Overview
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Hybrid electrical vehicles (HEV) have become a key contender in the quest for future low pollution transportation systems. An urgent need exists for an alternative means of transport especially in the larger metropolitan areas. Progress with the development of a commercially viable small size passenger HEV has been slow and is still much reliant upon the availability of a low cost, reliable and environmentally safe high power battery. The leading candidates, nickel metal hydride and lithium ion, have shown their limitations and the opportunity is still wide open to secure a good proportion of this potentially large market.
The sodium/nickel chloride battery which utilises a ceramic electrolyte for its functioning can meet the necessary requirements. A new cell concept capable of producing the high power required for a parallel HEV is now proposed. The opportunity has been identified by the SME proposers to demonstrate the viability of such a high performance battery through a co-operative effort. The battery will be constructed from cells which incorporate a monolithic ceramic electrolyte produced by extrusion techniques. For the parallel HEV application the battery is expected to receive all its energy from the fuel tank via the engine and no provision is made for mains charging. In order to compensate for the 30 Watt heat loss from the battery box it is planned to utilise a modified commercially available fuel combustion heater commonly used as cab heater and based on the principle of hot air circulation in trucks and vans.
The battery will also be equipped with an electronic control system for the measurement and control of all the parameters required during the simulated HEV tests and to ensure proper and safe operation of the battery under all circumstances.
The prime industrial/economic objectives were:
- technical improvements: Specific power > 500 W/kg ; Specific energy >55 Wh/kg ; Power density >700 W/l ; Energy density >75 Wh/l;
- rapid increase in the demand for HEV Sodium/nickel chloride batteries - up to 30% of the market by 2010;
- reduction in overall production cost of the battery to be competitive with other battery systems for this application;
- utilisation of the lowest cost route for the production of the key ceramic electrolyte component - cost per ceramic unit not to exceed 0.85 Euros in large scale production;
- reduction of cost to maintain battery at operating temperature when unattended for long periods - fuel consumption of the combustion heater not to exceed 25 litres over 5 months of continuous operation;
- reduction of the cost of the battery management system by up to 10% utilising simplified control algorithms;
- independence of ambient temperature for effective operation. As the cells are thermally insulated fluctuation of ambient temperatures between -40°C and +70°C have no influence on its operation.
Social objectives were:
- a major contribution to the reduction of pollution of the atmosphere by low emission vehicle;
- a product that will be socially acceptable as it contains all the features of the familiar internal combustion engine(ICE) vehicle;
- no battery maintenance such as overnight charging is required as the battery receives its charge from the engine and is thermally controlled by combustion heat from the fuel tank;
- highly safe product - the Sodium/nickel chloride battery shows an extremely good safety record and has passed all the safety requirement tests laid down for passenger and road vehicles;
- utilisation of only environmentally friendly materials. The essential materials consist of nickel metal, sodium chloride, aluminium oxide and aluminium chloride;
- development of a novel high surface area monolithic ceramic electrolyte for an intermediate temperature high power sodium/ nickel chloride batterydevelopment of a novel high surface area monolithic ceramic electrolyte for an intermediate temperature high power sodium/ nickel chloride battery
Research Approach and Methodology
The characteristics of a single sintered beta-alumina ceramic tube via the extrusion route involving a variety of ceramic pastes and extrusion conditions had to be qualified as a first step using conventional Sodium/nickel chloride single cell assembly and testing technology. The optimum organic debonding and firing temperature profiles were to be investigated at this stage.
Based on the above information an extrusion mould should be designed and manufactured to produce the first prototype monoliths for debonding and sintering. Theseshouldagain be qualified in single cell and small multi-cell module tests.
The technical data from these tests should be used to design and construct a full size HEV battery, the final deliverable.
This data should also be incorporated in the design and construction of the thermal and electrical management systems of the battery. A sufficient number of ceramic units should be supplied together with all the other cell components needed for the battery. Bench cycling tests should be performed using prescribed duty cycles for a typical parallel HEV application.
The final deliverables should be:
- The extrusion technology and mould design.
- A prototype high power Sodium/nickel chloride battery with thermal and electrical control for HEV application.
- A techno-economical report to demonstrate the viability of the above.
Funding
Results
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The main objective of this project was to develop a battery with a power:energy ratio high enough for use in electric vehicles in a timescale which will fit in with the expansion plans of the present ZEBRA battery factory. The CHEETAH battery depends upon the development of an extruded honeycomb solid electrolyte made of beta alumina.
The first phase of the project was to demonstrate that extrusion could produce a ceramic electrolyte with the same properties as that produced by conventional isostatic pressing methods. This was achieved by extruding a thin walled tube (0.5mm wall thickness) from an extrudable ceramic/organic polymer paste with a high ceramic content. Once the shrinkage data was available from the work on single tubes, a mould was designed and manufactured for producing honeycomb monoliths with 50 electrode compartments.
After several optimisation steps, success was achieved in extruding monoliths both of alpha alumina and beta alumina. It was found that the beta alumina monoliths cracked during the de-bonding process. The de-bonding process was optimised and turned out to have a small process window. A limited number of monoliths were sintered but the final density was much less than theoretical. The reason for this was found to be the use of an additive which interfered with sintering process. It was decided to change to aqueous based pastes which do not require such additives but it was too late in the project to develop this system for extrusion of monoliths.
An alternative cell concept based upon four tubes attached to an alpha alumina header was developed. The tubes are manufactured by iso-static pressing instead of by extrusion. The tubes were sealed to the header plates and assembled into cells. These cells were used to build
Technical Implications
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The timescale for introduction of fuel cells has been put back to eight to twelve years, and it is now generally accepted that they will be used in conjunction with rechargeable batteries in hybrid vehicles because of the slow start-up time and the inability to accept regenerative braking. The CHEETAH battery will be ideally suited to such an application if fuel cells can be developed into a reliable, cost effective power source.
For an ic engine/battery hybrid, the CHEETAH battery offers advantages over nickel/metal hydride batteries (an operating window of 80% compared to 40%) and lithium batteries (lower cost, proven safety