A Solar System was installed to power a new fridge and freezer, and it works great. It's been in for about 6 months now and aside from an intial hiccup with the solar regulator (repaired under warranty) it has been trouble free.
A further comment was made saying: this isn't cheap up front, $4400 was spent on a fridge/freezer pair (“Fridge: 413 L;Freezer: 324 L. Being being the brand that was chosen, these are Nett values rather than the more inflated Gross values sometimes used. They are big!”) and $8000 on the solar system. The system is over sized for the needs though, there is plenty of excess power and energy to use when the sun is shining. That's an important distinction there, power comes from the size of your inverter and batteries, while energy comes from the size of your solar array.
The first decision that needs to made is on which fridge/freezer set you will use. The decision was made that because there is grid energy available, that to have AC appliances so they could be powered off the grid if the solar system failed. If grid energy was not available, then the more efficient DC units with the Danfoss compressors would have been used. The most efficient AC units that could find were used. It's critical to get high efficiency appliances, because the more energy they use, the bigger your solar system needs to be, hence it's more expensive. Extra money spent on efficient appliances saves you more on solar panels.
It is pointed out that they are seperate free-standing fridge and freezer, so the discussion will revolve around our two appliances. Fridge: 150W power, 200kWh/year. Freezer: 150W power, 298kWh/year. Total: 300W power, 598kWh/year = 1.64kWh/day average. The locality is a relatively hot place most of the time so a rounded up figure of 1.8kWh/day average was used.
Once a decision to have AC appliances is made, the next decision is what size inverter is to be used. An inverter is a device for generating 240V AC mains voltage from a low-voltage DC source, such as a battery bank or solar panels. Refrigeration is a difficult load for an inverter, due to the high surge currents when the compressors start. Most inverters are rated with both their continuous power and surge power ratings, but unfortunately very few appliances list their surge power requirements. A rule of thumb that was used is 4x the power rating of the appliance, which for our 150W rated fridges is 600W. You shouldn't discount the possibility of both fridge and freezer starting up at the same time either, one obvious point is when you first apply power (if both units are plugged in) but considering they're plugged in and running permanently, the odds are that at some point they will both turn on together. If your inverter can't handle it, it'll shutdown, and Murphy's law states that this will happen when you stand to lose a lot of food. So to take this case as an example, an inverter with 300W of continuous power and 1200W of surge power available was used.
The next figure of merit for an inverter is it's output waveform. As may be known? mains AC voltage is mostly shaped like a sine wave, with smooth transitions between full positive and full negative voltage. This kind of wave is quite difficult to produce from a DC source, which is a stable "flat" voltage of one polarity (ie. it has definite positive and negative terminals). Inverter manufacturers confuse the issue somewhat with their terminology, but the two main types of inverter are "Modified Sine Wave" and "Pure Sine Wave". As it's name suggests, Pure Sine Wave tries to replicate as closely as possible the nice sine wave shape of mains voltage, while Modified Sine Wave is, more correctly, a square wave, which is switched at the appropriate times so as to contain the same amount of energy as a sine wave. There are some appliances which don't like modified sine wave, and won't operate correctly on it. These include microwaves and flourescent lights, most devices that are either motorised, or have a switch-mode power supply will work fine from modified sine, but they will consume a bit more power and/or run hotter than they should. All-in-all, the extra cost of a Pure Sine Wave model is worth it to be able to power any and all appliances without any drawbacks.
In the end, considering the plans to eventually go fully solar, it was decided to spend a bit more and get one inverter that will cover the power needs for the future. It's a pure-sine wave 3000W unit with a surge rating of 9000W, and it runs from a 24V DC source. It has happily powered our fridge/freezer while at the same time running the dishwasher or washing machine or bread-maker, etc. on days when we have the extra solar input to cover it. $1800 was paid for it. A great feature is the in-built solar regulator, it eliminated the need for an extra piece of gear.
Another big decision: how big should the battery bank be? Because the site is a rental, approval can't be had for a grid-interactive solar system. This means a battery bank was needed to store the excess solar energy during the day for use at night and during cloud cover, etc. It was decided that about 2 days of battery-only operation would be suitable for the needs. The locality is rarely overcast for the whole day, and poor weather rarely lasts more than a few days. That, combined with the ease of switching back to mains power during extended cloudy periods, meant we could get away with a smaller battery bank than we otherwise would. If there wasn't mains, then you need be looking for at least a weeks worth of backup?
So two days of refrigeration with this system is approx. 3.6kWh of energy on average, more during summer and less during winter. Considering that most extended solar outages happen during winter, Using 1.8kWh/day figure is a safe figure to use, with the knowledge that there's probably some headroom in that. The key equation here is the power equation Power(W) = Voltage(V) * Current(A), 3.6kWh (3600W-h) divided by the nominal voltage of our inverter (24V) gives 150A-h. A-h, Amp-Hours, is the standard unit that batteries are sold in. 1A-h basically means you can draw 1A from that battery for an hour before it's depleted (It gets more complicated than that, but a line needed to be drawn somewhere or this huge post will become book sized). Of course if 150Ah batterys were used it could be disappointed, because there are inefficiencies in wiring and inverter, standby currents, etc. to be taken into account. Basically the 150A-h figure is a baseline to start from, the next size above that was chosen, which for the batteries chosen turned out to be 240Ah and cost around $800 for them. So there is 24V 240Ah battery bank in which to store the solar energy, but how many panels are needed?
Starting with the 1.8kWh/day figure for the system, there is a need to pad the figure a bit to account for inefficiencies of the inverter and battery. Use 30% as a ballpark figure, so it was increased it to 2.3kWh/day average. If a reference to a map of "Peak-Equivalent Solar Hours" for Australia is used, it will be seen that this locality is around 6 hours average. Now this nice thing about using the average values, is that when the peak solar hours reduces in winter, so does the kWh consumed by the refrigeration, it's a nice thing which makes the decisions easier.
So... We have the equivalent of 6 hours of peak sunshine to use. What this means is that the total energy created by the solar panels during an average day will be 6x their peak power rating. We need 2.3kWh of energy, which divded by 6 is ~383W peak. Rounded up to a nice value this is 400W of peak solar power at 24V DC nominal. So our original spec was to be 4 of BP's 100W 12V solar panels in a series-parallel connection to output 24V. At current prices of ~AU$8/watt, this is about $3200 in panels. Another $350 bought enough steel to make two adjustable angle frames for our garage roof, with room for 2 panels on each. There was also about another $400 on cables and hardware, including the all important fuses for the battery bank and solar panels.
At about this point the solar panel supplier was out of stock of 100W panels, so the decision was made to use 4 of BP's 160W 24V panels, connected in parallel for a total of 640W peak at a cost of around $5200.
The final piece, now that there is peak solar power available, is the solar regulator. These are fairly simple devices that connect between the battery bank and the solar panels, and regulate the current input, preventing the batteries from being overcharged and damaged. Lucky the inverter chosen contained a regulator already, but if it didn't, this is the process to choose one...
The basic figure of merit for a regulator is it's current handling ability. You choose the voltage based on your battery bank (in this case, 24V) and you choose the current based on your peak solar input. In BP's documentation, they suggest that in optimum conditions, the panels can exceed their ratings by 20%, so it was assumed they are working with a 768W array. We return again to the power equation, P = V * A, to calculate current: 768/24 = 32A. So the regulator needed to be capable of handling 32A at least.
So how does it perform? In a word, flawlessly. The batteries are charged by about 11am every day, and unless a use is found for it, the rest of the days energy is just lost. So the extra power is used by plugging the dishwasher or washing machine in, or transferring the computer and stereo in the lounge room over to solar. the output of the inverter wired into a 4-way outlet in the laundry, and a few extension cords take it further around the house for opportunity use.
Try and wrap up any extra use by about 3pm, giving the batteries enough time to be full before the sun goes down. Also try and be a gentle to them as possible, batteries don't last forever, they are a consumable item in solar installations and the less they're discharged, the less they will cost. To this end try and avoid opening the freezer at all after sunset, and minimise the opening of the fridge. Do shopping in the morning so that it has the whole day to be chilled/frozen. If any food is bought at nighttime that needs freezing, then chill it first in the fridge. If it's like juice and can wait until the next morning to be chilled, we leave it out. As a rule don't put warm food straight in the fridge, rather let it cool on the bench until it cool enough to put in and we try and defrost food in the fridge, so that the energy used to freeze it isn't wasted.
Sometime soon another pair of 160W panels on a frame will be added, there is no ideas at present for an automated system to use up our excess energy in the day, but switch some loads back to AC at night, so that a small battery bank can be maintained. The brilliant part about solar systems is the modularity, there's plenty of scope for small increases, if there is mains available there's no need to buy an entire system at once, The final goal can be reached in small increments.
Overall it has been a very worthwhile project. It is recommended to anyone who has the financial ability and can turn a screwdriver.
The battery discharging is heavily dependent on how much they are discharged before they're recharged again. This is (funnily enough) termed "depth of discharge" (DoD) and it's usually expressed in a percentage of the batteries full capacity.
In this case the bank capacity is 240Ah, and a typical nights use will drain about 60Ah from them, for about 25% DoD. According to the manufacturer, they should be good for 1500 cycles at 25% DoD, which is about 4 years.
It's important to use "Deep Discharge" or "Main Power" batteries for solar installations. Some types of lead-acid batteries are only designed for standby applications, and don't cope well with repetitive discharge-charge cycles, as a consequence they have short lives when used that way.
The good news is around 97% of the materials used in a lead-acid battery are recyclable, and the manufacturers accept battery returns for this purpose, making them directly into new batteries for a much smaller energy cost than acquiring brand new material.
I'll start by listing a few that we've thought of so far related to our system, which is basically just solar powered refrigeration, with some spare outlets.
* Do your grocery shopping in the morning.
When you bring home a load of groceries and stack it all in the fridge and freezer, they have to work extra hard for a while to chill all the new stuff down to their working temperature. If you do your shopping in the morning, you have all those hours of sunshine with which to chill everything down. By contrast, coming home in the evening and loading them up will only drain the batteries down further as the new food gets chilled and frozen.
*Connect some small loadsOur inverter uses about 25W of idle power when nothing else is drawing from it, just to produce the sine wave output. When connecting up some small loads like battery charges, mobile chargers, etc. I noticed that this doesn't increase the idle power drawn. In a way some portion of that 25W is like a minimum-spend, use it or lose it.
* Less discharge = longer battery life
Anything you can do to reduce the power drawn while the sun isn't shining (or wind blowing, etc.) will directly save you money by increasing the life of your battery bank. Things like being very choosy with when you open the fridge, not opening the freezer at all (get your defrosting out in the morning), and using the energy intensive appliances (washer, dishwasher, etc.) during the peak of the day or after your battery bank has charged.
Extra information – not from the original source:
1.. A totally solar powered home with 20 x 120 w panels will run the fridge, chest freezer, computers, washing machine, TV, water pump etc - a "normal household".
Cooking is with bottled LPG gas, and a wood burning stove.
All power is switched off at the board each night at retiring time, to ensure there are no standby loads anywhere (although the lights can be turned on), this also includes the fridge and chest freezer, which do not seem to suffer much... everything is turned back on in the morning.
Eco' Home Essay
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