Hot Water Heating Options for the Milking Parlour
It is projected that during 2020 Ireland will produce up to 8.8 billion litres of milk; this will require approx. 370 GWh of electricity. On farm dairy electricity related CO2 emissions may be 162,000 tonnes by the end of 2020 unless mitigation strategies are implemented.
Teagasc Energy Specialist Barry Caslin has some advice.
Heating water accounts for 23% of the energy costs on a typical dairy farm, meaning savings can quickly be made by operating the most efficient system. Even though electricity is the most expensive form of energy it is still the most common method of powering water heating systems on Irish dairy farms. There are other systems which are cheaper to run – a solar thermal or PV system has virtually no running costs – but of course there is the capital investment to consider.
Average figures suggest that a dairy cow uses 350 kilowatt hours (kWh) of electricity a year, of which 40% is for heating water. The most common approach with electric boilers is to use night-rate electricity in combination with an immersion heater. As the night rate is often half the day rate, it is beneficial to operate electrical water heating systems entirely using the cheaper tariff, provided water tanks are well insulated. It makes sense to synchronise the water heater with the night rate. Water heaters should be connected to a timer to ensure water is not heated unnecessarily and, where possible, heat all the water you need for the day using cheap-rate electricity. This may require buying a second heater to avoid using daytime electricity, but the capital outlay should be recouped quickly through the energy cost savings. Electricity heating systems are easy to install, with a low maintenance requirement. Supply interruptions can be supplemented by using a generator. Electricity is one of the costs on dairy farms which continue to increase. Carbon taxes and renewable Public Service Obligation (PSO) levies to fund renewables are likely to put upward pressure on electricity prices. It is also important to consider water treatment technologies to combat hard water supplies in such areas.
Depending on farm water pressure, there are gas boilers which can achieve a reliable and consistent 70 litre’s of hot water at 80ºC within seven minutes. Gas systems proide hot water at the flick of a switch with no waiting time associated with water heating. Our challenge with gas is that Ireland is not self-sufficient in gas supplies and prices are dependent on global market forces.
Oil-fired water heating systems can provide large volumes of hot water very quickly, although the maintenance requirements. The capital costs can be higher than those of electric systems. The use of oil to heat water is not restricted by night rate electricity rates. For oil heating systems a separate boiler and fuel storage tank will be required. The price of oil can fluctuate, with sudden spikes caused by surges in demand, weather conditions and geopolitical factors. As oil is delivered by road, supply deliveries could be interrupted due to poor weather conditions such as snow and ice. Its important to consider that oil is cheaper than daytime electricity, with costs similar to cheap rate electricity.
Heat Recovery Unit
Cooling milk in the bulk tank releases heat that is normally evacuated into the air by the condenser of the refrigeration unit. The refrigerant leaving the compressor of a milk cooling system is at 21-27ºC – energy that can be used to pre-heat water before it enters a water heater. A heat recovery unit (HRU) transfers this energy from the refrigerant to water in a storage tank, raising the water temperature. The most common type is essentially a large, insulated water tank that recovers heat from the refrigerant whenever the compressors run. The water heater is filled from the HRU, which in turn is fed by cold water from either the mains or a borehole. An HRU can pre-heat the water to 45-60ºC.
One of the key factors influencing the efficiency of an HRU is whether a mains water Plate Heat Exchanger (PHE) is used with the milk cooler. This reduces the amount of refrigeration-based cooling required and, therefore, means there is less heat to recover. A HRU can be retro fitted into existing installations. The heat coming from cooling a litre of milk from 35ºC to 4ºC can heat a litre of water to 50ºC. Installing an HRU is a specialised job and should be done by a registered refrigeration technician with experience of heat recovery equipment. Incorrect installation will stress the compressors and drive higher power consumption as well as decreasing compressor life. Installation by anyone other than the original manufacturer/commissioner of the system will almost certainly void the warranty. A guarantee should be sought for heat recovery installations.
Solar energy is a clean and efficient sustainable source of energy. A solar hot water system has the following main components: collectors, which absorb as much heat as possible from the sun’s radiation; a heat transfer system made up of insulated pipework containing the heat transfer fluid; a water pump (which requires electricity); a heat exchange system; measurement equipment and a control system; and a water storage vessel. The most common system is a pressurised indirect primary circuit – the hot water from the solar collectors is pumped through a heat exchanger in the boiler to pre-heat the water. Lower-cost options are available and include thermo-siphon systems, which integrate the collector with the storage tank. Solar heating systems have a long life with low maintenance – many work reliably for at least 20 to 25 years. On sites with high hot water demand, such as dairy units, the system can pay for itself two or three times over during its working life. The initial capital outlay is higher than that for other systems. A solar thermal system normally connects into any existing setup, taking most of the energy load from an immersion heater. There is no grant support for solar thermal systems and the payback is approx. 10 years.
Solar Photovoltaic (PV)
Solar PV generates renewable electricity from the sun. It qualifies for a TAMS grant for up to 11 kWp system. An 11 kW PV system can save about 4.5 tonnes CO2 per year. Similar to other energy efficient and renewable technologies it qualifies for accelerated capital allowances. A diverter can divert excess electricity to heat the immersion tank (buffer tank). This mean that the immersion tank acts as a battery store to allow excess electricity produced at times of the day when it cannot be used and allow it to heat the hot water tank. PV has a payback at about 6 years for grants at 40%. Young trained farmers can receive a grant of 60% which gives a 4.5 year payback.
Considerations when replacing your old water heater
- Water storage tank should be sized to provide enough hot water for one milking as a minimum, according to the number of cows being milked – Irish Standards require a minimum of 18 litres of boiler capacity per milking unit, irrespective of the method of cleaning used.
- Farm energy costs should be calculated before selecting a heating method
- Look for a system with a high Energy Factor rating
- Weigh up the price of the heater against the annual running costs
- Test water for hardness – a water softener should be installed in hard water areas
Top Tips for running water heaters
- Ensure time clocks are set correctly.
- Check the thermostat setting.
- Check for leaks.
- Insulate your water heater – a well-insulated heater will lose less than 5% of its heat compared with 50% from water stored in a bare copper cylinder.
- Insulate the first six metres of pipe running from the heater.
- Purge 9-14 litres from the water heater’s drain tank twice a year.
- Avoid heating to too high a temperature – 80ºC is adequate.