Date: Mon, 31 Oct 94 15:28:38 CST
Sender: Vanagon Mailing List <vanagon@vanagon.com>
From: "J. Walker" <JWALKER@ua1vm.ua.edu>
Subject: solar panel article
Motorhome Magazine, September 1994
Panel Power
by Joel R. Donaldson
As many seasoned boondockers (and extended weekenders) already know,
keeping the coach batteries charged while away from hookups can be a
real challenge. Some people tackle the problem by running an AC
generator for several hours a day. Since the DC output of most
generators is miniscule (typically only a few amps), this approach is
usually just a waste of time. Other folks buy a large battery charger
to use along with the generator, or run their motorhome engine at fast
idle for several hours. Each of these schemes has a serious disadvantage.
The crux of the problem is that, even if you are using a very powerful
recharging system, your coach batteries are only capable of accepting
a rapid recharge of about 75 percent of their maximum capacity. From
that point on, the internal resistance of the batteries rises,
preventing them from absorbing as much current. Attempting to
compensate for this characteristic by forcing in extra current doesn't
help, because the batteries just dissipate this extra energy as heat,
which can quickly ruin a battery.
So, you have three options: You can put up with several hours of
engine wear and noise, fumes and fuel consumption while your batteries
take their own sweet time reaching a full charge. Or, you can live
with chronically undercharged batteries, along with the reduced battery
capacity and sulphation-induced premature battery death that inevitably
follows. Or, you can install solar panels, which produce a slow steady
charge that is almost perfectly matched to the charging requirements of
a battery. Take your pick!
In initially considering the purchase of solar panels (or photovoltaics,
as they are frequently called) many motorhomers are dismayed by the
high up-front cost, which, for the largest solar systems, can easily
equal the purchase price of a moderate-sized AC generator. However,
bear in mind that the panel is capable of continuously delivering this
current for many hours each day, accomplishing a surprising amount of
recharge by day's end. If you actually add up the total daily amp-hours
produced from a solar system and then compare them with the amount of
time you would need to run a generator each day to achieve the same
level of battery recharge, the cost-effectiveness of a solar system
quickly becomes apparent.
Besides being a cost-saver in many applications, solar panels also have
other great advantages. THey are completely silent, require virtually
no maintenance, no fuel storage and handling, no oil checks or changes,
no tuneups or adjustments, and they can safely be left operating while
the motorhome is unattended (which is great for periods when it is in
storage). They also produce no exhaust fumes, and will probably outlive
the motorhome on which they are installed.
Solar Electric Basics
In operation, a solar-panel power system is simplicity itself. You
connect the panels to your coach batteries (either directly or through
a charge controller) in much the same manner as you would connect a
battery charger. When exposed to sunshine, the panels generate a DC
current that flows into your batteries. This power is then avaialble
for direct use by any 12-volt DC equipment that may be present in your
motorhome, or an inverter can be used to transform it into 120-volt AC
power. Even on overcast or rainy days, the panels usually continue to
produce a charging current, albeit at a much slower rate.
The only limits on the amount of solar power that can be obtained are
the space available for mounting panels, the weight and space available
for any additional house batteries and the cost of the system components.
Many people start out with a small solar system, and add panels and/or
house batteries as their needs or budget dictate.
Selecting a System
Buying a motorhome solar power system is like buying a stereo system.
You can buy a complete package, or you can select the system components
individually. Packaged systems are convenient and usually well-designed.
On the other hand, putting together a system yourself is often cheaper,
and perhaps more satisfying. Either way, it helps to be familiar with
the individual components, so that you can be sure the resulting system
will meet your needs. Since most system packages consist of just a few
items (one or more solar panels, wire, mounting hardware and maybe a
charge controller), it doesn't take too much research to turn the
average motorhome owner into a well-informed prospective buyer.
Two distinct solar panel technologies are currently available:
amorphous and crystalline. The panels using amorphous technology are
usually constructed as one large photovoltaic cell, while the
crystalline models are comprised of approximately three dozen small
individual photovoltaic cells, all connected together and encapsulated
in a weather-tight, clear-glass or plastic panel.
Amorphous technology is a recent development that is showing much
promise for panels available in a few years. However, the present
generation of crystalline panels provide around two times the full-sun
charging current of equal-size amorphous panels. Also, unlike
crystalline models, present-generation amorphous panels lose 10-15
percent of their power output in the first few months of use. (Most
panel manufacturers acknowledge this fact by conservatively specifying
the panel's power output at an "after break-in" level.)
On the other side of the coin, amorphous panels are available in smaller
panel sizes, and their power output is less affected by overcast
weather conditions and partial shading. Consequently, amorphous panels
are practical for motorhome applications that require small amounts of
power (e.g., anything under about half an amp of charging current),
while crystalline panels are currently the most cost- and space-effective
choice for higher-power requirements (e.g., half an amp to several tens
of amps). However, look for that trend to shift significantly in favor
of amorphous panels in the next decade.
Solar panels are frequently rated in terms of watts of power produced.
This specification can be misleading, because in the real world, the
panel is rarely capable of actually producing this much power because
the voltage is down until shortly before the battery becomes fully
charged. Consequently, it is preferable to look at a panel's rated
charging current specification, which stays fairly constant under any
state of battery charge (and is thus a more reliable measure of the
panel's true battery-charging ability). Therefore, in comparing
different solar panels, look for panel models that produce the most amps
per dollar of cost, rather than the most watts per dollar.
Crystalline panels are currently available from five major manufacturers
(BP, Kyocera, Siemens/Arco, Solarex and Solec/Hoxan), as well as from
several smaller specialty firms. Most of these manufacturers also have
a line of amorphous panels. UniSolar, a sixth major manufacturer,
produces a line of panels that are constructed of many small inter-
connected amorphous cells, somewhat resembling conventional crystalline
panel construction. Due to the inherently simple mechanical design of
any solar panel, the product quality from major manufacturers is
consistently high, with 10- to 15-year warranties being typical.
In terms of obtaining the most power for the money, the best values are
currently in the crystalline panel models that produce 3 to 4.5 amps
(or 45 to 70 watts), currently selling for around $90 to $120 per amp.
Panels smaller than this size (crystalline or amorphous) invariably
cost much more per amp.
Siemens and Solarex have recently introduced new crystalline-technology
models that offer slightly lower cost per amp than their predecessors
(8-15 percent less, depending upon model and dealer); these products may
be attractive for motorhome applications.
Used crystalline panels are also available, with top-quality models under
10 years old typically selling for $40 to $80 per amp. Many of them are
sold with a short warranty. These panels can be an excellent value,
since the life expectancy of a new solar panel is generally considered
to be at least 30 years. THeir biggest downside is that since they are
usually earlier-generation designs, they may not provide as much power
per square foot as the newest models. On a motorhome rooftop, space
may be limited, so be sure to measure your roof before shopping for
panels.
In choosing new or used crystalline panels, try to avoid models rated
at less than 15.9 charging volts (e.g., constructed with less than 33
internal cells). "Self-regulating" 14.6-volt (30-cell) panels are widely
available, but they may be incapable of fully charging a battery,
particularly during very hot weather. (Since each cell in a panel
contributes to the panel's total output voltage, the number of cells in
a panel greatly influences its maximum voltage output.)
All crystalline-technology solar panels produce less voltage in hot
weather, but the extra cells in the 33- and 36-cell models provide
enough reserve capacity for just about any weather condition. Also,
mixing panels with greatly dissimilar output voltages is undesirable,
because it may reduce the maximum voltage to that of the lower-voltage
panel in the group. This problem is especially common when several
used panels containing differing numbers of cells are connected together.
A charge controller can be very important in assuring the proper
operation of a solar-power system. All charge controllers work by
measuring battery voltage, which gradually rises as the battery becomes
more charged. As soon as the measured battery voltage indicates that
charging is complete, the controller prevents any further power from
reaching the battery.
If the combined maximum output current of all the solar panels is less
than 2 percent of the total amp-hour capacity of all the connected
batteries, a charge controller may be unnecessary. However, in moderate
to large solar-power systems, the charging current from the panels can
be high enough to quickly ruin the coach batteries if a charge controller
is not present.
When selecting a charge controller, look for a model that is capable of
handling the peak output of at least four large solar panels (around
15 amps). You may only want to install two panels right now, but the
cost of the extra controller capacity is very minor, and will prevent
any need to buy a beefier controller if you ever add extra panels.
Series, shunt and diversion controllers all work well in motorhome
applications. Look for a controller with an ammeter and an accurate
voltmeter. THe ammeter is handy for determining at a glance if the
panels are receiving unobstructed sunlight, and an accurate voltmeter
is helpful in determing the battery's state of charge. To be useful,
the voltmeter should be capable of measuring voltage differences down
to 0.2 volts or less. On an analog meter, this means an expanded scale
(i.e., 10 to 16 volts) is necessary. In the absence of a good voltmeter
in the charge controller, you should consider purchasing a hand-held
digital test-meter for making the measurements yourself.
Finally, some controllers include a provision for adjusting the charge
cut-off voltage. Since gel-cell, golf-cart and conventional marine/RV
batteries each have a different ideal charge cut-off voltage, a
controller with this feature will allow you to maximize the life of
your coach batteries by avoiding under- or over-charge. This is
particularly important with gel-cells, which are very intolerant of
high voltages.
System Sizing
With these basics in mind, some consideration can be given to the actual
size and number of panels required for your particular installtion. A
time-tested rule-of-thumb is to install one large solar panel per large
coach battery. If you are currently satisfied with the size of your
motorhome's battery bank (e.g., it meets all your present energy needs
for several days between recharges), then installing one 3- to 4.5-amp
panel for each existing battery should prove to be acceptable.
Most people find that they need at least two good-sized house batteries
in order to comfortably camp for any appreciable period away from AC
hookups, so this means that most motorhomes will require a minimum of
two large panels. Three or four large panels will meet most of the
energy needs of motorhome households with moderate appliance use. In
sizing your system, remember that you can easily add more panels and/or
house batteries at a later date.
Determining a Mounting Configuration
Solar panels can be successfully used while mounted flat on the motor-
home roof, although considerably higher daily power output (up to 40
percent) is usually obtained if they are tilted toward the noontime sun.
Tilting is particularly helpful in the winter months, when the sun never
gets very high in the sky. Some motorhomers accomplish this by mounting
the panels on hinged brackets that can be propped-up at an angle, using
removable braces. This works well in situations where the motorhome can
be parked in a direction that points the panels due south. Changing the
horizontal direction of the panels in order to track the sun as it moves
across the sky also helps improve daily power output somewhat, although
it may be difficult to justify the increase of complexity and cost that
this capability involves. Most motorhomers obtain perfectly adequate
performance by leaving the panels flat in the summer and tilting them
south when ever parked extended winter periods.
It is very important that no part of the solar panel be shaded for any
significant period of the day. If even a 1-square-inch shadow falls
on some part of a panel, the power output of the panel drops drastically.
Take particular care to avoid shadows from air-conditioners, vents, TV
antennas, luggage racks and compartments.
Some shading during the early and late hours of daylight may be
unavoidable, but the panels should have an unobstructed "view" of the
sun during the peak power generation hours from 10 a.m. to 2 p.m. To
avoid exposing the entire motorhome to sunlight during the hot summer
months, it may be desirable to mount all the panels in one corner of
the roof, so that the rest of the motorhome can remain under any
available shade. Finally, the panels should be mounted slightly above
the roof, so that some air can circulate underneath them.
System Installation
Several solar-supply dealers offer panel-mounting kits (and complete
solar-system packages, with your choice of new or used panels) that
are specifically designed for motorhomes. These mounts have the
advantage of being well tested, and can save considerable installation
time.
Most panels are installed with brackets that screw into the motorhome
roof. On motorhomes built with light duty roofs, the brackets are
attached directly to the rafters; on heavier roofs, the brackets can be
attached anywhere. In all cases, considerable care must be taken to
avoid drilling or screwing into any hidden electrical wiring, since
a shock or fire hazard could result. Mastic sealant is applied at each
screw to prevent water leaks.
The wires from the panel are frequently routed across the roof to the
refrigerator exhaust vent, which makes a convenient entrace point to
the motorhome's interior. The charge controller is mounted inside the
motorhome, where it connects to these wires. Additional wires are then
routed from the controller to the house batteries. All of these
connections can be made with relatively small-gauge wire, since the
combined currect output of even four large panels will normally be under
15 amps. However, the wire should be rated as being sunlight-resistant
(types USE and UF are frequently used), or it must be otherwise protected
from exposure to direct sunlight.
Since most charge controllers are factory-adjusted to work right out of
the box, no installation adjustments are immediately necessary. However,
some users adjust the cut-off voltage to match their coach batteries.
For example, most gel-cell batteries work best if charging ceases at
about 14.3 volts and most conventional RV/marine batteries prefer 14.4
to 14.8 volts. Consult the battery manufacturer for the exact voltage.
Daily System Use and Maintenance
You will probably soon develop a feel for how much power you can use
without overtaxing your batteries. On sunny days, that will probably
mean as much electricity as you want. On the other hand, after a week
of continuous rain, you will probably want to consider some supplemental
battery recharge method.
Many motorhomers find that the installation of solar panels makes the
use of some new appliances attractive. For example, if extenseive
camping in a hot, dry climate is planned, a 12-volt vent fan or
evaporative cooler can be use to great advantage. Hot climates provide
an excess of sunlight that can be used to keep the motorhome cool,
while still meeting routine battery-charging needs. Many solar-equipped
motorhomes also eventually buy an inverter, alllowing the use of
ordinary household appliances.
As previously mentioned, maintenance on a solar system is practically
nil. You will need to clean the panels every month or so, using glass
cleaner or a garden hose. Unless you are using sealed or gel-cell
house batteries, you will also need to check the battery's water level
at least every month. Use only distilled water (never tap water), and
observe all safety precautions when working around them. Every year or
so, it's also a good idea to inspect the panel mounting brackets for
water tightness, in order to avoid future roof leaks. Other than these
three tasks, you can ignore the entire system. Try doing that with your
generator!
-----------------------------------------------------------------------
Solar Equipment Suppliers
Alternative Solar Products Backwoods Solar
27420 Jefferson Avenue, 8530 Rapid Lightning Creek Road
Suite 104B Sandpoint, ID 83864
Temecula, CA 952590-26668 (208) 263-4290
(800) 229-SOLAR (229-7652)
Camping World Photocomm, Inc.
Three Springs Road 930 Idaho Maryland Road, D3
Bowling Green, KY 42102 Grass Valley, CA 95945
(800) 626-5944 (800) 544-6466
Real Goods RV Solar Electric
966 Mazzoni Street 14415 N. 73rd Street
Ukiah, CA 95482-3471 Scottsdale, AZ 85260
(800) 762-7325 (800) 999-8520
Siemens Solar Industries Solar Electric Specialties
4650 Adohr Lane P.O. Box 537
Camarillo, CA 93010 Willits, CA 95490
(800) 233-1106 (800) 344-2003
Star Power Energy Systems Sunlight Energy Corporation
28121 Front Street 4411 W. Echo Lane
Temecula, CA 92590 Glendale, AZ 85302
(909) 699-1855 (800) 338-1781
United Solar Systems
1100 W. Maple Road
Troy, MI 48084
(313) 362-4170
----------------------------------------------------------------------
This is a wiring diagram for a typical motorhome solar panel installation
and it shows the simplicity of the circuit. For larger capactities,
simply add more batteries or more panels until all your electrical needs
or budget are met.
charge controller
solar panels (optional with very
_____ _____ small systems)
| | | | _________
| (+)------(+)-----------| |
| | | | | |----(-)------------------:
| (-)------(-)-----------|_________| alternator :
| | | | : ______ :
|_____| |_____| : | | :
(+) :-----(+)--| | --(-)---:
: : |______|
-------------------------: :
: : : : :---------:
(+) (-)--- (+) (-)--- _:____:_____:_ (+) (-)------:
_:___:_ : _:___:_ : | | _:___:_ :
| | : | | : |______________| | | :
|_______| : |_______| : isolator |_______| :
house : house : start :
battery : battery : battery :
:------------:--------:---------------------------:
:
---------
----- system ground
-
