Thermodynamic analysis of organic rankine cycle waste heat recovery system for an oil tanker

In today's world where energy demand has increased day by day and the negative effects of current energy usage on the environment has already reached noticeable dimensions, energy efficiency, energy saving, renewable and sustainable energy have become the primary issues. More than 90% of the world's trade is carried out by international shipping, so that the concepts of energy and emissions for ships are the topics on the agenda as well. It is estimated that by 2050, the atmospheric carbon dioxide (CO2) concentration will be between 480 ppm and 550 ppm, and this range will increase the global temperature from 0.5 °C to 2.5 °C. Transportation by sea has the share about 3.3% of global CO2 emissions. International Maritime Organization (IMO) has set limits on emissions gradually, and has set regulations on energy efficiency for ships. It was aimed to reduce emissions by 80% for nitrogen oxide (NOX) and around 90% for sulfur oxide (SOX) compared to the first limits in emission control areas (ECAs). The mandatory measures have been taken to reduce emissions of greenhouse gases and increasing energy efficiency of ships, are the establishment of the "Energy Efficiency Design Index" for new vessels, adopted for Annex VI of the International Convention for the Prevention of Pollution from Ships (MARPOL) and the preparation of the "Ship Energy Efficiency Management Plan" for all vessels greater than 400 GT has been put into effect. The loss of energy in ships is largely in the ship's main engines. After the combustion process, the power used for propulsion is only about 30%, heat load of cooling of main engine and exhaust energy are about 50% of the total fuel energy. While the exhaust gas has the share about 25% of total fuel energy. Scavenge air, jacket water and lubricating oil have the share about 16%, 5% and 3%, respectively. Therefore, it is an important step to recover this energy to increase energy efficiency and to decrease operational costs and emissions. After the strict measures of International Maritime Organization, engine manufacturers and researchers have been developing different technologies on these issues. Classical Rankine cycle, Kalina cycle, exhaust gas turbines, thermo-electric power generation and combined heat and power systems are the common solutions for waste heat recovery technologies. One of the most promising technology is the Organic Rankine Cycle system that recovers the waste heat of the main engine to generate electricity. Unlike conventional Rankine cycle, ORC uses organic fluids in closed system. Main reason for using ORC is that the system can recover the low-grade waste heat of the sources. Considering the quality of ship waste heat, ORC technology is an innovative and strong option to recover the waste energy. ORC system performance is the most important issue for operations so that many studies focus on improving efficiencies. One of the key point of increasing cycle performance is selecting an appropriate working fluid. It should have good thermodynamic properties and should give high performance outputs. Moreover, considering the International Maritime Organization regulations and ship's safety, fluid should be environmentally friendly as well. It should have low global warming and ozone depleting potential, non-corrosive, chemically stable, non-toxic, non-flammable, non-explosive, cheap and accepted by the industry. In this study, waste heat analysis of the main engine of a chemical/oil tanker was made and the waste heat recovery models of Organic Rankine Cycle were established by using different waste heat sources according to the heat loads and source temperatures. Jacket cooling water, scavenge air and exhaust gas were determined as waste heat sources. In addition, it was aimed to investigate the ORC system performance by using the waste heat of the medium-range tanker's main engine to meet navigation electricity requirement. During the thermodynamic analysis, R245fa was selected as the working fluid for the ORC power generation systems. Jacket water, scavenge air and exhaust gas waste heat recovery models were analyzed separately then three waste heat sources were combined as a unique model. Waste heat analysis indicated that jacket water waste heat had the lowest quality. However, high amount of mass flow rate was made remarkable the source's waste energy to recover. Scavenge air waste heat was very sensible to the engine load. When the engine load increased, its heat load increased rapidly as well. This situation effected the cycle performance and assumption criterias. Exhaust gas has a very big potential waste heat based on its high temperature and heat quality onboard ships. However, minimum exhaust temperature is recommended as 140 °C by engine manufatures due to the acid forming problem in funnel, so that it's limited to absorb the full energy of exhaust gas. The major advantage of the exhaust gas waste heat that it enables the ORC system to work in higher pressures and evaporation temperatures. According to the ORC models' analysis result the jacket cooling water ORC system has the lowest power capacity as 103.2 kWe comparing other recovery models. The gain was calculated as as 312.9 kWe by the exhaust gas waste heat ORC system. This value for the scavenge air waste heat ORC system was 526.9 kWe. The highest capacity was obtained from the combined waste heat ORC system as 759.2 kWe. Results also indicated that at the economic navigation speed of the ship, the combined waste heat ORC system produces 589,2 kWe of electricity meaning that the ship can cruise without ever running the generators by using the ORC system. Analysis results show that by using the combined waste heat ORC system, up to 874,11 tons of fuel can be saved annually. This value for jacket water, scavenge air and exhaust gas waste heat ORC systems are 113,06 tons/year, 606,21 tons/year and 355,73 ton/year, respectively. This means that total maximum fuel saving from individual systems can be calculated as 1074,95 tons, annually. Although this case seems more fuel-efficient, the individual systems are 24.16% more expensive than the combined waste heat ORC system and the pay-back time is 21.7% longer then the unique model. Estimated pay-back times were calculated as 7,28 year for jacket water, 6,13 years for exhaust gas, 4,9 years for scavenge air and 4,33 years for combined waste heat ORC models. As a result, lower power output capacity resulted in higher investment cost and longer pay-back time. The integrated use of ORC systems with the main engine had an improving effect on overall thermal efficiency of the main engine. The total thermal efficiency can be increased up to 1.13% by jacket cooling water, 3.17% by exhaust gas, 4.56% by scavenge air and 8,1% by the combined waste heat ORC systems. In conclusion, combined waste heat ORC system is able to meet the navigation electricity load when the main engine loads are above 68%. This result, which corresponds to the most efficient operation range of the engine, is an innovative and important step towards saving fuel, reducing operational costs, fulfilling IMO regulations and increasing energy efficiency of the ship.