Electricity Production

a) Generator:
We are not operating within a conventional environment, so the method of supplying electricity will not be the same as large grids. Operating off-grid presents a set of unique challenges. One of the largest challenges faced, is to make sure that devices used are robust enough to survive in a remote region, far away from any technical support. Another challenge is to provide electricity in a useful form for the end users. The final main challenge to be addressed is to make sure that the system is not only economically viable, but also economically attractive to potential sponsors. One way this can be achieved is by having a low initial cost for the project.

i) AC or DC
When designing a power generation system, you must consider what kind of power is most applicable to your scenario, and most easy to implement. The implications of this choice impact the applications of the system and the type of generators that will be used, which each have varying characteristics. For very small systems (less than 300 Watts), DC is the only viable type of electricity to be generated, purely because DC can be directly stored in a battery, however at this scale, the whole system may not be economically viable. For our system we must use AC because systems larger than 300 Watts should generally use AC, unless storage is required. This is because AC generators are widely available and fairly cheap to purchase. The main relevant issue to this project is reliability; we find that AC generators are far more reliable than DC generators. This is directly related to the design of each, DC generators use split ring commutators, which involve a moving brush contact that can wear out, requiring maintenance⁴. This is generally not an option, and so AC is found to be far more suitable. To implement an AC system, some conditions must be met or further designing is required; the load and the water speed must not vary, otherwise a control system must be implemented. We are aiming to create symptoms capable of producing more than 3kW, as a result we will most likely be using AC.

ii) Number of Phases
Since we have decided on an AC system, we use this to generate either single or three phase power. Single phase is the only viable option below generation levels of 3kWs. Above 3kW, single or three phase can be used, with the choice depending on what the loads are going to be and how they could be balanced. Single phase could be cheaper since it only requires control and protection for one phase rather than three. For our scenario, single phase is the best choice and is probably going to be the more cost effective solution.

iii) Type of Generator
As we need to produce AC power, so we must decide what kind of AC generator we must use. There are two possible type of generator that can be used, synchronous or induction. Synchronous generators are a type of AC generator that uses permanent magnets to create the magnetic field, to produce power. Induction generators are effectively induction motors that produce power when their rotor is turned faster than the synchronous speed. Induction generators are not self-exciting; they do not create their own magnetic field. Induction motors are generally cheaper and more widely available; however, induction generators are not really a viable choice since they need to be connected to some kind of grid to excite them. Current research has gone toward trying to make induction generators useable in isolated areas, but until then we will have to use synchronous generators⁴.

iv) Safety
We are running an unmonitored system in a remote region so system protection is required for the longevity of the system as well as protecting the end user. This must consist of components that can prevent large currents from running through the system, and the generator from running over its maximum speed. This can be done using a series of circuit breakers⁴. Circuit breakers are safety devices that act as switches. When the system has too much current flowing through it, the circuit is broken, giving an open circuit. No current can flow once a circuit breaker has been tripped, this means that damage to the system or users will be limited.

v) Transmission
Since we are working on a small scale, it is not required to engineer the transmission cables to a very high standard, as losses will generally be small over such a small distance. The main concerns when designing the cables will be robustness and safety. This must be done on a case-by-case basis to make sure that the cables will survive for a reasonable amount of time, whilst not being dangerous or being overly expensive.