Agricultural Engineering · Energy, Structures & Environment · also Greenhouse Technology, Precision Farming for Protected Cultivation, Plastics in Agriculture
Designing greenhouses and controlling the climate inside them, with the plastics and soilless systems that go with them.
In the exams: ICAR AICE subject 40 Unit 9 and subject 43 Unit 5; NCEES PE greenhouses; skill modules and electives in the ICAR 6th Deans' Committee B.Tech
Protected cultivation means growing a crop inside a structure or under a covering that changes the environment around it: a glass or plastic greenhouse, a shade-net or insect-proof net house, a walk-in polytunnel, a low tunnel over a row, or a plastic mulch on the soil. The engineer's job is to decide how much of the environment to control and to size the parts that control it: the frame and its foundations, the cladding, the vents and fans, the cooling pads or fog lines, the heaters, the carbon dioxide supply, the irrigation and fertigation system, the soilless growing system, and the controller that ties them together.
The subject matters because a protected crop is an expensive crop. Capital per square metre is many times that of an open field. A greenhouse that overheats at noon, drips condensation on a rose crop, runs out of carbon dioxide on a winter morning, or salts up its substrate loses that margin quickly. Most such failures are failures of design arithmetic.
Worldwide the topic is examined in two ways. Licensing exams such as the NCEES PE Agricultural and Biological Engineering paper and the Philippine ABE board treat greenhouses as a facility to be designed: heat loss, ventilation and evaporative cooling, structural loads. Postgraduate and fellowship entrance exams in India (ICAR AICE-JRF/SRF, Soil and Water Conservation Engineering Unit 9 and Irrigation and Drainage Engineering Unit 5) mix recall of types, materials and soilless systems with short calculations. This manual follows the B.Tech syllabus unit by unit, works the standard calculations in full, and flags where practice differs by climate and country.
Any cover changes four things at once. It cuts the radiation reaching the crop, because no cladding is perfectly transparent and the frame casts shadows. It traps heat, partly because many claddings, glass above all, block the long-wave thermal radiation that the warm crop and soil emit, and mostly because the enclosure stops wind carrying the warm air away. It raises humidity, because transpired water is held in. The same mechanisms that give a winter crop warmth make a summer crop overheat, which is why ventilation and cooling usually dominate design in tropical and subtropical regions while heating dominates at high latitudes.
Protected structures are commonly ranked by how much control they give. A low-cost or naturally ventilated polyhouse has a plastic film cover, roll-up side vents, perhaps a roof vent and an insect net, and nothing powered. A medium-technology house adds fans, pads or fog and simple thermostats. A high-technology house has an integrated climate computer, heating, screens, carbon dioxide enrichment and soilless culture. Shade-net houses, insect-proof net houses, walk-in tunnels and low tunnels sit below all of these.
A lean-to greenhouse uses an existing wall as one side; it is cheap and the wall stores heat but light comes from one side. An even-span house has two roof slopes of equal pitch and width and is the classic free-standing glasshouse. An uneven-span house has slopes of different width, used on hillsides. A ridge-and-furrow, or gutter-connected, greenhouse joins many spans along their eaves at shared gutters, so a large floor area is covered with fewer outside walls and less heat loss per square metre of floor. A sawtooth house is a multi-span house whose roofs are offset so that each span has a vertical vent opening above the next; it is popular in warm climates because the vents work without fans. The Quonset or hoop house uses curved pipe arches covered with film; the gothic arch is a pointed variant that sheds snow and condensation better and gives more headroom at the sides. The Venlo-type glasshouse, typical of north-west Europe, is a multi-span house with narrow roof spans, light glazing bars and alternating hinged roof vents. Frames are usually hot-dip galvanised steel, with aluminium glazing bars for glass.
Orientation is chosen to maximise winter light and to avoid fixed shadows. Classic North American guidance is that at latitudes above about 40 degrees a free-standing single-span house receives more winter light with its ridge running east to west, because the long south-facing roof then intercepts the low winter sun. Gutter-connected multi-span houses are usually oriented with ridges north to south so that the shadows of gutters and trusses sweep across the floor during the day instead of falling on the same rows all day. Nearer the equator the sun is high all year and orientation matters less for light; it matters more for ventilation, and vents are placed to face the prevailing wind.
A good site is level or gently sloping and well drained, open to the sun on the south side in the northern hemisphere and the north side in the southern hemisphere, sheltered from strong wind by a windbreak placed far enough away not to shade the house, and supplied with ample good-quality water and reliable electricity. Water quality is often the deciding factor: sodium, bicarbonate, boron and iron levels limit soilless culture and clog drippers and pads.
A greenhouse is a light structure with a large surface, so wind and snow govern its design rather than self-weight. The design loads are the dead load of frame, cladding and fixed equipment; crop loads from plants hung on wires, which for tall vine crops can be substantial; installation and maintenance loads from people and trolleys; wind pressure and uplift; and snow where it falls. They are combined with load factors set by the governing code. Examples of codes are EN 13031-1 for commercial production greenhouses in Europe and IS 14462 in India; North American practice draws on national building codes and industry design manuals. Many codes allow film houses a shorter design life than permanent buildings, which lowers the design wind speed.
The basic wind quantity is the velocity pressure, q = 0.5 ρ V², where ρ is air density (about 1.2 kg/m³) and V the design wind speed. At 30 m/s this gives 0.5 × 1.2 × 900 = 540 Pa. The code then multiplies q by pressure coefficients for each face; on curved and pitched roofs much of the load is suction, so film houses are more likely to fail by uplift than by crushing. Anchorage therefore matters as much as member size: posts are set in concrete footings or screw anchors, and film is locked into continuous aluminium or steel profiles.
Sunlight reaching the ground covers ultraviolet (below 400 nm), photosynthetically active radiation or PAR (400–700 nm), and near-infrared (700 nm to roughly 2,500 nm). PAR carries a little under half of solar energy. Plants, soil and the cladding itself emit long-wave thermal radiation, mainly between about 3 and 50 µm, with a peak near 10 µm at ordinary temperatures. A cladding is judged by how much PAR it lets in, how much ultraviolet it blocks or passes, and how much long-wave radiation it stops leaving at night.
Plant light is counted in photons, not watts: photosynthetic photon flux density (PPFD) in µmol m⁻² s⁻¹, and the daily light integral (DLI) in mol m⁻² d⁻¹. DLI = mean PPFD × hours of light × 3,600 ÷ 1,000,000. A mean PPFD of 500 µmol m⁻² s⁻¹ for 12 hours gives 21.6 mol m⁻² d⁻¹. A widely quoted rule of thumb from north-European glasshouse work is that each 1% of light lost costs roughly 1% of yield in light-limited crops such as tomato and cucumber; it is a winter, high-latitude rule and overstates the loss in high-light climates, where shading is often added on purpose.
Hemispherical transmission, for light from all directions, is lower than the perpendicular figure and more relevant to a whole house. Diffusing claddings scatter light so that it reaches lower leaves and reduces leaf scorch, with little loss of total transmission. Transmission falls with dirt, with condensation formed as droplets, with the frame's shadow and, for plastics, with age. Two layers in series multiply: two layers each transmitting 0.90 pass about 0.81 of the light.
Horticultural float glass, usually about 4 mm thick, has high PAR transmission when clean, lasts for decades and is almost opaque to long-wave thermal radiation, which reduces night radiative loss. It is heavy, breaks under hail and needs a stronger frame with closely spaced bars.
Twin-wall and multi-wall polycarbonate sheets are light, very impact-resistant and insulate better than single glass because of the trapped air layer, but transmit less light and yellow slowly under ultraviolet unless the surface carries an ultraviolet-protective co-extruded layer. Acrylic (polymethyl methacrylate) twin-wall sheet transmits more light and ages well but is brittle and burns readily. Glass-fibre reinforced polyester (FRP) panels were common in the past; they lose transparency as the surface erodes and fibres are exposed, and they are combustible.
Typical heat-transfer coefficients (U-values) quoted in greenhouse engineering texts are about 6–7 W m⁻² K⁻¹ for single glass or single film and about 3.5–4.5 W m⁻² K⁻¹ for double-inflated film or twin-wall sheet. Real values rise with wind speed and fall with condensation, so they are design figures, not constants.
Most of the world's protected area is covered with polyethylene film: low-density (LDPE), linear low-density (LLDPE) and ethylene-vinyl acetate (EVA) copolymer blends, usually co-extruded in three or more layers. Plain polyethylene degrades quickly in sunlight, so greenhouse film is ultraviolet-stabilised, mainly with hindered amine light stabilisers (HALS); life is commonly one to four years depending on the stabiliser package, climate and pesticide use, since sulphur and chlorine compounds attack HALS. In India the common specification for polyhouse film is 200 µm UV-stabilised film.
Additives tailor the film. Infrared (thermic) additives, or an EVA layer, absorb long-wave radiation and cut night losses; plain polyethylene transmits much of it, so a clear night under a single film can leave the crop colder than the outside air. Anti-drip (anti-condensation) agents make condensation spread as a film that runs down instead of forming light-scattering drops that fall on the crop. Anti-dust agents keep transmission up. Ultraviolet-blocking films cut the wavelengths many insects use for vision and reduce whitefly and thrips activity and sporulation of some fungi, but they can also disturb bumblebee pollination and reduce colour in some flowers and red-leaved crops.
Two layers of film inflated by a small blower to hold an air gap roughly halve heat loss compared with one layer, at the price of lower light.
The choice weighs light transmission, insulation, life, weight and frame cost, fire behaviour, hail risk and price per square metre per year of life, not price per square metre. Glass suits long-lived high-technology houses in cool, cloudy climates where every unit of light counts. Polyethylene film suits low- and medium-cost houses almost everywhere, especially in the tropics and subtropics where light is abundant and replacement every few years is accepted. Rigid plastics suit end walls, hail-prone areas and retail houses. Where insects carry viruses, a UV-blocking film combined with insect-proof side nets is often a stronger case than better light.
A greenhouse heat balance lists every path by which energy enters or leaves the air and crop. Energy enters as solar radiation transmitted by the cover, as heat from heating systems, and from lamps where they are used. It leaves by conduction and convection through the cover, by air exchange (infiltration through gaps, or deliberate ventilation) carrying both sensible heat and the latent heat of water vapour, through the floor and perimeter, and by long-wave radiation to a cold sky. Inside, much of the absorbed solar energy is turned into latent heat by crop transpiration and evaporation from wet surfaces, which is why a well-watered full crop runs much cooler than an empty house.
For design, two steady-state balances are used: a night or winter balance with no sun, to size the heating system, and a midday summer balance, to size ventilation or cooling. Thermal storage makes the real house lag behind these figures; dynamic simulation is a research tool, steady balances the design tool.
The night loss is the sum of a cover term and an air-exchange term:
Q = U Ac (Ti − To) + ρ cp N V (Ti − To) / 3,600
where U is the overall heat-transfer coefficient of the cover (W m⁻² K⁻¹), Ac the total cover area including roof, side and end walls (m²), Ti and To the inside set point and outside design temperature, ρ cp the volumetric heat capacity of air (about 1,200 J m⁻³ K⁻¹), N the air-exchange rate (air changes per hour) and V the greenhouse volume (m³). Some texts add a perimeter term for heat lost through the ground around the edge, expressed per metre of perimeter.
Greenhouse design guides give leakage rates that are well under one air change per hour for new double-film houses and several per hour for old, poorly maintained glasshouses. The outside design temperature is taken from local climate data, usually a temperature exceeded on all but a small percentage of winter hours, not the record low.
Worked example. A 30 m × 10 m house with average height 4 m has volume 1,200 m³ and cover area 520 m². With U = 6.2 W m⁻² K⁻¹, Ti = 18 °C, To = -2 °C and N = 1 h⁻¹: cover loss = 6.2 × 520 × 20 = 64,480 W; air-exchange loss = 1,200 × 1 × 1,200 × 20 / 3,600 = 8,000 W; total ≈ 72.5 kW. The cover term dominates, which is why double layers and screens pay back faster than sealing alone in most film houses. Using floor area in place of cover area is the most common student mistake; for this house it would understate the total loss by more than a third (45.2 kW instead of 72.5 kW).
On a sunny day the greenhouse gains τ Is Af, where τ is the cover's solar transmittance, Is the outside solar irradiance on a horizontal surface and Af the floor area. A fraction E of the absorbed energy goes into evapotranspiration; for a full, well-watered crop it is often taken as about half, for bare or dry floors much less. The remaining sensible heat must leave through the cover and with the ventilation air if the inside is to stay at a chosen temperature rise above outside:
τ Is Af (1 − E) = U Ac (Ti − To) + ρ cp Qv (Ti − To)
so that Qv = [τ Is Af (1 − E) − U Ac ΔT] / (ρ cp ΔT), with Qv in m³/s.
Worked example. τ = 0.7, Is = 800 W m⁻², Af = 1,000 m², E = 0.5, Ac = 1,400 m², U = 6 W m⁻² K⁻¹ and an allowed rise ΔT = 4 K. Sensible solar gain = 0.7 × 800 × 1,000 × 0.5 = 280,000 W. Cover loss = 6 × 1,400 × 4 = 33,600 W. Ventilation must remove 246,400 W, so Qv = 246,400 / (1,200 × 4) = 51.3 m³/s, or 51 L s⁻¹ per m² of floor. The equation shows the three levers a designer has: reduce τ with shade, raise E with a healthy crop and fog, or accept a larger ΔT. Halving the allowed rise roughly doubles the air needed, which is why ventilation alone cannot bring a greenhouse below outside temperature; only evaporative cooling or refrigeration can.
Natural ventilation moves air through openings without fans, driven by two pressure sources. The wind effect comes from pressure differences around the house: positive on the windward face, suction on the leeward face and over most roof openings. The stack or chimney effect comes from buoyancy: warm inside air is lighter than outside air and rises out of high openings, drawing cooler air in through low ones. In most greenhouses the wind effect dominates at wind speeds above roughly 2 m/s, while the stack effect matters in still conditions and in tall houses with both side and roof vents.
Two standard building-ventilation formulas, both in the ASHRAE Fundamentals handbook, serve for design estimates. For wind:
Q = Cv A u
where A is the free area of the inlet openings (m²), u the wind speed (m/s) and Cv the effectiveness of the openings, about 0.5–0.6 for wind perpendicular to the openings and 0.25–0.35 for diagonal wind. For stack:
Q = Cd A √[2 g Δh (Ti − To) / Ti]
where Cd is a discharge coefficient (about 0.65), Δh the height between the centres of the lower and upper openings, and Ti the inside absolute temperature in kelvin. When both act together, the flows are often combined as the square root of the sum of their squares, not simply added.
Worked examples. Roof and side vents with 40 m² free area, perpendicular wind at 2 m/s and Cv = 0.5: Q = 0.5 × 40 × 2 = 40 m³/s. In still air, 20 m² of opening split between side and roof vents 3 m apart, inside 30 °C and outside 25 °C, Cd = 0.65: Q = 0.65 × 20 × √(2 × 9.81 × 3 × 5 / 303.15) = 12.8 m³/s. Entering 30 instead of 303.15 for Ti is the classic slip; it inflates the answer about threefold.
Roof vents release the hottest air and work with wind from any direction; continuous ridge vents and alternate-flap vents are used on glasshouses, and roll-up or hinged roof flaps on polyhouses. Side vents, usually roll-up curtains on film houses, give a strong cross-flow in wind and supply the low inlet for stack flow. Combining side and roof vents gives the best rate in calm weather because it maximises Δh. The usual design index is total vent area as a percentage of floor area; warm-climate guides ask for much larger ratios than cool-climate ones, and the figure must be raised when insect nets are fitted.
The width of the house limits how far side-vent air can reach. The centre of a wide multi-span house depends on roof vents alone and runs warmer. Sawtooth houses solve this by giving every span its own vertical opening.
Where whitefly, thrips or aphids transmit viruses, vents are covered with insect-proof nets. Net mesh is described by the number of threads per inch in each direction, such as 40 or 50 mesh; finer nets exclude smaller insects but have lower porosity, so they create a larger pressure drop and cut airflow, often sharply. A net that halves the airflow through a vent means that the vent area must be roughly doubled, or the house runs hotter. The designer has to balance the exclusion that a fine mesh gives against the higher temperature and humidity it causes, and often adds fans or fog where nets are fine. Double doors with an entry vestibule, kept closed, complete the insect barrier.
Fan ventilation uses exhaust fans in one end or side wall and inlets (often motorised shutters or the cooling pad) in the opposite wall, so that air sweeps the length of the house. The ASAE engineering practice for greenhouse heating, ventilating and cooling (EP406) gives a base summer rate of 0.04 m³ s⁻¹ per m² of floor area, about 8 cfm per square foot, for a house at low elevation under bright light with a temperature rise of about 4 °C from inlet to fan. This base rate is then multiplied by factors for elevation, light intensity and allowed temperature rise. For a house with an average height of 2.4 m it equals one air change per minute, the older rule of thumb. Winter ventilation for humidity control is a small fraction of the summer rate.
Worked example. A 1,800 m² house (30 m × 60 m) needs 72 m³/s at the base rate. Fans rated at 10 m³/s each at the working static pressure give 7.2, so 8 fans are installed; rounding down would leave the house short of air on the hottest day.
Fans are selected from test data at the static pressure they will actually work against, typically a few tens of pascals once pads, shutters and insect nets are included, not from free-air ratings. Fan efficiency is compared as airflow per watt. Air power is Q × Δp, so 72 m³/s against 30 Pa is 2.16 kW of air power, and about 4.3 kW of shaft power at 50% fan efficiency. Fans are spaced evenly, never blow toward a neighbouring house's inlets, and have shutters against backdraughts. Horizontal air-flow (HAF) fans mounted in the canopy are a separate device: they stir air to even out temperature and humidity and to dry leaf surfaces but do not exchange it with outside.
In a fan-pad system the inlet wall carries a wet pad, and the exhaust fans pull outside air through it. Water evaporating from the pad takes its latent heat from the air, so the air leaves the pad cooler and more humid along a line of nearly constant wet-bulb temperature. The pad's saturation (cooling) efficiency is
η = (Tdb,out − Tpad) / (Tdb,out − Twb,out)
where Tdb,out and Twb,out are the outside dry- and wet-bulb temperatures and Tpad the dry-bulb temperature leaving the pad. Corrugated cellulose pads 100–150 mm thick commonly reach 75–85%; aspen-fibre, coir and grass pads used in low-cost houses are less efficient and need more frequent replacement.
Worked example. Outside 38 °C dry-bulb, 24 °C wet-bulb, pad efficiency 80%: leaving air = 38 − 0.8 × (38 − 24) = 26.8 °C. The cooling available depends on the wet-bulb depression, so fan-pad systems work well in hot dry weather and poorly in humid weather.
Pad area is found by dividing airflow by the pad face velocity recommended for the pad type, about 1.25 m/s for 100 mm corrugated cellulose and higher for 150 mm pads. With 72 m³/s at 1.27 m/s the pad needs 56.7 m² of face. Water is pumped to the pad top, distributed evenly, and returned to a sump; a continuous bleed-off removes water to limit the build-up of salts, which otherwise crust and block the pad. Air warms again as it crosses the house, so the far end is hotter than the pad end, which is why pad-to-fan distances are kept modest and crops are arranged with the gradient in mind.
A high-pressure fog system pumps filtered water at pressures of tens of bar through fine nozzles, producing droplets small enough, of the order of 10 µm, to evaporate before they reach the crop. Because cooling happens throughout the house, not only at one wall, fog gives a more uniform temperature than fan-pad and works with natural ventilation as well as fans. It also raises humidity, which reduces the vapour pressure deficit and so reduces crop water stress and transpiration. Water must be clean and often softened or demineralised, because nozzles block and leave white deposits on leaves. Low-pressure misting produces larger drops that wet foliage; it is used in propagation, where wet leaves are wanted, but in a production house it encourages fungal disease. Fog is controlled by humidity as well as temperature, so that the air is not driven to saturation.
Heating keeps night and winter temperatures above the crop's set point and lowers humidity by warming the air. The peak heating capacity is the night heat loss from the heat balance with a margin. The main systems are as follows. Hot-water pipe heating circulates water from a boiler through steel pipes laid low between rows, often as a pipe rail that doubles as a trolley track, with additional growing pipes in the canopy; it gives even, gentle heat near the crop and is standard in large glasshouses. Forced-air unit heaters burn gas or oil and blow warm air, often through perforated polyethylene ducts along the house; they are cheap to install and suit small and film houses. Infrared radiant heaters warm plants and surfaces directly rather than the air. Floor, bench and root-zone heating warm the growing medium, which suits propagation and pot plants. Combined heat and power units generate electricity and supply the waste heat, and their cleaned exhaust supplies carbon dioxide.
Any combustion heater inside a greenhouse must be vented to outside and supplied with combustion air. A leaking flue or a burner starved of air releases carbon monoxide, which endangers workers, and ethylene, which harms crops at very low concentrations, causing flower drop, leaf curling and abortion of buds. Sulphur in fuel produces sulphur dioxide, which burns leaves.
For seasonal energy, the degree-day method multiplies the house's heat-loss coefficient UA (W/K, including the air-exchange term) by the heating degree-days for the location and set point and by 24 hours. Fuel is then the heat divided by the fuel's calorific value and the heater efficiency.
Worked example. UA = 3,000 W/K and 2,000 heating degree-days (°C·d) give 3,000 × 2,000 × 24 = 144,000,000 Wh = 144,000 kWh, or 518.4 GJ. With natural gas of 38 MJ/m³ burnt at 85% efficiency, fuel = 518,400 MJ / (38 × 0.85) = 16,050 m³. Forgetting the 24 hours, or using kJ where MJ is meant, gives answers out by factors of 24 or 1,000, both common exam traps.
The degree-day method ignores solar gain on winter days and the change in air exchange with wind, so it gives a planning figure.
Because the cover term dominates, the most effective savings come from the cover. A second inflated film layer roughly halves conduction loss. Thermal or energy screens of woven or knitted strips, some aluminised, are drawn horizontally across the house at night; they trap a still air layer, cut radiation to the cold roof and can save a large share of night heat when closed tightly at the edges and gutters. Sealing gaps, doors and vents reduces air exchange.
The passive solar greenhouse developed in northern China is a well-known example of design for cold, sunny winters: a thick north wall of earth or masonry stores daytime heat, the south face is a sloping film roof, and an insulating blanket is rolled over the film each night.
Shading reduces the radiation load so that leaf and air temperatures fall and the ventilation or cooling system can cope; it also protects shade-loving crops such as many foliage plants and orchids from scorch. Every unit of shade also removes light the crop could have used, so shade is applied only when it is needed, ideally with a movable system.
The main methods are external shade nets, fixed or retractable; internal retractable screens, often the same screen used for energy saving at night; and whitewash or shading compounds sprayed on the roof, which are cheap, are removed by rain or cleaning agents, and cannot be adjusted through the day. External shade is more effective at reducing heat load because the absorbed energy stays outside the house; an internal screen absorbs solar energy inside and re-emits part of it as heat. Aluminised screens reflect more and heat less than dark ones.
Shade nets are usually knitted from UV-stabilised high-density polyethylene tape or monofilament and are sold by nominal shade percentage, such as 35%, 50% or 75%. The light reaching the crop is the product of the transmissions in series: outside light × cover transmittance × (1 − shade fraction). With 100 klux outside, a cover passing 0.85 and a 50% net, about 42.5 klux reaches the crop. Coloured (photoselective) nets also change the spectrum; their crop effects are still being studied.
A shade-net house has a net roof and walls and no film. It reduces radiation, wind and hail damage and raises humidity a little, while staying close to outside air temperature. It is widely used for nurseries, for leafy vegetables and flowers in hot summers, and for hardening seedlings. An insect-proof net house uses a fine net, such as 40 or 50 mesh, on all sides to exclude virus vectors such as whitefly, with a double door. Because it has no film it does not trap heat as a polyhouse does, which makes it suitable for vegetable seedlings and virus-free planting material in warm climates. Its weak points are torn nets, open doors and the reduced airflow of the fine mesh.
A walk-in polytunnel is a single-span hoop house covered with film, ventilated by opening the ends or rolling up the sides.
Low tunnels are small hoops of wire or bamboo over a row or bed, covered with thin transparent film, used to raise crops such as cucurbits weeks ahead of the open-field season in cold winters. They are ventilated by cutting holes or raising the edges as the days warm, and removed when the outside temperature allows. Floating row covers of lightweight spunbonded non-woven polypropylene lie directly on the crop; they give a few degrees of frost protection and exclude insects while passing light, air and water.
Carbon dioxide is a substrate of photosynthesis, and for C3 crops, which include almost all greenhouse vegetables and flowers, the rate of net photosynthesis rises with CO2 concentration well above today's outdoor level of a little over 400 ppm. In a closed greenhouse on a bright winter morning a dense crop quickly draws CO2 down well below the outdoor level, so photosynthesis is limited exactly when light is good. Enrichment restores and raises the concentration. Set points of roughly 700 to 1,000 ppm are common for tomato, cucumber, pepper and cut flowers when vents are closed; the best value depends on crop, light, temperature and the price of CO2. Reported yield gains vary widely with crop and season, so the economic return must be judged locally.
Enrichment pays when the house is closed. Once vents open wide for cooling, added CO2 escapes almost at once; growers then lower the set point to about the outdoor level, aiming only to prevent depletion. This is one reason heated, well-sealed houses at high latitudes enrich routinely, while naturally ventilated houses in hot climates rarely do.
Pure CO2, delivered as a liquid and vaporised, is clean and easy to control but is the most expensive source per kilogram. Burners using natural gas, propane or low-sulphur kerosene placed in the house produce CO2 and heat together; their fuel must be clean and their combustion complete, because incomplete combustion yields carbon monoxide and ethylene, and sulphur in fuel yields sulphur dioxide. Large houses use cleaned flue gas from the boiler or a combined heat and power unit, storing surplus heat in a hot-water tank so the boiler can run for CO2 by day.
Stoichiometry gives the CO2 yield of fuels. Methane, CH4 + 2O2 → CO2 + 2H2O, gives 44/16 = 2.75 kg CO2 per kg of fuel. Propane, C3H8 + 5O2 → 3CO2 + 4H2O, gives 3 × 44/44 = 3.0 kg per kg.
The CO2 needed is a mass balance: the amount to raise the house to the set point, plus the amount that leaks out with air exchange, plus the amount taken up by the crop. At 20 °C and 1 atm, CO2 has a density of about 1.83 kg/m³, and 1 ppm by volume is 10⁻⁶ m³ per m³ of air.
Worked example. Raising a 1,000 m³ house by 600 ppm needs 600 × 10⁻⁶ × 1,000 = 0.6 m³ of CO2, about 1.1 kg. Holding that 600 ppm excess against an air exchange of 0.5 h⁻¹ then needs 0.5 × 1,000 × 600 × 10⁻⁶ × 1.83 ≈ 0.55 kg/h before crop uptake. Crop uptake on a bright day usually exceeds the leakage term in a well-sealed house.
CO2 is distributed through perforated tubes laid near or within the crop and measured with non-dispersive infrared (NDIR) sensors, sampled at crop height and calibrated regularly. People are at risk at high concentrations: the occupational exposure limit in many countries, including the United States, is 5,000 ppm averaged over an eight-hour day, so set points stay far below it and alarms guard against faults. Burner exhaust must be monitored for carbon monoxide.
Soilless culture grows plants without field soil, with roots in a nutrient solution or in an inert or organic substrate wetted by it.
In the nutrient film technique (NFT), a thin film of solution flows down shallow sloping channels, the roots form a mat in the channel and the upper part of the mat stays in air, so roots get both water and oxygen. It suits lettuce, herbs and other short crops. Its weakness is that there is almost no buffer: if the pump stops, roots dry within hours. In deep-water culture (deep-flow or floating-raft systems), plants sit in rafts floating on a deep tank of aerated solution, which buffers temperature and pump failures; it is widely used for leafy greens. Aeroponics suspends the roots in a closed chamber and sprays them with fine mist at short intervals; root aeration is excellent and water use is low, but a failure of pump, nozzles or timer kills roots very quickly, so it needs back-up power and alarms. Substrate culture places plants in slabs, bags or pots of rockwool, coconut coir, perlite, expanded clay, sand or gravel, fed by drippers; it is the standard for tall vine crops such as tomato, cucumber and pepper, and for roses and gerbera.
Systems are open (run-to-waste), where the drain is discarded, or closed (recirculating), where the drain is collected, disinfected, topped up and reused. Closed systems save water and fertiliser and avoid polluting groundwater, which is why several countries require them, but they can spread root diseases such as Pythium through the whole house unless the drain is disinfected by ultraviolet light, heat, ozone or slow sand filtration.
A complete nutrient solution supplies all mineral nutrients: nitrogen, phosphorus, potassium, calcium, magnesium and sulphur as macronutrients, and iron, manganese, zinc, copper, boron, molybdenum and the other micronutrients in small amounts. The Hoagland solution of the 1930s is the classic laboratory recipe; commercial recipes are tuned for each crop and growth stage.
Two measurements run the system. Electrical conductivity (EC), in dS/m or mS/cm, which are the same, tracks the total salt concentration; most crops are grown in roughly the 1.5 to 3.0 dS/m range, lower for lettuce and higher for tomato. pH is held roughly between 5.5 and 6.5 so that phosphorus, iron and manganese stay available. Meters that report ppm or TDS convert EC with a factor, commonly 500 or 700 ppm per dS/m depending on the instrument, so readings from different meters cannot be compared directly.
Concentrated stock solutions are kept in two tanks, usually called A and B, because calcium salts precipitate with sulphates and phosphates at stock strength: calcium nitrate goes in one tank, sulphates and phosphates in the other, and each is diluted by injectors into the irrigation water. Iron is supplied as a chelate; Fe-EDTA loses effectiveness as pH rises above about 6.5, so DTPA or EDDHA chelates are used in alkaline conditions.
Worked example. To supply 200 mg/L nitrogen from calcium nitrate containing 15.5% N, the final solution needs 200 / 0.155 = 1,290 mg/L of the salt. With a 1:100 injector the stock must be 100 times stronger, 129 g/L. In a closed system, nutrients the crop takes up less than it receives (such as sodium and sulphate) build up, so EC alone stops telling the truth; periodic laboratory analysis and partial dumping of the solution are part of the routine.
Roots respire, so dissolved oxygen matters; warm solution holds less oxygen and encourages Pythium, which is why solution temperature is kept moderate and deep tanks are aerated.
With no rain and with less radiation and wind than outside, a greenhouse crop transpires less than the same crop in the open, and all its water comes from the irrigation system. Daily demand is estimated from a crop coefficient applied to a reference evapotranspiration calculated or measured inside the house or from accumulated solar radiation. Substrate growers also weigh a slab on a scale or measure the drain volume, and adjust so that a planned share of the water supplied drains, often 20 to 30%, which flushes salts.
Drip irrigation is the standard: laterals along each bed or slab with pressure-compensating drippers or drip tape, giving uniform delivery along long rows and on slopes. Pot plants use ebb-and-flow benches, capillary mats or booms; misters serve propagation.
Worked example. A 1,000 m² house with crop water use of 4 mm/d and an application efficiency of 90% needs 1,000 × 0.004 / 0.9 = 4.44 m³/d. With 2,500 plants each on one 2 L/h dripper, system flow is 5 m³/h and the daily run time is 4.44 / 5 × 60 ≈ 53 minutes, split into several pulses through the day so that the root zone never dries out or waterlogs.
Fertigation injects dissolved fertiliser into the irrigation water. Three injector types are common. A fertiliser (bypass) tank is a pressure vessel through which part of the flow is diverted; its concentration falls exponentially as water passes through, so delivery is not uniform, and about four tank volumes of water flush out most of the fertiliser. A venturi injector uses the pressure drop across a constriction to suck stock solution in; it has no moving parts but needs a significant pressure difference across it, which costs head. Dosing pumps, either water-powered proportional injectors or electric pumps, deliver a set ratio such as 1:100 regardless of flow; they are the standard in greenhouses, and a 1:100 injector on a 5 m³/h line draws 50 L/h of stock.
The injection point is placed upstream of the main filter so that any precipitate is caught before the drippers. A check valve and vacuum breaker, or a reduced-pressure backflow preventer where the law requires one, must stop fertiliser flowing back into the water source. Fertilisers are tested for compatibility in a jar before mixing; calcium with sulphate or phosphate is the usual problem. Acids, usually nitric or phosphoric, are injected to neutralise bicarbonate in the water, keep pH in range and dissolve carbonate deposits in drippers.
The main greenhouse crops worldwide are tomato, cucumber, sweet and coloured pepper, lettuce and other leafy greens, strawberry, melons, and cut flowers such as rose, gerbera, carnation and chrysanthemum, plus pot plants and nursery seedlings. Tomatoes and cucumbers are grown as single stems trained up strings to an overhead wire and lowered and leaned along the row as they grow, which keeps the head in the light for many months. Greenhouse cucumbers are usually parthenocarpic, setting fruit without pollination, so bees must be kept out to prevent seedy fruit in some types; tomatoes need help to pollinate, either from bumblebees or by vibrating the trusses.
Soil-grown houses suffer a build-up of soil-borne disease and salts; growers rotate, use grafted plants on resistant rootstocks, solarise or steam the soil, or move to substrates. Methyl bromide, formerly the standard soil fumigant, has been phased out under the Montreal Protocol because it depletes stratospheric ozone. Integrated pest management combines exclusion with nets and double doors, monitoring with sticky traps, and biological control, such as the parasitic wasp Encarsia formosa against greenhouse whitefly and the predatory mite Phytoseiulus persimilis against two-spotted spider mite, with selective pesticides as a last resort.
Control is only as good as measurement. Air temperature and humidity are measured in an aspirated, radiation-shielded box at crop height near the centre of the house, because a sensor in direct sun or still air reads several degrees high. Temperature sensors are thermistors, platinum resistance thermometers (such as Pt100) or thermocouples; humidity sensors are usually capacitive polymer types, which drift and need regular checking against a reference. PAR is measured with a quantum sensor in µmol m⁻² s⁻¹ and total solar radiation with a pyranometer in W/m². CO2 is measured with non-dispersive infrared sensors. A weather station on the roof reports outside temperature, humidity, radiation, wind speed and direction and rain, which the controller uses to close vents in rain or high wind and to anticipate heating demand. The root zone is monitored with EC and pH probes in the feed and drain, water-content sensors in substrate, and scales.
Humidity is increasingly controlled through the vapour pressure deficit (VPD), the difference between the saturation vapour pressure at air temperature and the actual vapour pressure. Using the FAO-56 form, es(T) = 0.6108 exp[17.27 T / (T + 237.3)] kPa; at 25 °C es is 3.17 kPa, so at 70% relative humidity VPD = 3.17 × (1 − 0.70) = 0.95 kPa. Very low VPD invites fungal disease and calcium deficiency in fruit and leaves because transpiration stalls; very high VPD causes stomatal closure and stress.
The simplest controller is a thermostat with on/off action and a differential (hysteresis), so that a fan switching on at 28 °C does not switch off until, say, 26 °C, which prevents rapid cycling. Staged control adds equipment in steps as temperature rises: first vents open, then fans, then more fans, then the pad pump or fog; heating stages work the same way on the cold side. A dead band between the last heating stage and the first cooling stage prevents heating and cooling at the same time, which wastes energy. Proportional, proportional-integral and PID controllers position vents, valves and screens continuously according to the error from set point, giving smoother climate than steps.
Integrated climate computers combine all of these with schedules and logic. Set points change with time of day and with radiation, for example raising the day temperature on bright days. The difference between day and night temperature (DIF) affects stem elongation in many crops, and a cool period just after dawn is used to restrain height without chemical growth regulators. Temperature integration lets the temperature wander within limits so long as the 24-hour average is met, which saves heating energy. Interlocks close vents in rain or high wind and stop CO2 dosing when vents open wide.
A fan-ventilated house with its fans stopped on a sunny day overheats within minutes, and a heated house that loses its boiler on a frosty night can lose the crop in hours. Automation must therefore fail safe: vents that open when power fails in summer, standby generators sized for fans and pumps, battery-backed controllers, alarms for high and low temperature, high CO2 and power loss sent to a telephone, and manual overrides for every motor.
Plasticulture is the use of plastics in agriculture. The main polymers are low-density and linear low-density polyethylene (LDPE, LLDPE) for films, high-density polyethylene (HDPE) for nets, pipes and geomembranes, polyvinyl chloride (PVC) for pipes and some liners, polypropylene for non-wovens and some nets, and ethylene-vinyl acetate for thermal films.
Plastic mulch is a film laid over the soil surface, usually over a raised bed with a drip lateral beneath it. It conserves moisture by stopping evaporation, controls weeds, warms or cools the soil, keeps fruit clean and reduces leaching of fertiliser. Colour decides the effect. Black film blocks light, so weeds cannot grow, and warms the soil moderately. Clear film warms the soil most but lets weeds grow underneath; it is the film used for soil solarisation, in which moist soil is covered in the hottest weeks to kill pathogens, weed seeds and nematodes. White-on-black and silver films reflect light and keep the soil cooler, which suits hot seasons, and silver films repel aphids and some other insect vectors. Thinner film is used for short crops and thicker film for long-duration and perennial crops.
Mulch is laid by hand or by a tractor-drawn mulch layer that shapes the bed, lays the drip line and buries the film edges. The mass of film follows from area × thickness × density: covering 7,000 m² with 25 µm polyethylene of density 920 kg/m³ needs 7,000 × 25 × 10⁻⁶ × 920 = 161 kg. Used film is contaminated with soil and must be collected and recycled; fragments left in the field accumulate, and burning releases harmful smoke and is illegal in many countries. Biodegradable mulch films, usually based on blends of biodegradable polyesters and starch, are designed to be tilled into the soil after the crop; in Europe EN 17033 sets requirements for their biodegradation in soil.
Plastic liners stop seepage from farm ponds, water-harvesting tanks, reservoirs and canals, where losses through permeable soils can be most of the water stored. LDPE film is cheap but degrades in sunlight and is easily punctured, so it is laid under a protective cover of soil or masonry. HDPE geomembrane, typically a fraction of a millimetre to a few millimetres thick and carbon-black stabilised, resists ultraviolet light and can be left exposed; its panels are joined by thermal welding and tested for leaks.
Good practice: shape the excavation with stable side slopes, compact the subgrade and remove stones, roots and sharp objects, treat with a soil sterilant where weeds can push through, lay a geotextile cushion on rough ground, anchor the liner in a trench around the top, and provide an inlet silt trap and an outlet that do not tear the liner.
Liner area for a square pond: top 20 m × 20 m, depth 3 m and side slopes 1.5:1 (horizontal:vertical) give a bottom of 20 − 2 × 1.5 × 3 = 11 m. Each sloping face has a slant length of 3 × √(1 + 1.5²) = 5.41 m and an area of (20 + 11)/2 × 5.41 = 83.8 m². Total = 121 + 4 × 83.8 = 456 m², before adding the anchor trench, overlaps and wastage.
A greenhouse is judged as an investment. Fixed costs include the structure, cladding, climate equipment, irrigation and fertigation, soilless system, land development, water source and electrical connection, each with its own life: film a few years, pads a few years, structure one or two decades. Operating costs include planting material, fertiliser, crop protection, energy, labour, cladding replacement, maintenance, packing and marketing.
To compare a capital cost with annual returns, it is converted to an equivalent annual cost with the capital recovery factor, CRF = i(1 + i)ⁿ / [(1 + i)ⁿ − 1], for interest rate i and life n years. For example, a capital cost of 1,200,000 currency units over 10 years at 10% has CRF = 0.1627 and an equivalent annual cost of 195,294. Simple payback is capital divided by annual net return; it ignores timing and life beyond payback. Net present value, benefit-cost ratio and internal rate of return use discounted cash flows and are the standard project appraisal measures for banks and subsidy schemes. Break-even yield is the annual cost divided by the expected price per unit of produce.
Sensitivity analysis matters more than any single figure: greenhouse returns swing with yield, price, market access and the cost of energy and labour. Subsidies change the private payback but not the need for a reliable market.
A B.Tech student should be able to sketch the main greenhouse types and say where each suits; compare glass, polyethylene and polycarbonate on light, insulation and life; write and solve the night heat-loss and daytime ventilation balances; size exhaust fans and a cooling pad from a floor area; compute leaving air temperature from pad efficiency; work out CO2 needed to enrich a house, a stock-solution strength for a 1:100 injector, a drip run time, a mulch film mass and a pond-liner area; and explain why an NFT pump failure is more dangerous than a deep-water one. These are the calculations licensing and PG entrance papers set.
An engineer five years into practice designs and commissions houses for growers and government schemes. Typical tasks: choosing a structure and cladding for a site and budget, checking a supplier's wind and anchorage design against the code, sizing fans, pads, fog lines and heaters, specifying fertigation units with backflow protection and filtration, and setting up controller stages and alarms. The common field problems are under-ventilated net houses, pads crusted with salt because bleed-off was omitted, drippers clogged by bicarbonate, and controllers with sensors in direct sun. Writing commissioning checklists and training growers is half the job.
With 25 years of experience the work shifts to project and programme level: setting standards for a state or company, approving designs for subsidy, diagnosing why clusters of greenhouses fail, and advising on energy, water and market strategy. Decisions include whether a region should build naturally ventilated houses or fan-pad houses, how to manage film recycling, how to keep growers on closed hydroponic systems profitable, and how to structure financing so that cladding replacement and maintenance are funded. Senior engineers also review collapse and crop-loss cases.
A researcher with 25 years in the field works on dynamic greenhouse climate models, computational fluid dynamics of ventilation with insect nets, semi-closed greenhouses with heat storage, model-predictive and optimal control linking climate to crop growth models, light-selective and energy-generating claddings, low-energy cooling for hot humid climates, recirculating soilless systems with disease control, and life-cycle assessment of plastics. Such a researcher designs field trials with proper replication, publishes validated models, supervises students and advises standard-setting bodies on greenhouse design codes and plastic waste policy.
This manual gives design-level equations and widely quoted coefficients, not the full tables of any code or standard: structural design must follow the governing national code (EN 13031-1, IS 14462 or local building codes) and the current ASABE EP406 for ventilation factors. It does not give cladding prices, film lifetimes for specific products, crop-by-crop climate set points or nutrient recipes, because these vary with supplier, climate and cultivar. Artificial lighting (supplementary LED and HPS lighting, vertical farms) is mentioned only in passing. Psychrometric chart use is assumed from the heat and mass transfer course. Subsidy rates and scheme rules are left out because they change often and differ by country and state.
Original practice questions written to the syllabus and pattern of the exams named in each question's examTags. They are not past papers and do not reproduce any board's questions.
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