Photothermal and Photovoltaic Utilization for Improving the Thermal Environment of Chinese Solar Greenhouses: A Review

Abstract

A Chinese solar greenhouse (CSG) is an agricultural facility type with Chinese characteristics. It can effectively utilize solar energy during low-temperature seasons in alpine regions. The low construction and operation costs make it a main facility for agricultural production in the northern regions of China. It plays an extremely important role in “Chinese vegetable basket projects”. Energy is one of the important issues faced by CSGs. The better climate resources in the northern regions of China make it possible to apply solar energy as a green and sustainable energy source in the production of CSGs. Faced with the increasingly serious environmental problems of the new century, the Chinese government has made a decision and put it into practice to improve the rate and efficiency of solar energy utilization in agricultural facilities. In this paper, we summarize the research on the application of photovoltaic power generation and solar thermal technology in CSGs. The application of these advanced solar technologies has made great progress. With the further improvement of economic benefits and the establishment of relevant support policies and incentive mechanisms, the combination of CSG and solar energy technology will have certain application prospects and satisfy China’s requirements for long-term sustainable development.

1. Introduction

In recent years, the global greenhouse horticulture field has always focused on “energy and water resources”, especially when the continuous increase in energy prices has brought tremendous operational pressure to the growers [1]. Meanwhile, the implementation of the Kyoto Protocol and the Paris Climate Agreement has increased the pressure on the production of greenhouses in terms of environmental and ecological protection. Energy conservation and the broadening of the renewable energy range have become the subjects of greenhouse energy research [2]. Since 2006, renewable energy sources, such as solar energy, wind energy, biomass energy, etc., have been put into commercial operation in part of the greenhouses in Europe and America [3]. Related application research is also being comprehensively developed. In the “New energy greenhouse design contest” of 2007 in the Netherlands, a total of three schemes were awarded, including a Sunwind greenhouse and two other greenhouses designed by the universities of Wageningen and Benk [4]. These three greenhouses used solar energy, geothermal energy, and wind energy with related technologies to explore the technical mechanism and commercialization approach of reducing the future dependence on fossil energy in greenhouses. Related projects have been demonstrated in Bleiswijk with the government’s support.

Facility horticulture in China started relatively late, due to strong and rigid demand for agricultural products (particularly vegetables) in the northern regions during the winter. The facility horticulture area in China has developed rapidly over the past 30 years, and the current facility horticulture area reaches 3.7 million hectares, which makes it the country with the largest facility horticulture area in the world [5]. In facility horticulture, the development of plastic greenhouses and Chinese solar greenhouses (CSGs) is the most rapid, while the multi-span greenhouse is slow (Figure 1). The light-transmitting covering material of the multi-span greenhouse comprises a film, glass, and a plastic plate. Research showed that the coal consumption of the multi-span greenhouse is about 90–150 kg/m² in a year, and the coal cost accounts for 30–50% of the whole production cost [6]. Therefore, the multi-span greenhouse is difficult to develop on a large scale, even if the economy of the century develops to a certain level. The plastic-covered tunnel mostly uses bamboo and steel as a framework and is a simple arched greenhouse covered with plastic film. It is built when in use and dismantled when not in use. The little requirement for additional energy makes it mostly used in southern areas of China [7].

Chinese agriculture is changing from traditional agriculture to modern agriculture. Although plastic-covered tunnels and CSGs are still important production facilities [9], high-yield and high-efficiency modern greenhouses are one of the directions of future Chinese agriculture development. With the continuous promotion of the modernization and industrialization of agriculture in China, the degree of dependence of agricultural production on energy has continuously increased. Compared with traditional agriculture, facility agriculture has a stronger dependence on energy. As shown in Figure 2, the climate and energy use patterns of agricultural facilities in China are very different from those in countries with advanced facility horticulture (http://solargis.com/maps-and-gis-data/download (accessed on 12 January 2023)). It is difficult to reduce energy consumption under complicated climate conditions in China without modifying Venlo-type multi-span greenhouses introduced from abroad.

A CSG is a facility type independently developed in China. It is a passive greenhouse with three wall sides and one light-transmitting side. Generally, solar energy is its only source of heat. At present, the overwintering production of vegetables in the northern regions of China completely depends on greenhouses, but the heat accumulated by the north walls of traditional CSGs is very limited [10]. Therefore, most CSGs need to be heated by coal. However, this heated mode faces problems of low efficiency, high pollution, and high investment and operation costs. Thus, the economic benefit of greenhouse cultivation is restricted. Especially in recent years, many cities in the north of China have come up with relevant policies to cope with haze disasters. As a result, the traditional coal-fired small boiler is banned. In this regard, it is important to select solar energy to replace coal for heating greenhouses [11]. Generally, illumination and temperature are considered the main environmental factors of the CSG, so the future development trend focuses on the research of structural optimization and heat insulation performance of the CSG.

In China today, the capacity of solar water heaters and photovoltaic cells ranks first in the world. In recent years, the solar cell industry has faced the problem of excess capacity under the anti-dumping complaints of Europe and America on Chinese photovoltaic export products. The solar cell industry in China develops diversified consumption markets for destocking by combining related industries. The application demonstration of distributed photovoltaic power generation and the photovoltaic poverty relief policy promoted by the National Energy Bureau make the solar energy and agriculture industries complement each other [12]. After combining photovoltaic cells with CSG, the power generated by the solar energy system can be used for the functions of intelligent temperature control, irrigation, and light supplementation. Moreover, the residual electric quantity can be sold back to a power grid company to create additional benefits [13]. In addition, the photovoltaic CSG can effectively utilize land resources and avoid the problem of farmland occupation. In the critical period of vigorously promoting the development of modern agriculture in China, the development of the photothermal/photovoltaic CSG not only facilitates agricultural production, but also improves the agricultural technology level to a great extent.

This review discusses the limitations of conventional technologies and how hybrid configurations can help to overcome those barriers to boost the implementation of solar systems in protected agriculture. Section 1 introduces the motivation and objectives of this review article. Section 2 presents the different kinds of solar technology systems in CSG. Section 3 presents the most common conventional solar heating systems in CSG. Section 4 presents the most common conventional solar photovoltaic systems in CSG. Section 5 discusses the solar energy supply and energy demand in CSG. Finally, Section 6 presents the conclusions of this article.

The aim of the present work is to analyze the potential of hybrid configurations of PV and solar heating technologies to improve the light and thermal environment in CSG. This work is also a contribution for solar project developers; it helps to encourage the proliferation of solar systems in agricultural engineering.

2. Basic Conditions of CSGs

2.1. Conditions Related to Solar Greenhouses in the Representative Area

At present, in North China, vegetables consumed in winter are mainly supplied by means of facility agriculture, while in all the facilities, more than 90% are produced from CSGs [14]. The continuous popularization and construction of CSGs can increase the labor income of farmers, playing an important role in supplying fresh vegetables in northern areas in the winter. However, the distribution of the CSGs in China is extremely unbalanced; the CSGs in North China, Northeast China, and the Huanghuai area develop faster, while the CSGs in Shandong, Henan, and Liaoning account for the vast majority of the CSGs in China. The proportions of the provinces of Xinjiang, Qinghai, Gansu, Ningxia, and Shaanxi in the northwest are very small compared with the eastern region of China. However, the northwest regions are vast, and the undeveloped, uncultivated land has abundant resources and sufficient photothermal.

The distribution of Chinese solar energy resources has obvious geographical characteristics. Except for the two autonomous regions of Tibet and Xinjiang, the western region is higher than the eastern region, and the southern region is substantially lower than the northern region. The distribution characteristic reflects the restriction of solar energy resources by conditions such as climate and geography. The Chinese Academy of Meteorological Science divides China into four bands of solar resources, as shown in Figure 3c [15]. The climate is partitioned according to the requirements of the thermal engineering design (Figure 3b). The climate factors of the air temperature, the average temperature of the coldest month (January), and the hottest month (July) are taken as the main indexes. The annual average temperature between 5 °C and 25 °C is taken as the auxiliary index. Thus, China is divided into five zones: severe cold; cold; hot in summer and cold in winter; hot in summer and warm in winter; and temperate regions [16]. Compared with Lanzhou and Shenyang (Figure 3d), Shouguang has better temperature and illumination resources, which ensure the basic photothermal environment requirements in the CSG.

2.2. Solar Technology for Chinese Solar Greenhouses

The poor heat-preserving performance of the current Chinese solar greenhouses and the shadowing effect in PV greenhouses have directed a series of studies in academia on utilizing advanced solar technology for PT (photothermic) and PV utilization technology, which are the main subjects studied by Chinese scholars, as shown in Figure 4.

3. Application of Photovoltaic Technology in CSGs

3.1. Development of Photovoltaic Technology in CSGs

In photovoltaic applications combined with buildings, the photovoltaic system can be combined with various buildings, structures, and envelope structures that can receive solar illumination. Photovoltaic technologies are widely used in production in conjunction with various agricultural infrastructures, such as photovoltaic systems, integrated glasshouses, and tunnel greenhouses [20]. In view of the special conditions of strong light during the day and low temperatures at night in most areas in northwest China, the solar greenhouse is suitable for photovoltaic applications. Currently, photovoltaic systems are still in the startup and exploration phases for CSGs [21]. The CSG is usually located in a suburban county or a remote area away from the town. Generally, the cost of power transmission and supply is high, and some remote areas even have no power supply. Power utilization facilities in the advanced CSG are numerous, so a stable power supply is needed if the economic and efficient operation of the CSG is to be satisfied [22]. Related technologies are applied to traditional CSGs, such as illumination, temperature and humidity regulation, ventilation, CO₂ regulation, and irrigation spraying systems. Photovoltaic cell panels covered on the south slope surface of the CSG are usually thick and heavy, and the greenhouse is required to be supported by better stand columns and beam frames. The southern slope angle of the existing CSG is gentle, and the generating efficiency of the photovoltaic cell panel is influenced to a certain degree.

The sunlight incident on the surface of the CSG film comprises three types: ultraviolet light, visible light, and infrared light. Visible light is the most suitable for plant growth, wherein the blue light within 400~520 nm and the red orange light within 610~720 nm are effective energy regions for photosynthesis [23]. Meanwhile, the greenhouse structure absorbs NIR more than the plants. As a result, the temperature of the greenhouse air increases, and this leads to a reduction in crop production. Thus, NIR is better utilized by semi-transparent photovoltaic modules to produce electricity (Figure 5).

In common solar photovoltaic cells (Table 1), the light transmittance of the thin-film cell can be customized to any percentage, such as 10%, 20%, and 30%. The light-transmitting principle is realized by laser scribing and the small-hole imaging principle. The power of the film assembly decreases with increasing light transmission, and the size of the assembly can be selected or customized based on the original structural features and modulus [25]. Most photovoltaic components used in the current Chinese photovoltaic agricultural greenhouse are thin-film components, with the advantages of good weak light properties, low cost, long power generation time, and a light transmission spectrum beneficial to plant growth. However, the thin-film components have low photoelectric conversion efficiency and a large annual attenuation rate, so the thin-film components have a short service life, generally 10~15 years. Compared with other related component materials, polysilicon has higher economy and practicability due to its advantages of maintenance-free properties, long service life, and low cost.

The dual-glass assembly is composed of three raw materials: glass, ethylene vinyl acetate (EVA) film, and polysilicon (Figure 6). EVA is a kind of thermosetting adhesive film with the advantages of adhesive force, durability, and optical characteristics. So, the EVA adhesive film is increasingly being applied to novel photovoltaic assemblies. The photovoltaic cell glass adopts toughened glass with the advantages of high strength, impact resistance, and bending resistance. Its bending strength is 3~5 times that of common glass. It has good thermal stability and light transmission. One of the features of the dual-glass assembly is variable optical transmission. Therefore, different light transmittances can be customized according to the illumination requirements of different plants in the greenhouse. It can prevent ultraviolet rays from damaging plants in the daytime and excessive heat from entering a greenhouse, while at night it can also prevent indoor infrared heat from radiating outwards for heat preservation and insulation [26].

Figure 6. Difference between a dual-glass assembly photovoltaic cell and a conventional photovoltaic cell.

Figure 6. Difference between a dual-glass assembly photovoltaic cell and a conventional photovoltaic cell.

In order to assess the impact of transparent solar cells on plant growth, the average action spectrum of 27 herbaceous plants (including common greenhouse crops such as tomatoes, lettuce, and cucumbers) was used. Figure 7 shows the photon flux density, b_s(λ), and the photon flux density weighted by the plant action spectrum, b_s(λ)a(λ). This modified spectrum can be understood as the photon flux spectrum required for optimal plant growth.

The impact of the absorption by the organic photovoltaic (OPV) device on crop growth was determined by calculating a crop growth factor, G(x), as a function of active layer thickness, x, which can be defined as

$$G(x)=\frac{\int T\left(x, \lambda \right)b_s\left(\lambda \right)a\left(\lambda \right)\mathrm{d}\lambda }{\int b_s\left(\lambda \right)a\left(\lambda \right)\mathrm{d}\lambda }$$

(1)

where T(x, λ) is the total transmission of the complete simulated solar cell stack. The rate of photosynthesis in a crop is governed by the integral of the solar spectrum and action spectra. G(x) therefore represents the ratio of the rate of photosynthesis under a clear sky to the rate under a greenhouse material (such as an OPV covering) with spectrally dependent transparency, T(x, λ).$b_s\left(\lambda \right)$ is AM1.5 spectrum irradiance. $a\left(\lambda \right)$ represents the average action spectrum of 27 herbaceous plants (including common greenhouse crops such as tomatoes, lettuce, and cucumbers). The growth of crops in the greenhouse is subsequently assumed to be reduced by 1% for every 1% reduction in the rate of photosynthesis [27].

Figure 7. Relative action spectrum for plants (solid blue line), AM1.5 photon flux density (dotted red line), and action spectrum modified photon flux density (dashed black line) [28].

Figure 7. Relative action spectrum for plants (solid blue line), AM1.5 photon flux density (dotted red line), and action spectrum modified photon flux density (dashed black line) [28].

In order to achieve a certain light transmittance, photovoltaic assemblies of solar greenhouses have the following two options [25]. First, by adjusting the interval between the opaque solar cells, which can be realized by silica-based thin-film solar cells and crystalline silicon solar cells. The shortage lies in the fact that the solar cell part is sheltered from the sunlight of the full wave band, which has a certain influence on vegetation. In addition, the silicon-based thin-film solar cells can achieve certain light transmittance through laser scribing or a transparent back electrode to satisfy the lighting requirements of the photovoltaic greenhouse. Moreover, plants can uniformly receive light through this application mode. In addition, the silicon-based thin-film solar cells can reduce ultraviolet rays, which are unfavorable for plant growth. If the transparent back electrode assembly is adopted, unijunction silicon-based thin-film solar cells can be selected. In this way, the defect that the silicon-based thin-film solar cells have strong absorptivity for light with a wavelength of 440 nm (Figure 5) can be overcome, which is active in photosynthesis.

Combining the above two modes, the best solution is to install the silicon-based film light-transmitting assembly according to the former, so that the photovoltaic greenhouse can generate electricity without influencing the growth of crops. So, the income of farmers is increased [29]. Solar power generation is combined with shade-tolerant crop production, and the economic value exceeds 30% compared with that of traditional agriculture [30]. The investment recovery period of the photovoltaic greenhouse is estimated using the electric energy generated by the photovoltaic assembly, so that the sustainable production of greenhouse crops can be better promoted. The calculated recovery period for the dynamic photovoltaic greenhouse under clear air conditions in Italy is 6 years [31]. The annual return rate of photovoltaic CSGs varies from 9% to 20%, and the investment return period is less than 9 years [32]. With the development of photovoltaic technology, the combination of photovoltaic assembly and CSGs will be a mature choice in the near future.

Table 1. Common CSG photovoltaic component materials [33,34].

Comparison Item	Single-Crystal Silicon	Polycrystalline Silicon	Amorphous Silicon Thin Film	Double-Glass Assembly
Technical maturity	Maturity reached	Currently, the technology of ingot-casting polycrystalline silicon is commonly used	Becoming mature	Becoming mature
Photoelectric efficiency conversion rate	13~18%	12~16%	5~9%	Improved by about 4% compared with common assembly
Price	Materials and manufacturing procedures are complicated; cost is high	Simple material manufacture, power consumption savings, and a lower total production cost than that of monocrystalline silicon	Production process is relatively simple, and total production cost is low	Cost is higher than that of crystalline silicon solar cells
Light transmitting or not	No	No	Yes	Yes
Illumination adaptability	Output power is directly proportional to illumination intensity	Output power is directly proportional to illumination intensity	Good weak light response and high charging efficiency	Output power is directly proportional to illumination intensity
Temperature adaptability	Insufficient exertion of efficiency under high-temperature conditions	Insufficient exertion of efficiency under high-temperature conditions	Requirement on the ambient temperature is lower	Heat dissipation performance is better than that of solar cells with the back plate
Operation maintenance	Extremely low failure rate of the assemblies	Extremely low failure rate of the assemblies	Easy to deposit dust and difficult to clean	Good surface aging resistance and easy maintenance
Service life	Guaranteed 25 years of service	Guaranteed 25 years of service	Decay is rapid, service life is only 10~15 years	Guaranteed 30 years of service life
Appearance	Black, atroceruleous	Irregular dark blue	Dark blue	Various colors

Table 1. Common CSG photovoltaic component materials [33,34].

Comparison Item	Single-Crystal Silicon	Polycrystalline Silicon	Amorphous Silicon Thin Film	Double-Glass Assembly
Technical maturity	Maturity reached	Currently, the technology of ingot-casting polycrystalline silicon is commonly used	Becoming mature	Becoming mature
Photoelectric efficiency conversion rate	13~18%	12~16%	5~9%	Improved by about 4% compared with common assembly
Price	Materials and manufacturing procedures are complicated; cost is high	Simple material manufacture, power consumption savings, and a lower total production cost than that of monocrystalline silicon	Production process is relatively simple, and total production cost is low	Cost is higher than that of crystalline silicon solar cells
Light transmitting or not	No	No	Yes	Yes
Illumination adaptability	Output power is directly proportional to illumination intensity	Output power is directly proportional to illumination intensity	Good weak light response and high charging efficiency	Output power is directly proportional to illumination intensity
Temperature adaptability	Insufficient exertion of efficiency under high-temperature conditions	Insufficient exertion of efficiency under high-temperature conditions	Requirement on the ambient temperature is lower	Heat dissipation performance is better than that of solar cells with the back plate
Operation maintenance	Extremely low failure rate of the assemblies	Extremely low failure rate of the assemblies	Easy to deposit dust and difficult to clean	Good surface aging resistance and easy maintenance
Service life	Guaranteed 25 years of service	Guaranteed 25 years of service	Decay is rapid, service life is only 10~15 years	Guaranteed 30 years of service life
Appearance	Black, atroceruleous	Irregular dark blue	Dark blue	Various colors

3.2. Photovoltaic Cell on the Top of the North Wall

Taking the combined benefits of the shading of the adjacent greenhouse on the southern side with the solar panels into account, solar panels are usually placed on top of the broad north wall of CSG. To ensure the lighting interval between the north and south adjacent CSGs, the photovoltaic assembly bracket and the CSG structure are completely independent. Photovoltaic power generation and greenhouse production can be simultaneously performed [34]. However, due to the height of the photovoltaic panels (Figure 8a), the interval between adjacent CSGs is correspondingly increased. This reduces the effective utilization of the land by agricultural production compared to conventional CSGs. Therefore, in order to improve the land utilization rate, the greenhouse is designed in the form of a synergy CSG, with vegetables planted in the south and mushrooms planted in the north. The shorter photovoltaic panels above the north wall (Figure 8b) can avoid shading the plants inside the north greenhouse. The installation can make full use of the north wall passageway or skeleton. The mode has a north wall-enclosing channel, so the cleaning and the appearance inspection are convenient and fast, with low maintenance costs [35].

3.3. Photovoltaic Cells on the South Slope of CSGs

3.3.1. Coverage Area of the Photovoltaic Assembly

The design mode of combining the photovoltaic panel with the front roof framework of the CSG is adopted, with the greenhouse structure fully utilized and structural materials reduced [36]. The greenhouse roof correspondingly increases the area of the photovoltaic panel and the photovoltaic power generation amount. However, because the photovoltaic panel blocks the lighting of crops in the greenhouse while generating power [37], the light becomes a competitive resource required by both photovoltaic power generation and agricultural production, besides planting shade-loving crops (such as mushrooms), which do not need light or need weak light in the greenhouse.

When the solar radiation is strong in the summer, the photovoltaic assembly can play a good role in shading sunlight. In order to ensure that crop growth is not affected as much as possible, the Italian government stipulates that the installation ratio of photovoltaic assemblies on the roof of the greenhouse must not exceed 50% for different coverage rates of crystalline silicon solar cells on the glass greenhouse [38,39]. Compared to foreign countries, Chinese scholars have less research on the combination of crystalline silicon photovoltaics and CSGs. Zan et al. use ECOTECT software to establish models of monocrystalline silicon photovoltaic greenhouses with roof coverage rates of 7.61%, 15.22%, 22.83%, and 30.44%, respectively, based on Kunming climate characteristics. Compared with CSGs without photovoltaic assembly coverage, the lighting coefficient is reduced by about 16%, 38%, 49%, and 58%, respectively [40].

In terms of the effect of the amorphous silicon photovoltaic assembly coverage area on the CSG, Zhao et al. monitored the light environment and tomato growth conditions in a thin-film photovoltaic CSG and a plastic-film CSG (Figure 9) in Yanan city (36° N, 108° E). Under the layout with a ratio of 1:1 between the area of the film photovoltaic and the PC board, the total radiation transmittance within 2 h before and after noon on a clear day in the summer is 38.7%, and the photosynthetic effective light quantum flux density transmittance is 38.9%. They are, respectively, 30.3% and 17.6% lower than that of a plastic-film solar greenhouse. And during this period, the thin-film photovoltaic solar greenhouse can block 3949.8 kJ/m² more heat into the room than the plastic thin-film solar greenhouse on a clear day [25]. Zhao et al. also experimented on the southern slope of a CSG in Yangling, Shaanxi (34°16′ N, 108°04′ E). Every three rows of film photovoltaic assemblies and PC sunlight plates are laid from west to east at intervals of 1:2 and 1:3. It is found that the total solar radiation with the 1:3 ratio in winter is increased by 50.3 W/m², and the average transmittance of the total solar radiation is increased by 9.1%, but the transmittance of the average photosynthetic active quantum flux density is almost unchanged. The average light transmittance of the inclined plane formed by the amorphous silicon cell assembly and the PC board is changed within a range of 34.7~41.7% (Figure 10c) [41].

The research shows that the film photovoltaic assembly can significantly block solar radiation and reduce indoor temperatures in the summer. This has little influence on the photosynthesis of plants and light transmittance compared with a PC plate. However, the coverage rate of the photovoltaic assembly of less than 50% is determined according to specific planting objects. When the coverage rate of the crystalline silicon photovoltaic greenhouse is 30%, the crystalline silicon photovoltaic greenhouse has an obvious inhibition effect on photosynthetic effective radiation, but the adverse effect of a thin-film photovoltaic CSG with the same coverage rate is much smaller [38]. The coverage area of the photovoltaic assembly can directly influence the power generation amount, so the photovoltaic coverage areas can be designed in a photovoltaic CSG that does not take power generation as its main purpose. For example, if the electricity generated is only used by the system itself and the coverage rate is low, the quality and yield of agricultural production can be ensured with reductions in the investment of grid-connected equipment and the energy loss in the grid-connected process.

3.3.2. Arrangement of the Photovoltaic Assembly

On the premise of the same coverage area, the influence of the arrangement mode on the environment in the greenhouse is very important. Zan et al. establish a monocrystalline silicon photovoltaic CSG model with one row of compact, two rows of compact, two rows of chessboard, and four rows of chessboard arrangements and compare the one row of compact and two rows of chessboard type greenhouses, respectively, under the condition that the roof solar cell coverage rate is 7.61% [40]. The results show that during the winter daytime, the temperature in one row of a compact greenhouse is higher than that in two rows of a chessboard-type greenhouse, with a maximum temperature difference of 2.5 °C. The results are close to those of the two rows of compact and four rows of chessboard-type greenhouses, with a coverage rate of 15.22% for roof photovoltaic cells. The lighting coefficients of the bottoms of the two rows of compact and four rows of chessboard-type greenhouse are, respectively, superior to those of one row of compact and two rows of compact type (Figure 10 a,b).

The comparisons between the compact type and the chessboard type show that the sealed heat preservation effect of the compact greenhouse is better than that of the chessboard-type arrangement. However, chessboard-type arrangement lighting is better than the compact type of arrangement. This is more favorable for the uniform irradiation of illumination because the photosynthesis and quality of crops are less influenced [43].

In the aspect of research on the arrangement of the amorphous silicon thin-film photovoltaic assemblies, Zhao et al. find that the average light transmittance of the thin-film photovoltaic CSG is 33.0% in January, which is 11.6% lower than that of the plastic thin-film CSG. By counting the days of lowest temperature during the test period, it was found that the number of days below 8℃ in the photovoltaic greenhouse (2 days) was significantly less than that in the plastic-film greenhouse (10 days, 33%) (Figure 10c) [25]. The film photovoltaic assembly covering 9.8% of the roof area of the CSG cannot influence the yield and quality of tomatoes. However, in the crystalline silicon photovoltaic CSG, the light transmittance of the checkerboard-arranged film solar sun is obviously better than that of a straight-line arrangement to improve the quality of crops. So, the checkerboard arrangement of the photovoltaic assembly can enable the illumination in the CSG to be more uniform and more suitable for the growth of crops. In addition, the low-temperature days in winter show that the winter temperature environment of the photovoltaic CSG is obviously better than that of a plastic-film CSG, and the photovoltaic CSG is more favorable for fruit and vegetable cultivation in winter.

The whole greenhouse roof area is often covered with opaque conventional PV panels to maximize energy production [44]. However, this scenario is unsuitable for green plant cultivation (Figure 11a). In fact, on average, among the most common PV greenhouse types, the annual global radiation decreases by 0.8% for each additional 1.0% of PV coverage on the roof. The greenhouse internal light environment varies greatly according to whether the PV modules are concentrated as a single array (Figure 11b,c) or dispersed over the roof (Figure 11d–f) when the roof is partially covered with PV modules. PV panel shadows are dissected and scattered on the wider area of greenhouse interior space when the PV modules are distributed in a dispersed manner on the roof (Figure 11d–f). Stripe (Figure 11d,e) or checkerboard (Figure 11f) arrangements of PV arrays using conventional opaque PV panels have been studied. An important implication exists related to the stripe direction. Straight lines of PV modules aligned east–west, mounted on the south roof of an east–west oriented greenhouse, are suitable for electricity production (Figure 11d) [37]. The inclination of south-facing roofs of east–west-oriented greenhouses in the northern hemisphere usually contributes to PV electricity production. By contrast, all plants in the greenhouse receive direct sunlight frequently during a sunny day when the strings of PV modules are aligned north–south, irrespective of the greenhouse orientation (Figure 11e) or if the PV modules are placed in a checkerboard arrangement (Figure 11f) [45,46].

4. Application of Solar Thermal Technology in CSGs

4.1. Profile of Solar Thermal Energy Utilization in CSGs

Solar energy is a renewable, clean energy source, and it is also a major source of light and heat for plant growth in CSGs. The solar heat collector is applied to the greenhouse active heating technology, so the requirement for fossil energy can be reduced. Moreover, the requirements for energy conservation and emission reduction are satisfied in an environmentally friendly way. The application principle of the active heating technology in CSG is that the solar heat collectors in different forms are used for storing solar radiation energy in the daytime, such as a heat-preserving water tank, a wall body, and soil. The stored heat is released into the greenhouse to increase the indoor temperature at night or in low-temperature weather, such as rainy days. When studying the heating effect of the solar heat collector in greenhouses, Chinese scholars mostly adopt the mode of installing the solar hot water collector outdoors to collect more solar heat (Figure 12a).

To overcome the influence of the cold weather on agricultural production, different modern heating modes are researched on the influence of indoor temperature, relative humidity, air flow rate, soil temperature, and indoor air quality on plants, as shown in

Table 2. Modern CSG heating mode.

Heating Mode	Measures	Disadvantages	Advantages	References
Boiler heating	Electric boiler, coal-fired boiler, or gas-fired boiler.	(1) Combustion products, such as smoke dust, sulfide, and nitrogen oxide, are easy to generate and have inevitable harm to the environment; and (2) energy consumption is too large.	Mature technology, high efficiency, and higher freight costs.	[47]
Solar heating	Direct utilization of solar energy; solar radiation panels.	The solar energy resource shows periodic variation, which causes the energy source of the greenhouse to be unstable.	Clean energy, both heat and light sources, and abundant resources.	[48,49]
PCM heating	Applied to greenhouse building materials to realize heat storage–release processes.	(1) Utilization efficiency is low; (2) investment is large; and (3) phase-change materials applied to the internal parts of the greenhouse are not mature at present.	Realize that short-term heat storage occupies less space.	[50,51]
Cross-seasonal heat storage	Solar cross-seasonal heat storage and solar cross-seasonal phase-change heat storage technology.	(1) Investment is large; (2) requirement for the heat preservation of the greenhouse is higher; (3) revenue is not directly proportional to investment; and (4) heat storage technology has a higher requirement.	Clean energy and large-scale greenhouse requirements.	[52]

.

The low ground temperature is an important environmental factor that restricts the normal production of vegetables in the CSG in the winter. Particularly, the difficulty of increasing the ground temperature of the greenhouse is higher after continuously cloudy days. Due to the fact that the soil has thermal inertia, the heat required to increase the ground temperature is relatively large. The heat is directly obtained from the air, so the heat transfer efficiency for storing the heat in the soil is low. Therefore, in practical production, the method of heating indoor air is rarely actively adopted to increase the ground temperature. Some researchers directly lay the radiator on the ground or in the soil so that the heating efficiency is higher (Figure 12b).

There are many methods to increase the ground temperature of greenhouses, such as heating with electric heating wires, biological reactors, underground heat exchange, and hot water. The heating of the soil with electric heating wires is the most direct and effective method, but the method has a large power consumption, so the method is essentially not adopted in large-scale vegetable production except for the use of greenhouse seedling culture or local heating in the greenhouse [53]. The biological reactor technology is a soil temperature-increasing form that has been widely popularized and applied in CSGs in recent years. It can improve the CO₂ concentration in the greenhouse besides the ground temperature. However, the time difference in contradiction with the requirements of the greenhouse in the early and later periods of planting is larger [54]. Underground heat exchange technology has been transferred from Japan to China since the 1980s. It is mainly suitable for areas with better sunlight and stronger soil heat storage capacity [55]. The solar energy underground heat exchange device fully utilizes solar energy, and the relatively surplus energy in the daytime is transported to the underground through fans to heat the soil in the greenhouse and store the energy. When the temperature is low at night, fans are used to carry the heat stored in the soil to the air, so that the temperature in the facility is increased. Low-temperature hot water (the water temperature is mostly below 50 °C) is easy to obtain, simple to control, and uniform in soil and indoor temperature distribution. So, it is one of the most common methods for soil heating. The solar panel collector is used for collecting solar radiant heat. Water is used as a heating medium and directly supplied to the ground or soil of the greenhouse in an environmentally friendly way.

4.2. North Wall Materials to Improve the CSG Thermal Environment

The north wall body of the CSG plays an important role in forming an indoor thermal environment. Tests on the temperature distribution and change rule of the wall body along the thickness direction in the CSG show that in a general solid wall body structure, the inner surface of the wall body is greatly influenced by solar radiation and the indoor thermal environment. The diurnal temperature can clearly fluctuate, but the fluctuation is also attenuated quickly [56]. For example, for a brick composite wall with a thickness of about 500 mm (Figure 13), the wave motion is obviously reduced to 200 mm from the inner surface of the wall. At 300 mm, the temperature of the wall body is close to stable, which indicates that the heat storage and release positions of the wall body are mainly concentrated in the thickness range within 200 mm of the inner side of the wall body, and other wall body materials are little or not directly involved in the heat storage and release process [57]. The limited heat storage area, insufficient heat storage capacity, and too low temperatures at night occur in the wall body of the CSG. On the other hand, the phenomenon that the temperature of the greenhouse is too high on sunny days and the waste heat is often removed by ventilation is undoubtedly a contradiction. Therefore, the materials in the deep part of the wall body are transferred to participate in the heat storage and release process by changing the structure of the wall body. In this way, the total heat storage and release amount of the wall body is increased, the solar energy utilization efficiency of the CSG is further improved, and the greenhouse environment is improved.

In different regions of China, due to different climates, the construction forms of greenhouse walls are different. And due to different solar radiation distribution conditions, the construction forms of the greenhouse wall are more diversified. The CSG wall is developed into a composite heterogeneous wall from a simple substance wall. The influence of a combination form on the thermal engineering performance of the wall is gradually considered. The CSG wall is further developed into various walls constructed with different combinations of materials and in different positions.

Table 3. Construction form of typical CSG wall [58].

Applications	Wall Structure Diagram (Left Is South, Right Is North)	Construction Parameters	Remarks
Shouguang		The wall body is in an isosceles trapezoid shape, which is made of a soil wall. The length of the upper bottom edge is 2 m, and the length of the lower bottom edge is 7 m.	In order to satisfy the growing environment of plants in the greenhouse, people in cold regions increase the temperature of the greenhouse by increasing the thickness of the wall body.
North of Jiangsu province		Straw brick (0.49 m) and color plate (2 mm).	The heat preservation effect is good.
International standard		Brick wall (0.37 m), brick wall (0.24 m), and polystyrene board (0.1 m).	Has the functions of heat preservation, heat accumulation, and heat insulation.
Passive phase-change heat storage		PCM board (5 cm), brick wall (0.8 m). and insulation board (5 cm).	According to an ideal wall structure, a novel phase-change heat storage type of greenhouse wall is constructed.

shows several typical construction forms of the solar greenhouse walls in China.

Table 3 shows four typical applications of the greenhouse wall: Shouguang type, northern Jiangsu type, Chinese standard type, and novel phase-change heat storage type. It develops from a simple substance wall body to a composite material wall body. There is a breakthrough in the construction mode and the phase-change material for the greenhouse wall body. The wall body of Shouguang type is made of the soil wall, which is a typical means for increasing the thickness of the wall body to increase the indoor temperature at night. However, the land utilization rate of the CSG is low in this way. The novel phase-change heat storage type of CSG is constructed according to an ideal wall structure [59]. The application of phase-change latent heat storage in the CSG is realized. At present, four typical greenhouse walls are verified, applied, and popularized to build a thermal environment suitable for plant growth.

According to the theory of thermal engineering, it is better to place a latent heat energy storage material (such as a phase-change material) with strong heat storage capacity on the inner surface layer of the wall body in the solar greenhouse to ensure that the projected solar radiation energy can be efficiently received and stored. In the CSGs, the material with large thermal resistance and good heat preservation performance (such as light polystyrene board) is placed on the outer surface layer of the wall body, so that the heat loss from the outer surface of the wall body to the outdoor environment is reduced to the maximum extent. Sensible heat energy storage materials (such as heavy building block bricks) with strong heat storage capacity are placed in the middle layer of the wall body of the CSG to further improve the structural stress and the overall heat storage capacity of the wall body [60].

To accumulate more surplus heat energy in the daytime for heating at night without consuming energy sources such as electric power, Zhao et al. adopt the structure of a hollow wall body (Figure 14). The natural force is formed by utilizing the temperature differences and gravity differences of the air in the greenhouse and the wall body to introduce the greenhouse air into the hollow part of the wall body. The surplus heat energy from the Sun is stored in the deep part of the wall body, so that the large-area contact heat exchange between the air and the wall body is utilized. The heat accumulation and release effects are comprehensively exerted through the inner wall body material, the outer wall body material, and even the roof material at the top part. The purposes of storing solar heat energy more effectively, increasing the heat release capacity at night, and increasing the temperature at night have been achieved [61]. The upper and lower parts of the inner side of the hollow part of the wall body are uniformly provided with a plurality of vent holes along the horizontal direction. The vent holes communicate with the interior of the wall body and the indoor space. In this way, a new path is provided by forming air circulation flow around the inner side wall body. The height difference between the upper vent hole and lower vent hole is set at 2 m to ensure sufficient natural thermal pressure difference for air flow.

In Figure 15a, in the process of heat storage in the wall body by natural convection circulation of air, the temperatures of the indoor surface and the two wall surfaces of the hollow layer are higher in the daytime. Meanwhile, the air temperature of the hollow layer is higher than that of the two wall surfaces. This shows that the area of the wall body participating in heat exchange is not only the indoor surface but also the wall surfaces on both sides of the hollow layer. That is why the heat exchange area of the wall body has increased. The temperature of the convection air is lower than that of the north and south wall surfaces of the hollow layer at night. This indicates that the convection air is heated in the circulation process at night. On 2–4 February 2017, it was cloudy, and on 5–6 February, it was sunny. In Figure 15b, under the action of natural convection of air, the heat storage amount and the heat release amount of the hollow layer in the heat storage wall body account for 31% and 35% of the total heat storage and release amount of the greenhouse, respectively. Compared with the control wall body, the wall body structure has the advantages that the daytime heat storage capacity is improved by 15.1%, and the nighttime heat release capacity is improved by 14.7%. These advantages show that the wall body structure can improve the heat storage and release capacity of the wall body [62].

Pebbles are good sensible heat energy storage materials. Broad pebble beds are generally buried in underground soil in the middle of greenhouses. As heat storage materials for CGSs in China, pebbles are mainly used in two forms: one is directly used as a wall material, and the other is used as a heat storage bed. The heat transfer rate of the pebbles is relatively high. The pores among the pebbles are also favorable for convection heat transfer. However, the temperature in the first half night is high, and the temperature in the second half night is low due to the fact that the pebbles release heat too fast in winter. Therefore, it is necessary to study how to slow down the release of the heat accumulated by the pebbles or how to transfer the heat accumulated by the pebbles to other materials for storage [63].

Many studies have assessed the selection and preparation of phase-change materials (PCMs) [64,65,66], their packaging [66], and their applications in greenhouses [67]. The main screening materials used included paraffin [68], Na₂SO₄·10H₂O [69], CaCl₂·6H₂O [64], Na₂HPO₄·12H₂O [66], and fatty acids [70]. The packaging methods used included blend soaking [69], block packaging [69], rice husk adsorption [65], and graphite adsorption [70]. The method used to combine the materials with the greenhouse was mainly to place the packed phase-change material on the north wall of the CSG [64] or use the plate or block directly built on the inner side of the CSG north wall [67] (Figure 16).

The phase-change heat storage material for CSG not only needs to overcome the problems of the material itself, but also needs to satisfy the conditions required by plant growth [72]. The following requirements should also be satisfied: (1) The phase-change temperature should be maintained within a temperature range suitable for crop growth. In general, most crops grow at an optimum temperature of about 25 °C, so the melting point of the phase-change material is preferably around this temperature range. The phase-change process of the phase-change material requires a certain amount of time. However, the adjustment to the environment is somewhat delayed. So, the selection of the phase-change temperature of the material between 20 and 25 °C is more practical and can better play the role of the phase-change material. (2) Large latent heat and small volume change in the phase-change process. (3) No toxicity, no corrosiveness, and no flammability. (4) Low cost and wide source.

The phase-change material has good heat storage and release characteristics. For example, the change in the temperature of the environment in the greenhouse can be slowed down, improving solar energy utilization efficiency. The phase-change heat storage material is also suitable to be applied to the CSG. However, there is still much work to be perfected on the aspect of popularization and application of phase-change technology in CSGs, such as the establishment of optimal phase-change temperature intervals of phase-change materials in different regions, the determination of optimal thicknesses of phase-change heat storage material layers of different types of greenhouses, and the determination of optimal laying areas [73]. The work of reducing plant shading to achieve the optimal input-output ratio is still under further study.

4.3. Equipment to Improve the CSG Thermal Environment

As shown in Figure 17, the active solar heat storage–release (AHS) system of the CSG is composed of heat collection/release, heat storage, and control devices. In the daytime, the solar radiant heat is absorbed and stored, while at night, the heat is released to warm the greenhouse. Zhang et al. adopted an aluminum alloy finned tube as a heat collector to improve the stability and reliability of the CSG initiative heat accumulation and release system operation. Results show that under three different solar radiation intensities, the temperature difference between the south and the north of the test area is large, with a maximum average temperature difference between the south and the north inside a plant population of 2.8 °C, 2.6 °C, and 2.4 °C, respectively [74]. By calculating the heat transfer process model of the CSG and the active heat storage and release system, for a polystyrene board solar greenhouse of 640 m² in the Beijing area, if the average temperature is 12 °C at night, at least 118 m² of a heat collection plate is required to be configured with a water supply volume of 10.4 m³. When the design value of the air temperature increases by 1 °C at night, a 5 m² larger area of heat-collecting plates and 0.2 m³ more volume of water are required [75]. Xu instead uses black hollow plates as heat collectors (Figure 17b). In January in the Beijing area, the daily heat collection quantity of the heat collector with an area of 84.4 m² is usually 0.40~0.88 MJ/m² (calculated according to the ground area of the greenhouse). The requirement that the night temperature of the CSG with the 400m² polystyrene board wall be increased by 3~5 °C can be satisfied. In addition, the water storage volume configuration of 0.02~0.03 m³/m² (calculated according to the ground area of the greenhouse) can satisfy the heating heat requirements on 2~3 continuously cloudy days. The system can raise the temperature of the tomato canopy obviously, to ensure the overwintering production of tomatoes and promote their growth. In this way, the yield is increased, and the fruit can mature in advance and come into the market [76].

The main function of the solar greenhouse frame is to support the pressure of the solar greenhouse covering and to bear the external pressure (wind load and snow load) caused by nature. It is an innovation for the solar greenhouse to use galvanized steel pipe roof trusses as a heat collector and a radiator [77]. However, because the steel pipes are close to the outside, when the night system operates, a large amount of heat is dissipated to the outside, as the heat preservation measures are not implemented well when the system radiates to the inside (Figure 18).

The heating capacity of a single air source heat pump is rapidly reduced along with the reduction of the environmental temperature, particularly when the environmental temperature is reduced to be below 0 °C. The heating efficiency cannot satisfy the temperature adjustment requirement, even when the machine is shut down due to a serious frosting phenomenon. The single solar heat pump temperature regulating system completely depends on sunlight irradiation, and solar energy is used as intermittent energy and greatly changes along with day and night and weather conditions. So, the single solar heat pump temperature-regulating system cannot satisfy the temperature-increasing requirements of the greenhouse. At present, composite heat pump temperature-regulating systems, such as solar energy combined ground source heat pumps and water source heat pumps, have the problems of complex equipment and large construction investment costs [78].

The multi-surface solar air collector with double receiver tubes (MSC-DRT) developed by Chen et al. for the CSG has the characteristics of simple operation and maintenance, small occupied space, and high heat collection efficiency (Figure 19a) [79]. The light inlet of the multi-curved-surface heat collector is narrow, which has little influence on the shading of the rear greenhouse. It is arranged on the north wall of the CSG with a high integration degree with the greenhouse (Figure 19b). Under the conditions that the daily average temperature of the outdoor environment is lower than −1.2 °C and the average inlet air temperature is lower than 3 °C, the daily accumulated heat collection amount of the heat collector per unit area is about 4.84 MJ/(m²∙d), and the daily average outlet temperature can reach 56 °C, which can provide favorable conditions for the active heat storage of the wall body of the CSG (Figure 19c). The length of the heat collector can be considered within the range of 16 m~18 m. In practical applications, the outer curved surface of the heat collector and the air supply pipeline have a certain amount of heat loss. Especially, the heat loss of the air supply pipe along the outdoor part is larger.

Figure 20a shows the three-dimensional schematic of the developed novel MSC-DRT, and Figure 20b presents its cross-section with an indication of its dimensions. The main components of the novel MSC-DRT include a glass cover with high transmittance, a multi-surface reflector with side and bottom surfaces, and two receiving tubes, as shown in Figure 20a. The concentrating ratio C is the ratio of the collector aperture to the heat absorption areas. For the described dimensions of the concentrating collector, C is approximately 1.4.

In order to improve the concentration and sunbeam convergence ratio of the MSC-DRT, the multi-surface reflector was designed with the off-axis location of the receiver tubes in mind. Two tubes were used to split the airflow and increase the heating surface/volume ratio. In addition, this design allows the enhancement of the convergence rate of the direct and reflected sunbeams over a wide range of incidence angles. The three parabolic curves AB, CD, and EOF, which make up the reflecting surfaces of the collector, can be described in the coordinate system shown in Figure 20b as follows:

$$AB:y=\frac{1}{0.92}{(x+0.23)}^2$$

(2)

$$CD:y=\frac{1}{0.92}{(x-0.23)}^2$$

(3)

$$EOF:y=\frac{x^2}{0.16}$$

(4)

where x and y are coordinates in units of length (meters).

Most current studies of the application of solar energy heating in greenhouses involve short-cycle heat storage. Despite the fact that short-cycle heat storage requires low investment, there are often more than a week of continuous cloudy days in the winter when the solar system cannot work. Li et al. carried out experimental and numerical investigations on underground heat storage using conducting oil as heat transfer fluid for growing plants in cold climates [52]. A curved Fresnel lens was used to maintain a temperature of 8 °C in a volume of 716 m³. The pipe depth varied from 0.5 to 3 m. The corresponding time required for the soil surface to reach a stable temperature was 7~100 days.

In order to save the land utilization rate of the CSG and effectively utilize the non-planting area on the top of the CSG, Wu et al. adopted a Monte Carlo ray tracing method to research the performance of the CPV/T system of the cylindrical surface Fresnel lens heat collector (Figure 21) [81]. To achieve a maximum total monthly average direct radiation daily amount, a round surface-paraboloid combined south roof structure is selected with a space utilization rate of 18.2% for the non-planting area. The experimental study is carried out by using a gallium arsenide high-concentration solar cell as a receiver. The test result under the sunny weather condition shows that the maximum generating efficiency of the system at noon is about 18%, and the maximum heat collection efficiency of the cooling water is about 45%. During noon (11:00~13:00), the total thermoelectric efficiency can reach 55%. Through the test, scattered light not utilized by the lens system is found, and the whole-day illuminance of the plant canopy in the test area is reduced by about 10~40% compared with that in the non-test area. In addition, the proportion of scattered light in plant canopies on cloudy days can be increased, and the heat collection efficiency of the corresponding device can be reduced.

5. Solar Energy Supply and Energy Demand in CSGs

Solar energy utilization technology is an important component of the sustainable development of the CSG. It can reduce the consumption of the conventional energy sources of the CSG in cold areas during the winter [82]. Solar energy technology is increasingly applied to greenhouse production, particularly two types of technologies such as those shown in Figure 22. One type is that solar energy is directly combined with photothermal heat storage technology, and heat is transferred to greenhouse soil and air through a heat exchanger [83]. The other type is that solar energy is converted into direct current to supply power to a storage battery by utilizing photovoltaic power generation technology, and the direct current can also supply power to greenhouse electric equipment through an inverter. The solar heat storage system mainly obtains low-grade energy (heat energy), which is mainly used to improve the heat preservation and heat storage capacity of the CSG in winter, while the solar power generation technology can obtain high-grade energy (electric energy), which can be directly used for illuminating lamps, light supplement lamps, rolling blinds machines, irrigation equipment, and plant care equipment in the greenhouse and provide energy for greenhouse environment regulation equipment [84].

However, the optical mechanism between photovoltaic power generation and the normal growth of plants in the CSG is still an urgent problem to be solved in the solar greenhouses. Solar energy is not only an indispensable energy source for the photosynthesis of green plants, but also a main heat source for the CSG. Therefore, when designing a solar energy system for the CSG, the problem of lighting the greenhouse should be solved first, and the photosynthetically active radiation (PAR) should be transmitted into the greenhouse to the maximum extent.

6. Summary

An important aspect of greenhouse sustainability and efficiency is energy-efficient operation. The north wall body of the Chinese solar greenhouse plays an important role in the formation of an indoor thermal environment. The heat storage and release positions of the wall body are mainly concentrated in the thickness range within 200 mm of the inner side of the wall body. In the vast northern areas of China, due to the cold climate in late autumn, winter, and early spring, the temperature difference between day and night is large. Without heating, the requirement for crop growth is hardly satisfied by the temperature of a CSG at night. As for heating, the traditional heating method needs to consume a large amount of non-renewable fossil fuel, generating a large amount of harmful gas to pollute the environment simultaneously [85]. Solar energy is a clean energy, large in quantity and wide in distribution, that can be applied to greenhouse warming. It can replace non-renewable fossil fuels for heating and is one of the most promising energy sources [86]. In the last two decades, China has performed more relevant research [87]. The heat collection mode of external solar heat collectors can collect solar heat outside the CSG, but it occupies outdoor land. Moreover, the heat loss in the conveyance of hot water is large and requires the installation of costly indoor heat release devices. The heat-collection mode of internal solar heat collectors can collect surplus heat energy indoors, but it has the defect of competing for light and heat with plants. Therefore, the existing solar heat collector cannot be completely used for collecting heat energy inside and outside the CSG. A new heat-collection mode and a new system are designed by combining the change rule of the CSG structure and the internal environment.

Photovoltaic solar greenhouses combine greenhouses with photovoltaic power generation systems to achieve integration of solar power generation with facility production, attracting increasing attention in the facility horticulture industry [88,89]. The annual rate of return of photovoltaic solar greenhouses varies from 9% to 20%, with a return-on-investment period of less than 9 years. Exploratory construction and operation of the photovoltaic power generation in CSG have also been carried out in China in the last decade, but due to the lack of research on the facility’s luminous environment aimed at the crop mechanism, the influence of the photovoltaic CSGs on crop production is uncertain. Meanwhile, in China, most PV agricultural companies specialize in PV businesses and lack experience in crop planting.

Because of the characteristics of the severe cold in winter in the north of China and the national conditions of more people, less land, an energy shortage, and emission reduction pressure, the CSG will be the most practical and widely applied mainstream horticulture facility in China for a period of time. Unfortunately, due to cultural and linguistic differences, CSG has long been the focus of only Chinese researchers. One of the advantages of CSGs is low-cost production, so the principle must be followed by the energy equipment invested. From the perspective of improving energy utilization rate, a sensor that is simple to use and low in cost and a low-cost intelligent control scheme should be developed to control heat preservation quilts, roller shutters, natural ventilation dehumidification, and temperature fluctuation. Moreover, the existing control equipment and energy equipment should be gradually upgraded. To make this feasible, researchers and designers should continue to investigate the unique characteristics of greenhouses and conduct interdisciplinary research that aims to maximize renewable energy.