AJ's Technical Tips Inverters in Small Solar PV Systems

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					                                                                                     TECHNICAL TIPS CORNER



AJ’s Technical Tips: Inverters in
Small Solar PV Systems
S
      imon Nyukuri wrote me recently with two         is any way to do the job with a DC appliance         that has a rated power output that is high
      important questions. The first question         instead of an AC one. Sometimes it takes an          enough to meet the power requirements of
      is about the use of inverters with small        extra effort to find a DC appliance that will        all of the AC appliances that will be connected
solar PV systems. He wrote:                           work, but if you can stick with DC it is almost      to the inverter at one time. For example, let
                                                      always worth the effort.                             us say that you want to select an inverter to
I earlier had installed a solar system that                 If it is absolutely necessary to use an        power a 21" color TV that uses 70 Watts of
included an 80W module, a 75 amp-hour                 inverter in a small solar PV system, then here       AC power when it is on and a small
battery, and a Sollatek SPCC6 charge                  are my recommendations for selecting an              refrigerator that uses 100 Watts of AC power
controller. It really did well. Then recently         inverter. I will discuss nine different factors      when it is on. This is a total of 170 Watts of
the owner decided to convert the system to            that you should consider in selecting an             continuous power when both the TV and the
AC. He bought a Trace DR 2424 inverter from           inverter. These are (1) input voltage, (2)           refrigerator are on. This means that you
where he knows himself. I added it on the             output voltage, (3) rated power output, (4)          should get an inverter that is rated for at least
system, then we latter noticed that the system        surge capacity, (5) standby power use, (6)           170 Watts of continuous power output. In
had some interference when he tuned in to             efficiency, (7) inverter wave form, (8) brand        practice you may want to get one that is even
FM radio stations. Could the inverter be              name, and (9) price.                                 a bit larger (for example, 200 or 300 Watts)
affecting the frequency radio waves? Also,                  Before I discuss these points, I will begin    in case other appliances are added to the
since inverters are often located inside the          by explaining that an inverter is a device that      system. You definitely do not want to get an
houses, is there any possible health hazard           converts DC (direct current) electricity into        inverter that is too small to run the two loads,
to be considered?                                     AC (alternating current) electricity. DC             because an under-sized inverter can be
                                                      electricity is the type of electricity that is       damaged by overheating that happens when
      First, it is, of course, important to install   produced by solar modules and is stored in           the appliances need more power than the
the inverters following safe electrical               batteries. AC electricity is the type of             inverter can deliver.
installation practices, but there are no safety       electricity that is used with the electrical grid.
or health issues associated with inverters            Most appliances are made for use with AC             Surge Capacity: The surge capacity is the
beyond the normal issues related to working           power, but many are also available for use           maximum amount of power that the inverter
with AC electricity.                                  with DC electricity. Appliances that are             can deliver for a very short period of time -
      The interference that Simon’s customer          commonly available for use with DC power             for example, one second. The surge capacity
got when he tuned in FM radio stations is a           include lights (both bulbs and fluorescent           should be listed along with the rated power
common problem with a type of inverter that           types), black and white TV sets, radios, and         on the inverter specifications. The surge
is called a “modified sine wave” inverter. The        mobile phone chargers. Some other                    capacity is always higher than the rated
Trace DR 2424 inverter is of this “modified           appliances, such as color TVs, refrigerators,        power. The surge capacity is important
sine wave” type, and it is definitely the cause       and others, are also made for use with DC            because some appliances draw a lot of power
of the interference. It is possible to get a          power, but they can be hard to find. This is         for a very short time when they are first turned
“true sine wave” inverter that will not cause         one big reason that people sometimes choose          on.
this problem, but this type of inverter is more       to use an inverter.                                        For example, electric motors often use
expensive. I will explain a bit more about the              Now, back to the main things that you          5 to 7 times more power when they first start
different types of inverters below.                   should consider when purchasing an inverter:         up than under normal operation, while TV sets
      First, however, I will say that my                                                                   often draw up to twice as much power at
recommendation about the use of inverters             Input Voltage: The input voltage for the             startup than during normal operation. The
with small off-grid solar electric systems is         inverter must match the voltage of the battery       inverter that you choose should have enough
that you should never use an inverter unless          bank in the system. Most small systems are           surge capacity to start the appliances that
it is absolutely necessary. If possible, it is        wired for 12 volts, but some larger systems          you want to use with it. In the example that
always best to use DC appliances. An inverter         use 24 or 48 volt configurations. If you have        we used above, the refrigerator might draw
should only be used if the appliance that is          a 12 volt battery bank then you need an              600 Watts during startup, while the TV set
to be used in a system is only available for          inverter that accepts a 12 volt input voltage.       might draw 140 Watts. If both the TV and the
use with AC power.                                    If you have two or more batteries that are           refrigerator start at the same time then the
      I try to avoid the use of AC appliances         wired in parallel so that you have a 24 volt         surge power could be 740 Watts in this
and inverters in small solar PV systems for           battery bank, then you need a 24 volt inverter,      example. This means that you should buy
several reasons. First, inverters introduce           et cetera.                                           an inverter that has surge capacity of 740
inefficiency into the system and this wastes                                                               Watts or more. In order to avoid damage to
some of the precious electricity generated by         Output Voltage: The other voltage to consider        the inverter it is always important to make
the solar PV system. Second, adding an                is the output voltage of the inverter. Most AC       sure that the startup power required by the
inverter makes the system more complicated,           appliances used in Kenya require 240 volt            appliances is not higher than the surge
and this is one more component that can               AC electricity at a frequency of 50 Hertz. This      capacity for the inverter!
break and cause a problem. Third, some                is almost always the type of inverter that you
types of inverters - the modified sine wave           want to use. Inverters sold for use in the           Standby Power Use: Inverters use a small
type - cause interference problems like the           United States and some other countries have          amount of power when they are turned on
one that Simon’s customer experienced.                120 volt AC output at a frequency of 60 Hertz.       even if no AC appliances are in use. Some
      I know that in this case the customer           These generally are not sold in Kenya, but if        inverters draw a few Watts when they are in
went and purchased the inverter on his own            you get an appliance that requires 120 VAC,          standby, while others draw 10 Watts or more.
without consulting with Simon. In such a case         60 Hertz power you may need to order one.            You want to select an inverter that has the
there is nothing that you can do except help                                                               smallest standby power use possible in order
the customer to the best of your ability.             Rated Power Output: This is the maximum              to avoid wasting electricity.
However, if a customer does ask about adding          amount of power that the inverter can deliver            In addition, you should always advise your
an inverter, my first question is to ask if there     continuously. You should select an inverter          customers to turn the inverter off when no AC

                                                                                       SolarNet Vol.6 No.1 August/December 2004 27
                                                                                  TECHNICAL TIPS CORNER

appliances are in use. An inverter that uses 5     higher DC to AC conversion efficiency than                As a final note on inverters, I will again
Watts of standby power can use up 120 Watt         modified sine wave inverters. Unfortunately,         emphasize that it is much better to use DC
hours of electricity in one day if it is left on   they can be significantly more expensive than        appliances with small solar PV systems
continuously! This is a waste of energy that       modified sine wave inverters, so they usually        whenever possible. You should only use an
can be avoided if people are careful about         only get used when they are necessary.               inverter if it is absolutely necessary!
turning off the inverter when it is not in use.                                                              To close, I will return to the second part
                                                   Inverter Brands: There are a number of               of Simon Nyukuri’s question (you will
Efficiency: Inverters also use some                different brands of inverters, and some are          remember that his question had two parts).
electricity in the process of converting DC        better than others. High quality inverter            In the second part of the question he asked
electricity into AC power. The efficiency of       brands include Trace, Xantrex, Hart, Exeltech,
the inverter is the ratio between the AC output    and ProWatt. When buying an inverter I               I would like to know if there is any special
power and the DC input power. For example,         usually try to stick to the high quality brand       possible position for mounting a solar panel
let us say that we have a 200 Watt inverter        names that I list here, but there are some           for best sunshine harvest despite the fact that
that is used to run a 21" color TV set that        other brands of inverters that are available         most of us just mount depending with the
uses 70 Watts of AC power. In this example         in Kenya that may also work well (I have not         roofing direction.
the inverter that we are using is 75% efficient.   tested some of these other brands so I cannot
This means that the AC output (70 Watts)           give a recommendation based on my                    I discussed this point in one of my earlier
divided by the DC input is 0.75. This would        experience). In addition, there are a few Jua        Solarnet articles (Solarnet Vol. 4, No. 1 -
mean that the DC input would be 93 Watts.          Kali electronics technicians in Kenya who also       January to May, 2002). As I said in that article,
This comes from 70 Watts ÷ 0.75 = 93 Watts.        make reasonably good inverters. If you are           for most locations in Kenya (and other places
The 93 Watts is the amount of power that           unsure if the brand you are considering is           close to the equator) it is usually best to
the solar system must provide to run the TV.       good quality, I recommend that you ask               mount the solar panel so that it is almost flat,
      Small inverters usually have a lower DC      around to see if it has a good reputation. As        with just enough tilt for rainwater to run off
to AC conversion efficiency than larger            always, I am more than happy to get letters          of the panel. When the panel is mounted so
inverters, and low quality inverters of any size   asking about the quality of a particular brand.      that it is almost flat the direction that the
can also have a low efficiency. High quality       I will do my best to answer any questions as         panel is facing is not so important. The main
inverters that are larger than 1000 Watts often    quickly as possible!                                 thing is to install it so that it is almost flat.
have a DC to AC conversion efficiency of 80%                                                                  I thank Simon for sending in these
to 90%. Smaller inverters are often 60% to         Inverter Price: The price of the inverter is         excellent questions, and I encourage the rest
80% efficient, although low quality units can      an important consideration when selecting            of you to write to me as well. As always, I
have efficiencies as low as 50%. You want          one for purchase. However, the lowest priced         hope that this information is useful to you
to get an inverter in your price range that is     inverters are not always the best ones to            all, and I look forward to writing again in the
as efficient as possible.                          choose. If it is possible to use a good quality      next issue of Solarnet. Until then, kwaherini!
                                                   brand name modified sine wave inverter
Inverter Wave Form: I started this article         instead of a true sine wave inverter this can
by talking about a “modified sine wave”            help keep the cost down. However,
inverter that was causing an interference          sometimes it is necessary to go for a more
problem with an FM radio. I also said that a       expensive true sine wave inverter in order to
“true sine wave” inverter would not cause          avoid FM radio interference issues or other
this problem. These two types of inverters -       problems.
“modified sine wave” and “true sine wave” -              These are some of the main factors that
have different types of “wave forms”. The          you should consider when purchasing an
wave form refers to the quality of the AC          inverter. I will also give a few tips to keep in
output signal from the inverter. There are         mind during installation. First, you should
actually three main types of wave forms for        ALWAYS install a DC fuse between the inverter
inverters. These are (a) square wave, (b)          and the battery. The amperage of the fuse
modified sine wave (sometimes called               should be small enough to protect the
                                                   inverter, but large enough to allow the surge        250 Watt Modified Sine Wave Inverter by
“modified square wave”), and (c) true sine                                                              ProWatt
wave (sometimes called “pure sine wave”).          capacity current to pass through without
     Square wave inverters are generally low       blowing the fuse.
quality and I do not recommend their use in              Second, you should be especially careful
most applications. Modified sine wave              not to reverse the polarity between the
inverters are appropriate for use with a           positive and the negative terminals for the
number of appliances, including lights, TV         DC input voltage when you are connecting
sets, computers, electric motors, and many         the inverter. If you reverse the connection
other common appliances. They are the most         even for a brief instant you can permanently
common type of inverter used with small solar      damage the inverter. Many manufacturers
electric systems, and they are less expensive      will not replace inverters under warranty if
than true sine wave inverters. However, they       they are damaged in this way, so you can
do cause interference problems with FM             lose a lot of money in just one second by            1100 Watt True Sine Wave Inverter by
radios (this can be true even if the FM radio      making an incorrect connection!                      Exeltech
runs on DC power and is not connected                    Third, you should be careful not to install
directly to the inverter!). They can also          an inverter in the same enclosure as the
damage certain types of sensitive electronic       battery unless the space has good ventilation.
equipment.                                         The acid fumes from the battery can damage
     True sine wave inverters have an AC           the electronic circuits in the inverter. Inverters
wave form that is more or less the same as         should be installed in a well ventilated space
the wave form for electricity from the             that is at least a meter away from the battery.
electrical grid. These inverters can be used             Fourth, you should be sure to observe
with any type of appliance, and some types         safe AC wiring practices when you install an
                                                   inverter. 240 volt AC electricity is                 2400 Watt Modified Sine Wave Inverter by
of sensitive electronic equipment require their
                                                   considerably more dangerous than 12 volt DC          Trace Engineering. This inverter is similar
use instead of a modified sine wave inverter
(for example, laser printers, CD players for       electricity, and you should always follow safe       to the one that Simon Nyukuri asked about
stereos, and others). They also often have a       wiring practices when working with inverters.        in his letter.

28 SolarNet Vol.6 No.1 August/December 2004