Launching my tiny solar power station

I was intending to write about electrical connections, but I think I will let that for another time because I have a recent update on my project.

• Problem in the first solar panel:

I checked earlier on my first solar panel mounted on the roof, and I found that my idea of holding the solar cells to the glass with a PVC tape is a total fail. Unfortunately I did not account for the elasticity of the PVC tape. The PVC tape stretched due to heat during day hours and finally they got dry and crunchy and finally lost its glue and separated from the panel. The result for this problem is the solar cells were sagging and exposed to environmental changes (humidity, dust, rain, ...etc).

• Accounting for the problem in the second panel:
  

I decided to go for another material for insulating the solar cells and holding them straight at the same time. The first material I thought of was the acrylic, but it was very expensive that I could pay EGP 500 (72 USD) for one sheet. Next material I thought of was wood. I wend to the wood store, but I would buy a very big sheet and I would not use half of it. After that I had an idea, what about the thin fiber sheet that is used in aluminium kitchen doors?. I found a leftover at the aluminium workshop that was barely the size I needed. This fiber sheet is thin, somewhat flexible and covered from both sides with a water proof layer and cheap that I paid 110 EGP (16 USD) for both solar panels backing. This was exactly what I wanted.

• Solving the problem in the first panel:

I replaced the new panel with the old one. I took the old one down to the working place and removed as much I could from the ruined PVC backing and mounted the fiber sheet with silicone sealant. Fortunately it was not damaged and it measurements were OK.

• Adding the second panel:

I finally added the old (modified) panel in parallel to the new panel on the roof. I now have a generating power of 140 Watts x 2 = 280 Watts.

Note that I increased the inclination of the solar panels to 45 degrees because we already passed September where the sun goes below the equator. The solar panels are directed to the true South.

• More numbers:
  

I can now operate the TV and the PC+Monitor (166 Watts) easily all day. The extra juice is stored in the battery for some time of night operation, and when these devices are turned off, the battery is charging even more quickly.

I bought the 65 Ah battery to have a total storage capacity of 72 Ah. In ideal case (for calculation) when my panels produce 16 A, then to recharge the battery bank from empty (never happens) to full capacity, then I would need 72 Ah/ 16 A = 4.5 hours. By observing once, it took about 5 hours to charge the battery from 30% to 90%.

Practically, I operated the TV and PC for about four hours on the battery. This really made my day.

The battery bank, when full, holds a usable (30% DOD) power of (72 Ah x 12 V DC ) / 30% = 600 Watts. The solar panels can produce daily (assuming 8 hours of bright sun) 280 Watts x 8 hours = 2.2 KWh.

After two days, I found that I consume daily about 1 KW from solar energy. That is divided between day and night. Then the losses in the system must be 50% (this is a big number, but I don't know if this is normal accounting for efficiency of the battery charger 95% and the power inverter 70% and the battery bank itself 85%. adding theses losses surprisingly results in 50%, but I'm still not sure about this).

I had to go around my basic design more than once to fulfill the budget which has reached now about 8000 EGP (1143 USD).

To conclude this post, I feel special because, in my country no one does as I do. I have my tiny solar power station which produces free clean sustainable energy. I sometimes keep looking at the power meter while it counts the used solar energy and I dream about a bigger station which operates all my home appliances off grid. All my effort in the past four months (whenever I have spare time) were fruitful. I am really happy and have a feeling of accomplishment.

The battery and the charger

I have purchased a cheap Chinese solar battery charger for my system. As I mentioned before this
was not chosen by calculation, but by price. I have a 20A MPPT charger.
20A means that is can handle a max of 20A solar panels input. My panel outputs 8A (by design), so I'm okay if I added another panel, then the current will be 16A, but I can't add a third panel except when I buy a bigger charger. This charger works with input power from the solar panel and charges the battery by regulating this power.

There are two types of battery chargers, PWM and MPPT as mentioned before.

As long as the sun is bright, the battery is charged, and when the sun goes down, the battery charger consumes the battery and routes it to the load.

This type of charger has an instruction that I should first connect the battery then connect the solar panels, but I ignored this instruction and connected only the solar panels, then I had an over current. Fortunately it has an electronic fuse and I haven't burnt anything. I wired the output to my inverter to test the output AC power, but the charger refused to power the load directly from the panels, so I thought the charger was faulty.

I then assumed that the battery has something to do with the operation of the charger, so I went down and bought a test battery of 7Ah (a cheap one of 175 EGP). The make of the battery is MxVolta and it's made in Korea. It is a VRLA (Valve Regulated Lead Acid) battery that can be discharged up to 60% DOD (Depth of discharge) for 2000 cycles. This suits the solar application.
I connected the battery to the charger and to the inverter and I was very pleased when the system worked flawlessly.

Sure a 7Ah has a power of 12V DC x 7A DC = 84 Watts/hour when it is fully charged. I was powering the PC, monitor and fan and the power consumption was 150 Watts, so 84/150=0.56 hours. Adding a losses of 20% then the time this battery can supply is 0.45 hours. But I must take into consideration that I don't want to fully discharge the battery (actually it's a function in the inverter and the battery charger as well). I will aim a 50% DOD, so this battery will have a time of 0.45/2=0.23 hours which is 13 minutes on condition that the battery is fully charged.

The charging time of the battery also counts to decide how big my battery bank will be. I now have a charging capacity of 120 Watts per hour. Considering 10 hours of bright sun I  can get 1.2 Kw of energy everyday. Thus I can charge a battery bank of 1200/12=100Amperes. I intend to buy a 65Ah battery for a start (easily available at my area) and it costs 1150 EGP.

Next time I should write about the cabling and the ATS (Auto Transfer Switch)

Building solar panels

I have stopped writing on my blog for more than two months due to my work heavy load, so I was very tired, but now, I have received the rest of my solar system's components and I again have the urge to write. At this moment when I'm writing these lines, I have my PC fed from solar energy. YES! I have completed my solar UPS and I have made a successful test run.

But first I should resume from where I stopped earlier. I will talk about building my solar panel.

I purchased a a pack of 108 solar cells from ebay (on the right picture and I have to say I expected them to be larger) and I have also purchased tabbing wire to connect solar cell together (this was a time consuming job). The solar cells I bought has a dimensions of 6"x6" each produces 0.5V DC and 8A DC (that what was written on ebay).

I wanted to assemble some of them to produce 18V DC (as standard panels produce) so the calculation will be: 18V DC/0.5V DC = 36 cells. 

Note: These cells are extremely fragile and brittle, I broke about a dozen, but they are still functional.

First thing I did is to put one in the sun and measure how much power does it produce. I measured a 0.55V DC and 1.1A DC. I thought something was wrong as it should produce 8A DC, but I think I could not hold it right or my hand shadow affected the current. Anyway I decided to go on with experimenting. I connected two of them and measured 1.1V DC and also 1.1A. I thought that since they are imported from China, they should be not good, but I couldn't stop at this phase. In the picture the measurement is away from the sun.

I had to connect those 36 cells in a rectangular pattern of 9x4 by connecting each positive side (dark side) to its neighbor's negative side (shiny side)

The frame is made of aluminium angles (3cm x 3cm thickness 3mm) joined together and in the middle is a glass sheet (thickness 6mm) glued to the frame with silicone. I had to line up the cells to be neat and have a nice look. I welded all the 36 cells and extracted the far terminals to be the positive and negative. I covered the back of the cells with a PVC tape to isolate them from humidity (I should use a piece of plexiglass or any isolating sheet for the back of the panel because the PVC tape is not solid, thus sagging a little bit).

The final panel look is in the picture on the right. I measured the voltage output and it was 18.8V DC. I measured the short circuit current and I was very pleased to read 6.5A DC.

My panel initial design was 144 Watts, but practically it is 122 Watts. It's okay for me and this result satisfies me for my first home built solar panel.

After doing some research about which direction to point my panel and which angle is the most suitable to extract the biggest amount of juice from the sun, I reached some very easy numbers. Since I live in Egypt in the Northern hemisphere, then I should point my panel to the true South. Also in Cairo the suitable tilting angle in summer is 15 deg and in winter is 45 deg (I have made an extension leg to increase the tilting angle to 45deg). Sure a tracking device will be much more efficient, but I don't want to bother myself with mechanical designs at this stage yet.

Second practical excercise

For the second day in row, I received the second item I purchased from ebay. Behold the power

inverter. As discussed before, the power inverter converts DC power to AC power to feed the ordinary house appliances. I had already purchased an over-sized inverter for future use. It has a continuous power output of 2000 Watts and surge output of 4000 Watts, that means it can output power of 2000 Watt all the time, but when I turn on my appliances (specially inductive loads like fans), they take a lot of current to start-up then return to their rated current draw. This happens in less than a few seconds, but the inverter should handle this situation or else it will shutdown due to overload or rather be damaged.

As discussed before, I designed my system to use a voltage of 12 V DC, so the input for my inverter is 12 V DC. The output is 220 V DC, as the voltage of the grid in Egypt. This inverter has some capabilities like shutdown on overload, short circuit, very high battery voltage and very low battery voltage. It also has a buzzer to give me a warning to shut down my appliances normally and safely when the battery is critically low.

As soon as I received my inverter, I took it down to my car, opened the hood and connected it to the battery for a test run. I connected the positive (red) terminal of the inverter to the positive terminal of the car battery, and connected the negative (black) terminal of the inverter to the negative terminal of the car battery. I was terrified when I heard a spark when the circuit is closed. I tested it with an AVO and yes!, 240 V AC was on the screen. I haven't tested it on a load, but I will wait till the rest of mt items arrive. I will buy the batteries the last thing, because they tend to lose their charge by time and I don't want them spoiled before using them.

This type outputs a pure sine wave, so that I'm free to power any device as long as it does not draw more than 2000 Watt (1800 for safety) continuously, and also doesn't draw above 4000 Watts (3600 for safety) at start-up.

This power inverter weighs about 5.5 Kg, and is bigger than I imagined, so I have to decide on where to put it. it must be put in a ventilated area to circulate air (it has fans inside), and also it must be put away from batteries to avoid heat or fire due to gasses (very small amount) emitted from the batteries.

• Recalculation

As discussed in the previous post, practically I use 385 Watts to power my target room. It is recommended to design for an inverter double the required power (that will be expensive), so if I would size my system for it's present power, I would have bought a 600 Watt inverter.

My inverter costed me 280$ and shipping 80$ = 360$ which equals 2520 EGP. I was charged 435 EGP for customs and other fees (services), then the total will be 3000 EGP.

A 800 Watt inverter would have costed me about a 130$ less, which will be about 2090 EGP. I will consider that in my calculation irrespective what power inverter I bought, because I want to calculate the real price for my target system.

First practical excercise

At last, today I received the first item from my purchase list from ebay, it's the power meter. That devices is very handy in design an analysis of my system. I bought it from www.eco-worthy.com for 13.55 $ from an auction on ebay. It was shipped to me from Singapore, but I think it's made in China, and I received it after 16 days of my payment. It costed me 135 EGP because of customs and my local mail office also charged me for services (which I don't know).

It is very interesting to watch by my own eyes exactly how much power my appliances are drawing.
This power meter can display Watts, Watt-hours, Amperes, Voltage, Frequency and Power factor. it also displays electricity usage cost, Max power and Min power> It can support a maximum of 3680 Watts before displaying Overload.

I connected my appliances (except the CFL lamp) and I was pretty surprised from what reading appeared on the power meter. I switched on the TV, receiver, fan, PC, monitor and speakers. I started a heavy graphics game (Medal of honor - Warfighter) - very nice game - and the maximum consumption was 350 Watts - 1.6 Amperes. Remember that I calculated my load power of 575 Watts.
These are really good news for me because I will have more juice to use from the batteries for more time. But what mistake did I make in my calculation? I made no mistake, but I have a PC power supply of 500 Watts, and I'm sure I'm not using them all. If I calculated the CFL lamp as 35 Watts, then my true consumption is 350 + 35 = 385 Watts, and 575 - 385 = 115 Watts. Then I can say that I'm using a maximum of 385 Watts from my 500 Watt power supply, but the other appliances I made my calculations from the data on their nameplate.

I'm happy today, I'm going to look at the power meter every now and then to see for real what my consumption costs are, and I'm going to re-size my solar energy system components.

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.

Design - 4

• Batteries

Connecting my panels directly to the inverter to power my appliances means that I can have power only while the sun is shining, so I need to store the excess power for using it at night (or in my case at the time of power outage).

Battery bank should have enough capacity for the designed working hours without charging. The capacity is described as Ah (Amperes hours) which means the amount of amperes I can extract from the battery bank in a time of some hours. If I have a battery of 12 Ah, that means I can get a current of 12 Amperes for one hour, or I can get a current of 1 Ampere for 12 hours, o a current of 6 Amperes for 2 hours, ...etc.

If I'm going to power my 575 Watt/Hour system for 4 hours after sunset, then I will need my batteries to store a power of 575 x 4 = 2.3 kWh. That means 2300 Watt/Hour / 12 V DC = 192 Ah and that is the capacity of the battery bank needed (without calculating losses)

According to my previous calculation in Design - 1, my requirements are day is 3.2 kWh. 3200 / 12 = 266 Ah. Adding losses of 20%, the result is 320 Ah.

If I used a battery bank of 320 Ah, then I will use all of this capacity every day, and I will need to fully recharge the batteries also everyday.

Discharging the batteries to the last drop will damage them, therefore I have to decide on the DOD (Depth Of Discharge). To know how much I can discharge my battery bank, I need to know about:

• Battery types

At first I thought that I can just buy enough car batteries and install them as my battery bank, but then I figured out that they can be used only on certain conditions and they are not recommended. Car batteries are designed to give a very high current in a very short time (cranking the car to start it up), and are designed to be discharged not less than 80% of their capacity or else their life expectancy is shortened much. In the car, after cranking, the dynamo is responsible of charging the battery to 100%.

If I want to use car batteries, then I will use them for a DOD of 20% only, then I need a battery bank of 320 / 20% = 1600 Ah. If the average 100 Ah car battery price is 1,000 EGP, then I need 16,000 EGP.

In my research  I will categorize batteries into two sections; Deep cycle and Shallow cycle (car batteries).

Deep cycle batteries are designed to give constant current while discharging, and to be charged more quickly. These batteries have many types, some of them are:

1. Flooded: These are positive and negative plates which have liquid acid to act as an agent for
   
   moving charges (current). This is the cheapest type of the deep discharge batteries. It has thicker plates to withstand deep discharging without corrosion. The bad thing about these batteries that they produce hydrogen gas when being charged. This makes them not suitable for mounting in a confined area because hydrogen is flammable and poisonous. Also these batteries need constant distilled water refill to compensate the hydrogen (Charging the batteries decomposes the water into hydrogen and oxygen).
2. (Semi)Sealed Lead-Acid: These are as the above type, but are sealed, hydrogen gas is
   
   significantly reduced (not eliminated) and does not need water refill. These are also called VRLA (Valve regulated Lead Acid). These have a moderate price.
3. AGM and GEL: These two have the acid absorbed in a fiberglass mat (AGM) and in the form of gel (GEL). These have the advantage of mounting horizontally or vertically.

Deep cycle batteries can withstand regular DOD of 40% and occasionally DOD of 80%. It is adviced not to go below 50% to lengthen their life expectancy.

• Battery bank capacity

To cope with a DOD of 50%, I need to double my calculated battery bank size, therefore I have to get 320 x 2 = 640 Ah batteries. If the average VRLA battery price of 100 Ah is 2,000 EGP, and I need 640/100 = 7 batteries, they will cost me 14,000 EGP.

Comparing this to a normal car battery, I will need 640/20%= 3200 Ah which will cost me 32,000 EGP.

• Wiring the batteries together

In my design, I went for 12 V DC battery charger and inverter (which was not the best choice). To have

 a battery bank of 12 V DC, just connect all the positive terminals together and all the negative terminals together. I will have then a 12 V DC, 640 Ah battery bank.

To have a  24 V DC battery bank, connect each half of the batteries as a standalone 12 V DC bank, then

connect one positive terminal from bank #2 to one negative terminal from bank #1. I will have a left one positive terminal from bank #1 and negative terminal from bank #2 which will have a potential of 24 V DC, 320 Ah. Note that voltage and current change by changing the battery setup, but the constant is the power. P = V x I (Power = Voltage x Current).

These batteries should be connected to the battery charger and the inverter.