Written by Tony Clark
In 2000, my wife and I sold our home and purchased another, in a different suburb. At our new home, my wife wanted to install a swimming pool; I wanted to install Solar panels and a Grid Connect Inverter. We both ended up having what we wanted, although I ended up being the one to clean the pool.
We installed a 1500Wp system on the roof of the garage, which comprised 15 100Wp Shell solar panels and a Selectronic SEAG 1500 inverter. The solar panels were installed on a frame, which allowed them to be elevated in winter, and lowered for summer, to maximize power generation during those seasons.
This was very reliable, even surviving a thunderstorm, which destroyed the power meter at our house.
In 2006, we changed homes again, buying a “solar oriented” rammed earth home. An environmental engineer built this house in 1984.
The buyers of our previous home did not want to pay extra for the solar power system, so I removed it an reinstalled the solar panels at our new home.
I retained the ability to elevate and lower the panels to maximize seasonal power generation.
Stand alone Solar Power
I was unable to get the Grid connect inverter working satisfactorily, and we were on a “spur line” which had no alternate route for the power feed, so I decided to build a stand alone power system, using second hand flooded batteries, rated at 600AH over 10 hours, from a telecommunications site. These batteries were already 15 years old, but should still have some life left in them for this application.
I installed the batteries in 2 rows in our garage (a double width garage) but with the batteries in place, there was only room for one vehicle (my wife’s).
I purchased an Outback MX-60 MPPT battery charger to manage the power from the solar panels and charge the 48V battery bank. This allowed me to retain the previous configuration of the solar array as 3 series panel strings (5 in parallel). The Maximum Power point for this array was approximately 90V (dependant on solar insolation, temperature and time of day).
I also purchased a Selectronics SX 42 inverter and current shunts to allow its “state of charge” function to provide this information to us. This allowed us to draw up to 4.2KW for up to 30 minutes before it would de-rate to 3.6KW output. When my wife used the Clothes drier and the electric kettle (2.5KW) at the same time, it shut down due to overload. My wife has adjusted to using the kettle on the gas cooker for heating water.
Non-solar powered loads
As our home has 3-phase power connected and we use 3 phase power for water pumps for drinking water and for garden water (from a bore hole), we decided to retain the mains for these pumps, the oven, air conditioner and for the solar hot water booster element. As we have a wood fuelled heating fire fitted with a “wet back” to provide water heating during winter, electric boosting is not used at all.
I devised a switching system, where all phases were connected to the AC Distribution in our house when the switch is set to “Mains Power”, but selected loads are connected to the inverter source, when the switch is set to “Solar Inverter”. An “Off” position is used between these, to prevent connection of the mains to the inverter output (this would destroy the inverter as it is not rated for mains connection).
Our neighbour is a licensed electrical contractor who specializes in industrial electrical contracting, but he was interested in my project, so he offered to wire up the inverter and switches.
The 1.5KWp solar array was not able to supply our daily power usage, so I have added solar panels in 2 stages over 3 years, to bring the total stand alone capacity to 3.6KWp. This easily provides between 10 and 14 KWH to our battery bank on clear days, even in winter.
On clear days, at any time of year, this system provides more than enough power for our usage. Where a cloudy day is followed by clear days, the battery capacity is recovered within 2 or 3 days of fine weather. Where there are 2 or 3 cloudy days in a row, we switch to Mains during the “Off Peak” periods of the Time of Use metering scheme, to minimise the cost of power used.
If there are 4 or more cloudy days following each other, we switch to “Solar Inverter”, only for “Peak” periods, of the Time of Use metering scheme. In this way we can avoid the high cost power periods, even when the solar power is not generating enough power for our needs.
Where mains power is not available due to network faults and t clouds prevent solar charging of the battery, we can connect a single-phase generator and switch to that in place of the Solar Inverter.
The battery voltage depends on the number of 2V cells in series, which make up the battery. 24 cells in series make a nominal 48V battery (operating voltage between 47V under load at less than 50% charged and 60V when nearly fully charged (absorb mode of MX-60). By adding one more cell, the nominal battery voltage changed to between 49V and 62.5V, which is still within specification for the inverter we have. This provides and additional 4% battery capacity.
I have sourced more of the used telecommunications batteries and built a 2-tier stand to support them in the same floor space as 24 cells occupy. When complete, the additional batteries will provide a total of 56 KWH of energy storage. I try to minimise battery discharges greater than 50% of battery capacity, so at 50% discharge this would provide about 4 days of supply to our home (even more if we further limit usage).
A few of these cells have failed in the 4 years we have had the first bank in use. Fortunately I was able to remove one cell, (bringing the battery back to 48V nominal) until a replacement cell was obtained. Luckily, we have not had 2 cells fail concurrently and my supplier of used cells has been able to provide them at short notice. His supply is becoming harder to obtain as most telecommunications providers are now using Sealed Lead Acid batteries in preference to flooded cells. I plan to stockpile a few flooded cells, setting them up as a 12V battery in my workshop, charged by a small solar array and MPPT charger. Faulty cells in the main battery will then be more readily replaced and I will have a much longer time period to source replacements.
Battery Replacement Plan
As most of our power is sourced from the Solar System, I have decided to set aside the amount that we would otherwise pay for power, as a reserve for eventual battery replacement. When my source of flooded cells is no longer available, I will purchase new replacement cells (perhaps 5 at a time) to upgrade the battery banks. These cells cost over Au$700 each, but we would spend over $2000 annually for mains power, without the solar system, so we should easily be able to replace up to 5 cells each 2 years. 10 years for one whole battery bank. 10 years battery life is achievable and with the second battery bank, longer battery life is possible (discharges below 50% capacity occur less often would result in longer battery life).
Our Grid Connect Solar System
Three years ago the Australian government was offering significant rebates for the installation of grid connected solar power systems and the Western Australian Government was offering a Feed–in Tariff of 40 cents per KW fed to the Grid. We signed up for a 1KWp system and paid only $1500 for it, complete, installed and commissioned. This now offsets our mains power usage for pumps, oven and air conditioning. Our power bill is now between Au$50 in credit or debt, depending on cloud cover during the 2-month billing period.
When the Feed-in Tariff expires (politicians decide to stop it), I can connect the Grid connect solar panels to the Stand Alone system. I believe that we will then be able to use the oven and air conditioner on the Stand Alone power system. I would still need to have Mains power for the pumps, but the main heavy loads would be running from our solar power system.