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Need power when the sun don’t shine . . . or when the grid fails? A large, “real world” HYBRID Solar System A major drawback of conventional (grid-tied) solar power systems is that they deliver no power when the grid fails. So if you have a blackout during and/or after a natural disaster, such as a bushfire, flood, cyclone or severe thunderstorm, you may be without electricity for days or even weeks. But if you have a hybrid solar system you can produce your own electricity when other people have none. You can even have “solar generated” electricity at night. . . By LEO SIMPSON 22  Silicon Chip siliconchip.com.au W ith a hybrid solar system, in addition to the PV panels and grid-tied inverter normally found in a typical domestic solar system, an inverter/ charger and battery bank are required. Even with a quite moderately sized battery bank, a hybrid system can typically double a household’s “selfconsumption” of solar-generated energy. An appropriatelysized system that can supply a household’s needs until after 10pm offers the possibility of moving to time-of-use metering, which can halve the price of purchased power. This system can easily supply this amount of power. If you can run the household or business from the solargenerated power during the day and recharge the batteries to carry the load until 10pm or later, you can have very real savings in the cost of your purchased power. The trade-off, of course, is the much larger initial investment in a hybrid system, particularly in storage batteries – in this system, around $50,000 worth! However, recent developments are likely to bring significant reductions in the prices of batteries in the future. A hybrid solar power system can deliver enough power to satisfy normal household loads during the day when the sun is shining. At the same time, assuming enough power is coming from the solar panels, it charges its batteries. Any excess power can be exported to the grid. When the sun sets, the batteries provide the household electricity needs and depending on the load and state of charge of the batteries, some power may be drawn from the grid. When the sun rises in the morning, the cycle repeats. The first priority is to supply any household loads, then The ~$50,000 bank of 24 x 2V gel cells, all connected in series to achieve a 48V DC, 75kWh battery. Inset at top is the label on the cells. Mounted below the solar panels and through the wall from the battery bank is the electronics: the three red boxes are the 4kW grid-tied inverters, with isolating switches underneath. Their 230VAC outputs are combined and fed to the charger (yellow box) as well as to the ATC contactor (underneath), thence to either the household load or via the smart meter back to the grid. The amount which goes to each is prioritised, with household use taking precedence, then battery charging, then feed back into the grid. Note the heavy steel posts which protect the electrics in case of an errant vehicle/trailer/etc. siliconchip.com.au October 2015  23 Block diagram of Geoff Woodman’s hybrid system installed on his farm outside Yass, NSW. Geoff’s background as an electrical engineer helped plan and install the $70,000 system which has a projected payback period of less than ten years; perhaps as low as seven depending on how electricity prices rise in the future. The SMA energy meter measures energy flows within the system; the smart meter is bi-directional and also has time-of-day metering. UTILITY GRID 230VAC HOUSEHOLD LOAD DATA SOLAR PANELS (16kW; 355V) DC DC ATS CONTACTOR & SMA ENERGY METER (Grid disconnected during blackouts) 4kW GRID-TIED INVERTERS 230VAC UTILITY COMPANY SMART METER 6kW INVERTER CHARGER DC INTERNET SUNNY HOME MANAGER 48V 75kWh BATTERY to recharge the batteries and when the batteries are fully charged, export any excess power to the grid. Sound simple, doesn’t it? In practice, it is a lot more complicated and the system is a lot more expensive than a basic grid-tied solar system of similar capacity. This article came about as a result of the Publisher’s Letters in the March & August 2015 issues on the drawbacks of grid-tied inverters, and the resulting letters in the Mailbag pages of subsequent issues. A real hybrid system Long-time reader and electrical engineer, Geoff Woodman, sent in some details of his hybrid solar system which had been installed on his property near the New South Wales country town of Yass. This type of system is quite new to Australia so I recently visited him for a closer look. This system is still grid-tied and so does not necessarily need a very large battery bank nor the option of a diesel generator to charge those batteries during possible long periods of inclement weather, when the solar panels may produce little power. And while the system is grid-tied, there is a limit of 2.5kW on the power that can be exported to the grid at any time . This is set by the voltage losses in the power lines to the property. This system could quite easily be converted to a wholly stand alone (“off grid”) system but would then lose the ad24  Silicon Chip vantages of having a backup grid supply and being able to sell excess power back to the grid. But at the derisory rate of 6c/kWh, the latter is not a huge incentive. It would, however, save the “standing” or “supply availability” charge which the utilities charge everyone who is connected – even if you use virtually none of their power. Currently, this charge is between about 70c and $1.75 per day, depending on your supplier and location – so it could be as high as $600 or more each year. Geoff’s system has 54 300W LG solar panels and three “Sunny Boy” SB4000 inverters made by SMA Solar Technology AG. These are normal grid-tied inverters rated at 4000W each (AC side). Each inverter has two DC inputs, each with its own MPPT (Maximum Power Point Tracking) controller. This provides six MPPT inputs. The maximum DC input to each inverter is 12kW, so it is possible have a PV array with a peak output well in excess of the inverter’s rated 4000W AC output. In fact, it’s good practice to oversize the PV arrays by about 30% relative to the AC output of the inverter, as most fixed panels only deliver their maximum output for a limited time each day, in bright sunlight; on cloudy days, they may deliver much less output. The Sunny Boy inverters have Bluetooth and/or serial data ports that provide data on the power delivered by the inverter and provision to limit the power output by adjustsiliconchip.com.au Three “Sunny Boy” SB4000 inverters take the DC from the solar panels and feed 230VAC into the system. They have a user-friendly LCD screen to show the power being generated, are virtually noiseless in operation and can be used in grid-tied, stand-alone and hybrid systems. The “heart of the system” is the Sunny Home Manager. It provides an overview of all energy flows within the solar installation and uses this information to direct energy to the location which needs it, in order of priority (household power has highest priority, then battery charging, then output to the grid). ing the output voltage. The AC outputs of the three Sunny Boys are effectively wired in parallel. The solar panels are mounted in six strings of nine panels each. Each string is connected to a separate MPPT DC input on one of the Sunny Boy inverters. So there is a total of 2 x 2700W (or 5.4kW) feeding each 4000W Sunny Boy inverter. Each panel has a maximum open-circuit voltage of 39.5V and a short circuit current of 10A. Maximum power output of each panel is 300W: 32.0V X 9.42A (NB: in a practical system solar panels are never operated opencircuit or short circuit). can be used in off-grid or “island” systems). It provides bidirectional energy conversion between the 48V battery bank and 230VAC. The battery bank may be either lead-acid or lithium ion, but in this case a lead-acid battery is being used. For lead-acid batteries, the charger provides 3-stage charging, ie, initial bulk current charging, followed by a constant voltage phase and then a float charge phase. Every 14 days, there is a 2-hour boost charge and full equalisation charging is performed every 90 days, for a duration of 12 hours. The charging algorithm is quite complex and has been optimised to maintain the battery’s state of health over multiple charge/ discharge cycles. Output from the charger is 6kW. The operation of the Sunny Island inverter is critical to the overall operation of the system. It can operate as a grid-tied inverter or stand-alone. When tied to the grid, it works in a similar manner to a grid-tied PV inverter, ie, it is synchronised to the grid and can export energy from the Batteries & battery charging The battery bank consists of 24 Sonnenschein 2V 1959Ah (C120) lead-acid cells connected to give a nominal 48V. The 48V battery is charged by an SMA “Sunny Island” SI8.0H Inverter/Charger (presumably, it is so named because it Two screens taken from the Sunny Portal which show real-time data from Geoff’s solar installation. The data was read on quite low usage days. Basically, green means power being generated on-site by the 45 solar panels – the screen at left showing 7.88kW – and also energy consumption. Red, on the other hand, shows energy being supplied from the grid – 2.06kW on the left screen, which would cost between 16 and ~60c per hour, depending on time of day. The screen above is even better, with just 0.06kW (maybe half a cent’s worth!) being purchased. Compare these with the screen grabs overleaf. siliconchip.com.au October 2015  25 The Sunny SRC20 Remote Control allows the system to be monitored and controlled remotely – a definite advantage on mid-winter nights in Yass! The four-line display gives current system status at a glance, and a memory card can be inserted to store all data. These are the battery fuses and DC disconnect unit. In this installation, it has redundant fuses as it is designed to support three Sunny Islands, as would be used in a 3-phase system. batteries to either the household loads or the grid. Presently, in Geoff’s system, the battery is only used to supply the home needs but there is now the possibility of the “grid export” feature, whereby the local grid operator can remotely control the operation of the Sunny Island inverter and instruct it to export to the grid at periods of peak demand (obviously, the system owner will get paid appropriately for this feed in.) If the system detects grid failure, then the Sunny Island inverter/charger reverts to stand-alone or “island” operation, and provides a 230VAC reference to which the PV Inverters can synchronise and thereby supply the household load. At the same time, the batteries can be charged if sufficient solar power is available. Both the three Sunny Boy PV inverters and the Sunny Island Inverter/Charger all have serial data ports, allowing energy flows and battery state of charge to be measured. Critical to system operation, the output voltage of the Sunny Island inverter can also be controlled, so that its contribution to the system output power can be set. There is a smart meter which has “time-of-day” tariff and this sits between the system and the utility grid, and can measure power flow to and from the utility grid. It also has a serial (modified Ethernet) data output. Finally there is the Sunny Home Manager, effectively the control computer that looks after all the energy flows in the system. It does this by adjusting the output voltages (and thus power) of the PV Inverters and the inverter in the Inverter/ Charger. It also reports lots of system information to the internet for remote analysis and viewing by the system owner (in this case, Geoff Woodman). There is also an Automatic Transfer Switch (ATS) which contains a contactor to isolate the system from the utility grid in the event of a grid failure, so it can run “stand alone” as an island grid. This does not provide instantaneous changeover in the case of a blackout because many of these are very short, often <1s. Instead, there is a delay of about five seconds between the grid going down, the contactor isolating the grid and then the Sunny Island powering up to provide 230VAC from the battery bank. Grey means no power is being generated from the solar panels, as you would expect at night. But the good news is that only 0.08kW is being purchased, the vast majority is coming from the near-fully-charged battery bank. And here’s what you really want to see: all green, meaning no energy is being purchased. Usage is significantly higher here at 2.00kW; only 0.01kW is going back to the grid but at least it is going in the right direction! 26  Silicon Chip Daily operation So let’s consider a typical day: The house has been running off the batteries overnight, with 230VAC generated by the Sunny Island inverter/charger. Before dawn, there is no output from the PV panels, so the Sunny Boy inverters siliconchip.com.au are asleep. The Sunny Island inverter/charger is locked to the utility grid and its output voltage has been adjusted so that it is supplying all the power to the household loads, but no export to the grid. It does this by constantly adjusting its output to be identical to the utility grid voltage, so there is no power flow in either direction through the SMA smart power meter. At sunrise, the batteries are discharged (say) 20%, and are thus at 80% State of Charge (SOC). After sunrise, assuming a cloudless sky, the PV panels start producing power, and the Sunny Boy inverters wake up and synchronise to the utility grid. As they start producing power, their output voltages are adjusted to match the utility grid voltage, so that all the power they produce flows to the household loads; there is no power to or from the grid at this time. As the output of the Sunny Boy inverters rises with increasing output from the PV panels, the power drawn from the batteries decreases, as the household loads are supplied more and more from the increasing output from the PV panels. Then, as the output of the Sunny Boy inverters continues to increase through the morning, the Sunny Island switches its mode and starts charging the batteries. The Sunny Home Manager continuously adjusts the output voltages of the Sunny Boy inverters so there is no power feed to/from the utility grid. All the solar generated power excess to household consumption is used to charge the batteries. When the output of the Sunny Boy inverters rises above the total household demand and the maximum that can be used for battery charging (6kW), the output voltage is adjusted so that excess solar generated power is fed into the utility grid. If the output of the PV inverters exceeds the sum of the battery charging requirements, household loads and permissible grid-feed of 2.5kW, the Sunny Home Manager reduces the The Sunny Portal doesn’t just give statistics – it can give forecasts and recommended actions, as seen in the graph at the bottom of the screen. Of particular interest on this mid-winter graph is that it is forecasting some solar energy generation even after 5pm, contrary to popular belief which says you won’t get anything after about 4pm. siliconchip.com.au Grid Connected, Off-Grid AND Hybrid Many people are confused about the different types of solar power installations which you can install. GRID CONNECTED: as its name suggests, you are always connected to the electricity grid and when it goes down, so does your supply. You do not normally have any batteries to charge because any excess power you generate from your system is usually sold back to the electricity supplier. However, in new installations the amount paid is much less than what they charge you – typically, about 6c to 8c per kWh (they charge you as much as 50c per kWh!) The vast majority of domestic solar power installations are grid connected. OFF GRID: again, as its name suggests, you are not connected to the electricity grid at all. This is sometimes referred to as “islanding”. Your system will normally have a bank of batteries which are charged by the solar panels and you take power from the batteries, invert it to 230VAC mains, and use it to power your home. If you generate more power than you can use or to charge your batteries, it is normally wasted. Off Grid installations have been popular if you are a long way from the power lines. HYBRID: this is a mix of the two – you remain connected to the grid but your solar panels generate enough power to run your home and to charge batteries. If the grid goes down (a “blackout”), your system will switch over and you will have power even if everyone else is in darkness! If you generate more than you can use or to charge your batteries, it can be sold back to the utility. However, like grid-connected, the price they pay you is very small compared to what they charge you for the same power. The other major disadvantage is that you will continue to pay the electricity “availability” charge, even if you never actually use any power from the grid. AC voltage output of the PV inverters to keep the grid-feed limit to its permitted maximum. As the day progresses, the output of the solar system peaks and then begins to decline. Grid-feed is progressively reduced, household loads are supplied as a priority and any excess is used to float charge the batteries. When the output of the PV system falls below the household load requirement, the Sunny Island switches to “inverter” mode and starts drawing the “shortfall” power (ie, the difference between the PV generated power and the household demand) from the batteries. As the sun sets and/or the output of the solar panels falls, the proportion of the household power supplied by the Sunny Island inverter/charger continues to increase until it reaches 100%, and thus is all coming out of the batteries (up to the power limit of the Sunny Island inverter). If it has been a heavy overcast day, with reduced output from the solar panels, the batteries may receive little or no charge. If this is the case, the batteries will continue to supply the household loads via the Sunny Island inverter/ charger. When the battery SOC decreases to 65% (or whatever the limit has been set to) the inverter is effectively switched off and the household load is supplied directly from the utility grid. When power from the solar panels is again available, it will first supply the household loads (decreasing the power drawn from the utility grid), and then begin charging the batteries as/when there is sufficient output to do both. The 65% SOC lower limit has been set to ensure a cycle life of 4,500 cycles for the lead acid batteries in Geoff’s system. October 2015  27 Lithium ion batteries in some hybrid systems are routinely discharged to 20% SOC for a similar life cycle. Installation 18 of the 54 solar panels in this installation are mounted on each side of a large shed’s hip roof (ie, 36 in total) while the remaining 18 are on a relatively flat skillion roof. The panels are then grouped and fed to the three Sunny Boy inverters to more-or-less equally share the load across all the panels, although some panels will be generating larger amounts during the day, depending on their orientation and the sun’s position in the sky. The fact that the total panel capacity is about 30% higher than the Sunny Boy inverters can actually fully use (see above) means that there should be plenty of generating capacity even on light overcast days. All of the inverters, circuit breakers, the Sunny Home manager, smart power meter and other gear is mounted on the back wall of a section of the shed which is also used to garage a car (with suitable barriers in front of the inverter gear to stop the unthinkable collision of a car with all that expensive electronics). The large battery bank is accommodated in another section of the shed, with plenty of space around it. Geoff can check the overall operation of the entire system at any time by logging into his individual pages at www. sunnyportal.com All of the screen grabs in this article were taken from that site as this article was prepared in late August. Note that this was during a succession of cloudy days so I did not see the solar panels and Sunny Boy inverters generating their maximum capacity of 12kW. The typical maximum during this period was around 9.5kW, possibly because the total Just as important in a solar power installation, the Sunny Portal also gives you historical data of power generation, purchased power, grid feed in and your own consumption; everything from the last few minutes to the last year. This data can help users modify their energy consumption to achieve maximum efficiency with their installation. 28  Silicon Chip demand meant the Sunny Home Manager had limited the output of the Sunny Boys. Geoff can also check the operation of the three Sunny Boy 4kW inverters via their LCD panels, which show the output of the panels connected to each MPPT input. Overall, the system is very impressive in its engineering. The overall domestic load is probably somewhat higher than you might expect because all heating and cooling in the home is via reverse cycle air-conditioning. At normal ambient temperatures RC air-conditioning is very efficient as a heat pump but once the outside air temperature drops below 5°C, it becomes quite inefficient and arguably no better for household heating than electric radiators; perhaps even less so. Investment & return This is a far bigger and much more expensive system than the typical “domestic” grid-tied solar system with no storage, which currently are routinely advertised at about $5,000-$20,000, depending on size. The 24 storage batteries alone would leave little change out of $50,000. In the first 12 months of operation, the system was 84% self-sufficient. Of the 16% bought from the grid, 12% was at off-peak rates (16c/kWh) and the remaining 4% at peak and/or shoulder rate (30c/kWh). Allowing for the $10,000 rebate from Renewable Energy Certificates (RECs), the total investment in the system has been about $70,000. Geoff calculates that, in a year of operation, he has saved about $5000 in his energy bills. That is a yield of about 7%. But if you consider that yield is free of income tax, an equivalent “before tax” yield could be above 12%. With those points in mind, the payback period in today’s dollars is about eight years, without factoring in And there’s even more data available should you wish to take advantage of it: this Sunny Portal screen shows the current power (9729W), the current consumption (1101W), energy used today (20.54kWh), CO2 avoided (21kg), the solar panel power (12.60kWp) and its commissioning date; even the local weather and the installation location. siliconchip.com.au PrOfEssIONAl sysTEM sOlUTIONs any inevitable increase in energy tariffs. Some readers may question why there are so many solar panels in the installation. This was a judgment based on the figures from Climate Data Online on the Bureau of Meteorology website, which give the average June insolation in Yass as 2.0kWh/m2. This varies from year to year, but typical daily minimums are about 0.9kWh/m2 and maximums are about 3.0kWh/m2. Also, there is restricted north facing roof space so Geoff chose to mount the panels on a low-pitched roof that faces east and west. Because of the low pitch, the output of the panels is basically the same as if they were lying flat, ie, 16.2kW x 2.0 = 32.4kWh on an average day; 16.2 x 0.9 = 14.6kWh on a poor day and 16.2 x 3.0 = 48.6kWh on a good day. Daily consumption in June 2015 averaged 29.9kWh, mostly due to house heating with the reverse-cycle air conditioning. This means that, on an average day, the yield from the panels is only just greater than the average demand. If you factor in the losses involved in charging and discharging the batteries, there isn’t enough PV on an average June day to make the system grid independent. The output of the panels in summer is, in fact, higher than if they were facing north, so the system has a high degree of autonomy over the summer months and a high export factor to the grid. SC siliconchip.com.au ICOM2005 The ATS contactor (labelled Q2 ) which senses, and disconnects the grid in case of dropout. Alongside is an SMA energy meter (bottom right). A data port connects to the Sunny Home Manager. IC-f1000/f2000 sErIEs Introducing the new IC-F1000/F2000 series VHF and UHF analogue transceivers! The IC-F1000/F2000 series is a compact portable radio series with convenient features such as built-in motion sensor, inversion voice scrambler, channel announcement and IP67 waterproof and dust-tight protection. To find out more about Icom’s Land Mobile products email sales<at>icom.net.au WWW.ICOM.NET.AU October 2015  29