전산유체역학을 이용한 간척지 내 자연환기식 연동 온실의 환기량 평가

학위논문 (석사)-- 서울대학교 대학원 : 생태조경·지역시스템공학부 지역시스템공학전공, 2016. 2. 이인복.Greenhouse area has increased continuously making possible for intensive cultivation, high quality agricultural product in South Korea. However, there is no enough space for installing large-scale greenhouse complex in South Korea, because the land area that account for 70% of South Koreas territory consist of mountain area. Reclaimed lands which are newly developed have great worth as agricultural lands because reclaimed lands can be used deliberately regardless of near terrain. Recently, Korea government made a public announced about the development plan of large-scale greenhouse complex in representative reclaimed lands. However, wind environments of reclaimed land are entirely different from those of inland. Natural ventilation studies for greenhouses built on reclaimed land should be conducted with proper wind profiles of the reclaimed land. Meanwhile, many standard documents for ventilation design did not describe quantitative design standard of natural ventilation which is commonly used. Therefore, natural ventilation rates were computed and analyzed to suggest standard for ventilation design of multi-span greenhouse built on reclaimed lands. In this study, natural ventilation rate of multi-span greenhouse was computed and analyzed according to greenhouse type (Venlo, Wide-span, 1-2W), number of spans (two, five, eight), wind speed (1.0, 2.5, 5.5 m•s-1), wind direction (90, 45, 0°), and vent opening (side vent, side-roof vent, roof vent). Computational Fluid Dynamics (CFD) simulation was used for the purpose of overcoming the limitation of field experiments, such as laborious and cost consuming, requirement of equipment for multipoint measurement, vulnerability to unstable weather condition, and so on. An analysis of the wind environment was performed to design the wind profiles of reclaimed lands in South Korea. CFD simulation models for multi-span greenhouses were designed based on a design method for a CFD simulation model, which was validated by Ha (2015). The designed wind profiles of the reclaimed lands were applied to the CFD simulation models. Two methods, mass flow rate (MFR) and tracer gas decay (TGD), were used to calculate the natural ventilation rates of multi-span greenhouses. The natural ventilation rates computed by these two methods were evaluated by comparing them with the ventilation requirements. As the result of comparing the natural ventilation rates computed by the two methods, it was judged that the tracer gas decay method evaluated the actual natural ventilation more closely than the mass flow rate method. In analyzing the overall ventilation rates, the results also showed that the natural ventilation rates were influenced considerably by wind speed and wind direction. The natural ventilation rates increased linearly as wind speed increased. As the number of spans increased, the natural ventilation rates generally decreased. It was observed that the effect of ventilation through the roof ventilators of the 1-2W type greenhouse was higher than in the Venlo and wide-span greenhouses. As a result of analyzing the homogeneity of the local ventilation rates, it was found that the homogeneity was mainly influenced by wind direction and the configuration of the ventilators except for wind direction. The results of analyzing the overall ventilation rates will be used as basic data to establish design standards for multi-span greenhouses built on reclaimed lands. Additionally, the results of analyzing the local ventilation rates are expected to be utilized for controlling the microclimate in large-scale greenhouses uniformly. The charts for expecting the natural ventilation rates will be used for designing the ventilation of greenhouses and the guidance of maintenance control. The natural ventilation rates were computed and evaluated according to various conditions as basic research to create design standards for natural ventilation. A study evaluating the effects of crops and buoyancy on natural ventilation is still required. When crops exist in a greenhouse, the air flow patterns and natural ventilation rates are completely changed according to the arrangement of the crops, variety of the crops, crop height, etcetera. Also, when wind speed is low, buoyancy-driven ventilation is more important. With additional research, it will be possible to suggest quantitative standards for designing the ventilation of greenhouses that will be practical for the managers and designers of greenhouses.1. Introduction 1 2. Literature Review 5 2.1. Studies on Greenhouse Ventilation 5 2.2. Standards for Greenhouse Ventilation Design 7 2.3. Studies on Greenhouses Built on Reclaimed Lands 8 3. Materials and Methods 10 3.1. Target Greenhouses 11 3.2. Target Reclaimed Lands and Weather Data 12 3.3. Computational Fluid Dynamics (CFD) 13 3.4. Evaluation Methods of Ventilation Rates 15 3.4.1. Ventilation Requirements 15 3.4.2. Natural Ventilation Rates 16 3.5. Analysis Procedures 18 3.5.1. Design of the CFD Simulation Model 18 3.5.2. Analysis Method of the Ventilation Rate Using CFD 24 4. Results and Discussion 27 4.1. Wind Environment 27 4.2. Ventilation Requirement 30 4.3. Analysis of Natural Ventilation Rates 32 4.3.1. Overall Ventilation Rates of Greenhouses 32 4.3.2. Local Ventilation Rates of Greenhouses 53 4.4. Suggested Charts for Expecting Natural Ventilation Rates and Comparison of the Ventilation Requirements and Natural Ventilation Rates 66 5. Conclusion 71 Reference 73 Abstract in Korean 80Maste

Cooling Strategies for Greenhouses in Summer: Control of Fogging by Pulse Width Modulation

The possibilities for improving the control of greenhouse fogging systems, were studied by comparing several combinations of ventilation cooling techniques, shade screening and low-pressure fogging. The study was divided into three parts: experiments, modelling and simulations. In the first part of the paper, ten combinations of five cooling techniques were tested during the summers of 2002 and 2003 in a 132m2 greenhouse with a steel structure and a single-layer methacrylate cover located in Madrid, Spain. An analysis of variance of the climatic parameters was carried out to determine which combinations produced significant differences in inside temperature or relative humidity. Comparing the values for the inside to outside temperature difference, the combination of a shade screen and above-screen fogging achieved a difference in temperature almost the same as that for under-screen fogging, but the relative humidity was significantly lower. In the second part of the study a dynamic model was developed (2002) and validated (2003). The mean absolute error obtained for inside temperature was similar in the fit and the validation and it was less than 1.5 1C in both cases. The model was used to simulate the inside air temperature for a fog system working without shading, and above and under a shade screen. Control algorithms were developed for reducing system water consumption. In the three cases a simple on/off control with a fixed fogging cycle was compared with a pulse width modulation (PWM) strategy, in which the duration of the fogging pulse was increased as a function of inside temperature. The strategies with PWM applied to the fog system were able to reduce water consumption by 8–15% with respect to the strategies with a fixed fogging cycle

Greenhouse technology for sustainable production in mild winter climate areas: Trends and needs

Greenhouse production in the near future will need to reduce significantly its environmental impact. For this purpose, elements such as the structure, glazing materials, climate equipments and controls have to be developed and wisely managed to reduce the dependence on fossil fuels, achieve maximum use of natural resources such as solar radiation and water, and minimize the input of chemicals and fertilizers. This paper discusses the most relevant developments in greenhouse technology for mild winter climates. Regarding greenhouse structures, recent studies based on computational fluid dynamics have been conducted to investigate the effect of parameters such as ventilator size and arrangement, roof slope and greenhouse width and height on the air exchange rate. Next generation greenhouses are expected to incorporate some of the innovations derived from recent ventilation studies. Covering crops with screens is becoming a common practice. Main advantages and limitations of screenhouses are discussed in this paper. Thermal storage is increasingly applied in closed or semi-closed greenhouses. Under some conditions semi-closed greenhouses could mitigate day/night while reducing the use of water and the entrance of pest. Photo selective films that reflect a fraction of NIR radiation are effective at lowering greenhouse temperature and, in some cases, may be cost effective. NIR reflective films have side effects of major importance in greenhouse production. The CO2 enrichment strategy in computer-controlled greenhouses is based on determining the benefits of increasing the CO2 concentration against the cost of it. No clear strategies have been defined for the application of CO2 in unheated greenhouses, where most of the time the source of carbon dioxide is the external air. Some authors suggest ventilating as little as possible and fertilizing with bottled carbon dioxide at least up to the external concentration. Improving greenhouses by introducing new technologies may have an additional impact on the environment. From an environmental point of view, the incorporation of technology needs to increase yield to compensate for its associated environmental burden. Previous results have shown that forced ventilation and heating are the main reasons for the increase in environmental impact in climate controlled greenhouses. Additional results on the area of technology and its associated impact are discussed in this pape

Use of supplementary lighting top screens and effects on greenhouse climate and return on investment

Discomfort caused by light pollution from greenhouses that apply supplementary lighting is an issue in Dutch society nowadays. At this moment Dutch legislation requires an opaque screen that reduces light transmission of the greenhouse wall by 95%. In 2008 also the light transmission of the greenhouse roof must be reduced equally and supplementary light will be limited to 15,000lx(180¿mol/m2/s), unless light emission is totally prevented. The objective of this research was to calculate the economic consequences of installing reflecting, light emission reducing or blocking screens by considering crop yield and costs. A mathematical correction equation was developed to approach the light gain for the crop as a result of internal reflection. Greenhouse climate and tomato crop growth were simulated for a reference greenhouse with supplementary lighting and without an emission blocking screen and for a low-light-emission greenhouse with a blocking screen. The supplementary lighting level was set at 180¿mol/m2/s. Results show that the greenhouse climate below the screen remained manageable, but that the desired DIF of 2°C was affected. The light gain was on average about 3% and resulted in production increase. A small net yearly profit resulted based on direct and indirect effects of the screen. In conclusion, the simulation suggested that stopping light emission at the source with help of reflective opaque screens is economically feasible if screen operation is included in planning the lighting scheme

Overall Energy Analysis of (Semi) Closed Greenhouses

Natural ventilation to discharge excess heat and vapour from the greenhouse environment has serious drawbacks. Pests and diseases find their way through the openings; carbon dioxide fertilisation becomes inefficient and the inescapable coupling of heat and vapour release results often in sub-optimal conditions for either temperature or humidity. The present trend, therefore, is to reduce ventilation as much as possible, also in Mediterranean conditions. This relies obviously on improved means for diminishing the heat load and proper use of cooling equipment. Especially the latter can combine the benefits of cooling the greenhouse air with serious energy conservation. However, opposite to the clear benefits there are also serious investments associated with active cooling of greenhouse. Therefore, there is a growing demand for some computational tool that enables quantitive comparisons between the vast number of alternatives with respect to the different components of (semi) closed greenhouse systems. The benefits in terms of improved production (quality, ornamental value and quantity) are quite difficult to quantify, due to the complexity of the biological processes involved. On the energy side of the balance, however, since the physics of greenhouses, climate controllers and horticultural hardware can be described very well, it is quite possible to develop such a tool for predicting the energy consumption of a (semi) closed greenhouse for a wide range of horticultural and outside climate conditions. This paper gives an outline of such a tool and discusses some results. Just as an illustration, a number of quantitative effects are shown of changing the fraction of closed green¬house surface in a 1 hectare enterprise that consists of closed and non-closed compartments. This analysis is made for both a Dutch climate situation and a Mediterranean weather data set

Numerical and Experimental Study of Fan and Pad Evaporative Cooling System in a Greenhouse with Tomato Crop

An experimental greenhouse equipped with fan and pad evaporative cooling is simulated numerically using a commercial CFD code. The main aspects of evaporative cooling systems, in terms of heat and mass transfer and both the external and internal climatic conditions were integrated to set up the numerical model. The crop (tomato) was simulated using the equivalent porous medium approach by the addition of a momentum and energy source term. Preliminary calculations were carried out and validated by experimental measurements, in order the pressure drop occurred in crop model due to air flow, to be determined as a function of leaf area, stage of crop growth and cultivation technique. The temperature and humidity of incoming air and the operational characteristics of exhaust fans were specified to set up the CFD model. The numerical analysis was based on the Reynolds-averaged Navier-Stokes equations in conjunction with the RNG k- turbulence model. The finite-volume method (FVM) was used to solve the governing equations. The 3D full scale model was solved in several differencing schemes of various orders in order to examine its accuracy. The simulation results were validated with experimental measurements obtained at a height level of 1.2 m above the ground in the middle of the crop canopy at 23 and 8 points, concerning air temperature and air humidity respectively. The correlation coefficient between computational results and experimental data was at the order of 0.7419 for air temperature and 0.8082 for air relative humidity. The results showing that the evaporative cooling system for greenhouses could be effectively parameterized in numerical terms, providing a useful tool in order to improve system’s efficiency

Building-integrated rooftop greenhouses: an energy and environmental assessment in the mediterranean context

A sustainable and secure food supply within a low-carbon and resilient infrastructure is encapsulated in several of The United Nations’ 17 sustainable development goals. The integration of urban agriculture in buildings can offer improved efficiencies; in recognition of this, the first south European example of a fully integrated rooftop greenhouse (iRTG) was designed and incorporated into the ICTA-ICP building by the Autonomous University of Barcelona. This design seeks to interchange heat, CO2 and rainwater between the building and its rooftop greenhouse. Average air temperatures for 2015 in the iRTG were 16.5 °C (winter) and 25.79 °C (summer), making the iRTG an ideal growing environment. Using detailed thermophysical fabric properties, 2015 site-specific weather data, exact control strategies and dynamic soil temperatures, the iRTG was modelled in EnergyPlus to assess the performance of an equivalent ‘freestanding’ greenhouse. The validated result shows that the thermal interchange between the iRTG and the ICTA-ICP building has considerable moderating effects on the iRTG’s indoor climate; since average hourly temperatures in an equivalent freestanding greenhouse would have been 4.1 °C colder in winter and 4.4 °C warmer in summer under the 2015 climatic conditions. The simulation results demonstrate that the iRTG case study recycled 43.78 MWh of thermal energy (or 341.93 kWh/m2/yr) from the main building in 2015. Assuming 100% energy conversion efficiency, compared to freestanding greenhouses heated with oil, gas or biomass systems, the iRTG delivered an equivalent carbon savings of 113.8, 82.4 or 5.5 kg CO2(eq)/m2/yr, respectively, and economic savings of 19.63, 15.88 or 17.33 €/m2/yr, respectively. Under similar climatic conditions, this symbiosis between buildings and urban agriculture makes an iRTG an efficient resource-management model and supports the promotion of a new typology or concept of buildings with a nexus or symbiosis between energy efficiency and food production.Postprint (published version

Optimal greenhouse cultivation control: survey and perspectives

Abstract: A survey is presented of the literature on greenhouse climate control, positioning the various solutions and paradigms in the framework of optimal control. A separation of timescales allows the separation of the economic optimal control problem of greenhouse cultivation into an off-line problem at the tactical level, and an on-line problem at the operational level. This paradigm is used to classify the literature into three categories: focus on operational control, focus on the tactical level, and truly integrated control. Integrated optimal control warrants the best economical result, and provides a systematic way to design control systems for the innovative greenhouses of the future. Research issues and perspectives are listed as well

Cover materials excluding Near Infrared radiation: what is the best strategy in mild climates?

Only about half of the energy that enters a greenhouse as sun radiation is in the wavelength range that is useful for photosynthesis (PAR, Photosynthetically Active Radiation). Nearly all the remaining energy fraction is in the Near InfraRed range (NIR) and only warms the greenhouse and crop and does contribute to transpiration, none of which is necessarily always desirable. Materials or additives for greenhouse covers that reflect a fraction of the NIR radiation have recently become commercially available. Besides lowering greenhouse temperature, a NIR-excluding cover has quite a few side-effects that may become quite relevant in the passive or semi-passive greenhouses typical of mild climates. For instance, the ratio of assimilation to transpiration (the water use efficiency) should increase. On the other hand, by lowering the ventilation requirement, such a cover may hinder in-flow of carbon dioxide, thereby limiting the photosynthesis rate. In addition, there are obviously conditions where the warming up caused by NIR may be desirable rather than a nuisance. NIR-reflecting materials are becoming available in forms that are suitable for various types of applications, such as permanent, seasonal or mobile. By means of a simulation study, we discuss in this paper the best form of application in relation to the external climate and climate management options availabl

Technical solutions to prevent heat stress induced crop growth reduction for three climatic regions in Mexico

In the last 15 years a significant increase in greenhouse area has occurred in Mexico, from a modest 50 hectares in 1990 to over 2,000 hectares in 2004. The rapid increase in greenhouse area is a result of an attractive export market, USA. Mexican summer midday temperatures are well above crop optimum and cooling is needed if heat stress induced crop growth reduction is to be prevented. The objective of this study was to determine the effectiveness and feasibility of greenhouse cooling systems for tomato culture under desert, humid tropic and temperate Mexican weather conditions. These climate regions are represented by Mexicali, Merida and Huejutla respectively. The cooling systems included a variety of passive and active systems, which through an engineering design methodology were combined to suit the climate conditions of the 3 regions. The evaluation was conducted via simulation, taking into account the most important temperature effects on crop growth and yield. The results showed that the cooling systems were effective in decreasing heat stress to plants. Investment costs of greenhouse with cooling equipment were under USD 50 m-2 and operational costs were under USD 10 m-2 for all equipment combinations and treatments except for the humid tropic climate of Merida. Solutions for Merida were both economically and physically not feasible due to too high humidity levels. This model study clearly indicates that cooling is feasible in desert and moderate climate regions of Mexico but in humid tropic climate regions feasibility is a problem. Application of design methodology and design evaluation with help of simulation greatly contributed to pointing out effective and non-effective solutions to reduce heat stress in hot climates