Modelling renewable electricity generation for energy- autonomous dairy farms in New Zealand

In this paper, we analyse farm energy needs and renewable on-farm energy resource potential and present a model of a generic 600-cow dairy farm aiming for energy autonomy based on a combination of on-farm biogas, solar and wind technologies, as well as energy-storage technologies. The wind and solar potential was evaluated at four New Zealand locations with contrasting climatic conditions and representative of the majority of NZ dairy regions. For a Southland location, which indicated the best wind but poorest solar resource, energy balances were modelled at a finer scale. For this site the yields of a 30 kW wind turbine, a 30 kW solar set-up and the biogas resources available from the cow shed effluent of the 600-cow dairy farm were calculated. The energy yields were balanced against the farms projected energy demand and the buffer available from the biogas resource to determine the level of achievable energy autonomy. Wind generation yields at farm scale were found to be very location specific and variable and even at the most favourable location in Southland, low compared to larger scale wind projects. The modelled 30 kW wind turbine could provide 30% of farm energy needs with weekly totals varying from 2–78%. Solar generation was found to be more consistent and reliable at all 4 locations. On average, 38% of the Southland farm energy demand could be met with a 30 kW solar set-up, with weekly totals varying from 9– 63%. Biogas based electricity generation was able to supply 33% of the total energy demand of the model farm. In combination with the 32% of farm energy needs that could simultaneously be satisfied with biogas generator waste heat of 2/3 total farm energy needs could be met reliably and on demand with biogas. Analysis of weekly farm energy demand indicated that nearly full energy autonomy could be achieved for most of the year with a combination of biogas and solar based generation in conjunction with some further established energy storage technology at the Southland farm. The combination of biogas and wind generation yielded poorer results. For the combination of biogas and solar at Southland, the modelled energy shortfall (5.7% of total) occurring over 20 weeks of the milking season could be met with a biogas generator back-up fuel such as LPG. For the Southland farm 1,587 kg LPG per year would be needed to achieve 100% energy autonomy with the described biogas and solar set-up. Increasing the solar generation capacity to meet the shortfall was found to be impractical as the PV set-up would have to have its capacity quadrupled to meet the shortfall occurring in the last week before the dry season starts. This initial modelling exercise showed that the concept of energy independent dairy farms based on primarily renewable energy resources is feasible in principal. More detailed work needs to be carried out to quantify the energy balances on a finer time scale (e.g. hourly) and required capacity of secondary energy storage equipment. Although outside the scope of this work it is indicated that the cost of the outlined alternative energy provision scheme will be favourable, provided it enables the farm to avoid the expense for a new electricity grid connection.