Throughout the thesis, a number of assumptions and simplifications were made. In this section, the ones estimated to be most significant are discussed and their impact on the calculations is analyzed.
General scope definition of the thesis
The broad definition of the subject matter was the primary reason why many assumptions had to be made in the first place. As this many technologies, some of which are based on completely different phenomena, were included in the scope there simply was no time or room to accurately analyze some significant factors within the limitations of a master’s thesis.
Fuel gas composition
For the calculations it was estimated that the fuel gas consists entirely of methane. In reality, this is not the case as natural gas also includes a varying percentage of other hydrocarbons. The impact of this assumption on the results obtained is estimated not to be very significant.
Ideal pumps and estimated losses
As the pumps were estimated to be ideal, no losses from them are included in the calculations.
The same applies to the heat exchangers, which were estimated to have a fixed and relatively small pressure loss regardless of other values. However especially with the higher pressure levels of some systems the losses of pumps, and the larger and varying more realistic pressure losses in heat exchangers could have become a real impact on the results.
Turbine efficiency and operation
The efficiency used for the turbines in this thesis causes inaccuracies for the calculation, even though it is based on a real turbine. The efficiencies of turbines in the temperature range and working fluids can be different to those of components designed for air, exhaust gases, water,
or steam. Additionally, the turbines in this thesis were estimated to generate power whenever the net power was positive, when in reality the turbines would be likely to have a cut-off limit at lower performance values. As the turbines in the thesis function a large percentage of time at partial loads, these factors have a significant impact on the results. The pressure differences defined for expansions were also high especially for the ORC system, which would be likely to demand splitting processes to avoid problems with machinery.
Splitting processes and cascading cycles
To simplify calculations, heat exchangers and turbines were kept as single components in calculations throughout this thesis. In reality, the heat exchangers would be extremely likely require more than one unit to fulfil their performance requirements, or at least to improve their performance. For example, by splitting the regasification process the requirements for the associated working fluids could be loosened slightly – possibly resulting in better overall performance and reduced costs. The same goes for the turbines, many of the case studies found that expansion in multiple stages has superior performance. For example, a 2-stage direct expansion system was found to have a 6,3 % better efficiency and 60 kJ/kgLNG better nominal energy production than a 1-stage system (Dorosz et al. 2018, 15.). Cascading multiple cycles was also found to improve the total performance of the system significantly. (Bao et al. 2017, 577-582.)
Scale of calculations
Most sources used for this thesis were from regasification terminals, that handle mass flows significantly higher than those of marine engines in a cruise vessel. This adds uncertainty to the calculations, because the main parameter used to evaluate profitability was calculated with investment cost equations from a larger facility. The obtained values were also difficult to benchmark as not many real-world references exist for the same or similar scale scenario as the one in this thesis.
Capital cost equations and total costs
The methodology used for the investment costs of components has several points that cause inaccuracies. With the highly limited access to any comparable information, benchmarking the results is difficult. For example, the equation used for the direct expansion turbine was
originally sourced to a turbine in a Brayton cycle utilizing natural gas as its fuel. The difference between the equations were coefficients changed in the first term without further explanations.
The original equation can be found in (Baghernejad & Yaghoubi 2010, 2202.).
The scale of the results is also a factor causing slight uncertainty. With the equation used for the ORC-turbine in this thesis, a cost of 387 000 USD was obtained. This amounts to about 2 500 USD/kW, which is within the range generally associated with ORC systems according to (Lemmens 2016, 5.). For the direct expansion turbine, the investment cost was calculated to be 8 000 USD, which appears small in comparison, especially because the outputs, mass flows, and pressure levels are comparable to one another.
The calculation of the heat exchangers falls in this same category, as the U-values for each type of heat exchanger was estimated to be a fixed number, and some temperatures used for surface area calculations were approximations. Both of these factors cause uncertainties to the reliability of the stated investment costs and payback periods. The turbine equations are more significant of the two, because the associated costs are larger especially with the ORC-system.
With the amount of specific system planning done, the added costs of piping, valves, and other similar components was done very roughly. As the costs of individual components vary widely, this can cause a significant error to the results especially as it is relatively safe to assume that components on systems with higher pressure levels cost more on average. Additionally, classification society acceptance is required for most systems, adding to total costs.
Calculation of obtained savings
The calculation of savings was done by assuming that the fuel mass flow remains the same to simplify calculations. In reality, the change in fuel consumption through savings would impact the availability of the LNG heat sink, which in turn would change the fuel savings, and so on.
To obtain accurate results several calculation loops or the creation or a more complex calculation tool would have been necessary and was deemed unnecessary for this thesis. The magnitude of impact this has on the results is unknown.