An Automatic Drying System Combining Photovoltaic, Photothermal, and Energy Storage

A solar air collector is composed of double-tube vacuum tubes, and the air collector is designed as an automatic drying system. Compared with the fossil fuel drying system, this system has better economic and social benefits. After theoretical calculation and experimental research analysis, the total energy saving of the drying system is 127564 MJ/year, with a cost saving of 21685.88 yuan/year and a static recovery period of 2.3 years; The experiment of drying 100 catties of sweet potatoes showed that the system can meet the needs of users for drying agricultural products. This system uses solar energy as its energy source, with zero pollution and zero emissions, and has certain promotion and application value.


Introduction
At present, most common drying systems use methods such as electric radiation heating, heat pump heating, oil or gas boiler heating, etc.This technology enables the heating temperature and fluctuation performance to meet the requirements, and the drying quality is good.The disadvantage is that it consumes energy, is not environmentally friendly, and the drying cost is high.From an economic and environmental perspective, it is not suitable for individual farmers to use.
Under the guidance of the national "dual carbon target" policy, this article mainly studies the technology of using solar energy to dry agricultural products.In order to improve the current situation of high energy consumption and serious pollution emissions in the crop drying industry, an automatic drying system composed of air collectors, heat storage devices, solar panels, fans, and other devices, which combines photovoltaic, photothermal, and thermal storage, is proposed.The system's heat collection, thermal storage, energy conservation, and drying are also studied.This system uses solar energy as a heat source, replacing conventional energy sources such as coal, oil, and natural gas, with significant energy-saving advantages.The drying system is equipped with universal wheels, which can move and dry agricultural products anytime and anywhere, reducing transportation losses and making it more economical.

Basic requirements
To solve the structural design problem of an automatic drying system that combines photovoltaic, photothermal, and energy storage, the following design requirements are proposed based on existing conditions: (1) Usage area: Most regions in China have abundant solar energy resources, with an average annual sunshine time of over 2200 hours, making it easy to promote the use of solar drying systems; (2) Drying materials: China is a major agricultural country with a rich variety of agricultural products, mainly targeting flake, block, and granular agricultural products (3) Drying cycle: The material characteristics of different agricultural products determine the difficulty of drying, and the drying cycle varies in length.Therefore, materials with high starch content, such as potatoes and sweet potatoes, are planned to be selected for drying experiments; (4) Drying weight: When using sweet potatoes as the drying material, the drying weight should not exceed 10 kg; (5) Hygienic conditions: When conducting drying experiments, avoid adverse effects of external dust, insects, and ants on the drying materials.

System Design
The traditional solar vacuum tube is a structure with one open and the other sealed.The heat inside the vacuum tube relies on the natural convection and circulation of the medium inside the tube, and the speed of natural convection has a significant impact on the efficiency of solar heat utilization.In the case of water leakage, the vacuum tube is prone to being in a high-temperature state, which affects the heat conversion efficiency of the vacuum tube.Based on this, a large-diameter solar dual pipe is used, which is a structure with two interconnected ends, as shown in Figure 1.The medium flowing inside the vacuum tube is air, which exchanges heat with the inner wall of the vacuum tube.At the same time, there is an internal turbulence structure that allows air to fully absorb heat when passing through.This structure improves the efficiency of heat exchange and can obtain different airflow and temperature through the series and parallel connection of collectors, expanding their range of use.

Figure 1. Structural of double pipe with openings at both ends
The automatic drying system that combines photovoltaic, photothermal, and energy storage includes a solar air collector, solar panel, supply air duct, drying room, return air duct, cooling fan, circulating fan, pry base, universal wheel, and other structures.The schematic diagram of the automatic drying system is shown in Figure 2.
The solar automatic drying system is placed under sunlight, which will heat the air inside the vacuum tube.At the same time, the solar panel will generate electricity, driving the circulating fan to start.At this point, the hot air inside the solar air collector is sent into the drying room for drying.The power generated by another solar panel will drive the cooling fan to start, thereby cooling the circulating fan and preventing high-temperature damage to the fan.All equipment is installed on a pry block with universal wheels, allowing the drying system to move as a whole without affecting system stability.The on-site physical image is shown in Figure 3.
The energy used in this drying system is solar energy, with a high photothermal conversion rate, no carbon emissions, and no operating costs.The drying output is large, conserving manual labor.Combining the unstable characteristics of solar energy, magnesium oxide is selected as the heat storage material, which can make the temperature in the drying room stable and dry for a long time, realizing the "peak shifting and valley filling" of solar energy.The application of the photovoltaic battery-independent power supply and heat dissipation fan is not affected by market noise, and it is convenient to use.It can provide heat dissipation protection for the main working fan, ensuring system reliability.

Estimation of Structural Fatigue Life of Double Tube Glass
The fatigue life of glass structures under load can be estimated using the following recommended formula: ɑ--The depth of Crack; N--Number of fatigue load cycles; C--Constant; ∆K--Stress intensity factor amplitude.Suggestions for the structure of circular tube glass:  1) and ( 2), it can be concluded that:

Calculation of hot air pipeline diameter
For general areas with an altitude of<500m, the calculation formula can be as follows: (4) D --Pipe diameter, m; Q t --Air volume under general regional operating conditions, m 3 /h; V --Pipeline wind speed, m/s.

Analysis and evaluation indicators
The analysis and evaluation indicators of solar hot air systems include the total annual energy-saving amount of the solar drying system, the annual energy-saving cost of the solar drying system, and the pre-evaluation of the investment payback period.
(1) Annual total energy savings of solar drying systems △Q save =ACJT(1-η L ) η cd =75×4252.14×(1-0.2)×0.5=127564.2MJ △Q save --Annual energy savings of solar hot air systems, MJ; η L --Pipeline heat loss rate, %; we take it as 20%.AC --Collection area of direct system solar energy, m 2 ; The heat collection area of a single project is approximately 75 m 2 .ηcd --Collection efficiency of solar collectors,%; Take 50%.JT --Annual total irradiation amount on the day lighting surface of the solar collector, MJ/m 2 ; The average annual total radiation exposure in Shandong region is approximately 4252.14 MJ/m 2 .
(2) Annual cost savings of solar drying systems W J =C c △Q save =127564.2×0.17=21685.914WJ --Simple energy-saving cost of solar hot air system, yuan; C c --Conventional energy heat price in the year of system design, yuan/MJ; Take 0.28 yuan/MJ.
(3) Pre-evaluation of investment payback period Static payback period calculation: Y t =W z /W j =50000/21685.914=2.30Y t --Investment payback period of solar hot air system; W z --increased investment in solar hot air systems compared to conventional heating systems; (i.e., investment in the heat collection system and control system, including: heat collectors, system pipelines, pipeline accessories and equipment, system insulation, and fixed materials) W j --Annual energy-saving cost of solar hot air system.

Experimental Analysis
The analysis and evaluation indicators of solar hot air systems include the total annual energy-saving amount of the solar drying system, the annual energy-saving cost of the solar drying system, and the pre-evaluation of the investment payback period.Experiment 1: Using 100 catties of sweet potatoes as the drying object, the moisture content was 73%, 42%, 48%, and 53% after 8 hours of drying by natural air drying, electric heating, coal-fired heating, and solar drying, respectively.
Experiment 2: The moisture content is controlled to 10%.By comparing natural drying, electric heating, coal-fired heating, and solar drying, the drying times used were 72, 15, 17, and 19 hours, respectively.
Experiment 3: The moisture content is controlled to 10%.By comparing natural air drying, electric heating, coal-fired heating, and solar drying, the operating cost of the solar drying system is saved by 150 yuan compared to manual air drying, 86.4 yuan compared to electric heating, and 61.625 yuan compared to coal heating, which has high economic benefits.
The comparative experiment results of solar drying and natural sun drying of sweet potatoes show that solar drying not only shortens the drying cycle, but also reduces the phenomenon of "sugar production" caused by direct sunlight.The solar drying system has good drying efficiency and high drying quality.Overall analysis shows that solar drying systems have advantages over other drying methods in terms of energy savings, carbon emissions, and operating costs.

Conclusion
Theoretical analysis and experimental research were conducted on the automatic drying system combining photovoltaic, photothermal, and energy storage.The results showed that the total energy saving of the drying system was 127564 MJ/year, with a cost saving of 21685.88yuan/year and a static recovery period of 2.3 years.The energy used in this system is solar energy, with a low failure rate, simple system, stable operation, low cost, zero pollution, zero emissions, and can meet the needs of users for agricultural product drying.It has certain promotion and application value.

2σ a 2 )
σ--One load design value; ɑ 0 --The depth of initial crack; ɑ c --The depth of critical crack.According to Equations (

Table 1 .
The product parameter.

Table 2 .
Efficiency Analysis of Different Drying Methods