Performance analysis of flat plate heat pipe and heat pump heating device based on solar energy

To solve the problem of poor heating performance of air source heat pumps when the air temperature is relatively low, this paper proposes a flat heat pipe and heat pump heating device based on solar energy. The device can store solar energy and continuously replenish the heat pump. The performance of the device is studied through experiments and simulation calculations. The results show that the difference in temperature between the evaporating section and the condensing section of the flat heat pipe is about 1°C, which shows that the heat pipe has good thermal conductivity. The device can produce a temperature rise of 7~61°C for the air entering the heat pump, which can realize the purpose of heat replenishment for the heat pump. Under the premise of no heat supply, the device can continuously heat -5°C fresh air to 5°C within 3.6 hours through the storage of solar energy. It shows that the device has good heat storage capacity. The above research shows that the device can realize the function of continuous replenishment of air source heat pump by solar energy, and has certain practical application value.


Introduction
Air source heat pumps have gained widespread attention due to their advantages such as low cost, no pollution, and free installation, but they are prone to frost at low temperatures and have poor heating performance [1] .Solar energy has the advantages of being rich, renewable, clean, and pollution-free [2] , but due to time, weather, and other factors, solar energy has intermittency, instability, and other problems [3] .Therefore, to solve the above problems, this paper puts forward a kind of flat plate heat pipe and heat pump heating device based on solar energy (FPHPD), which can use solar energy to heat an air source heat pump.Then it can give full play to their respective advantages and overcome their shortcomings.
Methida et al. [4] proposed a solar water-heating system with a flat heat pipe as a heat transfer device, and the experimental results showed that the thermal conductivity of the flat heat pipe was better than that of the U-tube, thermosyphon, and closed-loop pulsating heat pipe.Lim et al. [5] applied the heat pipe to the evaporator end of the ground source heat pump, and the evaporator was placed in the heat pipe radiator, which improved the performance coefficient by 10.3% compared with the ordinary direct expansion type.Lou et al. [6] proposed a flat-panel heat pipe applied to a solar-air dual-energy heat pump system.The results indicate that the equivalent thermal conductivity of the flat plate heat pipe can reach 6.8×10 5 W/(mꞏ℃) at low temperatures, and the COP of the dual heat source operating mode is 5.7% higher than that of the solar single-mode system.
Through the above research, it can be found that the flat heat pipe has good thermal conductivity, and the application in the solar heat pump has improved the performance of the system, but the application mode is mostly embedded in a certain part of the system, which leads to a complex structure.

Experimental setup
In this paper, a simple structure of FPHPD is proposed, and the performance of the device under different operating conditions is analyzed through experiments and simulation calculations, which provides theoretical support for the practical application of the device.

Experimental composition and principle
The FPHPD performance test platform is mainly composed of the heat pipe, fin, air duct, fan, electric heater, heat storage water tank, and circulating pump.The experimental schematic diagram and measuring point arrangement are shown in Figure 1.In this experiment, an electric heater was used to heat the water in the water tank to simulate the process of solar energy collection.After the experiment began, the water in the water tank was heated by the electric heater, and then the working medium in the evaporation section of the heat pipe was heated and evaporated.The evaporating working medium reached the condensing section in the air duct, and when the cold air was met, the condensation and heat were released.The condensing working medium returned to the evaporation section under the action of gravity, and the heat transfer process was completed by repeating.The outdoor air flowed through the heat pipes and fins to achieve temperature rise and then entered the heat pump evaporator.The block diagram of the experiment process is shown in Figure 2. IOP Publishing doi:10.1088/1742-6596/2728/1/0120063 26 cm.The fan is an SF axial flow fan with a rated power of 120 W and a rated air volume of 1, 300 m 3 /h.It is connected to an external fan governor, and the air volume and speed of the fan can be adjusted according to demand.The rated power of the circulating pump is 115 W, the rated flow is 60 L/min, and the head is 6 m.The heat pipe material is aluminum, and the heat conduction medium is an acetone mixture, with a size of 650 mm×50 mm×3 mm.The condensing section length is 250 mm, and the evaporation section length is 400 mm.The material of the fin is aluminum, with a size of 50 mm×41 mm×8 mm, each fin has 26 teeth, and the fins are glued to the front and back sides of the heat pipe through the thermal silicone gel.Each side pasted 6 fins, that is, 1 heat pipe pasted the fin number of 12, and the heat pipe evaporation section does not paste the fin.There are two rows of heat pipes between the water tank and the air duct, with 12 heat pipes in each row, that is, the device has a total of 24 heat pipes and 288 fins.

Test conditions
In this experiment, the wind speed in the duct was controlled by the fan governor and thermal anemometer to about 1.5 m/s, and the water in the tank was heated by the electric heater to control the average water temperature of 33℃, 43℃, 53℃, 63℃ and 73℃.When the average inlet air temperature was 7℃ and 23℃, the temperature of the evaporation section and condensing section of the heat pipe and the outlet air temperature were measured.The specific test conditions are shown in Table 1.
Table 1.Description of test conditions.

Data acquisition and data processing
The T-type thermocouple with an accuracy of 0.4% was used to measure the temperature of the water tank, the inlet and outlet water temperature of the circulating pump, the surface temperature of the evaporation and condensing section of the heat pipe, and the temperature of the inlet and outlet of the air duct.The AR866A thermal anemometer with an accuracy of ±0.1 m/s and ±0.1℃ was used to measure the wind speed and temperature at the inlet and outlet of the duct.The data acquisition system consists of one computer and one Agilent34972A data acquisition instrument, which can realize automatic data acquisition and real-time recording.In this experiment, the difference in temperature ∆T 1 between the evaporation section and the condensing section of the heat pipe is used as the performance evaluation index of the heat pipe, and the temperature rise of air heated by heat pipes ∆T 2 and the heating quantity Q a are used as the heat transfer performance evaluation index of the device.The formula is as follows: where ∆T 1 is the difference in temperature between the evaporation section of the heat pipe and the condensing section; T 6 is the temperature of the evaporation section of the heat pipe (℃); T 2 is the temperature of the condensing section of the heat pipe (℃); ∆T 2 refers to the temperature rise of air heated by heat pipes; T 3 refers to the air outlet temperature heated by the heat pipe fin (℃); T 1 is the air duct inlet temperature (℃); Q a is the heat added to the air by the heat pipe (kW); C a is the air-specific heat capacity (J/(kgꞏ℃)); ρ a is the density of air (kg/m 3 ); V a is the volume flow of air through the air duct (m 3 /s).

Establishment of mathematical model
According to the measured data, a mathematical model is established by using Matlab to calculate the heat storage performance of the water tank.In the modeling process, the phase transformation heat emission process inside the heat pipe is ignored, and the temperature of the water tank is considered to be steady state.The model is set to stop heating after the electric heater heats the temperature in the water tank and turns on the fan.The wind speed is set at 1.5 m/s, the inlet air temperature is -5℃, and the outlet air temperature after heating is 5℃.The time required for the temperature of the water tank to drop to 30℃ is calculated.
The formulas required for modeling are as follows: ) where Q h is the change in heat storage of the water tank from 70℃ to 30℃ (kJ); C h is the water-specific heat capacity (J/(kg •℃)); ρ h is the density of water (kg/m 3 ); V h is the volume of water in the tank (m 3 ); T h is the initial temperature of the water tank (℃); T h ' is the final temperature of the water tank (℃).
) ( where Q A is the heat required to heat the air from -5℃ to 5℃ (kW); T a is the temperature of the outgoing air (℃); T a ' is the temperature of the incoming air (℃).
where Q A ' is the heat transferred to the air in the duct through the heat pipe, and q s is the rate of heat loss, taking 5% [7] .
where τ is the time required for the water temperature in the heat storage tank to decrease from 70℃ to 30℃ (S).
The specific calculation steps are shown in Figure 3.

Performance analysis of flat plate heat pipe
The temperature change of the evaporating and condensing sections of the heat pipe under different working conditions is shown in Figure 4.As shown in Figure 4, as the temperature of the heat storage tank increases, the temperatures of the evaporating and condensing sections of the heat pipe also continue to increase, and the temperature of the heat pipe under Condition 2 is slightly higher than that under Condition 1, which shows that the fresh air temperature has less effect on the performance of the heat pipe.Under Condition 1, when the temperature of inlet air is 7℃ and the water temperature is 63℃, the difference in temperature between the evaporating and the condensing section of the heat pipe is the maximum, which is 1.27℃.The average difference in temperature is 1℃, which shows that the flat plate heat pipe has good thermal conductivity.

Analysis of device heat transfer performance
The elevated temperature and heating amount of the air heated by the device under different operating conditions are shown in Figure 5.As can be seen from Figure 5, with the increase of the water temperature in the thermal storage tank, the temperature rise and heating capacity of the device to the air are also increasing, and the temperature rise and heating capacity of Condition 1 is greater than that of Condition 2, indicating that at the same water temperature, the lower the inlet air temperature is, the greater the temperature rise and heating capacity of the device to the air is.Under Condition 1, when the inlet air temperature is 7℃ and the water temperature is 73℃, the temperature rise and heating capacity are the largest, which are 61℃ and 8.9 kW respectively, and the average temperature rise under the two conditions is 27℃ and 41℃, and the average heating capacity is 4.4 kW and 6.1 kW respectively.For every increase of 1℃ in the average temperature of the water tank, the elevated temperature of this device in the air increases by nearly 1℃, which indicates that this device can achieve the purpose of supplementing the air entering the air source heat pump evaporator with heat.

Analysis of the thermal storage performance of the device
The change in water temperature and heat storage of the tank calculated according to the model is shown in Figure 6.As shown in Figure 6, when the original water temperature of the tank is 70℃, the time required for the temperature of the tank to drop to 30℃ is 216 minutes when the air inlet to the duct is heated from -5℃ to 5℃ by the device, and the change of the heat storage of the whole process is 2.1 MJ.When applying the device, if the solar collector heats the water in the thermal storage tank to 70℃ before sunset, the device can continuously heat the air entering the evaporator of the heat pump from -5℃ to 5℃ for 3.6 hours before the water temperature drops to 30℃.Similarly, if the solar thermal collector heats the water temperature of the thermal storage tank to 60℃ and 50℃ before sunset, the continuous heating air time is 2.7 hours and 1.8 hours respectively.It shows that the device has good thermal storage performance, and the air entering the air source heat pump evaporator can still be continuously replenished with heat, without solar energy.

Conclusion
Taking FPHPD as the research object, this paper analyzes the thermal conductivity of flat plate heat pipe, the heat transfer, and the heat storage performance of the device in detail.The experimental conclusions are as follows: The difference in temperature between the evaporation section and the condensing section of the heat pipe is about 1℃, indicating that the heat pipe has good thermal conductivity.For every increase of 1℃ in the average temperature of the water tank, the device increases the temperature rise of the air entering the air duct by nearly 1℃, and the maximum temperature rise is 61℃.The heating capacity of the air is 8.9 kW, indicating that the device can achieve the function of supplementing heat to the air source heat pump evaporator.The heat storage tank has good heat storage performance, allowing the device to continuously supplement heat to the air source heat pump without a heat supply, which has a certain application value.

Figure 1 .
Figure 1.Experimental schematic diagram.In this experiment, an electric heater was used to heat the water in the water tank to simulate the process of solar energy collection.After the experiment began, the water in the water tank was heated by the electric heater, and then the working medium in the evaporation section of the heat pipe was heated and evaporated.The evaporating working medium reached the condensing section in the air duct, and when the cold air was met, the condensation and heat were released.The condensing working medium returned to the evaporation section under the action of gravity, and the heat transfer process was completed by repeating.The outdoor air flowed through the heat pipes and fins to achieve temperature rise and then entered the heat pump evaporator.The block diagram of the experiment process is shown in Figure2.

Figure 2 .
Figure 2. Experimental process block diagram.The heat storage water tank is made of stainless steel with a size of 0.6 m×0.3 m×0.8 m, and the external is wrapped with thermal insulation materials.The air duct is made of an XPS fire insulation board with a size of 1.25 m×0.4 m×0.3 m.The electric heater is temperature-controlled heating, and the adjustable temperature range is 30~110℃.There are two electric heaters with rated power of 2 kW and 4 kW respectively.The installation mode is side-mounted, and the length inserted into the box is

Figure 3 .
Figure 3. Flow chart of mathematical model calculation.

Figure 4 .
Figure 4. Temperature change of evaporation and condensation section of heat pipe.

Figure 5 .
Figure 5. Plot of temperature rise and heating volume variation of heat pipe to air temperature.

Figure 6 .
Figure 6.Variation of water temperature and heat storage in water tanks.