12% of water consumption, and 38% of carbon dioxide emissions. [1] Figures for Europe are similar. [2]

In a less tangible shift, the natural world is being marginalized and green space is increasingly remote as people live and work in ever-taller structures.

Agriculture has an equally significant impact on our world. Modern agriculture feeds billions of mouths every day, but is the world’s largest consumer of land and water, the source of most water pollution, and the source of 15% of the world’s greenhouse gas emissions. [3]

Once grown and harvested, fresh produce travels an average of 2500 km to reach U.S. cities, adding to traffic congestion, air pollution, and carbon emissions. [4] Exacerbating the strain, global warming is predicted to lead to widespread shortages of food, water, and arable land by 2050.

Today’s notion of green building is not green enough, nor widely enough applied, to reverse these trends. A more aggressive solution is well within reach.

Growing food crops on buildings can reduce our environmental footprint, cut transportation costs, enhance food security, save energy within the building envelope, and enrich the physical and psychological comfort of building occupants.

The Vertically Integrated Greenhouse:
The Vertically Integrated Greenhouse (VIG) is a highly productive, lightweight, modular, climatically responsive system for growing vegetables on a vertical curtain wall facade. The system is achievable with extant technology.

Hydroponics, the culture of plants in water, is a technically sophisticated commercial practice in most regions of the world. As publicly demonstrated by New York Sun Works at the Science Barge greenhouse in Manhattan, recirculating hydroponics can produce premium-quality vegetables and fruits using up to 20 times less land and 10 times less water than conventional agriculture, while eliminating chemical pesticides, fertilizer runoff, and carbon emissions from farm machinery and long distance transport.

In the building sector, the double skin façade (DSF) is an innovation which can reduce the energy used for space conditioning in modern high rise buildings by up to 30%. [5, 6] A DSF consists of a vertically continuous void space enclosed by a second curtain of glazing over the entire facade. A DSF provides solar heat in winter, buoyancy-driven cooling flows in summer, and allows opening windows year round. Despite these advantages, DSF applications remain limited due to economic concerns and the need to install a large shading system within the cavity to realize the full benefits.

The Vertically Integrated Greenhouse combines a DSF with a novel system of hydroponic food production, for installation on new high-rise buildings and as a potential retrofit on existing buildings. In addition to producing food, plants can reduce building maintenance costs by providing shade, air treatment, and evaporative cooling to building occupants. [7]

System Details
The Vertically Integrated Greenhouse is applicable to a broad range of buildings. For demonstration purposes, a net zero impact high rise building conceptualized by Kiss + Cathcart and Arup, known as the 2020 Tower, was chosen to model the system.

Key features of the VIG:

• A glazed curtain wall (a “double skin”) is located 1.5 m outside the southern façade. The void space behind this curtain wall is the VIG, containing stacked rows of hydroponic vegetable crops.
• The VIG is structured in modules that are 40 m high. Crops are cultivated in innovative plant cable lift (PCL) systems, composed of two wire cables looped around pulleys, driven by a computerized motor on the farming level. Shallow trays of plants, 2.0 m long, are suspended between the cables by swiveling clamps at each end.
• The PCL design is based on a well-established hydroponic method called nutrient film technique (NFT). A thin film of water runs along the bottom of each tray, delivering nutrients to the roots of leafy plants, before flowing down to the next tray. The solution is recovered at the farming level for reuse. Transpiration is limited to 10% of the flow rate by design.
• Seeds are germinated in flat trays on the bottom level, and planted into the bottom tray. The trays rise up the front of the facade, pass over the pulley, and down the back, returning to the bottom for harvest. The entire trip takes approximately 30 days.
• The vertical alignment of the front and back trays can be controlled by a slight turn of the pulleys, similar to adjusting a Venetian blind. This feature allows the VIG to track solar elevation in real time throughout the day and year, optimizing light capture. Occupants can see out of the building through the ‘slats’ formed by the dual row of plant trays.
• Vertical spacing between trays on the cable can also be varied. Rows will be more tightly spaced in winter, when the sun is lower, resulting in steady yields year-round.
• In winter, the VIG is an effective solar capture device, warming and insulating the glazed façade of the building. On winter nights, exhaust air from the building can be ducted to the VIG to maintain plant temperatures.
• In summer, the VIG shades the interior of the building, and provides a source of fresh air to occupants with opening windows. The VIG reduces solar heat gain by absorbing energy as latent heat, through transpiration. The VIG mitigates the urban heat island effect like a green roof, but over a much larger area.

Economic Viability
A south facing, vertical glazed façade in New York City admits a remarkably even distribution of sunlight throughout the year. [8] Compared to a conventional greenhouse, the VIG provides increased production in winter, when produce prices peak.

Each PCL is expected to yield 3000 kg of fresh produce per year in New York City, based on the orientation, spacing, and light levels of the VIG.

Research on existing buildings in Europe indicates that a DSF adds approximately 5% to the cost of a new high rise. [5, 6] Other studies indicate that a ‘green’ work environment raises productivity by 1.0 to 1.5%, representing a net present value in the U.S. of $400 to $600 per m2 of floor area. [9]

Table 1
VIG costs and benefits, per unit area of rentable space [10]
*All figures are per m2 per yr*
VIG costs:
($11.25) DSF system [5, 6, 9, 11]
($2.50) PCL systems [12, 13]
($14.25) VIG farm operation [13, 14]
VIG benefits:
$4.00 Energy savings [5]
$43.75 Crop value [13, 15]
$50.00 Human productivity [9]
Net VIG value:
$19.75 Direct benefits only
$69.75 With productivity gains

These results can be used to calculate the net present value (NPV) of a full scale VIG implementation. To deploy the VIG across a façade 60 m wide (e.g. the 2020 Tower) for a height of 50 floors represents a $13.0 million investment. The present value of the direct benefits totals $22 million, and the potential productivity advantage adds an additional $23 million. The NPV of this full scale VIG is strongly positive, with a range of $9 million to $32 million.

This system would include 135 PCLs producing over 400 tons of crop annually, slightly exceeding the total fresh vegetable consumption of all of the 3,000 tenants occupying those floors.

To these economic benefits must be added the ecological value of local food production. Each of the 135 PCLs in this system would conserve 300 tons of fresh water per year, avoid up to 3.75 tons of CO2 emissions, and replace 1/10th of a hectare of cropland, reducing habitat impact and agricultural runoff. Finally, no chemical pesticides will be necessary in the VIG.

Implementation:
The VIG could be achieved at commercial scale within five years. If the VIG wins the BFI competition, the prize money will be applied as follows:

Arup, Kiss + Cathcart, and New York Sun Works will each claim 30% of the prize money and apply these funds to a collaborative process to pursue a real-world application of the VIG.

Arup will provide thermal, HVAC, and structural analysis for a variety of climate zones. Kiss + Cathcart will provide architectural planning and building configuration. New York Sun Works will provide hydroponic system designs for a variety of crops.

The Vertical Farm Project will claim the remaining 10% of the prize money and use these funds to promote the VIG and strengthen the technical base of the Vertical Farm concept.

Finally, the team will work together to formally propose the VIG to an appropriate set of building developers in North America, northern Europe, the Persian Gulf, or China.

Notes
[1] U.S. EPA (2004), Buildings and the Environment: A Statistical Summary.
[2] Balaras, C. A. et al (2007), European residential buildings and empirical assessment of the Hellenic building stock, energy consumption, emissions and potential energy savings. Building and Environment 42/3, pp 1298-1314.
[3] Netherlands Environmental Assessment Agency, Emission Database for Global Atmospheric Research.
[4] Pirog, R. and Benjamin A. (2003), Checking the Food Odometer: Comparing Food Miles for Local Versus Conventional Produce Sales in Iowa Institutions, University of Iowa.
[5] “Environmental Second Skin Systems” at http://www.battlemccarthy.com/.
[6] Streicher W. (Ed.) (2005), Best Practice for Double Skin Façades, at http://www.bestfacade.com/.
[7] For some early work on the potential benefits of plants in curtain walls, see Stec, W. J. Modeling the double skin façade with plants, Energy and Buildings, Volume 37 (2005), Pages 419-427.
[8] U.S. National Renewable Energy Lab (NREL), Solar Radiation Data Manual for Buildings.
[9] Kats, G. (2003) The Costs and Financial Benefits of Green Buildings: A Report to California’s Sustainable Building Taskforce, available at http://www.cap-e.com/.
[10] Capital charge rate =10% (20 year lifetime, 7% discount rate).
[11] DSF cost calculated using net building construction cost and marginal charge for DSF from the cited references.
[12] Based on commercial greenhouse equipment at $10 per ft2.
[13] The 2020 Tower floorplate depth of 15 m and floor height of 4 m were used for these calculations.
[14] Operations cost estimate for PCL systems $5 per ft2 vertical.
[15] Premium leafy greens or herbs at $5 per kg. Crop yield is 8.75 kg per m2 of building floor area.