What is the best use of small plots of urban land?
Gardening is presently a hot topic in many metropolitan areas around the world. Small open spaces — from rooftops and patios to unused parking spaces and disused building sites — are actively being turned into vegetable, herb, and decorative gardens. Terms like 'square meter gardening', 'parking space gardening', and 'micro-gardening' seem to be blooming everywhere. Self-styled 'guerrilla gardeners' even occupy public and private strips of land to plant their greenery and vegetables.
The advantages of small city gardens are obvious: they bring more green into the city, it is a pleasurable pastime for many individuals, and often provides a cheap source of produce. It is surprising in fact how much food a small urban garden can produce. Proponents argue that a single 30m2 piece of land is enough to feed one person for one year. In Singapore, for example, one quarter of all of the vegetables consumed are products of inner-city gardens.
Now suppose you are living in a large city and take the decision to stop using a privately owned vehicle and rely instead upon a shared car, public transport, or bicycling. Assuming you had off-street parking, what is the best use of your former parking space: gardening or solar electricity?
If your point of view is more heavily oriented towards aesthetics and leisure activities, then the garden will probably be your preferred option. But what is the economic and ecological balance between these two options?
The energy balance
One square meter of land in a middle European city such as Paris, London, or Brussels receives approximately 1,000 kWh of solar energy per year (1). A garden can transform about 2% of this energy into food energy (20 kWh/m2/year). Photovoltaic (PV) panels typically have an efficiency of around 10% for turning that same level of sunlight into electrical energy. If half of the surface of the parking space is filled with PV panels, the yield will be approximately 50 kWh/m2/year. Consequently, from an energy and climate change point of view, the balance is in favour of solar electricity.
The economic balance
What about the economic balance however? Suppose PV panels do produce 50 kWh per m2 per year, which is the equivalent of 4 kWh per m2 per day. With an electricity price of €0.25/kWh and €0.15/kWh in government incentives, a 30m2 piece of land will yield €1.60 per day of solar electricity. From this figure, we still have to deduct the investment cost of the PV panels. Suppose a cost of €1,000/m2, of which 30% is regained by tax credits, and further suppose a life span of 40 years. This results in an investment cost of €700/40/365 = €0.048/m2/day. The net yield of the solar panels will be €1.55 per day.
This 30m2 is exactly the surface area required to produce a year’s supply of food for one person. If this food had come from a typical vegetable vendor, that would cost you an average of €15 per week, or €2.14 per day. This conclusion corresponds with calculations made by the Belgian business newspaper De Tijd concluding that a 40m2 garden yields €920 of vegetables per year (2). It follows from these figures that from an economic standpoint, a vegetable garden is a better choice than PV panels on the same piece of land. That assumes, of course, that the labour for maintaining the garden is not charged and the cost of seeds and gardening tools is minimal.
Saving on transport energy
There is also another line of reasoning that can be followed. We need both food and electricity anyhow, so the question is rather which product of these two choices is most reasonably produced locally. In other words, which requires the least energy for transport?
Suppose you have a 30m2 plot of land. PV cells could produce about 50 kWh/m2/y * 30 m2 = 1,500 kWh/y on this surface. If we take into account the average grid losses of 7%, the annual energy savings by producing the electricity locally are 1,500 * 0.07 = 105 kWh.
How much transport energy do you save when you opt for growing vegetables on this 30m2 garden? The average food product in the US travels 1,500 miles or 2,400 kilometres (3). The energy consumption for goods travel is 0.65 MJoule/ton/km for cargo ships and 0.69 MJoule/ton/km for a heavy-duty truck (4). Suppose food travels half the way by ship and half by truck, that results in an average energy consumption of (0.43 MJ/ton/km)/(3.6 MJ/kWh) = 0.12 kWh/ton/km. An average person in the US eats 250 kg food per year (5), resulting in an annual transport energy of 0.25 ton * 2,400 km * 0.12 kWh/ton/km = 72 kWh. Conclusion: you save more transport energy by producing solar electricity than by growing vegetables.
A final question regarding growing vegetables in city environments: to what extent will their nutritional and health–giving benefits be affected by the high concentrations of pollutants typical of metropolitan air quality?
(1) Solar Electricity Handbook
(2) Netto / De Tijd
(3) Sustainable Table
(4) Kristensen, H.O., Cargo Transport by Sea and Road — Technical and Economic Environmental Factors, Marine Technology, Vol. 39, No.4, October 2002, pp. 239–249
(5) Ambio, a Journal of the Human Environment