Design - 5

• Inverter

I have now stored power in the form of DC voltage and DC current. I can power some DC devices such as LED lamps, DC motors or some appliances that have DC input. Apart from that, I will need to convert the DC power to AC to power my appliances.

I will divide the types of inveters into two types from the waveform point of view:

1. Modified sine wave: This inverter takes the DC input and converts it to AC output, but with a waveform between the conventional sine wave and the sharp square wave. This can be tolerated by some devices which have their own power supply to change AC to DC like PCs. But most inductive loads like fans or ACs will produce a humming/buzzing sound, produce more heat, draw more power and shorten their life expectancy. Other devices like TVs can have incorrect signal with impure image. This inverter type is the cheaper one.
2. Pure sine wave: This inverter outputs conventional sine waveform that will power anything the grid can power. This type is more expensive.

Inverters can be categorized into two types from the functional point of view:

1. Grid-Tie: This type of inverter has its output can be plugged directly to the grid through any
   
   mains outlet in my apartment. It can synchronize its wave with that of the grid to achieve the same frequency, voltage and angle. This type will take power from the solar energy system to directly power my appliances. If I draw more power than the generated power from solar system, then the extra need is compensated by the grid. If I draw less power than the generated power from solar system, then I actually feed the grid. If I have a mechanical power meter, then it will rotate backwards saving me some money. Some countries (but not Egypt) use this technique to solve a part of energy demand problems, so they encourage people to generate their electricity and sell it to the grid. This type of inverters will cutoff output power if the mains electricity is cut. this is called (islanding technique). This happens for safety because power can be cut off for some electricians need to do some maintenance in the area feeding box, so that they don't get an electric shock from the energy injected by the inverter.
2. Off-grid: This type will not be connected to the grid. It outputs its power to operate appliances
   
   that are connected to it only. This can have backup batteries to extract power when the sun is not in the sky.

Having a grid tie inverter seems to be more reasonable to have because I can sell power when I'm not using it, but I don't need to have power cutoffs. A hybrid system can be achieved by having a grid-tie inverter connected to the grid and another off-grid inverter where some loads (emergency loads that I need to be up all the time like my PC, Internet router and some lighting) connected to it.

As for my calculated load of 575 Watt, I need an inverter with an output that can power that load. Therefore I need at least a 600 Watt inverter. I must take losses into consideration. Adding 20% losses due to inverter efficiency and cabling, then I need at least 575 x 1.2 =690 Watt (700 Watt inverter). i don't think I ever saw an inverter of a power rating between 600 and 800, then the least I can get is a 800 Watt inverter. As per my design, I intended from the beginning to have one room off-grid.