Site Selection for PV Array, Battery Bank and Other System Components

Array

It is very important to properly select the site for installation of array/modules. If the system is relatively small, consisting of few modules only, then these can be easily installed on the roof-top. Large array requires large field or roof-top area for installation.

The site for array installation should not be shaded by nearby trees, buildings, etc. from approximately 09:00 to 15:00 on the shortest day of the year, which in the northern hemisphere is December 21.

It is necessary to orient the array south or north depending on where in the world the installation site is. Array located in northern hemisphere need to orient towards south and that in southern hemisphere towards the north. The array should be oriented within 15⁰ of true south or north. The best way to find the true south (for northern hemisphere) is to locate the magnetic south by using magnetic compass and then calculate the true south by using the magnetic declination data of the location.

The array field must be as close as possible to the battery bank and control room.

Keeping this distance minimum will lower the cost in cables.

Battery Bank

The battery bank must be installed in well ventilated shed or room protected from temperature extremes. It is advisable to keep the banks within 3-5 meters from the CR if possible. While arranging the batteries in the bank, it is a good idea to have air gap between them to cool and equalize temperature.

Control Units

It is a good practice to assemble CR, sub-array circuit breakers, PV disconnects, Battery disconnects and load breakers in a single housing (load center) made of corrosion resistant enclosures with clear marking of each of the breakers and disconnects. The sub-array combining boxes could be located near the modules. The load center must be positioned, within the permissible distance from the battery bank.

The exact locations of system components must be designed after inspecting the installation site and consulting with the owner of the system.

Example 9.8.1

Let us consider the load requirements as worked out in example 9.1.1. The total load in winter and summer are 3620 Wh and 4620 Wh respectively.

The system voltage is 24 V DC for all loads except the color television.

This unit is to be powered from an inverter of appropriate rating.

Array Sizing

For the purpose of array sizing, let us assume that the location of the site is 28⁰ N (Latitude), the minimum yearly average insolation at the site is 4500 Wh/m²/day, and the temperature variation is 40C minimum to 36⁰C maximum.

The total required array current can be calculated using formula 9.2.1 assuming derating factor and columbic efficiencies to be 0.9 and 0.95:

I array (winter) = 150.8 / (4.5 x 0.9 x 0.95) = 39.27 A I array (summer) = 192.5 / (4.5 x 0.9 x 0.95) = 50.13 A

It is evident from the above figures that a single module cannot generate required current; therefore number of modules has to be connected in parallel. Suppose a 65 Wp module with Imp = 4 A is selected. Then the number of modules to be connected in parallel, Np, would be (formula 9.2.2):

Np (winter) = 39.27/4 = 9.81, say 10 Np (summer) = 50.13/4 = 12.53, say 13

Now since the system voltage is other than the module voltage, number of parallel-connected module strings is to be parallel-connected in series. The required number of strings in series, Ns is (formula 9.2.3):

Ns = 24 V/12V = 2

Finally the total number of modules, Nt, in the array would be (formula 9.2.4):

Nt (winter) = 10 x 2 = 20 Nt (summer) = 13 x 2 = 26

Here we can decide to use all 26 modules so that the energy demand is met for entire year. This option would mean that we would be spending more money for 6 additional modules that may not be required for 6 winter months. While calculating the array current, it was assumed that the value of peak-sun (4.5) was at average tilt angle equal to the latitude of the site, i.e. 28⁰. Instead of aligning the array in fixed angle, we can make seasonal adjustment of the tilt angle (39⁰ for 6 winter months and 17⁰ for six summer months) to intercept more radiation and subsequently more array output power. Assuming that seasonal adjustments of the tilt angle results in an average of 20% increase in output power, we may come-up with 20% less modules for six summer months. With this assumption we conclude that 22 modules (instead of 26) would meet the load demand of summer months.

Finally the array configuration would be:

string 1

string 2

string 11

Array out Module 1

Module 22

Battery Sizing

For battery sizing it is better to consider the peak load throughout the year. In our case it is the summer load and is equal to 192.5 Ah@24 V. Moreover as the supply is used for hospital we may consider the load to be critical and therefore assume days of autonomy (DOA) to be at least 5. It is evident that the battery bank should again come to the full state of charge some days after continuous discharge for 5 autonomy days. For this to happen, the daily energy supplied by the array should exceed the daily energy consumed.

In other words, the array to load ratio (ALR) of the system should always be greater than 1.

The ALR for summer and winter months can be calculated using the following:

ALR (summer) =

= (11 modules strings x 4 A x 5.5 peak-sun x 0.9 derating factor x 0.95 columbic efficiency)/energy consumed

= 206.9/192.5 = 1.07

In above calculations the peak-sun is considered to be 5.5 instead of design value of 4.5. It is done to take care of the 20% gain in array output power resulting from the seasonal adjustment of the tilt angle to optimum level.

Repeating the same calculations we get ALR (winter) = 206.9/150.8 = 1.37

The relatively low value of ALR (1.07) for summer months is an indication of the fact that loss of load may occur for some time if next session of no-sunshine days follows immediately after the first session of 5 autonomy days. In such case we may advise the client to turn of some of the non-critical loads like TV and fans.

Alternately we may decide to increase the number of modules in the array if the client is not comfortable with first option.

The capacity of the battery bank can be calculated using formula 9.3.1. Assuming the allowable DOD to be 80% (deep cycle batteries are considered) and the charging efficiency to be 0.8, the required battery capacity would be:

C = (192.5 x 5)/(0.8 x 0.8) = 1503.9 Ah, say 1500 Ah.

In the above calculation we have considered the summer load of 192.5 Ah because it is the highest amount of energy drawn from the battery.

Now we may decide to use 12 V block batteries of appropriate capacity (say 100 Ah) or use 2 V cells of higher capacity (say 500 Ah). In the first case the total number of 100Ah, 12 V batteries required is 30. In the second case 36 numbers of 500 Ah, 2V cells are required. It is now up to the designer to select the type of the battery based on the cost issues.

Charge Regulator Sizing

The CR to be used must handle the full short circuit current from the array.

Assuming the short circuit current of the selected module to be 4.6 AP 30, the total maximum charge current would be:

I _array = 11 strings x 4.6 A = 50.6 A

The charge current handling capacity of the selected CR should be greater or equal to 50.6 A. A CR with 60 A charge current handling capacity would meet our requirement.

The maximum possible load current is calculated using formula 9.4.1:

I _lmax = {10 W lamps x 10 nos. + 200 W lamps x 3 nos. + (480 Wh refrigerator/24 hours) + 20 W fans x 10 nos. + 40 W TV x 1 no.} / 24 V system voltage

= 860 W / 24 V = 35.8 A

The load current handling capacity of the selected CR must exceed 35.8 A. A CR with 40 A load current handling capacity would meet our requirement.

Since the system voltage is 24 V, the operating voltage of the selected CR should also be 24V.

Other parameters (e.g. regulation type, regulation algorithm, LVD and HVD set-points, RFI, protections etc.) of the selected CR must be verified using the technical specification of the CR.

Wire Sizing

For wire sizing the NIPQA standard may be followed as detailed in formula 9.5.2.

For the purpose of wire sizing let us assume the following length of wire required for installation:

 Array to load center (allowable voltage drop 3%) - 20 m (cable S1)

 Load center to Battery bank (allowable voltage drop 1%) - 5 m (cable S2)

Then according to formula 9.5.2:

S1 = (0.3 x 20 m x Iarray)/3 = (0.3 x 20 x 50.13)/3 = 100 sq. mm.

S2 = {0.3 x 3 m x Imax (load or array current whichever is the highest)}/1 = = (0.3 x 3 x 50.13)/1 = 45.11 sq.mm.

Since the size of the cable available in the market may not be exactly equal to the size calculated, the cables with cross-sectional area just exceeding the calculated value may be selected.

Wire size of other segments of installation can be calculated in the similar manner.

Inverter Sizing

In the example the only load powered from AC is the 40 W color television.

Assuming that the TV draws three times its nominal current during power ON, the capacity of the selected inverter should be:

P = 40 W x 3 time (surge) / 0.8 power factor = 150 VA.

The input DC supply voltage of the selected inverter should be 24 V DC. Since the load of the inverter is not very sensitive to the wave shape of AC, we may decide to choose high efficient square-wave inverter.

Example 9.8.2

A solar PV power system is to be designed for the operation of VHF telecommunication equipment. Followings are the available data:

 Site location: 28⁰ N (Latitude)

 Minimum yearly average insolation: 4 kWh/m²/day

 Temperature variations: 300C (max.) to 40C (min.)

 Power consumption by VHF set in talk/receive mode: 50 W

 Power consumption by VHF set in stand-by mode: 10 W

 Average talk/receive time (24 hours average): 4 Hours

 System Voltage: -48V DC Load Calculation

The total energy consumed by the VHF set in 24 hours (a day) is:

W = 50 W x 4 hrs. (Talk/receive duration) + 10 W x 20 hrs. (Stand-by mode) = 400 Wh

The total energy consumed in Ah is:

A = 400 Wh / 48 V system voltage = 8.33 Ah

Since the VHF is in operation 24 hours a day and 365 days a year continuously, there is no seasonal variation in load profile.

Array Sizing

Total array current required is:

I _array = A, Ah / (4 peak-sun x 0.9 derating factor x 0.95 efficiency) = 2.43 A A single module with Imp >= 2.43 A can supply required current. Therefore the number of modules to be connected in parallel is one. Let us select a 50 Wp module with Imp = 2.9 A.

The number modules to be connected in series is:

N_s = system voltage/12 = 48/12 = 4

Therefore the array configuration would be just four 50Wp modules connected in series. The array may be tilted to the angle equal to the latitude of the site to intercept optimum radiation throughout the year.

Battery Sizing

Battery sizing for the critical load like telecommunication equipment requires proper assumption of days of autonomy (DOA). To be in safe side we may take DOA to be 7 or even higher. In the first example (hospital) we considered DOA to be 5 only, because in that case we did have the opportunity to turn of some of the non-critical loads like TV and fan in case of continuous no sunshine days for more than 5 days. But in the present example we have only one load and turning it off means no communication at all.

The design value of ALR also becomes vital in critical load case. In the case under consideration, the ALR is:

ALR = 2.9 A x 4 peak-sun x 0.9 x 0.95 / 8.33 = 1.19

This is a comfortable figure and we can expect the batteries to be in normal state of charge after few days from the lapse of design DOA. If more reliability is the concern over the price of the system, higher capacity modules may be selected.

Finally the capacity of the battery (deep cycle) bank is:

C = (8.33 x 7)/ (0.8 x 0.9) = 80.98 Ah.

We therefore select 90 Ah, 12 V deep cycle batteries for this case. Since the system voltage is 48 V, four of these batteries have to be connected in series.

Charge Regulator Sizing

The maximum possible current from the array is the short circuit current. Assuming that the short circuit current of the selected module is 3.4 A, we may select the CR with charge current handling capacity exceeding 3.4 A.

The maximum load current drawn by the VHF set is:

I lmax = 50 W/48 V = 1.04 A.

The load current handling capacity of the selected CR should exceed 1.04 A.

Finally the operating voltage of the selected CR should be 48 V. Moreover since the load is sensitive to the RFI, the selected CR must produce minimum or zero RFI.

Wire Sizing

The required size of the wires can be calculated as in the example 1.

Review Questions

1. Accurate knowledge of the load is essential in PV system design to a. reduce the cost

b. increase the module power c. increase the cost

d. increase the reliability

2. In solar PV system sizing, the load is expressed in a. Amps

b. Watts c. Volts-hours d. Ampere-hours

3. The basic criteria for selecting number of modules to be connected in series in an array is

a. total array power b. load current c. system voltage d. tilt angle

4. The required number of modules in an array is not a function of a. the capacity of an inverter

b. peak sun c. location d. total load

5. While sizing the battery, the required battery capacity decreases with increases in a. number of autonomy days

b. daily load

c. allowable depth of discharge d. system voltage

6. One of the following is not the essential criteria for battery sizing a. type of the battery

b. latitude of location c. surrounding temperature d. cost

7. The rated load current of a charge regulator is not a function of a. total power consumed by appliances

b. system voltage c. array current d. all of above

8. The size of the cable increases with decrease in a. maximum allowable voltage drop b. length

c. maximum permissible current d. cross sectional area

9. The daily load requirement for a remote household is 375 Ah at 48 V system voltages. If the yearly average peak sun in the locality is 6, calculate the total array size and number of module in series and parallel.

10. The distance between the array output and the charge regulator input is 10 meters;

the maximum array current is 50A. Determine the size of the cable in SWG if the maximum permissible voltage drop is 3%.

CHAPTER 10

In document Training Manual for Engineers on Solar PV System (Page 186-196)