I've been meaning to write about the challenges and opportunities with fuel cells (for power generation, not transportation) for a while now. Here is a note I had written about a year ago.
(1) The core issue with any energy source is COE (cost of electricity), which is composed of 3 parts
a. capital cost
b. efficiency
c. lifetime
(2) Capital cost is high because of high operating temperatures (800 - 900C). This affects
- fuel cell cost
- Metallic interconnect cost - Coming up with an oxidation resistant metal for 5 year life @ 850 C is generally accepted to be the showstopper.
- Balance of Plant (BOP) cost - again, high temp requires more expensive pumps, blowers, valves etc. BOP forms > 70% of system cost, the other 30% being the stack itself Bloom claims that they have a regenerative SOFC (which generates oxidant internally) which can operate at slightly lower temperature, may be 700C. Typically 800C is a big break point in terms of metal cost/life, so this is an advantage for them. The issue is - I am not sure how much improvement they are actually reaching.
Fuel cell cost itself is generally high because of the long sintering process which is a batch operation. Just like going to 12" Si wafers in semiconductor industry, the ability to make larger area fuel cells in one sintering batch is key. GE was able to demonstrate a 12" planar fuel cell.
Siemens went the other way. They raised the temperature and replaced metals with ceramic conductors. They made good technical progress but could never sell the product - it was simply too expensive. Even they are rewinding the clock and looking at alternatives.
Several companies (GE, Siemens included) also believe that they have to do away with the sintering process, and have a "spray-on" fuel cell to minimize cost. You spray a ceramic just as you spray thermal barrier coatings onto turbine blades.
Some numbers: $1500/kW would be a good starting point. US DoE, under their SECA program, set a long term target of $400/kW for stack cost, and $800/kW for system cost. Having been involved in costing, I found that this requires some extreme assumptions on how thin your metals will have to be, and how large your individual cells will have to be.
(3) Efficiency
- This can be improved at higher temperatures, but I have listed the issues above, under cost. Durability also drops with higher temperature
- Hybrid fuel cells are the dream. You take the exhaust hot air at 900C and send it into a gas turbine. At GE we decided that the whole thing wouldn't make $$sense unless we hit a combined efficiency of > 75%. Siemens has demonstrated a 250kW hybrid system with I think > 55%. This is pretty good... you will need to go to 5MW to reach 75%.
- Regenerative SOFC may help improve efficiency - this is where I don't have enough data. It is likely that it is an advantage. However, I don't know if it can go close to 70%... they claimed it can.
(4) Durability
The accepted standard is 5years (40k hrs), with 10 shutdown cycles each year.
(a) Again, here, some companies claim that they can operate at a lower temperature and thus extend life. This may be possible.
(b) Another aspect of life is thermal cycling. Each shutdown-restart of a system takes a heavy toll on the stack. This is why many (myself included) believe that SOFCs form good base base load machines, or generators for utilities/substations, rather than as portable devices, because they involve too many shutdown cycles.
Other business models
One option for fuel cells is Combined Heat and Power (CHP). Some PEM fuel cells manufacturers worked on that for a long time -with the idea of setting up a 5kW fuel cell heat generator either in your home, or a bank of these near an apartment complex for "district heating"... they eventually ended up in the back-up generator space. They tried selling CHP systems to European customers, but the efficiency requirements were rather high, and it was challenging. Now - SOFC can do better at heat generation because as such operating temps are much higher. 85% efficiencies are possible if you account for both heat and power.
Another concept is about making H2 in a reformer along with the fuel cell (from propane or natural gas) and fueling your car with it. Plug Power & Honda are working on a home-refueling system... But again you know my thoughts about the whole H2 economy.
Now... thinking of SOFCs - the application for SOFCs that makes sense to me is remote power generation, specifically for cellphone towers etc. The reason is that conventional PEM fuel cells may not operate very well in India's hot environment. I'm taking off from your Acme Green Shelter example. Some US PEMFC companies started with the idea of remote power generation using propane tanks... but found it tough to break into. Again, installed cost was an issue, and as such there are only a few farms and ranches in the US that don't have grid connection... market was not large enough.
So one can explore this further. What should be kept in mind while working on these is that in the space of flexi-fuel sub-100kW, your competitor is not grid, but diesel gensets. They can also run on fossil fuel, and they are cheap as hell...
Plug Power (www.plugpower.com) sells their fuel cells to telecom companies as back-up. PEM Fuel cells are good power back-ups because they can electronically come on instantly, and in one package replace the need for battery-back-up+Diesel-genset-kicking in. It made their life much easier to go telecom because they would need to produce only 48V. In fuel cells, the voltage is proportional to stack height. Tall stacks are hard to build. FCE's SOFC stack is I think 80V or something - 100 cells tall (@ 0.8V/cell). Inverters are fine, but not there could be issues at high power ratings.
