Editor’s Note: This is Part V of a series. To see all parts, click here.
Mick Jagger sang “I can’t get no satisfaction”, and that’s exactly what will happen if you use more than you produce in any renewable self sufficient system. It’s exactly the same for both energy or food as anyone who has worked towards self-sufficiency quickly realises. I know that even with my extensive and diverse orchard, a dozen vegetable beds and nine chickens, I wouldn’t want to have to eat only things produced on my farm as I’d eventually starve.
With a self-sufficient renewable energy system though you have no option but to live within your production means or somehow increase your production of energy. Living within your means may be something as simple as only running lights and a refrigerator rather than, say, having a computer running 24/7 for entertainment. You do get free power from the sun, wind or water but perhaps it’s not as much, or not delivered in the way that you are used to and it requires you to ultimately adapt your expectations.
Solar panels generate electrical power whenever the sun is shining. In A Solar Powered Life – Part III you’ll see that the sun doesn’t shine all day every day and that some days you just don’t generate as much power as you use. So when you are not connected to the electrical grid, you have to store the energy that you generate in batteries so that you can use it later. So, it’s the batteries that actually provide a constant source of energy in an off grid solar power system. The solar panels work as a source of energy to the batteries so that they can be topped up. If you only relied on the solar panels to provide energy without battery storage, you’d end up being only able to run your lights during the daylight hours and this would be a mostly pointless activity!
The solar power system so far:
- Solar Panels – connect to – Batteries
Charging batteries can be a tricky business. Remember in A Solar Powered Life – Part IV, it was pointed out that pretty much all off-grid solar power systems used lead acid batteries and that the acid used was generally sulphuric acid. The more technically minded readers will note that sulphuric acid has a molecular formula of H2SO4 (take special note of the 2 parts Hydrogen and 4 parts Oxygen).
Too Fast – If you charge a battery too fast it will release both hydrogen and oxygen gas at the same time and this is a truly explosive combination. If you are in any doubt, have a look at some YouTube footage of the space shuttle disaster or the explosion of the Hindenberg airship. The space shuttle used hydrogen as propulsion fuel, whilst the the Hindenberg airship used hydrogen (which is lighter than air) to help the airship stay in the air. Both incidents were fatal for all involved. Charging batteries can also have the same lethal outcome unless done carefully.
Too slow – If you charge a battery too slowly, you’ll simply end up using more energy from the battery than it has stored, and it will eventually go flat. If you’ve ever had a flat battery in a car and tried to start the engine, you’ll understand this feeling. Try then to imagine how it would feel if you had no lights at night or the refrigerator simply stops working and everything in the freezer defrosts, or even worse, if you had no access to the Internet! Completely draining a battery can damage the materials in the battery which shortens its usable life and can possibly permanently damage it.
Charging conditions change – When you fill a motor vehicle fuel tank the flow of fuel into the tank will be at the same flow rate right up until the fuel tank is full and hopefully then the fuel stops flowing or it will spill out of the tank and go everywhere. However, it’s a very little known fact that batteries don’t work this way. A battery will take as much energy as you can put into it up until it is around 85% full. If you continue to charge the battery at a high volume after this point, it will begin releasing Hydrogen and Oxygen and this is a bad thing (remember the Space Shuttle disaster). After the battery is around 85% full, you can only charge the battery at a very slow rate. On my own solar power system this can mean an increase of only 2.5% per day regardless of how much power the solar panels are generating. In the real world, this means that it can take up to 6 days of strong sunlight to charge your batteries from 85% to 100%.
What monitors the battery charging? – You can’t possibly monitor the charging process in your home solar power system 24 hours a day / 7 days per week so you employ the services of what is known as a regulator (or also sometimes a solar charge controller). This clever device sits in the system between the solar panels and batteries and all it does is monitor how much power is being generated and then decides whether the batteries can absorb all, part or none of the energy being generated.
The solar power system so far:
- Solar Panels – connect to – Regulator – connect to – Batteries
More alert readers will note that it is very possible to be generating energy from your solar panels that may not be used or stored by the system. In fact this happens all the time over summer and not very often over winter. The energy in this case, if it is not used and not stored, is lost.
Before readers begin sending in rude comments about the deficiencies of solar power systems, you need to first understand that this same effect happens on a much larger scale with the electricity grid. It is very hard for a large power station to quickly raise or lower the amount of electricity generated and there is no possibility to store any surplus energy so it too is lost. Unlike solar power though, the scale of that loss is huge. Also, with the large scale generation of power, it is mainly at night where energy is lost. This is because it takes a while to slow down the production of power at the generators, to the amount required for the night-time by the population, and then they have to ramp up the generators again the next morning. This is the reason for the existence of peak (day-time) and off peak (night-time) rates for grid supplied electricity. In fact there is probably quite a significant spare capacity in the existing electricity grid to provide power for the charging of quite a few electrically powered vehicles off peak. This may not be the case however in either extreme cold or extreme heat as the excess capacity will be used up by people heating and cooling their houses.
I’m often asked the question, how much battery storage would you need in an a off-grid solar power system? The simple answer is that you can have too little battery storage, but you can never have enough. As a minimum though, I would suggest that three days requirement is the absolute minimum storage capacity of your batteries (although for my own system, I have 8 days storage). For the average household, like that described in A Solar Powered Life – Part I, which uses 17kWh per day, they’d require a battery which could store 51kWh. This is a massive and expensive battery system, but is commercially available! For more modest requirements such as 3.5kWh per day, this would mean a much smaller and cheaper battery system of 10.5kWh.
Just to confuse people completely, battery manufacturers and suppliers never supply battery specifications in kWh. On the other hand appliance manufacturers and suppliers generally supply specifications in terms of kWh or Wh so you have to be able to convert the numbers.
A battery will be described by a retailer as so many Ah (Amp Hours) at a certain V (Voltage). It’s simple to convert this into kWh as all you have to do is multiply the Amp Hours (Ah) by the voltage (V). For example a 200Ah battery at 12V has a storage capacity of 2.4kWh (200 x 12). It’s that simple.
Retrieving stored energy
The funny thing about batteries is that because they are basically a chemical reactor and not a fuel tank, they have some unusual ways of delivering that stored energy. It’s very different from a fuel tank because if you emptied the contents of that fuel tank (whether petrol or hydrogen etc) and ignited it, all its stored energy would be immediately available. Batteries don’t release energy this way. The simple rule is that the more energy you try to extract from a battery, the less overall energy will be available and this can often be far less than the advertised storage capacity.
So if you can’t access all of the advertised total stored energy in a battery immediately, what do you actually get? When a retailer supplies information about a battery, then it is usually the amount that can be retrieved from the battery, but only over a 10 hour period! What this means in the real world is that with the above example battery (200Ah at 12V or 2.4kWh), you could only retrieve 240Wh over a 10 hour period to extract the full 2.4kWh of stored energy.
It all comes back to Mick’s song. If you understand the limitations of the system and can both accept them and live with them, you’ll have an independent self-sufficient power system that can bring you years of no power bills. Otherwise, you can’t get no satisfaction.
The next article will look at how you can convert the stored energy in batteries into something you can easily use, and also asks the more important question — should you do this?
Below are some explanations of terms for the very technically minded only:
To understand the terms Volts, Amps and Watts it easiest to think about how water is delivered in a pipe.
Volts – is the pressure in the pipe. A high voltage means a lot of pressure only.
Amps – is the flow of water in that pipe, but not at one point in the pipe. It’s the same flow rate at all points in the pipe. High amps equates to a faster flow, not more water in the pipe.
Watts – is a measure of power, i.e. an instantaneous measure. Watt hours is an amount of energy, equivalent to the total amount of water that has moved through the pipe.