Separaciones conyugales

Since William Grove demonstrated the first fuel cell in 1839, the reversibility of the water splitting and recombination reaction has been well known. Interest in fuel cells grew rapidly from the 1950s onwards particularly to generate electricity for spacecraft using safe, compact and relatively lightweight technology. Early fuel cells used in the US and Soviet Union’s space programs utilised hydrogen and oxygen stored on board. But it was soon recognised that a reversible cell capable of both electrolysis and power generation could be the central component of a closed-cycle regenerative energy storage and power supply system that used solar energy to split the stored water into hydrogen and oxygen. These gases could then be stored and used as required in a fuel cell to produce power, together with water once again that could be returned to the water storage tank for reuse. The only external input to the system would then be the solar energy.

An early proof of the URFC concept was provided by General Electric in 1973 when a lifecycle test of a single-cell PEM URFC showed that reversible operation was feasible without significant degradation of the cell membrane (similar to DuPont's Nafion 120) and catalyst (Mitlitsky et al., 1996a).

In 1992, The Lawrence Livermore National Laboratory (LLNL) working with AeroVironment Inc., identified URFCs as the ideal energy storage solution for the High Altitude Long Endurance Solar Rechargeable Aircraft (HALE SRA) program being funded by the Ballistic Missile Defence Organization (BMDO). Electricity from solar panels located on the model aircraft’s elongated wings was used to drive the propellers directly during the day, with the surplus fed into a URFC in E-mode to produce hydrogen and oxygen for on-board storage in transparent polycarbonate tanks (Figure 4). The hydrogen and oxygen gases were then fed back into the URFC in FC-mode to generate power to keep the propellers turning and the plane moving during the night (Mitlitsky et al., 1999a and b).

Figure 4:The model solar-powered aircraft with a URFC-based energy storage system built in the High Altitude

Long Endurance Solar Rechargeable Aircraft program in the USA in the mid 1990s. Top - on Rogers Dry Lake bed before flight; bottom – the aircraft in flight at an altitude of 60 m. Source: Energy and Technology Review, 1994.

In 1993, LLNL, worked with Hamilton Standard to design and construct the first portable URFC demonstration unit (Mitlitsky et al., 1999b). Several copies of this URFC propeller toy (shown left in Figure 5) were built to demonstrate that from an initially inert state (just low pressure water), electrolysis can commence (using solar power) to produce pressurised hydrogen and oxygen in storage tanks (all embodied in transparent polycarbonate). After a charging (pressurisation) period of seconds to minutes, a switch is toggled to place the load (motor) across the fuel cell, so that it is powered by the URFC operating as a fuel cell. This URFC demonstration proved to be so popular that its copies were invited to attend meetings and are sought by politicians, funding agents, and teachers worldwide (Mitlitsky et al., 1999b). Proton Energy Systems (now called Distributed Energy Systems) recognised the sales value of such URFC toys early, and has produced its own toys (shown right in Figure 5) for sales purposes, which may be useful in portable applications below ~5 Watts (Mitlitsky et al., 1999b).

Figure 5: LLNL/Hamilton Standard URFC Demo unit (left), Proton URFC demo unit (right) Source: (Mitlitsky

et al., 1999b)

In the latter half of the 1990s, similar URFC technology began to be developed for space applications. LLNL adopted a reversible aerospace PEM technology, available only from Hamilton Standard (HS) before 1998, in order to solve the challenging problem of propelling a solar power aircraft through the night (Mitlitsky et al., 1999a).

research and development on RFCs as an enabling technology for HALE SRA program under its Environmental Research Aircraft and Sensors Technology (ERAST) program. This research was leveraged by the U.S. Department of Energy to develop similar systems for ground transportation and utility applications (Mitlitsky et al., 1998b).

The hydrogen and oxygen generated by a URFC were the ideal rocket propellants combination used by launch vehicles, such as the main engines of the Space Shuttle. The benign nature of water as a precursor for made-on-board rocket fuel created the need for another generation of LLNL demonstrations, as shown in Figure 6 . This water rocket demonstration or “torch toy” electrolysed gases from initially ambient water up to 100 psi (0.69 MPa) hydrogen and oxygen. Once pressurised, the system was capable of demonstrating safe combustion in conference rooms, and had been designed to be combined with the URFC propeller toy to demonstrate hybrid energy storage and propulsion (Mitlitsky et al., 1999b).

Figure 6:LLNL/Hamilton Standard URFC Demo unit. Source: (Mitlitsky et al., 1999b)

Mitlitsky et al., (1996b) at LLNL developed a concept and much of the componentry for the so-called Integrated Modular Propulsion and Regenerative Electro-Energy Storage System (IMPRESS) for Small Satellites. In IMPRESS a single URFC stack of cells produced hydrogen and oxygen by using solar cells to electrolyse water carried on board. These gases

were then used either directly as fuel for small rocket boosters for satellite navigation, or to generate electricity for the satellite’s electronic systems using the URFC stack operating in FC-mode. URFCs were the heart of IMPRESS, which produced power and electrolytically regenerates its reactants using a single stack of reversible cells ((Mitlitsky et al., 1996a).

In the mid 1990s, a primary fuel cell (FC) test rig with a single cell 46 cm2 active area was modified and operated reversibly as a URFC by LLNL (Mitlitsky et al., 1996a). URFC systems with lightweight pressure vessels were also designed by LLNL for zero-emission terrestrial vehicles (Mitlitsky et al., 1996a and b), and utility and remote power energy-storage applications, with funding support from the US Department of Energy and Proton Energy Systems Inc. as a prime contractor the technology development and testing of URFC stacks (Mitlitsky et al., 1998a and b; 1999b).

In a DOE-funded industrial demonstration effort, LLNL directly supported Distributed Energy Systems as prime contractor in its technology development by testing URFC stacks. These systems included high altitude long endurance (HALE) solar rechargeable aircraft (SRA), zero emission vehicles (ZEVs), hybrid energy storage/propulsion systems for spacecraft (Mitlitsky et al., 1998a).

Distributed Energy Systems has subsequently developed and prepared for commercialisation its Unigen range of URFC systems (Figure 7) for energy storage applications in space, defence systems, remote-area and uninterruptible power supply, zero-emission transportations systems, and possibly peak-shaving applications on utility grids, under research contracts from Electric Power Research Institute, the NASA Glenn Research Centre, and the US Missile Defence Agency (Mitlitsky et al., 1999b; Smith, 2000). Distributed Energy Systems offer both URFC and DRFC systems depending on the application. Unigen URFC stacks are capable of being operated to pressures of over 10 bar (Molter, 1999; Smith, 2000). In addition the company has been involved in a joint development program with Marconi to incorporate Proton's reversible fuel cells (initially with 70 kWh storage capacity) in Marconi's power systems for the

(Power Engineering, 2003). The US companies, United Technologies Corporation and Hamilton Standard (Space and Sea Systems Division) have undertaken research and development on URFCs primarily for aerospace applications (Mitlitsky et al., 1999c).

Figure 7: Unigen URFC systems developed for commercial sale by Proton Energy Systems (now Distributed

Energy Systems): the complete unit (left) and the URFC stack (right). Source: Mitlitsky et al., 1999b; Smith, 2001

In December 2003, Distributed Energy Systems was awarded two contracts for Regenerative Fuel Cell Development from NASA and the U.S. Missile Defense Agency for development of lightweight URFC technology for unmanned aerial vehicles and high-altitude airships (PR Newswire, 2003).

Other US companies, such as Giner Inc. and Lynntech Inc. have also been developing URFCs, and offer for commercial sale suitably-catalysed PEM electrode assemblies for URFCs. The company, Electric Boat, is working with the University of Connecticut on a seven-cell URFC stack coupled to a metal hydride storage system for use in submarines (University of Connecticut, 2005).

A range of other governmental, university and private sector research groups have been working on experimental RFC systems around the world. One of the largest R&D projects on URFCs outside of the US is currently the Revcell project in The Netherlands to develop autonomous energy supply systems and uninterruptible power supplies using PV arrays and a

URFC-hydrogen based system for long-term energy storage for PV stand-alone systems. This project is being conducted by Hynergreen Technologies, Spain, Fraunhofer-Gesellschaft, Germany, NedStack Fuel Cell Technology, and others (Nedstack, 2007).