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.
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.
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