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.
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.
The 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.