This is only a preview of the May 2015 issue of Silicon Chip. You can view 29 of the 104 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
Items relevant to "Appliance Earth Leakage Tester":
Articles in this series:
Articles in this series:
Items relevant to "Balanced Input Attenuator For Audio Analysers & Scopes":
Items relevant to "4-Output Universal Voltage Regulator":
Purchase a printed copy of this issue for $10.00. |
Home Solar Panel
(PV) Electricity:
Is it worth it?
By
Dr Alan Wilson
In these days of soaring energy prices, generating your own electricty
from solar panels on your roof seems like a great idea. But is it? One
reader “took the plunge” a few years back and has kept records
since. His report might help others make that (quite costly!) decision.
F
ive years ago I looked into household photovoltaics
(PVs) for generating electrical energy and decided
the technology, and cost, was then at a point where
it was worth considering.
Living in Melbourne, a bit on the marginal side for solar
energy, I decided to go for the biggest system I could. Thus,
a bit over four and a half years ago I had a 5kW system in-
Fig. 1 Current (black) and Voltage (blue) curves for an
autumn day with scattered cloud.
38 Silicon Chip
stalled on my roof. This comprises 27 panels and takes up
most of the roof space. An inverter is mounted on the side
of the house. Apart from an upgrade for the circuit breaker
(25A, up from 20A) the system has worked flawlessly.
Temperature and PV Efficiency
Why was a 20A breaker inadequate? It was an interesting
Fig. 2 Daily energy produced (ie, fed back into the power
grid - pink) and consumed (blue) as indicated by my smart
meter. The black lines are smoothed versions of the same
data for clarity.
siliconchip.com.au
(Above): the meter box with old style fuses below and smart
meter on the left. At the top are the PV circuit breaker/
isolator (left) and then the control block for the hot water
system comprising the circuit breaker, timer and timer
contactor. The large bare area used to hold two (peak and
off-peak) rotating style power meters.
(Right): the inverter and the grid isolation and PV isolation
switches. The small wire at the bottom connects to an
RS232 port.
(Opposite): the 27 solar panels installed on the north-facing
roof, with the evacuated tubes for the solar-assisted hot
water on the wall at the bottom right. Shadowing of the
lower section of panels starts around 4pm in summer. A
single-storey neighbouring house ensures the evacuated
tubes are never shaded, even in winter. At the bottom left of
the evacuated tubes it is just possible to see the single small
solar array which provides power to the controller and
water pump.
‘fault’. The solar panels worked fine for the first nine months
after installation, through winter, spring and summer and
then one day in autumn I discovered they were offline.
The circuit breaker had tripped so after checking for
obvious causes – and finding none – I simply turned it on
and all seemed fine. But it happened again a few days later
and then I had my suspicions.
The recent weather had been cold but with patchy cloud
and times of quick, bright sun when it broke through gaps
Fig.3: total energy produced less energy consumed. The
original downward trend shows net consumption of electricity which has been turned around after the installation
of solar assisted hot water in November 2013 (red arrow).
siliconchip.com.au
in the cloud cover. So on a similar day I hooked into the
RS232 port on the inverter and started logging.
Murphy must have been on vacation because I was lucky
first try! As the sun broke through the clouds I obtained a
great set of data showing the current peaking above 20A
(see Fig.1 opposite).
The problem was due to the temperature dependence of
the PV panels. They are more efficient at lower temperature
and on a cold day, if the sun bursts through the clouds at
full strength, the current can peak above normally expected
values before the cells heat up and the efficiency reduces.
I called my installer with data ready and prepared for an
argument but was pleasantly surprised when they quickly
agreed with my analysis and sent a person around the next
day with a replacement 25A breaker.
Since then the breaker has not tripped but on similar
days the inverter has temporarily shut down due to excess
power generation. The maximum I have ever noticed was
5.3kW on a cool but sunny autumn day. Compare this to
my typically observed peak powers of 4kW on hot, cloudless summer days and the effect of temperature on the PV
efficiency is very obvious.
First two years
Over the years since installation I have not been ‘fanatical’ about monitoring the operation of the solar panels and
May 2015 39
Evacuated tube solar-assisted hot water
Evacuated tube, solar-assisted hot water consists of a number
of evacuated glass tubes which contain a light-absorbing material
coated onto a metal structure, which in turn transfers heat to an
internal heat pipe. The top of the heat pipe is in a heat exchanger
manifold which has the water to be heated circulating through it.
This system is more efficient than the more-well-known flat
panel systems and is particularly suited for colder climates where
the evacuated tubes largely isolate the heated elements from the
environment. Another advantage of the tube construction is the
circular geometry automatically ‘tracks’ the sun.
The system I installed includes thirty 2m-long tubes, a conhave only recorded the energy taken from the grid and
energy sent to the grid, as measured by my smart meter,
on a roughly weekly basis.
This gives me an indication of how much energy my
system is producing and also assures me that all is functioning correctly. Note that this gives no idea of total energy
generated since the power I use directly from the panels
is not included in these measurements.
Fig.2 shows the energy as a daily production and
consumption (obtained by averaging from the previous
measurements) with an added smoothed line for each,
plotted against the date. When production is greater than
consumption, I am winning. Whether I am doing better
overall is not obvious from this plot so the second graph,
Fig.3, is for the same time but plotted as the total energy
produced, less the total consumed. It is now clear that up
until December 2013 I was still a net consumer of electricity.
Addition of solar-assisted hot water
I run an all-electric house except for gas hydronic heating.
(That’s where gas-heated hot water is circulated through
the house to heat it).
So, to reduce my electricity use I modified my off-peak
electric hot water heater in November 2013 to include
Evacuated Tube Solar Assisted Hot Water (see breakout box),
shown by the arrow in figs.2 & 3. The ‘consumed’ graph in
fig.2 shows an immediate drop at this point.
The interpolation of the straight line in Fig. 3 through
the peaks of the graph shows where the next peak would
be expected and it is even clearer that the solar assisted
hot water system is giving an immediate pay-back. Of more
interest is the final peak for this year which has reversed
40 Silicon Chip
troller, temperature sensors, small PV array and motor. The PV
array provides all the power required and the controller is set up
to start pumping water when the temperature difference between
the manifold and the hot water storage tank exceeds 8° and stop
when it falls below 4°.
Even in winter the evacuated tubes make a significant contribution to my hot water heating, shown by the ~2.5kWh drop in the
July-August consumption peak in Fig.2 overleaf. In spring and
summer they provide all the heating required, corresponding to
the close to 4kWh drop in consumption in the January-February
period.
the downward trend and is indicating I am now a net producer of electricity.
Going ‘off-grid’?
There is a significant period of time over the middle
months of the year when the production from the 5kW
system (plus solar-assisted hot water) falls significantly
below consumption.
Going ‘off-grid’, often promoted as the nirvana of alternative energy, would require a currently impractically large
amount of storage capacity to cover this time. The only way
to reduce this would be to increase the size of the PV array;
eg, the data for July 2014 would indicate a 15kW PV array
might just cover the energy requirements for that period.
Of course, some smaller amount of energy storage would
still be required for night usage and to cover multiple overcast days. Thus the peak daily consumption of ~18kWh
would need to be available from storage for a number of
days.
This is much more tractable with 20kWh lithium-based
battery packs and built-up modules now readily available,
along with indications that costs may fall below US$100/
kWh in the next few years. However, installing 80 solar
panels on an average suburban rooftop is not feasible, so
for the moment, I will remain connected to the grid.
Those pesky blackouts
The electricity supply where I live has been very dependable but just last month a random lightning strike took out
a chunk of Melbourne suburbs for more than 20 hours: long
enough to prompt me to frantically ship the contents of my
freezer to a friend with power.
siliconchip.com.au
Why is “anti-islanding” important?
FULL DUPLEX
COMMUNICATION
OVER WIRELESS
LAN AND IP
NETWORKS
Wouldn’t it be nice if a solar PV system could be used
to provide power in these circumstances (during the day
at least) and avoid this sort of angst?
Unfortunately, grid-connect systems (with no local electrical storage) cannot do this, even if disconnected from the
grid (so they are not feebly trying to power up everyone) to
only provide power for their own household.
Apart from the anti-islanding feature built into all gridconnected systems (see panel), another problem is one
of consistent supply. With variations in sun level due to
clouds the power available can vary wildly – from nearly
nothing to full supply. Imagine the effect this could have
on electrical appliances, particularly electric motors which
might try to operate with inadequate power available, possibly drawing large currents but with insufficient voltage to
turn over properly. Thus, for safety reasons, grid-connect
PVs will not operate in isolation – if they cannot detect the
presence of mains (ie, a blackout), they simply shut down.
Conclusions
Even in Melbourne, a 5kW solar panel installation plus
solar-assisted hot water (or gas hot water) appears capable
of producing more electrical energy than a household uses
over a year. This obviously reduces energy bills and also
reduces the amount of CO2 emitted by coal-powered electrical generating plant. Solar panels would be expected to be
an even better proposition in more northern and sunnier
climes of Australia.
A nice way for future household solar panels to go would
be the inclusion of some local energy storage and the capability to operate from this stored source during blackouts.
Also, given the currently very low feed-in tariffs available
for new installations, it would make sense to store as much
locally produced energy and use it before the expensive,
grid provided energy. 10kWh of battery storage would
cover most of these requirements and with the expected
drop in battery storage costs it will become more feasible
in the near future.
SC
siliconchip.com.au
IP 100H
See the
review in
SILICON
DecemberCHIP
2014
(ask us for
a
copy!)
Icom Australia has released a revolutionary
new IP Advanced Radio System that works
over both wireless LAN and IP networks.
The IP Advanced Radio System is easy to set
up and use, requiring no license fee or call
charges.
To find out more about Icom’s IP networking
products email sales<at>icom.net.au
WWW.ICOM.NET.AU
ICOM5001
When the electricity grid fails (eg, a blackout or even a
brownout) the solar panel array doesn’t know that – and keeps
on producing power as long as it is being irradiated.
It could therefore be regarded as an “island” in a sea of unpowered electrical lines.
This could be quite dangerous in many ways: (a) anyone
working on an apparently “dead” circuit could be electrocuted;
(b) without a reference voltage, the system could produce far
more voltage than it is designed to handle; (c) conversely, a
small solar installation could be trying to power a whole suburb
or town so it could be massively overloaded and (d) the inverter
may not operate correctly in either case – when there is no grid
power or when the grid comes back up.
To prevent islanding, power inverters connected to solar panels
almost invariably check for a live grid. If they don’t find one, they
simply don’t start up. (For more information, see “Mailbag” in
SILICON CHIP, July 2011 issue, page 8, 9 & 10.)
For this reason, solar panel inverters designed to feed into
the electricity grid cannot normally be used as “stand alone”
systems which will charge batteries and supply power – for
this you need a system designed specifically to be “off grid”.
And that ain’t cheap!
May 2015 41
|