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Table 1. Fuel-Product Table of the food chain (GJ) 0 F 1 F 2 F 3 F 4 F 5 F Total P0 80 7 15 13.5 0 115.5 P1 0 0 60 0 0 0 60 P2 5 0 0 51.5 3.5 0 60 P3 0 0 0 0 0 1.9 1.9 P4 0 0 0 0 0 3.1 3.1 P5 3.6 0 0 0 0 0 3.6 Total 8.6 80 67 66.5 17 5 Equations (3) and (5) allow computing the production cost. These equations could be also used to decompose the costs considering the different types of resources. The resource cost vector can be separated into two parts: fossil biomass fossil C0 = C0 + C 0 , where the cost due to fossil fuels in terms of energy resources is C 0 = (0, E02, E03, E04,0) , and biomass the cost due to biomass is C 0 = ( E01,0,0,0,0) , therefore the production cost due to fossil fuel could be computed as: fossil CP= * P fossil C0 (8) and in the same way the production cost due to biomass is: biomass CP= * P biomass C0 (9) Similar expressions could be used to compute the unitary production cost. Table 2. Efficiency and production costs of the processes for the base case Fossil Fuels Biomass Total Process κ P c (GJ/GJ) P C (GJ) P c (GJ/GJ) P C (GJ) P c (GJ/GJ) C P (GJ) 1 1.33 0.00 0.00 1.33 80.00 1.33 80.00 2 1.12 0.12 7.00 1.33 80.00 1.45 87.00 3 35.00 11.06 21.01 36.14 68.67 47.20 89.68 4 5.48 4.49 13.91 1.51 4.67 5.99 18.58 5 1.39 9.70 34.92 20.37 73.33 30.07 108.25 Total 13.43 35.50 80.00 115.50 In table 2 the efficiency values of each system process are shown. The ratio of the total fossil fuel required per fossil calorie consumed is approximately 10:1 ( c P,5 = 9.70 ), a clear indicator of the high inefficiency of the current food chain. Also note that the production of meat (process 3) requires 2.5 times more fossil energy resources (11.06 vs. 4.49) than the vegetal production (process 4). By far, the most inefficient process (identified by κ ) is the production of meat. Starting with the base case we will analyze several scenarios, in order to demonstrate the capabilities of the thermoeconomic approach for the evaluation of the environmental impact of the food system, and for the identification of potential improvements. 3.2 Recycling biomass residues Our aim now is to identify and quantify possible optimization options of the food chain system. In the first scenario, we are going to analyze the effect of recycling 10% of crop residues (2 GJ) and reusing them as fuel in the next process. The unit consumption of the first process is reduced to κ 1 = 1.29 and the external resources of the second process to C 02 = 5. We assume that the distribution ratios and the final demand of the system do not change. Using Eqn. (3) it is possible to compute the production costs. Since biomass energy and the cost distribution matrix * P does not change, the production costs due to biomass do not change either. 890
Proceedings of ECOS 2009 22 nd International Conference on Efficiency, Cost, Optimization Copyright © 2009 by ABCM Simulation and Environmental Impact of Energy Systems August 31 – September 3, 2009, Foz do Iguaçu, Paraná, Brazil Table 3. Production costs due to fossil fuel, recycling 10% biomass Fossil Fuels Process P c (GJ/GJ) C P (GJ) 1 0 0 2 0.08 5 3 10.15 19.29 4 4.45 13.79 5 9.19 33.08 Total 33.50 Table 3 shows the new costs due to fossil fuels. As it can be seen, recycling 10% of crop residues reduces the fuel consumption by 2 GJ, or what is the same, the ratio between the fuel required and the energy consumption is reduced by 5.25%. 3.3. Process efficiency impact In this scenario we will assume that the efficiency of each single process is increased by 10% without modifying the final demand and the system structure (junction ratios). Eqn (7) is used for this purpose, with Δ κi = 0.1. The impact in resources consumption is decomposed into the part coming from biomass and from fossil fuels (see table 4). Table 4. Impact of resources consumption with a 10% increase on the process efficiency Process 0 C Δ Fossil Fuels (GJ) 0 C Δ Biomass (GJ) 0 C Δ Total (GJ) 1 0.000 0.00% 6.000 7.50% 6.000 5.19% 2 0.627 1.77% 7.881 9.85% 8.507 7.37% 3 0.060 0.17% 0.216 0.27% 0.276 0.24% 4 0.254 0.71% 0.094 0.12% 0.347 0.30% 5 2.514 7.08% 5.808 7.26% 8.322 7.21% Eqn. (10) is used to compute the fuel impact of each efficiency simulation. The cost of fuel is evaluated for the simulated scenario and the production corresponds to the base case. i 0 Δ C0 = cF, iΔ κi Pi (10) Note that improving the efficiency of the last stage of the food chain has an important impact on the fossil fuel consumption. On the other hand improving the efficiency of the first stages of the process has an impact only on the reduction of biomass requirements. An important conclusion can be thus drawn: for improving the energy efficiency of the food chain, major efforts should be focused on the last production stages. 3.4. Change in the food diet The last scenario analyzes a change in the diet. As it has been shown, the production of animal food requires much more resources than vegetable food. In the base model a person consumes 62% of vegetables and 38% of animal food. What would happen if we reduced the consumption of meat by 10% maintaining the final demand of energy per person in 3.6 GJ? To simulate this scenario we must change the junction ratios q 35 = 0,38 → 0,318 and q 45 = 0.62 → 0.682 . The rest of the parameters are kept constant. Eqn. (7) can be used to compute the resources consumption impact for this simulation as: Δ C0 = ( cP,3Δ q35 + cP, 4Δ q45) F5 (11) Using Eqn. (11) with the unit production cost due to fossil fuel and biomass, the change implies a saving of 10.74 GJ (13.42 %) of biomass resources, and 2.04 GJ (5.73 %) of fossil fuels. On the other hand, if we would reduce the energy demand per person by 10%, the associated saving would be equal to 3.49 GJ (9.84%) of fossil fuels and 7.33 GJ (9.16 %) of biomass, which could be computed using the expression: Δ C0 = cP, 5Δ E50 (12) In these simulations we have differentiated between fossil fuel and biomass resources. The first one has an explicit cost: it has a market price, an environmental impact, externality costs, etc. The second one is a different case, since biomass energy is provided by the sun which is “free”. Nevertheless, the solar energy provided to biomass is proportional to the harvest area. Furthermore in this model we are considering the energy consumed per habitant, 891
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