2.2.4. Simulation

After validating our proposed model, further simulations were performed to investigate the effect of di fferent greenhouse design parameters from an energy conservation point of view. Table 6 shows details of all the studied parameters. Firstly, an analysis was carried out to determine the effect of di fferent orientations on the heating and cooling loads of the greenhouse that maintain the desired temperatures of 18 ◦C and 30 ◦C inside the greenhouse for heating and cooling, respectively. Further analysis was carried out with di fferent covering materials and double glazing using the same greenhouse operating conditions that were used for validation (i.e., those presented in Table 5). Further analyses were performed to evaluate the e ffect of using no screen and using di fferent thermal screens on the cooling and heating loads of the greenhouse. Comparisons were made to find the best solution. Moreover, the total annual energy load, including heating and cooling loads, was estimated with a fully closed and naturally ventilated greenhouse to predict the cooling energy reduction due to the use of natural ventilation. The e ffect of north wall insulation was estimated for the winter season only and heating load was calculated with and without insulation and a comparison was made. Finally, a venlo type and wide-span greenhouse heating and looing load was estimated.


#### **Table 5.** Summary of reference greenhouse conditions.



## **3. Results and Discussion**

Figure 7a,b shows the summer and winter validation results. The computed results were obtained with the same physical and operating conditions as those of the experimental greenhouse, and are detailed in Table 5. Specifically, the internal greenhouse temperatures were compared. The statistical analysis of both validation results (given in Figure 7a,b) indicate R<sup>2</sup> values of 0.84 and 0.63; RMSE values of 1.8 ◦C and 1.3 ◦C; and rRMSE values of 6.7% and 7.4%, respectively. The R<sup>2</sup> value of 7a indicates that the model is accurate enough to calculate the inside temperature. In the case of the 7b, the value is a little less at 0.63, which is because the temperature fluctuations are much less. The RMSE

value for the prediction of the greenhouse's internal temperature is less than those of the studies conducted by Ahamed et al. and Vanthoor et al. [26,27]. Their studies indicated that an rRMSE value less than 10% is reasonable. According to that, our model prediction is sufficiently accurate. The good agreemen<sup>t</sup> between the computed and experimental internal temperatures encourages the adoption of the proposed multi-span greenhouse BES model.

**Figure 7.** Computed versus measured internal air temperature of the greenhouse for validation in (**a**) summer, and (**b**) winter.

After successfully creating and validating the multi-span greenhouse BES model, simulations were carried out for all the multi-span greenhouse design parameters presented in Table 6 (simulation section). Further analyses were conducted with the same greenhouse description and operating conditions presented in Table 5. (validation section). Only heating and cooling set points were changed to 18 ◦C and 30 ◦C for winter and summer, respectively. Firstly, we evaluated the e ffect of using a number of screens in the greenhouse during heating and shading screens during cooling, and made a comparison. Figure 8 shows the monthly heating and cooling energy demand when thermal and shading screens were used. The months for heating and cooling were selected based on South Korean weather and crop needs. In the periods November to March we used heating, while cooling was used for June to September. The results showed significant heat energy savings during the winter period by using screens and compared with the no screen greenhouse case. Moreover, when using three screens inside the greenhouse, the heat energy demand was 70% and 40% lower than when the single and double layered screens were used. Normally, in greenhouses, one or two thermal screens are used. Geoola et al. [28,29], in two studies and our previous study (Rasheed et al. [30]) on the U-value of greenhouse cladding with thermal screens, reported that the use of thermal screens can reduce heating energy demand by 50–60%. Taki et al. [31] also reported a reduction in heating energy demand using thermal screens in comparison with cases without thermal screens. Their results are for the particular screens used in the current study, and can be di fferent for other screen materials, as heat loss characteristics depend on the screen's properties. Furthermore, during summer months the shading screen showed 25% less cooling energy demand for the greenhouse. Therefore, during winter, thermal screens reduce heat loss to the ambient environment. Conversely, during summer, shading screens reduce the amount of solar radiation entering the greenhouse. This causes energy savings to be made and using screens in the greenhouse causes a volume decrease and hence a reduced energy demand. Ahmed et al. [32], who conducted a review on greenhouse shading for greenhouse energy savings improvement, and Abdel-Ghany et al. [33] who predicted the potential of di fferent shading methods for greenhouses, also reported the same trend. In addition, Figure 9 shows the proposed model's capability in evaluating the thermal and shading screens e ffect on the greenhouse's energy demand under dynamic outside weather conditions and dynamic control of the screens (as in the real greenhouse).

**Figure 8.** Comparison of heating and cooling loads of the greenhouse using thermal and shading screens.

Further analyses were conducted to estimate the maximum heating and cooling demands for the reference multi-span greenhouse with and without screens, and the results are presented in Table 7. The maximum heating demand occurred at 8:00 a.m. on 9 January 2019 when the outside temperature was −10.7 ◦C and the solar radiation was 0 kJ·h−1·m<sup>−</sup>2. The maximum cooling demand occurred at 3:00 PM on 7 July 2019 when the outside temperature was 35.7 ◦C and solar radiation was 3570 kJ·h−1·m<sup>−</sup>2. An estimation of the maximum heating and cooling demand of the greenhouse with fully controlled systems helps to design an energy providing facility for the greenhouse. Lee et al. [34,35] conducted two studies to estimate the maximum heating and cooling demand of a multi-span greenhouse to design a facility to fulfil its energy demand. They assessed the performance of the facility in fulfilling energy requirement in peak times.

**Figure 9.** Screen control with outside solar radiation.


**Table 7.** Details of studied parameters.

Different kinds of thermal screens are available in markets all over the world. Our screen analyses can help researchers and growers to choose the right screens according to their specific needs. For this reason, the heating energy demand of the different thermal screen materials available on the South Korean market were evaluated and compared. Their properties are described in Table 3. Figure 10 shows that Ph-77 combined with Ph-Super gives lowest least energy demand 139 MJ·m<sup>−</sup><sup>2</sup> compared with the others including Ph-77+Luxous, Ph-77+Polyester, Tempa+Ph-Super, Tempa+Ph-Luxous, Tempa+Polyester, which give demands of 142, 163, 145, 147, 171 MJ·m<sup>−</sup>2, respectively. This is due to the fact that Ph-77 and Ph-super's emissivity values are lower than those of the others. Our previous study Rasheed et al. [28] and that of Ahamed et al. [30,36], on the sensitivity analyses of the effect of material properties on energy demand, confirmed these results.

Greenhouse covering material is also an important factor, as there are many covering materials available on the market. It has a direct effect on the solar gain and the energy requirement of the greenhouse, and choosing the most appropriate one according to crop needs can help in minimizing energy costs [15]. Figure 11 shows the monthly heating and cooling demand of the most commonly used greenhouse covers including, PE, PVC, HG, PC, PMMA. All the conditions were same as in the

previous analysis and only side walls were replaced with different covering materials. A comparison was made of the estimated energy demand for all of them. The PC-16 mm material gave the smallest heating demand of, 20%, 19%, 7%, 4% during the heating months (Jan, Feb, Mar, Nov, Dec) than the other materials.. Furthermore, PE gave the smallest cooling demand of 2%, 7%, 5%, 4%, lower than the others during summer months (June, July, August, September).

**Figure 10.** Heating demand of greenhouse using different thermal screens.

**Figure 11.** Monthly heating and cooling energy demand for the multi-span greenhouse using different greenhouse side wall covers. PE: Polyethylene, PVC: Polyvinyl chloride, HG: Horticulture Glass, PC: Poly carbonate, PMMA: Polymethylmethacrylate.

Further analyses were carried out for the investigation of the effect of side wall multi-glazing on heating and cooling energy demand for the multi-span greenhouse. The triple-, double-glazing materials PC, and PMMA with different thicknesses available on the South Korean market were evaluated. Figure 12 depicts the results for triple-layered PC-16, and double-layered PC (10, 8, 6, 4 mm), and PMMA (16, 10 mm) thickness. Each sheet of material was 0.8 mm thick, and hence the total

thickness depends on the air gap between two sheets. The results in Figure 12a showed that the smallest heating demand is for the PC-16 mm material, and that the heating demand increased significantly for lower thicknesses. This is due to the fact that a thicker material serves better in preventing heat loss. Moreover, PC-16 mm gave 4% less heating demand than PMMA-16 mm. In Figure 12b, the cooling demand trend is the inverse of the heating demand and the lower thickness of PC-4 mm showed the smallest cooling demand. This is due to the fact that a higher thickness will resist heat loss to the ambient weather, thereby causing the internal temperature of the greenhouse to increase more than with the thinner material. Consequently, the cooling demand is increased. Figure 12c shows the total annual energy demand including heating and cooling for all the selected double glazing. The PC-16 mm material is more energy efficient than the double-glazed material.

**Figure 12.** Annual energy demand of greenhouse with double-glazing (**a**) Heating (**b**) Cooling, (**c**) Total.

Greenhouse orientation has a significant effect on passive energy saving techniques. The effect of different greenhouse orientations was studied with the physical operating conditions described in Table 5. The total annual energy demand, including heating and cooling, was estimated for each orientation and the results are shown in Figure 13. The results indicate that the E–W orientation has the smallest annual energy requirement. This is due to the fact that, in winter, the E–W orientation receives more solar radiation than the other orientations, and the reverse happens in summer. Figure 14 shows the average daily solar energy gain inside the greenhouse during January (winter) and June (summer). The result indicates that, in January, the average daily solar gain of the E–W (0◦) orientation was 11%, 14%, 15%, 12%, and 7% higher than the 15◦, 30◦, 45◦, 60◦, 75◦, and 90◦ (N–S) orientations, respectively. Moreover, during June, the E–W (0◦) orientation received 7%, 11%, 13%, 15%, and 15% less average solar gain than that of the 15◦, 30◦, 45◦, 60◦, 75◦, and 90◦ (N–S) orientations, respectively. A similar study conducted for energy efficient design of a multi-span greenhouse also confirmed this trend [1].

**Figure 13.** Annual energy demand for the greenhouse with different orientations.

**Figure 14.** Annual energy demand for the greenhouse with different orientations.

Natural ventilation is widely used to decrease the internal temperature of greenhouses, especially in the summer. In this study, we evaluated the effect of natural ventilation on the cooling demand of the multi-span greenhouse. We estimated a monthly cooling demand of a fully closed and naturally ventilated greenhouse and the results are shown in Figure 15. Again, all physical and operating conditions are same ones presented in Table 5. The results indicate a 50% reduction in cooling demand during the whole summer season using natural ventilation combined with cooling. Figure 16 shows the internal temperature of the greenhouse with natural ventilation and a fully-closed condition. It can be seen that in the fully-closed condition with no ventilation, the greenhouse's internal temperature increases to 60 ◦C. However, with natural ventilation, it reduces to 40 ◦C, which causes a reduction in the cooling demand in the natural ventilation condition. The TRNSYS model successfully described the natural ventilation effect combined with a greenhouse thermal model.

**Figure 15.** Comparison of monthly cooling demand of the greenhouse with natural ventilation and fully closed conditions.

**Figure 16.** Comparison of greenhouse internal temperature with natural ventilation and fully closed conditions.

In the northern hemisphere, the sun stays at the south side and north wall of the greenhouse and contributes less to the solar heat gain inside the greenhouse. Other studies reported that the use of an opaque north wall can reduce 25–30% of the heating requirement in winter when compared to using a transparent north wall during the daytime. Therefore, it is recommended that insulation should be used on the north wall of the greenhouse to reduce heat loss, especially in colder regions during winter [4,37]. Commonly, multi-layered thermal screens are used for north wall insulation in South Korea. To simulate the specific insulation, we need to have the physical, optical and thermal properties of that material as described in Table 3. Due to lack of availability of these properties, for this specific reason (to show the model's capability) the available Ph-77 material was used as north wall insulation. Figure 17 shows the comparison between the heating demand of the multi-span greenhouse with north wall insulation and without insulation (transparent wall). The conditions are the same as those in aforementioned analyses. The overall results indicate that a 5% reduction in heating demand during the whole winter season was achieved using north wall insulation. The reduction of the heating demand purely depends on the material used for the north wall. Moreover, the specific energy saving is di fferent as location and available solar radiation are di fferent and the insulation material can also affect energy savings. Other similar studies for greenhouse design parameters, conducted by Gupta and Chandra [7], and Ahamed et al. [4], also confirmed this trend.

**Figure 17.** Comparison of monthly heating demand of the greenhouse with and without north wall insulation.

Greenhouse roof geometry has a huge influence on the annual energy demand of the greenhouse, as solar radiation received by the greenhouse influences its internal temperature [12]. Two multi-span greenhouse roof geometries (venlo and wide-span) were selected in order to compare their annual energy demands. The designs were selected according to South Korean standards for multi-span greenhouses. The results of both greenhouses' annual energy demand are shown in the Figure 18. The results indicate that during the winter season the venlo type greenhouse requires 25% less heating energy than the wide-span. In contrast, during the cooling season, the wide-span greenhouse requires 35% less cooling energy than the venlo type. This is due to the fact that the solar energy gain of the greenhouse with the wide-span roof geometry is less than that of the venlo type greenhouse. Figure 19 shows the monthly solar energy gain of both greenhouses. The venlo type greenhouse received more solar energy than that of the wide-span greenhouse. The solar gain inside the greenhouse was di fferent due to the fact that the wide-span greenhouse has a lower angle of incidence—which causes low transmission—than that of the venlo type greenhouse with high angle of incidence—which causes high transmission. A study was conducted by Ha et al. [17] with the same type of greenhouse but the outcomes of the study show the wide-span greenhouse needs less annual energy than that of venlo type greenhouse. In that study, the greenhouse dimensions were not the same, so comparisons cannot be made. In our study, we used the same greenhouse dimension with but with a different roof geometry as shown in Figure 5.

**Figure 18.** Comparison of monthly heating and cooling demand for selected greenhouse roof geometries.

**Figure 19.** Comparison of monthly solar radiation gain for selected greenhouse roof geometries.
