27/11/2015. How does a greenhouse work: humidity. Energiezuinig ontvochtigen. Climate control, humidity

Transcription

1 Energiezuinig ontvochtigen How does a greenhouse work: humidity Masterclass 26 november 2 HAS Den Bosch BOGO project Klimaat en energie: nieuwe low input teeltsystem in de tuinbouw F. Kempkes en S. Hemming, Wageningen UR Glastuinbouw humidity inside Plants give: Vapour The need for humidity control may cool the greenhouse more than necessary Relative Humidity or vapour deficit? Absolute humidity amount of water in air [g/kg or g/m ] For the control Absolute Humidity difference in- & outside is most important Abshum in >> Abshum out open the window Abshum in <= Abshum out increase T inside Outside air ( dryer always ) Latent and sensible heat loss Minimise sensible heat loss (winter) ventilation Cover condensation cover temperature Condensation heat stays inside Mechanical dehumidification condensation cold surface What cold source? Production costs energy Temperature below dew point else no condensation at all as well loss of sensible heat (pre heat back to air temperature?) 1

2 Energy fluxes Transpiration needs energy If warm it cools the greenhouse If cold extra heating energy required Condensation releases energy Melting vs freezing of water Botrytis Chance for Botrytis is high when crop (parts) become wet Wet crop caused by Condensation Guttatie (root pressure) Chance for infection increases with duration of wet crop Crop cooler than dew point of the air Vapour content in air can also be described in terms of dew point, that is the temperature at which air with given vapour content would be saturated saturated vapour function actual vapour sun radiation radiative losses transpiration T dew temperature T a The crop at night is cooler than the air, but it should be warmer than dew point... Crop temperature air temperature, but... 2

3 Effect of temperature on RH T greenhouse =.5 o C Each kg air exchange is 5.6 g moisture released RH greenhouse = 88 % Abshum. greenhouse =.7 g/kg T TR outside =.1 o C A RH T outside = 86 % Abshum. outside =.1 g/kg Transpiration how much we have? Effect of artificial lighting (18 µmol/m 2 /s) on transpiration. Mainly related to: Radiation Humidity LAI CO 2 T air = 16.1 o C RH air = 1 % Abshum. air =.6 g/kg T air = 1.6 o C RH air = % T air =.7 o C RH air = 6 % T air = 6. o C RH air = 76 % Abshum. ait =.5 g/kg Abshum. air =.7 g/kg Abshum. air =.5 g/kg 25 2 Transpiration: full grown tomato crop 2 a gram/m 2 /uur at the lower side is not eq min transpiration [ o C] verd [l/m2/uur] tkas setp [g/m ] 1 globale straling [W /m2 ] vd sp vd Transpiration: full grown tomato crop [g/m ] Humidity problems vd sp vd verdamping Transpiration [gram/(m2 [g/m 2.hour] uur)] afternoon morning globale straling [W /m2 ] Inside straling radiation binnen [W/m [W/m2] 2 ]

4 RH (%) temperature (C) Condensation danger time kaslucht bij plant Morning, crop T lower time RV RH kaslucht air RH RV bij plant plant Afternoon, crop T higher RH air is controlled RH plant shows problem Good plant temperature measurement is difficult Plant, where to condensate? Plant temperature Cover temperature & screen control Radiative heat losses (night) Effect of cover material (glass / plastic, τ ir ) Plant shape (gerbera) Plant health Plant type (pot plants versus tomato) Plant part (leave or flower bud) Ventilation and temperature and dew point 6 8 W/m 2 sun radiation o C unventilated Temperature range allowed by ventilation management Temperature management = regulable ventilation 8 o C 28 2 well-ventilated 18 Effect of low humidity on crop Small leaves Blossom end rot Delayed growth Smaller buds Smaller fruits (better taste) Less fungi Loss of production Generative growth Effect of high humidity is in general positive for crop production unless diseases become a problem outside 1 ventilation 1 does 1-Oct 2-Oct not 21-Oct dry the air

5 Dehumidification costs energy - because Dehumidification Sensible heat loss during ventilation Water vapour Latent heat uptake by the crop How to reduce losses Reducing the transpiration rate during heating periods Reduce the Sensible/Latent ratio of the ventilation air Both measures imply growing at higher humidities OR Make use of the cold roof top (condensation) Energy efficient system Condensation Condensation on inner side of roof Glass temperature < dew point temperature of greenhouse air Saving on energy for dehumidification Crop evaporation [kg/(m 2 year)] control on a higher setpoint 8 kg/(m 2 year) reduction Evaporation in periods with heating Evaporation in periods without heating (8 kg/(m 2 year)) Tomato crop 1 l/(m 2 year) Setpoint for relative humidity [%] 5

6 Saving on energy for dehumidification Accurate humidity control Humid air leaving the greenhouse The humid and warm air is pushed through the screen and through small window openings At humidity setpoint of 85% kg of moisture and 185 MJ/(m 2 year) (Tomato in Dutch weather conditions) (18% of heating demand) Crop is dried at the most vulnerable parts with dry outside air Outside air is dry, but cold 5 MJ/(m 2 year) decrease (%) Preheating in the air treatment unit Sensible heat loss due to dehumidification Sensible heat [W/m 2 ] 2 5 mm diameter every other gutter for a 12 meter deep path 1 5 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 185 MJ/(m 2 year) 6

7 Three ways to reduce this heat loss Regain out Balance ventilation system Condensation by cooling 1 in Greenhouse air out Outside air in Hygroscopic drying Efficiency van de regain unit Balance ventilation 6 to 8% regain of sensible heat [ o C] 25 2 Dehumidifier on T outside T gh extract T gh entrence dehumidification 185 * 7% = 1 MJ/(m 2 yr) energy saving 1 5 Efficiency calculated Dehumidifier off Efficiency of this regain unit is about 82% (sensible heat) Switching on /of reduces the efficiency (vary the fan speed) 7

8 Condensation on cold surface keeping sensible and latent heat inside Greenhouse is closed in winter Experiments show 22 MJ/(m 2 yr) energy saving Hygroscopic dehumidification keeping sensible and latent heat inside Hygroscopic dehumidification principle Ordinary water Brine of 8% mass CaCl 2 RV=1% RV=55% 8

9 Hygroscopic dehumidification orienting experiments Comparison of capacity air circulation rate: 1 m /(m 2 hr) Dehumidification capacity [gr/(m 2 =85% Hygroscopic brine Cold surface Expected savings: 25 MJ/(m 2 temperatures brine in advantage Conclusions: (holding for tomato in NL weather) Or exchange air from above the screen Allowing high humidities is a first step to save on dehumidification costs. A proper system to safely enable a high humidity control saves around 2 MJ/(m 2 yr) The simplest system to further decrease energy costs for dehumidification retains sensible heat and saves around 1 MJ/(m 2 year) When using a cooling system to condens the moisture excess sensible and latent heat can be recovered, saving 22 MJ/(m 2 year) When using a hygroscopic dehumidification system sensible and latent heat can be recovered more effectively expected saving of 25 MJ/(m 2 year) Vents can be opened to reduce dew point temp. cover

10 Ventilation jets: (local system) Dehumidification & Air circulation Dehumidification else than by vents means forced air movement (distributed or free Air circulation lowers ALWAYS the difference between crop and air temperature It cools if crop is warmer It warms if crop is cooler Thus it certainly may help when there is risk of condensation Systems for dehumidification Air distribution /central system 1

11 Systems for dehumidification Air distribution /central system Climate control, Air distribution Air takes way of less resistance Solution of poor distribution is in general not to circulate more m High air speeds increases the transpiration Climate control, Questions? Vertical temperature distribution Be aware mechanical systems can change natural air flows in greenhouse systems Dehumidification systems are not for cooling (too low capacity) Cold air is heavy (stays at bottom) For natural air mixing bring cold air in above the crop Bringing in cold air from below (in case dry air is not preheated up to greenhouse air temperature) can change (negatively) the vertical temperature distribution (cold feet / warm head) and can create a dead climate 11