本研究主要目的為評估包含副產品之生質能源製程的資源消耗與製程轉化效率。本研究以同時生產糖與生質酒精之製程為例。
糖蜜是製糖過程中的黑棕色黏稠狀的廢棄物。在現代化的製糖工業當中，糖蜜通常被當作是便宜的二次原料販售，而成為製糖產業中的副產物。糖蜜可以再製成有價值的產品像是生質酒精或是味精。糖蜜生質酒精可以取代非再生的石化燃料，例如，汽油以及柴油，並且不與製糖工業競爭料源。對於一個像是台灣這樣的國家來說，超過99%的初級能源皆為進口的，並且可耕作的面積也受限於自然，糖蜜生質酒精的製程可視為一個能夠改善能源安全的解決方案，且不與糧食競爭。
本研究強調利用三個個案分析在不同的甘蔗品種、種植條件以及生質酒精製程情況之下的比較討論。傳統的指標(Energy Profit Ratio EPR)再評估上遭遇到一些困難，1)無法公平的分析包含有副產物之生質酒精製程與一般的生質酒精製程，2)且無法考慮到再生可用能(exergy)投入的效率。相對來說，可用能分析(exergy analysis) ，例如，可用能平衡(exergy balance)分析與可用能為基礎的指標(exergy-based indicator)，允許多樣化的投入與產出在一個能源的基礎上作比較，並可評估再生及非再生能源的使用。類似於生命週期評估(life cycle assessment)，本研究所評估的範疇從甘蔗種植至無水酒精製程均包含在內，且關於原料的部分也採用從搖籃到大門(cradle-to-gate)的概念，將源頭的原料也包含在評估的範疇之內。本研究應用可用能分析於以一公頃一年之甘蔗產出為基準評估，平衡不同的耕作模式(例如，在台灣3年為一個耕作系統)。
可用能流轉圖(exergy flow diagram)顯示在甘蔗種植時，最主要的可用能投入為太陽輻射，此為一再生可用能投入。而柴油及肥料為在甘蔗種植時期，最大量之非再生可用能投入。在生質酒精製程當中，能源投入(例如，電力和蒸氣)是最主要的可用能投入大約佔總可用能投入的90%。大部分的能源都可以被氣電共生系統所產生的能源替代，並且氣電共生系統所使用的燃料為壓榨蔗汁製程所殘留的蔗渣。在甘蔗種植時期，唯一的可用能產出就是甘蔗，甘蔗的葉與根的部分留在田裡當作肥料，同時甘蔗的莖被用於後續的製程。在生質酒精製程中，蔗渣為最大量的可用能產出。而粗糖為第二大的可用能產出，尤其在Case T與Case J的粗糖產出佔總產出超過20%以上。反觀生質酒精的可用能產出大概只有佔3-5%且約等同於糖損失(sugar loss)的可用能產出而已。
本研究使用三個以可用能為基礎的指標，可用能效率(exergy efficiency)、非再生比(non-renewable fraction)與單位製程的可用能流失(exergy loss per exergy production)，來評估。其中單位製程的可用能流失(exergy loss per exergy production)為本研究所建立，用以輔助其他的指標。
此新指標(exergy loss per exergy production)揭露在甘蔗種植時期大部分的再生可用能投入都是被浪費的。約有99%的再生可用能(太陽輻射)是被流失的。比起在製程中的可用能流失，減少種植時的流失更為重要，且以增加種植時的可用能產出量來減少流失，就此觀點，比起其他作物甘蔗為最有潛力的作物。
個案在非再生比(non-renewable fraction)的結果相似於文獻上其他作物的結果。相較於其他的生質能源製程，糖蜜生質酒精的可用能產出遠遠大於其他製程。糖蜜生質酒精製程比起其他的生質能源製程在相同大小的土地上更有潛力去獲得更多的可用能產出。然而可用能效率(exergy efficiency)相對低於其他製程約只有40-50%，這使得非再生比的結果不如其他的生質能源製程好。為找尋其原因，將製程細分成製糖製程與生質酒精製程作可用能效率(exergy efficiency)評估，發現製糖製程的各別可用能效率就是整體可用能效率較低的原因。那即是，在副產品製程當中，製糖製程有較低的效率。這個例子告訴我們，當可用能效率(exergy efficiency)被誤用就有可能忽略了副產品製程當中的可用能損失(exergy loss)差異。
總結以上，有較好的可用能效率(exergy efficiency)及非再生比(non-renewable fraction)結果，並非代表那就是一個好的生質能源製程。還需要再評估可用能流失(exergy loss per exergy production)，以避免無效的再生可用能投入。
尤其是針對於台灣與日本，以下的建議為有效的推廣自給自足的能源，1)增加甘蔗產量以達到只需一次的糖萃取就可產生所需要的量(以減少在糖損失上的可用能產出)，2)例如Case M，開發最佳化的甘蔗品種以提高糖及酒精的產量，3)開發創新的耕種方式以期提高作物產量並減少肥料使用。

This study aims at evaluating resource consumption and conversion efficiency in production of bio-fuels coproduced with other products. Bio-ethanol coproduced with sugar is taken as an example throughout the study.
Molasses are the dark-brown viscous residue from raw sugar extraction. In modern sugar making, molasses are treated as a by-product sold as an inexpensive secondary feedstock that can be converted into valuable products such as ethanol and chemical seasonings. The molasses-derived bio-ethanol substitutes non-renewable fossil fuel (i.e. gasoline and diesel) without compromising sugar production. For a country like Taiwan importing over 99% of its primary energy and has limited area of arable land, molasses-derived bio-ethanol production becomes an important candidate to improve energy security without directly influencing ability to produce food.
Variations in sugarcane cultivar, cultivation conditions and bio-ethanol production processes are highlighted using three cases from Japan and Taiwan. The typical indicator, (Energy Profit Ratio EPR) suffers from shortcomings such as 1) failure to evaluate energy production systems with non-energy products, and 2) disregarding of efficiency of utilization of renewable exergy inputs. In contrast, exergy analyses, i.e. analysis of exergy balance and exergy-based indicators, allow comparisons of processes with varied inputs and outputs on an energy basis focusing on both renewable and non-renewable resource utilization. Similar to comparison made in Life Cycle Assessment, the scope of the analyses include processes ranging from sugarcane cultivation to production of anhydrous ethanol, together with cradle-to-gate of resources used in each processes. In this study, exergy analyses were conducted based on sugarcane produced using one hectare of farm for a year, taking various cropping systems (ex. 3 years cropping cycle in Taiwan) into account.
The exergy flow diagram of each case shows that the dominant exergy input to the entire system is the solar radiation, i.e., renewable exergy inputs, in cultivation process. For non-renewable exergy input in cultivation process (i.e. farm), diesel oil and fertilizers were the largest items. For production processes (in factory), utilities (i.e. electricity and steam) accounted for 90% of the total exergy input occurred. Most of the utilities can be supplied by cogeneration system fueled by bagasse, the fibrous residue from sugarcane juice pressing. In cultivation, the only output is sugarcane, separated it into stem, leaves, and roots. Leaves and roots remain at the field as fertilizer, while stems are utilized in the following production process. In production, the bagasse was the largest output. The second large output was raw suger, in case T and case J, accounted for more than 20% of exergy output. Bio-ethanol only occupied 3-5%, almost equal to exergy of sugar loss.
Three exergy-based indicators, i.e., exergy efficiency, non-renewable fraction, and exergy losses per exergy production were used in the study. Exergy losses per exergy production was established by this study to aid the shortage of the other indicators.
The new indicator, exergy losses per exergy production reveals that most of the renewable exergy is wasted in cultivation process. The only renewable exergy input was solar radiation, about 99% of which was lost. Comparing with renewable exergy losses in factory processes, reduction of this loss (equivalent with increase of exergy yield in a farm) is much more important. In this regard, sugarcane is the most advantageous among the compared crops.
The results of non-renewable in molasses-derived bio-ethanol represented by our three cases are similar to that of other crops presented in literature. Compared to biofuels from other crops, exergy output of molasses-derived bio-ethanol process was significantly higher. It shows that the molasses-derived bio-ethanol process compared to biofuel production from other energy crops have potential for obtaining higher exergy from the same area of land. However, the exergy efficiency around 40-50% was relatively lower, leading the result of non-renewable fraction became inferior to processes derived from other crops. To elucidate the reasons in behind of this, the production process was divided into sugar and bio-ethanol production parts. The exergy efficiencies in the respective parts showed that sugar production was the reason of low overall efficiency. Namely, sugar production is less efficient production among the production process of coproducts. This example suggests that misuse of exergy efficiency could lead to overlooking of the difference in exergy losses in production of coproducts.
To summarize, better result in exergy efficiency and non-renewable fraction does not necessarily mean the better crop for biofuel production. The crop and process should be evaluated from exergy losses to avoid inefficient use of renewable exergy input.
In particular for Taiwan and Japan, the following is suggested to efficiently promote self-sufficiency of its energy demand: 1) to enhance production of sugarcane so that the required sugar production is achieved even with only once of sugar extraction, (to reduce the exergy output of sugar loss in production) 2) like in case M, explore sugarcane cultivar that is optimized for both sugar and ethanol production, 3) explore innovative cropping systems to enhance the yield while reducing fertilizer uses.

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