Greenhouse Heat-Sink

Yep, an update is well overdue.

Ok, we had the 96hrs data and we also have some further 2 or more weeks. Not had a chance to analyses this yet, but we are quite confident it will show similar results to the 96hrs data. All the data is only based on 2 sensors, as mentioned in the previous answer this was a shipping issue and we had to make do with what we had.

We have some good findings as far as material testing and possibly best to mix materials forming layers i.e. small granite chips and then some larger rocks and small granite chips onto and so on and so forth.

The glass and granite are small chips, the concrete was larger blocks all cooled down to 2C I think overnight, then warmed and then cooled. The concrete larger chucks held onto their core temperature for longer, so having a mix of materials gives allows a more balanced temperature gradient.

We have been looking into temperature sensors and I have found a very cheap option, already sealed in a 1m package, these can be combined and produce accurate readings which can be logged or just used in any calculation for the fan PWM control.


The block diagram above shows a very basic system, there are a few additional parts, but this gives a good overview of the intended system.

The bad part is costs of the solar panel and deep cycle battery, some preliminary calculations based on a location of Odense in Denmark, for all year round operation with a 1W load running 24/7 are are a 50W solar panel and 50AH deep cycle battery. This is much more favourable for UK and I think the panel could be a 30W and the battery 30Ah. It's preferable to use a larger panel and smaller battery as the panel is far more reliable, so running costs will be substantially reduced.

As far as size we designed the proof of concept prototype heatsink to be near on 10% volume of the prototype greenhouse. This then allows an easy calculation of the greenhouse size, then the heatsink size for the measured effect we observed. We have spoken to an expert on the principles at work here, and he said if you double all the dimensions on the heatsink (effectively making the volume 8 times as large), you should double the effect of the heatsink.

I could write a lot more as very interesting and intend to structure and display all this information on my site, will have more time over summer to do this.

The key finding I think and we have a presentation next week, is the heatsink is not a viable business! However the effect does work and if you built a system scaled correctly, it will cost more initially but over time will save when compared to paraffin or gas burners in the long term. The heatsink should have low running costs, no smelly residue or Carbon Monoxide release, no storage issue for the heater and the fuel etc.

Another option is to have a smaller heatsink and supplement with conventional heaters, the heatsink will draw the heat from the apex and recycle it, slowing the effect of lost heat.

Actually a couple of questions for anyone who uses a paraffin or gas burner:

How much paraffin do you use a year?
How much gas do you use a year?

Regards,
Ant

 

Hi Ant
I push air through it using a big fan equipped with a speed control. Running flat out it can put the entire greenhouse air volume underground once a minute but most of the time it only runs 20%-50% of the fan capacity. I use a differential thermostat to control when the fan runs and a second thermostat to reverse the soil/air sensors which automatically switches the system between cooling and heating.. I built a few other features into the design to enhance the performance on marginal days/nights but i haven`t used them much so far..I dont keep track of the power these days as the fan only draws 150w flat out. During the first season i was regularly seeing a COP of over 30..

 

Hi John
As long as we dont go sub zero for a week with zero sunshine in the daytime it`ll stay above freezing. The emphasis is more on cooling in summer for me than heating in the dead of winter as i dont keep tender or tropical plants. Its important to mention that the plants themselves are a big part of the system so a full greenhouse works much better than an empty one. A sunny winter day can provide enough heat to keep the roots warm overnight, if the roots are growing in the thermal mass. A little extra heat in the rootzone can make all the difference even if the air temperature is low. An inert concrete/glass/gravel heatsink wont provide that benefit unless the roots are growing in it. Not forgetting the water that condenses out underground is put back into the rootzone where it started from.
It takes over 600w to convert a litre of water into a gas, luckily plants do it all the time (transpire water)using energy from the sun. when the water condenses out you get most of that water and energy back. There`s a lot more going on here to benefit the plants than just keeping the gh warm or cool.