Cold frame under soil heating with solar panels

[javaMan]

Apologies if this has already been suggested.

Long been fascinated by methods to extend growing season, particularly using hotbeds, and was intrigued to see hotbeds being made in the old BBC series Victorian Kitchen Garden. The insides of a cold frame is excavated to quite a depth (can’t remember exact depth, but at least 60cm – possibly 1m), filled with horse manure, and then capped with a layer of soil. The decomposing horse manure provides heat to keep the cold frame warm, and extends the growing season by a good month or so.

However the average urbanite has limited access to a ton or so of horse manure, and then there is all the effort in building the hotbed. Hence this idea to combine modern technology to achieve the same effect – I hope.

Basically, the cunning plan is to use a solar collector to provide under soil heating. I figured it would be better than pumping the warm water through pipes in the cold frame.



The cheapest solar panels would, I guess, be DIY jobbies, made with radiator panels or the back of a fridge. I’m not sure about the cost effectiveness of using technology like vacuum tubes. Power for the pump would come from a small solar cell panel, perhaps combined with a small home made vertical axis wind turbine.

One of the biggest problems I see is ensuring that the pipes under the soil in the cold frame are not pierced by spade, fork or trowel. Not sure if burying them deeply would work, as it might lessen the amount of heat getting to the cold frame. So protecting the pipes (or radiator) with something like slate or paving slabs would be a good option. The latter might work as a storage radiator.

Any pipes to from the solar panel would have to be insulated, and protected with study plastic pipes.

In general, during winter, the sunny days tend to be the freezing cold days/nights, and the cloudy, wet days tend to be mild. This doesn’t always hold true. I remember many a cursed winter when the nights would be clear and frosty, and as dawn approaches the cloud rolls in only to clear away come evening time.

I doubt this set up would heat a greenhouse, but in the absence of cold frames, it might be used to provide localise under soil heating.

Angus

[grahamhobbs]

This seems basically a good idea. The main problem I can see is evening out the fluctuations in heat generated by the solar panel and a pump to circulate the hot water.
The first problem is easily overcome if the pipes (or another radiator) are buried under a layer of concrete slabs, gravel or the such like, to provide a heat store releasing heat more evenly. It will also protect the pipes as you suggest. I would insulate under the pipes and up the sides of the bed so as not to loose heat.
Providing a pump is more difficult on an allotment, although a solar powered one could be considered. Otherwise the solar collector needs to be below the hot bed, so that the water circulates by natural convection.
Being a completely sealed system you can use anti-freeze in the water.

[Harasimow]

Hi Java,

Im planning on making a couple of hotbeds in the Spring using the maure method. Got a good supply of manure some well rotted and some fresh. Used for starting seedlings off a month early.

[grahamhobbs]

Hi Harisimow, it will be very interesting to hear the results of your trial, I can imagine some crops will respond and crop earlier but others may not. Would love to get some feedback.

[Big Al]

Would the length of day have anything to do with this ? I ask in the thought that the days being a lot shorter you might not have long enough to get the water up to a reasonable, worthy temperiture. Still a good idea and I will watch with interest.

[grahamhobbs]

I am sure you will find that the length of day and not mearly temperature will affect progress. I imagine what are essentially plants from hotter climes will respond better, not only because of the warmth, but because they are not tuned to day length (day length hardly varying from winter to summer close to the tropics).
But the practical results of the trial will be very interesting.

[Colin]

Make sure to insulate the hotbed soil from the surounding soil otherwise any heat you do gain from the panel will be lost in short order. Extruded polystyrene (xps) is the only insulation that can be used below ground.

[Cheezy]

I have long thought about this.

A couple of things.

A small radiator painted black would suffice. (I have two in the garage awaiting time!.)

However the principal used in these gravity systems is based on hot water rising, cold water sinking. Basically you need to put the solar collector below your pipe work, then the hot water will raise up to the heat exchange pipework, cool and sink back to the radiator.
You will need a pipe therefore at the top of the rad going out, and the return pipe in at the base.

Secondly, yes you will need to insulate the base so the heat goes up to the seeds. I would cover as previously said with gravel or those clay peas which have a large density and will release the heat, the seed trays would then sit on these. You will also need to cover the radiator with a clear cover to prevent rain and snow cooling it, and provide some insulation.

THe rad should be angled at 45' south for most solar incidence.