24 vs 48 Volts; or, Fable of a Dead End (with a moral)
This tale exemplifies the experimenting we had to do in our efforts to maximize the efficiency of our system. To skip the technical jabber and get the key point, cut straight to the bold print below.
We originally installed a fuel cell with 24 volt output, to match the 24 volt battery bank. Using pure H2 we got only 85% (0.85 kw) output from the fuel cell. We finally decided this was because ReliOn preset the high voltage shut-off at a non-adjustable 26.2 volts. Presumably ReliOn set their output voltage to suit their telecom customers' needs. However, in a home power system where the fuel cell is charging batteries, 26.2 is a "float" voltage, used to keep full batteries topped off without boiling them. A higher "bulk" voltage, about 29.2, is used for all-out charging. ReliOn's low voltage setting effectively reduced our maximum fuel cell output by 15%, because you need a high voltage difference (relative to the battery bank) to get maximum output.
To extract this extra 15% output we traded our 24 volt FC for a 48 volt model, and added an Outback charge controller (same as the one on the PV array) to maximize the fuel cell output. The charge controller worked extremely well: By setting its parameters for maximal output we were able to get 1300 watts out of our 1000 watt fuel cell (!). ReliOn, however, advised against this as a steady-state. We had to adjust the charge controller to a deliberately less efficient level to keep the fuel cell from exceeding 1kw output.
On start-up the fuel cell needs a small amount of DC power. The 24 volt FC simply used the 24 volt battery bank. But when we went to a 48 volt model we had to set up a dedicated 48 v battery bank for fuel cell start-up. Besides the cost of the extra gear, we had the extra work of setting a relay to switch the FC output from the 48 v batteries (which it charges first) to the 24 v battery bank. A bad battery in the 48 v bank caused trouble with this system.
Our final, clear conclusion was that the 48 volt/charge controller route was a bust, the gain in output not worth the cost and hassle. It is much easier to deal with the issue at its source:
The key is to be sure your fuel cell has an adjustable max voltage setting, or is preset to 29.2 v or so. This higher setting is important for maximum fuel cell output.
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