Electricity use in the farm dairy
Dairy farmers suffered substantially increased energy bills during the 1970's, at a time of herd expansion and modernisation of equipment to allow bulk milk refrigeration and storage on the farm. Little was known of the levels of electricity use in the dairy farming sector, but extrapolations had suggested a figure of 300 to over 400 kWh/cow/annum. Farmers were requesting quantitative estimates for the potential of conservation equipment, particularly plate heat exchangers and heat recovery units. An energy audit of dairy farms in South Devon is described. Over a period of two years, data were collected relating to energy use by each of the major components of a milking parlour and dairy, for a range of parlour sizes, levels of production and the ambient conditions. Analysis revealed the factors most influencing variations in energy use. An equation was developed to describe the energy use by a bulk milk tank, given the level of production and the ambient temperature. The bulk tank accounted for some 40% of the total energy used. The bulk tank has been studied in detail. The stages of heat transfer from the milk to the chilled water and the resulting effects upon the ice bank have been modelled. Laboratory investigations were carried out to determine some parameters empirically. The model's limits, sensitivity and validation are reported. Typical levels and ranges of energy use are suggested. A mean of approximately 250 kWh/cow/annum resulted from the audit, but 200 kWh/cow/annum was achieved by the most economical of farms without resort to conservation equipment, and this level is proposed as a target for the conscientious farmer. The factors affecting energy use in the farm dairy are identified as political, environmental, technical and managerial and these are discussed. The farmer's influence has to be directed mainly at the last of these categories. Investment in energy conservation equipment should not be considered until consumption is down to the proposed target level.