A post a while back asked whether our basement dehumidifiers could be run off a renewable source of electricity, specifically solar panels. This idea seems to make sense since the dehumidifiers run mostly in the summer when solar intensity is at its peak.
Based on what we know from the foray into solar reported in the past few posts, the real question is: "What would it take to run the dehumidifiers off of batteries, and to recharge the batteries using solar?"
We can answer this in two parts: (a) how much battery capacity is needed to run the dehumidifiers? and (b) how much solar capacity is needed to recharge the batteries to allow the system to run 24/7?
Using what we've learned in the previous several posts, the place to start is to figure out how much energy the dehumidifiers use in 24 hours, then figure out what batteries are needed to hold this amount of energy (with some to spare to cover cloudy or excessively humid days), and then figure out how much energy solar panels need to collect to recharge the batteries fully.
The Kill-A-Watt measurements of the dehumidifiers estimate that two units consume 7+ kWh per day. Let's say 8 kWh to be on the safe side. Just to provide some perspective, this is like a 1500 watt hair dryer running continuously for 5 to 6 hours.
The 8 kWh, or 8000 watt-hours is distributed over 24 hours. Let's assume evenly. This means that the power consumed by two units is 8000 watt-hours / 24 hours equals 333 watts, or 167 watts per unit.
To figure out how much battery capacity we need, it is necessary to figure out the current that will be drawn from the batteries. To get current, divide power by voltage: 333 watts / 120 volts equals 2.7 amps. Round up to 3 amps to be on the safe side.
Three amps. Not much at all, eh? Here's where I think it's easy to get lulled into a sense of false security.
The 3 amps is AC. Recall that the AC current is produced by a DC-to-AC converter, and that typically the current is much more substantial on the DC side. Assuming a 12-to-120 volt converter, the current on the DC side is ten times the current on the AC side. Accordingly, we need 30 amps running continuously from our batteries.
So will our 250 amp-hour battery do it? Clearly not -- 250 amp-hours / 30 amps equals 8.33 hours means that our battery-powered dehumidifier system will run for about 8 hours. This suggests that we need 3, maybe 4 (or more), batteries for a system that runs 24/7. Given that each battery costs $500, we're already up to $2000, and we haven't yet even added the solar panels!
Damn the cost! Full speed ahead, even if it is out of curiosity about how many solar panels are needed for this system.
To fully replenish 8000 watt-hours per day, and given that there is only about 4 hours of suitable solar intensity, we need to generate about 8000 watt-hours / 4 hours equals 2000 watts. In other words, our solar panels need to generate about 2000 watts. Before running out and installing a 2000 watt solar system, recall that the 2000 watts is the actual power you need, not the "rating" by the manufacturer. The "rating" needs to be significantly more than the actual power, probably something like 3000 watts.
Recall that you can now get solar panels that are rated to produce 400 watts (the fictitious number, not the actual amount). The actual amount will be probably, under the best circumstances, 300 watts. We need 2000 actual watts, which means that we need 2000 watts / 300 watts per panel equals 6.6 panels. Since no one will sell you 0.6 of a panel, round up to 7. Heck, why not round up to 8?
Now recall that each panel is $1000, making the cost of the solar system upward of $8000. You'll also need a charge controller and some pretty hefty wiring, and probably a permit from the town to install this. It probably is also advisable, and most likely legally required, to have the entire system set up professionally.
BTW, out of curiosity, how much current would be coming from the solar panels? Let's say they are all in parallel, so the output voltage is much like the Harbor Freight system -- ranging between 13 to 18 volts. 2000 watts / 13 volts equals 153 amps! This level of current will make your hair stand on end if you are anywhere near it, and heaven help you if you get a short. The good news is that it is possible to reduce the current by increasing voltage, and to increase voltage we put some of the solar panels in series. Let's say we create two series of four panels each (now you see why I was keen on going with 8 panels ;-) ). The voltage of each series is now 4 x ( 13 to 18 volts ) equals 52 to 72 volts. With the increased voltage, the current is less: 2000 watts / 52 volts = 40 amps. This is still a very substantial amount of current, but much safer than 150 amps (for reference, the circuit to the typical electric kitchen range can handle 50 amps).
Will this solar configuration replenish the batteries? An issue is that we have 12 volt batteries, but the solar system is now configured so the voltage across its output cables is 52 to 72 volts.
Recall that's where the charge controller comes in. It will smooth out the voltage, and produce an output of about 55 volts and 40 amps. To match up the charge controller voltage with the batteries, we put them in series as well. In a series configuration, the four 12 volt batteries combine to create one 48 volt battery bank. To charge the batteries the voltage from the charge controller needs to be a bit higher than the battery voltage (in the Harbor Freight system, the battery is 12 volts, while the charge controller output is 14 volts), so applying the 55 volts from the charge controller to the 48 volt battery configuration will actually be just what is needed to charge the batteries.
So to answer the question, "Will this solar configuration replenish the batteries?" Assuming 4 hours of suitable sunlight x 55 volts x 40 amps gives us a bit more than 8000 watt-hours. We're in business! That's assuming we want to spend $12,000 to $14,000 to run two basement dehumidifiers.
A consideration: producing 1 kWh of conventional electricity creates 2 lbs of carbon dioxide. 8 kWh per day then is responsible for 16 lbs of carbon dioxide each day. Over 100 days, we're up to 1600 lbs, or nearly a ton. Thus we can reduce our electricity carbon footprint substantially -- in my case, by 15% -- by using the solar system. I'm not including the carbon dioxide generated by the manufacture of the batteries, solar panels and related gear, and the energy involved in the installation. But even so, maybe $12-$14K is worth it after all.
If I'm interested in reducing my carbon footprint, is this the place to start? Actually not. Transportation is a much bigger component of my energy usage. The next post will go into some detail about how to reduce my transportation carbon footprint.
No comments:
Post a Comment