EXCEL BASED CALACULATOR

[alitheking]

Hello All,

Glad to be a member. It is my first post in here. I have been busy recently with preparing an excel based calculator specially for off-grid, battery based systems , single phase. In fact, i am new to this field. I hope you all try the file attached and use it from time to another for a reference. Surely there will be some mistakes or questions somewhere, please do not hesitate all to post a suggestion or a question or a help in here. I want to develop it. many things i want to apply but still restricted by my excel knowledge limitations!

The questions on my mind:
1. when having an off-grid system. We have two things to account for; first, the load and the daily operation. also, the back-up load and the wanted duration. When designing the PV array, we depend on the battery back-up load info? For example, if the load itself is 3000KW, and the operation hours daily =10h. but the battery backup required is to be for the 2000kw running 5h a day, what info i shall select to size my array? - i think it is the back-up info , is that right? waiting your replies.

Another thing to mention, All you want to enter in the file is the components' spec you want to use and the site lowest and highest temp. Excel will do the calculations accordingly including the conductor and the OCPDs sizing. The conductors are based on copper 90c temp rating only.

You will notice that the calculated conductor size will be one level smaller than requested. For example, if the conductor size shown is #8; that is in fact , #6 and so on!:happysad: Here the problem will arise when calculating the voltage drop which surely depends on the conductor size! it is a long story to talk about, i will leave you with the file and waiting for your building suggestions and opinions!

Note: In order to track the voltage drop in the system,the longest PV source circuit is from the PV strings to the combiner box, and the longer PV output circuit is from the combiner box to the inverter. we calculate them separately and add them together later.

I tried to attach the file, but couldnt! could any one tell me why ?

 

[jaggedben]

From what I know there is really no 'one size fits all' standard for sizing a system. However, a typical approach is to size it at roughly twice the size that is calculated to be necessary in the winter months. (The calculation would be done with PVWatts or similar software.)

The doubling is done with both the batteries and the PV array. It's done with the batteries because lead-acid batteries do not last nearly as long if they are deep-cycled. It's much better if the batteries are only drained by 50% or less each day. On the PV array side, you oversize in case you have a few cloudy days in a row. The required size of the array depends on your location and system design constraints.

If you want to treat the batteries better, you might get even more of them.
If you want more insurance against cloudy days, you'll add even more solar panels.
If you are willing to take risks to save money in the short term, you might go with less on either.
It really all depends on how critical it is not to have operation interrupted, how much money you have to spend, and how much time you have to spend on the project. I think most off-grid system owners deal with a fair amount of trial and error.

The WindSun forum is a big hangout for off-grid people...
http://www.wind-sun.com/ForumVB/activity.php

 

[alitheking]

thanks

thanks

Thanks alot for your reply. But i am not giving fixed sizing in the attached photos that you might have seen. The user shall enter the spec of the components of his selection and the calculations will be done so he/she can compare the results. If does not like the outputs, just change the inputs. To sum up, the numbers shown there in the attached photos depend on the random numbers i entered but are never fixed.

I wanted to upload the file but unfortunately no succeed.

i hope to hear from you again about the array sizing, depending on the battey back-up size and daily duration required? or based on the basic load and daily operation just as the example i have given up there?

Thanks again , i am checking the link you offered~

 

[dereckbc]

Well I cannot read what is on the spreadsheet but here is what I practice to get the basic components, then fine tune once equipment is determined. It is based on what John Wiles of NMSU has taught me and tweaked on personal experience.

Recommended Design Practice of Off Grid Solar PV Systems

The design process is a fairly simple and straight forward that doesn't require a lot of technical knowledge. What follows is based on using MPPT Charge Controllers because they are the most efficient and economical solution for systems requiring a 200 watt or higher solar panel array. The initial steps are:

• Determine the load in energy for a 24-hour period. Not the watts, but the watt-hours.
• Determine the size of the solar array to be used.
• Determine the battery size and type.
• Determine charge controller size

DESIGN EXAMPLE

The following example is a rough estimate to take to a system designer to discuss cost and objectives. He/she will then fine tune the system based on actual components, cable distance, etc? Our basic objective in this simple example is to provide power to a 250 watt load (light bulb) for 24 hours per day in two different cities (Tucson AZ, and Seattle WS) with 90 % availability. FWIW this is a system I have designed many times using remote cell radio sites. The transmitter is continuous 250 watt load, and the system will be a typical sized system that can be used in a home application. The only difference is radio sites are designed for 99.99% availability and this one will only be 90%. Getting from 90 to 99.99% greatly increases the cost with a larger solar array, larger batteries, and a standby generator set.

1 DESIGN FOR WORST CASE

In this example the worst case is simple to determine because the load is continuous 24 x 7 x 365 of a 250 watt light bulb. So the worst case is the months of December and January when the Solar Insolation is at its lowest point of the year. In some instances, the worst case for the load might be in the summer. Therefore you make two designs, one for winter and one for summer, and then use the larger of the two systems.

So in this example we need to determine the energy needed in a 24 hour period. This is done with watt-hours. To determine the watt-hours is straight forward of Watts x Time (in hours). So 250 watts x 24 hours = 6000 watt-hours or 6 Kwh in a day or 24 hours. Make note of this number as it will be needed latter.

2 IS THROW IN A FUDGE FACTOR

To account for overall system losses in the wiring, charge controller, battery charge efficiency, and inverter you multiply the total 24 hour load energy by 1.5 So 6000 x 1.5 = 9000 watts or 9 Kwh. Now take note of this figure. FWIW if using a PWM Controller the Fudge Factor is 2

3 DETERMINE SOLAR INSOLATION IN HOURS

Most solar map data are given in terms of energy per surface area per day. No matter the original unit used, it can be converted into kWh/m2/day. Because of a few convenient factors, this can be read directly as "Sun Hour Day? The number you want to use in this example is for December since December days are the shortest. Tucson is shown to receive 5.6 kWh/m2/day in December.. For Seattle, the number is 1.2 Kwh/m2/day. So we need to note 5.6 and 1.2 for our Sun Hour Day as it will be used to determine the solar panel array wattage.

4 DETERMINE THE SIZE OF THE SOLAR PANEL ARRAY.

The size of the array is determined by the adjusted daily energy requirement using the Fudge factor number divided by the sun-hours per day. So for Tuscon 9000 wh / 5.6 h = 1607 watts, 1600 will work. For Seattle 9000 / 1.2 = 7500 watts. Note the huge difference; it is because of the Solar Insolation. Location matters and will greatly affect system cost.

5 DETERMINE BATTERY SIZE

Determining battery size is very simple. All batteries will last substantially longer if they are shallow cycled. That means discharged only by about 20% of their capacity in a given day. Whereas deep discharge means that a battery is discharged by as much as 80% of its capacity. Second point is no lead acid battery should ever be discharged more than 50%. Below 50% soft lead sulfate crystals begin to harden on the plates which reduce capacity and shorten battery life substantially. So with this said the battery capacity is calculated to be 5 days minimum autonomy. So in real application you have 2.5 days of usable capacity to allow for cloudy days before reaching the 50% discharge point.

To figure the daily load, go back to the original load number before the fudge factor?that is, 6000 watt hours. Battery capacity = Daily Watt Hours x 5. So we need 6000 watt hours x 5 days = 30,000 watt hours or 30 Kwh.

Now that we have the battery capacity in Watt Hours we need to convert to Amp Hours. To find the Amp Hours we need to select a battery voltage. Amp Hours = Watt Hours / Voltage. To select battery voltage is based on panel wattage vs controller size. MPPT charge controllers have maximum panel wattage input vs the controller?s current rating in AMPS. MPPT controllers typically come in 20, 40, and 80 amps. So selecting battery voltage is very important. As a general rule you want to run the battery voltage as high as economically possible.

I have some general rules of thumb for battery voltage selection:

• Never use 12 volts. 12 volts is for toys and RV?s
• Panel wattages 300 watts to 2000 watts use 24 volt battery
• Panel wattages higher than 2000 up to 4000 use 48 volt

Based on those rules Tucson will use a 24 volt battery system, and Seattle a 48 volt system. So the battery capacity at Tuscon is 30Kwh / 24 volts = 1250 AH, and Seattle = 30 Kwh / 48 volts = 625 Amp Hours. Note no problem making Tuscon a 48 volt system if desired as that will allow for expansion later on if needed.

6 DETERMINE CHARGE CONTROLLER SIZE IN AMPS

It is very simple, Charge Controller Output Amps = Panel Wattage / Battery Voltage. So for Tucson the minimum MPPT Charge Controller is; 1600 watts/ 24 volts = 66 amps. So Tuscon requires a 80 amp MPPT Charge Controller.

OK for Seattle you are in for a huge expense because 7500 watts / 48 volts = 156 amps. There is no such thing as a 156 amp MPPT Charge Controller. MPPT charge controller?s typical sizes are 10, 20, 40, 60, and 80 amps. So in Seattle you will need 2 units of 80 Amp MPPT Charge Controllers supplying a common battery. That means you need to break the 7500 watt panels into two separate 3750 watt systems each with its own 80 amp MPPT Charge Controllers. Remember location matters.

7 CONCLUSION & LAST COMMENTS

We need to re-visit batteries for a moment. Flooded Lead Acid (FLA) are the least expensive and last the longest of the lead acid chemistry. However they have one drawback and that is they have the highest internal resistance. What this mean is the maximum charge rate they can be charged with is about C/8 where C = the battery 20 hour discharge rate Amp Hour Capacity. So the maximum current we can apply to a FLA 625 AH battery is 625 AH / 8 h = 78 amps. So for the Tucson we can use FLA batteries.

For Seattle things change again because of location as the charge rate exceeds C/8 for FLA batteries. 156 amps is a C/4 charge rate. So in Seattle you are looking to use AGM batteries. AGM is a lead acid battery which is a Sealed Lead Acid (SLA) or sometimes called a Valve Regulated Lead Acid (VRLA). AGM batteries have low internal resistance but they do not last as long as FLA batteries. AGM batteries cost roughly twice that of FLA.

That gets you in the ball park to look at budget. Once actual equipment and wiring runs are determined you can then use the actual efficiency of all the parts, but it will not change much.

 

[jaggedben]

Nice writeup!