Co-simulating greenhouses in buildings
Urban agriculture is becoming increasingly popular in cities, in all its forms. From high-tech vertical farms to long waiting lists on community gardens, there is a recognition that growing food closer to point of use can have greater environmental, social and economic benefits. Now, isn’t that the triple bottom line every proponent of sustainability researches for?
The problem is that quantifying this benefit is challenging. Plants have a complex interaction with their environment, as crops simulataneously are affected by and affect the environment that they are in.
We have developed a method to combine greenhouse modelling and building modelling to estimate how waste flows in buildings could be reused in a greenhouse to improve resource use efficiency. We find that integrating a greenhouse in a school building can halve the energy demand of a greenhouse, while only increasing the school’s energy consumption by 3%. This could yield 6 tonnes of leafy vegetables such as lettuces in a year, enough to feed the entire school with lettuce for a year, no transport required.
This article reviews the motivation behind studying urban integrated agriculture, explains the method used, and presents our estimated results.
Interest in urban integrated agriculture
Food security
Most of our arable land is already used intensively to grow crops. In fact, it is estimated agriculture uses 38% of the land’s surface. Significant problems with agriculture lie in water scarciy for irrigation, eutrophication of waters, and soil erosion.
The first, water scarcity relates most directly to climate change. Our warming climate, coupled with our dependence on rivers and groundwater for irrigation, are leading causes for increased water scarcity. In Spain for example, groundwater reserves are expected to be xx% reduced.
Eutrophication of rivers results from our fertiliser use in conventional agriculture. Fertiliser that is not taken up by the plants, seep through the ground to reach river waters, causing excessive algal blooms in rivers due to the high nutrient content. The disturbance to the river balance can lead to ecosystem damage through negative feedback loops such as less fish, less sediments.
Finally, the major worry of farmers, soil erosion. While this is a natural process, whereby the topsoil is moved away by water or wind, unsuitable agricultural practices have sped this up. The main activities are ploughing, overgrazing, pesticides, and deforestation. Again, this causes casacading effects such as nutrient loss, loss of habitats and wildlife, thereby reducing the capacity of the soil to be regenerated.
These combined pressures on agriculture are compounded by the increasing population and decreasing malnutrition. Of course, it is possibl to feed the world with the resources available, but significant changes need to be made to feed everyone sustainably and equitably. However, of the 3.5 extra billion people on this planet by 2050, 2.6 billion of these will be in cities. So would it not make sense to also think of an urban or peri-urban way to grow food sustainably?
The promise of hydroponics
Hydroponics is the process of growing plants in water without soil. The substrate which replaces soil can be fibrous material like rockwool, or granular material such as perlite or vermiculite. The system is irrigated with nutrient rich water which feed the plants. The benefit of this process is that typically 90-95% less water is used than with conventional agriculture, as the water can be recycled through the system. Fertilisers can also be recycled through the system so their use can be more targeted and they do not end up in wastewater. This system can operate in controlled indoor environments, allowing several yields a year. As such, hydroponics can have much more significant yields per unit area than conventional agriculture, making them particularly interesting in cities where cost of land is expensive.
Urban-integrated hydroponics
Hydroponics in schools
Co-simulation model
I have designed a model that can combine the thermodynamics of plants and that of buildings, in order to estimate the potential impact of the indoor environment on plant growth and vice versa. By combining these two models, we can work out which flows and resources we could reuse from a building to make an urban greenhouse more efficient.
Summary of the method
The details of the method can be found in my thesis and the paper. Here is a brief explanation about how it works.