The Concept
The idea of a greenhouse heat sink is not new and many examples can be found on the internet, using various technologies and different novel approaches. The heat sink is used to help regulate the internal temperature of the greenhouse, the main benefits of building a heat sink is to extend growing season and provide frost protection.
The principle relies on the sun to generate warm air in side the greenhouse, the warmest air then collects at the greenhouse apex.
The heat sink we aim to build will be based around transporting the warm air from the upper parts of the greenhouse, then transporting this air down into an underground chamber. The chamber will be filled with a material that possess good thermal conductivity properties, offers a good surface area for the air to pass over and also a large enough thermal mass to ensure sufficient energy is stored.
The warm air will be drawn into a pipe or pipes using a fan and forced down into the heat sink, where it will passed into the chamber with the heat sink storage material. As the air passes through the material the warm air is cooled by the material as the heat is transferred from the air into the material, thus storing the heat.
Depending on the heat-sink size this system should help to regulate and keep the greenhouse temperature with an improved yearly mean value compared to the ambient temperature. A larger heat sink will allow a larger amount of energy to be stored, but the key factor here is the sun as any stored energy will be constantly bleeding from the system, so a long period of no sun will allow the system to cool significantly.
The image below was created to visualise what a finished system could look like.
Goals
The aim is to make the entire system as accessible as possible and share it as Open Source:
Growing Season Extension: allow plants to be sown earlier and grown later due to improved frost protection and higher mean average temperatures.
Heat Sink Volume: provide some data based on a heat sink to greenhouse size ratio of 1:10, this should give other users a potential benchmark to follow.
The system should also be fairly easy to install, use recyclable materials where possible, use easily accessible parts and run off a solar panel and battery ideally.
How
Our aim is to validate a heat-sink concept through material testing, as well as using two similar sized greenhouses: the first will act as a control providing base-line readings with no heat sink and the second will have a heat sink system in operation.
A simple data logger built from an Arduino will monitor and log temperatures at various points. The logged values will then provide a data set to be analysed and the heat sink performance evaluated.
As well as testing the systems performance and efficiency, all current air heat-sink systems lack a control element, the fan driving the air is kept at a constant speed. Implementing a more intelligent system based on a microcontroller will allow a simple algorithm to be used that controls the speed of the fan depending on the various temperature measurements. The fan speed control will allow the heat sink stored energy release to be throttle depending on the ambient temperatures so the stored heat energy is used more effectively.
The Team
Andrei, Daniel, Jens, Ant and Christian (not in the image)
Which material did you use in the heat sink underground?
Hi, we ended up using 20-30mm granite chippings as easy to source and spread bag in a colleagues drive once we dismantled the prototype, brocken up concrete is a good option,free rubble is often available and you are recycling a material.
Ant
I’m thinking of building a greenhouse and incorporating a heat sink, so I’m really interested in the results of this experiment. Are they posted anywhere? I’d love to know how effective it was.
Any news on this? 🙂
Hi,
Sorry for a long overdue reply, yes there is I updated the prototype details page and will try and have the full 3 week test data up soon
I have a greenhouse ive been working on, i ran it year around all last season, (central nebraska) i did double layer 6mm walls, a 220 volt garage heater and it worked but was very expressive, not worth the cost at all. I am now digging a heat sink and alot of what i have read about says to bury pipe cover with gravel, bricks, anything that obsorbs and holds heat value. I also hear people say use 55g drums with water, it takes up way to much space to work for any kind of small, 15x7x7 GH, i have looked up if anyone has tried to use a underground water tank to hold the heat and have not seen anyone try this.
My idea was to coil RV pipe in a 300 gallon water tank, suck the heat out of the GH, through the pipes to heat the water and blow out cool air for summer and hot air for winter, if my idea worked and i still needed extra heat for winter i could drop a water tank heater into the water through a pvc pipe to add extra heat. Water ( from what ive read) holds heat better than anything else. The problem is, if it doesnt work, thats a very big hole to redig!
Hi,
Running it off electricity is definitely not cost effective, I guess this is ultimately down to the value you and others place on your crop. As you say anything that absorbs heat and is of sufficient density to provide a good thermal mass will work.
A very basic water heat storage system to test would be to simply place a dark coloured water tank inside the greenhouse (sunniest spot), this would then act as a thermal mass and requires no digging. This might be a simple way to test the effects of the system, but you need to have some baseline measurements to test the systems effectiveness, as it may provide heat for a few hours after sunset when you need it most a few hours before sunrise. A more complex system could be to use some type of absorber plate/s around the water tank which thermosiphon water during the day and are shutoff at night, the tank could then be insulated on the sides which face the greenhouse external panels and the sides which face into the greenhouse can remain uninsulated allowing infrared radiation to more easily radiate out.
Water is a good option and something I want to explore, I currently don’t have a garden but wanted to dismantle an old refrigerator and up-cycle the heat exchanger to flow water through. This will need a basic control system so the heat can be used at the most advantageous times and cooling would simply be the reverse.
Ultimately these systems rely on solar energy so depending on your latitude will have a reduced effect in winter months the nearer you are to the poles. For cooling in summer the best system for a greenhouse is to use the small hydraulic window openers in combination with some sort of shade system.