科研之友
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摘要
The common cover materials of greenhouses are glass and transparent plastic film. Generally speaking, they have no significant advantage for adjusting incidence lights of greenhouses. Given that the light intensity above the light saturation point is meaningless for plant growth, as in summer, the strong sunlight at noon is surplus, that is, surplus light is unused, so a new kind of entity Compound Parabolic Concentrator (CPC) transparent greenhouse cover is proposed. The cover consists of many entity CPCs, meaning that the material is plexiglass (polymethyl methacrylate) and has high transparency. This cover can change the light paths, and the compound parabolic cells that stick to the bottom of the CPC can generate electricity. Lights of different incidence angles are tracked by optical simulation software; and, the relationship between the lights of incidence angles and transmittance has been analyzed. A test sample has been produced and a test system has been built. Also, an experimental study of transmittance and power generation under actual weather has been conducted. The test system consisted of an integral box (using the integrating sphere principle), a transparent entity CPC cover material (size: 30 cm × 27 cm × 2 cm), a light meter, a voltmeter, an ammeter, and so on. Test method: 1) Put the test device on horizontal ground and make sure that the front edge of the cover is vertical in the south direction; 2) Raise the back of the cover and make sure that the angle between the normal of the cover and ground is the biggest solar elevation angle (33.8°) on the test day, and ensure that the incidence angle is 0° at noon. It can be seen from the transmittance test that the transmittance is lowest at noon. With the increase of the absolute value of the incidence angle, the change of the transmittance curve presents a trend that is rapid before becoming slow. The turning point of the transmittance curve appears at 10:15 AM and 14:15 PM, while the incidence angle is about 30 deg, and is in good agreement with simulation results. An integral box illumination test shows that the greatest solar irradiance appears at about 12:00; the change trend of the integral box illumination in one day is hump-shaped; and that peak values appear at 9:00 AM and 14:30 PM, respectively. When the irradiance is strongest, at noon, the illumination in the box is low. Experimental and simulation results match well and show that the lowest transmittance is only 32, and appears when there is strong sunshine at noon; while in the morning and afternoon, the transmittance of cover material can reach 60. Although the covering material has almost no influence on the light intensity entering a greenhouse in the morning and afternoon, while substantially reducing light intensity at noon, it can improve the status of uneven illumination in a greenhouse in one day. Also, because the photovoltaic cells are stuck to the bottom of the CPC, it can receive the surplus light to generate electricity; the power per unit area is 6.2 W/m2. This new cover can control light well and because it can adjust the thermal environment in a greenhouse, it is conducive to the growth of plants; furthermore, because surplus light can be used to generate electricity and the system makes good use of solar energy, it has good application prospects.
