Heating

Can I heat the greenhouse?

My first idea was to see if it is possible to maintain a 5 degree C temperature gradient between the inside and outside of the greenhouse. I wondered how much power I would need to do this. Below is a calculation to do just that but getting the constants right is pretty tricky and ambiguous at best.

The calculation is based upon this tutorial and these websites give a good idea of what the constants for the windows might be.

Engineering edge - list of heat transfer coefficients
Engineering edge - double glazed window heat transfer coefficients
Bre Group - is bigger always better - triple better than double?

k_glass = 1.1 # https://www.bregroup.com/insights/is-bigger-always-better-triple-better-than-double/
h_inside = 5 # can vary from 5-37 for free air convection . examples seen have used 8 - 10 
h_outside = 15 # can vary from 10 to 100 depending of wind speed outside. examples seen have used 15-40

L = 28e-3 # thickness of window 
A = 15 #m2 area of window

R_cond = L / (k_glass * A)
R_conv_i = 1 / (h_inside * A)
R_conv_o = 1 / (h_outside * A)

R_total =  R_cond + R_conv_i + R_conv_o

T_inside = 5
T_outside = 0 

Q_dot = (T_inside - T_outside) / R_total

print(f'{Q_dot:.2f}W')

This gives 256.74W but if you play about with the numbers you’ll quickly see this can vary quite a lot.

What does this mean? Well, I can assume that something between 300W and 600W is required to maintain a positive temperature differential. After some googling I found these. At the time the site said they were 300W and operated on 24V with a built in fan. If you click on the link now it says 250W… They cost £25 on amazon and I bought two to be delivered the next day to my parents house. DO NOT BUY FROM AMAZON. You can find these on aliexpress in the £7 range.

I have never measured the power consumption of these devices but I can assume they are in the 250-300W area. Here they are mounted on the wall.

How are they operated?

To control the heaters a switching device is required. After looking for a cheap commercial option on the internet I came to the conclusion that a custom board was required for switching loads above 10A at 24V. Behold the four channel relay PCB. It features optically isolated inputs and 16A relays that operate off a 24V coil with an LED indicator. Version 2 did include current monitoring but the project became corrupted and so I ended up with version 1. Here is the datasheet for the relay.

The PCB can be seen in the lower right of the electronics canvas. I should also mention that each heater has its own breaker in case of failure.

Concluding Thoughts

This works for now but I would like to add in high side current monitoring to the relay board for additional telemetry.