Green Energy > Energy From Agriculture
Here we explore several methods of harvesting energy from agriculture including; biomass for processing into biodiesel or biogas; biomass for burning; heat exchange from milking parlours.
The vast majority of biomass is used for burning to create heat and hot water. It is becoming more common to use it for generating electricity – especially biogas and biodiesel and bio-oils to run combined heat and power units.
The key drivers for interest in biomass based fuels include: recognition of the damage caused by fossil fuels (pollution and climate change); energy security (unstable supply, peak oil); Farm Diversification; Increased structural support in the form of EU grants and national grant schemes; and market demand from domestic customers as well as larger scale users like Government and business.
Miscanthus and willow are the most popular crops that are used for burning to create hot water for heating.
Miscanthus plants last for 20 years, yielding about 20 o.d.t. p.Ha (Oven Dry Tonne per Hectare). The crop is harvested in November each year when it is dry. A large part of its attractiveness is its easy management, from planting to harvesting all of which can be done with conventional farm equipment, and requiring virtually no inputs in terms of chemicals. There are some complications with uncontrolled burning of the dried miscanthus such as burning in wood chip burners, and it is recommended that it be processed into briquettes or pellets where it has lime added and it burns very cleanly with a low-ash content.
Willow is a native tree and many of its varieties are particularly well suited to growing in the cool, wet maritime climate typically found in Ireland. It is easy to establish and grows extremely rapidly. Willow can be coppiced, meaning that in the winter it can be cut back to ground level and the ‘stool’ left in the ground will resprout the following Spring.
Willow does need intensive preparation and management, including weed control. It is harvested on a three-year cycle giving a willow plantation a life of approximately 30 years. Yields average out at about 10 to 12 dry tonnes per hectare per year. The willow is harvested with a forage harvester to produce wood chip.
This chip is about 50% moisture at the time of harvest and needs to be dried to reduce moisture to less than 20% for storage and utilisation. It is also possible to harvest willow as billets or even whole rods. If left in the field these will dry naturally to approximately 30%. However, a second handling procedure will be needed to produce wood chip. Site selection is crucial and some parameters have to be observed. The machinery for harvesting is heavy and uses a lot of fuel.
Right now the economics are volatile, along with the energy market and payback periods vary. Willow sells at £80 to £90 (~€100) per dry tonne – (therefore annual income of £2400 per hectare) and will provide 4800kw hours per tonne (same as wood pellet) Currently this equates to a seven-year payback for the investment needed. Added benefits are the establishment grants that are available from Governments in both Northern Ireland (£1000 per hectare) and Ireland (€1450 per hectare). Energy crops are also able to benefit from certain tax incentives that further reduce the payback period.
Miscanthus is a better earner at approx £65 (€113) per tonne for an energy return of around 5600kw hours per tonne. See here for further information. While few people are processing miscanthus currently, the economics mean it is likely to quickly become more established. The grants available for miscanthus establishment are the same as those for willow. The payback will be shorter than for willow.
Another increasingly viable option for farm energy production is biogas. It is attractive because it processes many farm materials including manure and crops grown for the purpose. Not only does it produce electricity and heat, but also a digestate or fertilizer that can be used on the land.
Biogas is produced by anaerobic digestion (AD) and typically consists of:
– methane (50 – 75 %)
– carbon dioxide (25 – 50 %)
– trace gases (including hydrogen sulphide).
It can be burned to create heat and power, or compressed to become compressed natural gas (CNG) which can be used to power vehicles.
Anaerobic digestion works by breaking down biodegradable organic matters with bacteria, in the absence of oxygen. It requires management of factors such as temperature, moisture, nutrient contents, and pH. On-farm AD is normally achieved at temperatures of 30-40°C, also known as mesophilic digestion. A more intensive process is used on bigger plants. There are different digesters depending on your feedstock. Here are the most common:
Complete mix digesters or continuously stirred tank reactors (CSTR) are silo-like digesters of steel or concrete in which the biomass is agitated and heated, and part of the content is periodically removed to be substituted by fresh substrate, so that the biogas production is almost steady. It generally operates at mesophilic temperature, and can be fed with feedstock with solid content between 2% and 10%.
Plug flow digesters are horizontal cylindrical tanks. The biomass or manure is continuously or semi continuously fed at one end, and is mechanically pushed to the other end, where the digestate and biogas is collected. It also operates at mesophilic temperature and can handle feedstock with solid content of 10 to 14%.
Batch digesters are also silo-like tanks. Periodically, the content is removed and the digester is filled with untreated manure. Usually about 10% of the former batch is let to increase the bacteria content of the fresh substrate. Since the biogas production from one reactor is not continuous, several reactors usually operate together and are filled at different times to obtain a relatively continuous biogas production. Batch digesters can handle feedstock with a solid content up to 25%. They generally operate at mesophilic temperature as well.
Energy output: 6 kWh/m3 on average (one third electric, two thirds heat)
Fuel oil equivalent: approx. 0.62 l fuel oil per one cubic meter of gas
Electricity yield factor: approx. 1.8-2 kWhel/m³ biogas
There are several sources of income including selling electricity into the National Grid; selling Renewable Obligation Certificates to power companies and charging gate fees to accept domestic, municipal and industrial organic waste for safe disposal.?Other financial incentives include the possibility of selling excess heat produced from the process into community-based or small-scale, local industrial heating systems and off-setting the cost of compound fertilisers.
Table 1: Data of typical farm scale biogas site (Source: German Biogas Association):
Small farm scale energy crop Co – digestion plant Parameter
The UK has embraced the concept of “feed-in tariffs,” (as used in Germany,)as part of the Energy Bill. In Germany, small producers with microgeneration are paid for each unit generated, depending on the technology. In Germany investors are guaranteed fixed rates for their electricity sales for a 20-year period. This is an important factor when securing finance from the banks. The German Government also offers further bonuses with additional funding for:
These different compensations can be combined so that a total compensation of up to 21.5 eurocent per Kilowatt-hour (kWh) can be achieved. For example a biogas plant with an installed capacity of 150 kW, using energy crops as substrates, providing heat and using new technologies could achieve this rate.
Dairy parlours have hot water needs that are normally met by electricity, either at standard tariff or on overnight, reduced tariffs like economy 7. They also use electricity to cool milk.
In an amazing alignment of requirements, one litre of cooling milk provides the energy for heating one litre of water. Therefore with an efficient heat exchanger you can meet your hot water needs while cooing the milk. Dairies require approximately 10 litres of hot water per cow for a milking session, including cleaning of equipment and cow preparation.
If you have excess water and the farmhouse is nearby then you can export hot water to the house.
On most dairy farms the milk heat available can preheat more water than is required for nominal wash-up and sanitising. The problem then is one of choosing a ‘heat recovery system’ that will provide only the warmed water needed on an individual farm, with a reasonable payback.
To maintain milk quality, milk must be cooled from about 39° C (cow body temperature) to 3° C for safe storage. Milk is normally cooled by a refrigeration unit acting as a heat pump moving heat from a source (the milk) to a sink (air or water) using a carrier called a refrigerant (Freon gas). Heat recovery systems do not change the basic refrigeration cycle. They simply change the type or combination of condenser(s) used to remove heat from the refrigerant.
The average preheated water temperature from a watercooled condensing mechanism is likely to be in the 50° to 60° C range. Starting water temperatures for milking equipment washing should be 75° C. Thus all preheated water used for equipment cleaning must be further heated using conventional energy sources. However, many auxiliary water uses such as: cow prepping, calf feeding, employee showering, can use the preheated water without further heating.
Before investing in any type of heat recovery equipment, potential preheated water usage should be estimated. It will probably be around ten litres per cow. Milk heat is a reliable source, occurring every time the cows are milked. It must be used as produced or stored temporarily because the milk must be cooled quickly to maintain quality standards. Thus it is not a constant source for space heating (i.e. the calf shed) and is not required on a year round basis for that type of application.
The economic attractiveness of heat recovery from milk must be based primarily on purchased energy displacement for water heating. For a common (35 milking cow) dairy unit producing 750 L of milk a day and using hot water at the rate of 250 L/d (7 L/cow/d). Only 33% of the milk heat can be used to preheat that amount of hot water, therefore the risk is that you have a lot of wasted hot water.
In contrast, a much larger dairy operation producing 2150 L of milk a day and having a high percentage (70%) warm water to milk usage, has an excellent opportunity to benefit from milk heat recovery with a payback perhaps in the region of three years.
Heat exchangers cost around £500 but prices can vary dramatically, while electricity costs between 7 pence per KwH (economy 7) and 13.5 pence per KwH (standard rate).