Now that SB 489 has removed this regulatory barrier, financing for projects which use crop residues for on‐farm energy generation will be facilitated . In this policy context, a case study of the mitigation potential of on‐farm renewable energy generation is very timely. The main objective of this paper is therefore to evaluate the mitigation potential of current and future renewable energy generation on a single farm . This case study examines an organic walnut production and processing operation that uses rooftop solar photovoltaic panels and producer gas derived from the pyrolysis of walnut shells to generate electricity and partially offset the farm’s consumption of grid electricity. A secondary goal of this study is to demonstrate a modeling tool, the Long‐range Energy Alternatives Planning 7 system, which can be used to plan energy activities at the farm scale. This study also uses the LEAP model to examine how the removal of regulatory barriers might facilitate on‐farm electricity generation by the state’s agricultural producers and processors.The study was conducted at Dixon Ridge Farms, located close to the city of Winters, California. The farm was established in 1979 and is owned by Russ Lester and his family. Of the 1,250 acres owned by the farm, approximately 400 acres are in walnut orchards that are distributed across several ranches. The majority of the walnut production is certified organic, with the remaining at various stages of transition,vertical hydroponic nft system as it takes three years to transition to U.S. Department of Agriculture certified organic status. Of the various farm operations, only spraying is contracted out to “custom operators.”
The operation is atypical in that it both produces and processes walnuts, i.e., small producers typically sell their walnuts to large processors . Dixon Ridge processes walnuts from their own orchards and those from approximately 60 other organic growers. With a strong focus on sustainability, the farm has adopted practices such as no‐till cover cropping, electricity‐driven irrigation pumps , energy‐efficiency improvements for chilling and lighting demand, on‐farm electricity generation from both solar PV panels and a bio-energy plant, and waste‐heat recycling used to dry walnuts. The current capacity of the solar PV unit is 17 kilowatts , with plans for an additional 100 kW of solar PV based on available roof area. The current bio-energy plant is designed for 50 kW, but is run sub‐optimally at 28 kW for most of the year because of load considerations and the legal inability of connecting the electricity output to the grid. Much of the energy and emissions mitigation potential comes from a walnut‐shell bio-energy plant that is already in place. The bio-energy plant at Dixon Ridge was developed and installed by the Community Power Corporation with a grant funded by the California Energy Commission. A schematic of the bio-energy plant is shown in Figure 6.1. Walnut shells, which are a byproduct of the processing operations, are the feed stock for the bio-energy plant. After the walnut meat is removed, shells are placed in the feed hopper every four to five days with a forklift. From the feed hopper, shells are metered at 175 lb/hr into the gasifier of the Biomax plant, which heats the shells using a controlled air stream to gasify the carbon in the shells. The gasification of the shells results in a stream of producer gas and a biochar by‐product. The producer gas stream is then run through a heat exchanger to recover heat used in the drying of walnuts.
The biochar is applied to soils in the orchard as a means of sequestering C and improving soil physical and chemical properties . The producer gas is used in one of two ways. For most of the year, it is used to fuel an internal combustion engine, which powers a generator to produce between 50–55 kW of electricity. During harvest when drying is a major energy demand, the producer gas is burned in walnut dryers. This offsets a substantial amount of propane that would have typically been used.Propane , diesel , and electricity are the dominant fuels being modeled in the BAU scenario, which assumes no on‐farm generation from renewable sources . The total annual energy demand at Dixon Ridge Farm under the BAU scenario is approximately 14,000 gigajoules . Energy consumption is dominated by petroleum products used in the growing operations, while electricity accounts for 25 percent of the total energy consumption . The total GHG emissions under the BAU scenario are estimated at 919 tons CO2e . A breakdown of emissions by fuel type indicates that propane combustion is responsible for 45 percent of emissions, followed by diesel , grid electricity , and other petroleum products. The REN scenario represents the current status of the farm’s energy portfolio, including on‐site energy generation from solar PV and one Biomax unit running on producer gas. Figure 6.3 compares the fuel sources under the BAU and REN scenarios. The Biomax unit displaces over half of the propane requirements for drying, equal to over 3,000 Gj . Electricity demands continue to be met by predominately by grid electricity, but the PV and Biomax units are able to displace close to 20 percent of total electricity demand . Accounting for the displacement of propane and grid electricity by producer gas, as well as a reduction in waste transportation emissions, renewable energy sources reduced emissions by 245 t CO2e yr‐1. Approximately 81 percent of the emission reductions in the REN scenario come from the replacement of propane with producer gas, which is renewably generated from walnut shells. Under this scenario, the solar PV and Biomax units generate 187 kWh of electricity annually and together reduce emissions by 37.3 t CO2e yr‐1per year relative to the BAU scenario.As described earlier, the M1 scenario assumes expansion of solar PV generation to 100 kW and increased bioenergy production by installing one additional Biomax unit with the generator operating at 28kW in 2015. The 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 of the propane required for walnut drying. The remaining producer gas is also used to generate 700 kWh of electricity on site. This is equivalent to 64 percent of the total electricity demand . Including the solar PV, 73 percent of electricity demand can be met by on‐farm renewable energy sources under the M1 scenario. Overall, the M1 scenario reduced emissions by approximately 62 percent relative to the BAU scenario .
The M2 scenario includes all mitigation measures planned and possible with the passing of SB 489, which will enable the Biomax units to run at design load . The new state policy also facilitates the addition of two more Biomax units, for a total operating capacity of 150 kW . Since walnut shell supply would have to increase to support increased bio-energy generation, the energy demand for both growing and processing increases with increased production of walnuts, to a total 17,265 GJ yr‐1, by 2015 . Under the M2 scenario, renewable energy sources meet 81 percent of electricity demand,nft hydroponic system with producer gas supplying 74 percent . As with the M1 scenario, all propane is able to be replaced with producer gas. Thus, by 2015, producer gas accounts for slightly more than 50 percent of the total fuel share. Overall, the M2 scenario reduced emissions by 62 percent relative to BAU levels in 2015 . These emissions reductions are mostly obtained by using producer gas for drying operations instead of propane . Decreased use of grid electricity and avoided walnut shell waste transport make up the remainder of the emission reductions.High energy prices and volatile energy markets have been a longstanding concern among California farmers. Studies also suggest that farmers in California are more inclined to adopt GHG mitigation practices that also offer direct private benefits; prime examples being measures to reduce their energy consumption and/or install technologies to generate renewable energy on‐farm . As such, the progression of California’s net metering laws has helped to spur investment in on‐farm renewable energy projects; particularly solar, wind, and biogas. The recent passing of SB 489 will allow California farmers who generate electricity from agricultural residues to participate in these net‐metering programs, thus providing a strong incentive for them to consider the benefits and trade offs of using agricultural residues to supplement their energy needs, rather than for another purpose, such as animal feed. The results of this study suggest that solar PV and producer gas derived from agricultural residues have significant potential to reduce reliance on purchased electricity and propane used in growing and processing operations , while also reducing their GHG emissions . At Dixon Ridge Farms, upwards of 70 percent of the total energy demand and more than 80 percent of electricity demand could be met through the addition of 100 kW of solar PV and an expansion of bio-energy generation to three Biomax units running at full capacity . These measures would also allow for nearly 40 percent growth in the farm’s processing operations, while reducing total emissions by 62 percent relative to the scenario that assumed no renewable generation and 50 percent relative to the farm’s current suite of renewable generation measures . On‐farm emissions reductions are achieved primarily through the complete displacement of purchased propane for drying, reduced reliance on grid electricity, and eliminating the need to transport walnut shells to a distant waste facility. It is important to note the scope of the analysis presented here.
We have focused on current and planned on‐site renewable energy measures and associated emissions reductions. The mitigation option with the largest potential to reduce GHG emissions is expanding the number of Biomax units to generate more producer gas to replace propane in drying operations and generate electricity on‐site. Our emissions analysis includes the waste transportation emissions that occur due to the expansion of the on‐site biomass power plant. Although we have included waste transport emissions, those have proved to be quite small. While we include in our analysis the emissions related to grid‐electricity consumption , we do not attempt to address transmission losses or upstream processes related to grid electricity generation. These were beyond the scope of the study. Despite the private and public benefits demonstrated above, farmers’ eventual decisions to invest in on‐farm renewable energy must be driven by a careful analysis of the financial costs and benefits. In this context, it should be noted that the LEAP decision support software used in this project has the capacity to include financial routines that can facilitate economic analysis. Future LEAP studies could combine on‐farm energy data with crop‐specific cost and return studies, and could be made available through the University of California Cooperative Extension, helping farmers who grow and process a wide range of agricultural products to consider the potential for integrating renewable energy projects into their operations.In 2006, California signed the Global Warming Solutions Act of 2006, or Assembly Bill 32 , into law.AB 32 aims to reduce California’s greenhouse gas emissions to 1990 levels by 2020 through a variety of regulations promulgated by the California Air Resources Board including a cap and trade program.The cap and trade program already includes four offset programs that give capped entities the opportunity to meet their emissions limitations in the most economically efficient way available. Whereas the level of emissions in a compliance obligation that may be accounted for through an offset program is capped at 8%, the number of existing offset programs that may be used to reach this 8% can be increased to include additional offset programs.Adding offset programs may be attractive to covered industries and government alike because offset programs allow covered entities to determine what the most economically efficient way to comply with emission limitations is for that specific entity while still complying with the overall program emissions cap. CARB’s implementing regulations for AB 32 do not address California’s large agriculture sector as directly as other sectors, as its dominant strategy to reduce agricultural emissions is to encourage dairies to voluntarily install manure digesters. However, California’s agricultural sector, primarily manure and cropland management, provides ample opportunity for offset programs.