This note shows the amount by which the carbon pollution prevented by a solar farm exceeds the carbon pollution which would be generated by importing onions and cereals, to replace the loss of productive land caused by using the land for a solar farm. It uses the example of onions at the proposed solar farm at Bishops Itchington. But weights are similar for cereals, and the numbers will scale to any large solar farm. See here for a broader argument for the solar farms, including views of the site.
1. Carbon saved by a 50 MW solar farm
Solar electricity has grid priority. This means that all available solar electricity is fed into the grid. Because of this, less fossil electricity is produced. So, obtaining a figure for the amount of solar electricity generated is the first step.
Heart of England Community Energy’s 3 solar farms deliver on average 15,124 MWh pa from an installed capacity of 14.69 MW (Ref 1) so 50 MW of installed capacity will deliver:-
15,124 x 50/14.69 = 51,477 MWh
Each MWh of electricity produced by burning gas produces 0.4 tons CO2e (Ref 2)
So 50 MW of solar avoids release of 51,477 x 0.4 tons CO2e pa = 20,591 tons CO2e per annum (pa).
Note that, due to the release of CO2 during manufacture of solar panels etc, solar electricity is not zero carbon so strictly speaking the carbon saving is the difference between the footprint of gas-fired electricity and solar electricity. Using this approach, and the table in the Wikipedia reference included under ref 2, a MWh of solar electricity might save slightly less than 0.4 tons CO2e. However, the table also shows that the difference could be higher than 0.4. The footprint of gas depends on where it has come from and since the UK is importing increasing amounts of liquefied natural gas, the footprint of UK gas-fired power is likely to be rising.
2. Carbon emitted transporting Onions from New Zealand or from Spain
2a From New Zealand
Distance from New Zealand to UK (Aukland to Felixtowe) = Between 11,286 and 13,700 nautical miles, depending on route, with 12,155 nautical miles as a middle distance route via Cape Horn. Ref 3
Converting to km gives 20,902 to 25,372 km with 22,511 km as a middle distance route.
The carbon footprint of bulk sea transport is 10 to 40 g per ton km. Ref 4.
Onions produced per hectare in UK = 60 tons per hectare pa. Ref 5.
Area of proposed Bishops Itchington 50 MW solar farm: 82.5 Ha
Weight of onions that would be produced from 82.5 Ha = 82.5 x 6 = 4,950 tons pa.
Carbon released in shipping 4,950 tons of produce from New Zealand to Felixtowe:
Max = 25,372 x 40 x 4,950 grams = 5,024 million grams = 5,024 tons CO2e
Mid = 22,511 x 30 x 4,950 grams = 3,342 tons CO2e
Min = 20,902 x 10 x 4,950 grams = 1,034 tons CO2e
2b. From Spain
By sea from the southernmost point in Spain. For simplicity, assume this is Gibraltar to Felixtowe = 1,296 nautical miles = 2,397km.
Carbon released in shipping 4,950 tons of produce from Spain:
Max = 2,397 x 40 x 4,950 grams = 475 tons CO2e
Mid = 2,397 x 30 x 4,950 grams = 355 tons CO2e
Min = 2,397 x 10 x 4,950 grams = 119 tons CO2e
It’s possible that onions come to the UK from Spain by road, a distance of 1,685 km. This was obtained from ref 6 in which Gibraltar and Felixtowe are not recognised so Granada and Harwich were used instead.
The carbon footprint of road freight is 60 to 150 grams CO2e per ton km (ref 4).
Carbon released from transport by road of 4,950 tons produce from Spain:
Max = 1,685 x 150 x 4,950 grams = 1,251 tons CO2e
Mid = 1,685 x 110 x 4,950 grams = 917 tons CO2e
Min = 1,685 x 60 x 4,950 grams = 500 tons CO2e
These are higher than for sea transport but remain below transport from New Zealand except when the least carbon efficient road transport is compared to the most carbon efficient shipping.
All of these transport-induced carbon pollution values are considerably less than the carbon saved by the solar farm. In the improbable worst case, by the longest possible route from New Zealand in an inefficient ship, this would reduce the carbon saved by the solar farm from 20,591 to 15,567 tons CO2e pa, still leaving a substantial and worthwhile saving. Note however that in practice savings would certainly be higher than this because onions are only imported to the UK from New Zealand when European onions are not in season (ref 7). The worst case impact for European sourced onions reduces the solar farm carbon saving to 19,340 tons CO2e pa.
3. Onion import sources
To see where imported UK Onions come from see ref 7. In addition to Spain, some European supplies come from Italy, which could be 5% further than from Spain, but supplies also come from Germany and Holland which would be a much shorter distance. The UK imports onions from New Zealand only when European onions are not in season (ref 7) so assigning UK land to a solar farm would not result in increased imports of onions from New Zealand, but even if is did, carbon pollution is still drastically reduced by building the solar farm.
4. Other crops and agricultural land grade
In the UK, crops such as cereals have much lower yields per hectare compared to onions in terms of tons. e.g. wheat yields are about 8 tons per hectare (ref 8) so transport carbon would be only 8/60 = 13% of the values set out above. Grade 3b agricultural land is best suited to cereals (ref 9), so from a carbon perspective is highly suitable for solar farms.
The carbon pollution saved by creating a solar farm in the UK is much greater than the carbon pollution created by shipping onions and cereal crops from as far away as New Zealand. From a carbon perspective, it is better to sacrifice low grade agricultural land in favour of solar farms.
Whether onions come from Spain or elsewhere in Europe, by sea or by road freight, the carbon balance is massively in favour of solar farms.
Ref 1. HECE due dil TA report greencat 2019. pdf (The figures quoted are derived from actual performance over 3 years. They are reduced to a basis of average annual solar radiation so not affected by any particularly good or bad years in the performance data.)
Ref 2 How Bad are Bananas (the Carbon Footprint of Everything) by Prof. Mike Berners- Lee. 2020 edition. Page 52. See also https://en.wikipedia.org/wiki/Life-cycle_greenhouse_gas_emissions_of_energy_sources
Ref 3. https://sea-distances.org/