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Sustainable Energy: Energy Distribution
Being so large, the domestic battery is usually installed outside - either in its own small outbuilding or buried in a large cylindrical hole. The electricity must therefore be somehow conveyed from the battery to the devices requiring it. This is the job of the Distribution System.
Natural energy supplies are intermittent. The sun shines only part of the time. The Stirling engine is only operable when the stove is lit. The wind does not blow every day. Electricity must be stored while it is being generated for use while it is not. This requires a large battery. A battery operating at 115 volts or 240 volts DC (direct current) is too dangerous and impractical. A battery is therefore normally designed to provide electricity at 12 volts and sometimes 24 volts.
For some devices, such as quartz-halogen lighting, the raw 12 volt DC supply is ideal. However, to avoid electrolytic effects such as the corrosion of contacts and the pitting and cratering of switches, it is advisable to reverse the polarity of the supply on a regular basis. This could be done automatically at 2 AM each morning on supplies such as lighting which are not adversely affected by changes in polarity. However, other types of device, especially those containing solid state electronics, cannot tolerate a change in polarity.
For other electrical devices a 12 volt DC supply is neither efficient nor practical. For these, the 12 volt DC supply must be converted to other forms of supply.
Devices containing motors of over 300 watts are much more efficient and cheaper to produce when designed to operate from a 115 or 240 volt supply. To supply such devices, an inverter is required to produce a 115 or 240 volt supply from a 12 volt DC input. The process by which the inverter produces its 115 or 240 volt output causes the electricity to appear first at the higher voltage as AC (alternating current) at 50 to 60 Hertz (cycles per second). Since what are essentially DC motors can almost just as easily use AC, there is no need to rectify it into DC at the higher voltage.
For devices involving complex motor control like automatic washing machines, the controller should be redesigned to operate directly from the original 12 volt supply. However, I prefer the functionality within such controllers to be unbundled. This allows individual sub-units to be replaced independently of the rest of the system as they wear out or become obsolete.
For motors of over 1000 watts, which must provide constant speed at variable torque, a 3-phase 440 volt AC supply is best. These would be in things like large machine tools and static farm machinery. This would require a different kind of inverter.
Devices containing solid-state electronics (such as computers, radio equipment and various types of monitoring and control equipment) require filtered supplies of -12 0 +12 and -5 0 +5 volt DC supplies. Today these devices contain their own internal inverters or power supply units which convert 115 or 240 volts 50 to 60 Herts AC to the kind of supplies the individual device requires internally. However, where, as in our case here, the original supply is at 12 volts DC, it is wasteful first to convert from 12 volts DC to 115 or 240 volts AC and then again to -12 0 +12 and -5 0 +5 volts DC. These internal power units should therefore be replaced with inverters which convert straight from 12 volts DC to -12 0 +12 and -5 0 +5 volts DC. Each should also incorporate a 12 volt backup battery where the supplied device could suffer adversely from a sudden loss of the main supply.
Small low energy discharge lamps are very efficient. They can be made to operate at 12 volts. However, they require it to be supplied as AC at quite a high frequency. I am not sure what the optimum frequency is but I have an idea it is around 500 to 1000 Hertz. If you know about these, please email me. Their supply would require yet another type of inverter and would have to be distributed through coaxial cable.
Where an inverter supplies a single device, switching on the inverter effectively switches on the device. When the device is not in use, the inverter is off. However, where an inverter supplies a distribution bus to which many devices are connected or can be plugged in, the inverter must be on when any one of the devices it supplies is on. To avoid such an inverter having to be on all the time, a common signalling wire (in addition to the power wires) must run between the inverter and the devices it supplies. Through this signal wire, the inverter provides a 12 volt supply from a very high impedance source. Whenever one of the devices it supplies is switched on, that device grounds the signal wire. This tells the inverter to switch on to supply power to the bus. This can be achieved using 3-core cable to supply such devices. One core would supply the power, one would provide the 'power required' signal, the 3rd core being the common return for both. For safety, all 3 wires of the supply bus would not be referenced to ground (earth). There are other ways of doing this without a signal wire, but I think using a separate signal wire is less complicated, less prone to faults and much easier to trouble-shoot.
Finally the question of priority must be addressed. It is vital that certain devices remain running no matter what. Other devices can tolerate irregular interruptions in supply. Vital devices must therefore be supplied with power from what is called an 'essential supply bus'. The others can be supplied from a non-essential supply bus. Below is a sample list of the various types of supply and the types of device each would supply.
- essential very low power - clocks, sensors, low level or emergency lighting, standby radio receivers.
- essential light power - refrigerators, freezers, heating pumps, computers, communications equipment.
- normal lighting - halogen and discharge lamps.
- normal power - domestic appliances, cookers, hand power tools.
- heavy power - industrial and agricultural machinery.
Because of the different supply requirements of devices of the same priority, some of the items above may require more than one physical supply bus.
Heat is distributed around the home by pumped water and ducted air. The water-based heating operates by circulating water from the PV array or wood-fired boiler through vary large radiators at about 40 to 45°C. These radiators are integral to parts of the walls of the house and are many times the area of conventional central heating radiators. The low operating temperature allows the heating system to act as an efficient heat sink for the PV array.
© January 2001 - Robert John Morton