運用海水冷卻空調系統取代淡水之研究

近來根據許多氣候變遷的研究，台灣的降雨漸趨極端，在乾季的降雨量頻率比以前少，雨季則降雨頻率比以往還高。雖然台灣每年平均降雨量高達2500mm，是全球平均降雨量的2.6倍，但卻因為地形的影響，約有70%降雨直接流入大海，僅26%的降雨量能夠被儲存下來。台灣已被列為全世界第18個缺水國家，必須尋找關鍵技術以解決未來缺水的情況。台灣四周環海，海水的供應不虞匱乏．不受降雨的影響，利用此先天環境的條件，引入海水做為淡水冷卻水的替代水源，利用海水做為空調冷卻水，可以減少對淡水的需求，並可以節省能源及降低CO2、Sox、NOx排放量。現今，海水淡化及海水冷卻的發展，已成了許多區域解決缺少淡水方法，並且可以以合理的費用穩定取得海水。減少淡水的使用，大量使用海水取代淡水作為冷卻水，將可能是台灣未來解決缺水問題的辦法。本研究的目的是利用海水做為冷卻水應用於臨海工業區或城市，選擇彰濱工業區(CHCIP)做一研究分析及初步概念設計。評估之關鍵項目，如環保法規、施工方式之選擇，主要海水取、排放水管線規劃項目、投資費用分析及預估施工時程。在過去，位於沿海地區的許多工業，常引進海水作為設備的冷卻水，例如火力及核能發電廠、石化廠、煉鋼廠等主要採用直流式(Ones Through Type)系統，引進非常大量的海水經過熱交換器冷卻設備。此種開放式冷卻系統直接引進海水經過熱交換器或冰水機(Chiller)後做完熱交換後，將設備產生的熱透過海水排放回大海，由於排放熱量大，將對海洋生態系統產生了極大的負面影響，例如珊瑚、魚蝦因溫排水而大量死亡。因此在環保法規對工業循環冷卻水的使用和排放標準漸趨嚴格的情況下，限縮採用此種直接排放的方式．根據新的環境保護標準，溫排水在排放之前需經過冷卻過程以降低水溫。加入冷卻水塔(Cooling Tower)利用蒸發散熱的方式達到控制排放水的溫度，是一個節省設備費用經濟的作法，不但可以兼顧國際及當地政府的規定，也可避免對海洋生物產生熱衝擊(Thermal Shock)。由於台灣位於大陸棚邊緣，東岸瀕臨太平洋，離岸數公里海深即可達到五百公尺。西岸台灣海峽平均水深僅六十公尺，以海水做為空調冷卻水，東岸可採用深層水低溫特性，直接與熱交換器做熱交換即可應用於空調冷卻使用，此為直流式的一種形式，西岸屬淺層海水，並沒有深層海水低溫的優點，水溫較高，經過熱交換器及冰水機做熱交換後可移除之熱量較少，須配合冷卻水塔，利用蒸發的方式與海水做熱交換，進一步將熱移除並降低排放水溫後再予以排放，此為蒸發式(Evaporation Type)海水冷卻系統。由於東岸和西岸地形不同關係，其應用方式不同，本研究將提供選擇冷卻形式的方法、環保法規之要求、海岸地形限制及系統設計需求，以經濟考量選擇適合的規劃及設計。依據選擇的結果，預估各項設備、工程建造及運轉的費用，並依各評估項目提出整個計畫從設計、環評(EIA)至施工完成各階段所需時程表，作為投資海水冷卻於經濟上是否可行之參考依據。

關鍵字

海水冷卻水塔；直流式；蒸發式；熱衝擊；熱交換器

並列摘要

According to numerous recent climate studies for environmental changes in Taiwan, Taiwan's rainfall situation becomes increasingly polarized, in that rainfall occurs less frequently in the dry season but more in the wet season. Although the average amount of rainfall in Taiwan reaches 2,500 mm per year, which is 2.6 times the amount of the global average, but 70% of the rainfall has flown into the ocean due to geographical limitations. The actual available and retainable amount accounts for 26% of the annual rainfall amount. Taiwan has been decreed as the 18th country suffering from water shortage. So, it is crucial for us to seek innovative technologies which can be the more promise for addressing water shortages over the next decades. Taiwan is an island and embraced by ocean with unlimited resource of seawater. Seawater is one of few rainfall-independent water resources. Such congenital environmental resource can be greatly utilized as a substitution of freshwater for the use of human consumption, irrigation and industry cooling media. Furthermore, utilization of deep water air cooling can decrease the demand for freshwater. It also saves energy, decreasing the harmful emission. To date, development of desalination and seawater cooling plants have been a solution to water scarcity, especially at the location where has a uniform and steady quality of seawater can be easily abstracted at a reasonable cost. Efforts to cut down on the large scale use of valuable fresh water in water cooling systems and switch to the use of large quantities of available ocean water as a replacement, might be Taiwan's future in solving its water supply shortage. The purpose of this study is to conduct a feasibility analysis and conceptual design for seawater cooling system in the areas of coastal industrial parks or cities. Changhua Coastal Industrial Park (CHCIP) was selected for this study. Evaluations of seawater cooling systems under various aspects were discussed, such as environmental regulation, option of construction methodology, major project planning of seawater drawing and waste water pipeline, and the analysis of investment costs and construction duration. In the past, many process industries in Taiwan were located along the coastal areas had used seawater as cooling water to replace freshwater, such as plants for fossil-fuel, nuclear power plants, petrochemical, steel mills, etc. Most of plants used the cooling technology called “Once Through System”. This type of cooling system requires taking tremendous amount of seawater through a heat exchanger or a chiller to transfer heat from equipment to water and releases hot water into the ocean. However, such systems have a significantly negative impact on the marine ecosystem, when discharging the water back to the sea at a higher temperature. More recently, stringent environmental regulations on industrial cooling water usage and discharge have made such once through systems redundant. According to the new environmental regulation standards, the resulting warm water must go through cooling to lower the water temperature before discharging back, to avoid a thermal shock to marine life. The installation of cooling towers, which use evaporative cooling, can be used to control the temperature of released water. This is a cost effective method that is not only complies with international and local regulations, it can also prevent the thermal impact on ocean organisms. Since Taiwan is located in the edge of the continental and east coast faces he Pacific Ocean, water depth can reach 500 meters just a few kilometers offshore. Hence, the east coast can use deeper cold water for deep water air cooling, which draws cold water directly through a heat exchanger. This is another method of the “Once Through System”. In contrast, average depth of West Taiwan Strait is only sixty meters. Because of this, the seawater temperature can be abstracted is relative higher. The amount of heat that can be transferred from equipment through heat exchangers is limited. As a result, it is necessary then to use cooling towers in conjunction with the heat exchangers to further cool down the emitted water from the heat transfer process, this is called the “Evaporation Type” seawater cooling system. Because of different terrain between east and west coast, selection of cooling application is different. Hence, the purpose of this study will guide people on selection of seawater cooling method depending on its location, understanding of environmental regulations and coastal terrain to come up with an appropriate planning and design. Based on the results of the selection, the costs of equipment, construction and operation can be estimated. In the meantime, this report can be used as a reference for the project assessment from the stage of design phase, Environmental Impact Analysis (EIA), construction to completion. Duration of each vital stage and whether the project is cost effective can be evaluated.