Let's get around to looking at some calculations. I suggest that you sit down with a piece of paper and a pen so that you can work through this example with me.

The problem:

During a 4 hour Eskom power failure, you want to run your computer for 4 hours and it uses 300 Watts (per hour); you also want to run 4 lights for 4 hours. Each light draws 8 Watts. You don't want to use a noisy, smelly and pollution generating generator. You want to build a system yourself and one day add a solar panel or solar array to it. You are prepared to spend a bit extra to make that "start" in Renewable Energy.

Renewable Energy "Thinking" Notes: Eskom energy is still much cheaper than generator energy, so we'll start with charging the battery using Eskom energy. Eskom energy isn't clean, but we need a starting point. We can add solar and other technologies to this system later. My Eskom energy (electricity) at the moment is about 70 cents per kwh. This in May 2009. With an expected 35% increase this year, it will go to 95 cents per kwh. I understand that running a generator costs R30 per kwh although I haven't checked this. Can anyone who sells 4KW generators let us know what the cost is of fuel per hour? Plus maintenance and the cost of the generator.

Purists tell me that I shouldn't use Eskom's electricity in these systems. I am not a purist. I am a realist. I know people want to get started and I want to help them/you. I started with the kind of system shown below. Why shouldn't you? Note that my first system was a 12 Volt system with a 40 Amp Hour battery. It uses a 3.6 Amp Benton charger at R350 plus VAT, plus a 40 Amp Hour battery at R818 plus VAT plus a 1000 Watt modified sine wave inverter at R1000 ex VAT. Total cost of approx R3000 including cabling and a 60 Amp circuit breaker which acts as a fuse on the positive line between the battery and the inverter. These are prices that you can get at places like Yebo electronics in Bellville. I got this system going in May 2008. Note that with the wiring sizes and circuit breaker on this system we draw a maximum of 200 Watts from the system.

The solution (part 1: battery sizing; inverter sizing; electric charger sizing):

A) work out the total watt hours required during the power failure:

Computer: 300 Watts * 4 Hours = 1200 WattHours.

Lights: 8 Watts * 4 Hours * 4 lights = 128 WattHours.

Total WattHours needed from the battery = 1328 WattHours.

1328 WattHours = 1.328 kwh (kilowatthours).

B) What kind of battery do we need?

Note that for this type of system, one needs a Deep Cycle Battery, not a car battery. A car battery is designed to release a large amperage for a short amount of time and to be recharged immediately. A Deep Cycle Battery is typically designed to release its charge over 5 or 20 or more hours and to be charged relatively slowly over time. A Deep Cycle Battery should be fully charged at least every 4 days.

C) I've heard that Deep Cycle batteries should not be discharged less than 50% of their total capacity. So what's this about 50% of the battery life?

We don't want to take the system below 50% of the battery life, so if we calculate that we need 100 Amp Hours of time (see calculation in D), we would want 200 Amp Hours of battery time.

Although one can go down to 20% of battery life, it is recommended that one stay above 50% of the battery capability so that the battery's life is not diminished.

The 50% is achieved at a certain voltage. I will look at what the voltages should be in the next Part of this course, ie what voltage is shown on a 12 Volt or a 24 Volt battery when the battery is full. What voltage is shown at 50%, etc. I'll also look at Aborb, Float, Bulk and other battery terms.

The 50% is sometimes called State of Charge (SOC). Initially one can think of SOC in terms of voltage because voltage is easy to measure. If you are a purist, then SOC is measured as Amp Hours In minus Amp Hours Out.

D) What size of battery do we need?

Assuming a 12 Volt system: 1328 Watt Hours / 12 Volts = 111 Amp Hours. Note that one normally allows for an 80% efficiency which means that you would take 1328 / 10 Volts = 133 Amp Hours. (10 is approximately 12 Volts * 80%). The 80% efficiency factor allows for the inverter inefficiency (92% - 96% efficient) and the fact that we can't take 100% of the available charge out of the battery.

Now we don't want to take the system below 50% of the battery life, therefore we would want 260 Amp Hours of battery time. This would mean 3 x 100 Amp Hour batteries. Each of these batteries is about R1,000.

Assuming a 24 Volt system. 24 Volts * 80% efficiency is approximately equal to 20 Volts (I use the 10 Volts or 20 Volts when I do calculations in my head). 1328 Watt Hours / 20 Volts = 66 Amp Hours. Multipled by 2 = 132 Amp Hours. Two 100 Amp Hour Batteries in Series would give 24 Volts at 100 Amp Hours. Two sets in parallel would give 24 Volts at 200 Amp Hours.

Looking at this backwards: a) 200 Amp Hours at 20 Volts = 4000 Watt Hours. 4000 Watts / (1368WattHours/4Hours) = 4000 / 342 = 12.5 hours * 50% = 6.25 Hours. Or b) 133 Amp Hours at 20 Volts = 2660 Watt Hours. 2660 / 342 = approx 8 hours * 50% = approx 4 hours, ie what we wanted in the first place.

E) Charging the battery

Note that 12Volt 102 and 105 Amp Hour batteries are a "standard size". There are also 12Volt 60 Amp Hour batteries; 12 Volt 18 Amp Hour batteries, etc.

A battery should be charged at between 5% and 20% of its rated Amp Hour rate if possible. Therefore a 300 Amp Hour battery at 12 Volts would need at least 15 Amps of DC power to charge it at 12 Volts (300 * 5% = 15). A 300 Amp Hour battery at 24 Volts would need at least 15 Amps of DC power. I have found an excellent 24 Volt charger that works at up to 14 Amps DC and so am designing small systems using this charger. There are other cheaper chargers around and on one BLOG someone says he found one for under R500.

The CTEK XT14000 charger is R3750 ex VAT and one can buy them from Aztec batteries in Joburg. See [url]http://www.aztecelectronics.co.za/index_files/page0081.htm[/url] Speak to Liz. Note that this is quite an expensive charger. For example a 3.6Amp Benton Charger is R350 ex VAT. This is 12 Volts and charges my 40 Amp Hour battery in about 12 hours. 40 Amp Hours / 3.6 = 11.1 Hours.

F) Series and Parallel: what does this mean?

Note that wiring in series increases the voltage of the battery bank.

Wiring the batteries in parallel increases the amp-hours of the battery bank.

In series the + battery pole on the first battery is connected to the - battery pole on the next battery.

In parallel, the + on the one battery is connected to the + on the other battery and the - is connected to the -.

4 batteries in series and parallel diagrammatically look like this:

+ 12V - ===== + 12V -

= ^^^^^^^^^^^^ =

= ^^^^^^^^^^^^ =

+ 12V - ===== + 12V -

The = signs show the cables. If the batteries are 100 Amp hours each and are 12 Volts each, then in this configuration we have 24 Volt "batteries" at 200 Amp Hours. The ^ characters are just fillers so that I could line up the = sign on the + and - poles on the batteries.

Note also that the higher the voltage the smaller the cable that is needed to wire the batteries. The thicker the cable the more expensive the cabling part of the system is. The thickness of the cable is essentially dependent on the amps going through the cable. The amps can be thought of as the flow through the cable. If you "force" more amps through a cable, there is more heat and the cable must be thicker. There is also something called "voltage drop". The longer the cable the thicker the cable must be.

G) But my system is 220 Volts AC and you are generating 12 or 24 Volts DC. How do I convert my system from DC to AC?

You need an inverter.

There are two essential kinds in this scenario:

1) modified sine wave

2) pure sine wave

A modified sine wave is ok for running a TV and a laptop (which has its own charger and battery inside), but a pure sine wave is needed for motors, fridges and electrical equipment like computers. A modified sine wave inverter produces a square sine wave, whereas a pure sine wave changes the voltage rapidly and produces a sine wave that looks smooth. See graphs and more descriptions at [url]http://www.energymatters.com.au/renewable-energy/inverters/[/url] Energy Matters only sell Pure Sine Wave inverters and recommend that you buy one of these, but you need to make up your own mind. Note the important point about the lower efficiency with the modified sine wave inverter, especially once you have a solar system. In this case, you would want the most efficient inverter as all the different efficiency losses on the system add up and you wouldn't want to lose so much of the potential energy you have in your battery.

A 24 Volt 300 Watt pure sine wave inverter is R1786 ex VAT.

H) You also need a fuse and a switch on the line between the positive of the battery and the positive of the inverter. We'll look at what size switch another time.

In the meantime adding all this up, we have:

4 x 100 Amp Hour batteries at R1000 each;

1 x 24 Volt CTek battery charger at R3750;

1 x 300 Watt Inverter at R1786.

Total = R9536 + VAT = R10,871. This excludes cabling, the switch and fuse and my time if you want me to do it for you.

I) "I" is a good place to end. You are probably thinking that R10,871 is a lot of money and it is, so one could consider replacing their 300 Watt desktop computer with a 50 Watt laptop. This arrangement would give 2 hours of time from the big batteries plus 2.5 hours from the laptop battery. Second hand laptops are available for around R4000.

Total system cost approximately:

50+32 (for the lights) = 82 Watts / 10 Volts (12 volt system) = 8 Watts;

8 into a 40 Amp Hour battery = 5 * 50% (depth of discharge) = 2.5 Hours;

40 Amp Hour battery = R1000.

Benton charger = R350

Modified sine wave inverter = R1000.

Total system cost including the laptop < R7,000.

You use the charger for the first 1.5 hours and then use the balance of the battery time for the lights and the internal laptop battery for the balance of the 4 hours.

This shows that the place to start spending money in an RE system is often on efficiency. ie using a 50Watt laptop instead of a 300 Watt PC. It is much cheaper to save electricity than to make it.

Note: if you have a laptop, it might only draw 50 Watts and it also has its own "UPS" built in, ie it has its own battery backup.

Note 2: A full DIY system design costs about R2,500. Prices excl VAT and must be paid up front. The design includes looking for equipment but excludes installation time. A typical design (for a house) takes between 2 and 3 days.

In part 9, I'll look at how to connect the solar panels as well as solar panel costs and solar charge controller costs.

In part 10, I'll look at battery terms; In part 11, I'll look at cabling; In Part 12, I'll look at efficiencies; In Part 13, I'll look at shading and site analysis; In Part 14, I'll look at Grid Tie vs Off Grid Systems.

If you want me to look at anything else or discuss anything else please let me know.

Systems like this are complex and if you don't know what you are doing, please get help from an experienced system designer and installer.

## Wednesday, 6 May 2009

### Part 8: Renewable Energy System Design - "DIY Sizing"

Labels:
battery,
Cape Town,
Renewable Energy,
solar,
wind

Subscribe to:
Post Comments (Atom)

## No comments:

Post a Comment