There is increasing interest in climate change, reducing food miles and, perhaps more importantly, reducing the carbon footprints of farm products. But complete life cycle analyses of embedded carbon are very complex and before looking at the energy used in manufacturing tractors, fertilisers, pesticides etc, there is a need to first quantify the direct energy used by farmers and growers. This has been the subject of a recent Defra-funded study led by Warwick HRI and carried out in collaboration with FEC Services.
Government statistics suggest that direct energy use in agriculture (including forestry) accounts for 1.26 million tonnes of carbon dioxide (expressed as carbon). However, this figure excludes electricity use, and possibly over-estimates the emissions from diesel use in farm machinery. We have estimated that direct energy use in agriculture (excluding forestry) accounts for 1.19 million tonnes of carbon (multiply by 3.66 to convert to CO2) or 0.8 percent of total UK emissions.
While this might seem small, it should be born in mind that agriculture is also responsible for a sizeable proportion of methane and nitrous oxide emissions which are more potent greenhouse gases. Some estimates indicate that, overall, agriculture is responsible for around seven percent of the UK’s greenhouse gases, and that the whole food chain accounts for 20 - 30 percent of emissions.
Unfortunately none of the government’s statistics break down direct energy use to the various sectors of agriculture, and some of the quoted figures include forestry and fisheries. Therefore, we took a bottom-up approach to quantifying energy use across agriculture and horticulture using 2005 as the base year. Electricity use was converted to a primary equivalent taking into account losses from power stations during generation and in transmission.
Our study suggests that horticulture accounts for around 28 percent of the total energy used in agriculture, arable crops account for 37 percent and livestock accounts for 35 percent.
The protected crops sector accounts for most of the energy used in horticulture, with horticultural field crops estimated to use just three percent of the total direct energy use. Protected crops differ from the other sectors of agriculture in that the bulk of the fuel use is for heating, principally by gas, oils and LPG, whereas diesel and electricity dominate use in the other sectors.
Around 37 percent of direct energy use in agriculture is for heating, and 61 percent of this is expended in the protected crops sector for greenhouse heating and humidity control. Potential energy saving measures include the adoption of temperature integration regimes, a greater use of thermal screens (particularly for edible crops), making greater use of correctly sized, insulated thermal stores, and an increased uptake of CHP technology. Furthermore, it was estimated that increased use of CHP could reduce carbon emissions by 60,000 tonnes. The technology is proven in the UK, but the capital expenditure, problems associated with grid connections, and ‘spark spread’ (the difference between gas and electricity prices) are key barriers restricting uptake and need addressing.
While it would not be unrealistic to reduce the heating energy used in agriculture by 20 percent from the 2005 base year by 2015, bigger reductions in carbon emissions could potentially be made by integrating low carbon/renewable energy sources. However, while the environmental benefits are potentially large, the payback is currently often less favourable. Additional government support is needed in the short to medium term to encourage a shift to low-carbon technologies.
Geothermal, solar thermal and ground source heat pumps are currently too expensive to be exploited on any major scale. One option where the payback can be more favourable is the use of waste heat from industry or possibly waste incineration. It is estimated that the chemical industry alone produces enough waste heat to meet all of the heating demand of agriculture. The protected crops sector is well suited to use low grade heat, although clearly businesses need to be co-located, and this is only an option for businesses that are looking to build new glass.
Overall, biomass has the greatest potential for reducing carbon emissions since this could provide heat for most sectors of agriculture. However, there are barriers to uptake for the protected crops sector such as the suitability of flue gasses for CO2 enrichment, capital expenditure, availability of suitable biomass and boiler maintenance. Greater support is needed to enable this technology to develop in the UK. Longer term, biomass CHP has great potential to reduce carbon emissions but this technology is still in its infancy.
Another technology that is under-utilised in the UK is anaerobic digestion (AD). Organic matter is broken down to produce methane (and carbon dioxide) which can be collected and used to power a CHP. Germany has hundreds of on-farm AD systems, while Sweden and Denmark have a number of centralised AD plants. The key to a successful AD plant is the availability of suitable local organic material so as to minimise transport costs. The UK has one such centralised AD plant at Holsworthy, Devon.
Around 36 percent of direct energy use in agriculture is gas oil/diesel for field operations. The largest users are the arable crop (66 percent) and livestock (31 percent) sectors; the horticultural sector uses just three percent. Savings can be made by the producers themselves by correct tractor ballasting, tyre selection and implement matching, but it is thought unlikely that these measures have the potential to save more than 10 percent of the total fuel used (720 GWh, 2,592 TJ). Biodiesel or straight vegetable oil (SVO) could be used as a substitute for conventional diesel with little or no capital cost implications. However, biodiesel and SVO is currently considerably more expensive than red diesel.
A further 28 percent of direct energy used in agriculture is electricity used to power ventilation, refrigeration, lighting and motive power applications. This comprises around 85 percent of the total electricity used in agriculture, with the remainder being used for heating. Whilst large-scale wind farms may be cost effective as a source of renewable electricity, intermittency of supply and economies of scale make the technology best suited to supplying the grid. The opportunities for micro-hydro are limited, and small-scale wind turbines and solar photovoltaic panels are currently only cost effective for remote applications where a grid connection is not available.
The horticultural industry has made good progress to date in reducing energy use and meeting CCL targets. However, that is only the start if the UK is to meet the Climate Change Bill target of reducing CO2 emissions by 60 percent by 2050.
Reductions of this magnitude cannot be met simply by improving energy efficiency, and therefore renewables will have to play an increasingly important role. This will no doubt present challenges, although compared with many industries, agriculture/horticulture is well placed to integrate low carbon technologies; low grade heat can be used, and biomass and AD can potentially be used locally.
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