04 February 2012

Solar Power 101

To the RV owner who cherishes independence from RV Parks and campgrounds and who hates the noise and smell of running a generator, solar power is, without a doubt, one the greatest technological advancements to hit the RV industry in the last twenty years.

What makes solar power so great for RVs? What are the benefits? First, the system requires practically no maintenance, as there are no moving parts. It's also lightweight, mounts primarily in out of the way locations on the roof, and doesn't take up valuable storage space. Solar power also adapts well to the electrical systems found in RVs and is relatively easy to add. Perhaps more importantly, the solar power system doesn’t require fuel like a generator, is clean and quiet, and utilizes a free and renewable source of energy that's good for the environment.

An RV solar power system isn’t that complex either. You don't have to be an electrical engineer to design a system or even install it. The solar power system consists of one or more solar panels to generate a charging current, a charge controller to regulate that charging current, and one or more deep cycle batteries to store that current. It also consists of ancillary components such as a roof top combiner box for connecting multiple solar panels to the system, mounting brackets for attaching the solar panels to the roof, wiring to connect everything together, fuses to protect your system, and an optional battery monitoring system. Now let’s take a look at each:

I. SOLAR PANELS

The key component that makes solar power possible, the solar panel is rated in watts and is constructed of numerous photovoltaic (PV) silicon cells that convert the sun’s energy into Direct Current (DC). Each individual cell consists of a positive and negative side and a middle area called the P-N junction. When the sun strikes the surface of the cell the electrons in the P-N area become excited and generate a voltage and current. Each cell generates approximately .5 volts, so you’ll need two cells to generate a single volt of DC.

Rooftop mounted solar panels.

Twelve volt solar panels are designed to produce a maximum voltage anywhere between 17 and 21 volts. This may seem strange, but when you consider that there are environmental factors at work that reduce this output, and that you need at least 13.8 volts to provide an effective charge for a 12 volt battery, it makes sense. Any excess voltage is regulated by a charge controller, anyhow, so no damage will occur to your battery. Solar panels come in the following forms: 

Monocrystalline:These solar cells are simply the best that money can buy. The manufacturing process is the most expensive and employs the use of very pure silicon and a complex crystal growth process that involves a single block of crystals. Mono-crystalline solar panels are more expensive, but they possess the highest efficiency, at 15 to 17%, and last the longest of the three. 

Polycrystalline: These are an excellent alternative to mono-crystalline cells. The difference with poly-crystalline cells is that they are grown from a large block of crystals rather than a single block. This produces the distinctive shattered glass appearance that is so characteristic of poly-crystalline solar panels. Poly-crystalline solar panels are only slightly less efficient, at 14 to 16%, but cost less than its more expensive "mono" cousin. 

Amorphous: These solar cells are created not by using crystals, but by depositing a thin layer of silicon over a base material such as glass or metal. Amorphous solar panels are much cheaper, react better to diffuse and fluorescent light, and work better at higher temperatures, but they are also the least efficient, at 6 to 8%, and require much more physical space than crystalline solar panels. 




As mentioned, several environmental factors affect the efficiency or electrical output of a solar panel. These factors include shading, overcast skies, and temperature. How shading and overcast skies effect sunlight is self-explanatory, but temperature isn't. As temperature increases the voltage of the panel is reduced, this is especially true when temperatures hit the triple-digits, like here in the Southwest. The negative impact of high temperatures can be alleviated, somewhat, by mounting the panel an inch or two from the roof to allow cooling air to circulate underneath. Since heat reduces the efficiency of each panel, it follows that the panel will perform better during winter months. Unfortunately, cooler weather also means that the sun will be lower in the sky, thus reducing the angle and the amount of sunlight the panel will receive. Tilting mounts, however, can be used to mitigate this low angle and improve the system's performance during the winter. 

Shading obviously reduces the amount of sunlight that strikes the solar panel, so care is needed in choosing the best location on the roof of your RV to mount it. Avoid placing the panel near air conditioning shrouds, satellite domes, storage pods, and collapsible television antennas. The same applies where you park your RV when you camp. Avoid camping next to trees or other terrain that can shade your RV and solar panel from the sun’s rays.

II. CHARGE CONTROLLERS

The “brain” of the solar power system, the charge controller regulates the voltage from the solar panels to prevent the battery from being overcharged, and in the case of a wet cell battery, boiled dry. Unlike solar panels, which are sized by watt, charge controllers are sized by amps with higher amperage models generally costing more. While charge controller features vary by make and manufacturer, there are two primary types of charge controllers used in the RV industry today: Pulse Width Modulation and Maximum Point Power Tracking.

Pulse Width Modulation (PWM): A simple, yet time-tested and proven design, the PWM charge controller works by sending a series of short, variable duty cycle charging pulses to the battery--like a very rapid “on-off” switch. The controller constantly checks the state of the battery and automatically adjusts how long the charging pulses will be to the battery. When the battery is fully charged the duty cycle almost falls to zero or "off" nearly 99% of the time. Conversely, when the battery is fully discharged, the pulses of the battery stay "on" nearly 99% of the time.

Strengths of the PWM charge controller design include the fact that it is built on a time-tested and proven technology, is inexpensive--a single unit capable of handling 25 amps can be purchased for less than $100--and is durable. The PWM charge controller comes in various sizes up to 60 amps and can be used in all but the largest systems found in an RV (to give you an idea how large a 60 amp system is, a single 120 watt solar panel produces about 6.5 amps).



The Morningstar Sunsaver Duo PWM 25 amp controller and Remote Meter.

While the PWM design is simple and rugged, there are some inherent flaws with the design. For one, PWM charge controllers are less efficient due to the fact that the controller connects the solar panel to the battery directly. This reduces the voltage developed by the solar panel from the nominal 17 volt output to the battery's voltage, which lowers the power available from the solar panel. Another flaw with the design is that the pulses generated by the device can create interference in radios and TVs. This is due to the lower frequencies typically used in PWM charge controllers compared to the higher frequencies used in the MPPT controller. 

Maximum Power Point Tracker (MPPT): The newest technology and the latest rage, the MPPT charge controller combines the most effective features of a PWM controller with additional functionality, a PWM controller on steroids, if you will. Instead of connecting the solar panel directly to the battery like a PWM controller, it uses a "buck converter" stage, a DC to DC conversion before the PWM charging stage. The buck converter ingests the solar panel voltage and transforms it to the optimum battery voltage or "Maximum Power Point." Secondly, the MPPT charge controller uses an embedded microprocessor driven algorithm to scan or Track the PV array for where the voltage and current are optimized between the solar panel and the battery (the Tracking portion of Maximum Power Point).



The Morningstar Tristar 45 amp MPPT charge controller.

The pros of the MPPT charge controller are pretty significant. It can increase the charging efficiency of the system up to 10% and in some cases even higher. Moreover, the MPPT charge controller is totally compatible with solar panels of various voltages, such as 24 volt and 36 volt models (just make sure all of the solar panel voltages are the same in order for the tracking function to properly work). These higher voltage solar panels can be wired in series or parallel and are generally less expensive per watt than 12 volt models, thus giving you greater flexibility in the panels you can buy. The MPPT charge controller can also be sized up to 80 amps and beyond, providing greater flexibility for system growth.

Cons of the MPPT controller include greater cost, two to four times more to a comparably sized PMW controller, and greater physical size. However, the pros associated with the unit's efficiency and features far outweigh the cons. There's no doubt about it. If you use solar power a significant amount of time throughout the year and cost isn't an issue, then the MPPT controller is the way to go. If your system is used perhaps six to eight times a year, then a PWM controller will more than suffice and provide you with years of excellent service.

One final note about PWM and MPPT charge controllers. Each is also an extremely effective three-stage battery charger, meaning each can perform bulk, absorption, and float charging just like high quality 120 volt AC battery chargers.

III. BATTERIES


Storing all of that energy being generated by the solar panel is the function of the 12 volt battery.
First and foremost, the 12 volt battery you get for your RV should be a true deep cycle battery. Avoid buying an automotive starting battery or a RV/Marine battery (a hybrid of the deep cycle and starting battery) as neither are designed to withstand severe discharges on a repetitive basis. Indeed, deep cycle batteries are designed to be discharged up to 80% or more numerous times, and still provide amperage at its rated capacity. When it comes to the battery's ratings, amp hours are the key and you want more of them. That means buying the largest battery or batteries that will fit in your battery compartment. The typical Group-27 deep cycle battery provides about 100 amp hours of service.

What does the term "cycle" mean as it relates to your batteries? A cycle is simply one complete discharge and recharge cycle. A "deep" discharge is typically lower than 20% (11.58 volts) of a battery, while a 100% recharge is achieved when the battery's resting voltage reaches 12.7 volts. The number of times and how deep your battery is discharged directly relates to its lifespan, so you should try to prevent your battery from discharging more than 50% (12.06 volts) in order to prolong its life. Avoid running your battery completely dead (10.5 volts) at all costs as this can cause irreparable harm to it.

Like solar panels temperature has a positive and negative effect on batteries. All batteries are rated at 77 degrees F, the optimum operating temperature. As the internal temperature of the battery increases a battery's amp hour capacity increases, as temperature falls its capacity decreases. For instance, at 40 degrees the battery's capacity in amp hours drops to about 80%, at 0 degrees F the battery's capacity falls to about 50%, and at 110 degrees F, capacity rises 20%. Battery voltage is impacted in the same manner by temperature. Anyone who has tried to start a car in freezing temperatures knows this from experience. This is why you should have temperature compensation on your solar system's charge controller, especially if your controller is inside and if your battery is in an outside compartment. If you like to camp in freezing temperatures, I recommend insulating your battery compartment and buying a quality battery warming system. Doing so will not only improve the performance of your batteries, but will also improve their life.

One popular alternative among RV owners is to employ 6 volt golf cart batteries rather than 12 volt Group-27 batteries. This approach has merit. Six volt batteries offer more amp hours than a Group-27 12 volt battery, have heavier plates, can suffer more discharge cycles than a regular 12 volt battery, and when you wire two in series you have, in essence, one huge 12 volt battery. Going this route, however, does carry a small risk in that if one fails you won't be able to power your 12 volt system. Fortunately, this risk can be mitigated by installing more than two 6 volt batteries. This way if one 6 volt battery fails you still have a pair that you can rely upon to power your 12 volt system.

RV owners should understand a few basic electricity rules when it comes to DC circuits and when wiring 12 volt and 6 volt batteries. The rules are pretty simple. Amp hours are additive when 12 volt batteries are wired in parallel, while the voltage remains the same. Conversely, voltage is additive when 6 volt batteries are wired in series, while the total amp hours remains unchanged. Examples of optimum wiring for 12 volt and 6 volt batteries are provided below:


In RV applications there are basically two types of deep cycle batteries from which to choose: Flooded, lead-acid (or wet cell for short) and Absorbed Glass Mat (AGM) (yes, Gel-Cell batteries are technically a third alternative, but in my opinion these shouldn't be considered as they don't work well in RV deep cycle environments). Let's take a closer look at each:

Wet Cell Battery: These are the cheapest and most common batteries found in the market today, and are available in numerous sizes. You can't go wrong with wet cell batteries, but the big negative with them is that they require periodic maintenance and proper charging. Overcharging can boil out the electrolyte in them and warp the plates, while undercharging will leave sulfate on the plates which reduces storage capacity. The maintenance consists of periodic gravity checks using an hydrometer, periodic equalization charges to remove sulfate from the individual battery plates, and careful monitoring of electrolyte levels to ensure the plates are covered at all times (make sure to use distilled water only when maintaining electrolyte levels). All things considered, wet cell batteries work great and are well worth the cost. With proper care and maintenance, they will provide many years of reliable service. 

AGM Battery: In sharp contrast with wet cells, AGMs require no maintenance or watering as they are sealed. AGM batteries are still technically lead-acid batteries, but the electrolyte in them is encased in a fibrous glass mat that can't be spilled. Since AGMs contain no liquid they are practically impervious to freeze damage and can be mounted on their side, an important benefit for some battery and storage compartments. Most AGMs, like those made by Lifeline, also have very thick positive plates and can suffer more discharge cycles. More importantly, they charge up to five times faster and have a slower self-discharge rate, about 1-3% a month. Unfortunately, AGM batteries cost two to three times more than a regular wet cell, but the benefits to your solar power system make them well worth the cost.

IV. ANCILLARY COMPONENTS

Wiring: Choosing the right size wire for your solar power system is critical. Undersized wiring will reduce the efficiency of your system, and with environmental factors already working against your system, you obviously want to avoid this. Many professional installers use 10/2 wire (10 refers to the gauge and 2 refers to the number of wires) to connect the solar panels to the combiner box, and 8/2 wire to connect the combiner box to the charge controller, and the charge controller to the battery. This approach will work for most solar power systems installed in all but the largest RVs with the largest systems, but you should still size the wiring in your system by using a tool like Blue Sea's excellent Circuit Wizard.

Fuses/Circuit Breakers: In-line fuses or circuit breakers are important to protect the wiring and components in your system from shorts and other catastrophic failures. Place one on the positive wire within a foot of your battery and another in between the combiner box and the charge controller. The size of the fuse or breaker depends upon the size of wire used in your system. Place no larger than a 30 amp fuse or circuit breaker for 10-AWG wire; no larger than a 48 amp fuse for 8-AWG; no larger than a 74 amp fuse for 6-AWG; and no larger than a 120 amp fuse for 4-AWG wire. 

Combiner Box: Some kind of system wire interface or combiner box will be needed on the roof to connect the solar panel to the solar power system. Most RVs with factory installed solar power systems use a simple two-pronged plug as a roof top interface. This kind of interface works fine, but can accommodate just one panel, so if you want more than one solar panel in your system, you'll need a combiner box. The combiner box offered by AM Solar can connect up to seven solar panels to your system.

Mounting Brackets: There's not a whole lot to say here, except that you'll need some kind of way to mount your solar panel to your roof. There are two basic types of brackets from which to chose: simple aluminum Z-brackets, which are fixed and immovable, and aluminum tilting brackets, which allow you to raise and tilt the panel to provide a better angle to the sun. Z-brackets are extremely cheap and a set of four can be purchased for around $8. Tilting brackets cost much more and come in various styles and provide the added benefit of allowing you to raise the panel to perform maintenance underneath.

V. SIZING YOUR SYSTEM


At this point you may be wondering how many solar panels you'll need for your RV. Figuring this out isn't too difficult. You can either calculate the amp ratings of the devices you use and how often you use them during a typical day, or you can conduct an experiment and boondock using all of the 12 volt lights and appliances you normally use when you camp and see how long it takes to discharge your battery. If it takes, say, three days to fully discharge your battery, and you have two Group-24 batteries rated at 160 amp hours total, that means you used about 53 amp hours per day. Only about 75 percent of a battery's rated capacity is usable, however, so in this particular case you actually used about 40 amp hours per day, not 53 amp hours.

Now that you've determined your daily amp hour usage, you'll need to determine how many solar panels you'll need to replenish the 40 amp hours consumed in your average day camping. A 120 watt solar panel can produce up to 6.8 amps and during spring and summer this usually means you'll get four to five hours of peak performance. So a 120 watt solar panel will produce at least 34 amp hours on a clear day. Two 120 watt solar panels will double that, providing nearly 70 amp hours a day. As a general rule, I like to oversize the system to provide additional amperage in case it's ever needed like on extra cold days when you need to run the furnace a lot during the night.

Another basic sizing rule used by some is to simply match each 100 amp hour battery in your system with a 100 watt solar panel. So if you have four Group-27 12 volt batteries (4 x 100 amp hours), you'll need four 100 watt solar panels. This surprisingly effective rule assumes you'll use an average amount of amp hours a day, but it works. It's the rule I use. 


Details on the installation of my 240 watt solar power system can be found here.



14 comments:

  1. The only major catch 22 with permanently mounted solar panels is that you need to be out in the sun, which if the weather isn't cool/cold, means you need to consume more energy to keep the interior of the camper cool, usually by powering an air conditioner and then yer back at square one again.

    However, the inverse of this is, in cold weather (40 degrees and less) where you're heating alot, they do come in a permanent mounted scenario as you want the solar energy to warm the camper as it also generates power.

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    1. I considered this problem, when I put my rig together, in 2008. My solution? I pull a 7X14 Motorcycle trailer, behind my Hardside Truck camper and have Solar (600w) installed on it's roof. I can set my Truck Camper up in the shade and leave the trailer out in the sun. They are connected just like you would if you were hooked up to shore power, as the 6volt bank of batteries are in the trailer (928AmpH). A second bank of batteries are still in the Truck Camper, so I have redundancy, which I feel is an important part of being prepared for any worst case scenario, when out in the middle of nowhere, for weeks and sometimes months at a time.

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    2. That's a pretty impressive solution, Bryan. Trailers really do give you more storage in more ways than one. Never thought about using one for my solar. Ingenious. Do you use your trailer for storing anything else like water, for example?

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  2. 12/2 7.5 fuse 100w solar.i use my dvd player dc9.5v 1.2a at6hr is that ok

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  3. Lots of good information wish I could find an RV dealer who knew all this and could install them

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    1. I'm surprised there isn't. Where do you live?

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    2. I have the same problem here in MISS. need a dealer that knows how to hook this up properly.

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  4. I so much want solar, but know so little. Any information I can obtain helps. Thanks much.

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  5. When I think about added electrical devices that run when I'm not there, I get the uneasy thought about a failure. One that leads to a fire.

    Have you heard about any controllers heating up and igniting?

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    1. No, because we're talking about DC current here, relatively low voltages. The only thing the controller is hooked up to are the solar panels on one end and the 12v batteries on the other.

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  6. Honestly, I have been very interested in learning more about solar for my airstream motor home. Our local dealer, posted a video that explains more. We are considering getting solar panels but need to save some more money first. I heard that people had some issues with refrigerators and such, have you had those issues??

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  7. Greatly enjoy your blog abnd the comments

    Are you considering lithium-iron-phosphate batteries in the future? We have 1420 W of solar panels, 4.0 Magnum PSE inverter and 720 amp-hours (12 V nominal) of LFP (this is over 600 amp-hours of usable energy). This has permitted us to be completely solar autonomous since elder son designed and fabricated the system. We have used line power once since last June. The batteries (Manzanita) weight 250 pounds and are equivalent to perhaps 800 pounds of PbS Maximum charge to controller has been 1200 W.

    I would think that LFP will become far more popular as the technology matures.
    Reed Cundiff
    reed.cundiff@gmail.com

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    1. Yes, Reed, I am. When it comes time to replace my AGMs, I will be taking a hard look at the lithiums.

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    2. Reed
      The LFP batteries are interesting. Do you have any links to more information and sources?

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Comments are welcome and encouraged.