This project evolved out of part whimsy, part solid logic. Which is how a lot of projects here have evolved. The full story is in this post.
The greenhouse climbs a southeast-facing hillside and, along with the solar panels just above it, gets the most winter sun of anywhere on the quinta.
The back wall is solid bedrock, giving it a large amount of thermal mass. It’s also terraced internally with schist walls and levels, one of which contains a pond, adding considerably to the surface area and volume of thermal mass present. Consequently it has a lot of heat storage potential.
A central bed forms the main growing area for tropical and semi-tropical plants. The 4.5m available height allows us to grow things like bananas, while round the base of the bananas there’s room for heat-loving summer vegetables like sweet potatoes, capsicum peppers, chillies and aubergines. These don’t do so well on most of the quinta because its soils are shaded and slow to warm in spring
Subterranean heating and cooling system
Even greater use of the potential of all the thermal mass in the greenhouse is made with a subterranean heating and cooling system (SHCS).
This works by forcing air through underground coils of perforated tubing. The coils are supplied at one end by vertical ducts containing fans. At the other end, they discharge air back into the greenhouse at or near floor level.
The fans are controlled by the temperature difference between air and soil.
During sunny days when the air is hotter than the soil, the fans come on and suck hot air down the vertical ducts into the coils underground where it cools and the moisture condenses out of it. It emerges back into the greenhouse cooler and drier.
During cold nights when the air temperature falls below that of the soil, the fans come on again and the reverse happens. The cool air is taken underground where it’s warmed and moistened. The system is designed to take some of the extremes off our annual 50°C temperature range and runs off the solar system.
The height of the geodome allows us to stack growing areas vertically as well as horizontally. We use 3 linked scaffolding towers round the periphery of the lowest part of the greenhouse. These accommodate several layers of seed cultivation, while above, on the top of the scaffolding, are aquaponics tanks.
These tanks are planted up with annual vegetables and recycle the water from the fish pond. Dirty pondwater is pumped round the tanks which flood and drain in a regular cycle. The plants growing in the tanks clean the water at the same time as being fertilised by it. The clean water returns to the fishpond again once it’s drained through the growing tanks.
Why grow in a system like this when there’s plenty of land for growing on the quinta and a good warm temperate climate?
It’s a closed-loop system. Once set up, the only additional water it needs is to replace evaporation and transpiration losses. With water becoming more and more of an issue as Portugal desertifies, a system for growing food which doesn’t require irrigation is a very useful thing to have.
Bathroom and sauna
The greenhouse also contains an open bathroom area, designed as a ‘wet room’ type space with a solar-heated shower draining into the central bed. The solar heating coil for the shower sits on the rainwater storage tanks on the slope behind the greenhouse, so the shower operates on a simple gravity feed.
Bathrooms and greenhouses work well together. Bathrooms provide water, warmth and moisture. Greenhouses can clean the grey water while making good use of it and help heat the water for the bathroom.
A small sauna is planned as well. The sauna will provide heat and moisture for the greenhouse, especially in winter, making good use of the fuel used to heat the sauna. Having the sauna in the greenhouse will mean less fuel needed to bring it up to temperature.
The middle of the central bed and its banana circle contains a mulch pit. The water from the shower drains into it. The vegetative waste from the greenhouse goes into the mulch pit and also into an underfloor worm farm beneath part of the pathway around the central bed.
Juices from the worm farm are collected and used to feed the greenhouse plants. The banana circle mulch pit feeds the central growing bed directly.
A compost tea-making station produces 250-litre batches of actively aerated compost tea for use in the greenhouse and elsewhere on the quinta.
Fires don’t play nicely with plastic. Especially not fires on the scale and ferocity we experienced in October 2017. Before leaving the quinta, I’d set a sprinkler going on the roof of the greenhouse, but it was one of those cheap Chinese ones that jam almost as soon as they start. I didn’t have time to try to fix it.
Had I had the benefit of hindsight and/or been using my brain earlier in the afternoon, it would have been so simple to remove the greenhouse cover and put it somewhere likely to be safe from the flames. Although predicting where would be safe from the flames wasn’t quite so easy. Other stuff I put away in what I thought was a safe place (the bottom room in the wee house) turned out to be entirely unsafe, so you win some you lose some …
The greenhouse burned of course. It wasn’t a particularly hot fire – the 4,000 litres of rainwater storage in the tanks above it were released as soon as the pipes burned through and put out the fire below. As a result, we have a lot of melted plastic everywhere which elsewhere was vaporised.
All the aquaponics tanks melted and/or vaporised, either wholly or partially. The compost tea-making tank disappeared completely and the air pump was destroyed. All the electrical wiring burned out, though the control box for the SHCS remains intact. The vertical ducts and fans for the SHCS system melted and ended up at all sorts of interesting angles. The plumbing for the bathroom and aquaponics systems burned. The wooden worm farm floor survived with one burnt hole which is easily repaired. All my tools where I’d been working in the bathroom area burned.
The banana survived. And has survived the cold winter following the fires with no protection. Its will to live is an inspiration and keeps me going when the daily panorama of devastation occasionally gets too much.
It was a while before I could face the clean-up in the greenhouse. We eventually got around to it in Spring 2018. At some point I will attempt to raise the money to replace the cover and start again, but at the moment it’s not top of the list of priorities.
Meanwhile, there’s some drone footage of the greenhouse as it was in this promo video (made for Liam who made the dome).