Off-Grid Power Systems
Are You Off-Grid?
The conventional power grid supplies 230 volt AC (Alternating Current) to the average household. Thousands of homesteads in Australia do not have access to a centralised electricity grid. Even when connection is physically possible, costs of poles, wires, transformer and labour can be prohibitive and quickly go over $50,000. Many of these consumers are dependent on diesel or petrol generators with occasional battery storage and/or renewable energy systems added. Others may depend mostly on renewable energy systems with possibly a petrol, LPG or diesel generator with a battery charging facility as a backup. These systems may have a conventional AC type of supply and/or a DC (Direct Current) battery supply.
Types of Systems
Combined DC and AC system
There are three basic types of stand alone renewable power systems. All of these systems may incorporate any combination of solar, wind and hydro as the primary energy sources.
DC Only System used in vehicles, boats, sheds, caravans, cottages etc to power lighting and low voltage appliances. The power is usually stored in a battery bank (usually 12 volts) which is regularly or intermittently recharged. This system should incorporate meters to monitor and fuses to protect the system. It may or may not include a charge regulator (RPC designed systems always include a regulator).
Combined DC and AC System is as above except that it contains a DC to AC (e.g. 12 volts to 230 volts) inverter to enable the use of commonly available 230 volt appliances. The inverter should be carefully matched to the loads (see Inverter section).
AC Only System where all the loads are run on AC via an inverter. For larger systems, this is the most cost effective solution. As such systems would come with a 48V battery bank, a DC-DC down converter can be added if 12V DC is needed.
If you are charging a low voltage battery bank to meet your power requirements, we do not recommend that you then run your house as a conventional 230 volt AC household through an inverter (an AC Only System) as this would necessitate a much larger and more costly installation as would otherwise be the case. It would dictate a substantial battery bank powering an appropriately sized inverter to cope with everything being on at the same time. It would also require a substantially larger charging source (eg solar panels) to put back into the battery what the inverter is taking out.
If you refer to the graph, we have presented a fairly typical performance curve of a very typical battery bank to 230 volt AC inverter. You will notice from this that at its peak you may expect 85% efficiency from a modern solid state inverter. The further the wattage rating of the appliance is from this peak, the more inefficiently the inverter will deliver that power.
If you look at the type of appliance that you may wish to have in your home you may find some electronic appliances dependent on getting their power supply via a transformer. The transformer uses the high voltage to generate a much lower voltage. We may be able to operate these directly by the low voltage DC power supply without needing the transformer or inverter. It is generally recognised that low voltage motors have more torque than a 240 volt motor of the same wattage rating (power consumption).
One of the more annoying aspects of some inverters, particularly inverters which are either square wave or stepped wave, is that they can produce an annoying hum on your stereo equipment. With some effort this can be minimised or filtered out, but at the same time introducing another level of inefficiency. If you can run the sound system directly from a low voltage DC supply or from a sine wave inverter you shouldn't get any hum.
In the average 230 volt household little attention is paid to how much power an appliance uses. With refrigerators, for example, the emphasis seems to be on space saving rather than efficiency, hence they have thinner walls but need more power to stay cool.Ideally, to maximise on efficiency, the refrigerator should be of the low voltage compressor motor type and preferably be top opening (so that the cold air doesn't fall out when you open it).
Alternatively, you could use a gas or kerosene powered fridge. An LPG refrigerator of 120 litres capacity should use about 500 grams of gas per day.
A 230 volt refrigerator running via an inverter from a battery bank is generally not recommendable. Considering the frequent starting and stopping of the compressor motor and the very high starting current of the motor it would be a costly practice in terms of the size of the battery charging system, battery bank and inverter that would be required. A standard 230 volt AC 220 litre fridge/freezer will consume 1.5 to 4 kWh of electricity per day. 12 or 24 volt compressor motor fridge/freezers do not have a high starting current and will operate with a power consumption of between 0.3 and 1.5kWh per day. A 12 volt 220 litre fridge/freezer uses between 25 and 90 amp-hours per day. An equivalent 230 volt fridge connected to a 12 volt to 230 volt inverter would use between 150 and 400 amp-hours per day from the battery bank.
For appliances that are designed to generate heat, such as a stove (be it for cooking or for room heating) and a hot water system, there are alternative ways to generate that heat. There may be a number of options to choose from including bottled gas, fire-wood, sunshine, bio-gas or producing the heat electrically if the electricity can be generated cheaply enough. With the present price of photovoltaic panels (solar electric panels) this option is not in the race.
Another argument in favour of low voltage is the safety aspect. Extra low voltage DC is safer than AC. Anything less than 120 volts DC is not considered lethal, whereas with AC you need to come as low as 32 volts! There is still a fire hazard, however, with low voltage DC, and so you must protect the system with the appropriate fuses and/or circuit breakers.
In the average 230 volt household little attention is paid to how much power an appliance uses. With refrigerators, for example, the emphasis seems to be on space saving rather than efficiency, hence they have thinner walls but need more power to stay cool. Ideally, to maximise on efficiency, the refrigerator should be of the low voltage compressor motor type and preferably be top opening (so that the cold air doesn't fall out when you open it).