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Biomass to cogeneration
02-SEP-2005

for New Zealand dairy farms

Dairy farms could form an ideal application for cogeneration plants fuelled by biogas produced on-site by the anaerobic digestion of manure. Such a solution would cut farmers’ electricity bills and help to solve a waste disposal problem, writes Ian Bywater.

Dairy farming in New Zealand has expanded rapidly, with dairy cattle numbers growing by 23% between 1999 and 2002, and more growth expected. A less welcome result is that a lot more manure has to disposed of at milking time.

Between 6% and 12% of the manure produced by a New Zealand dairy herd each day accumulates in the dairy shed area. This quantity is much higher in countries such as the UK, where the animals are not out grazing everyday as is the case in New Zealand. Therefore if the New Zealand system is shown to be viable it will be of even more use in countries where animals are housed for part of the year. This is because more manure is then available for collection.

From the 1970s, New Zealand farmers have been encouraged to dispose of effluent from dairy sheds in a two-pond treatment system. The ponds receive the waste water effluent daily after milking. They contain manure, spilt milk, water from udder and machine washing, chemicals used during milking, mud and grit. About 30% of New Zealand dairy farms still use this system, but it does not always dispose of the raw effluent or destroy pathogens to today’s environmental standards.

THE ISSUES

In the 1990s, following the passing of New Zealand’s Resource Management Act, the Ministry of Agriculture and Fisheries stopped promoting effluent ponds. It now recommends disposing of diluted effluent by spray irrigation of open pasture, sometimes after storage in an effluent pond if conditions are not suitable for daily dispersal. Problems with this solution include the contamination of groundwater, leaching of nutrients from the soil, delay in grazing, and unpleasant odours. In some situations, spray irrigation is impractical or constrained by soil type. This means that ponds must be used, preferably those with an advanced design and function.

Dairy farming places a considerable load on the electricity grid once or twice daily at peak times

Dairy farming also requires large quantities of water each day to keep the dairy shed and environs clean, and cold water is needed to pre-cool the milk in an amount estimated at 50 litres per cow per day. Groundwater is first used to pre-cool the milk and then held for the shed hosing operations.

Dairy farming also places a considerable load on the electricity grid once or twice daily at peak times, although twice-daily milking is losing favour to once-daily. New Zealand cows produce more than 3,500 litres of milk each per year, requiring 116 kWh per head of electricity for harvesting and processing. Some 60% of this power is used to heat water and to chill the milk, in roughly equal amounts, while 40% is used to power the milking system, to pump water and effluent, and to provide lighting, etc.

Milk is not always collected daily and must be kept chilled below 7°C. More than 80% of dairy farms have refrigerated vats. Some water can be pre-heated by recycling the heat removed from the milk. Simple heat exchangers, such as plate coolers, are used to cool the milk before it enters the refrigerated vats.

Simple measures such as insulating the milk vats, recycling hot water and using non-peak electricity to heat water may help a farm’s profitability. However, an integrated energy system could save more energy costs and reduce demand on the electricity grid while conserving water and reducing the odour and other environmental problems of effluent disposal. Figure 1 shows the operation of one solution, under development by Natural Systems Ltd, called BioGenCool.

WINNING TEAM

In its New Spirit Challenge competition, the Institute of Electrical Engineers in the UK recognizes individuals worldwide whom it judges to be making an innovative contribution to sustainability. The author had a winning entry in the 2003 competition that outlined an integrated energy system to use dairy-shed wastes to cogenerate the heat and electricity needed to cool milk and provide hot water. The system combines three core technologies for which a patent filing has been made.

First, an anaerobic biodigester to convert the manure waste into biogas and biosolids. Secondly, a cogeneration technology, for example a Stirling genset, to use the biogas as a fuel to produce on-site power and heat. Thirdly, a cold-storage medium, such as an ice bank, with a capacity to cool the milk from cow body heat down to the required safe milk-storage temperature.

BIODIGESTER

The dairy-shed wastes are fed into a biodigester system in which they will be converted into biogas in a process analogous to that occurring in the rumen (the first stomach of a cow) in which organic matter is broken down anaerobically by mixed microbial populations. A biodigester is essentially a heated tank into which the manure and water slurry is directed. Oxygen is excluded to allow anaerobic bacteria to liquefy the volatile organic compounds in the mixture and then convert the resulting simple organic acids into a methane-rich biogas. Anaerobic conditions allow methane-producing bacteria to flourish while inhibiting those that produce foul odours. The undigested solid residue has not lost any nutrient value and is suitable for storage followed by land application, or for sale as compost or soil conditioner. The supernatant liquor is generally pathogen free and can be used for pasture irrigation without the drawbacks associated with raw effluent dispersal.

Anaerobic digestion does not significantly reduce waste volume, so the same amount of waste that enters the biodigester leaves it each day. The entry pipe can be closed to prevent detergents, medications and other contaminants entering the biodigester. The biodigester is usually heated to provide optimal conditions for bacterial growth, and maintained at pH 6.6 to 7.6. A proportion of the heat output of the cogeneration unit is fed to the biodigester to maintain optimum thermal conditions.

An integrated energy system could save energy and reduce demand on the grid

The biodigester design will need to be standardized to minimise production costs, although some site engineering will be required. A two-stage design is being considered with the ability to keep the biodigester activity alive year-round, even when some farms’ herds are in the period when milk production ceases for a time, usually over winter. This way, the dairy farmer can have biogas production at the start of the new milking season and not suffer a delay in starting biodigester activity.

COGENERATION

Biogas is best suited for continuous stationary operation because of its low energy density (about 60% of that of natural gas) and low level of corrosive contaminants, and the more or less constant production rate from the biodigester. It is especially suited to fuelling a Stirling engine or appropriate fuel cell. Capital costs will most likely dominate the choice of cogeneration plant, which today is more likely to be an internal combustion engine and generator. However, generators driven by Stirling engines and ceramic or molten-carbonate fuel cells that offer high efficiencies will be used once they become cost competitive.

Table 1. Estimated energy benefits for New Zealand of using integrated energy systems on dairy farms
New Zealand dairy herd statistics in 2003/04 (www.lic.co.nz)					Analysis of BioGenCool yield on a per-site basis (estimates from dairy energy evaluation modelb)
Herd size	Number of herds	Percentage of herds	Number of cows	Percentage of cows	Electricity load factor during season (%)	Electricity generated (kWh per year)	Water heating electricity displaced (kWh per year)	Cooling energy displaced (kWh per year)	Peak Load Reduction (kVA)
10-200	4312	33.8	623,399	16.2	0	0	0	0	0
200-300	3662	28.7	879,065	22.8	58	16,800	3800	7200	6
300-400	2042	16	687,786	17.8	80	23,100	3800	7800	8
400-500	1083	8.5	475,342	12.4	93	26,900	3800	12,900	11
500-600	625	4.9	336,024	8.8	100	28,900	3800	25,300	14
600-700	384	3	244,387	6.4	75	43,600	6700	29,000	19
700-800	227	1.8	167,182	4.4	83	48,000	6200	30,200	20
800-900	140	1.1	117,023	3.0	90	51,900	6200	37,800	26
900-1000	91	0.7	84,627	2.2	96	"center">55,700	10,700	38,600	28
1000+	185	1.5	236,467	6.1	100	57,900	13,100	42,200	29
Total	12,751	100	3,851,302	100.1a	National benefits: 358 GWh per year and 81 MVA
a Rounding error b Model developed by Natural Systems

Using biogas in an internal combustion engine presents some problems. It must be of a sufficient energy value and cleaned of corrosive contaminants. This will usually mean scrubbing out the carbon dioxide and hydrogen sulphide content. This can be done chemically. For an internal combustion engine, the biogas will also need to be compressed to give it a pressure suitable for induction. The compressor can be mechanically driven off the cogeneration set.

An added advantage of having a cogeneration plant on the farm is that it can also be used in electricity supply emergencies to allow milking to continue. With the large numbers in modern dairy herds in New Zealand (up to 1000 head), it is no longer possible to contemplate hand milking! At those times when the biogas is insufficient, LPG back-up cylinders or LPG bulk supply tanks can be installed to maintain emergency generation facilities.

In the BioGenCool system the electrical output is mains synchronized and runs continuously to provide power to the refrigeration unit used to provide the ‘cold storage battery’ for milk cooling. In general, no electricity is exported to the grid, but a biodigester does offer the opportunity to add more biomaterial to increase the biogas production and hence create surplus electricity.

Natural Systems has developed a full analytical model for New Zealand dairy farms so that the benefits of the BioGenCool system can be quantified (see Table 1). Recent analysis of a 50-bail rotary dairy showed that 28% of power is used for water heating, 26% for milk chilling, 9% for farm water supply, 9% for the variable-speed vacuum pump, 9% for the wash-down pump, 4% for lighting, 2% for effluent pumping and 13% for other needs. The model is sufficiently robust to enable it to be configured for dairy farming in other countries.

It is worth noting that over 50% of New Zealand herds have less than 300 cows, which suggests reduced energy benefits of on-site cogeneration. However, application of BioGenCool offers significant benefits in terms of reduced water use and reduced peak demand due to water pumping and chiller operation.

The integrated energy system’s estimated net electricity benefit to New Zealand would be 358 GWh per year

The estimated net electricity benefit to New Zealand (assuming a 100% uptake) is 358 GWh per year, and demand reduction of the order of 81 MVA during milking periods. This demand reduction would coincide with network demand in the morning and mid- to late afternoon, although some (3–12 kVA) may already be reduced because electricity network companies remotely switch off water heaters at peak load times by using ripple control systems.

COOLING

Cows produce milk at a body heat of 37°C, which means it must be cooled rapidly to prevent spoilage by microbial action. New Zealand standards are that the temperature must be reduced to 7°C or less within three hours of entering the storage vat. Usually this is accomplished by using cold ground water from a bore through a primary heat exchanger. This exchanger, which typically removes 60%–80% of the milk’s heat is used only once and discarded (it is usually then stored and used for washing down the shed), greatly adding to the volume of water used by a dairy farm.

Storage vats must keep the milk cool until it is collected by a tanker, which may not call every day. Milk tankers are not refrigerated, so the milk must be cool enough when collected at the farm to withstand the trip to the plant without losing its quality. A farmer is penalized if the milk quality is below standard.

A prototype 1 tonne ice bank has been designed by Thermocell Ltd of Christchurch. It incorporates stainless steel, flat plates refrigerated by using the heat-pipe principle (see photograph opposite).

Ice banks and chilled water tanks are used on some dairy farms in New Zealand. Only 4% of dairy farms used them in 1996, using mains electricity for power. Electricity generated by the cogeneration set in the BioGenCool system will power the ice bank on a 24/7 basis to produce ice by running the refrigeration system continuously at a capacity of 6 kW (cooling) or about 2 kW (electrical) per 300 cows. For small herds, this may displace the bore water required for pre-cooling of milk.

In operation, the ice bank circulates water to the plate heat exchanger. The warmed water is passed back to the ice bank while the cold milk is stored in a vat. More ice will melt to produce water at 0°C to be circulated to the plate heat exchanger.

At the end of a milking, most if not all the ice will have been melted, and ice formation will continue to build until the next milking session. It is estimated that by this means the refrigeration capacity will be reduced to one sixth the normal size used to refrigerate a milk vat. Alternatively, a glycol system of cooling could be used, although the equivalent fluid storage volume would be 10 times greater than for an ice bank.

HEAT

Hot water will be produced for washing use in the dairy shed primarily from the cogeneration set, supplemented by heat from the refrigeration plant and from any hot water solar collectors on the dairy-shed roof. Some of that heat is used to maintain optimum thermal conditions in the biodigester system.

Alternatively, there is the possibility to use an absorption refrigeration plant fuelled directly by the biogas plant, or to use a heat engine to mechanically drive compression refrigeration plant for ice making or glycol cooling.

ON-FARM TRIAL

The South Island Dairying Development Centre is in discussion with Natural Systems to trial the system at its 650-cow demonstration farm at Lincoln University near Christchurch. Detailed design work, once approved, will begin in the third quarter of this year, with the expectation of it being installed half-way through the southern hemisphere milking season. A demonstration of the system will be a precursor to the commercialization phase.

Dairy farming is increasing throughout South Island, where some electricity network companies experience summer peaks that are much higher than winter peaks because of intensive irrigation and milking operations. A system such as BioGenCool has many features that make it attractive to these rural network companies.

In summary, the BioGenCool system encapsulates the sustainable practice and sensible application of using an on-site biowaste as a fuel resource for cogeneration, sized to the specific needs of the production taking place, i.e. milking and storage of milk. BioGenCool has other possible applications, particularly for developing economies in which a mains supply of electricity is poor or non-existent and where a food product is being processed, for example fish farming, or fruit or vegetable preparation.

Ian Bywater is an independent energy consultant and a director of Natural Systems Ltd, Christchurch, New Zealand. Fax: +64 33 65 41 46
e-mail: bywateri@woosh.co.nz

This article is based on an article by Claire Le Couteur, published in 2004 in the magazine of the Institution of Professional Engineers of New Zealand.

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