Editor’s Note: As many of you will have noticed, Alex has been making some great practical updates on the work going on at the Strawberry Fields Eco-Lodge. The following article is another good example. I thought I’d mention that if you want to soak in some excellent experience at this site, Steve Cran will be leading a great course beginning July 1, 2011 that you might want to attend if you can.
The heat chimney for the solar fridge
The solar fridge is a new system which we have now managed to get set up after months of pondering, trying, adjusting, tweaking and trying again. We think we have finally got it kicking and pretty well integrated into the other functions of the kitchen area, so we can demonstrate permaculture principals with it pretty nicely.
The system is based on an old design for desert/dry-land cool storages which makes use of a heat chimney to create an up-draft which then sucks cool air in to the storage chamber from below. This air may pass through a long tunnel in its way to the storage chamber and hence be cooled by the ground on the way to the chamber. In order to enhance the cooling of the air on the way to the chamber, if possible, water, by evaporating will take in thermal energy, causing the temperature of the air to fall further. The main logistical issue to deal with, as usual, in building the system, was getting the theory to work in practice using available materials. Most of the construction work on this project was done by one of our long term volunteers, Duncan Colquin from Herefordshire, England, so a big thanks to him.
A 1000 litre water tank is ready to be buried in insulation
The chamber of the fridge is an old 1000 litre water tank. The wooden tower that was holding up this tank collapsed in heavy rains two years ago and the whole top face was ripped off. However the other five faces of the cubic tank were pretty much intact. We turned the tank upside down and put it in the trench which we had dug for it. It was then covered with old plastic trash (bags, flip-flops, wrappings, bubble wrap etc.) as insulation. This was then all buried and a terrace built around the whole thing. A door was fitted onto one face of the tank, which opens up-wards, and this faces north to avoid receiving direct sunlight.
Fitting a good door on was a tricky part
Kusse the master terrace builder
The engine of the system though is the heat chimney. This does part must get direct sunlight. It is a just a piece of 1mm sheet steel rolled into a tube 6m long. It is painted black to catch the heat of the sun. When the tube warms up so does the air inside it and this causes it to rise, creating an updraft which causes a flow of air though the whole system. The tube is very long to increase the heating effect and also to stick up into the wind. Wind will also create a draft through system when there is no sun – so it is actually both solar and wind powered (multiple elements support the function)!
But perhaps the most important and definitely the most challenging part of the project was the cooling tunnel. The question was how to make a tunnel which has porous walls, to let moisture seep through into the air as it travels along the tunnel, but is strong enough to stand the pressure of being buried under ground. Finally we came up with the idea of rolling heavy wire mesh into a tube and wrapping sack cloth around this, which could then be buried in sand. The first time we tried this, the tunnel collapsed, so we had to dig it up again and re-enforce it. To do this we bent 8mm rebar into rings and wired them into metal poles made from old deck-chairs, to create a rib cage which was then wrapped inside the wire mesh and the sack cloth before being buried again. The tunnel is 17m long in total, but the trench is only 12m as the tunnel doubles back on itself as it runs under the washing area.
A wire mesh roll will be wrapped in sack cloth. This later had to be reinforced
to stop it from collapsing under-ground. The porous wall allows water to
evaporate into the air passing throught he pipes so cools it down.
The tunnel is a wire mesh role, re-inforced with rings of re-inforcement bar,
wrapped in a hessian sheet, which is porous.
The washing up area is all important in the design. We needed a source of water to keep the sand around the tunnel cool. In permaculture every function should be supported by multiple elements, so we make use of several water sources for this. One is the drip dry water from our dish-drying rack.
Starting to look good now all the terraces are in!
The outflow from this runs along another buried pipe which is buried just above the cooling tunnel. It is perforated at intervals to moisten the soil along the length of the cooling tunnel. This is also amended with grey water from a sink used by guests and staff. Since we eat with our hands in Ethiopia we always wash hands before and after eating, so plenty of waste water to use here!
This dirty water passes though a sand filter before entering the same perforated outflow pipe. Thirdly, on top of this whole apparatus, we have (mostly) set up a little clay-pot garden. This is a terrace which sits just above the cooling tunnel. It is filled with a mix of sand and compost about 30-40cm deep, inside which are buried 12 clay pots. Since clay is a porous material, water seeps through it. As is well known this leads to a cooling effect, the same one used to cool the fridge itself. But since these pots are buried in the sand-compost mix we can plant tomatoes around them which will take up the water as it seeps out. Furthermore we can put drink bottles into the water in some of the pots as this will keep them cool too, saving us space for veggies to go in the fridge itself! And of course the clay pot-irrigation helps keep the ground damp above/around the cooling tunnel too. A lot of mutually supporting connections in there!