The US Department of Energy announced that, as part of its Vision 21, it was setting a target of achieving 70 per cent combined cycle efficiency by 2010. Technologists have been less than convinced that this target is achievable, but it can be unwise to underestimate the DoE. David Flin
Just a short five minutes walk away from the US Department of Energy is the Smithsonian Institute, where those interested can see examples of key developments in the history of flight. The centre piece of the Smithsonian exhibition is, quite rightly, the Apollo 11 command capsule. The dramatic success of the Apollo mission has influenced US thinking on R&D. That is very much in evidence with Vision 21, the DoE’s plan for the future development of the power industry. According to Bill Richardson, Secretary of State for Energy, one of the objectives of Vision 21 is to have a combined cycle gas turbine plant with an efficiency of 70 per cent in commercial operation by 2010.
The Apollo project is used by the DoE to justify setting such an ambitious target. “President Kennedy set the target of putting a man on the moon within a decade. We are setting a similarly ambitious target, an extra ten per cent efficiency within a decade.” The DoE also points out that it has a duty to distribute public money effectively, which means setting ambitious targets. “If the targets weren’t ambitious, then private firms would do thhem without our help.” However, is 70 per cent combined cycle efficiency an excessively optimistic target? Is it even theoretically possible? There are certain theoretical limits that can be applied to the calculation. We can assume stoichiometric conditions at the inlet, ambient conditions at the outlet, and with no losses due to friction. Under these impossibly perfect conditions, apart from the fact that NOx emissions would rise to stratospheric levels, the efficiency would be about 75-80 per cent.
The immediate conclusion that can be drawn from this is that achieving 70 per cent in gas turbine combined cycle efficiency is simply not viable. When I discussed the DoE’s suggestion with a number of technologists, especially those from the gas turbine field, their response to the target varied, although their attitude was pretty much the same. I suppose a feeling of the general attitude that was expressed can be summarised in the most common phrase the technologists used: “Don’t quote me on this, but…” In general, people cynically assumed that the DoE was simply setting an unrealistic goal for purely political ends. After all, it is a nice, convenient, easy-to-remember target that sounds tough but achievable. It is, in fact, quite simply good politics.
It is easy to make such assumptions about politicians, but it is also worth remembering that Kennedy’s prediction and target for NASA was both a highly politicised statement, and an accurate prediction. The DoE may have set an ambitious target, but it would be very rash indeed to simply assume that the target is not achievable.
The second thing that people said to me was, after they had complained about how unrealistic the target was, to try and work out how it could be achieved. Typically, they would come around to suggesting using fuel cells as a hybrid with a gas turbine.
At the recent ASME conference in Munich, the DoE indicated the direction in which its researches were leading. Recognising the major technical problems that were associated with achieving 70 per cent efficiency with traditional approaches to combined cycle power plants, the DoE is investigating less traditional approaches. In particular, it has decided to focus on the development of hybrids involving fuel cells.
The DoE studies have indicated that the hybrid fuel cell-gas turbine concept has the potential of efficiencies of 80 per cent. The DoE has set a series of targets to achieve this, including the milestone of 70 per cent efficiency by 2010. Other milestones targets include the following:
Developing a 60 per cent efficient 1MWe plant to be commercialised during 2003-2005. This will be developed by the National Energy Technology Laboratory (NETL) and Siemens Westinghouse. Siemens Westinghouse plans to use gas turbines ranging from 150-220 kWe to be integrated with its solid oxide fuel cell.
NETL is designing and carrying out a feasibility study for systems of up to 20 MWe with at least 70 per cent efficiency and producing electricity at costs 10-20 per cent below today’s conventional plants. Five feasibility studies have been initiated (Table 1). NETL envisions these systems to be available for demonstration by 2003, and be commercially available by 2007.
NETL believes that the next generation of fuel cell systems will be the UltraFuel Cell concept. A hybrid using this concept would, in turn, represent the next generation of hybrid systems. NETL predicts that these systems could be up to at least 80 per cent efficient and could be commercially available by 2015. It hopes that the concept will be demonstrated by 2010.
By applying the same development technique that was able to put a man on the moon within a decade, the DoE has mapped out a scheme that technologists agree is an ambitious one, but which is feasible. Which is considerable progress from their initial instant reaction.