Technology: Greenhouse plans to reap the sea wind


By ANDY COGHLAN Tomatoes, lettuces and other juicy vegetable crops could soon be thriving along some of the hottest, most barren coastlines in the world. If all goes according to plan, they will grow in the first greenhouses in the world to be irrigated with seawater. Researchers from Britain, Portugal, Spain and Greece have joined forces to develop the greenhouse as part of a £750 000 project supported through the pan-European Framework Programme of collaborative research. They hope to erect a prototype greenhouse next year on Lanzarote, one of the Canary Islands. ‘The goal is to develop and demonstrate a cost-effective means to provide water for agriculture from the sea,’ says Charlie Paton of Light Works, the company in London that is coordinating the project. If it succeeds, he says, communities living along barren coastlines will be able to grow their own salad vegetables instead of importing them. Hot, moist sea air and strong prevailing winds make the goal achievable, says Paton. ‘We want to desalinate water the way God does it, by taking water vapour out of the air,’ he explains. Essentially, the greenhouse will act as a funnel for warm, moisture-rich air blowing in from the sea. A wall of evaporative filters facing into the wind traps the moisture and acts as a wind block to protect the plants. Condensed water drips down the filters and is transferred into the greenhouse, raising the humidity. The double-layered roof serves a dual function, shielding the vegetables below from scorching heat yet admitting the wavelengths of solar radiation which are vital for photosynthesis. Paton says that commercial filters capable of doing this are too expensive, so he is experimenting with cheap materials that are, he says, almost as effective. The best candidate is a mixture of Ribena and Campari bonded with salt and sugar. ‘Unless the solution is cheap, it’s not worth bothering about,’ he says. A shallow cavity dividing the upper, transparent roof layer from the solar filter layer below traps heat and raises the moisture of the air sucked inside. The humid air swirls down to a heat exchanger at the rear of the greenhouse. Humid air arriving both from the roof cavity and the greenhouse itself condenses on aluminium pipes carrying cold seawater piped in from 3 kilometres offshore. ‘We’ve calculated that at 13 degreeC it will only be 1 degreeC warmer by the time it’s reached the greenhouse,’ says Paton. The system also allows for capture and recycling of the water vapour discharged by plants as they transpire, so virtually no water is wasted. Any surplus freshwater from the heat exchanger is used to irrigate plants and trees grown on the leeward side of the greenhouse, sheltered from the wind. Paton says that most of the energy for pumping up the cold seawater would be harvested by windmills mounted on the roof of the greenhouse at the front, and supplemented by the energy from solar cells. ‘In Lanzarote, the prevailing wind is very prevailing, with 80 to 90 per cent of the wind coming from the northeast throughout the year,’ says Paton. ‘We would therefore have the largest surface area (of the greenhouse) facing the wind, with wind protection in the lee.’ So far, Paton and collaborators have focused on the thermodynamics of the system, evaluating the trade-offs between such variables as temperature, relative humidity, air flow, light intensity and wind speed. They have also examined the aerodynamics of the system with the help of computational fluid dynamics and tests on scale models of the triangular greenhouse in a wind tunnel. Ultimately, he says, they hope to develop a ‘black box’ which accepts data from monitors and sensors around the greenhouse and optimises conditions to suit the vegetables. It will also make as much fresh water as possible when conditions allow, and store the excess. Another, slightly more ambitious objective is to isolate plant nutrients and fertilisers such as nitrogen, phosporus and potassium from the cooling water. Paton explains that the seawater for cooling would be piped from depths of around 100 metres. Sunlight cannot reach such depths,
  • 首页
  • 游艇租赁
  • 电话
  • 关于我们