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!