solar heating for greenhouse

[starloc]

somewhere on the internet ( it may have been on here ) , i saw someone who had set up a powerless solar heater for a greenhouse, using convection to circulate the water,
it was very simple setup, maybe not easy to get right, but it was easy to set it up

Does anyone know where it could have been?, or know how to do it

its something similar to a car radiator for the solar panel and tubes taking the hot water upwards to circulate the heat into the greenhouse and back down after its cooled?

[T8Ter]

Not seen that one Starloc, but if using solar power, wouldn't you want to store the heat somewhere (as per Dick Strawbridge heat sink) as surely if the sun is hot enough to power a panel the greenhouse would be hot anyway?

[starloc]

Ive been given a 100ft by 20ft greenhouse ( will be up as 50 by 40 ) but im putting it up in Bulgaria so during the summer it will be very very hot ,so i can solve that easily... open all the doors around the edge of it to keep it cool,
but in the winter it can drop very low so im going to need to warm it by 15 to 20 degrees C , the sun is hotter in bulgaria, but the temperature is usualy lower i the winter

Im trying to avoid any large bills for heating it,i could use a generator but its noisy and fuel costs are high, Bulgaria doesnt have very good electric supply if last year is anything to go by and where we are there is no gas, just propane bottles , everything is done by wood

i can use a heater run on wood or similar to heat it by having a hollow floor and letting hot air circulate

Due to the temperature outside , im going to have to cover the glass to prevent sunlight getting in if the temperatures are cold inside, so its probably easier to insulate it all with thin polystyrene sheet all over to prevent heat loss, it has wooden shutters to close inside, i will insulate these

hopefully the only electric i will use will be from a vertical wind generator

Ive seen people say they have heated up the greenhouse from outside with a car radiator, the heated water during the day circulates to radiators higher up inside the house so the water circulates , but never seen any figures on how much heat they can produce area for area, any plans for a system they have used that works would be good!

for storage ive seen mention of using a drum of water in the circuit that heats up, some say an old chest freezer works and is insulated ( but would look bad! )

[King Carrot]

Very low tech I know, but barrels of water along the sides - you make the tops of the barrels into benches. I don't know the exact thermal properties of the set up but you can look it up and its free as the water stores any heat during the day - might off set any other heating costs considerably.

[starloc]

Very low tech I know, but barrels of water along the sides - you make the tops of the barrels into benches. I don't know the exact thermal properties of the set up but you can look it up and its free as the water stores any heat during the day - might off set any other heating costs considerably.

Id forgotton about that one, some time ago there was a post on the citrus growers forum where someone did that in the winter to stop frost damage

Its probably a good idea to use it even with heating as it will reduce costs and probably make temperatures more stable

[Potstubsdustbins]

If you are talking about heating water on a hot sunny day and storing this to use over night with out using any electricity then the setup would have to be gravity circulation.

I know this works as a friend and I did an experiment a few years ago just for fun and not in a gardening situation.

First we obtained a second hand single panel radiator 4` long x 2` high. This was set on end in a purpose made wooden box that had been painted black inside. (not sure if that made any difference).

We then led the 22mm flow (hot) pipe from the top of the radiator to a plasitc 10/4 gallon tank some 3ft higher than the highest point on the radiator and connected this pipe as the top most connection some 2/3 of the way up the tank. The 22mm return (cold) pipe was lead from the bottom of the tank into the bottom of the radiator. The setup was filled with a hose pipe. It was a hot sunny summers day and the 8 gallons of water reach hand washing temperature in about 90 minutes.

The thing to remember when dealing with a gravity fed system is they are prone to air locks to avoid this the flow pipe must rise continuously from the heat source to the storage vessel and the return pipe must likewise fall continuously.

The main problem that I can foresee is that at night the tank becomes the heat source and therefore this must be lower than the heat emitter's it is going to serve.

Hope this gives you some ideas Colin

[teakdesk]

I've been interested in this topic for a while but the weather always seemed to work against it.

From my school days, if I take a 40 gallon water butt (180 litres) and I manage to get the temperature during the day to 30 degC then I have a heat source that will give out heat as it cools.

If I want to keep my greenhouse at 10 degC minimum then my water butt will cool by 20 degC.

Each litre of water gives out 4.2 kilojoules of energy for each degree it cools so my water butt will give out 4.2 x 180 x 20 = 15,120 kJ of energy.

If I want my water butt to last all night - say 14 hours in winter/spring - then I can convert the 15,000 kJ of energy into watts.

15,000 / 60 secs / 60 minutes / 14 hours = 0.3 kW = 300 watts.



So if I can raise the temperature of my water butt to 30 degC every day from its starting temperature of 10 degC I would have a reasonable heater.



The problem comes from the heat loss from the greenhouse over night.

If I replace all my glass in my 10 x 6 greenhouse with double glazing this 300 watt heater will keep the inside at 10 degC only when the outside temperature is above 5 degC. So I would need to use 2 water butts both heated to 30 degC every day to keep the plants at 10 degC when it gets to freezing point outside during winter.



The physics says it should/will work (providing you double-glaze the greenhouse with proper double-glazed panels) but only if you can get the water butt temperature up to 30 degC on dismal, overcast winter days.

And I've never seen any DIY plans that would guarantee this in England's climate without spending an absolute fortune!!!!!!!




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[teakdesk]

Pots.... I've got an old radiator rusting in the garden... I feel a project coming on!!!!!





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[rana]

Rather than a water system I would be tempted to go along the 'storage heater' route by painting some stone blocks black, placing them in a sunny part of the house where they collect energy in the afternoon and radiate heat after the sun goes down. I assume in Bulgaria it is sunny in winter/spring but cold at night.

Because a A 50x40' greenhouse is a lot of space to heat it would be better if only a small portion of the space was heated so any use of gas, electric or wood as a backup would be minimised.

[starloc]

Ive just found the post about using drums on the citrus growers forum theres photos of a massive polytunnel owned by ` millet `, from colorado he has vast amounts of drums of water in his polytunnel , temperatures there drop very low in the winter and his citrus and other plants seem to do fine

from the pictures he seems to have rows of drums used as benches, about 5 in a row, 8 rows and more dotted around, theres also a row down one side, each drum has a plant on top, the heat from the drum keeps the pot warm in cold weather, he says hes at 5440 ft altitude so it gets very cold in the night , his plants look very happy , theres got to be something like 70 drums in there

Its probably worth using the drums to keep it warm as it seems to work, and also adding something to heat the air inside using external solar panel heat boxes that work on air convection heating, but i presume im going to have to use something like wood in the coldest weather

[Potstubsdustbins]

Forgot to mention last night. If possible the radiator should 4 x 1/2" tappings this makes cross connections easy i.e. as you stand looking at the radiator if the top (flow) connection goes in on the right then the bottom (return) connection goes in on the left, the other two tappings are of corse plugged. This way the whole of the radiator heating surface is used to collect energy.

This is definitely NOT scientific but a 2` x 4` radiator will emit approx 1100 watts an hour with the water temperature at 55`C. Someone much cleverer than me might be able to work back and give some idea of absorbtion.

Colin

[teakdesk]

Cheers Pots, yes I sussed the cross-plumbed entry-exit for the radiator....

I now have an old 6 foot radiator at the back of my greenhouse (inside) with hoses connected to a smallish tank on the bench. I've lashed it together with a bit of hose (think I got them positioned so no air locks!).

Thermometer in the water tank just waiting for some sun!!!!!




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[Kristen]

Dunno if it helps your sums chaps ... but I have solar thermal [evacuated tube] panels facing due South (hopefully more efficient than a black radiator 'coz for sure they cost a bit more than one!). Each panel is about 2.5 sq.m. and has averaged 2.6 kWh per day since the beginning of March, and the best day so far was 7 kWh (it might be that "best days", being clear, coincide with "Frosty nights" - which would be good!)

From 3rd to 6th then generated nothing

So you will still have to deal with cloudy spells - large tanks to store heat probably not practicable?

Now ... if you were talking about June they would give you all the heat you could want ...

[rana]

The penalty for good thermal capacitance in water is inertia. Once the water has gone cold it needs a heck of a lot of energy to warm it up again. This may be OK on a clear sunny day but when you only get a few hours of afternoon sun, if the water is in barrels or tanks it will not warm up nearly enough. That is why traditional solar collectors are thin panels with a large area and low water volumes.

[starloc]

This guy uses the barels of water and gets good results

Citrus Growers Forum :: View topic - Millet's Greenhouse

Quoted from `millet` on the citrus forum ( caluculations he has ( from this post Citrus Growers Forum :: View topic - Water structure ))

Quote from `millet` on citrus forum...."I have twelve benches pluse various raised beds, which are used for planting in the ground. Of the twelve benches, eight of the benches are made with black plastic 55-gallon barrels filled with water. Each bench is comprised of seven water filled 55-gallon drums placed in a row, that are black on the south side which faces the sun, and painted white on the north side to reflect light around the greenhouse. A gallon of water weighs 8-lbs., so a 55-gallon drum contains 440-lbs. of water, or a total of 3,080-lb. of water per bench. In total there is 24,640 -lbs. of water in the 8 benches. Also The outside walls are lined with black plastic 55-gallon drums which also act as a long bench. The outside rim is made up of an additional 40 drums. In total the greenhouse "benches" are made up of 96 X 55-gallon drums, which contain 5,280 gallons of water, which is equal to 42,240-lbs. of water. This gives a lot of free heat. By defination, one BTU is the amount of heat required to raise one pound of water one degree F. Or, when one pound of water loses one degree of heat, it gives off one BTU. Therefore in my greenhouse every time my water barrels drop one degree in temperature during the night, I gain 42,240 BTU's of heat. At this time of year when the sun is nearly at it lowest level in the southern sky, the average water temperature in the barrels at dusk is 74F. In the morning the average water temperature is 66F +or -. This is an average drop of eight degrees during the night X 42,240-lbs. of water = 337,920 BTU's of free heat per night. Or 10,376,600 BTU's of free heat per month during the winter. months During the summer the water storage works in reverse and helps cool the greenhouse. All this is free heat, plus it give me the use of 8 two foot wide, 15-ft long, 4-ft. high benches to set my plant on. I'll take some pictures and put it on this forum in a couple days. - Millet"