Building of the week – the passive solar greenhouse

Generally speaking, eating local food is going to be better for the environment, as it won’t have been transported long distance. There are exceptions though, and among them are out of season salads and Mediterranean vegetables. British growers can now produce crops such as tomatoes all year round, but in winter that relies on extensive use of artificial light and heat.

There are all sorts of interesting ideas in farming under glass – combined heat and power or rainwater harvesting, for example – so I don’t want to be dismissive of the industry. But it is possible to build passive solar greenhouses that don’t need external energy sources. They’re a proven technology that has been around since the 1970s, but we’ve never really made use of it here.

To see passive solar greenhouses, we should look to China. 800,000 hectares of them have been built in the last 30 years, says Kris De Decker of Low Tech Magazine.

china greenhouse

Unlike the greenhouses we know, the passive solar variety has walls on three sides. They were originally built with rammed earth or with cavity bricks, but are now designed with modern insulating techniques. The southern side and the roof are open to light and covered with transparent plastic foil.

The basic principle is that the walls absorb the sun’s warmth during the day and release it at night, while also blocking cold north winds. To keep the heat in, insulating blankets are rolled down over the greenhouse at night.

Research shows that the passive solar greenhouse can produce crops all year round with no external energy source, even if temperatures drop below zero.

china greenhouseThe downsides? They require more space. You need to leave room for the walls and a sufficient space between greenhouses that they don’t shade each other, so you can’t apply the techniques to the vast land-maximising glass hangars like Thanet Earth. There’s only so far north you can take the concept, although it’s further than you might think. And you don’t have the benefits of additional CO2 that fossil fuel warmed greenhouses have.

Those considerations make it unlikely that commercial growers in Britain will be building passive solar greenhouses anytime soon. But smaller scale farmers and growers could adopt them, and in places where land is cheaper and energy is more expensive, they may well offer a valuable solution – just as they have already in China.

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3 Comments on “Building of the week – the passive solar greenhouse”

  1. marcusasdf February 5, 2016 at 2:27 pm #

    interesting – I didn’t know about these. It’s important to remember that, as the article author points out “The performance of the Chinese greenhouse depends on its design, the latitude, and the local climate.”. Other options are available, and may do better in given circumstances.
    Richard Nelson and others have demonstrated double-skin tunnels where soap bubbles are blown into the space to provide adaptive insulation (at night, on the non-insolated side during slanting-sun daylight, or all over if the insolation is getting too much), and/or water is trickled in a film on the inside of the cover to absorb energy, which is later released when the same solution is used to blow the bubbles. See http://solaroof.org/wiki/ and http://www.solarbubblebuild.com/overview/overview.php (although these sites and newsgroups have gone suspiciously quiet in recent years). This system clearly works, and apparently demonstrators have maintained high uniform temperatures in northern latitudes like Canada, but it’s not clear whether the heat saving/storage outweighs the energy used in pumping the water film or bubbles – or at least whether the system outperforms regular polytunnels which maintain a water heat reservoir/and/or heat pumping technology. I believe some more rigorous research evaluation programmes started, but i’ve not seen any reported results.

    • Jeremy Williams February 8, 2016 at 1:14 pm #

      Yes, this is going to be a useful idea in some places and not in others. I hadn’t come across the bubble insulation technique. That’s quite ingenious and I’ll look that up.

Trackbacks/Pingbacks

  1. Building of the week: the solar PV greenhouse | Make Wealth History - September 16, 2016

    […] featured greenhouses before in my building of the week posts, including the passive solar greenhouses that are common in China. Here’s an other form of solar greenhouse, produced by a […]

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