Ways in which you can heat your polytunnel naturally?

It is a massive undertaking to use, say, manure to generate heat for a greenhouse. It was OK in Victorian times when there were plenty of horses "on the estate" and the labour for the huge effort of digging out the old manure, and replacing it with fresh, as cheap.

The sun has only 10% of the power in the Winter that it does in the summer. So whatever you do with black water drums, or "passive solar" is only going to reduce the rate of heat loss, not provide an alternative to supplemental heating.

Insulate. That's the cheapest form of heating - e.g. line it with bubble wrap. Plant a hedge, or erect a fence, around it to keep chilling wind off the structure.

Conservatory attached to the house, particularly a lean-to with a relatively large surface area of brick wall at the back, will need far less heat than a stand alone structure.

Get a really accurate thermostat (you won't get one for less than £50) for whatever heater you do use, so that the hysteresis is kept really tight. That way you won't be paying for fuel to overheat the greenhouse by several degrees before the [cheapie] thermostat cuts out.

You need to heat the greenhouse when we get night minimums of -10C for several days at a time, even a week sometimes, during which time the greenhouse probably won't climb (with sun's rays alone) above 0C during the day. No passive heating or black water barrels is going to heat that, all they will do is mean that initially the temperature falls slower than a greenhouse that doesn't have those features.

It is a futile task in a polytunnel as the majority of heat is lost through the thin polythene and the gaps around the doors and vents if you have them. Passive solar heating or a 'hot bed' system would only work in a well insulated greenhouse with polycarbonate or glass panels. The temperature difference between the outside and inside of a polytunnel in deep winter is often only one degree, and it is the protection from wind which gives the advantage for growing and protecting semi hardy winter crops.

A better approach if you are concerned about over wintering tender plants in your tunnel, would be to make an inner greenhouse within it, such as a large cold frame, and to heat this. I have successfully used several dozen old plastic paraffin containers filled with water against a south facing wall as a passive source, but this was inside a timber and polycarbonate 'solar' house. Polythene really will not give you sufficient protection.

I'm experimenting this year by putting a cheap blowaway greenhouse inside my main polytunnel and having a fresh manure hotbed surrounding it. I know this will only slow down heat loss, but unless we have an extended cold spell, I'm hoping I can keep this 'core' just above freezing.

It will help insulate but you need to be careful to avoid rot in a damp atmosphere due to lack of ventilation etc.