Session 2 Greenhouse Control Getting the basics right first 24 January 2018, Bordon Hill FEC Energy

Transcription

1 Session 2 Greenhouse Control Getting the basics right first 24 January 2018, Bordon Hill

2 What can we control? Temperature Air Exchange Air Movement Light Temperature and air exchange have biggest effect on humidity

3 Topics What affects greenhouse heat loss Cost of keeping a greenhouse warm Good housekeeping Heating system Vents Screens Measuring boxes Computers and graphs

4 Keep a greenhouse warm? What affects heat loss from a greenhouse? Outside temperature h Heat Loss i Greenhouse temperature h Heat Loss h Wind speed h Heat Loss h Greenhouse structure Fixed h Heat Loss h Rain / snow

5 Greenhouse structure 5 C 20 C High heat loss (no screen) 8W/m² C 0.45W/m² C for a house Example Greenhouse temperature 20 C Outside temperature 5 C 1 Ha needs 1.2MW of heat (120W/m²) Can be reduced Screens Sealing

6 The cost of 1.2MW 1.2MW = 1,200kW MW & kw are the rate (speed) of energy use 430 kettles We buy gas by the kwh 1kWh = 1kW x 1 hour 1.2MW x 1 day =?? kwh 1.2 x 1,000 x 24 = 28,800kWh Cost of gas, say 1.7 p/kwh Cost to keep 1Ha at 20 C when it is 5 C outside? 490/day ( 3,430/week)

7 What would I save by growing 1 C colder? 5 C Heat loss (no screen) 8W/m² C Greenhouse temperature 19 C Outside temperature 5 C 1 Ha needs (19-5) x 8 x 10,000 = 1,120kW (1.12MW) 19 C 1,120 x 24 = 26,880kWh 26,880 x 1.7p/kWh = 457/day ( 3,199/week) 33/day - 7% less

8 Heat emission W/m Heat output from a pipe Well defined scientifically Translates reliably into practice Pipe temperature - o C

9 Minimum pipe temperature Many growers like to use minimum pipe more than they really should It helps them to sleep at night But it can make them cry when the gas bill arrives UK many years ago Minimum pipe temperature rarely <40 C no matter what time of year or day Anybody who did was a risk taker UK now (without Biomass!!!) Many will turn the pump off

10 Heat emission W/m What does 30 C minimum pipe cost? Average greenhouse 1.25m of 50mm pipe per 1m² Air temperature 20 C Heat emission = 20W/m =25W/m² On for 6 hours/day when not required (25/1000) x 6 hours x 7 days = 1kWh/m² per week = 170/Ha/week Pipe temperature - o C

11 Heating systems

12 Final heating loop Do you quickly, and accurately, achieve the pipe temperature that you ask for? If not, the mixing valve set points may need to be changed When the mixing valve is fully closed, does the pipe temperature stay high? If so, the valve seat or seals may need replacing Does the return temperature respond and stabilise within 20 minutes? If not, the circulation pump may be getting tired All of these can cause unstable and inaccurate greenhouse temperature and humidity control

13 Don t forget the sun heat gain 100W/m² outside Gives 70W/m² inside = 45 C pipe temperature Higher light levels = lower heat demand Note A lot of this free heat is used for transpiration 20 C greenhouse temperature, 1.25m 50mm dia pipe per m²

14 Air exchange humidity Air exchange can be a useful way to control humidity Important to monitor incoming air Improved humidity control at expense of heat

15 Heat Loss W/m 2 / o C Wind Wind quite literally sucks heat out kW/Ha Wind Speed m/s 10 deg C pipe

16 Wind Can minimise this effect by Well sealed doors Good close on vents Roof screens Side curtains Temporary screens/ bubble wrap Replacing broken panes

17 Glass

18 Vents and doors Do they close properly? Do the vents all open by the same amount when you ask for <5%? When were the vents last calibrated?

19 Vents Slack in the wires Sticking sliders Bent frames Position sensor accuracy

20 What is the effect of a screen? Solid energy screen High light transmission Reduces heat loss by 43% Solid shade screen High shade level (lots of foil) Reduces heat loss by 75% Voided shade screen Reduces heat loss by 25% But they are rarely closed all day

21 Screens

22 Climate control computer Upgrading to a new system with User friendly interface Advanced data recording and graphing capabilities Remote access Will deliver energy savings of 10% PLUS BUT Better environmental control A better crop An easier life for you You need to dedicate time to using it properly You need training in how to use it Otherwise all it will be is a very expensive thermostat

23 Climate control computer training Product support and on site training can suffer When suppliers are competing on price When the local market it too small to justify local expertise Most state of the art control systems are complex Require staged training Require continued use to remember how to use them

24 Climate control Why spend 10,000 s on a computer 100,000 s on energy THEN Put your measuring box where it makes life easiest Only check it when the alarm goes off

25 HD - g/m 3 Location of measurements Top Bottom Time

26 So where do you put it then?

27 Weather station Location Away from artificial light sources In open air Not in the wind shadow of another building Or too close to the glasshouse roof When was it last calibrated? Does it really matter that much? Storm, frost & rain settings Light influences on heat & vent temperature Modern computers use weather data all the time Calculate heat demand Vent position Open/close screens

28 Don t be complacent Every compartment is different What about different structures? Old glass vs new glass Different screen materials Exposure to the wind What about different plants? Energy to transpire evaporate water Energy for humidity control

29 Good housekeeping When costs are rising & profit is under pressure Fewer staff Everybody is busy Less time to do the simple jobs There is a tendency to focus on the big issues To achieve big effects But they usually have a high cost or risk Can take a long time to implement Good housekeeping Low cost Easy to implement No/low risk

30 Good housekeeping Broken & slipped panes of glass Badly fitting / damaged door seals Vents not closing properly Damaged or missing pipe insulation No insulation on pump bodies and valves The heat loss from a gate valve = 1m of pipe The heat loss from a pump = 2m of pipe Old and damaged or worn out screens Climate computer Weather station

31 That s all for this session Any more questions at this stage?