Changing Energy Sources
A Case Study of Dixon Ridge Farms
How are state policies making it easier for farmers to produce renewable energy from agricultural residues?
The Renewable Energy Equity Act (SB 489), which was signed into law in October 2011, allows farms to participate in California's Net Energy Metering (NEM) Program. The law will permit them to produce electricity from agricultural residues and connect any excess power to the utility grid via net metering accounts.
How can the use of agricultural residues for renewable energy production help reduce greenhouse gas emissions?
- Electricity generated from biomass can reduce consumption of grid electricity, a large fraction of which is derived from fossil fuel combustion.
- Agricultural residues can be used in place of fossil fuels consumed on-farm (i.e. propane, diesel)
- Generating energy on-farm avoids emissions associated with transportation and disposal of agricultural residues
How can modeling help farms increase renewable energy use?
This study utilized a model referred to as LEAP (Long Range Energy Alternatives Planning system) to help Dixon Ridge Farms, a walnut growing operation in Solano County, California, estimate its greenhouse gas (GHG) mitigation potential. This study was conducted by Vishal Mehta, Victoria Clark and Ryan Haden in 2011 F . LEAP is a tool that can be used to quantify energy demand and supply, as well as GHG emissions across any number of scenarios. Utilizing this tool, requires a characterization of the energy demand and supply for a given production scenario.
Energy demand on Dixon Ridge Farms can be divided into two main categories: growing and processing. On the growing side, energy is required for running equipment, irrigation pumps, and drying. Processing activities include chilling, shelling and transport of waste walnut shells.
Electricity at Dixon Ridge Farms is comes from three sources: the grid, rooftop solar panels, and an on-site bioenergy plant (Figure 2). The bioenergy plant uses walnut shells as feedstock to generate producer gas, which is burned in a generator to produce electricity and in walnut dryers in place of propane.
Figure 1. Schematic of the bioenergy plant at Dixon Ridge Farms.
What mitigation scenarios were evaluated in the Dixon Ridge Farms study?
The study examined the mitigation potential on Dixon Ridge Farms according to four different future scenarios:
Business as Usual (BAU)
This scenario represents on farm emissions in the absence of mitigation options. This scenario assumes all demand met by conventional fuel sources (e.g. diesel, grid electricity, propane, etc.).
Renewable Generation (REN)
This scenario describes the current state of Dixon Ridge Farms in 2011 which includes rooftop solar PV (17 kW), one bioenergy plant operating at (28 kW). Producer gas and waste heat from the bioenergy plant is assumed to partially displace propane used in drying operations.
Mitigation 1 (M1)
This scenario combines all measures currently in place under REN above, while simulating the expansion of renewables as follows:
- Expansion of rooftop solar PV panels from 17 kW to 100 kW.
- Addition of a second bioenergy plant operating at a capacity of 28 kW. This additional capacity increases heat and producer gas production, allowing propane to be fully displaced in drying operations.
Mitigation 2 (M2)
This scenario includes all mitigation measures from Mitigation 1, above, and those made possible through SB 489. SB 489, the California Renewable Energy Equity Act, allows for net metering of bioenergy production. On Dixon Ridge Farms, this would mean the ability to increase the running capacity of a single bioenergy plant from 28 kW to a full 50 kW capacity.
This scenario explores the possibility of increasing the walnut processing to produce enough feedstock to run three on-farm bioenergy plants.
Which scenarios reduced GHG emissions the most?
The total annual energy demand on Dixon Ridge Farms was estimated at 14,000 GigaJoules (GJ). Energy consumption is dominated by petroleum products used in growing operations while electricity accounts for 25% of the total energy use. Diesel (for equipment and irrigation), electricity (for irrigation), and propane (for drying) are the dominant fuels being used on Dixon Ridge Farms.
The total GHG emissions under the BAU scenario are estimated at 919 tonnes carbon dioxide equivalent (CO2e), the majority of which is attributed to CO2 emissions from propane (45%) or diesel combustion (32%).
Figure 2: Percent contribution of energy sources to energy demand at Dixon Ridge Farms under (a) BAU scenario (total BAU demand = 14,019 GJ) and (b) REN scenario (total REN demand is 13,913 GJ).
Figure 2 compares fuel mixes under the BAU and REN scenarios. The REN scenario represents the current state of Dixon Ridge Farms including 17 kW of rooftop solar generation and 28 kW of bioenergy generation. The operation of a single Biomax unit is able to displace over half of the propane requirements for drying, equal to over 3000 Gigajoules of energy. Electricity demands continue to be met mostly by grid electricity, but the biomass and PV units are able to displace close to 20% of total electricity (Figure 4).
Figure 3: Amount and source of electricity for business as usual (BAU), renewable energy generation (REN), mitigation 1 (M1) and mitigation 2 (M2) scenarios in 2015.
Overall, propane and grid-based electricity displacement by bioenergy as well as avoided waste transportation emissions, represents an annual saving of 245 tonnes CO2e for Dixon Ridge Farms. Biopower and solar PV generate 187 kWh annually, equating to 37.3 tonnes of carbon dioxide equivalent (CO2e) saved per year when compared with the BAU scenario.
As described earlier, the M1 scenario assumes expansion of solar PV generation to 100 kW and the addition of one Biomax unit operating at 28kW. Energy demand remains the same as in previous scenarios, as no expansion in production or processing is assumed. Under the M1 scenario, producer gas from the two Biomax units can replace all propane required for walnut drying with the remaining used to generate 700 kWh of electricity on site. Including the solar PV, 73% of electricity demand can be met by on-farm renewable energy sources under the M1 scenario while reducing emissions by approximately 62% relative to the BAU scenario.
The M2 scenario includes all mitigation measures planned and possible with the passing of SB 489, which will enable running the Biomax units at design load (50 kW) and the addition of two more units for a total operating capacity of 150 kW. Since walnut shell supply would have to increase to support increased bioenergy generation, the energy demand for both growing and processing increases with increased production of walnuts, to a total 17,265 GJ yr-1. Under the M2 scenario, renewable energy sources meet 81% of electricity demand, with producer gas supplying 74%. As with the M1 scenario, all propane can be replaced with producer gas, accounting for slightly more than 50% of the total fuel share by 2015. Overall, the M2 scenario reduced emissions by 62% relative to BAU levels. These reductions are mostly obtained by using producer gas for drying operations instead of propane (72%), while decreased use of grid electricity (25%) and avoided walnut shell waste transport (3%) make up the remainder of the emission reductions.
Figure 4: Electricity generation requirements by source for (a) Mitigation 1 scenario (M1) and (b) Mitigation 2 scenario (M2).
Where can I get more information about on-farm renewable energy?
SB 489 Renewable Energy Equity Act by Senator Lois Wolk
|||Jackson, Louise, Van R. Haden, Stephen M. Wheeler, Allan D. Hollander, Josh Perlman, Toby O'Geen, Vishal K. Mehta, Victoria Clark, John Williams, and Ann Thrupp (University of California, Davis). 2011. Adaptation Options for California's Natural and Managed Ecosystems . California Energy Commission. In prep.|