COOLING AND HEATING OF GREENHOUSE

1. BIRSA AGRICULTURAL UNIVERSITY
Protected Cultivation and Secondary
Agriculture
LECTURE 8: COOLING AND HEATING OF GREENHOUSE
BY
DR. PRAMOD RAI
DEPARTMENT OF AGRICULTURAL ENGINEERING

2. Why Greenhouse cooling is needed
Solar radiation is the “heat input” for the earth.
Up to 85% of this radiation may enter the greenhouse (most of the IR heat
becomes trapped inside and greatly increases the greenhouse temperature).
An effect caused by the existence of a cover characterized by its low
transparency to far infrared radiation (emitted by the crop, the soil and the
inner greenhouse elements), but its high transparency to sunlight.
A confinement effect, resulting from the decrease in the air exchanges with
the outside environment
Mechanism is needed to remove this trapped heat.
Fig. 1: Working of GH

3. GH cooling
GH Cooling Systems
1. Ventilation 2. Evaporative Cooling 3. Heat Prevention 4. Composite System
1.1. Natural
Ventilation
1.2. Forced
Ventilation
2.1. Fan-Pad System
2.2. Fog/Mist System
2.3. Root
Evaporative Cooling
3.1. Shading
3.2. Radiation Filters
3.2.1. NIR-
Reflecting Film
Covers
3.2.2. Fluid
Roof Covers
4.1. Earth-to-Air Heat
Exchanger System
(EAHES)
4.2. Aquifer Coupled
Cavity Flow Heat
Exchanger System
(ACCFHES)

4. 1. Ventilation
High temperature inside GH require constant heat
removal from the GH.
This may be accomplished by replacing the existing air
in the GH with cooler air from outside the structure.
The upward and outward movement of warm air pulls
in cool air from side or end vents. This system is most
effective in the winter, spring and fall.
It is limited in its effectiveness for summer cooling
since the incoming solar load and the outside air
temperature may be too high for the capabilities of this
system during summer months.

5. 1.1 Natural Ventilation
If the outside temperature is low enough, and if the
temperature inside the GH is not too high, warm air is
exhausted passively through the greenhouse vents
(natural ventilation).
The effectiveness of this system depends on the
temperature difference between inside and outside the
GH (bouncy effect) and on the wind speed outside the
structure (wind effect).
The external cool air enters the GH through the lower
side openings while the hot internal air exits through the
roof openings due to density difference, lowering of
temperature in the GH.

6. Fig. 2: Different types of ventilation opening

7. 1.2 Forced ventilation
At low wind speed, exhaust fans are needed to induce air circulation
through the vents (forced ventilation).
Systems like exhaust fan, blower, etc. can supply high air exchange
rates whenever needed.
These are simple and robust systems and significantly increase the air
transfer rate from the GH and allow maintaining inside temperature to
a level slightly higher than the outside temperature by increasing the
number of air changes.
Fig. 3: Fans for GH forced ventilation

8. 2. Evaporative cooling (EC)
It is based on the conversion of sensible heat into latent heat of
evaporated water.
As water evaporates it takes away heat from the air thus reducing its
temperature.
During the process, the total heat (enthalpy) of the air remains the
same.
Fig. 4: Psychrometric Chart

9. 2.1 Fan-pad system
This system consists of a fan on one sidewall and pad on
the other sidewall of the GH.
The principle of evaporative cooling is applied by
running a water stream over the pad and consequent
withdrawal of air through it by fans on the opposite side.
The air becomes cooler and its humidity is also raised.
More effective when outside air humidity is low.
Fig. 5: Pad (left) and fan (right) greenhouse cooling system

10. Evaporative cooling
Generally the system performance of EC
process can be based on the saturation efficiency
Where
Tdbin = The dry bulb T at inlet, 0C
Tdbout = The dry bulb T at outlet, 0C
Twbin = The wet bulb T at inlet, 0C
Leaving air condition
Tdbout = Tdbin – ηsat(Tdbin – Twbin)

11. Air flow rates
The mass flow rate of air through the EC is a
function of the air velocity and is calculated on
the basis of frontal area of the cooler. The density
and velocity of air at the entry. The mass flow
rate of air, ma
ma = ρAVa
Where,
ρ = density of air at the entry of the cooler, kg/m3
A = frontal area of the cooler’s opening, m2
Va = air velocity at the entry of the cooler, m/s

12. Cooling capacity
The cooling capacity
Qc = macp (Tdbin – Tdbout)
Where,
cp is the specific heat capacity of air
Water consumption
The water consumption is essential because it indicates
the amount of water needed to operate the system
QCo = ma (H2 - H1)
Where,
H: Humidity ratio

13. 2.2 Fog/misting system
It is based on spraying water as small droplets (droplet diameter of 2–
60 micro meters) with high pressure nozzles.
Cooling is achieved by evaporation of droplets. Free fall velocity of
these droplets is slow and the air streams inside the GH easily carry
the drops.
This can result in high efficiency of water evaporation combined with
keeping the foliage dry.
Provides more uniform spatial air temperature and RH than fan-pad
system. Less expensive to install and operate.
Fig. 6: Fog system used for greenhouse cooling

14. 2.3 Roof evaporative cooling
It is sprinkling of water onto a surface of the roof
so as to form a thin layer, which results in an
increase of the free water surface area and
consequently increases the evaporation rate.
This causes the water temperature to fall to the
wet bulb temperature of the closely surrounded
air.
Fig. 7: Roof sprinkling of water

15. 3. Heat prevention
The spectral distribution of global solar radiation flux, that is incident
on or transmitted into a GH can be divided into ultraviolet radiation
(UV: 200–400 nm; about 5% of global solar radiation), visible light
or photosynthetically active radiation (PAR: 400–700 nm; about
45%), and near infrared radiation (NIR: 700–2500 nm; about 50%).
The NIR is less absorbed by the plants but it is absorbed mainly by
the GH floor soil, installations, and construction elements of the
greenhouse. Then it is released again to the GH air as converted heat,
that increases the GH air temperature.
Accordingly, NIR is the main source of heat load that should be
removed to prevent the overheating problem in summer in many
sunny areas worldwide.
Through heat prevention methods, the radiative heat load can be
eliminated or reduced before entering the GH by either absorbing
and/or reflecting a portion of the incident radiation on the GH cover.
This is accomplished by using commercial shading devices (curtains,
clothes, or plastic nets) or by using a radiation filtering roof (blocking
the NIR via reflection or absorption and transmitting the PAR).

16. 3.1 Shading
Shading the roof of a GH is usually performed by various
conventional methods such as whitening the roof, external shade
cloths, deploying plastic nets of various colors, and movable
refractive screens or curtains.
Whitening (white shading paint) can be achieved by spraying the
exterior cover surface with an aqueous solution of hydrated Calcium
oxide [Ca (OH)2]. Whitening the GH roof is inexpensive, has positive
effects on both microclimate and crop behavior, and can be
considered an efficient means for alleviating the large heat load
during summer.
However, it reduced the average GH transmittance to solar radiation
from 0.62 to 0.31. The whitening is washed away if rains fall over the
GH and its shading density cannot be changed once applied.
The external shade cloth is usually applied by deploying wet or dry
shade cloths on the outer surface of the GH roof.
An external or internal shade can also be obtained by using movable
plastic nets, curtains, or refractive screens applied above or below the
roof of the GH.

17. All shading methods regulate the amount of solar energy
entering the GH and reduce the heating load in summer.
Disadvantage of shading system that use curtain or screen
below the roof of the GH is that when fully deployed, it will
decrease the effectiveness of the natural roof ventilation and
negatively affect the GH microclimate.
Moreover, presence of shading materials deployed in the GH
absorbs a portion of solar radiation, reemits it again in the GH,
and reflects back a portion also inside the GH. Therefore, the
effect of internal shading on reducing the GH air temperature
is expected to be small.
All the aforementioned shading methods significantly reduce
solar radiation across the whole solar spectrum including the
PAR (400–700 nm) which is essential for plant growth.
Therefore, recent studies have focused on developing more
selective covers that can transmit PAR and block NIR.

18. Control of light intensity in GH using shading
Shading should be done very carefully, especially permanent
shading.
It reduces both the temperature & the light intensity.
Therefore it is important to consider both factors when
deciding which shading technique to be utilized.
Application of external shading compounds: It will diffuse
light rays & reflect heat, come either in white or green and
can be thinned using paint solvent, difficult to apply & to
remove the uniform coating, semi permanent and during
cloudy & dark days of summer; the plants in the GH may
receive insufficient light.
Installing a shading screen or net over/under the GH
frame: reduce light level is to block out light with some
shading screens made of cloth, polypropylene, polyester or
aluminum-coated polyester, inside screen serve two purposes;
reduce light in the summer and act as a thermal blanket in the
winter.

19. Fig. 8: Application of external shading compounds
Fig. 9: Shading screen or net over/under the GH

20. 3.2 Radiation filters
For GH in hot and sunny regions, scientists and
companies have worked for many years to develop GH
covering systems that is able to reduce the heat load as
well as the air temperature in the GH.
Two systems have been introduced for filtering out the
incident solar radiation at the GH cover:
 NIR-reflecting film covers
 Fluid roof covers

21. Infrared (IR) Additive
Minimize temperature fluctuation:
During the day, slightly decreases temperature inside GH by
blocking near infrared radiation (NIR: 700- 3000 nm). It is the part of
the solar spectral that is hardly used by the plants for photosynthesis;
it is mostly substituted into heat (sensible and latent) in the GH. This
can be an advantage in a country with a colder climate and a
disadvantage in a GH located in warm country.
During the night, increases temperature inside GH, by creating a
barrier to far infrared radiation (FIR: 3000-100000 nm) reflected by
the soil. It is not caused by direct sun radiation, but it is heat radiation
transmitted by each heat body in the GH. This radiation is very
important in GH; since it causes a part of the greenhouse effect.
Fig. 10: Working of IR additive in GH film

22. 3.2.1 NIR-reflecting film covers
The NIR (700- 3000 nm) can be rejected by applying
absorption, reflection, or interference pigments to the
polymer during manufacturing the covering materials.
Fig. 11: Working of NIR-reflecting film cover

23. Fig. 12: Low cost natural ventilated GH with NIR-reflecting film cover

24. 3.2.2 Fluid roof covers
One of the first attempts to eliminate the maximum
temperature of air in a GH was carried out by flowing a
water film of 0.5 mm thickness on the roof, and a drop of
4-50C in the inside air temperature could be achieved.
A water film up to 10 mm thickness did not reduce the
PAR transmission significantly; however, it blocks (via
absorption and reflection) only about 5% of the NIR.
A concentration of 1.5% ∼2% CuSO4-water solution as
LRF (liquid radiation filter) flowing through a hollow-
channeled, rigid plastic (polycarbonate or acrylic) roof of
semiclosed GH has been examined.
Based on these studies, the fluid-roof covers can remove,
via absorption, more than 50% of solar energy incident
on the GH cover; thus the radiation heat load in the GH
can be reduced.

25. Fig. 13: Working of fluid roof cover

26. 4. Composite system
A number of studies have been reported in literature
where advanced composite system are integrated with a
GH for cooling and heating of GH:
Earth-to-air heat exchangers (EAHES)
Aquifer-coupled cavity flow heat exchanger systems
(ACCFHES)

27. 4.1 Earth-to-air heat exchange system (EAHES)
Using the earth mass, the earth-to-air heat exchanger
system is well established in cooling & heating.
The ground potential of the earth can also be used for
cooling the GH in summer conditions due to its
constant year round temperature (26–280C).
The hot air is circulated through the buried pipe (2–4 m
depth) for dissipation of heat to the underground soil.
Though earth-to-air heat exchanger system can lower
the inside air temperature to a remarkable extent, but
the major disadvantage of using EAHES is the initial
cost involvement and less longevity of the metallic
pipes due to corrosion.

28. Underground heat exchanger
Also called:
Earth-Air Heat Exchangers
Air-to-soil Heat Exchangers
Earth Canals etc.
Fig. 14: Earth-to-air heat exchange system

29. Earth acts a source or sink
High thermal Inertia of soil results in
air temperature fluctuations being
dampened deeper in the ground
Utilizes Solar Energy accumulated in
the soil
Cooling/Heating takes place due to a
temperature difference between the
soil and the air
Earth-to-air heat exchange system (EAHES): Principle
Fig. 15: Variation of temperature with soil depth

30. EAT can be used in either:
Closed loop system
Open loop system
Open Loop system:
Outdoor air is drawn into tubes and delivered to
AHUs or directly to the inside of the building
Provides ventilation while hopefully cooling or
heating the building interior
Improves IAQ
Closed Loop system:
Interior air circulates through EATs
Increases efficiency
Reduces problem with humidity condensing
inside tubes.
Tube Arrangement
Fig. 16: Tube arrangement in Earth-to-air heat exchange system

31. Calculating benefits from EAT is difficult due to:
Soil Temperatures
Conductivity
Performance of EAT can be calculated as:
where;
To = Inlet Air Temperature
To (L) = Outlet Air Temperature
Ts = Undisturbed ground temperature
EAHES Efficiency

32. COP based on:
Amount of heating or cooling done by EAT (Heat
Flux)
Amount of power required to move the air through
the EAT
Q = Heat flux
W = Power
COP decreases as system is operated
COP can be integrated into system control strategies
When COP down to a certain point, EAT should be shut
down and conventional system should take over
Co-efficient of performance (COP)

33. 4.2 Aquifer coupled cavity flow heat exchanger system
(ACCFHES)
It is used for cooling and heating of GH.
The system use deep underground aquifer water from
irrigation tube well at the ground surface at almost
constant temperature of around 240C (year round).
The integration of ACCFHES with GH helps in
maintaining the inside GH temperature 6-70C below
that of ambient.

34. Conclusion
The natural ventilated GH is normally used only 8 to 9 months
during the year due to high inside temperature of GH during
summer season.
The critical factors affecting the performance of natural ventilated
GH are rate of air exchange through natural convection and which
depends on total area of vents, wind speed and temperature
difference between inside and out air.
The selection and operation of the system is based on various
parameters such as type of climate, crop to be grown, cost,
maintenance, ease of operation, reliability, life of the system,
dependency on electricity, etc.
Normally for cooling the GH the combination of natural
ventilation, inside shade net materials/thermal screen and
fogging/misting is used.
So the most suitable technology for GH cooling is that which
meets most of the desired conditions of the farmer to grow
offseason crops in order to fetch maximum returns.

35. GH Heating
GH Heating Systems
1. Passive 2. Active
1.1. Water Storage
1.2. Latent Heat Storage
Material
1.3. Rock Bed Storage
1.4. North Wall Storage
2.1.Heating 2.2. Radiant Heat
System
2.3. Composite System
2.1.1. Local
2.1.2. Central
2.3.1. Earth-to-Air Heat
Exchanger System (EAHES)
2.3.2. Aquifer Coupled Cavity
Flow Heat Exchanger System
(ACCFHES)

36. Need for GH Heating
Temperature is one of the most important factors
in the production of horticultural crops.
Solar energy on sunny days is often enough to
keep a GH warm, even in cold weather.
During the night time, air temperature inside GH
decreases.
The heat is always lost from the GH when the
surroundings are relatively cooler.
The requirements for heating GH depend on the
rate at which the heat is lost to the outside
environment.

37. To time the production for a specific market and to have some
control over crop quality & yield, growers need to heat their GH
whenever its temperature drops below the recommended
temperature for their specific crop.
Except of raising the air temperatures to the desired level, heating
is also applied in cases where there is a need to reduce air
humidity (e.g. to reduce the probability of condensation on plant
organs and thus reduce development of fungal diseases).
In cold countries GH are heated during most of the year, while in
mild climates the heating period is shorter and heating is usually
applied during the winter.
Heating costs are not only directly connected to profitability, but
in the long term they may determine the survival of the GH
industry.
In addition to the costs of high energy consumption, heating is
associated with environmental problems through the emission of
noxious gases.

38. Mechanism of Heat Loss
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.
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 GH
occurs through infiltration loss.
A third mode of heat loss from a GH is that of radiation.
Fig. 17: Heat loss from GH

39. Control of Heat Loss
Various methods are adopted to reduce the heat losses,
viz., using double layer polyethylene, thermo pane
glasses.
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 GH 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.

40. 1.1 Water Storage
The heat storage system can be placed inside the GH, in plastic
bags filled with water. Water containers used as solar collector and
heat storage.
The system absorb and trap the incident solar radiation during the
day. During the night, the stored heat is returned to the interior by
natural convection or radiation.
Fig. 18: Passive solar GH with water storage in (a) plastic bags
and (b) water containers

41. 1.2 Latent heat storage material
Latent heat materials are an alternative heat storage medium and like
CaCl2
.6H2O (with a melting temperature of 29.7oC and a latent heat of 170
kJ/kg) have been successfully used in many GH.
Heat is absorbed by the latent heat storage material and stored for later
use. The material changes phase during this process. At night, cold air
from inside the GH is circulated through the storage and is heated before
returning into the GH. The latent heat material then returns to its initial
solid phase. This process may result in a humidity increase during the
night periods.
Fig. 19: GH with rock bed storage

42. 1.3 Rock Bed Storage
A popular and economical heat storage material is a rock bed,
which consists of 20-100 mm diameter gravel. The storage area is
place under the GH at a depth varying between 40-50 cm.
During the day, excess heat is transferred from inside the GH to the
underground store. A ventilator can be used to transport GH air
(using a fan at a rate of 5 m3/min m2) to the heat storage area.
At night, the process is reversed. The cool air is moved through the
store, where heat is transferred from the gravel to the colder air and
then return to the GH.
Fig. 20: Passive solar GH with latent heat storage material

43. 1.4 North wall storage
To reduce construction cost resulting from the implementation of
the previous systems, it is preferable to use insulated sides for
reducing heat losses and a north storage wall.
This wall is externally insulated and internally painted black which
operates as a heat storage.
A simple, north side storage wall is of small cost and applicable to
commercial application where the heating needs are not very high.
Fig. 21: Passive solar GH with north wall storage

44. Heating needs
There are various ways to calculate GH heating needs (Hg) (W).
The simplest is
Hg = UA (Ti-To) (1)
Where
U = heat loss coefficient (W m-2 K-1)
A = exposed greenhouse surface area (m2)
Ti = inside air temperature (K)
To = outside air temperature (K)
Note that the estimation of GH heating needs using Equation 1 did
not take into account heat loss due to leakage. However it is a
simple formula which can be used in order to estimate heating
needs according to the GH covering area and the desired
temperature difference between inside and outside air.

45. Types of Heating system
2.1.1 Local heating systems: It are usually placed
at one end of the GH. They can be unit heaters,
convection heaters or radiant heaters. Local heating
systems are more suitable for smaller GH.
2.1.2 Central heating systems: It consist of a
boiler in a central location. Boilers heat with steam
or hot water and can burn a variety of fuels. It is
used most commonly in larger commercial GH due
to cost.

46. Function of GH heating
There are four functions that must occur to heat a GH:
Conversion of fuel to heat energy: The conversion of fuel
to heat energy is typically accomplished through
combustion with a burner installed in a boiler or heater
combustion chamber.
Distribution of the heat energy: The heat energy is then
distributed through the GH through pipes, ducts, tubes,
or air.
Transfer of the heat energy: Once the energy is
distributed, it must then be transferred to the plants and
soil by convection, conduction, or radiation.
Conversion of the heat energy into useable heat by the
plant: Finally, once transferred to the plants and soil,
they must in turn absorb its energy and convert it to
usable heat.

47. Unit heaters
The unit heater is a fan equipped device with a means to heat the air.
The most common and least expensive is the unit heater system.
Warm air is blown from unit heaters with self-contained fireboxes.
Heaters are located throughout the GH, each heating a floor area of
180–500 m2.
Unit heaters are available in oil fired, electric, hot water or steam, and
gas fired. The most popular being the gas fired unit. Unit heaters are
typically suspended from the GH framing. Floor mounted units are
also available.
There are two main types of unit heaters that are used for space
heating in GH: vented and unvented.
The traditional vented, gas fired unit heater transfers heat from the
combustion gases to the air through a heat exchanger and exhausts
the combustion gases outside the GH through a flue pipe.
An unvented unit heater burns the gas and exhausts all combustion
gases directly into the GH, so virtually all the heat from the fuel is
used to heat the air.

48. Reason for using Unit heaters
They provide the air circulation needed and can be
used in conjunction with ventilation systems.
They can provide uniform bench top temperatures
and under the bench temperature.
They are comparatively the least expensive and
quick response to temperature changes.
They are easy to install and offer inexpensive
expansion for additions.

49. Fig. 22: Unit heaters in GH

50. Central heating
Steam or hot water is produced, plus a radiating mechanism in the
GH to dissipate the heat.
Unlike unit heater systems, a portion of the heat is delivered to the
root and crown zone of the crop, resulting in improved growth and
to a higher level of disease control.
Placement of heating pipes is very important as it is directly related
to heat loss; for example, the placement of pipes in the walls resulted
in high losses through the sides.
Boiler components
Firebox: where fuel is burned.
Flue: provides a way for smoke, from the
firebox, to vent to the outside air.
Heat exchanger: network of tubes either filled with or surrounded by
water.

51. Fig. 23: Arrangement of Heating Pipe Coils

52. 2.2 Radiant heat systems
These heaters emit infrared radiation, which travels in a
straight path at the speed of light. The air through which the
radiation travels is not heated.
After objects such as plants, walks and benches have been
heated, they will warm the air surrounding them. Air
temperatures in infrared radiant heated GH can be 3-6°C
cooler than in conventionally heated GH with equivalent
plant growth.
Grower reports on fuel savings suggest a 30–50 percent fuel
reduction with the use of low energy infrared-radiant heaters,
as compared with the unit heater system.
Fig. 24: Radiant heater in GH

53. Comparison to GH heating system
Type of system Advantages Possible disadvantages
Steam  Can transfer heat throughout large
ranges (GH physical plants)
without cooling
 Requires smaller piping than hot
water systems reducing installation
cost
 Makes steam available for heating
soil in GH
 Control of GH temperatures not as
subtle as with hot water and GH
may temporarily overheated
 Cost of maintenance greater with
steam due to damaging effect that
high pressure on pipes
Hot water  Permits more accurate and
responsive thermostatic control of
GH temperatures
 Distributes heat more evenly with
fewer hot spots to injure plants
 Less potential for danger to
workers if a hot water line ruptures
than if a pressurized steam line
breaks
 Larger piping required, increasing
cost of installation
 Steam still required for soil
pasteurization
Unit heater  Adaptable to small GH areas
 Excellent back-up systems in the
event of boiler breakdown or
power failure of large system
 Fuel cost (gas or oil) may be greater
than cost of expanding exiting steam
or hot water system
 Heat distribution uneven but can be
improved by attaching plastic
sleeves that extend length of house.
Bench level temperatures may still
be cooler than nearer roof.

54. Conclusion
The maximum temperature during summer month is
major challenge in India for round the year utilization of
GH not minimum temperature.
The minimum temperature is issue but it is confined to
few areas of country.
The selection of heating methods depends upon many
factors, some of them are size of GH, air and or soil
heating, efficiency of heating system selected,
application duration during year, passive or active system
etc.
The alternative technologies available for cultivation
during low temperature season.
The most suitable technology for GH heating which
allow farmer to grow offseason crops in order to fetch
maximum returns.

55. If you have any question/suggestion
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