Design - 2

Types of solar energy systems

• 1. Grid-Tie
  

In this system, the DC power comes out of the solar panels directly to the inverter (grid-tie inverter). This type of inverters convert the DC power to AC power and plugged directly in the mains and it synchronizes its wave form with that of the grid. It supplies your home with its nominal power (e.g. 500 Watts). If you are using more than 500 Watts than the rest is fed to your home from the grid. if you are not using all the 500 Watts, then you are feeding the grid itself. Your utility meter runs in backward direction. This happens as long as the sun shines, but after the sun sets, you are using the grid power only. Another feature in that inverter is when the grid power cuts off, the inverter power cuts off also. this is to prevent the inverter to feed the grid while workers are working and assuming no power is in the lines.

• 2. Grid-Tie with batteries
  

This is like the grid-tie system, but there are backup batteries that are charged using the charger inside the inverter and are used mainly to feed your DC loads, or can be connected to another (not a grid-tie) inverter to feed some AC loads

• 3. Off-Grid

In this system, the panels feed DC power to a battery charger which is connected to the battery bank to charge it while the sun is shining. An off-grid inverter is connected to the batteries to use their stored power to convert it to AC and feed AC loads. This type of inverters con not be plugged into the mains.

Components of solar energy system

• Solar Panels

The sun sends in one hour an amount of energy enough for human consumption for a full year. We need to collect that energy and make use of it either directly or indirectly. Solar panel is one way to do so. Solar cells which form solar panels are like a transistor. They are made from high purity silicon that has impurities to form a P-N junction.A P-N junction that has positive and negative charges on the barrier of that junction. These charges are waiting for excitation to move around forming electrical current. The excitation comes from the power of the sun in the form of photons.

There exists 3 commercial types of solar cells; mono-crystalline, poly-crystalline and amorphous.

The mono-crystalline are the most efficient and also most expensive. Efficiency 17-20%

The poly-crystalline are the mid-range in terms of efficiency and price. Efficiency 12%

The amorphous are the least efficient and least expensive. Efficiency 8%

More efficient cells means that they collect more solar energy in the same area. Amorphous has an advantage that they are like a thin film and can be put on any shape without breaking and they can resist heat, but also they are very inefficient, lose efficiency by time and I will need more space to mount them.

I found that the most suitable type is the poly-crystalline in terms of performance and price.

As I said before that the average price of a mono-crystalline panel of 100 Watts is about 200$, so if I used a poly-crystalline panel it will cost me less. A more better idea that I can buy single solar cells and wire them together and put them into a frame to compose my custom solar panel which will save me some money.

If I'm going to power my requirements of 575 Wh directly from the solar panels (without using batteries) then I will have to get a 575 Watts panels. Calculating losses for inverter of 20% : 575 x 1.2 then I require panels power of  690 Watts panels. This will enable me to extract electricity only during sunlight hours.

• Wiring solar cells
  

A solar cell of size 6" x 6" gives a typical of 0.5 V DC and 8 Amperes max (that equals 4 Watt). A solar panel gives a typical of 18 V DC. Then wiring 36 solar cell in series gives me that volt and keeping the current a 8 Amperes. This panel gives a maximum of 18 x 8 = 144 Watts. Therefore, I will need 900 / 144 = 7 panels of those. Wiring the panels together in parallel give me 18 V DC, 8 x 7 = 56 Amperes. That is equal to 56 Ah.

The solar cell has two sides, one is the negative (shiny side), and the one which is positive (matt). Wiring the positive side of  the cell to the next cell's negative side in series forms a panel. I can choose to either arrange than as 6 x 6 cells or as 4 x 9 cells. the latter will be better to save space.

I will the cells together under a protective surface such as glass or thick acrylic and over another piece of acrylic and insulate them to protect them from weather harshness like rain and dust. I will also put that setup inside an aluminium frame and fix them to another aluminium setup on the roof (my roof is horizontal, so I need to tilt them).

• Mounting solar panels
  

To get the maximum efficiency I can from solar panels, I must mount them either on an adjustable base or on a fixed base. Adjustable base moves through the day using servo motors and a tilting controller that calculates the position of the sun to direct the panels towards it to collect as much power from the sun as it can. this systems cost too much and I'm not going to use them in my system.

Putting panels on a fixed base requires calculating horizontal and vertical angles. Because Egypt is in the northern hemisphere, then my panels should be directed towards the South. Cairo lies geographically in the latitude line number 30. As a general rule, in the winter when sun is far on the South, the panels needs a tilting angle of (Latitude + 15 = 45) and in summer where the sun is over our heads (Latitude - 15 = 15). I have to make my base adjustable with two positions to change them once every 6 months to get the max power from the sun.