ORC Organic Rankine Cycle Power Systems Updates 2024

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Paper ID: 86, Page 2
Paper ID #86, page 2 of 10
a very low round-trip efficiency of 15.5% only, which is not high enough to compensate for the high capital cost required to implement this solution (Escamilla, 2023). This highlights that the incorporation of much more efficient power generation devices into these P2P-EES systems is the most critical route towards the development of cost-effective, small-scale P2P-EES solutions based on green hydrogen.
Micro gas turbines have power outputs below 500 kWe, typically, a range where the main competing technologies are Internal Combustion Engines (ICE) and Fuel Cells (FC), technologies featuring higher efficiency than mGTs and, for ICEs, also lower capital cost (CapEx). Therefore, despite the visible advantages of mGTs (low NOx, high-grade heat, flexibility, fast-response capability), this does not suffice to emerge as a more interesting prime mover. It is hence essential for mGTs to increase their electric efficiency in order to gain market share in a future energy system transitioning towards decentralized energy solutions (Tilocca, 2023). The Aurelia A400 engine features the highest electric efficiency of all mGTs in the market: 40.2% (LHV) (Aurelia, 2023) (Escamilla, 2022). This efficiency is comparable to ICEs and FCs of similar power output but cannot be sustained at smaller outputs because of the need to adopt simpler cycles or due to the impact of scale effects on component efficiency.
In the foregoing context, the current study aims to increase the efficiency of mGTs for a wider output range in order to enable their utilization in P2P-mGT ESS. To this end, past work by the authors concluded that the efficiency of the power conversion unit must be, at least, 45% since this boosts the round-trip efficiency of the resulting solution to 26% (in lieu of 15.5% for an mGT with 26.9% efficiency). The consequence of this is an almost 45% reduction in hydrogen consumption of the mGT, which translates into a footprint reduction of all systems involved in a P2P-mGT ESS, largely reducing the CapEx of the overall system. Two options to enable such efficiency increase are considered: higher efficiency mGTs and combined mGT + waste heat to power bottoming system. In the latter case, the exhaust gas from the mGT is used to drive a thermomechanical energy conversion system to produce additional electric power from the mGT waste heat. The two options are typically considered for WHP: Steam Rankine Cycle (SRC) and Organic Rankine Cycle (ORC). Amongst the two, steam is not cost- effective at a small scale, given the very low performance and high cost of steam turbines of a few hundred kilowatts (Vanslambrouck, 2012). Therefore, this research investigates the thermodynamic feasibility of integrating ORCs with different recuperative Brayton Cycle layouts, used by microturbines, to boost the power-to-heat ratio, and thus, the electric efficiency. A short literature review on the topic is presented below.
Bao et al. (Bao, 2013) presented a review of organic working fluid and expander options for ORC systems, screening the most prominent Key Performance Indicators for the selection of these two elements. Bonolo de Campos et al. (Bonolo de Campos, 2019) presented a review of the combination of micro gas turbines and ORC systems, concluding that the efficiency of a 100 kWe mGT could increase from 30% to 35% when combined with an ORC system, even if at the expense of 48% higher capital cost; it is to note that the work cited is a review and does not rely on calculations by the authors. Mago et al. (Mago, 2013) examined the potential of using the exhaust gas from microturbines in ORC systems, considering microturbines with different power outputs and different fluids in the ORC system, and basing the analysis on electric and exergy efficiencies. For this, the authors considered a set isentropic efficiency for both the ORC turbine and pump, and also a fixed electric efficiency for the mGT (ranging from 25% to 31%, depending on power output). The work gives a good understanding of which fluids might perform better whereas the present paper goes into more detail about the assessment of potential configurations of Brayton cycles and ORC systems that could help raise global efficiency, and therefore, the round-trip efficiency of the P2P-mGT ESS that the authors have previously investigated. This is of course not a new concept, as the forecited works have studied it already, but the integration of the mGT within a P2P ESS with the objective to break the 45% efficiency barrier and the improvements that are needed are innovative, as deduced from the literature review carried out by the authors. For that, systems forming both cycles will need to be dealt with in detail in the forthcoming sections.
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7th International Seminar on ORC Power Systems, September 4 - 6, 2023, Seville, Spain
7th International Seminar on ORC Power Systems, September 4 – 6, 2023, Seville, Spain