Regenerative Energy Storage System (RESS)

Abstract

A hydrogen based energy storage system allows the separation of power conversion and energy storage functions, enabling each function to be assessed individually for performance, cost and feasibility of manufacture. Energy storage systems that use hydrogen as their primary storage medium are already rivalling conventional energy systems in many aspects. The Regenerative Energy Storage System (RESS) currently under development at Brunel University is focusing on using advanced hydrogen technology in the design of small units capable of acting as buffers between the generation and use of electrical power, allowing intermittent power generation technologies to be employed in domestic and portable devices.


Introduction

The remarkable recent advances in wireless and portable communications devices (e.g. laptops, mobile phones, e-organisers) have fuelled the need for high-energy-density portable power sources for consumer use. In a similar way, interest in the portable power market has increased amongst users who currently are dependent on batteries or other logistic fuels. 

Today, fuel cells have reached a degree of development from which it is possible to envisage future commercial fields where technology could have a decisive role in many applications where electricity must be produced with high efficiency and low environmental impact. 

The Proton Exchange Membrane (PEM) fuel cell is considered to be a viable portable power source for the future user. A low temperature, reliable and lightweight fuel cell with a solid electrolyte, which when combined with a suitable fuel, can provide power far beyond the capacity of batteries. 

When used as an energy storage device, the fuel cell is combined with a fuel generation device, usually an electrolyser, to form a Regenerative Fuel Cell (RFC) system. The RFC can convert electrical energy so it can be stored and later use this fuel to produce electricity on demand. Most common RFC units use hydrogen as their storage medium, which is generated through electrolysis of water. 

Through separation of the energy storage systems functions, the effectiveness of the system can be valued and each function optimised for best performance, cost and feasibility of manufacture. 

RESS proposed concept

The proposed URFC system to date is a completely regenerative power unit where all the components are hermetically sealed into one product. The only external input needed is the electrical power to run the electrolyser to produce the hydrogen and oxygen for the fuel cell and makeup water for the electrolysis. This external electrical input could come from a renewable resource.

The system consists of a polymer electrolyte membrane (PEM) water electrolyser, a PEM fuel cell and two storage chambers for the hydrogen and oxygen. In the chambers, water is also used and stored for use during the electrolyser mode.

The system has been designed so that when electricity is needed; the system must first be charged (water electrolyser mode) before it can be discharged (fuel cell mode), it cannot be used in both modes simultaneously. It can, however, be charged again before the fuel and reactant gases have been used up. By pressurising the hydrogen and oxygen gases, the size of the system is minimised at the same time as the efficiency is improved. This leads to a small compact system that can be run forwards and backwards.

Membrane Electrode Assembly

To minimise the thickness and layers in the membrane electrode assembly (MEA) for a URFC, work is currently being aimed at printing bi-functional electrodes directly on to the membrane. These electro-catalyst bi-functional layers would work in both the evolution of hydrogen and oxygen as well as in the oxidation/reduction of the same gases. The development was awarded the PRIME Faraday SPARK award in 2003 and is in collaboration with Davison Chemographics Ltd.