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High insulation; how to control humidity
Seminar 23rd of October 2012, Gjennestad, Norwegen
Frank Kempkes
Reduction of energy losses
Double covering materials
●High insulation = less convection losses
●Specific coatings = less radiation losses
Screening
●More screens are more effective as one single super screen cavity-split standing air
●Up to three screens
●How to control ?
But with increase of insulation humidity will increase as well
Reduction of energy losses
Humidity:
Humidity is an increasing problem with increasing insulation
Decrease of condensation from 100l/m2/yr to about 10l/m2/yr
Search for alternative dehumidification system
General: Worse humidity control results in non optimal use of energy savings
Can we find an energy saving dehumidification system?
Energy flows
Removed moist has used energy to evaporate● In summer cooling of the greenhouse ++
● In winter often heating energy --
How to remove● Out side air (“always” dryer as greenhouse air abs. humidity)
● latent and sensible heat loss
● minimise sensible heat loss
● Cover● condensation cover temperature
● condensation heat remains in greenhouse
● Mechanical dehumidification● condensation cold surface
● what is source of this cold? energy
● temperature below dew point but as well loss of sensible heat re-heating uses (lots of) energy
Balance
Economic feasibility
Practical fit in (existing) greenhouses
Energy efficient
Ventilation
14 g kg-1
9
11.5
24°C
14
19
Ventilation for humidity control as now used
Controllability of ventilators (now the system follows and 1% today has different effect as 1% tomorrow)
Equal distribution
Often unintentional heat loss
Improvement of controllability
Mechanical ventilation!
Mechanical ventilation is: controlled movement of air
Complex in greenhouses:air is difficult to lead in the right direction (way of less resistance)
Influence on microclimate is not clear
In practice experience with several systems (mainly based on closed greenhouses)
Distinguish:● movement of air (MICROCLIMATE)
● Input of outside air (DEHUMIDIFICATION)
Movement of air (equal distributed en not to much) can help to create a “good” microclimate
(what is good?)
Introduction of fans●Electricity use●Avoid resistance or at least high pressure
On short distances it can help to level out temperature differences
Microclimate (mixing air, local)●Capacity: movement of air of mm/s, cm/s, m/s
and / or Dehumidification (exchanging air between in and
outside, transport & distribution)●Capacity: m3/m2/hr.
depended of crop transpiration
Mechanical ventilation is: controlled movement of air
Dehumidification:Capacity is balance between crop transpiration
and difference of absolute humidity between in- and out-side air
Do we know crop transpiration? (radish or tomato)Effect of soil in case of non soillessHow often we allow underperformance of the
system dehumidification by ventilatorsDehumidification system is not for cooling
Dutch tomato crop capacity of 5- 7 m3/m2/hour non soilless, single glass, lack of capacity in August/
September (warm nights small Δx)
Mechanical ventilation is: controlled movement of air
How did we start: principle
Cold, dry outside air
About the system
System installed at 1.2 Ha
In total 18 fans installed in side walls
Maximum capacity of fans is 3000 m3/h 4.5 m3/m2/hour
Energy screen: LS10 Ultra Plus
Holes in tube directed to heating pipe (no pre heating of out-side air)
No “official” outlet; air percolates through gaps and holes
Fans in side walls
Air tube below the gutter
And start running
Temperature along the tube
Tgh =15.5°CRHgh =88%AHgh =9.7 g kg-1
Tair =16.1°CRHair =41%AHair =4.6 g kg-1
Touts =3.1°CRHout
s
=86%AHouts =4.1 g kg-1
Tair =14.6°CRHair =44%AHair =4.5 g kg-1
Tair =9.7°CRHair =63%AHair =4.7 g kg-1
Tair =6.3°CRHair =76%AHair =4.5 g kg-1
airair
Conclusions of first experiment
Horizontal temperature distribution in compartment with system is better than in control where humidity is controlled with screen splits
In experiment temperature of out coming air (because of non pre heating) not equal but in this case no problem
●condensation at tube specially at beginning beside side wall (makes growers nerves)
●grower likes preheating because of creating a “good feeling” (no condensation nearby the crop) but it’s a perfect dehumidifier)
Vertical profile as in reference
Tube●distribution works (bring temperature at
greenhouse air temperature)●heating of greenhouse by input of hot air at
central point is not smart horizontal temperature distribution
Combined / mixing system including recirculation●in mix system a fixed flow is distributed through
the system. valves control outside air mixingd.− in practice often problems to control− pumping air ≠dehumidification− extra fans energy− suck in of greenhouse air can create problems+ regain of sensible (latent) heat possible
Lessons learned this experiment and past
In practice: an example of dehumidification system
Biological production (soil is in use) distribution system is lifted Combined with vertical fans for distribution in the crop
Recirculate inside air and / or distribute outside air
Controlled ventilation of greenhouse air
One tube each 6th span No pre-heating
In practice: an example of dehumidification system
By increase of insulation an increase of humidity as well
Combination of screen use and screen-splits for dehumidification far from ideal
By ventilation lots of energy can be lost good reason to control this as good as possible
Mechanical dehumidification with outside air can be a:
● simple system
● with or without pre heating
● with or without regain of sensible (latent) heat
● rather small capacity of 5 - 6 m3/m2/hour necessary
● by keeping screen closed maximum energy savings of screen when in use
● more screening hours extra energy saving
For maximising energy savings dehumidification system essential
So
Ready?
NO we need an energy efficient dehumidification
Balance ventilation can pre-heat the incoming air
● efficiency restricted because it mainly works on sensible heat (latent heat is lost)
● extra fan(s) is needed for the outgoing air
● easy and cheap heat exchanger is needed
Balance between economics of extra investments and extra electricity use vs energy saving for pre-heating
Can we combine functionality of heating and dehumidification system?
●air heating means low water temperature increase efficiency of boiler room or geothermal source
In Venlow energy: replacement of regain unit
Optimization of dehumidification system goals:
●reduction of electricity use
● improve regain efficiency
● low temperature heating system increase efficiency of heat pump, boiler house, thermal energy
master slave
Dehumidification & heating
master
slave
Temperature of heating systems
3 heating systems
●Master
●Slave
●Pipe rail
Water temperaturemainly < 45 oC
Opening of screen
Working of regain heat exchanger, may 5th
Minimum fan capacity of 25% due to equal distribution inside the greenhouse
●T exhaust 17.8 oC●T outside 8.2 oC }82% sensible heat
●T mixed 16.1 oC }
4 8 12 16 200
5
10
15
20
25
[oC]
t hoogt afvoert mengV1 & V2 vrijt buiten
T gh boxT exhaustT mixedFans onT outside
How to save more energy?
Energy use is about 0.1-0.2 m3/m2/week
Heat required between 02:00 tot 08:00
Nature (the sun) starts around 06:00
Always heat requirement
Energy saving compared to commercial farms
Winter: double glazing
Summer: growing concept+CO2
Venlow concept compared to practice
Special thanks to my colleagues:Feije de Zwart, Jan Janse and Jouke Campen
Takk skal du ha!