"Delta" Solar Heat Collector and Soil Warmer for the Greenhouse


Introduction: "Delta" Solar Heat Collector and Soil Warmer for the Greenhouse

About: I am a stone mason. My hobby is making new solar cooking and gardening stuff. I have used solar heat to cook soil for a couple of years. In mother earth news in January, i read that their compost expert does...

This project is at proof of concept stage. In my experiment, water at 8 degrees to 10 degrees C is dumped on the top of the dripwall and drips to the bottom. On its way down it collects heat from the material and the water temperature increases by 2 to 15  degrees C. (depending on if the sunshine is on the This warmed water drips into the soil and takes its heat to the bottom of the container,  The water returns to the bucket usually  a few  degrees warmer than when it left it.   Water is continuously pumped out of the bucket and onto the dripwall during sun hours. The Dripwall collects solar heat,  it transmits this heat quickly to the water.  The original idea was that it would collect heat from the air of an over heated greenhouse and transmit it to the water, thus cooling the air and storing the heat at the same time. Most people just open a vent when their greenhouse overheats but this involves serious energy loss. Some greenhouses can overheat in the winter and then later in the day, they need the power on to maintain the temp!   But if we stored instead of vented, it would be more logical.    However my greenhouse will not get hot until April or May and it is serving primarily now as a solar heat collector and to move that heat down under the plants to make them grow.   This is an attempt to cheaply solve some difficult greenhouse problems and to use solar energy effectively. It is not a finished product but it is several steps in the right direction.  (And it works!). Soil has rotten thermal conductivity,  you can heat the top of the soil and it will take ages for the heat to transmit down into the soil. But if you drip warm water on the soil, the problem is solved,  the water takes its heat with it down and in.  Also, water has really high heat capacity about twice that of soil so it is like little heat "bullets" going down into the soil.  Water has over 3000 times the volumetric heat capacity of air and this means that 3 liters of water has the same heat capacity as all the air in a little 6 by 8 greenhouse!



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    14 Discussions

    At the end, you make a statement, paraphrased hear, that a hydronic system will probably be more efficient. I will agree with that more especially if you seal to reduce the surface area of water exposed to air, because heated water evaporates carrying a lot of heat (heat of vaporization) to the walls of the green house where it condenses releasing that heat to the walls.

    Naturally you need some moisture in the soil with regular watering, but if you keep all your solar heating water sealed in the solar collector, pump, pipes and thermal storage water barrel with a cover over it, then you can reduce the humidity in your green house so that you don't loose as much heat to the walls, as well as keep the pump and pipes clean of debris.

    The soil can only hold so much heat, and thus it is good that your storing the extra heat in the mass of the water in the barrel.

    I am thinking in my design I will use a pump to pull warm water out of the bottom of a barrel and push it through a hose along the bottom of a plant bed, circling up to a sealed solar collector where it gets hot and then drains back to a barrel storing the excess heat. With a more efficient system and larger solar collector, I might consider a thermal switch to shut the pump off if the water is above a certain temp, maybe 90F or 32.2C, and let the natural plant respiration and soil evaporation warm the air.

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    I agree with you. Unfortunately this summer I was way too busy and the spider mites took care of everything for me. In Summer, in daytime, (and depending on the water temperature in the dripping system) you "might" be getting things backwards! If the water is cool enough, as it drips down, water from transpiration "may"  be condensing on it and drying and cooling the air in the greenhouse!  (This is totally dependent on the humidity of the air, and the temperature differential).   It is one of those counter intuitive things that probably happens at least  some of the time. I had planned to test it but I am too busy at work to ever get it done this year.   Perhaps you could test this with your hydronic system.  If it "Sweats"  this is probably happening.   Thanks for your input, sorry I could not do more.  I did get the earliest peas (in an unheated greenhouse) in Victoria and some of my gardening friends were jealous (because they are way better gardeners than me).  Brian

    This idea and this project is very young. I run the water constantly to see what will fail first. Here are some of the things I have learned. As you might expect, things in physics are often reversible. This is the real world with weather and temperature variations so I have to give ball park figures based on my very imperfect greenhouse. At night the window screen radiates heat. (So best to turn it off at night) Mine reduces the temperature of the water by about 1.5 degrees C from top to bottom at night. During the day there is about 3 degrees C of a rise. I expect a greater daytime rise when the sun shines over the tree and actually hits the screen. (later in the year).

    6 replies

    Pretty neat idea, if you color the screen black with high heat aerosol spray it should increase the heat absorbtion of the aluminum.

    I have a plain black piece of steel 150mm x 150mm x 2mm thick in my solar water distiller that I use to heat up frozen food. From 1pm to 5pm the steel gets to 103 deg C as opposed to the black glass base which reaches 75 deg C.

    Thanks, Not sure about painting it black, might affect the conductivity of the wire. My window screen is aluminum wire and that means it has a core of aluminum (SUPER heat conductivity!) with a thin layer of alu oxide on the outside. The alu oxide does not have good heat conductivity at all. (I have checked the heat conductivity properties of materials online and there are many surprises). So painting (adding a layer with (probably) poor conductivity) is something that side by side tests would show. Another thing that might be interesting would be black fiberglass insect screen. It is a great absorber for the air heating people. It should be a great absorber for this too but will it conduct the heat away to the little drips of water? Again, side by side tests are needed. Mother earth news magazine just shared one of the videos and comments have been extremely negative on the first day. It is scary when people quickly judge a proof of concept demonstration. There are hundreds of ways of doing this and I could never cover them all. Brian

    Great work. I also was thinking of using water to transfer heat to the soil and store it there.

    All aluminum exposed to water and air is coated with a thin layer of aluminum oxide; but the layer is so thin that its lower conductivity, then that of aluminum is practically negligible in most applications. Heat conductivity works well with heat sinks and heat exchanges, but aluminum's heat conduction does you no good hear. Even worse aluminum tends to reflect most light instead of converting it into heat.

    Instead, you want to create heat, so that the water can take it deep into the soil. What you need is any flat dark black surface to absorb sunlight. The black paint gets hot when exposed to the sun and transfers heat to water in contact with the paint over the surface area of your solar collector. Since water is a good conductor of heat, you really don't need the added surface area of a screen.

    I hear you, I was thinking that the blacking of aluminium heatsinks being more efficient, might work in a similar vein with your application. Perhaps a black etch primer for aluminium might overcome the alu/oxide issue.

    I understand your reluctance... if it doesnt work you might need a lot of paint stripper. :)

    With the screen as a back to lay things on, it was easy to test other materials. Best so far is oven pans. Things that were not good included plexiglass, aluminium foil, pond liner (heavy black rubber) and black landscape fabric and a black shopping bag. I am lucky because I have one thermometer that measures to 0.1 degree C and is very sensitive so I can quickly discover what works and what doesn't. Perhaps if I paint things black they will be better but only so much I can do. Latest vid is long, it is at


    The blackening might work great, I just don't have time to try it. My greenhouse is in the wrong spot to test this at this time. It has partial moving shade for the next couple of months so I would be mad to do side by side tests in it. But I do my best. I have to leave proper tests to others who are better situated. ( It might never be done). People are lazy. There are millions of retired people puttering around who could do this to make their life meaningful and to make the world a better place for their grandchildren.

    The heat collector is a work in progress. I changed to sheet metal instead of window screen (It gives much better heat transfer to the water.) So now I can get 8 to 10 degrees C of heat gain in the water when it is sunny. I have tried lots of different material at this stage and baking tins seem best. (for now)

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    I am guessing the reason sheet metal is working better than the screen are that aluminum reflects more light, and that the screen may allow some light to pass through it.

    Substantial changes are in place. So when you read this, I will probably have the video above changed to a more "How To" video. With slate or sheet metal as the drip wall, I get 2 to 5 C of warming when it is cloudy and 10 to 15 C of warming when the sun shines on the wall. That is at about 3 liters per hour water drip. Charred wood is also good but slightly poorer than the other materials. 10 to 15 is 18 to 27 F so nothing to be sneezed at.

    reference design delta dripwall heat movers.png

    What if you paint in black the screen?

    Maybe not thick paint, but that used for permanent markers.

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    Might work.. I will probably use aluminium kitchen foil behind it at some stage too (as a reflector), maybe even use foil instead of window screen. (easy to spray the foil black). The Window screen is really for later in the year to suck heat out of the air (in an overheated greenhouse) and transfer it to water. But kitchen foil probably might do the solar heat absorber thing effectively. Right now I am searching for the cheapest system possible. Not really concerned with efficiency. Economic efficiency often means that a cheap inefficient system has a quicker payback time than an expensive one. Thanks Brian