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Land use and Agro-solar options

 In search of net zero emissions some pretty odd ideas can sometimes be promoted. A post from the contrarian Saltbush group in Australia offered this pejorative list, mostly related to carbon removal options: 

1. Buy dodgy carbon credits from dubious foreigners.

2. Cover our grasslands and open forests with carbon-absorbing bushfire-prone eucalypt weeds.

3. Build costly energy-hungry carbon-capture schemes.

4. Chase the hydrogen mirage.

5. Log and replant old-growth forests. New trees will grow and extract CO2 faster than old mature trees. 

Leaving the slightly hectoring tone aside, there are some valid points here. Carbon credits are sometimes rewarded for poorly regulated carbon sequestration and offset activities, including bio-sequestration. Direct Air Capture is very energy intensive and, like reliance on biomass/trees to absorb CO2, opens a range of environmental and land use issues – notably, is there enough room to safely store all the CO2 we would need to trap to make much difference to climate change?  Or to store the CO2 produced when making hydrogen from fossil fuels? 

Uncertainties like this mostly involve environmental problems with carbon storage and new types of land use associated with carbon removal using biomass- for example undermining biodiversity, destroying natural carbon sinks and competing with food production.  However, there may be ways to reduce CO2 levels without impacting on land use, undermining natural processes or farming. Rather than trying to extract CO2 from the air, arguably the best options avoid producing it in the first place- either by cutting demand or by generating power from renewables. That’s assuming you don’t think that nuclear power plants are the way ahead - which Saltbush evidently do. Views differ, but at best nuclear is a low carbon options, not zero carbon, and there are a host of problems, not least waste production.   

By contrast most renewable energy systems are less problematic and are fully zero carbon. Most (hydro apart) only have minimal land use requirements, and offshore wind none at all. With on-land wind farms, the only area taken is that covered by the tower bases and any access roads, so they can be very compatible with continued crop farming and animal grazing. Solar PV is less so, with critics complaining that solar farms often misuse valuable agricultural land, blocking it from use for crop cultivation. In response new array layouts are being adopted, with solar modules mounted on stands, so that crops can be grown underneath, and undue sun shading of the ground plants being avoided. In fact though, some sun-shading can be beneficial for plant growth is some locations: ‘the intermittent shade cast by the PV panels in agrivoltaic systems ..does not necessarily diminish agricultural yield’. So a mix and food and energy framing may actually be best in some cases, and what we may need is an optimal agro solar farming balance. 

Compatibility with animal grazing is part of that, sheep being an easy option, they can keep grass and weeds down.  But, even without sheep, there can be an ecological gain: PV arrays can aid biodiversity, for example allowing wild flowers and wildlife to thrive under them.  That can also be the case in extreme locations like deserts, which are well suited to large PV arrays, but also have some local wildlife that needs protecting. However, there may be albedo changing issues to consider with very large arrays of black solar solar cell, although some may be positive- improving local micro climates.  

In some other locations, floating solar is an option, with arrays floating on pontoons on water (lakes and reservoirs) and benefitting from cooling, a big issue for PV cell efficiency in hot climates. In such locations, floating arrays of reservoirs also help to reduce evaporation. Integration with fish farming, or perhaps algae growing, might also be possible.  So it there may be a range of possible agro-solar mixes, depending on location, avoiding undue land use conflicts.  

What about using land for growing biomass energy crops? While solar-driven biomass growth is what land is naturally all about, now that we are mostly managing the land, there can obviously be conflicts between food growing and other land uses, including for energy production. Moreover, the energy yield of some types of biomass is very low, while over- concentration on some types of biomass growth can undermine local biodiversity. 

However, some types of non-food biomass may be less environmentally damaging as an energy source than others, and some can avoid new land use: e.g. with AD biogas generation from farm waste and domestic food waste. AD biogas production can also produce fertilizer to increase crop productivity. But, unless carefully integrated into the local ecosystem, growing energy crops may not be too good an idea environmentally, and, more generally, photo-synthesis is just too inefficient as a bulk solar energy collector/convertor unless you want cover vast areas of land. PV is better than biomass systems by a factor of ten or more in energy efficiency terms, so that energy crops take up a lot more room/ kWh generated. For example, the Bad Energy report I looked at in recent post claimed that solar PV generates 12–18 times more energy per hectare than maize or grass grown for AD biogas production. 

Growing biomass just for carbon storage might be a bit more viable, if space is available and if it can be secured against decay or fire, but, in general, that is not the case, and avoiding CO2 production seems far more sensible- though when deploying renewables to do that, we have to make compromises with other land uses.  Wind farms may be the most productive options in some contexts, and wind is a solar generated energy flow, but despite their lower capacity factors and higher land-use requirements, as indicated above, solar farms, suitably configured, may also be effective in some places. 

It also obviously possible to have both wind and solar on the same site. They fit together well physically and, wherever located, the maximum outputs from each usually are helpfully displaced in time- wind in the winter, solar in the summer.  Certainly, while there may at times be physical space conflicts with other land uses for both options, there may be strategic room for both wind and solar, either in the same or different locations: we need both. 


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