The influence of the ammonia concentration on the performance of OTEC power cycles

The influence of the ammonia concentration on the performance of OTEC power cycles

Kuikhoven, L.J.

Kirkenier, J. (mentor)
Infante Ferreira, C.A. (mentor)

Mechanical, Maritime and Materials Engineering

Process and Energy (P&E)

2017-06-07

The oceans cover over 70% of the earth’s surface and are therewith the largest source of renewable heat available on the planet. Ocean Thermal Energy Conversion (OTEC) is a process that exploits this low-grade oceanic heat. A heat engine is employed to generate electricity from the small temperature gradient that is available between the tropical surface water and deep seawater. The big advantage of OTEC is that it provides base-load power and it is especially interesting for tropical islands who currently rely heavily on diesel generators. The small temperature difference between the heat source and sink leads to a significant challenge to attain reasonable thermal efficiencies. The working fluid and the applied thermodynamic cycle significantly affect the thermal performance of the plant. In this work an analysis is made between pure ammonia and ammonia-water as the working fluid for an experimental OTEC set-up. Simulation software has been created and validated for the OTEC demo set-up in the Process & Energy laboratory of Mechanical Engineering at Delft University of Technology. The simulation software finds the operating conditions of the OTEC set-up with respect to the ammonia concentration of the working fluid, the geometry of the components and the mass flows of the system (cold & warm water and the working fluid). Each process unit is modelled separately to increase the flexibility of the model. The model is written in Python 2.7 and can use the REFPROP 9.0 database (Lemmon, Huber & McLinden, 2007) or CoolProp database (Bell et al., 2014) to determine the required properties of water and ammonia. For ammonia-water the thermodynamic properties are determined with the Ziegler & Trepp equation of state implementation by Rattner (Rattner and Garimella, 2016) and the transport properties by Condé (Condé, 2014). Water-water experiments have been conducted in order to empirically fit a heat transfer coefficient correlation. The result is a custom single phase water correlation - GoudKuik - for the heat transfer of water in the OTEC set-up. Component level validation with experimental data leads to the selection of the Donowski & Kandlikar heat transfer correlation (Donowski and Kandlikar, 2000) for the single phase behavior of ammonia(-water), and the two phase ammonia(-water) evaporation behavior is relatively well predicted by the Ayub correlation for direct expansion evaporators (Ayub, 2003). There are currently no two phase condensation correlations that predict the behavior of ammonia(-water) well enough. Therefore a surface area fraction correlation was fitted with the experimental data. An analysis of the experimental data leads to the conclusion that the saturation pressure of ammonia-water depends linearly on the liquid solution ammonia concentration. Therefore the power output also mainly depends on the liquid solution ammonia concentration. The power output is further decreased for a mixture due to a decrease in overall heat transfer coefficient in both the evaporator as well as the condenser. An increase in heat transfer area or Reynolds number of the flow can counteract this effect, then ideal Kalina cycle operating conditions can likely be achieved. A model optimization leads to the optimal operating conditions for the OTEC set-up for several ammonia concentrations. Though the difference between ammonia and ammonia-water (96%) becomes smaller in terms of power output and thermal efficiency, pure ammonia remains superior. The off-design performance of the set-up is analyzed with two case studies for different in- and outlet (sea)water temperatures for 96% ammonia-water and 100%. In both cases pure ammonia still outperforms ammonia-water, though there was one scenario where the thermal efficiency of 96\% ammonia-water was higher than pure ammonia. Ammonia-water also shows more steady behavior between different boundary conditions which can be beneficial for large fluctuations in (sea)water temperatures. From the results of this work an updated Kalina cycle configuration should be designed and compared to optimized Organic Rankine cycle design (both single as well as multi-stage) before a conclusive decision can be reached about the superiority of pure ammonia over ammonia-water.

Ocean Thermal Energy Conversion
working fluid mixtures
Kalina cycle
Organic Rankine Cycle

http://resolver.tudelft.nl/uuid:3d51f86a-4f03-4a59-bb6d-e1761d7a9a10

2022-06-07

Student theses

master thesis

(c) 2017 Kuikhoven, L.