Dairy farms decarbonization — turning livestock waste into resources via anaerobic digestion
Techno-economical analysis to assess the potential of anaerobic digesters in UK dairy farms
This project was conducted over my year at Imperial College London, as part of the MSc Sustainable Energy Futures.
The objective was to address the problem of the UK’s dairy farming sector decarbonization via identifying a sustainable solution and assessing its economical feasibility.
This article provides a short summary of the work performed, articulated around three sections:
- Understanding two main challenges faced by dairy farms: (1) greenhouse gas emissions and (2) electricity costs.
- Investigating a sustainable solution based on anaerobic digestion, a process that aims to produce biogas and biofertilizer from livestock waste.
- System dynamic modelling and techno-economical analysis to assess the solution’s potential to thrive.
- Study’s results and discussion.
A last section is dedicated to study’s conclusions and limits.
Challenges faced by dairy farms
Regulations of greenhouse gas emissions
In 2018, dairy farms contributed to just under 2% of the UK’s total annual greenhouse gas (GHG) emissions [1].
Most of these emissions come directly from livestock, as a result of enteric fermentation in the rumen and decomposition of organic manure under anaerobic conditions.
There are also indirect emissions imported into the system, including feed production for the herd, artificial fertilizers production, fuel, electricity and bedding materials.
The UK dairy industry has made great progress towards improving its sustainability, cutting its emissions by 24% between 1990 and 2015 [2]. However, further action is required to meet government targets of net zero of all GHG emissions by 2050.
High electricity costs
Beyond GHG emissions, another challenge faced by dairy farms is the economic burden associated with their high electricity consumption.
Nowadays, most of dairy sites draw electricity directly from the grid at a cost which depends on the regional rate structure. In UK, consumers are usually under time-of-use tariffs which means electricity cost depends on the time of day — this works as an incentive to discourage consumers from contributing to peak load times by charging their more money to use power at that time.
In the case of dairy farms, electricity is mostly used for milking process which usually occur between 13:00 and 16:00 in the afternoon [3]. This period matches with a high electricity demand on the national grid and dairy farmers are forced to purchase electricity at high prices.
Turning livestock waste into energy
A potential solution to both problems is to capture greenhouse gas emitted from cattle manure and turn it into energy.
Traditionally, manure is disposed off through direct application to soil or open-air composting, releasing large quantities of methane (CH4) — a greenhouse gas which, based on the IPCC AR5 values, has a global warming potential 28 times higher than CO2 [4].
The idea is to capture methane and other greenhouse gases emitted from cattle manure and turn them into useful energy. This can be done via combining two technologies:
- Anaerobic digester — used to process organic matter, including animal waste, into biogas and biofertilizer.
- Combine Heat and Power (CHP) plant — used to convert gas into power and heat with high efficiency.
Here, livestock manure is placed into an anaerobic digester (AD) which captures GHG. The biogas collected from the AD is in turn converted into power and heat using a CHP plant.
Therefore, installing this technology would allow dairy farmers to both manage their significant amounts of cattle waste and associated GHG emissions whilst enabling on-site electricity and heat generation.
Furthermore, the anaerobic digestion also produces a digestate which can be collected and used as a bio-fertilizer.
Techno-economical analysis
To assess the solution’s potential to thrive, a techno-economical analysis is conducted on the system described in the previous section.
Technical analysis
The model used for this study is described by the following sub-elements:
- Manure — On average, a cow produces 45 kg of manure everyday. Considering a herd of 150 cows, and assuming that 100% of this amount is recoverable, this yields 6,750 kg of manure per day.
- Anaerobic digester (AD) — The AD manure to biogas conversion ratio is roughly 0.86 m³/kg. Moreover, we can expect to get 0.53 kg of digestate (residual material left after the digestion process) for every kg of manure. With this in mind, the AD produces 5,805 m³ of biogas and 3,577 kg of digestate per day.
- CHP plant — based on the biogas collected from the AD, we consider an industry-standard CHP plant with a capacity of 45 KWe and 96 kWth (heat-to-power ratio of 2.1).
- Farm energy demand —The farm electricity and heat demands fluctuate throughout the day (e.g. peak electricity demand over milking) but also larger periods (e.g. seasonal changes in heat demand). These features are modelled by sine squared functions with a periods varying from one day to one year.
Now, let’s look at the economical structure of our system.
Economical analysis
Cash flows associated with the system implementation and operation are represented in the diagram below:
Moreover, a government grant as a fraction of the initial funding (CAPEX) will be eventually taken into account (positive cash flow).
Overall system modelling
Combining both technical and economical structures, the overall model is implemented on the Stella Architect software:
Stella Architect enables to understand the system’s dynamics over time and track key metrics such as project’s Net Present Value (NPV) and greenhouse gas emissions.
The full model is available in open-source on my GitHub.
It is worth mentioning that there are uncertainty around the value of certain parameters such as the amount of manure produced, the fraction of manure recoverable, the operation and maintenance costs and the evolution of feed-in-tariffs in the future, all of which would influence the results of the model.
Results
Economic feasibility
To assess the economic feasibility of the AD plant, the Net Present Value (NPV) is calculated over a 10-year period for several different debt interest rates, varying from 0% to 12%. For each simulation, a critical value of the grant fraction associated with a NPV equal to zero is calculated.
The graph below displays the fraction of the total capital investment (CAPEX) which must be funded through grants to get a null NPV, as a function of debt interest rate.
The graph draws the critical limit over which the project is whether economically feasible (positive NPV) or unsustainable (negative NPV).
We can observe that even for an interest rate equal to 0%, the grant fraction must be superior to 25%, which is considerable. This figure grows quickly as the debt interest rate increases and reaches over 55% for a 12% rate.
Emissions reduction
The AD plant enables to capture greenhouse gas emitted from cattle manure. Cumulated CO2eq savings are displayed on the graph below:
Considering the overall emissions of a typical UK dairy farm, this would result in cutting the carbon footprint by up to 18%.
Conclusion
The project’s objective was to address the problem of the UK’s dairy farming sector decarbonization via identifying a sustainable solution and assessing its economical feasibility.
Based on the study’s results, the following conclusions are raised:
- The solution has the potential to cut dairy farms emissions by up to 18% which is a substantial step towards government targets of net zero of all GHG emissions by 2050.
- However, incentives are required to make the project economically viable. These include favorable debt interest rates and significant grants to pay for the initial capital investments.
References
[1] RABDF — Greenhouse Gas emissions: the truth behind the dairy sector’s carbon footprint (2018)
[2] Olivia Godber — Reducing the carbon footprint of milk, ADAS (2020)
[4] Greenhouse Gas Protocol — Global Warming Potential values (2018)
[5] C. Alan Rotz — Modelling greenhouse gas emissions from dairy farms, Journal of Dairy Science (2018)
