Exergy recovery from lng-regasification for polygeneration of energy

Resumen

In the context of the current climate emergency situation, Liquefied Natural Gas (LNG) emerges as a key transition primary energy source. Apart from the environmental benefits concerning other fossil fuels, LNG is a valuable physical exergy source because of its cryogenic temperature and usually high regasification pressure. To exploit that exergetic potential throughout the regasification process, multiple applications and technologies have been investigated. However, at present, their deployment in receiving terminals worldwide is scarce because of major drawbacks such as their limited techno-economic viability, among other bottlenecks. The combination of different applications through cascaded polygeneration schemes is an attractive technical solution to achieve high efficiency. But the structural complexity and, therefore, the high capital investment increase considerably concerning single-applications configurations. This doctoral thesis tackles the development of innovative and sustainable polygeneration systems to harness the physical exergy of LNG (and other cryogenics fluids such as bio-LNG) with a competitive efficiency-complexity ratio both in large-scale and satellite regasification facilities. The different configurations proposed consist of non-combustion systems that regasify LNG and produce electric power and/or refrigeration at different temperatures as by-products. Unlike more sophisticated applications based on immature technologies and which offer services to a very specific type of customers, electricity is a versatile energy carrier. Also, to find neighbouring industries and buildings with a certain refrigeration demand (e.g., for foodstuff or air-conditioning applications) consuming considerable quantities of power will be quite common. Moreover, both services can be generated by combining well-known technologies (e.g., heat exchangers, pumps and turbines) which would increase their implementation opportunities. The first large-scale cascaded polygeneration plant proposed is integrated by three power generation modules (Rankine cycles) driven by seawater, low-grade waste and biomass, respectively, and a district cooling network that supplies refrigeration at -20oC, -10oC and 5oC. The plant is used as a case-study to preliminary estimate the exergy recovery potential and to address a major technical issue: the selection of the most suitable operating fluids. Despite the lack of an ideal candidate, a remarkable finding is the suitability of natural fluids even at different thermal boundary conditions. As for the simulation results, the plant reported an equivalent electricity production of 125 kWh/t-LNG regasified and an exergetic efficiency of 40%. The next part of the thesis puts the spotlight on the techno-economic feasibility. The subsequent large-scale polygeneration configurations modelled consist of different combined cryogenic power and cooling units built upon the structure of a commercial 6 MW cryogenic power plant. The reference system is integrated by a propane Rankine cycle and a natural gas direct expansion unit, and has been in operation since long time ago, providing a certain degree of technical reliability for the new configurations proposed derived from this power-only plant. It was gathered information regarding the operation and the equipment utilized by the reference unit to calibrate the modelling and estimate costs. The techno-economic analysis revealed that the combined production in cascade of electricity and refrigeration for both low-temperature (i.e., -50oC) and air-conditioning applications achieves a successful balance between efficiency and complexity. For the baseline scenario, the simulations reported a nameplate equivalent electricity production and an exergetic efficiency of 150 kWh/t-LNG and 42%, respectively, with an estimated payback period of five years. Despite a simpler structure with respect to the first polygeneration plant analysed in the thesis, the better performance of this plant is mainly due to the higher amount of refrigeration produced and also at a lower temperature. The last part of the thesis deals with the recovery of the low-temperature exergy of LNG for foodstuff refrigeration applications in satellite plants. These facilities usually receive fossil LNG from large-scale harbour facilities, although they are also suitable for regasifying bio-LNG. Two strategies are evaluated to refrigerate the cold rooms. One aims to improve the efficiency of refrigeration machines by using LNG as a heat sink instead of the ambient whenever possible; the other option is to supply the cold directly without conventional refrigeration devices. To model the energy demands, factors such as the application of the gas regasified, the activity schedules, the day of the week or the ambient conditions are considered. The results show that the techno-economic feasibility of the configurations developed is generally limited to temperate/warm climates and medium-large plant sizes (e.g., with nameplate regasification capacities above ∼1,500 Nm3/h in local satellite plants). Those plants with constant regasification rates encourage the direct utilization of the cold without back-up refrigeration machines. For the case of satellite facilities where the regasification rates fluctuate, hybrid configurations which keep the conventional refrigeration systems provided in place, turn to be a flexible solution to ensure refrigeration for cold rooms. In conclusion, the cascaded polygeneration configurations developed in the thesis boost the competitiveness of the physical exergy recovery of cryogenic fluids. While their viability ultimately depends on factors such as electricity tariffs or the carbon prices, the production without combustion of electricity and multi-temperature refrigeration as regasification by-products contribute to the sustainability of the current energetic model and, particularly, to a cleaner refrigeration sector. Finally, in certain scenarios satellite plants are an attractive gateway to the integration of cryogenic biofuels in the energy mix for the recovery of the low-temperature heat from the regasification.