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

Abstract

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

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, which comprises:

[0008] A photovoltaic photo-thermal hydrogen production and energy storage system based on nanofluid and phase change material, comprising a photovoltaic photo-thermal assembly, a water tank, a solar collector and a PEM electrolytic cell;

[0009] The photovoltaic photo-thermal assembly comprises, from top to bottom, a photovoltaic cell, a first thermoelectric generator, a nanofluid channel and a composite phase change material;

[0010] The solar collector comprises, from top to bottom, a second thermoelectric generator, a water fluid channel and a composite phase change material;

[0011] The mixed nanofluid in the nanofluid channel of the photovoltaic photo-thermal assembly enters the heat exchange coil in the water tank to heat the water in the water tank;

[0012] The water in the water tank first enters the water fluid channel of the solar collector to be heated, and then enters the PEM electrolytic cell to electrolyze water to produce hydrogen;

[0013] The mixed nanofluid is used for heat exchange and is composed of BN nanoparticles, MWCNT nanoparticles and pure water;

[0014] The composite phase change material is used for heat storage and is composed of BN nanoparticles, MWCNT nanoparticles and paraffin.

[0015] 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.

[0016] 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.

[0017] Preferably, the top of the PEM electrolytic cell is provided with a condenser for concentrating solar radiation on the upper part of the PEM electrolytic cell; the upper part of the PEM electrolytic cell 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 electrolytic cell 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 electrolytic cell is composed of a plurality of electrolytic cell single pieces connected in series; the side edge of the PEM electrolytic cell comprises, from inside to outside, a composite phase change material and an insulation layer;

[0018] The selective absorption coating is used to convert solar radiation energy into heat energy to improve the operating temperature of the PEM electrolyzer;

[0019] The upper and lower permanent magnets of the PEM electrolyzer 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;

[0020] The composite phase change material is used to collect and store the heat of the PEM electrolyzer, and when the temperature of the PEM electrolyzer is lower than the phase change temperature, heat is supplied to the PEM electrolyzer to avoid cold start of the PEM electrolyzer;

[0021] The PEM electrolyzer is also provided with a temperature sensor inside for monitoring the internal temperature of the PEM electrolyzer, and when the internal temperature is higher than the set temperature t1, the ventilation opening and the air outlet are opened for air cooling, and when the internal temperature is lower than the set temperature t2, the ventilation opening and the air outlet are closed, t1>t2.

[0022] Preferably, the power matching between the photovoltaic cell in the photovoltaic photothermal assembly and the PEM electrolyzer is realized by increasing / decreasing the number of series connection of the electrolyzer single piece in the PEM electrolyzer, and the calculation formula is as follows:

[0023] |A-B|=U×C+D;

[0024] 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 electrolyzer; U is the operating voltage of a single electrolyzer single piece; D is the remainder, D<U; C is an integer, when D≤U / 2, then the number of series connection of C electrolyzer single pieces is increased / decreased, when D>U / 2, then the number of series connection of C+1 electrolyzer single pieces is increased / decreased; wherein, if A is greater than B, the number of series connection of electrolyzer single pieces is increased, if A is less than B, the number of series connection of electrolyzer single pieces is decreased.

[0025] Preferably, the specific structure of the photovoltaic photothermal assembly is as follows:

[0026] 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;

[0027] The composite phase change material is bonded below the nanofluid channel by heat-conducting silicone, which is used to further recover 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;

[0028] All components are assembled through the frame, and the frame has the heat preservation function for the internal structure.

[0029] Preferably, the specific structure of the solar collector is as follows:

[0030] 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 through 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 the water in the water flow channel; the second thermoelectric generator generates electricity through the temperature difference between the upper and lower ends;

[0031] The upper part of the second thermoelectric generator is provided with a glass cover plate, and there is an air gap between the glass cover plate and the second thermoelectric generator, for reducing the environmental heat loss of the solar collector to the outside;

[0032] The composite phase change material is bonded below the water flow channel through heat-conducting silica gel, for further recycling 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;

[0033] All components are assembled through the frame, and the frame has the heat preservation function for the internal structure.

[0034] Preferably, the first gas-liquid separator and the second gas-liquid separator are arranged at the outlet of the PEM electrolytic cell, respectively for separating oxygen and hydrogen; the separated water is recycled into the water tank; the separated oxygen and hydrogen are respectively subjected to heat exchange with the water in the water tank through the first heat exchanger and the second heat exchanger, the heated water is returned to the water tank again, and the heat-exchanged oxygen and hydrogen are respectively stored in the oxygen storage tank and the hydrogen storage tank.

[0035] Preferably, the water tank is provided with a temperature sensor, a heater, a radiator, and a liquid level sensor;

[0036] When the liquid level sensor monitors that the water level in the water tank is lower than the set water level x1, the water tank is replenished until the water level reaches the set water level x2, and the replenishment is stopped, x1 < x2;

[0037] When the temperature sensor monitors that the temperature in the water tank is higher than the set temperature t3, the radiator is turned on for cooling until the temperature reaches the set temperature t4, and the cooling is stopped; when the temperature sensor monitors that the temperature in the water tank is lower than the set temperature t5, the heater is turned on for heating until the temperature reaches the set temperature t6, and the heating is stopped, t3 > t4 > t6 > t5.

[0038] Preferably, the operation mode of the system is as follows:

[0039] When the light intensity is sufficient, the electrical energy output by the photovoltaic cell is partly 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 from the two thermoelectric generators, it is converted into alternating current by the inverter to supply the user end.

[0040] When the light intensity is insufficient, the water electrolysis to produce hydrogen is stopped, and the electrical energy output by the photovoltaic cell is all 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.

[0041] The advantages of the present application are:

[0042] (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 based on multi-walled carbon nanotubes (MWCNT) and boron nitride (BN) and a composite phase change material. These new materials are combined with the PV / T-PEM coupled energy storage system, and through the power regulation of the intelligent terminal, high-efficiency production of electricity, heat and green hydrogen can be realized.

[0043] (2) The present application also provides a thermal management and magnetization hydrogen production scheme for the PEM electrolytic cell, aiming to further improve the energy utilization rate and safety of the system.

[0044] (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% compared with that 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.

[0045] (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% compared with that of paraffin.

[0046] (5) The present application realizes the synergistic thermal management of the hybrid nanofluid and the composite phase change material on the photovoltaic-thermal component. Compared with the ordinary photovoltaic component, the average temperature of the photovoltaic-thermal component can be reduced by more than 20 DEG C. Therefore, the photovoltaic-thermal component is used for PEM water electrolysis to produce hydrogen, which can realize the efficient output of thermal energy, electrical energy and hydrogen energy, and further improve the comprehensive utilization rate of solar energy. Compared with the ordinary photovoltaic component, the hydrogen production efficiency of the photovoltaic-thermal component is improved by 11.38%.

[0047] (6) When the PEM electrolytic tank of the present application is at low temperature, the composite phase change material arranged below and at the side of the electrolytic tank can release heat to avoid the risk of low-temperature operation of the electrolytic tank. When the PEM electrolytic tank is at high temperature, the intelligent terminal will open the ventilation opening and the air outlet for air cooling. The above measures improve the safety performance of the PEM electrolytic tank during electrolysis. The setting of the condenser (radiation plate) and the selective absorption coating makes the PEM electrolytic tank absorb the heat of solar energy, further improves the operating temperature of the electrolytic tank and increases its electrolysis efficiency. In addition, two permanent magnets are installed in the PEM electrolytic tank. The magnetic field formed by the upper and lower permanent magnets can provide energy to break the hydrogen-oxygen bond of water, reduce the overpotential in the process of PEM water electrolysis to produce hydrogen, and further realize efficient water electrolysis to produce hydrogen.

[0048] (7) The setting of the photovoltaic-thermal component and the temperature difference power generator in the solar collector of the present application can convert low-grade heat energy into high-grade electrical energy, improving the quality of system energy. In addition, the solar collector heats the water flow at the inlet of the PEM electrolytic tank, which also improves the operating performance of the PEM electrolytic tank. And compared with using electrical energy to heat water, the present application system reduces energy loss.

[0049] (8) The water tank of the present application recovers the residual heat contained in the hydrogen and oxygen produced by the PEM electrolytic tank, improving the heat energy utilization rate of the system.

[0050] (9) The water tank of the present application has the function of intelligent control of liquid level and temperature. When the liquid level is lower than 30%, the water replenishment operation will be performed, and when the water temperature is lower than 20 DEG C or higher than 90 DEG C, the heat regulation work will be performed. This function improves the automation degree and safety performance of the system.

[0051] (10) The energy storage system of the present application has multiple working modes: when the sunlight is sufficient, part of the electrical energy output by the system is output to the user end and the system electrical appliance end for use, and part of the electrical energy is used to electrolyze water to store hydrogen energy; 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 electrical energy when the power output by the system is lower than the power used by the user end and the system electrical appliance end. Therefore, the system realizes the complementation of solar energy and hydrogen energy, and improves the flexibility of energy utilization. The system is particularly suitable for running as an independent energy system, and has important engineering value. In addition, the thermal energy stored in the water tank can not only be used to supply the PEM electrolyzer for efficient electrolysis of water to produce hydrogen, but also can be used as hot water for users, enriching the form of energy utilization. BRIEF DESCRIPTION OF DRAWINGS

[0052] Figure 1 is a test result curve graph of the mixed nanofluid.

[0053] Figure 2 is a test result curve graph of the composite phase change material.

[0054] Figure 3 is a structural schematic diagram of a photovoltaic-thermal (PV / T) component.

[0055] Figure 4 is a structural schematic diagram of a solar collector.

[0056] Figure 5 is a structural schematic diagram of a PEM electrolyzer.

[0057] Figure 6 is a structural schematic diagram of the photovoltaic-thermal hydrogen production and energy storage system of the present application.

[0058] Figure 7 is a running logic schematic diagram of the photovoltaic-thermal hydrogen production and energy storage system of the present application.

[0059] Figure 8 is a working curve coupling schematic diagram of a photovoltaic cell and a PEM electrolyzer.

[0060] BRIEF DESCRIPTION OF DRAWINGS

[0061] 1-photovoltaic-thermal component, 2-first water pump, 3-heat preservation 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 supply port, 15-second temperature sensor, 16-heater, 17-radiator, 18-liquid level sensor. DETAILED DESCRIPTION

[0062] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0063] Example 1: Synthesis of Hybrid Nanofluids

[0064] The primary goal of nanofluid research is to achieve higher thermal conductivity than that of basic fluids. The hybrid nanofluid BN-MWCNT / H2O consists of BN nanoparticles, MWCNT nanoparticles, and pure water. A two-step method was used to synthesize the BN-MWCNT / H2O hybrid nanofluid. First, the nanoparticles were weighed using an analytical balance and added to pure water, with the mass ratio of BN to MWCNT nanoparticles fixed at 1:1. The nanoparticles were uniformly dispersed in the water for 30 minutes using a magnetic stirrer. Subsequently, sodium dodecylbenzenesulfonate (SDBS) surfactant was added to the hybrid nanofluid, followed by ultrasonic dispersion for 90 minutes. The proportion of SDBS surfactant used was one-tenth of the combined mass of the BN and MWCNT nanoparticles. Overheating of the instrument should be avoided during ultrasonic treatment to ensure complete dispersion. Following this procedure, hybrid nanofluids with mass concentrations of 0.1 wt%, 0.5 wt%, 1 wt%, and 2 wt% were prepared. The mass concentration refers to the ratio of the combined mass of BN and MWCNT nanoparticles to the total mass of the hybrid nanofluid.

[0065] Figure 1 In the figure, (a) shows the variation of thermal conductivity of the mixed nanofluids at different mass concentrations, and (b) shows the variation of Zeta overpotential of the mixed nanofluids at different mass concentrations. Figure 1 In (a) of the study, it was observed that the increase in thermal conductivity gradually leveled off when the mass concentration of the mixed nanofluid exceeded 0.5 wt%. The thermal conductivity of the 0.5 wt% and 2 wt% mixed nanofluids was 8.06% and 9.08% higher than that of water, respectively. Figure 1 (b) It was observed that after 30 days of standing, the absolute value of the Zeta overpotential of the mixed nanofluids at all mass concentrations was greater than 30 mV, indicating that the prepared mixed nanofluids possess theoretical stability. Furthermore, it was found that the absolute value of the Zeta overpotential of the mixed nanofluids decreased continuously with increasing mass concentration. This is because increasing the mass concentration promotes the aggregation of nanoparticles, causing instability in the solution. Based on the above data, the preferred mass concentration of the mixed nanofluid is 0.5 wt%.

[0066] Example 2: Synthesis of Composite Phase Change Materials

[0067] 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. The paraffin wax is melted in a constant temperature water bath at 60℃, and then MWCNT and BN nanoparticles are added. The mass ratio of the two kinds of nanoparticles is 1:1. Then, oleic acid is added 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.

[0068] Figure 2 In (a), the change of thermal conductivity of the composite phase change material under different mass concentrations is shown, (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.

[0069] From (a) in Figure 2 It is observed 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%.

[0070] From (b) and (c) in Figure 2 The DSC and TGA comparison curves of 2wt% composite phase change material and paraffin wax given in (b) and (c) show 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 be seen that the prepared composite phase change material has good thermal stability, and starts to decompose in small amounts only above 100℃.

[0071] Example 3, structure of photovoltaic-thermal component (PV / T)

[0072] As Figure 3 shown, the photovoltaic photothermal assembly includes photovoltaic cells, heat-conducting silicone, first thermoelectric generator, heat-conducting silicone, nanofluid channel, heat-conducting silicone, and composite phase change material arranged in order from top to bottom.

[0073] The first thermoelectric generator is tightly bonded to the lower side of the photovoltaic cell through heat-conducting silicone. The nanofluid channel is tightly bonded to the lower side of the first thermoelectric generator through heat-conducting silicone. By absorbing the heat of the photovoltaic cell, the upper side of the first thermoelectric generator serves as the heating end. The mixed nanofluid in the nanofluid channel cools the first thermoelectric generator, and the lower side of the first thermoelectric generator serves as the cooling end. The first thermoelectric generator generates electricity through the temperature difference between the upper and lower ends.

[0074] The composite phase change material is tightly bonded to the lower side of the nanofluid channel through heat-conducting silicone. The composite phase change material further recovers the heat of the photovoltaic photothermal assembly. In weak sunlight or winter, the composite phase change material plays a role in keeping the photovoltaic photothermal assembly warm and preventing freezing. In addition, the use of composite phase change material helps to improve the temperature uniformity of the photovoltaic cell. The composite phase change material is packaged with an aluminum foil bag or an aluminum alloy.

[0075] Finally, all components are assembled through the frame, which has a heat preservation function for the internal structure. The photovoltaic photothermal assembly realizes the synergistic thermal management of the mixed nanofluid and the composite phase change material on the photovoltaic cell.

[0076] The mixed nanofluid and the composite phase change material are prepared according to embodiments 1 and 2, respectively.

[0077] According to actual experimental tests, compared with ordinary photovoltaic assemblies, the average temperature of the photovoltaic photothermal assembly of the present embodiment can be reduced by more than 20℃, and the solar hydrogen production efficiency can be improved by 11.38%. Therefore, the photovoltaic photothermal assembly of the present embodiment has good performance and improves the hydrogen production efficiency of the system.

[0078] Embodiment 4, structure and composition of a solar collector

[0079] As Figure 4 shown, the solar collector includes glass cover plate, air gap, second thermoelectric generator, heat-conducting silicone, water fluid channel, heat-conducting silicone, and composite phase change material arranged in order from top to bottom.

[0080] The surface of the second thermoelectric generator is smeared with a selective absorption coating to enhance the absorption of solar radiation energy. By absorbing the heat of the solar radiation energy, the upper side of the second thermoelectric generator serves as the heating end. The water fluid channel is tightly bonded to the lower side of the second thermoelectric generator through heat-conducting silicone, and the water in the water fluid channel cools the second thermoelectric generator, with the lower side of the second thermoelectric generator serving as the cooling end. The second thermoelectric generator generates electricity through the temperature difference between the upper and lower ends.

[0081] 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.

[0082] The composite phase change material is bonded to the lower part of the water flow channel by 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.

[0083] Finally, all the components are assembled by the frame, and the frame has a heat preservation function for the internal structure.

[0084] The composite phase change material is prepared according to Example 2.

[0085] Example 5, structure of PEM electrolyzer

[0086] 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.

[0087] 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.

[0088] 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 electrolysis efficiency when used outdoors.

[0089] Permanent magnets are installed in the upper and lower parts of the PEM electrolyzer. 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.

[0090] 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.

[0091] 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 supply heat to the electrolyzer, thereby avoiding the risk of cold start of the electrolyzer.

[0092] 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 and prevent heat loss.

[0093] The PEM electrolyzer is internally provided with a first temperature sensor, which is used to monitor the internal temperature of the PEM electrolyzer and is connected to the intelligent terminal. When the first temperature sensor monitors that the internal temperature is higher than the set temperature t1, the intelligent terminal opens the air inlet and the air outlet to perform air cooling and heat dissipation. When the first temperature sensor monitors that the internal temperature is lower than the set temperature t2, the intelligent terminal closes the air inlet and the air outlet to stop air cooling and heat dissipation. The air cooling and heat dissipation can be achieved by connecting an air blower. In this embodiment, t1>t2, 95℃>t1>85℃, 75℃>t2>65℃. In this embodiment, t1=90℃ and t2=70℃.

[0094] The composite phase change material is prepared according to Example 2.

[0095] Example 6, structure of a photovoltaic-photothermal hydrogen production and energy storage system

[0096] Based on the above-mentioned Examples 1-5, a photovoltaic-photothermal hydrogen production and energy storage system based on nanofluid and phase change material has a structure as shown in Figure 6 .

[0097] 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.

[0098] 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 circuit is mixed nanofluid, and 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.

[0099] 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.

[0100] 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.

[0101] 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.

[0102] 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%.

[0103] 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℃.

[0104] Embodiment 7, operation logic of photovoltaic photo-thermal hydrogen production and energy storage system

[0105] Based on the above embodiment 6, the operation logic of the photovoltaic photo-thermal hydrogen production and energy storage system based on nanofluid and phase change material of the present application can be divided into two operation modes as shown in Figure 7

[0106] 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 produced 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.

[0107] 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 supply power to the system electrical appliances.

[0108] 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.

[0109] 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 the purpose. 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.

[0110] ​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.

[0111] 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.

[0112] 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.

[0113] 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 photo-thermal hydrogen production and energy storage system based on nanofluid and phase change material, characterized in that, The application relates to a photovoltaic photothermal assembly, a water tank, a solar heat collector and a PEM electrolytic cell. The photovoltaic photothermal assembly comprises photovoltaic cells, a first thermoelectric generator, a nanofluid channel and a composite phase change material arranged from top to bottom. The solar heat collector comprises 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 photothermal assembly enters a heat exchange coil in the water tank to heat water in the water tank. The water in the water tank first enters the water fluid channel of the solar heat collector to be heated, and then enters the PEM electrolytic cell 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. The top of the PEM electrolytic cell is provided with a condenser for concentrating solar radiation energy on the upper part of the PEM electrolytic cell. The upper part of the PEM electrolytic cell comprises a selective absorption coating, a composite phase change material, a permanent magnet and an end plate arranged from top to bottom. The lower part of the PEM electrolytic cell comprises an end plate, a permanent magnet, a composite phase change material and a heat preservation layer arranged from top to bottom. The middle part of the PEM electrolytic cell is composed of a plurality of electrolytic cell single pieces connected in series. The side edges of the PEM electrolytic cell comprise a composite phase change material and a heat preservation layer arranged from inside to outside. The selective absorption coating is used for converting solar radiation energy into heat energy to improve the operating temperature of the PEM electrolytic cell. The upper and lower permanent magnets of the PEM electrolytic cell are used for generating 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 for collecting and storing 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. When the internal temperature is higher than the set temperature t1, the ventilation opening and the air outlet are opened for air cooling, and when the internal temperature is lower than the set temperature t2, the ventilation opening and the air outlet are closed, t1>t2. The power matching between the photovoltaic cells in the photovoltaic photothermal assembly and the PEM electrolytic cell is realized by increasing / decreasing the number of electrolytic cell single pieces connected in series 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 is less than or equal to U / 2, then the number of electrolytic cell single pieces connected in series is increased / decreased by C, when D is greater than U / 2, then the number of electrolytic cell single pieces connected in series is increased / decreased by C+1; wherein, if A is greater than B, the number of electrolytic cell single pieces connected in series is increased, if A is less than B, the number of electrolytic cell single pieces connected in series is decreased.

2. The photovoltaic photo-thermal hydrogen generation and energy storage system based on nanofluid and phase change material according to claim 1, characterized in that, The mass ratio of the BN nanoparticles to the MWCNT nanoparticles in the mixed nanofluid 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 percentage of the mass sum of the BN nanoparticles and the MWCNT nanoparticles in the total mass of the mixed nanofluid is 0.5 wt%.

3. The photovoltaic photo-thermal hydrogen generation and energy storage system based on nanofluid and phase change material according to claim 1, characterized in that, The mass ratio of the BN nanoparticles to the MWCNT nanoparticles in the composite phase change material is 1:1, and 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 percentage of the mass sum of the BN nanoparticles and the MWCNT nanoparticles in the total mass of the composite phase change material is 2 wt%.

4. The photovoltaic photo-thermal hydrogen generation and energy storage system based on nanofluid and phase change material according to claim 1, characterized in that, The specific structure of the photovoltaic-photothermal assembly is as follows: The first thermoelectric generator is bonded below the photovoltaic cell through heat-conducting silica gel; the nanofluid channel is bonded below the first thermoelectric generator through heat-conducting silica gel; the upper side of the first thermoelectric generator serves as a heating end and absorbs the heat of the photovoltaic cell; the lower side of the first thermoelectric generator serves as a 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 through heat-conducting silica gel, is used for further recycling 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 through a frame, and the frame has a heat preservation function for the internal structure.

5. The photovoltaic photo-thermal hydrogen generation and energy storage system based on nanofluid and phase change material 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 radiation energy into heat energy; the water fluid channel is bonded below the second thermoelectric generator through heat-conducting silica gel; the upper side of the second thermoelectric generator serves as a heating end and absorbs the heat of solar radiation energy; the lower side of the second thermoelectric generator serves as a 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; The upper side of the second thermoelectric generator is provided with a glass cover plate, and there is an air gap between the glass cover plate and the second thermoelectric generator, for reducing the environmental heat loss of the solar collector; The composite phase change material is bonded below the water fluid channel through heat-conducting silica gel, is used for further recycling the heat of the solar collector, and 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; All components are assembled through a frame, and the frame has a heat preservation function for the internal structure.

6. The photovoltaic photo-thermal hydrogen generation and energy storage system based on nanofluid and phase change material according to claim 1, characterized in that, The first gas-liquid separator and the second gas-liquid separator are arranged at the outlet of the PEM electrolytic cell, and are respectively used for separating oxygen and hydrogen; the separated water is recycled into the water tank; the separated oxygen and hydrogen are respectively subjected to heat exchange with the water in the water tank through the first heat exchanger and the second heat exchanger, the heated water is returned to the water tank again, and the heat-exchanged oxygen and hydrogen are respectively stored in the oxygen storage tank and the hydrogen storage tank.

7. The photovoltaic photo-thermal hydrogen generation and energy storage system based on nanofluid and phase change material according to claim 1, characterized in that, The water tank is provided with a temperature sensor, a heater, a radiator, and a liquid level sensor; 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 until the water level reaches the set water level x2, and the replenishment is stopped, x1 < 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 for cooling until the temperature reaches the set temperature t4, and 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 for heating until the temperature reaches the set temperature t6, and the heating is stopped, t3 > t4 > t6 > t5.

8. The photovoltaic photo-thermal hydrogen generation and energy storage system based on nanofluid and phase change material according to any one of claims 1-7, characterized in that, The system operates as follows: When the light intensity is sufficient, 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; when the light intensity is insufficient, the electrolysis of water is stopped, and the electrical energy output by the photovoltaic cell is all 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 used to supply the system electrical appliances.

Images