrolyzer can also obtain solar energy to increase the operating temperature and the electrolysis efficiency when used outdoors.

[0059] The upper and lower parts of the PEM electrolyzer are respectively provided with permanent magnets. The upper and lower permanent magnets generate a stable magnetic field, because the Lorentz force of the magnetic field provides energy to break the hydrogen-oxygen bond of water, reduces the overpotential in the process of hydrogen production by electrolysis of water, and improves the hydrogen production efficiency of electrolysis of water.

[0060] The end plates of the upper and lower parts of the PEM electrolyzer are acrylic end plates, which can also be replaced by other non-magnetic end plates. It should be noted that the thickness of the end plate should not be too high to avoid weakening the penetration of the magnetic field.

[0061] The lower end and the side of the PEM electrolyzer are provided with composite phase change materials, which are used to collect and store the heat of the PEM electrolyzer. When the temperature of the PEM electrolyzer is lower than the phase change temperature, the composite phase change materials can supply heat to the electrolyzer, thereby avoiding the risk of cold start of the electrolyzer.

[0062] The lower end and the side of the PEM electrolyzer are also provided with thermal insulation layers, which are located outside the composite phase change materials and are used to reduce heat exchange between the inside of the electrolyzer and the external environment, thereby preventing heat loss.

[0063] A first temperature sensor is arranged in the PEM electrolyzer and is used to monitor the internal temperature of the PEM electrolyzer. The first temperature sensor is connected to the intelligent terminal. When the first temperature sensor monitors that the internal temperature is higher than a set temperature t1, the intelligent terminal opens the air inlet and the air outlet to perform air cooling. When the first temperature sensor monitors that the internal temperature is lower than a set temperature t2, the intelligent terminal closes the air inlet and the air outlet to stop air cooling. The air cooling can be realized by connecting an air blower. In this embodiment, t1 > t2, 95℃ > t1 > 85℃, and 75℃ > t2 > 65℃. In this embodiment, t1 = 90℃ and t2 = 70℃.

[0064] The composite phase change material is prepared according to the method in Example 2.

[0065] Example 6: Structure of a photovoltaic-photothermal hydrogen production and energy storage system Based on the above-mentioned Examples 1-5, a photovoltaic-photothermal hydrogen production and energy storage system based on nanofluid and phase change material according to the present application has a structure as shown in Figure 6 .

[0066] The system comprises a photovoltaic-photothermal component 1, a first water pump 2, a water tank 3, a solar heat collector 4, a second water pump 5, a PEM electrolyzer 6, a first gas-liquid separator 7, a second gas-liquid separator 8, a first heat exchanger 9, a second heat exchanger 10, an oxygen storage tank 11, a hydrogen storage tank 12, a third water pump 13, a water supplement inlet 14, a second temperature sensor 15, a heater 16, a radiator 17, and a liquid level sensor 18.

[0067] The photovoltaic-photothermal component 1, the first water pump 2, and the water tank 3 constitute a mixed nanofluid circuit. The heat exchange working medium in the pipeline of the mixed nanofluid circuit is mixed nanofluid. The mixed nanofluid in the nanofluid channel of the photovoltaic-photothermal component 1 enters the heat exchange coil in the water tank through the first water pump 2, thereby heating the water in the water tank.

[0068] The water tank 3, the solar collector 4, the second water pump 5, and the PEM electrolyzer 6 constitute a water circuit. The water in the water tank 3 first enters the water flow passage of the solar collector 4 for heating, and the heated water enters the PEM electrolyzer 6 for water electrolysis to produce hydrogen. Preheating the water in the water tank 3 by the solar collector 4 can improve the electrolysis efficiency of the PEM electrolyzer 6 and avoid the problem of cold start of the electrolyzer. At the same time, compared with electric heating, the solar collector 4 has better economic efficiency and environmental protection.

[0069] The first gas-liquid separator 7 and the second gas-liquid separator 8 are arranged at the outlet of the PEM electrolyzer 6 and are used for separating oxygen and hydrogen respectively. The separated water is recycled to the water tank 3 through a pipeline. The high-temperature oxygen at the outlet of the first gas-liquid separator 7 is introduced into the first heat exchanger 9 to exchange heat with the water flowing out of the water tank 3. The high-temperature hydrogen at the outlet of the second gas-liquid separator 8 is introduced into the second heat exchanger 10 to exchange heat with the water flowing out of the water tank 3. The high-temperature hydrogen and oxygen heat the water from the water tank 3 through heat exchange, and further, the heated water is returned to the water tank 3 again through a circulating pump. At the same time, the heat-exchanged oxygen and hydrogen are respectively stored in the oxygen storage tank 11 and the hydrogen storage tank 12.

[0070] The second temperature sensor 15, the heater 16, the radiator 17, and the liquid level sensor 18 are also arranged in the water tank 3 and are connected with the intelligent terminal. The intelligent terminal adjusts the water level and temperature of the water tank by receiving signals.

[0071] When the liquid level sensor 18 monitors that the water level in the water tank is lower than the set water level x1, the water replenishing port 14 of the water tank 3 will perform a water replenishing operation, and when the liquid level sensor 18 monitors that the water level in the water tank 3 reaches the set water level x2, the water replenishing operation is stopped. This operation avoids the risk of water shortage of the electrolyzer caused by the decrease of water amount. Wherein, x1 < x2, 25% < x1 < 35%, 90% < x2 < 100%. In this embodiment, x1 = 30%, x2 = 95%.

[0072] When the second temperature sensor 15 monitors that the temperature in the water tank is higher than the set temperature t3, the intelligent terminal will open the radiator 17 for cooling until the temperature reaches the set temperature t4, and the cooling is stopped. When the second temperature sensor 15 monitors that the temperature in the water tank is lower than the set temperature t5, the intelligent terminal will open the heater 16 for heating until the temperature reaches the set temperature t6, and the heating is stopped. This operation ensures that the water temperature entering the PEM electrolyzer is in a reasonable range. Wherein, t3 > t4 > t6 > t5, 85℃ < t3 < 95℃, 65℃ < t4 < 75℃, 15℃ < t5 < 25℃, 35℃ < t6 < 45℃. In this embodiment, t3 = 90℃, t4 = 70℃, t5 = 20℃, t6 = 40℃.

[0073] Embodiment 7, operation logic of photovoltaic photo-thermal hydrogen production and energy storage system Based on the above embodiment 6, the operation logic of a photovoltaic photo-thermal hydrogen production and energy storage system based on nanofluid and phase change material of the present application, as shown in Figure 7 , can be divided into two operation modes: Operation mode 1: when the light intensity is sufficient, part of the electrical energy output by the photovoltaic cell in the photovoltaic photo-thermal component 1 is converted into alternating current by the inverter to supply the user end, and the other part (excess electrical energy) is input into the PEM electrolytic cell to electrolyze water to produce hydrogen, and the prepared hydrogen energy is stored in the hydrogen storage tank. The electrical energy output by the first thermoelectric generator in the photovoltaic photo-thermal component 1 and the second thermoelectric generator in the solar collector 4 is used to maintain the operation of the circulating pump and other system electrical appliances. If there is excess electrical energy in the thermoelectric generator, it is also converted into alternating current by the inverter to supply the user end.

[0074] Operation mode 2: when the light intensity is insufficient, the electrical energy output by the photovoltaic cell in the photovoltaic photo-thermal component 1 is lower than the amount used by the user end, and the PEM electrolytic water production of hydrogen is stopped. The electrical energy output by the photovoltaic cell in the photovoltaic photo-thermal component 1 is all delivered to the user end. In addition, the hydrogen in the hydrogen storage tank is introduced into the hydrogen fuel cell to generate electrical energy, and the electrical energy generated by the hydrogen fuel cell is delivered to the inverter to convert it into alternating current to supply the user end. Similarly, the electrical energy output by the first thermoelectric generator in the photovoltaic photo-thermal component 1 and the second thermoelectric generator in the solar collector 4 is used to maintain the operation of the circulating pump and other system electrical appliances. When the power generation of the thermoelectric generator is lower than the power of the electrical appliances, the electrical energy generated by the hydrogen fuel cell is adjusted to power the system electrical appliances.

[0075] This set of energy storage and supply system can operate completely independent of the power grid, and is particularly suitable for independent energy systems in remote areas.

[0076] In addition, the power matching between the photovoltaic cell in the photovoltaic photo-thermal component 1 and the PEM electrolytic cell 6 can be adjusted by the intelligent terminal. The control method is to increase / decrease the number of series-connected single pieces in the PEM electrolytic cell 6 to achieve. Figure 8 Theoretical calculation is given to facilitate understanding of the operation logic. As shown in Figure 8 , the intersection of the PEM electrolytic cell working curve and the photovoltaic cell working curve is the coupling voltage value when they are working. When the number of single pieces is 10, the PEM electrolytic cell working curve is far away from the maximum power point of the photovoltaic cell. At this time, increasing the number of single pieces of the PEM electrolytic cell can make the working curve of the PEM electrolytic cell close to the maximum power point of the photovoltaic cell, so as to improve the electrical efficiency of the system.

[0077] Based on the above idea, in the embodiment, the operating voltage of each single PEM electrolyzer cell is about 1.8V. Therefore, the regulating ratio in the embodiment is set to 1.8V. The specific regulating formula is |A-B|=1.8C+D. Wherein, A is the voltage output value of the theoretical maximum power point of the photovoltaic cell at the current time. B is the coupling voltage value of the photovoltaic cell and the PEM electrolyzer, which is the voltage value when the output end of the photovoltaic cell is directly connected to the PEM electrolyzer for power supply. D is the remainder, D is less than 1.8, and C is an integer. When D is greater than 0.9, the final adjusted (increased / decreased) number of series of the single PEM electrolyzer cell is C+1. When D is less than or equal to 0.9, the final adjusted (increased / decreased) number of series of the single PEM electrolyzer cell is C. When A is greater than B, the adjustment scheme is to increase the number of series of the single PEM electrolyzer cell. When A is less than B, the adjustment scheme is to reduce the number of series of the single PEM electrolyzer cell.

[0078] For example, when A is greater than B, and the difference between A and B is 4, 2 series of single PEM electrolyzer cells need to be added. When A is less than B, and the difference between A and B is 5, 3 series of single PEM electrolyzer cells need to be reduced.

[0079] The scheme is simple to implement and can be realized without complex algorithms. Through the method, the performance matching degree between the photovoltaic-PEM electrolyzer can be improved, and the efficient hydrogen production of the system can be realized.

[0080] The above is only a preferred embodiment of the present application, and is not intended to limit the present application. Any modification, equivalent replacement and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A photovoltaic thermal hydrogen energy storage system based on nanofluids and phase change materials, characterized in that: include: Photovoltaic thermal modules, water tanks, solar collectors and PEM electrolyzers; The photovoltaic thermal component includes a photovoltaic cell, a first thermoelectric generator, a nanofluid channel, and a composite phase change material arranged from top to bottom; The solar thermal collector includes a second thermoelectric generator, a water fluid channel, and a composite phase change material arranged from top to bottom; The mixed nanofluid in the nanofluid channel of the photovoltaic thermal component enters the heat exchange coil in the water tank to heat the water in the water tank; The water in the water tank first enters the water fluid channel of the solar collector for heating, and then enters the PEM electrolyzer for electrolysis to produce hydrogen; The mixed nanofluid is used for heat exchange and consists of BN nanoparticles, MWCNT nanoparticles and pure water; The composite phase change material is used for heat storage and consists of BN nanoparticles, MWCNT nanoparticles and paraffin.

2. The photovoltaic thermal hydrogen energy storage system based on nanofluids and phase change materials according to claim 1 is characterized in that: In the mixed nanofluid, the mass ratio of BN nanoparticles to MWCNT nanoparticles is 1:1, sodium dodecylbenzenesulfonate is added as a surfactant, the usage ratio of the surfactant is one tenth of the total mass of the BN nanoparticles and the MWCNT nanoparticles, and the mass percentage of the total mass of the mixed nanofluid is 0.5wt%.

3. The photovoltaic thermal hydrogen production and storage system based on nanofluids and phase change materials according to claim 1 is characterized in that: In the composite phase change material, the mass ratio of BN nanoparticles to MWCNT nanoparticles is 1:1, oleic acid is added as a dispersant, the usage ratio of the dispersant is one-fifth of the total mass of the BN nanoparticles and the MWCNT nanoparticles, and the mass percentage of the total mass of the BN nanoparticles and the MWCNT nanoparticles to the total mass of the composite phase change material is 2wt%.

4. The photovoltaic thermal hydrogen production and storage system based on nanofluids and phase change materials according to claim 1 is characterized in that: The top of the PEM electrolyzer is provided with a concentrator for concentrating solar radiation energy on the upper portion of the PEM electrolyzer; the upper portion of the PEM electrolyzer comprises a selective absorption coating, a composite phase change material, a permanent magnet, and an end plate arranged from top to bottom; the lower portion of the PEM electrolyzer comprises an end plate, a permanent magnet, a composite phase change material, and an insulation layer arranged from top to bottom; the middle portion of the PEM electrolyzer is composed of a plurality of electrolyzer cells connected in series; and the side edges of the PEM electrolyzer comprise a composite phase change material and an insulation layer arranged from inside to outside. The selective absorption coating is used to convert solar radiation energy into thermal energy to increase the operating temperature of the PEM electrolyzer; The upper and lower permanent magnets of the PEM electrolyzer are used to generate a magnetic field. The Lorentz force of the magnetic field provides energy for breaking the hydrogen-oxygen bond of water to reduce the overpotential in the process of electrolyzing water to produce hydrogen. The composite phase change material is used to collect heat from the PEM electrolyzer for storage, and to supply heat to the PEM electrolyzer when the temperature of the PEM electrolyzer is lower than the phase change temperature, thereby preventing the PEM electrolyzer from cold starting; A temperature sensor is also provided inside the PEM electrolyzer to monitor the internal temperature of the PEM electrolyzer. When the internal temperature is higher than the set temperature t1, the vents and air outlets are opened for air cooling and heat dissipation. When the internal temperature is lower than the set temperature t2, the vents and air outlets are closed, and t1>t2.

5. The photovoltaic thermal hydrogen energy storage system based on nanofluids and phase change materials according to claim 4 is characterized in that: By increasing or decreasing the number of single electrolytic cells connected in series in the PEM electrolytic cell, the power matching between the photovoltaic cell in the photovoltaic-thermal component and the PEM electrolytic cell is achieved. The calculation formula is as follows: |A - B| = U×C + D; where, A is the voltage output value at the theoretical maximum power point of the photovoltaic cell at the current moment; B is the coupling voltage value between the photovoltaic cell and the PEM electrolytic cell; U is the operating voltage of a single electrolytic cell; D is the remainder, D < U; C is an integer. When D ≤ U / 2, the number of single electrolytic cells connected in series is increased or decreased by C. When D > U / 2, the number of single electrolytic cells connected in series is increased or decreased by C + 1. Among them, if A is greater than B, the number of single electrolytic cells connected in series is increased. If A is less than B, the number of single electrolytic cells connected in series is decreased.

6. The photovoltaic thermal hydrogen production and storage system based on nanofluids and phase change materials according to claim 1 is characterized in that: The specific structure of the photovoltaic-thermal component is as follows: The first thermoelectric generator is bonded below the photovoltaic cell through thermally conductive silicone; the nanofluid channel is bonded below the first thermoelectric generator through thermally conductive silicone; the upper part of the first thermoelectric generator serves as the heating end to absorb the heat of the photovoltaic cell; the lower part of the first thermoelectric generator serves as the cooling end, and the hybrid nanofluid in the nanofluid channel cools the first thermoelectric generator; the first thermoelectric generator generates electricity through the temperature difference between the upper and lower ends. The composite phase change material is bonded below the nanofluid channel through thermally conductive silicone, which is used to further recover the heat of the photovoltaic-thermal component, playing a role in heat preservation and anti-freezing for the photovoltaic-thermal component; the composite phase change material is encapsulated with an aluminum foil bag or aluminum alloy. All components are assembled through a frame, and the frame has the function of heat preservation for the internal structure.

7. The photovoltaic thermal hydrogen energy storage system based on nanofluids and phase change materials according to claim 1, characterized in that: The specific structure of the solar collector is as follows: The surface of the second thermoelectric generator is provided with a selective absorption coating for converting solar radiant energy into heat energy; the water fluid channel is bonded below the second thermoelectric generator through thermally conductive silicone; the upper part of the second thermoelectric generator serves as the heating end to absorb the heat of solar radiant energy; the lower part of the second thermoelectric generator serves as the cooling end, and the water in the water fluid channel cools the second thermoelectric generator; the second thermoelectric generator generates electricity through the temperature difference between the upper and lower ends. A glass cover plate is provided above the second thermoelectric generator, and there is an air gap between the glass cover plate and the second thermoelectric generator, which is used to reduce the environmental heat loss inside the solar collector to the outside. The composite phase change material is bonded below the water fluid channel through thermally conductive silicone, which is used to further recover the heat of the solar collector, playing a role in heat preservation and anti-freezing for the solar collector. The composite phase change material is encapsulated with an aluminum foil bag or aluminum alloy. All components are assembled through a frame, and the frame has the function of heat preservation for the internal structure.

8. The photovoltaic thermal hydrogen production and storage system based on nanofluids and phase change materials according to claim 1, characterized in that: At the outlet of the PEM electrolytic cell, a first gas-liquid separator and a second gas-liquid separator are provided, which are respectively used to separate oxygen and hydrogen; the separated water is recycled into the water tank; the separated oxygen and hydrogen respectively exchange heat with the water in the water tank through a first heat exchanger and a second heat exchanger, and the heated water returns to the water tank again. The oxygen and hydrogen after heat exchange respectively enter an oxygen storage tank and a hydrogen storage tank for storage.

9. The photovoltaic thermal hydrogen production and storage system based on nanofluids and phase change materials according to claim 1, characterized in that: A temperature sensor, a heater, a radiator, and a liquid level sensor are provided in the water tank. When the liquid level sensor detects that the water level in the water tank is lower than the set water level x1, the water tank is replenished with water until the water level reaches the set water level x2, then the replenishment is stopped. <x2; When the temperature sensor detects that the temperature in the water tank is higher than the set temperature t3, the radiator is turned on to cool down the water tank until the temperature reaches the set temperature t4, and then the cooling is stopped; when the temperature sensor detects that the temperature in the water tank is lower than the set temperature t5, the heater is turned on to heat the water tank until the temperature reaches the set temperature t6, and then the heating is stopped, t3>t4>t6>t5.

10. A photovoltaic thermal hydrogen production and storage system based on nanofluids and phase change materials according to any one of claims 1 to 9, characterized in that: Here's how the system works: When the sunlight intensity is sufficient, part of the electricity output by the photovoltaic cells is converted into AC power by the inverter and supplied to the user end, while the other part is input into the PEM electrolyzer to electrolyze water to produce hydrogen and store it in the hydrogen storage tank. The electricity generated by the two thermoelectric generators is used to maintain the operation of the system's electrical appliances. If there is excess electricity from the two thermoelectric generators, it is also converted into AC power by the inverter and supplied to the user end. When the light intensity is insufficient, the electrolysis of water to produce hydrogen is stopped, and all the electricity output by the photovoltaic cells is converted into AC power through the inverter and supplied to the user end; in addition, the hydrogen in the hydrogen storage tank is passed into the hydrogen fuel cell to generate electricity, and converted into AC power through the inverter and supplied to the user end; at the same time, the electricity generated by the two thermoelectric generators is used to maintain the operation of the system electrical appliances. If the electricity generated by the two thermoelectric generators is not enough to maintain the operation of the system electrical appliances, the electricity generated by the hydrogen fuel cell will be supplied to the system electrical appliances.

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