The commercial fuel cell comes a long step closer1 September 2009
The R & D effort to achieve an ultra environmentally-friendly means of generating electricity using fuel cells is gathering momentum, with one company making a significant contribution to the evolution of such a machine by successfully testing the prototype of a commercial 3 kW unit.
As the worldwide need to identify less polluting energy solutions becomes more acute, fuel cell technology, which derives energy from gasoline and other fossil fuels but provides a green alternative to burning them, looks increasingly like a solution that has the potential to result in the development of a cleaner, more efficient source of power, one that some believe is destined to revolutionise the energy sector. Fuel cell technology is tipped to be the largest ‘new’ industry of the 21st century, with fuel cells anticipated to become a common source of power for cars, trucks and buses and even ultimately, individual homes, public and commercial buildings. Cummins Power Generation – a leading company in the design and manufacture of distributed generation – has come to the forefront of research into fuel cells with its announcement that it expects to produce a small SOFC commercially within a few years.
Dr Xin Li, a technical specialist at Cummins, predicts (as his personal view) that on the basis of current progress solid oxide fuel cell based mobile power products sized at 10 kW will be commercially available in as little as 2-3 years, with higher powered (> 100kW) stationary power units becoming commercially available in 7-10 years. Is there a practical size limit? Probably not. The US DOE has projected that units of >100MW are feasible, and is basing its funding strategy on that goal.
Fuel cells can be used in a wide variety of applications from portable charging docks for small electronics to base load power plants, including electric and hybrid vehicles, auxiliary power, and off-grid power supply. SOFCs, which operate from a mixture of hydrogen and carbon monoxide, can also run on reformed diesel oil or petrol (gasoline) and are therefore more compatible with the existing fuel supply network including natural gas, and its only by-products are water vapour and a small amount of CO2.
Further R&D needs to be carried out to develop several key technologies. The stand-outs are a sulphur tolerant fuel cell stack, and for mobile power units, a waterless solution probably based on gas recycle technologies. Other difficulties that need to be overcome include, for vehicles, a satisfactory way of vibration testing the fuel cell, and proofing it against vibration damage.
Cummins PG’s history of research into fuel cell capability goes back to the 1960s. It received a shot in the arm in 2001 when the company began an association with the US Department of Energy’s Solid State Energy Conversion Alliance (SECA) programme. Cummins PG elected to focus its research and development on SOFC technology owing to its potential to be cost effective while operating efficiently on existing hydrocarbon fuels, and hydrogen too as it becomes more widely available. The DoE/SECA arrangement gave Cummins a good opportunity to gain familiarity with SOFC technology, and provided a solid foundation to the company for the development of its own fuel cell product. Cummins’ approach to SOFC is specific to them and therefore the R&D programme has generated several patents that it now owns.
In 2007 Cummins was one of six industry teams involved in the DOE SECA programme. The programme is in three phases, with Cummins electing to take part in Phase 1, the demonstration to successfully complete tests of SOFC prototypes in the size range 1-10 kW. This is a purely technical programme, not primarily involving price or marketing considerations, although there are price targets for each phase. The Phase 1 target is $800/kWe installed capacity, which, says Cummins, has already been met. In Phase III, at MW scale, the target is much lower at $400. Phase II, in which Cummins is not participating, is under way with a 2010 intermediate target. Cummins is currently concentrating its effort on the 2 kW diesel fuelled auxiliary unit for trucks.
So what are the aims and objectives of the first phase tests? The central project target is to produce a low power demonstration unit, probably a hybrid, possibly for CHP applications, by 2013 to 2014, with a field test 3 years later. The overall thermal efficiency target has been set by the DoE at 50-60%. The aim ultimately is to demonstrate, within ten years from now, 100 MW utility sized gas turbine-fuel cell hybrids, perhaps linked with coal gasification units.
Cummins’ Phase 1 target is limited to demonstrating small sized non-hybrids in the 2-10 kW range running on natural gas, propane etc for auxiliary power for recreational vehicles, leading to larger units in the 60 KW range suitable for the recreational powered boat market. Such units can have an overall thermal efficiency of around 30%, which is comparable to the diesel recips that currently supply that market. These kinds of recreational vehicles typically cost $300 000 to $600 000 and are at the luxury end, so the $10 000 projected cost of a fuel cell generator is not a significant barrier to sales.
In Europe such units should find a market in gas fired micro-CHP domestic applications, supplying perhaps 2kWe ‘baseload’ and hot water with make up power from the grid during high demand periods. The electricity from such units could be produced for 4 cents per kWh in the USA and Europe, (Cummins’ figures) compared to 10 cents/kWh for the public supply. And their fuel compatibility makes such units an easy plug in for existing gas, oil or propane systems.
‘These units offered the potential to be manufactured at costs approaching those of conventional stationary power-generation technology’, says Xin Li, whose brief included the design and creation of a low cost air supply system for the fuel cell, and whose significant contribution was to implement the requirement to use mass produced automotive components and evolve some innovative designs to meet the need for high accuracy and high resolution air flow measurement and control systems. The programme helped achieve overall system costs and performance to meet aggressive DOE targets.
In a complete fuel cell system, the major cost item is not the fuel cell stack, but the balance of plant, and the major BoP item is the air supply system. Moreover the efficiency of the system is determined mainly by the parasitic power required in the ancillary systems, and 80% of that energy goes into the air blower. An idea of the supply needed can be gleaned from a consideration of the stoichimetric ratios for a fuel cell and a conventional diesel cycle – around 3:1 compared to 14:1. The task then, and the one entrusted to Dr Li’s group, was the development of a low cost, efficient air blower that would run reliably for 10 000 hours.
The team opted for a specialised low pressure centrifugal design for its efficiency, and chose components already heavily employed in the automotive industry, because their high volume production makes them cheap, lowering costs by up to 90%, and necessity makes them reliable. These included the hot wire type air flow sensors, and the fan’s step motor.
A disadvantage of SOFC fuel cells in general is their high warm-up rate, of around 1 minute/kW rating. Thus even a 3 kW unit takes 3 minutes to warm up to operating temperature, so it was necessary therefore to develop a battery hybrid in which the battery provides power initially and is recharged by the fuel cell during normal operation.
The long start up time also has serious implications for the use of SOFC for larger scale power supplies, where a 5 MW unit would take well over a day to reach operating temperature. Baseload applications would not present a problem, but the technology could in its present state of development be used for only a very few back-up power applications and for no cycling applications at all.
To date new approaches to the sulphur poisoning problem have been undergoing investigation and testing, including a low temperature absorber, a high temperature absorber, and a sulphur tolerant anode design. Regarding the ‘waterless solution’ the concept of anode gas recycling has been tested by Cummins and several other companies, and incorporating an anode condenser has been conceptually tested. A difficult area still in the early stages of investigation is that of redox – oxidation of the anode side of the fuel cell during shut down – where some very limited testing has been carried out.
The resulting solid oxide fuel cell power system (which was developed in conjunction with Versa Power) has the potential to directly and seamlessly replace diesel powered generator sets in many applications and can provide virtually silent power with significantly lower fuel consumption and exhaust emission levels than existing generator sets. Additional projected benefits include higher reliability and lower maintenance than today's systems. The prototype unit tested for SECA produced 3 kW of electrical power while operating on commercial pipeline natural gas and ran flawlessly for over 2000 hours, says Cummins Power Generation, at its test facility in Minneapolis (Minnesota, USA) demonstrating a thermal efficiency performance of over 37%. This compares favourably with comparably sized small IC engine based generator sets, where efficiency is generally well below 30%.
Commenting on the advantages of the
SOFC system Xin Li says ‘The benefits of SOFC based energy are numerous. The technology represents a highly efficient, clean emission (no exhaust treatment required) source of high quality AC power, which is compatible with other energy resources such as diesel generator sets, solar and wind. The unit is quiet, making it socially more attractive than traditional engine driven generator sets, with low vibration levels, and the whole system boasts the added appeal of low maintenance.
In the case of CHP, in addition to the significant green credentials, the possible financial
savings to the consumer are considerable. For example, for home CHP applications the natural gas powered SOFC system can deliver over 70% efficiency, which when converted to current home pipeline natural gas prices represents half the cost of regular supply electricity.’