Application of biomimetics in Greenhouse design Natural ventillation and solar optimization Anurag Bhattacharya
1
1
Delft Univeristy of Technology, Netherlands 1
a.bhattacharya@student.tudelft.nl
Abstract This paper describes a method for the design of complex branching and streamlining morphology which can be applied to optimize ventilation and sunlight in a greenhouse. The research proposes that through the integration of digital form finding techniques and climatic factors that govern a greenhouse complex. The global climate change cautions and has a major impact on agricultural production across the globe, thus demanding a new approach for food production that is largely environment adaptive and therefore resilient to climate change. Accordingly, the research focus will be on the dynamic environment, ventilation, temperature and solar gain. Here biomimetic has an important role to play to reduce the dependency on conventional non-renewable energy sources. Biomimetic is a scientific approach to help achieving the goal by mimicking natural systems, processes, and models. (Pawlyn, 2011) Here termite mounds have been examined, in which an intersection of Branching and Streamlining Morphology has been adopted to design a greenhouse prototype. Custom written dynamic branching algorithm written in processing language was used to define the overall form and structure. A basic research in application of branching organization and its geometrical constitute in termite mounds defined the framework of the design process. In light of this some of the main factors which characterize and influence the greenhouse environment like lighting, location factors and ventilation are also taken into account. The custom written application became the primary form generating tool in the design process of the green house. In order to ensure structural stability in the emergent structure, spring physics dynamics and gravitational forces were considered during the form generation stage itself. Keywords. Biomimetic; greenhouse; dynamic branching; performative design; form finding; termite mound.
Figure 1: Outcome from one of the branching simulation.
Introduction The main objective of a greenhouse remains crop rotation and programming of crops depending upon spaces that have regulated access to sunlight and radiation alongwith controlled temperature and humidity levels. All the greenhouses till now although look relatively complex, the designs still contain a degree of regularity and repetition in order to simplify formwork production. The crucial aspects which constitute and influence the greenhouse environment are light, humidity, temperature, ventilation rate and CO2 concentration. Ventilation is the chief determinant of the greenhouse’s high temperatures. The performance of ventilation plays a pivotal role in the overall production, affecting the environmental conditions of the greenhouse, most importantly the qualitative and quantitative properties of the crop production as well. Natural ventilation have widely been used in building design since olden days, as it requires less energy, equipment and power than any other forms of ventilation. Though in the current available literature and papers there is not much information on the design of naturally ventilated greenhouses, mostly because of its less efficiency and other constraints. Recent works at the University of Arizona and others has seen experimentation into greenhouse production systems. Their experiment suggests that modifying the greenhouse design and its operation to meet performance standards, will always create subsequent consequences, some which are anticipated, but others unknown. For example, making a greenhouse more environmentally friendly may add burden to operational demands. Two trajectories of previous research have been combined to form the branching morphology in greenhouses that is the subject of this paper. First is the “Ventilation of termite mounds� by Judith Korb and Karl Eduard Lisenmair, in which they tested the open and closed ventilation mechanisms in cathedral and dome shaped mounds by investigating temperature, CO2 concentration and air currents. The second being the Green house production systems by Giacomelli, G.A.; Sase, S. and Cramer, R at the Wageningen UR Glastuinbouw which emphasize the current basics of greenhouse design and low-input systems to highly complex production systems. The proposed greenhouse prototype discussed here combines these techniques to develop a complex branching system algorithm for ventilation embedded into the structure developed for greenhouse. The project goal was to investigate how computation techniques help to provide an alternative ventilation system, enable real-time feedback loops in the design process, and to give an overview of the most pressing research tasks towards a prototype.
Termites Mounds organization can be applied into greenhouse design for enhancing its performance In nature, the termite colony acts as a complex system comprise of numerous individual agents, we can see each agent perform simple task at the same time response to one another, and through the collective behaviour of the agents, a complex and emergent system is built. These termite mounds are responsive to environmental changes which will enable the colony to survive as one system, where single termite will never be able to survive on its own. A complex system is the resultant of the numerous interaction of individual entity with its immediate neighbors and with the environment. As an individual agents, each termite collects information of the environment and feeds back to the system (Menges, 2010). The mound is the result of the collaboration of the agents influenced by the information of the surrounding environment. The technical implementation of algorithmic growth processes can vary significantly according
to system type and design strategy. In this paper the main focus lies in the climate form finding and temperature control based upon intersection of branching and streamlining morphology, which is an essential driver of the overall orientation and articulation of a greenhouse volume with regard to the sun path, deducing this principle from the termite mound.
Mechanisms Affecting Natural Ventilation Airflow Naturally ventilated greenhouses rely primarily on air blowing into a windward side opening and out the open roof vents. The cause of ventilation is due to the pressure difference across the opening driven by wind forces and thermal effect. It is caused due to external wind force (wind effect) and thermal effect based on the temperature difference between internal and external air (Roy, 2002). Natural airflow inside greenhouses is a combination of the effects of both thermal and wind forces. Rate of natural ventilation varies with external wind velocity and area of openings, and it also varies with the square roots of height of openings and temperature rise (Pontikakos, 2011). This is why natural ventilation has always been regarded as complex and more difficult in design of greenhouses. The advantages of natural ventilation: 1. Low energy requirements. 2. Pleasant internal working and shopping environment. 3. Unused greenhouse can be easily cooled in summer. 4. No ventilation restrictions on length of greenhouse. 5. Air temperature can be maintained very close to outside air. 6. Very high ventilation rates are possible. 7. Low temperature gradients across greenhouse are possible. The height of the greenhouse can have a significant effect on the ability of the heating and cooling systems to maintain a uniform air temperature within the structure. The larger the volume of air to be conditioned tends to minimize significant shifts in the interior air temperature, humidity, and CO2 concentration. In a greenhouse, if openings are at different heights and the inside temperature is greater than the outside, a pressure is generated causing the inside air to flow out of the higher opening and the outside air into the lower openings. The airflow varies with the difference in temperature between inlet and outlet as well as due to the difference in height. This difference in wind pressure creates a potential for the air to flow from a point of higher pressure to another point with lower pressure (Givoni, 1981). Similarly, when wind strikes a wall orthogonally to its direction, the wall surfaces goes through a pressure which is higher than atmospheric pressure. This difference in pressure causes the inside air to flow from inlet to outlet in building walls which are at lower pressure. These fluid principles are essential for natural ventilation in a greenhouse. There is an another augmentation, where if the measured pressure difference between the two openings is equal to zero, some airflow can still occur as a result of inertia from wind entering the window or along the height of each window (Ernest, 1991; Evans, 1979). If the climatic air temperature during much of the growing season is above 32째F (0째C), the primary requirement for environmental control within the greenhouse will be cooling (ventilation). If the climatic air temperature during much of the growing season is below 32째F (0째C), the primary requirement for environmental control within the greenhouse will be heating. In either instance, a totally different greenhouse design may be required to cope with exterior climatic extremes. Exposure to wind can significantly impact the heating and cooling requirements; therefore, having a windbreak can prove to be highly desirable. Having a windbreak can minimize the deposition of suspended material that might accumulate on the greenhouse surface or the immediate surrounding area. In addition, the immediate area around a greenhouse must be kept as inert as possible, with the minimum of activity from operations not bringing vehicular traffic close to greenhouse entrances.
Pressure Differences as a driver of natural ventilation According to the Bernoulli's principle, the static pressure on the outer skin of an object exposed to the wind becomes at its maximum positive pressure at the wind-ward side, and its minimum negative one at the zone parallel to the wind direction [1]. On the other hand, the amount of ventilated air is proportionate to the square root of the pressure difference between a concerned inlet and an outlet. In the case of two holes, when one is at the point of the maximum positive pressure and another is at the minimum negative pressure point, and two holes are lying on the same stream line, that airflows should become its maximum amount. For other air passages, it should be necessary to know how air behaves depending on relationships between positions of holes and the pressure differences (Hensel, 2010).
Figure 2: Natural ventilation driven by the pressure differences on the surface through the boundary layer of the wind.
Figure 3: Tilt of mounds towards the average zenith enhances the stack effect (Judith, 1999).
Natural Ventilation mechanisms of Termite Mounds Based on the research done on termite mounds and geometry, these inferences can be brought forth. 1. Round Horizontal Section of mounds induces the natural ventilation driven by the pressure differences on the surface through the boundary layer of the wind. 2. Branching Conduits Network makes the entire mound porous in order to utilise the pressure differences on the surface induced by the wind and organises the airflows directions with changing their roles in ventilation depending on their geometry, positions and the wind direction (Gabriel, 2012). 3. Tilting of mounds towards the average zenith enhances the stack effect. 4. Upward airflows from the core through the buoyancy effect; 5. The lateral air distribution from the mound chimney via lateral connections towards surface conduits;
Figure 2: Three representative characteristics from the morphology of termites mounds: Tree conduit and Shrub conduit (Susumu, 2009).
Functions of Conduits • Chimney Tree Conduits: Ventilate the interior air through the Venturi effect around the top side of the dome as well as enhancing ventilations within the wall through sucking the exterior air at the shaded side (Susumu, 2009). • Wind-catcher Tree Conduits: Introduce the cooler air at the higher altitude into the interior. • Shrub Conduits: Allow the dome to let the natural day light in through them as well as the air, which is an ability not to be needed for termite’s mounds.
Figure 5:Geometrical Constitution of Branching Region (Judith, 1999)
Figure 6: Mixing of Detached Airflows
Methodology For the experiment, representative area of Mikkeli lakefront in Finland has been chosen for the establishment of the greenhouse. A computational generative and evaluation methodology has been proposed to gather and process information datasets. Evolutionary problem solving techniques has been used in order to arrive at an optimized site specific geometry for outer form of the greenhouse through the directions of the wind and sun. According to the principle of termites' mounds' geometry, the latitude of a concerned site has been introduced for defining the position of the sun "attractor" as the average zenith. The wind direction has been set to the prevalent direction of the site during summer season. By this, it becomes a site-specific geometry. This method is an effective means to run computationally controlled results within the iterative design framework which allows architects and designers to generate optimized form that best meets design criteria. For this experiment, Galapagos, an evolutionary solver designed for Rhino/ Grasshopper has been used in conjunction with analysis tool “Ecotect” to optimize form based on energy data.
Setting an outer form based on sun-wind In this experiment, the inputs are two polygons that are lofted to form a faceted surfaces. Open from the bottom. The parameters being the number of sides in a polygon, height, scale are controlled by the sliders, thus manipulating the form and sending each iteration to “Ecotect”, an environmental simulator. Once the form reaches here, it is evaluated for average solar radiation within a specified environmental boundary condition. These values are then forwarded back to grasshopper, where they inform the algorithmic script to re-evaluate conditions that favor high solar radiation; hence, by each generation a new higher order of fit instance is generated, creating at each pass a more-optimized result [3]. The final outcome is a geometry that has all planes tilted towards the sun’s path in order to attain maximum exposure across the overall form. Continuing on with the research of improving CO2 absorption, a sky factor and photosynthetically active radiation (PAR) analysis [2] can be performed for the whole year for
the greenhouse form using the tool of “Ecotect�. The doubly glazed material used for the experiment were high density glass and ethylene vinyl acetate (Transmission 88% PAR). The photosynthetic rate is positively correlated to the CO2 concentration surrounding the plant; the extent of this concentration effect varies with plant species and light intensity. There is also a significant relationship between light intensity and the CO2 content of the air surrounding the plant. With increasing light intensity, the rate of photosynthesis reaches a plateau due to either CO2 concentration and/or leaf temperature (Jones, 2005). In tall greenhouses (30 ft or more) with roof vents open, CO2 can be introduced at the base of the canopy as it will then be carried slowly by the upward-moving air from the base of the plant canopy to its top, thereby enhancing photosynthesis.
Figure 7: The evolutionary toolset "galapagos"and cliamte analysis tool "ecotect" allowed a series of formation to be explored to achieve high solar radiation.
Figure 3: Greenhouse on the prevalent wind direction and the sun azimuth.
Figure 9: Photosynthetically active radiation(PAR) analysis.
By doing this experiment, the parts of the greenhouse which are highlighted with higher range of yellow to red color receive the most photosynthetically active radiation and would be considered better spots to plant crops which require more direct solar radiation. The greenhouse collector efficiency is calculated to about 14.6% for doubly glazed, where solar radiation ranges from 350 to 450 W/m2 at greenhouse temperature of 22 °C. This data was compared against the table 12.1 published in Practical Hydroponics, Issue 60. Once the outer form is set, the skin lines becomes the basis for branching conduits. Distribution of centers of holes are defined along the skin lines of the greenhouse prototype. Since site is in Finland, it suffers from cold winters and a warm climate with damp during the summers. The experiment was carried out for the summer season. The most important thing which could be done by ventilation could be expelling the hotter air at the higher altitude in the interior thus letting the cooler air in. This would allow the movement of inlet air upward firstly to the outlets and then to expel hotter air at the higher altitude out. After the distribution of holes are done, next comes selection of holes according to the prevalent wind direction of the site which was south and south-west, the leeward side facing north which allows the cooler air from north. After selection of the points, the seed points were set for the growth of branching conduits and were run through the dynamic branching algorithm written for this specific purpose.
Dynamic Branching algorithm The final ventilation conduits were generated on the principles for optimized branching morphology, as observed in Centroid Branching Algorithm and tree conduits [4]. Central Branching Algorithm defines the growth direction of branches through calculating a centroid of each cluster of points per each growth generation and setting that as an attractor, for a seed point to head toward targeted points including a structural ability.Through physical selforganization, the chains take forms that contain only tensile forces. The self-organizing process can be described by using Hooke’s law, which states that for elastic deformations of an object, the magnitude of its deformation (extension or compression) is directly proportional to the deforming force or load. Algebraically, Hooke’s law states that the applied force F equals a constant k multiplied by the displacement (change in length) x, thus: F = kx (Larsen, 2012). The formula is implemented through the physics class in the computer code written in processing. The application is able to simulate the self-organizational behavior, creating branches sprouting out of the given point based data set. By performing the simulation digitally the possibility of real-time adjustments of factors such as spring length, spring strength, gravity and damping can be provided. In turn, this provides a massive increase in the number of possible solutions that can be explored within a limited time span, as compared to physical models.
Figure 4: Left: Setting Seed Points for the growth of branching conduits. Right: Growth of branching conduit.
The script written in processing takes three datasets, one being the base (ground) points, the shell geometry seed points which defines the inlet and outlet holes. It simulates a dynamic branching and relaxation process, controlled by setting a list of essential variables, such as the relative rest-length, strength of the members and the damping of the system.
Figure 11: Dynamic Branching experiment 1. Left: Top view network. Right: Output Form.
Figure 12: Dynamic Branching experiment 2. Left: Top view network. Right: Output Form.
Figure 13: Dynamic Branching experiment 3. Left: Top view network. Right: Output Form.
Through iteration the system arrives at an equilibrium state, meaning that all the forces in the system have been balanced, and the velocity of the nodes is zero. The generated threedimensional conduit lines are then exported to a 3D modelling program. The above shown three experiments were executed by varying the variable values like spring strength, gravity (y-axis), minima and maxima for connections.
Findings The experiment demonstrated how digital form finding techniques and complex branching morphology could be used to optimize ventillation and solar gain in a greenhouse. It also demonstrated the method of translating natural ventillation as found in termite mounds into branching conduits of a greenhouse. The continuation of this work will include statistical CFD analysis of the inlets and outlets evaluating distributional pattern of the dynamic pressure on the greenhouse form and ventillation driven by the pressure difference. In the following experiment, interior pressure has been neglected which would definitely have an impact on definitions of inlets/outlets. Since the experiment dealt with summer scenario, it would be interesting to test the prototype in a winter situation. Despite the success in terms of implementing the branching algorithm under the digital tool “processing�, it should be pointed out that the structural forces were not simulated to its hilt. Despite having the capability to induce tension by varying the spring strength in the script, in principle it was not subjected to structural analysis tools like FEM or GSA.
Conclusion This paper is part of expanding the application in field of biomimetic in greenhouse design. The key characteristic here being understanding the application of natural ventilation and digital form finding in greenhouses, taking instances from nature, in this case being the termite mounds. The literature study clearly opened the door of possibilities in biomimetic, though never gave a concrete translation of the theory and arguments into design. The greenhouse prototype demonstrates how digital simulation and experimentation be applied effectively to define and develop forms for greenhouse based on solar optimization logic and natural branched ventilation. The greenhouse collector efficiency was calculated to about 14.6% for doubly glazed, which was remarkable in comparison to some of the modern greenhouses. The method for generating a branching
pattern through self-organization was successful in terms of reaching optimized conduit connections, though material characteristics were not embedded during this process, thus limiting structural stability aspect. There is still lot of scope for specific research to demonstrate the viability of biomimetic principles be it the termite mounds other plant morphologies in greenhouse design.
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