Planning the new dairy unit at CAFRE
When planning the new dairy unit, considerable thought was put into ways to reduce the cost for electricity and water use. These are ongoing costs of milk production and the aim was to have a system with as little demand for these natural resources as possible.
How much does electricity cost for a dairy cow in a year?
The results from the dairy herds that CAFRE benchmarked for their electricity usage show that a typical dairy cow uses one unit (kWh) of electricity each day, 365 kWh per year. With the average usage on the night tariff at 40 per cent, and electricity costing 13.5 pence/kWh by day, and 7.3 pence/kWh at night, this gives a total cost of just over £40 per cow per year.
What is the electricity used for?
Survey data from both GB and the ROI show that the highest electricity demand is for cooling milk (38 per cent), closely followed by water heating (31 per cent). The vacuum pump uses 20 per cent and the remainder goes on lighting and other smaller items.
The actual proportions used will vary considerably from farm to farm for a variety of reasons:
- those with ice bank tanks or ice builders feeding a plate cooler will use a greater percentage of electricity for milk cooling, though the price per unit will be less due to use of the night tariff
- modern water heaters tend to be better insulated than those installed years ago
- variable speed vacuum pumps give substantial savings compared to the more conventional vane pumps. For example, in the old Greenmount dairy unit the vacuum pumps accounted for only 11 per cent of the total usage
What is the cost of water in a dairy unit?
Water costs for dairy units will vary considerably for a number of reasons:
- the availability of borehole water
- the amount of drinking water obtained from watercourses; normally at least half of the water used in a dairy unit is consumed by stock
- the use made of the plate cooler water
- the amount used for washing the yards and parlour
If all the water is purchased at over one pound per cubic metre, the cost per cow per year would be at least £50.
Electricity and water use in the new dairy unit
All of the electrical equipment installed in the new dairy unit was selected for good energy efficiency. This includes low energy fluorescent lighting in the milking parlour and cattle handling areas and a variable speed vacuum pump for milking.
Three systems were installed that are new to the College dairy unit. These will give more effective management of water and electricity usage:
- milk is pre-cooled by a plate cooler using a high volume of water which is held in a 30,000 litre header tank. Water is drawn from this tank and circulates through the plate cooler, returning to the tank for further use
- heat is recovered from the second phase of milk cooling by circulating hot gas from the compressor through a tank of water. This gives two benefits - it marginally cuts the milk cooling costs but more significantly, the heat recovered produces partially heated water which cuts the cost of heating for the daily hot circulation cleaning of the milking plant
- rainwater from the roof is collected and stored in a large underground tank, which, in turn, feeds the 30,000 litre header tank in the plant room. This 'harvested' water feeds the plate cooler and is used for drinking water and also for volume washing
Milk cooling
The temperature will rise slightly during milking as warmer water returns to the tank from the plate cooler circuit. While this is a disadvantage for cooling milk, it is far outweighed by the benefit of a high water flow rate compared to the milk flow rate. This is anticipated to be a 3:1 or 4:1 water flow to milk flow ratio and studies of the system will be carried out in the coming months.
The system will be fine-tuned to ensure the milk reaching the bulk tank is at as low a temperature as possible.
Heat recovery
The recovery of heat from the cooling compressor is widely practiced in various industries and is becoming an economic option in our larger dairy units. The economic benefit will be greater in larger herds where greater amounts of heat can be recovered from the larger volume of milk. The Greenmount system will be investigated to determine the electricity savings and financial benefits achieved.
Water management
The overall water management in the unit is based around three sources of water. These are rainwater collection from the roof, borehole water and mains supply. The first call is on the harvested rainwater and this will be supplemented by the borehole supply when necessary. Normally, the mains water will only be used in the circulation cleaning of the plant.
The harvested rainwater is filtered to remove any solid debris. Water for stock drinking is also treated by an ultra violet (UV) light source which kills potentially harmful bacteria. The good operation and maintenance of this UV filter is essential to minimise the risk from water borne diseases.
As these systems are monitored and run as effectively as possible, they will give efficient use of both water and electricity in the unit and provide useful information which can be used in the design of other new systems.
For further information, please contact David Trimble, Renewable Energy Technologist, CAFRE at david.trimble@daera-ni.gov.uk