Effects of arch frame air supply and return mode on the indoor thermal environment of crop canopy area for solar greenhouse in winter

Solar greenhouses are one of the most important agricultural facilities for off-season vegetable production in northern China, particularly in the long-term, stable and safe supply of vegetables. The environmental control equipment of solar greenhouses is relatively simple with the low level of automation at present. Manual induction control can also result in low environmental control capabilities in the greenhouse. Thus, the traditional passive thermo-environment control mode cannot be used to regulate the air temperature and velocity in greenhouse crop canopy areas, due to the low energy efficiency. In this study, a new mode of air supply-air return was proposed to consider the enclosure structure using the arch frame of a solar greenhouse. A heat transfer model was developed with the crop porous media model for the solar greenhouse under the air supply-air return. Four evaluation parameters were selected, including 1) the non-uniformity coefficient of the air temperature and velocity; 2) the area ratio of the suitable zone for air velocity; 3) the energy utilization coefficient; 4) the cumulative effective accumulated temperature. A systematic evaluation was implemented to clarify the effects of three air supply-air return modes on the thermal environment of the indoor crop canopy area for solar greenhouse in winter. The results showed that the downside air supply-upper air return mode outperformed the other two modes (the intermediate air return and the upper air supply-downside air return modes), in terms of air temperature and velocity at various crop canopy heights. The greatest area ratio of the suitable zone was achieved in the air velocity, corresponding to the crop canopy heights. Once the insulation quilt was turned off, the highest cumulative effective accumulated temperature of the downside air supply-upper air return was 132.86 ℃·h, which was higher than that of the intermediate return air and upper air supply-downside air return by 1.57% and 8.89%, respectively. When operating at the downside air supply-upper air return mode, the area ratios of the suitable zone for the air velocity corresponding to the crop canopy were 37.2%, 40.7% and 43.5%, at the heights of 0.4, 0.6, and 1.0 m, respectively. For intermediate return air mode, the area ratios of the suitable zone for air velocity corresponding to the crop canopy were less than that of downside air supply-upper air return mode by 56.7%, 47.2%, and 55.4% at the heights of 0.4, 0.6 and 1.0 m, respectively. More importantly, the area ratio of the suitable zone was less than 1.5% at different canopy heights for the air velocity in the upper air supply-downside air return. Compared with the air velocity of 9 m/s, the energy utilization coefficients of the air velocity of 10 m/s increased by 0.31%, 0.53%, 0.49% and 0.22% at the crop canopy heights of 0.4, 0.6, 0.8 and 1.0 m. Similarly, the energy utilization coefficients of the air velocity at 10m/s increased by 0.37%, 0.37%, 0.32% and 0.32%, respectively, compared with the air velocity of 11 m/s. In downside air supply-upper air return mode, the air velocity of 10m/s of the air supply main pipe was recommended to maximize the energy utilization coefficient, which was 0.976, 0.982, 0.985 and 0.987 at the crop canopy height of 0.4, 0.6, 0.8 and 1.0 m, respectively. Also, the lowest non-uniformity coefficients of air temperature and velocity were achieved at different crop canopy heights, when operating at the recommended air velocity of 10 m/s. These findings can serve as a strong reference for the precise control on the thermal environment in solar greenhouses.