Energy supply and demand analysis of a biogas electricity plant for a dairy farm

Transcription

1 Energy supply and demand analysis of a biogas electricity plant for a dairy farm Introduction George Estcourt, Michael Jack and Per Nielsen (9 January 26) In this report we study a dairy farm generating its own electricity from the biogas produced from the dairy shed wash-down waste. We present a number of different scenarios for a dairy farm to partially meet its electricity demand by this type of biogas plant. An energy supply and demand analysis is carried out on each of the scenarios and they are compared on their ability to reduce electricity costs. We consider a biogas plant operating on a 4 cow dairy unit. The daily energy demand profile that we assume for the dairy unit is shown by the blue solid line in Figure 1. The biogas plant is assumed to supply a 6% methane, 4% carbon dioxide fuel mix to a gas engine with an electric generator. This biogas is produced by bacteria growing in a digester which is fed by the waste-water from the dairy shed wash down (and possibly a feeding pad). To increase biogas production the waste water is heated to greater than 35 C. We make the following additional assumption s which are common to all scenarios described below: Biogas production per day per cow is.1625m 3 Water used per day per cow is 35 litres Biogas heat value is 22.7 MJ Necessary to heat waste-water 15 C (a heat-recovery system is assumed). Monthly electricity usage is 792 kwh Annual electricity cost is $16, (assuming a set electricity price of $.25c/kWh) 7 6 Altered Usual Daily Electricity Demand Electricity Demand (kw) Figure 1: Electricity Demand Profile Forest Research/21-Jun-1 farm biogas GE per nielsen 1 Jan 6.doc/Page 1

2 Scenario 1 In Scenario 1, the plant will use a gas engine with a 6 kw el generator and will also have a 1 kw gas boiler. The engine exhaust heat, engine cooling water and gas boiler will supply the energy needed to raise and keep the temperature of the water to above 35 C. In this scenario, the plant is operating to supply the farm with electricity for its demand. As can be seen from the daily profile shown in blue in Figure 1, the daily electricity demand is characterized by two peaks at the milking times. Attempting to follow the uneven nature of the demand will cause increased wear and tear on engine due to start stop procedures, shortening its lifetime and increasing maintenance. In addition, the two electrical peaks a day are well outside the production rate of the relatively small electric generator. These peaks will need to be supplied by the electricity grid as a top up. Note that in this scenario electricity produced onsite will not be fed back into the grid at any time. Following Electricity Demand Engine Boiler El. Demand Max El. Prod. Heat (kw) Electricity (kw) Figure 2: Scenario 1 The reduction in electricity costs associated with this scenario has been determined by considering the possibility of purchasing electricity from the spot market. The average daily profile of the spot market prices (determined by averaging the spot market price throughout the year) that were used in our calculations are shown in Figure 2. It appears that savings of up to 75% could be made from simply shifting to the spot market without changing the way in which the plant is operating. There are also further savings due to the reduction of electricity bought from the grid. The reduction in electricity costs are from $16, down to $3912 (a reduction of 76%) due to Forest Research/21-Jun-1 farm biogas GE per nielsen 1 Jan 6.doc/Page 2

3 purchasing from the spot market and a further reduction down to $357 (a reduction of 8%) due to electricity self-production. An Average Day in Average Spot Price ($/kwh) Figure 3: Average daily spot-market profile Scenario 2 In Scenario 2 the same 6 kw el generator is used but no gas boiler is used. The engine exhaust heat and engine cooling water will supply the heat needed to keep the digester at or above 35 C. The prime objective of this scen ario is to supply enough heat for the digester and make electricity at a constant rate to smooth out the supply curve. This will result in a decrease in the wear and tear on the engine. The rate at which the plant can generate electricity full time is around 5kW. The results of the supply/demand analysis for this scenario are shown in Figure 2. The two daily peaks are well outside the production rate of the electric generator. These peaks will be supplied by the grid as a top up. On the other hand, electricity supply exceeds the demand a large percentage of the time and this will be sold back to the grid. As before the reduction in electricity costs have been considered via purchasing the electricity from the spot market. Due to the increased electricity production from Scenario 1, this scenario leads to a further reduction in monthly electricity costs down to $237 (a reduction of 86%). Forest Research/21-Jun-1 farm biogas GE per nielsen 1 Jan 6.doc/Page 3

4 Heat (kw) 1 5 Steady Electricity Production Engine Boiler El. Demand Max El. Prod Electricity (kw) Figure 4: Scenario 2 Scenario 3 In Scenario 3 we consider a larger gas engine with a 2kW el generator again, no gas boiler is needed. The engine exhaust heat and engine cooling water supplies the energy needed to keep the digester at or above 35 C. In this case, as in Scenario 2, the plants prime objective is to supply heat for the digester and make electricity at a constant rate to smooth out the supply curve. As before, the rate at which the plant generates electricity full time is set at around 5kW and the peak demand will be supplied by the grid. The difference between this scenario and Scenario 2 is that by having a 2 kw el generator on standby, it is possible to completely remove oneself from the grid in the case of a spot market spike. Spot market price spikes occur at random times of the year when electricity demand in a certain region far surpasses supply. Figure 4 shows the spot market prices in $/MWh in Dannevirke for the year 24, illustrating the spikes. In combination with a gas storage system of at least 5m³, the 2kW generator should be sufficient to run the operation for four hours with some minor changes in the schedule of some parts of the dairy operation. This enables the farmer to avoid the spot market spikes. A possible situation is shown by the dashed line in Figure 1, where a spike occurs just before morning milking and lasts for less than 4 hours. The effluent pump has been rescheduled till after milking to reduce the electricity demand so that the 2kW electric generator can cope with the load of running the milking machine and the cooler. Scenario 3 leads to the same cost reduction as scenario 2, but with the added security that the plant can supply the dairy productions electricity demands in the event of a spike. Forest Research/21-Jun-1 farm biogas GE per nielsen 1 Jan 6.doc/Page 4

5 Dannevirke Putative Price Over One Year (from 1/5/24) 1 9 Spot Market Price ($/MWh) Figure 5: Spot market spikes Findings In conclusion, we have considered the reduction in electricity costs for three scenarios involving a biogas electricity generation plant. The appropriate scenario will be determined by the particular operation at each farm, and the aims of the farmer. In general, however, it seems that a huge reduction in electricity costs can be made by simply purchasing electricity from the spot market rather than at the, on average, higher fixed rate. The additional saving from the on-site electricity generation from the biogas is minimal. Purchasing electricity from the spot market does have the drawback that it involves exposing the dairy operation to spikes in the spot market price. It is possible that electricity generation from biogas can play an important role in reducing this risk. Scenario 3 shows a way of avoiding this exposure by storing enough biogas to supply the dairy operations electricity demands in the event of a spike. Forest Research/21-Jun-1 farm biogas GE per nielsen 1 Jan 6.doc/Page 5