One of the areas creating great interest in the energy sector is the use of hydrogen fuel cells as a source of alternative energy. Eusebio Huélamo reports.
Spanish technology has been used in developing hallmark international projects such as the study on the dynamic behaviour of alkaline fuel cells being designed for the European Space Agency's Hermes space vehicle, or the experimental fuel cell plant built in San Agustin de Guadalix (Spain).
New achievements in the field of alternative energies are relying more and more on the use of simulations to design optimised processes for obtaining hydrogen.
In this context, the use of mathematical tools such as EcosimPro has been applied to a number of different cutting-edge technological projects such as the study on the dynamic behaviour of alkaline fuel cells being planned for the European Space Agency's (ESA) Hermes spacecraft.
To make this model, specific components had to be programmed for the fuel cells themselves and for the membrane separators. The system chosen for that model (see table) is based on immobile electrolyte alkaline fuel cells.
In each cell, the hydrogen and oxygen react and generate an electric current, producing water and heat as by-products. The hydrogen circulates by means of the jet compressor, aided at low speeds by the compressor. The water produced in the fuel cells is dragged by the hydrogen in the form of steam, and partially removed in the membrane separator. The pump is in charge of keeping the flow or the cooling circuit which, in turn, is in charge of removing the heat produced in the fuel cells.
The water passes through a heat exchanger making up part of the Heat Control System of the Hermes spacecraft.
There is a controller to regulate the water temperature at its optimum operation point by diverting some of the flow of water outside the heat exchanger. There is also a second control loop to regulate the temperature of the hydrogen at the outlet of the membrane separator, thereby ensuring that the concentration of electrolyte in the cells remains within acceptable limits.
At start-up, the system temperatures are normally below the normal operating points; the cooling water does not pass through the heat exchanger during this phase, completely bypassed at this stage, and also is pre-heated with help from the electric heater that in turn is powered from the energy produced by the fuel cell.
Operation in this situation is very inefficient, and results in a large amount of heat being produced, which helps the temperatures rise sharply. The complete bypassing of the heat exchanger means that water cannot be removed from the hydrogen stream, which can lead to concentrations that make the electrolyte inoperable. If the concentration becomes too low, the electrolyte may pass to the hydrogen loop, which would make the cell malfunction.
The main point here is that the start-up phase is relatively critical, and therefore is worthy of considerable study. This is the point where mathematical calculation tools become essential.
The simulation models correctly reproduce the real behaviour of all the relevant parameters of the systems. The results can be used to make a quick and easy study of how the systems respond during the critical phases of operation as well as analysing how they affect and interact with each other.
Composition of the chosen system
Main component Fuel cells
Hydrogen loop Membrane separator
Cooling circuit Pump
Joints or forks
Control loops Heat sensors
Eusebio Huélamo is Section Head, Thermal Hydraulics Simulation, Empresarios Agrupados (EA). For more information, visit www.empre.es