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John Hurst's Environment Page
The first two plots here show watt hours generated per day by
our solar panels and grid interactive inverter. The plot to the
left shows the daily cycle for today and the last 7 days, while
the second plot below shows the total daily energy every day
since installation. Clicking each plot will show more
detail.
The daily plot is restricted to the hours of 6am to 6pm, since input is negligible outside these hours. You will note the stepped nature of the plot - every 30 minutes the output appears to change slope, and output appears to limited by some factor other than insolation. I have not been able to find any explanation for this - there is nothing in the manufacturer's manuals, nor their web pages. If anyone can shed any light on why this occurs, I would be pleased to hear from them. My best guess currently (sorry!) is that it is some battery recharge regime.
There is a slight liberty taken with the data in the second plot. This was due to equipment failure over the summer of 2004/5. Data from the previous summer is substituted to maintain the shape of the graph. The daily plot is generated by our house computer, and updated automatically every 10 minutes. The long-term plot is generated manually (last update 20100210).
The solar assembly consists of 20x75W panels (1500W total) feeding into a Trace SW3024 E 3300W inverter system, buffered by 4 Sonnenschein 6v 330AH gel cells (7920WH). In sunny weather, the system generates enough power that the meter is usually spinning backwards! Two of the house power circuits (including the refrigerator and all computers) are fed from this inverter, giving about 24 hours of autonomy (at normal current levels) in the event of grid supply failure. (see footnote *)
Our current electricity drawdown from the grid is about 14KWh per day (averaged over the year), and the solar system supplies an average of 4.6KWh (see chart above). That gives an average fraction of about one quarter of our electricity needs being supplied by the sun. That translates a saving of approximately $0.84 a day (at 18.3c/KWh). The system cost about $6000 ($4000 for the inverter, $2000 for the rebated solar panels, not counting the batteries, which are only for UPS protection), and will therefore pay for itself (assuming no increase in electricity costs, an unrealistic assumption) in about 25 years! But of course, we are also saving about 2.5 tonnes of greenhouse gas a year, (roughly) the equivalent of not driving our car anywhere for the year.
(20090612) With recent rises in the cost of electricity ($0.15/KWH), the revised payback time is now 23.8 years.
(20100210) With recent rises in the cost of electricity ($0.183/KWH), the revised payback time is now 19.5 years.
We also have a rainwater system consisting of two 2250 litre Plastanks, fed from the house roof, and pressurised by a 240v Onga Riva-Flo TF30 centrifugal pump. This pump delivers 30 litres/minute at 8 metres head, and is quite adequate for our purposes. Delivery is to two toilet cisterns, and to a garden watering system (8 solenoid operated outlets and 2 standpipes). The garden watering system is driven automatically by cron jobs running on the house computer.
The plastanks are supplemented by a 4000 litre 'wootank'. 'Wootanks' are tanks made of wood, and this one is home-constructed from 12mm plywood and 100x50 hardwood. The hardwood is used to build an 'exoskelton', and the plywood is used to make a box shape, lined with a pond pool liner. This tank captures water runoff from the rear of the house, while the plastanks capture water from the front of the house, and there is a 3m difference in height of the base of the tanks. A submersible pump, non-return valve and separate ball valve allow water to be transferred in either direction between the tanks. In addition, overflow from the upper (plas-)tanks is piped into the wootank. Overflow from the wootank tops up the garden ponds, and overflow from these goes to waste.
On the left is a plot of the water level in the rainwater tanks monitored electronically by means of a water-capacitance oscillator bridge. At the moment, only 1 tank is monitored, the system has been calibrated in terms of total plastank capacity. Full supply level of approx 5000 litres is achieved by plumbing the overflow back into the tank through a 'snorkel' type fitting. This allows the overflow level to be significantly about the fitted overflow outlet.
Below is a plot of the same data as the left diagram, but drawn over 8 days, rather than 1.
On the right is a plot of the inside and outside temperatures
of the house. You can see the effect that the heating system
has. The data is collected from a Dick Smith wx200 weather
station and logged through an RS232 interface. The heating is
controlled by a relay driven from the house computer (which
also generates these plots). The cycling of the
"thermostat" (There is no such animal, it is all soft
controlled) can be clearly discerned when the heating is in
operation. The actual "thermostat" temperature is
controlled by a cron job, which can be overridden through a
web interface.
(footnote *): The irony of this is that while my wife and I were away on a week's holiday, my son and his band cronies plugged guitars and amplifiers into the UPS circuit, and played so loudly that they overloaded the inverter, tripping its overcurrent protection, and turning off the fridge. He simply moved to another power point and thought no more about it, until he noticed all the icecream running out the bottom of the fridge! We came home to a disaster! Just goes to show, such systems are not 'fool-proof'!
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