A small update to this post. We ran across a great deal on solar panels and have upgraded the system for the RV. We picked up a pallet of 230 watt panels that were rejected on a large installation for minor visual flaws. We tested each panel for voltage and current and each panel meets and usually exceeds the manufacturers specifications. Most are in storage until we need them but we kept 2 and added them to the RV system. Just 2 panels has increased our system by 8 times!
We now run laptops, charge gadgets, charge tool batteries, use our blender and vacuum cleaner during the day with no worries! The old panels are now charging a secondary battery that is powering LED work lights for the construction site. You can read about details on the system below in the original post…
This is a repeat of a tweet from last summer but I thought I would add it to the blog just the same! In an effort to get used to the idea of living offgrid and working with a solar system, we converted our construction trailer (a.k.a. RV) to a solar system and doubled the capacity.
The images below show you the modification to the original battery system in the RV. I replaced the original battery and added a second to bring the capacity up to about 180 amp/hours. Using a Rubbermaid container I made a box to house the second battery and a power inverter. It sits under the RV itself and has a 110V power outlet. We use the outlet to run our hand vacuum cleaner, charge laptops, IPhones, power tools etc. I also changed every light in the camper from incandescent to LED. This reduced the lighting draw down to about a tenth of what it was with the incandescent bulbs. Overall the new system meets all of our needs for now but we may bump it up as we move ahead with construction.
With deep cycle batteries it is recommended to always keep your batteries above 80%. This will maximize the life span of the batteries. This means that the batteries should never drop below 10.5 volts or so (max is around 13.2 volts). To help regulate this and avoid battery damage I also added a voltage/current meter to the system. This meter is proving very valuable in other ways as we can now see what each appliance or device is drawing and adjust what we use when to maximize our stored power.
The batteries are charged by 3 small 25 watt solar panels. The panels are outputting 24 volts so we always have enough headroom to the charge controller to supply a consistent charge voltage even on overcast days. Since solar panels are not 100% efficient and the sun doesn’t always shine I calculate the solar panel output at about 75%. So with this in mind we can now refer to our meter to get an idea of how long we can run things. For example, if I run our vacuum cleaner for 1 hour straight I will pull 5 amps out of our battery system. This will take a little over 2 hours of sun to re-charge. Here is the math in case you’re interested (P=VA 75 watt panel at 75% is 56 watts divided by 24 volts is 2.33 amps per hour of charging if the sun is shining the entire time). So you can see it is easy to drain the system if you do not monitor it and manage your power use. The power usage adds up quickly when we are running a water pump, charging IPhones and laptops and running the electronics of our propane fridge.
Two or three overcast days could be an issue. Luckily we also have a generator to quickly charge the system if we run into problems. Another option is to add more panels which we plan to do this year. I’ll try to add to this post when I upgrade the system some more.
Feel free to add comments or ask questions using the comments section below the post.
Hi, I found this post via your sister who, for some reason I can’t remember (urbanism?) I follow on twitter.
When I’m not tweeting about urbanism (walkable neighborhoods are the ultimate energy-saver!) I work with universities and remote communities building smart renewable microgrids. As my training is finance and policy, I’m not directly involved in the wiring up, but I’m keen to understand the issues and state of technology.
Bravo for your efforts so far! Here are my thoughts. I’m sure you know a lot of this, but I just thought I’d lay it out clearly so that I can understand your set-up. Please correct any bits that are wrong.
You talk about battery capacity in terms of amp-hours, but I usually think in terms of kWh. The battery capacity is 180 amp-hours, and a max 13 volts. So P=VA, you’ll get 2340 watthours or 2.3kWh. For comparison, the Nissan leaf battery is ~24kWh, so you have 10% of a Leaf.
I’m just thinking out loud here about how this relates to the 110V power outlet. I suppose the appliances will all draw 110V, but different currents (amps) and so burn through the stored power quicker. Electronics draw hardly anything, while motors like your vacuum – basically anything that involves moving atoms, making mass move – will draw a lot more. Vacuums seem to draw 12A so (12 x 110V) 1320w or 1.32kW. That means it will use up all your storage in a little under 2 hours (2.3 / 1.32 = 1.74 hours, 1h45mins, to be precise).
By contrast, typical laptops draw up to 65W, or a bit more than 1/2 an amp on 110V line. You could run the laptop (or an LED bulb, or charge a cellphone etc.) for 2.3 / 0.065 = 35 hours.
The water pump will be like the vaccuum, high power drawers. I presume you control the pump directly: maybe you could rig something up where you store water up high yourself, with a bucket and pulley? An electric fridge would run on a cycle: every 15 minutes or something the pump runs, cooling the inside down until the thermometer’s happy. I don’t know how propane fridges work. You might want to investigate a clay fridge like the mitticool: http://www.notechmagazine.com/food-storage/ Does the fridge heat up the RV, like the back of an electric fridge would (by ejecting the heat from inside)? There might be devices that let you concentrate that waste heat for hot water.
All these numbers are great in theory, but in reality there’s resistance in the wires and in the appliances so you’ll get less. There’s also reactive power and harmonics, which are the big focuses of the academic research in microgrids. I don’t exactly know how that shows up in a microgrid system, except as more lost power: worth reading up on, to see if there’s any clever way to manage them.
In terms of real-life datagathering, there might be low-cost (and low power: probably powered by their own duracell battery) data loggers you could use to produce excel charts of your power usage and draw. You could clamp one around the battery cable and one on the cable from the PV inverter. Onzo http://www.sentec.co.uk/case-studies/case-study-3 Cheap (chinese?) http://www.dhgate.com/10mm-sensor-clamp-for-wireless-energy-monitor/p-ff8080813e407f2c013e9cfaaf6f369e.html#s1-5-1 EnviR http://www.smartnow.com.au/products/envir-energy-monitor/envir-transmitter/
I presume it’s a cost thing, but since you have propane on site, did you look into a propane fuel cell instead of the diesel generator? A quick google turned these up: Protonex http://www.protonex.com/recreation/rv-power.aspx Sirius http://online.wsj.com/article/PR-CO-20130502-912634.html Apart from the noise and clean-ness, I think these would be more efficient than the diesel which will waste a lot of heat. The cost of diesel will probably be the influencing factor.
Let me know if any of that is new to you, tried-and-failed, or just flat out wrong.
And good luck with the project!
Thanks for checking out the blog! Great comments, thanks for taking the time to send me your ideas. I have added my responses and comments below.
Bob – Yes, your math is accurate, I used amp-hours in this post since most smaller deep cycle batteries use amp-hours. You can use either.
Bob – Actually the Earthship will use as many native DC components as possible. RVs are also usually equipped with many DC components as well. For example our refrigerator runs on propane with the ignition system and electronics running from the 12V supply.
Bob – Yes, your math here is correct for a stand-up vacuum. We would burn through our stored power pretty quickly. We are using a small 75W handheld unit since we only have about 30 sq/ft of carpet in the camper. On paper this draws about 6.25amps (at 12V) and drawing about 5 amps real world. We usually run it early afternoon when the batteries are fully charged so the DC voltage is a bit higher.
Bob – Yes, In fact what we do is charge the laptop batteries during the day when the sun is shining and run on the laptop battery after dark. Once in a while if I am busy I need to charge after the sun goes down. There is usually plenty of power for this.
Bob – We have two water sources, drinking water from a container and the water stored in the RV’s cistern. The cistern water is what goes through the pump. That is used to flush the toilet and run the shower where higher pressure is needed. I could rig something up but the retrofit to the RV would be a lot of work and I have no problems with the amount of power available.
Bob – Since our main goal is the Earthship, investing too much in the RV doesn’t make sense from a time or cost standpoint. At this point it is running quite efficiently for a 30 year old camper! I will be looking at alternative technologies for the Earthship so thanks for adding the link. Our Earthship has a built-in cold storage that pulls cool air from pipes run through the berm. This will be able to keep most of our fruits and vegetables fresh throughout the year reducing our cooling requirements significantly. We will have a sectioned portion of the cold storage that will be used in the winter as a fridge and freezer (weather allowing) so we will be using the propane fridge mostly during the summer months.
Bob – Actually since all of the numbers in my post are taken from the meter which monitors the batteries directly all of these numbers are real world values and include line loss. Given the size of the RV and gauge of wire used the loss due to wiring is negligible. The longest run in an RV is only about 30 feet. However, with the Earthship this is more critical and all DC wiring will be oversized for this reason.
Bob – Reactive power and harmonics are a problem associated with non-linear AC loads and don’t apply to DC systems. In microgrids these are present because of generators, inverters and ultimately the grid tie. From what I can tell, the research is primarily targeted at finding efficient ways to comply with grid tie standards. Since we are building a PV based off-grid home this is not an issue for us. If I decide to add wind (which we are considering) this may become a factor especially when selecting my main power inverter.
Bob – I use my Agilent 1271A for data logging now and will have data logging built into the control system of the Earthship.
Bob – The RV already has a gas powered generator that we rarely use. It is definitely noisy and not very clean but since it is rarely on it’s fine for now. I am looking at a propane generator (not fuel cell) for on-site construction and eventual backup for the Earthship. At this point the propane fuel cell generators are expensive but hopefully the price will drop by the time we get to that stage.
Great comments, thanks again!