Sustainability in power production requires the proper utilization of all – or most – sources. Despite the growth in solar and wind power, the majority of electricity is still generated from heat by various heat-to-power methods. Often, heat is produced explicitly for power generation, but numerous existing heat sources (including some renewable ones) could also be used for this purpose. Among these sources, the majority of them have low heat flow rate (max. a few tens of kWth, where th denotes thermal power), which would limit the production of more than a few kWel (el indicates electric power), even with advanced heat-to-power methods. In such cases, minimizing the device cost and the power losses could be more important than maximizing the cycle efficiency or the net power. A novel approach is presented here to introduce a simple but still efficient cycle. The new cycle – existing only on a conceptual level – is a trilateral trans-critical cycle. With fewer components, the expected costs and theoretical self-consumption are low. These properties make the cycle feasible to economically utilize heat sources with very low heat flow rates for power production. The conception of the cycle will be presented in detail by using carbon dioxide as working fluid.

In the last years the importance of natural gas in the global energy mix has increasedlargely, due to its role in the energy transition toward a decarbonization. While in 2000 natural gas contributed to the overall primary energy supply with slightly more than 20%, its share has increased up to 24.7% in 2020, corresponding to 137.6 EJ. The higher natural gas consumption has led to a large increase of interregional trades (940∙10⁹ m³ in 2020, with an annual growth rate of 4% over the last decade). Among natural gas transportation technologies, Liquefied Natural Gas (LNG) plays a dominant role, and it recently surpassing pipelines as main transportation technology. In fact, in 2020 LNG contributed to about 52% of the overall natural gas trades, growing from a share of 41% in 2010. In this context, an efficient operation of LNG terminals becomes a key aspect both for environmental as well as economic reasons. Especially the recovery of the cold energy available at receiving terminals during the LNG regasification process assumes a fundamental importance. As of today, at the regasification terminals a significant amount of cold energy (LNG is typically stored at about -160 °C and ambient pressure) is often wasted.

This paper explores the integration of the LNG regasification process with one or more topping cycles, in which the working fluid directly exploits the available LNG cold energy through its condensation process. Two different categories of topping cycles have been considered, depending on the heating source of the working fluid: a) a low-temperature one using seawater as a heat source and b) a high-temperature one using exhausts from a gas turbine as a heat source. The topping gas turbine has been assumed to be fed with a portion of the natural gas from the regasification process. The choice of working fluid is critical, as its condensation temperature is far below ambient temperature. This study evaluates two working fluids: CO₂ and an organic fluid (R125) have been selected as working fluids in this study, and their performance has been compared. CO₂ is analyzed in both subcritical and supercritical conditions, depending on the operating temperature range of the topping cycle The performance of the overall system has been calculated as a function of the pressure of both the regasification process and the natural gas distribution grid. Additionally, a gas turbine has been also introduced in the LNG regasification line in order to exploit the difference between regasification pressure and distribution pressure.

A medium-size regasification terminal (50 kg/s) has been chosen as reference, leading to a topping cycle power output ranging between 3 and 35 MW depending on the heat source and on the working fluid. The topping gas turbine is a medium- to large-scale gas turbine with a power output of approximately 70-80 MW.

Dedicated models have been developed using the Aspen Plus commercial software to simulate the regasification process the integrated topping cycles, and their mutual energy integrations.

The development of hydrogen turbines combine cycle is crucial for advancing the hydrogen economy. However, the performance of the combined cycle is significantly influenced by the selection of an optimal bottoming cycle. Among various options, the supercritical carbon dioxide (sCO₂) cycle has emerged as a promising candidate due to its superior thermal efficiency compared to traditional steam cycles. This study investigates the performance of hydrogen gas turbines integrated with an sCO₂ bottoming cycle. The Kawasaki M1A-17 turbine was selected as the gas turbine and the turbine model was corroborated by comparing the results with the KAWASAKI catalogue. The partial heating cycle layout of the sCO₂ us selected as the bottoming cycle, since the partial heating cycle has been shown to outperform other configuration in terms of waste heat recovery. The analysis employed energy and exergy perspective to provide a comprehensive assessment of each configuration. The exergy analysis revealed the optimum turbine inlet temperature is 400°C for the inlet pressure of 210 bar. At 400°C and a compressor inlet temperature of 35°C, the sCO₂ cycle produces net power of 834.31 kW. At these conditions, the H₂ turbine only efficiency is 28.8% while the combine cycle offers an efficiency of 40.9%. The sCO₂ cycle performance improves considerably at cooler temperatures. For a compressor inlet temperature of 15°C, the sCO₂ can produce net power of 931 kW and the 1.5%-pt improvement in the combined cycle efficiency can be observed.