Since our system has only a 7% efficiency (see Efficiency), and since the home is not continually occupied, we must produce hydrogen remotely. This ensures that our tank is always full of hydrogen when we need it, despite the low efficiency. Our remote operation system has two main parts: The first part is a series of voltage- activated relays which turn the electrolyzers on and off. The second part is a remote monitoring setup which allows us to track the system by internet.
The relays read the voltage off the battery bank. When the PV array is producing adequate power and the batteries are full, the battery bank will have a high enough voltage to trip the relays which turn on the electrolyzers. This ensures that a.) the PV array is producing enough juice to run the electrolyzers; and b.) the batteries have already been fully charged (they are first in line for solar power). Since the electrolyzers have a one hour warm-up period, the relays also trip a timer, which after an hour turns on the actual hydrogen production. To set up the electrolyzers for remote use, we had to hack into their switch units, since they were designed to work by push-button.
Another relay turns off the electrolyzers when battery voltage drops too low, since this is a sign that the PV array is not producing power (eg as the sun goes down). However, a passing cloud can cause a sudden, temporary drop in battery voltage; we do not want to turn off the electrolyzers in this case, because the electrolyzers would have to go through the warm-up period again as soon as the cloud passed. Thus we have a timer on the "off" relay, so that it waits about 20 minutes for voltage to rise again before shutting the electrolyzers off.
Right now we use a set of analog relays for this system. They are difficult to adjust. We may switch over to a PLC (programmable logic chip), which should be more skookum.
We also have an iBoot module, which enables us to switch the electrolyzers on and off by internet. In practice, this is an additional shut-off for safety. The electrolyzers are also shut off by low PV voltage and by the hydrogen sensor.
We have another gang of voltage shunts whose job is to monitor the system's performance. We installed monitoring gear from Brand Electronics. This unit (picture at top of page) is an overall system monitor similar to the Tri-Metric, a metering tool well-known among solar power users. The Tri-Metric has one shunt, measuring the DC voltage drop at the negative battery terminal, from which it derives its various data streams (eg voltage, amps, and % battery available). The Brand unit can monitor an arbitrary number of shunts, both AC and DC. We have nine, including three AC. We are thus able to track not only solar output and battery power levels but also fuel cell output, the AC draw of the electrolyzers, and the pressure in the hydrogen tank via a digital pressure transducer. Brand allows access to this data via the internet. We can store it for analysis, and use it to act in absentia in real time, eg by turning off the electrolyzers.
That's the theory anyway. But this remote monitoring business is buggy. We worked through some problems with the Brand gear, and it now does its job. Its web-access interface is not market-ready. Other companies working on remote monitoring units include Wattplot, Right Hand Engineering, and Fat Spaniel. Each unit caters to a different niche. In general, remote monitoring is a happening thing because it makes trouble-shooting so much easier, and saves on travel.
We also have a webcam watching the electrolyzers, partly to track when they are producing hydrogen, and partly for gratuitous amusement.
Above Left: Brand metering modules (remote monitoring) in the top box. Analog relays (remote operation) in the bottom box. (picture taken during testing)
Above Right: Brand One Meter; web hookups below.
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