Building-integrated rooftop greenhouses: An energy and environmental assessment in the Mediterranean context

Significance Statement

World urbanization is projected to reach 69% in 2050 and therefore, food production requires closer examination. Urban agriculture is seen as an innovative solution which can offer sustainable food supplies within the urban context especially due to limited access to green spaces. Therefore soil-free rooftop greenhouses are seen as a viable option for food production. The energy consumption in both buildings and greenhouses is high and recycling of energy, carbon dioxide and water can be achieved through the integrated rooftop greenhouse. The major challenge to this advancement is insufficient empirical data.

In a recent paper published in Applied Energy, Ana Nadal and colleagues reported measured data outlining the symbiotic relationship between the integrated rooftop greenhouse and the building in terms of energy, and quantified the heating energy recycled by the greenhouse from the building.

A 7-storey building which houses 4 rooftop greenhouses each measuring 128 m2 enabled this study. The research team focused on one of the greenhouses with a crop area of 84.34 m2, which uses low-density polyethylene curtains as well as a thermal screen. This improves the internal heat condition and insulates the space from the rest of the building. The thermal screen and curtains are both automatically operated as a function of the internal temperature of the greenhouse. The research period was between December 2014 and December 2015.

The authors noted that the average seasonal temperatures in the integrated rooftop greenhouse ranged from 16.5 °C during winter with a minimum of 6.3 °C, to 25.9 °C during summer with a maximum of 39.7 °C. This was within the recommended optimum range of 14-26 °C. At winter nights with outdoor temperatures dropping to -3.6 °C, the greenhouse temperature was 6.3 °C which was as a result of the thermal inertia that the concrete floor provided, as well as the application of the curtains and thermal screen to minimize thermal loss. This translates to better thermal conditions and energy savings when compared with conventional free-standing greenhouses.

It was further observed that the greenhouse’s thermal behavior closely resembled that of the building’s atrium as opposed to the outside conditions, with the greatest difference being in autumn and winter.

The authors compared the researched rooftop greenhouse with a simulated free-standing greenhouse and noted that the indoor climate of the research greenhouse met the optimum temperature range condition in over 76.3% of annual hours. However, the free-standing greenhouse under similar climatic conditions would meet this condition in 42.4% of annual hours in an unheated condition and 65.1% of annual hours when heated, which is due to higher instances of summertime overheating of a freestanding greenhouse. The simulation result shows a free-standing greenhouse would have required a total heating demand of about 43.78 MWh with a peak heating load of 66.62 kW, and this provides a scale of the total recycled heat by the actual building integrated greenhouse.

A comparison of a heated greenhouse and the research greenhouse, in terms of carbon and financial savings, shows that the former’s heating demand would produce 113.8kg CO2(eq)/m2/yr at a cost of 19.63 €/m2/yr when an oil boiler is used. These savings demonstrated in the study show the viability of integrating greenhouses into buildings.

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Reference

Ana Nadal, Pere Llorach-Massana, Eva Cuerva, Elisa López-Capel, Juan Ignacio Montero, Alejandro Josa, Joan Rieradevall, Mohammad Royapoor. Building-integrated rooftop greenhouses: An energy and environmental assessment in the Mediterranean context. Applied Energy 187 (2017) 338-351.

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