Sensitivity analysis of a Maisotsenko sub-atmospheric inverted Brayton cycle and comparison with relevant technologies

Details

Supervisors

Contino, Francesco

Faculty

Ecole polytechnique de Louvain

Degree label

Master [120] : ingénieur civil mécanicien, à finalité spécialisée

Abstract

Decentralised cogeneration appears as a versatile alternative towards a greater flexi- bility of our energy production system. Developing small-scale Combined Heat and Power (CHP) units is therefore required. The current market is mainly composed of Internal Combustion Engines (ICE). Still, micro Gas Turbine (mGT) cycles constitute another promising technology. Their current low market penetration is explained by their limited electrical efficiency (around 30% for a 100 kWe unit), which induces that heat supply is currently their main work field (with a considerable overall efficiency of 85%). This heat-driven aspect reduces their profitability when the consumer’s heat demand decreases. Indeed, the thermal power output of the unit is lost which makes the overall efficiency drop to the limited electrical efficiency. Increasing the commercial availability of mGT units requires thus to find a solution to decouple the heat and electricity production. Humidifying a mGT cycle is a potential way to use the available heat in order to improve the electrical performance. More than that, the Maisotsenko Cycle (M-Cycle) is a promising thermodynamic conception aiming at increasing the heat recovery by exchanging heat while taking advantage of the psychrometric energy available when evaporating water in the air. This best-performing humidified heat recuperator might allow to boost the electrical efficiency of mGT cycles thanks to its dew point cooling capacity. Unlike the typical Brayton cycle configuration used in most mGT systems, the Inverted Brayton Cycle (IBC) layout gives the possibility to reduce the power outputs of the cycle while keeping a modest efficiency. This downsizing asset is particularly interesting in the quest of small-scale power units. In this Thesis, the M-power component is numerically implemented and then in- troduced into a previously validated IBC model. The resulting cycle is called the Maisotsenko sub-atmospheric inverted Brayton cycle (M-SAB cycle). This model is thereafter used to assess the cycle performance and to understand how the M-saturator is influencing it. By drawing the related Sankey diagrams, the variation of the energy fluxes between each component is investigated. More than that, a sensitivity analysis is conducted to evaluate the impact of the M-Cycle performance parameters, the initial conditions of the working fluid and the design and control parameters. In order to adequately compare the electrical power output to the thermal one, an exergetic analysis is done using Grassmann diagrams. The results show that the M-SAB cycle offers the best electrical efficiency when the M-Cycle heat recovery is the highest (43.5% with an associated load of 30 kWe). Such efficiency exceeds that of current commercial small-scale CHP units. This latter is reached when fixing a wet-bulb effectiveness of 98% and injecting as much water as possible (48 g/s in that case, corresponding to a proportion of roughly one third with respect to the total mass flow through the cycle). These two parameters might constitute technological challenges in order to reach the performance achieved by our numerical model. Indeed, arriving at such effectiveness will probably require methods increasing the pressure drops, the size and the cost of the recuperator. At the same time, the high proportion of water injected might be restricted by the surge limit of the compressor. Another consideration associated with the discussed performance corresponding to the effectiveness of 98% is the low exergy content of the recovered heat which removes its cogenerative character. This latter is present with a lower effectiveness of 90%. But the electrical performance of the M-SAB cycle in that case (21.3% of electrical efficiency with an electrical power output of 20.1 kWe) is too low to justify the investment in such unit.