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| Date post: | 05-Apr-2017 |
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Engineering |
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Greenhouse technology is one way of the using solar energy. Ggreenhouses are the enclosures where crops, vegetables or flowers are provided proper environment under adverse climatic conditions for plant growth and production.So for growing off season crops, it is very much useful.
Figure 2. A greenhouse for flowers.
Passive Greenhouse
• The greenhouse in which there are no any moving components or active elements ie.
heating or cooling occurs naturally is called passive greenhouse.
Active Greenhouse
The greenhouses in which auxiliary energy is used for heating or cooling is called as active greenhouse.
Active systems employ pumps and other devices.
Greenhouse Drying
Post harvest life fruits and vegetables is low due to perishable nature.
Fruits and vegetables being high moisture foods are spoiled due to microbes, enzymes and biochemical reaction.
Low temperature storage could extend post harvest life but maintenance of cold chain is a must.
Hence fruits and vegetables can be dried to prolong shelf line and dried products can be kept under ambient conditions.
Moisture Content
Food Sample =
Mass of product = Mass of water in food + Mass of dry solids
+ Food Liquid Food Solids
Mass of dry solid
Food Sample
Mass of water in food
• A food is initially at a moisture content of 90% dry basis. Calculate the moisture content in wet basis.
Ans.: M: 567%
Water activity is an indicator of the availability of free water.Water activity is an important property for food safety. It predicts food safety and stability with respect to microbial growth, chemical/biochemical reaction rates, and physical properties.
• The ERH of a food product is defined as that relative humidity of the air surrounding the food at which the product neither gains nor loses its natural moisture; that is, it is in equilibrium with its environment.
• At a relative humidity above the ERH, the product will gain moisture and at a humidity below that level, it will lose moisture.
• A food with a aw of 0.6 will lose moisture at a relative a relative humidity below 60% and gain moisture above 60%.
Free water and Bound water Free water present in food acts as a
solvent. Water which is bound by such minute
forces that its vapour pressure is equal to the vapour pressure of pure water.
It can be found as free water, in cavities and wide capillaries. This can often be thought of as the second and subsequent layers of moisture attached to a surface.
A portion of the total water content present in a product is strongly bound to specific sites on the chemicals within the product .
It exerts a vapour pressure less than that of pure water.
It can often be thought of as the first layer of water molecules attached to a surface.
Total moisture content=bound moisture content + free moisture content
Bound MoistureFree Moisture
Drying is traditional method for preserving the food. It also helps in easy transport since the dried food becomes lighter because of moisture evaporation.
Drying prevents growth of fungi and bacteria.
The traditional practice of drying agricultural produce in the developing countries is sun drying, which is seasonal, intermittent, slow, and unhygienic.
• To overcome the problems of sun drying, mechanical drying is introduced with the following advantages:
• (i) fast drying • (ii) large volumes of produce can be handled
(iii) drying parameters can be controlled and quality of the produce can be maintained.
The energy demand of mechanical dryers is met by electricity, fossil fuels, and firewood are becoming scarce.
Solar energy can be an alternative source for drying of food and solar dryers are employed for the purpose.
The use of the greenhouse as a dryer is the latest development.
The drying capabilities of the greenhouse can be utilized for curing tobacco leaves, while guarding the harvest from rain damage.
In an efficiently managed greenhouse, there will not be any time gap between crops.
if crops are not grown in a particular period, the greenhouse can be utilized as a greenhouse dryer.
Importance of Drying
It can be processed during peak season and can be utilized during off season.
Due to low water activity, shelf life of dried product is more and it can be stored for longer duration at room temperature.
Total volume of produce is reduced and it will help in storage and transportation.
Added advantage is the large savings in packaging, storage space requirements and transportation costs.
In general, the produce is spread as thin layers in trays covering the greenhouse area.
The trays can be fabricated with sheet metal and wire mesh.
Trays should be arranged horizontally on existing growing benches or frames.
For better operation, proper ventilation should be provided by either forced or natural ventilation, to remove the moisture liberating from the produce and to control the air temperature inside the greenhouse.
The natural ventilation can be enhanced by using a black LDPE chimney connected to the greenhouse.
A dehydrated products remains stable only when it is protected from the exposure to air, water, sunlight, etc. hence appropriate packaging of a dried product is an important consideration.
Greenhouse HeatingTemperature is one of the most important
factors in the production of horticultural crops. Solar energy on sunny days is often enough to
keep a greenhouse warm, even in cold weather.
During the night time, air temperature inside greenhouse decreases.
The heat is always lost from the greenhouse when the surroundings are relatively cooler.
The requirements for heating greenhouse depend on the rate at which the heat is lost to the outside environment.
Heat losses can occur in three different modes of heat transfer, namely conduction, convection, and radiation.
Heat Loss
Various methods are adopted to reduce the heat losses, viz., using double layer polyethylene, thermo pane glasses.
Several different methods are used to heat greenhouses.
Heat must be supplied to a greenhouse at the same rate with which it is lost in order to maintain a desired temperature.
For the purpose of greenhouse heating, apart from conventional systems, solar energy can also be used and the heat can be stored using water and rock storage.
Most heat is lost by covering material by conduction.
Different materials, such as aluminum bars, glass, polyethylene, and cement partition walls, vary in conduction according to the rate at which each conducts heat from the warm interior to the colder exterior.
A good conductor of heat looses more heat in a shorter time than a bad conductor and vice versa.
There are only limited ways of insulating the covering material without blocking the light transmission.
A dead air space between two coverings appears to be the best system.
A saving of 40% of the heat requirement can be achieved when a second covering in applied.
For example greenhouse covered with one layer of polyethylene loses, 6.8 W of heat through each square meter of covering every hour when the outside temperature is 1oC lower than the inside.
When second layer of polyethylene is added, there is 40% reduction in heat loss.
A second mode of heat loss is that of convection (air infiltration).
Spaces between panes of glass and ventilators and doors permit the passage of warm air outward and cold air inward.
About 10% of total heat loss from a structurally tight glass greenhouse occurs through infiltration loss.
A third mode of heat loss from a greenhouse is that of radiation.
Heating Systems • The heating system must provide heat to the
greenhouse at the same rate at which it is lost by conduction, infiltration, and radiation.
• There are three popular types of heating systems for greenhouses.
• The most common and least expensive is the unit heater system.
• In this system, warm air is blown from unit heaters that have self contained fireboxes.
• These heaters consist of three functional parts.
Unit heaters
• In a unit heater the fuel is combusted in the chamber at bottom.
• Hot fumes rise inside the heat exchanger tubes, giving heat to the walls of the tubes.
• Smoke exists at the top. • A fan forces cool air of the greenhouse over
the outside of heat exchange tubes, where it picks up heat and warm air is circulated.
Heat Distribution Systems In the convection tube method, warm air from
unit heaters are distributed through a polyethylene tube running through the length of the greenhouse.
Heat escapes from the tube through holes on either side of the tube in small jet streams, which rapidly mix with the surrounding air and set up a circulation pattern to minimize temperature gradients.
Boiler
This system is used for very big greenhouses and is a centralized system of heating.
The fuel for boiler can be coal or fuel oil. The heating of the greenhouse is generally
done through hot water at 85°C or steam at 102°C.
Water or steam pipes are installed above the beds of crop and along the side wall.
The steam system is cheaper than hot water system.
To reduce the length of pipe to be used a number of hot water or steam pipe coils can be used and green house air circulated over them by blower for heating.
A central heating system can be more efficient than unit heaters, especially in large greenhouse ranges.
Infra-red HeatersThe fuel gas (LPG) is burnt and the fumes at a
temperature of about 480°C are passed in 10 cm diameter pipes kept overhead at a height of 1.5m above plants.
Reflectors are provided over the full length of pipe to radiate the infra red rays over the plants.
The plants and soil get heated.
The fourth possible type of system is the solar heating system.
Solar heating systems are found in small commercial firms.
Both water and rock energy storage systems are used in combination with solar energy.
Solar heating system Solar heating is often used as a partial or total
alternative to fossil fuel heating systems. Few solar heating systems exist in greenhouses
today. The general components of solar heating
system are collector, heat storage facility, exchange to transfer the solar derived heat to the greenhouse air, backup heater to take over when solar heating does not suffice and set of controls.
Normally flat plate collector are used.
This consists of a flat black plate (rigid plastic, film plastic, sheet metal, or board) for absorbing solar energy.
The plate is covered on the sun side by two or more transparent glass or plastic layers and on the backside by insulation.
The enclosing layers serve to hold the collected heat within the collector.
Water or air is passed through the copper tubes placed over the black plate and absorb the entrapped heat and carry it to the storage facility.
Based on the locations, the heat derived can provide 20 to 50% of the heat requirement.
Water and rock storage Water and rocks are the two most
common materials for the storage of heat in the greenhouse.
To store equivalent amounts of heat, a rock bed would have to be three times as large as a water tank.
A water storage system is well adapted to a water collector and a greenhouse heating system which consists of a pipe coil which contains a water coil.
Heated water from the collector is pumped to the storage tank during the day.
As and when heat is required, warm water is pumped form the storage tank to a hot water or steam boiler or into the hot water coil.
Flat plate solar heaters are used to heat the water during day time.
The hot water is stored in the insulated tanks.
The hot water is circulated in pipes provided along the length of the greenhouse during night.
Supplementary or emergency heating systems are provided for heating the greenhouse during cloudy or rainy days.
A rock storage bed can be used with an air-collector and forced air heating system.
In this case, heated air form the collector,
along with air excessively heated inside the greenhouse during the day, is forced through a bed of rocks.
The rocks absorb much of the heat.
The rock bed may be located outside the greenhouse, and it should be well insulated against heat loss.
During the night, when heat is required in the greenhouse, cool air from inside the greenhouse is forced through the rocks, where it is warmed and the passed back into the greenhouse.
A polyethylene tube with holes along either side serves well to distribute the warm air uniformly along the length of the greenhouse.
The water or rock storage unit occupies a large amount of space and a considerable amount of insulation is provided.
Hydroponics
What is Hydroponics?
Plants grow in water.Soil less growing!
Soil Less Growing?What is used as a growing
media? Gravel -Rockwool- Sand -Styrofoam Vermiculite - Anything Inert!
The term hydroponics was first used in the 1930s by a California researcher named W. F. Gerike.
It is a combination of two Greek words—hydro means “water” and ponics means “labor.”
Together they mean “water labor.”
Simply defined, hydroponics is growing plants with their roots in a medium other than soil.
Sometimes, hydroponics is referred to as soilless culture because soil is not used.
In recent years, there has been widespread expansion in hydroponic systems due to a better understanding of plant growth, nutrient needs, and technological requirements.
Advantages
Faster Growth- Hydroponics works by automatically getting the complete nutrient mixture and water to the roots without drowning the plant.
Plants get everything they need all the time, so they do not grow a lot of roots searching for nutrients.
AdvantagesNo Weeds or Pests- Gardening
without soil eliminates the weeds do you do not need weed sprays. Because hydroponics does not use soil, harmful insects that live in soils cannot damage hydroponic crops.
The amount of nutrients needed by plants can be adjusted as they grow. – As plants mature, the type and amount
of nutrients can be easily adjusted in a hydroponic system.
Hydroponic systems allow the pH levels available to plants to be adjusted quickly. – Adjusting the pH of the nutrient
solution helps in nutrient uptake.
Hydroponics allows for high-quality yields
Disadvantages Cost of initial investment on hydroponic systems is
high. Hydroponic production is capital and labor intensive. A high level of expertise is required. Daily attention is necessary. Specially formulated, soluble nutrients must always
be used. Some diseases can spread rapidly throughout a
hydroponic system. Some water born diseases can spread rapidly in
recirculation system.
Plant NeedsWhat is needed for a plant to survive?• Water• Sunlight• Air• Nutrients (usually soil)• Anchorage (root system)
Requirements For Plants To Grow• Hydroponically grown plants have the same basic
requirements as plants grown in soil. • All hydroponic systems must supply support, water,
nutrients, and air. • The major differences between hydroponic systems are the
way in which plants receive support and the method in which nutrients are made available.
1. Temperature—Since most hydroponic systems are in greenhouses or confined areas, specific temperatures can be set. – Each type of plant has an optimal
temperature range for maximum growth.
2. Light—All vegetables and most flowering plants need large amounts of light. – Hydroponically grown vegetables require 8 to 10 hours of
direct sunlight daily for healthy growth. – Commercial operations sometimes use high-powered
lamps to increase light intensity and duration.
3. Water—Providing plants with enough water is not a problem with water culture systems. – However, water quality can be an issue. – The pH of water should be tested and, if necessary,
adjusted for the particular crop being grown. – Softened water may contain harmful amounts of sodium
and should be avoided.
4. Oxygen—The most critical factor is supplying the root system with enough oxygen for healthy root growth. – Plants and plant root systems require oxygen for
respiration.
5. Nutrients—Hydroponically grown plants have the same nutrient requirements as those grown in soil. – Since hydroponic systems do not use soil,
essential nutrients must be provided with a water solution.
– The solution requires careful calculations to ensure that the optimal amounts of macronutrients and micronutrients are provided.
6. Support—Soil provides a firm anchor for plants to grow upright. – In hydroponic systems, artificial support can
be provided. – This can be accomplished through string and
mesh materials.
The term hydroponics is used to describe many different types of systems.
Generally, all systems can be classified as either aggregate culture or water culture.
• Aggregate culture involves the use of aggregate or substrate materials that help support plants.
• Such materials allow the plants to take root.1. Common substrates include sand, perlite, vermiculite, gravel, peat moss, and rock wool. – Rock wool is a spongy, fibrous material spun from
molten volcanic rock. – All these materials are considered inert. – They do not provide nutrients to the plants.
Soil Less Growing?What is used as a growing
media? Gravel -Rockwool- Sand -Styrofoam Vermiculite - Anything Inert!
Rock Wool
Perlite and Vermiculite
2. Solutions provide the plants with essential nutrients. – Common methods of supplying a solution
are through drip and trickle. – One method involves flooding the aggregate
for 10 minutes. – The aggregate is allowed to drain for 30
minutes and then flooded again.
• Water culture is also referred to as nutriculture. – In this type of system, no substrate is used. – Although plants may be started in rock wool,
most of the roots are growing in a nutrient solution.
– A system of this type has a continuous flow or mist of nutrient solution that is recycled.
– Such a system is referred to as a circulating system.
DRIP SYSTEMS Hydroponic Drip systems are the most widely used type of
hydroponic system in the world. Operation is simple, a timer controls a
submersed pump. The timer turns the pump on and nutrient
solution is dripped onto the base of each plant by a small drip line.
In a Recovery Drip System the excess nutrient solution that runs off is collected back in the reservoir for re-use.
The Non-Recovery System does not collect the run off.
The non-recovery system requires less maintenance due to the fact that the excess nutrient solution isn't recycled back into the reservoir, so the nutrient strength and pH of the reservoir will not vary.
This means that you can fill the reservoir with pH adjusted nutrient solution and then forget it until you need to mix more.
A recovery system can have large shifts in the pH and nutrient strength levels that require periodic checking and adjusting.
1. The water culture system most commonly used in commercial operations is called nutrient film technique (NFT). – In an NFT system, a continuous flow of nutrient
solution runs through a series of tubes or troughs. – A pump raises the nutrient solution to desired levels,
and gravity allows it to drain. – The system is constantly recycling the nutrient solution.– NFT is typically not used to produce root vegetables or
tuber crops.
The concept of Nutrient Film Technique (NFT) was originally developed by Dr Allen Cooper in the UK. Dr Cooper described the concept as follows:A very shallow stream of water containing all the dissolved nutrients required for growth is recirculated past the bare roots of crop plants in a watertight gully.
Ideally, the depth of the recirculating stream should be very shallow, little more than a film of water – hence the name nutrient film.
Nutrient Film Technique (NFT)It is a kind of hydroponic system.The NFT systems provide a constant film of
water and nutrients along the bottom of a channel.
In effect, part of the roots grow down in the water/ nutrients and parts of the roots above the water line getting fresh air and oxygen.
The nutrient solution is pumped into the growing tray (usually a tube) and flows over the roots of the plants, and then drains back into the reservoir.
2. Aeroponics is another type of water culture system. – In such a system, plant roots are suspended
in the air within a closed container. – Inside the container, spray nozzles mist the
roots.
• The aeroponic system is the most high-tech type of hydroponic gardening.
• The growing medium is primarily air. • The roots hang in the air and are misted with
nutrient solution. • The mistings are usually done every few
minutes. • A timer controls the nutrient pump much like
other types of hydroponic systems, except the aeroponic system needs a short cycle timer that runs the pump for a few seconds every couple of minutes.
