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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