A kind of photovoltaic solar greenhouse with lighting surface covered with photovoltaic panels and its performance (Part 1)
Improvement of greenhouse insulation performance
In order to improve the overall insulation performance of the greenhouse, measures should be taken to improve the insulation performance of the greenhouse on the basis of good wall and rear roof insulation. The thermal insulation performance of the greenhouse's front roof and gables, and the sealing performance of the greenhouse should also be given great importance.
Improvement of gable insulation
The gable wall of the greenhouse initially used light-transmitting PC hollow board (Figure 7a). First, it is hoped that the light-transmitting covering material can gain more light from the greenhouse gable wall and make up for the lighting surface due to the laying of photovoltaic panels. The amount of light entering is insufficient; secondly, the hollow PC board also has certain thermal insulation properties. However, from the perspective of production practice, this design firstly has insufficient thermal insulation resistance of the PC board and cannot match the thermal resistance of the rear wall and rear roof structure, thus becoming a "cold bridge" for greenhouse heat dissipation; secondly, the PC hollow board The light transmittance itself is not high, and the light entering the greenhouse from the gable wall affects a small area in the greenhouse, making it difficult to improve the overall light level inside the greenhouse. This kind of design scheme that takes into account both lighting and heat preservation can be said to be a failure for overwintering solar greenhouses.
In order to transform the greenhouse gable wall and enhance its thermal insulation performance, hollow bricks (Figure 7b) and thermal insulation color steel plates (Figure 7c) were used to replace the PC hollow boards in subsequent improvements. This kind of transformation, judging from the thermal resistance of the gable wall, has basically reached the same level as the rear roof. In fact, this kind of modification measure is also commonly used for gable walls in traditional solar greenhouses. It should be said that this kind of insulation performance modification of gable walls is applicable and feasible.
Improvement of rear roof insulation and sealing performance
The modification of the rear roof insulation of the greenhouse is mainly a measure taken to address the problem of loose sealing at the ridge of the greenhouse (Figure 6a). In order to fundamentally solve the problem of roof ridge sealing, the renovation measures directly moved the ridge of the greenhouse back, transformed the original sloping rear roof into an upright rear roof (Figure 8a), and moved the ridge to the end of the original structure cantilever beam part (Figure 8b), thus completely solving the problem of the structural cantilever beam passing through the rear roof panel, and the problem of sealing at the greenhouse roof ridge is easily solved.
Due to the light blocking of the photovoltaic panels on the top of the roof, this transformation actually did not affect the lighting distance between the front and rear buildings of the greenhouse. The color steel plates used for the transformation maintained the same specifications as the original rear roof, so they also maintained the same specifications as the original rear roof. Same insulation properties. From the perspective of structural stress, since the cantilever beams supporting the photovoltaic panels of the original structure are closed, the bending moment of the cantilever beams under the north wind load is also reduced, which also helps to improve the load-bearing capacity of the structure.
Add internal insulation to strengthen the insulation of the front roof of the greenhouse.
The photovoltaic solar greenhouse does not have an external insulation cover on the front roof. The heat dissipation of the front roof is huge at night in winter. No matter how good the insulation performance of the rear wall, gable wall and rear roof is, it cannot prevent it. The heat dissipation of the front roof and the good thermal insulation performance of traditional solar greenhouses are basically lost. Therefore, strengthening the front roof insulation of the solar greenhouse is a problem that must be solved to ensure the winter production of the greenhouse.
Add external insulation to the photovoltaic panels. First, the insulation must block part of the photovoltaic panels, which affects the power generation of the roof photovoltaic panels. Second, the reciprocating movement of the rolling shutter machine on the greenhouse roof may damage the glass or photovoltaic panels. Therefore, the traditional The insulation form of solar greenhouse insulation covered by the outside does not seem to be necessarily feasible in this kind of photovoltaic solar greenhouse.
Under the condition that external insulation cannot be achieved, internal insulation has become the only means to enhance the insulation of the front roof. In the early days of the photovoltaic greenhouse renovation design, it was considered to add an inner auxiliary arch in the greenhouse to specifically support and operate internal insulation. This is the best way to insulate. However, after economic analysis, it is believed that the cost of adding auxiliary arches is high. In addition, the inner auxiliary arch actually depressed the greenhouse’s growing space, so this method was abandoned during the construction. In the design, it was also considered to use the lower chord of the frame to install a hanging pulley on it to hang and pull the insulation curtain to achieve the requirements of internal insulation. However, after testing, the operation effect was not ideal, and it has not been promoted and applied in greenhouse construction.Two measures were adopted in the recent internal insulation renovation: one is to install fixed internal insulation on the rear roof; the other is to install movable internal insulation on the front roof.
The fixed internal insulation of the rear roof extends from the height of the back wall of the greenhouse to the upper edge of the top translucent glass plate on the roof. The thick insulation is fixed and installed on the lower chord of the greenhouse frame, forming a connection with the back room of the greenhouse. The triangular closed cavity between the panels (Figure 9a), on the one hand, the thick insulation quilt and the rear roof insulation board themselves have high thermal insulation resistance, which can slow down the loss of indoor heat from the rear roof to a great extent; On the other hand, the closed space between the thick insulation quilt and the rear roof panel also forms an insulated space, which effectively increases the heat transfer resistance of the rear roof. In fact, in this insulated space, the heat generated by the photovoltaic panel backplane during the day can effectively increase the air temperature in the cavity, reducing the temperature difference between the indoor air temperature and the air temperature in the cavity, thereby further reducing the outward transfer of indoor heat. speed. Intuitively, this transformation basically reaches or even exceeds the insulation of the rear wall. However, judging from the actual fixed internal insulation, because the thick insulation quilt and the rear roof insulation board did not form a seal at the ridge, vertical openings appeared in the room (Figure 9b), which greatly reduced the actual insulation effect. It is recommended to close the vertical opening as soon as possible to prevent the flow of indoor air in the cavity to maximize the insulation effect of the closed cavity of the rear roof.
After adopting the fixed internal insulation of the rear roof and forming a closed insulation cavity with the rear roof panel, although the insulation performance of the rear roof of the greenhouse has been greatly enhanced, the vents on the rear roof of the greenhouse have also disappeared. It is not feasible to ventilate a greenhouse solely through the vents at the bottom of the front roof of the greenhouse, whether in winter or summer. For this reason, it is recommended that after sealing the rear roof cavity of the greenhouse, a window opening mechanism is installed on the uppermost light-transmitting glass panel on the greenhouse roof to form a greenhouse ridge vent. Combined with the front roll film vent, a complete greenhouse can be formed. ventilation system.
Since the photovoltaic panel back panel generates a lot of heat, a large amount of heat will accumulate in the cavity after the greenhouse's rear roof cavity is closed. In winter, this heat accumulation is beneficial to improving the insulation performance of the greenhouse, but in summer, this heat accumulation is not only detrimental to greenhouse cooling, but the heat on the back of the photovoltaic panel cannot be eliminated in time, which will greatly affect the power generation efficiency of the photovoltaic panel. Therefore, retaining the vents of the original sloped roof on the outer insulation board will have a very positive effect on removing heat from the cavity, ensuring the cooling performance of the greenhouse and the efficient power generation performance of the photovoltaic panels. Of course, if the sealing port at the joint between the insulation quilt and the rear roof insulation board can be made movable, and the opening and closing of the sealing port can be automatically adjusted as the indoor and outdoor temperatures change, it would be a more ideal modification solution. .
After solving the problem of the insulation of the rear roof, let’s look at the solution of the insulation of the front roof. Just like the movable insulation curtain in a multi-span greenhouse, this greenhouse uses a curtain-type movable insulation quilt to insulate the front roof. The insulation is opened during the day to be illuminated by the greenhouse, and the insulation is unfolded at night to be kept warm by the greenhouse. Choosing thick insulation curtains or even using double-layer insulation curtains will have a very positive effect on improving the insulation performance of the greenhouse.
There are two ways to move the curtain-type movable insulation quilt according to the direction of movement of the insulation quilt: one is to move along the length of the greenhouse, and the other is to move along the span of the greenhouse.
The curtain system that moves along the length of the greenhouse divides the insulation quilt into several zones along the length of the greenhouse, and uses steel cable drive or rack and pinion drive to open and close the insulation quilt in the length direction of the greenhouse (Figure 9c). This kind of curtain system is not affected by the shape of the front roof of the greenhouse. One curtain system can control all indoor insulation quilts. However, this kind of insulation driven method that opens and closes along the length of the greenhouse will cause problems even if the insulation is fully opened during the day. Collecting the position of the insulation quilt will form a local fixed shadow zone in the greenhouse, affecting the lighting and uniformity of the greenhouse. In order to eliminate the indoor shadow zone caused by the shrinkage of the insulation, the subsequent renovation adopted a curtain system in which the movable insulation is opened and closed along the span direction of the greenhouse.The fixed edge of the insulation quilt is fixed on the back wall of the greenhouse, and the movable edge moves along the span direction of the greenhouse (Figure 10). Since this kind of curtain system cannot use one curtain motor to control the internal insulation curtain system of the multi-fold deformation roof (mainly the low front part of the greenhouse), it generally requires multiple curtain machines to move the insulation quilt along the span direction of the greenhouse. Divide it into several partitions for control. But this will increase the cost and will also create a fixed shadow band in the greenhouse along the length of the greenhouse. In order to solve the problem of thermal insulation of the low part of the bend in the front of the greenhouse, this insulation system adopts the method of drooping the front insulation curtain directly. It does not insulate the entire front roof of the greenhouse, but it can form a closed insulation with the ground. Although the front insulation curtain is reduced There is some planting space in the greenhouse, but the cost is low. After the insulation curtain is folded during the day, the hanging part will not affect the lighting of the crops in the greenhouse. It is especially suitable for planting relatively low leafy vegetables.
Of course, if you want to maximize the utilization of the greenhouse floor, the insulation can also be divided into two parts. When the insulation is folded, one part is folded to the back wall of the greenhouse, and the other part is folded to the front foundation position of the greenhouse. The thermal insulation quilt can be collected by rolling the quilt, or it can be collected by pulling the curtain. The folded insulation quilt in this way will completely eliminate the shadows that may be formed in the greenhouse crop area after the insulation quilt is folded, and can also overcome the heat loss caused by the lax sealing of the adjacent connecting parts after the insulation quilt is unfolded along the length of the greenhouse. Improve the thermal insulation performance of the greenhouse.
It should be said that by installing a movable internal insulation quilt on the front roof (whether driven along the length of the greenhouse or along the span of the greenhouse), the insulation performance of the greenhouse in winter will be significantly improved. In fact, the use of a movable internal insulation system driven along the span direction of the greenhouse can also eliminate the need for fixed insulation quilts on the rear roof of the greenhouse, saving construction and renovation costs from another perspective. Of course, if both are retained, the thermal insulation performance of the greenhouse will be more guaranteed.
Greenhouse production benefits
Planting benefits
Before the renovation, the photovoltaic greenhouse had poor thermal insulation performance, making it difficult for warm-loving fruits and vegetables, and even low-temperature-tolerant strawberries, to survive the winter. Therefore, the current planting is basically based on over-summer production, which to a certain extent affects the efficiency of greenhouse agricultural planting and greatly reduces the agricultural production function of photovoltaic greenhouses.
After the insulation performance of greenhouses has been improved, light has become the main factor restricting agricultural production. In order to explore planting varieties that may be suitable and have higher economic benefits under such photovoltaic conditions, the base has tried to grow a variety of vegetable varieties such as leeks, peppers, and zucchini (Figure 11). Judging from the planting situation, although there is an obvious shadow zone in the greenhouse due to the shading of the roof photovoltaic panels, the shadow zone is also in motion indoors with the movement of the sun. Judging from the growth of the crops, the indoor shadow formed by the photovoltaic panels The shaded zone did not have a big impact on the growth of leeks and courgettes, but it was obvious that the peppers were not growing well.
Chives are a crop that likes coolness and is tolerant to both cold and heat. In terms of light requirements, it is a medium-light plant with strong shade tolerance, while pepper is a temperature-loving and strong-light crop. Comparing the growth conditions of crops in the greenhouse also proves the problem of insufficient light for growing strong-light crops in such greenhouses. In actual production, shade-tolerant crops should be planted.
Judging from the price of vegetables sold in Qinghai Province at that time, the average wholesale price of leeks in the production area is 5.6 yuan/kg. Calculated based on the yield of leeks of 12kg/m2 and the commodity rate of 90%, the output value of greenhouse-grown leeks is 60.48 yuan/m2, after deducting costs. 20%, the actual income will be around 50 yuan/m2. In addition to edible fungi, leeks may be another technically feasible planting species with broad market prospects and good economic benefits that can be grown in photovoltaic greenhouses. There have also been cases of tea trees, seedlings, etc. being successfully grown in photovoltaic greenhouses. What other similar varieties are there? We look forward to further testing and exploration by the base.
Power generation efficiency
Judging from the number of photovoltaic panels installed in a single greenhouse, the installed capacity of a greenhouse is about 140kW. Calculated based on the local average annual effective lighting time of 1500h in Qinghai, the theoretical power generation of each greenhouse per year should be 210,000kW· h, calculated based on the State Grid's 0.95 yuan/kW·h, the income of a greenhouse photovoltaic power generation is nearly 200,000 yuan, and the equivalent income is 182 yuan/m2.
Judging from actual operation, the actual power generation of photovoltaic panels is only about 70% of the theoretical power generation. This is mainly due to factors such as pollution on the photovoltaic panel surface, rainy weather, and heating of the photovoltaic panel backplane. Even based on actual power generation, the greenhouse's power generation income can reach 127 yuan/m2 per year. This part of the income is basically a net income, because there are no more production materials and labor inputs in the photovoltaic power generation process. The income from photovoltaics is more than double the income from agricultural cultivation. From this, we can see the huge benefits of photovoltaic power generation compared with greenhouse cultivation. This is also one of the fundamental reasons why companies build photovoltaic greenhouses only for photovoltaic income and abandon agricultural planting.
Conclusion
During the climax of the development of photovoltaic greenhouses in the past few years, a large number of photovoltaic greenhouses have been built in China from south to north, especially in northern regions with good lighting resources such as Inner Mongolia, Qinghai, Gansu and Ningxia. huge. Most of these greenhouses can generate electricity normally and realize the designed functions of photovoltaics, but many of them have abandoned agriculture, which has ruined the original intention of agricultural production in photovoltaic greenhouses. Photovoltaic greenhouses have become a "cover" for companies to occupy agricultural land to develop photovoltaic power stations. , not only wasted agricultural land, but also seriously damaged the government's image. In view of this, the question of whether to develop photovoltaic greenhouses, especially the development of combined photovoltaic greenhouses that cover the lighting surface of the greenhouse with photovoltaic panels, is a big issue in academia, government management, and agricultural production. dispute.
How to put these existing greenhouses into normal agricultural production, maximize the value of land resources while obtaining power generation benefits, and do not reduce the benefits to agricultural production, so that photovoltaic greenhouses can truly balance both power generation benefits and agriculture Produce benefits from dual-benefit technologies and projects.
In recent years, photovoltaic panel manufacturers have developed photovoltaic panels with a certain light transmittance in photovoltaic panel products in response to the special requirements of agricultural production. There are even photovoltaic panels that can convert the wavelength of sunlight. This is a good attempt. . However, the presence of photovoltaic panels will always block crop lighting to varying degrees when using a combined photovoltaic panel layout for solar greenhouses operating in the low-light season in winter.
In addition to the contradiction between photovoltaic power generation and agricultural production, photovoltaic greenhouses that combine photovoltaic panels with solar greenhouses cannot install outdoor insulation quilts on the greenhouse roof, which seriously affects the thermal insulation performance of the greenhouse at night. A large number of photovoltaic solar greenhouses are abandoned in winter. They cannot perform the high-efficiency and energy-saving functions of solar greenhouses in winter, and it is difficult to obtain high benefits in off-season vegetable production in winter. This is also closely related to this technical limitation.
Qinghai Kaifeng Agricultural Science and Technology Co., Ltd., under the guidance of Qinghai University Professor Tang Qingchuan, faced with difficulties and problems, carried out research and practice on greenhouse insulation technology and greenhouse planting variety screening of photovoltaic panel combined solar greenhouses, for this purpose A technology with huge social controversy, we are looking for ways to organically combine photovoltaic power generation with agricultural production. The true agricultural-photovoltaic complementary technology solution for photovoltaic greenhouses has since been put into practice, creating a new way for photovoltaic greenhouses and even traditional solar greenhouse producers in trouble. A direction that can see a bright future. Their exploration is very worthy of our admiration. Their exploration is still ongoing. More and better photovoltaic greenhouse design and construction technical measures and greenhouse planting product solutions based on agricultural and photovoltaic complementation are worthy of our expectations.
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