S.K.B.P. EE How to Design Solar PV System

What is solar PV system?

Solar photovoltaic system or Solar power system is one of renewable energy system which uses PV modules to convert sunlight into electricity. The electricity generated can be either stored or used directly, fed back into grid line or combined with one or more other electricity generators or more renewable energy source. Solar PV system is very reliable and clean source of electricity that can suit a wide range of applications such as residence, industry, agriculture, livestock, etc.

Major system components

Solar PV system includes different components that should be selected according to your system type, site location and applications. The major components for solar PV system are solar charge controller, inverter, battery bank, auxiliary energy sources and loads (appliances).  •   PV module – converts sunlight into DC electricity.  •   Solar charge controller – regulates the voltage and current coming from the PV panels going to    battery and prevents battery overcharging and prolongs the battery life.  •   Inverter – converts DC output of PV panels or wind turbine into a clean AC current for AC   appliances or fed back into grid line.  •   Battery – stores energy for supplying to electrical appliances when there is a demand.  •   Load – is electrical appliances that connected to solar PV system such as lights, radio, TV, computer, refrigerator, etc.   •   Auxiliary energy sources - is diesel generator or other renewable energy sources.

Solar PV system sizing

1. Determine power consumption demands The first step in designing a solar PV system is to find out the total power and energy consumption of

all loads that need to be supplied by the solar PV system as follows:

     1.1 Calculate total Watt-hours per day for each appliance used.           Add the Watt-hours needed for all appliances together to get the total Watt-hours per day which must be delivered to the appliances.

     1.2 Calculate total Watt-hours per day needed from the PV modules.            Multiply the total appliances Watt-hours per day times 1.3 (the energy lost in the system) to get the total Watt-hours per day which must be provided by the panels.

2. Size the PV modules Different size of PV modules will produce different amount of power. To find out the sizing of PV

module, the total peak watt produced needs. The peak watt (Wp) produced depends on size of the PV module and climate of site location. We have to consider “panel generation factor” which is different in each site location. For Thailand, the panel generation factor is 3.43. To determine the sizing of PV modules, calculate as follows:

    

2.1 Calculate the total Watt-peak rating needed for PV modules           Divide the total Watt-hours per day needed from the PV modules (from item 1.2) by 3.43 to get  the total Watt-peak rating needed for the PV panels needed to operate the appliances.

     2.2 Calculate the number of PV panels for the system           Divide the answer obtained in item 2.1 by the rated output Watt-peak of the PV modules

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available to you. Increase any fractional part of result to the next highest full number and that will be the number of PV modules required.

Result of the calculation is the minimum number of PV panels. If more PV modules are installed, the system will perform better and battery life will be improved. If fewer PV modules are used, the system may not work at all during cloudy periods and battery life will be shortened.

3. Inverter sizing An inverter is used in the system where AC power output is needed. The input rating of the inverter

should never be lower than the total watt of appliances. The inverter must have the same nominal voltage as your battery.

For stand-alone systems, the inverter must be large enough to handle the total amount of Watts you will be using at one time. The inverter size should be 25-30% bigger than total Watts of appliances. In case of appliance type is motor or compressor then inverter size should be minimum 3 times the capacity of those appliances and must be added to the inverter capacity to handle surge current during starting.

For grid tie systems or grid connected systems, the input rating of the inverter should be same as PV array rating to allow for safe and efficient operation.

4. Battery sizing The battery type recommended for using in solar PV system is deep cycle battery. Deep cycle battery

is specifically designed for to be discharged to low energy level and rapid recharged or cycle charged and discharged day after day for years. The battery should be large enough to store sufficient energy to operate the appliances at night and cloudy days. To find out the size of battery, calculate as follows:

  4.1 Calculate total Watt-hours per day used by appliances.     4.2 Divide the total Watt-hours per day used by 0.85 for battery loss.     4.3 Divide the answer obtained in item 4.2 by 0.6 for depth of discharge.     4.4 Divide the answer obtained in item 4.3 by the nominal battery voltage.     4.5 Multiply the answer obtained in item 4.4 with days of autonomy (the number of days that you need the system to operate when there is no power produced by PV panels) to get the required            Ampere-hour capacity of deep-cycle battery.

Battery Capacity (Ah) = Total Watt-hours per day used by appliances x Days of autonomy(0.85 x 0.6 x nominal battery voltage)

5. Solar charge controller sizing The solar charge controller is typically rated against Amperage and Voltage capacities. Select the

solar charge controller to match the voltage of PV array and batteries and then identify which type of solar charge controller is right for your application. Make sure that solar charge controller has enough capacity to handle the current from PV array.

For the series charge controller type, the sizing of controller depends on the total PV input current which is delivered to the controller and also depends on PV panel configuration (series or parallel configuration).

According to standard practice, the sizing of solar charge controller is to take the short circuit current (Isc) of the PV array, and multiply it by 1.3Solar charge controller rating = Total short circuit current of PV array x 1.3

Remark: For MPPT charge controller sizing will be different. (See Basics of MPPT Charge Controller)

Example: A house has the following electrical appliance usage:

One 18 Watt fluorescent lamp with electronic ballast used 4 hours per day.

S.K.B.P. EE One 60 Watt fan used for 2 hours per day. One 75 Watt refrigerator that runs 24 hours per day with compressor run 12 hours and

off 12 hours. The system will be powered by 12 Vdc, 110 Wp PV module.

1. Determine power consumption demands

Total appliance use = (18 W x 4 hours) + (60 W x 2 hours) + (75 W x 24 x 0.5 hours)  = 1,092 Wh/dayTotal PV panels energy needed  = 1,092 x 1.3  = 1,419.6 Wh/day.

2. Size the PV panel

2.1 Total Wp of PV panel capacity needed

= 1,419.6 / 3.4

  = 413.9 Wp2.2  Number of PV panels needed = 413.9 / 110  = 3.76 modules

                                                                    Actual requirement = 4 modules          So this system should be powered by at least 4 modules of 110 Wp PV module.

3. Inverter sizing    Total Watt of all appliances = 18 + 60 + 75 = 153 W    For safety, the inverter should be considered 25-30% bigger size.     The inverter size should be about 190 W or greater.

4. Battery sizing    Total appliances use = (18 W x 4 hours) + (60 W x 2 hours) + (75 W x 12 hours)    Nominal battery voltage = 12 V    Days of autonomy = 3 days

    Battery capacity = [(18 W x 4 hours) + (60 W x 2 hours) + (75 W x 12 hours)] x 3                                                 (0.85 x 0.6 x 12)    Total Ampere-hours required 535.29 Ah    So the battery should be rated 12 V 600 Ah for 3 day autonomy.

5. Solar charge controller sizing    PV module specification    Pm = 110 Wp    Vm = 16.7 Vdc    Im = 6.6 A    Voc = 20.7 A    Isc = 7.5 A    Solar charge controller rating = (4 strings x 7.5 A) x 1.3 = 39 A    So the solar charge controller should be rated 40 A at 12 V or greater.

3-Core CablesThese cables are used generally for a perfect balanced 3-phase system. When the currents on the 3-live wires

S.K.B.P. EE of a 3-phase system are equal and at an exact 120° phase angle, then the system is said to be balanced. The 3-phase loads are identical in all respects with no need of a neutral conductor.An important example of 3-phase load is electric motor and that is why, they are fed through 3-Core cables in most cases.3.5-Core CablesA 3-phase system may have a neutral wire. This wire allows the 3-phase system to be used at higher voltages while it will still support lower voltage single phase loads.It is not likely in such cases that the loads will be identical, so the neutral will carry the out-of-balance current of the system. The greater the degree of imbalance, the larger the neutral current.When there is some degree of unbalance and the amount of fault current is very small, then 3.5 core cables are used. In these types of cables, a neutral of reduced cross section as compared to the 3-main conductors is used, which is used to carry the small amount of unbalanced currents.4-Core CablesWhen there is severe out-of-balance conditions, the amount of fault current will raise to a very high level. Generally in the case of linear loads, the neutral only carries the current due to imbalance between the phases.The non-linear loads such as switch-mode power supplies, computers, office equipment, lamp ballasts and transformers on low loads produce third order harmonic currents which are in the phase of all the supply phases.These currents do not cancel at the star point of a three-phase system as do normal frequency currents, but add up, so that the neutral carries very heavy third harmonic currents

THE 3.5 CORE CABLE IS POWER CABLE (NORMALLY HIGH LOAD).-3 LEADS WILL BE OF SAME SQ.MM-1 LEADS WILL BE,HALF OF THE ABOVE 3 LEADSTHE 4 CORE CABLE CAN BE USED FOR BOTH POWER AND CONTROL CABLE(NORMALLY LESS LOAD)-4 CORE WILL BE IN SAME SQ.MM

Neutral conductor must be at least 1/2 the size of other conductor.It is theoraticaly explained , that algebric sum of all the phases when in balanced condition then neutral current will be zero.

Now suppose one phase fail i.e. total unbalanced condition its neutral conductor will provide a return path and its current will be half the load on other phases so by this result on neutral conductor for every unbalanced condition its current will not exceed

the rated value (same as in phase conductors).

The selection of neutral conductor depends on the nature of loads, such as the load harmonics (more than 20% loads are electronic) or the load balancing (most of the loads are 3 phase motor) etc, etc.

So, there are few general cases to consider here:1. If the loads are almost balanced and there is no significant electronic loads, you can select 3.5 (3 + 0.5)

2. If the loads are not balanced and there is no significant electronic loads, you can select 4 (3 + 1)

3. If the loads are balanced and there is significant electronic loads, you can also select 4 (3 + 1)

4. If the loads are not balanced and there is significant electronic loads, you need to select even higher size for neutral (3 + 1.5)

With modern equipment it is getting to be a major problem especially with IT gear.Some cable manufacturers can now supply cable with a neutral 1.5 X phase conductor.

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Structured Design Process //

To achieve the best overall outcome in a lighting installation, it is important to avoid the tendency of rushing straight into luminaire selection before determining more broadly what is required from the system. The use of a structured design process helps to avoid this.

The key steps in the design process are:

1. Identify the requirements2. Determine the method of lighting3. Select the lighting equipment4. Calculate the lighting parameters and adjust the design as required5. Determine the control system6. Choice of luminaire7. Inspect the installation upon completion

(and if possible, a few months after occupation, to determine what worked and what didn’t. This is the only way to build up experience to apply to future designs)

The five initial stages are considered in more detail in the following lines.

1. Identifying the requirements

This involves gaining a full understanding of what the lighting installation is intended to achieve. This includes the following:

Task Requirements ? Illuminance Glare

Mood of the space Relation to shape of space Things to be emphasised Things to hide Direction of light Interaction of daylight

2. Determine the method of lighting

At this stage, consideration is given to how the light is to be delivered, e.g. will it be recessed, surface mounted, direct or indirect, or will up-lighting be used, and its primary characteristics, e.g. will it be prismatic, low brightness or mellow light.

Consideration should be given at this stage to the use of daylight to minimise the need for artificial light

3. Select the lighting equipment

Once the method of lighting has been selected, the most appropriate light source can then be chosen followed by the luminaire.

The following attributes should be studied when choosing the light source:

S.K.B.P. EE Light output (lumens) Total input wattage Efficacy (lumens per Watt) Lifetime Physical size Surface brightness / glare Colour characteristics Electrical characteristics Requirement for control gear Compatibility with existing electrical system Suitability for the operating environment

A number of factors also affect luminaire choice:

Characteristics of the light source and control gear Luminaire efficiency (% lamp light output transmitted out of the fixture) Light distribution Glare control Finish and appearance Size Accessibility of components for maintenance Ability to handle adverse operating conditions Aesthetics Thermal management

4. Calculate the lighting parameters

Lighting calculation methods fall into three broad categories:

1. Manual calculation methods2. Three dimensional modelling3. VisualisationPhotometric data for light sources and luminaires is commercially available to contribute to these calculations

4.1 Manual calculation methods

There are a wide range of manual computation methods for the calculation of different lighting aspects. These include complex methods for calculating the illuminance from a wide variety of shapes of luminous objects. The majority of these have now been superseded by computer programs (check our free software ).

The Lumen Method was the mainstay for interior lighting and has remained in use as a quick and relatively accurate method of calculating interior illuminance .

The Lumen Method calculates the average illuminance at a specific level in the space, including an allowance for the light reflected from the interior surfaces of the room. The calculation method has a set of assumptions that, if followed, gives a reasonable visual environment.Inadequate attention to the assumptions will produce poor results.

The basic assumptions are:

All the luminaires in the room are the same and have the same orientation The luminaires do not have a directional distribution and are aimed directly to the floor The luminaires are arranged in a uniform array on the ceiling and have the same mounting height

S.K.B.P. EE The luminaires are spaced less than the maximum spacing to mounting height ratio nominated in

the coefficient of utilisation tablesThe average illuminance produced by a lighting installation, or the number of luminaires required to achieve a specific average illuminance, can be calculated by means of utilization factors (UF), a UF being the ratio of the total flux received by a particular surface to the total lamp flux of the installation.

Lumen method formula 

The average illuminance E(h) over a reference surface s can be calculated from the “lumen method” formula.

where:

F – the initial bare lamp flux (lumens) n – the number of lamps per luminaire N – the number of luminaires LLF – the total light loss factor UF(s) – the utilization factor for the reference surface s of the chosen luminaire

Utilization factors can be determined for any surface or layout of luminaires. The “UF” symbol is normally shown followed by an extra letter in brackets, to denote the surface, for example,  UF(F) is the utilisation factor for the floor cavity and UF(W) is the utilisation factor for the walls.

Utilization factors are, in practice, only calculated for general lighting systems with regular arrays of luminaires and for three main room surfaces. The highest of these surfaces, the C surface (for ceiling cavity), is an imaginary horizontal plane at the level of the luminaires having a reflectance equal to that of the ceiling cavity.The lowest surface, the F surface (for floor Cavity), is a horizontal plane at normal working height (i.e. table height), which is often assumed to be 0.85 m above the floor.

The middle surface, the W surface (for walls), consists of all the walls between the C and F planes.

Although the lighting designer  can calculate utilization factors, lighting companies publish utilization factors for standard conditions for their luminaires. The standard method of presentation is shown below. To use this table, it is only necessary to know the Room Index and the effective reflectance of the three standard surfaces (floor cavity, walls and ceiling cavity).

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Room Index

The Room Index is a measure of the angular size of the room, and is the ratio of the sum of the plan areas of the F and C surfaces to the area of the W surface. For rectangular rooms the room index is given by:

Where:

L – the length of the room W – the width of the room Hm – the height of the luminaire plane above the horizontal reference plane If the room is re-entrant in shape, for example L shaped, then it must be divided into two or more

non-re- entrant sections, which can be treated separately. Spacing to Mounting Height Ratio (SHR)

The Spacing to Mounting Height Ratio (SHR) is the spacing between luminaires divided by their height above the horizontal reference plane.

It affects the uniformity of illuminance on that plane. When the UF tables are determined, for a nominal spacing to height ratio SHR NOM, the maximum spacing to height ratio SHR MAX of the luminaire is also calculated, and is a value that should not be exceeded if the uniformity is to be acceptable.

Three dimensional modelling

S.K.B.P. EE Although it was possible to calculate the luminance of all the surfaces in a room, the calculations were extremely laborious and could only be justified in the most special cases. However, the advent of computer modelling enabled a more flexible approach to lighting design and significantly increased the information available to the designer.

In contrast to the Lumen Method, lighting programs enable the lighting designer to broaden the assumptions:

A mixture of luminaires can be used The luminaires no longer have to be arranged in a regular array Directional luminaires can be modelled A large number of calculation points can be considered to give a meaningful uniformity calculation The illuminance and luminance of all surfaces can be calculate

This gives the lighting designer a much greater understanding of what is happening in the room

However there has been considerable research, experience and documentation over the past 80 years that has developed the current thinking in the adequacy of various illuminance levels for various tasks and functions.

Although there is some general understanding of the need for appropriate luminance distribution in the vertical plane, there is little information, experience or understanding for many designers to determine:

What the luminance of surfaces should be in varying situations What is an acceptable luminance uniformity Whether there should there be a maximum luminance uniformity What is the desired graduation in luminance At what point is the luminance distribution of the wall unacceptable

It is important in using a lighting calculation program that the output records the type of luminaire used, the location of the luminaires, the assumed lumen output of the lamp, the light loss factor and the aiming points. If this is not recorded you have a pretty picture of the installation and no way of making it a reality.

4.3 Visualisation

These are programs that create a perspective rendering of the space in levels of detail that vary from a block representation of the space, to photographic quality renderings, depending on the sophistication of the program and the level of detail of the interior to be entered.

The programs fall into two basic types:

Flux transfer or radiosity calculations Ray tracing calculations

S.K.B.P. EE The major difference being in how they interpret light from reflective surfaces.

A Lambertian surface is a perfect diffuser, where light is reflected in all directions, irrespective of the angle of incidence of the light such that irrespective of the viewing angle the surface has the same luminance. A specular surface is a mirror like surface, where the angle of reflection of the light is the same as the angle of incidence.

Left: Lambertian surface; Middle: Specular surface; Right: Semi-specular surface

A real life surface is a combination of both surfaces (semi-specular) and has both specular and diffuse characteristics. Some materials are more specular while others are more diffuse.

A flux transfer or radiosity program treats all surfaces as diffuse or Lambertian surfaces, as a result their rendering tends to appear flat with soft shadow details. It will tend to overestimate the uniformity. Ray tracing traces the individual rays of light from the source to the eye as it reflects from surface to surface around the room. As a result ray tracing can allow for the specular component of the surfaces.

Some programs calculate the entire lighting by ray tracing while others calculate the space on a flux transfer basis and have an overlay of ray tracing of specific areas to improve the quality of the rendering. When ray tracing is added, reflections are added in polished surfaces and shadows become sharper.

Visualisation programs are a useful tool in the presentation of a design, as a tool for the designer to check that the design is consistent with his own visualisation of the space, and to model specific lighting solutions. The programs are still calculation tools and not design programs.The programs can show the designer how a specific design will perform but that they cannot reliably be used to assess the acceptability of a design.

Irrespective of the form of the visualisation output, it is important that the program provides adequate information to enable the construction and verification of the lighting design.

The output should include:

Installation information  – the type and location of all luminaires and the aiming information. The lamp details should be included as well as the specific catalogue number of photometric file that has been used.

Light technical parameters  – the illuminance, uniformity and other parameters that have been calculated to achieve the design.

Verification information  – adequate details to enable the lighting calculation to be verified. This should include the luminaire type, the photometric file, surface reflectances that were assumed, light loss factors, lumen output of lamps and mounting and aiming locations.

Determine the control system

The effectiveness and efficiency of any lighting installation is affected as much by the control system as by the light sources and fixtures chosen.

S.K.B.P. EE Give consideration to:

Providing multiple switches to control the number of lights that come on at any one time. Using one switch to turn on all the lights in a large room is very inefficient.

Placing switches at the exits from rooms and using two-way switching to encourage lights to be turned off when leaving the room.

Using ‘smart’ light switches and fittings which use movement sensors to turn lights on and off automatically. These are useful in rooms used infrequently where lights may be left on by mistake, or for the elderly and disabled.

Make sure they have a built-in daylight sensor so that the light doesn’t turn on unnecessarily. Models which must be turned on manually and turn off automatically, but with a manual over-ride, are preferable in most situations. Be aware that the sensors use some power continuously,  up to 5W or even 10W in some cases.

Using timers, daylight controls and motion sensors to switch outdoor security lights on and off automatically. controls are particularly useful for common areas, such as hallways, corridors and stairwells, in multi-unit housing.

Using solar powered lighting for garden and security lights.

Using dimmer controls for incandescent lights (including halogens). This can save energy and also increase bulb life. Most standard fluorescent lamps cannot be dimmed, but special dimmers and lamps are available. If lamps are to be dimmed it is important to ensure that the correct equipment is used, especially when retrofitting more energy efficient lamps.

Choice of Luminaire

The performance of a luminaire should be considered just as carefully as its cost. In the long term a well designed, well constructed luminaire will be cheaper than a poor quality unit; and the salient features of a good quality luminaire are:

Sound mechanical and electrical construction and a durable finish Adequate screening of high luminance lamps to minimise discomfort and glare Adequate heat dissipation to prevent over-heating of the lamp, wiring and ancillary equipment High light output ratio with the appropriate light distribution Ease of installation, cleaning and maintenance

input informationThese are the input data for the following calculation:1. An office area has length: 20 meter; width: 10 meter; height: 3 meter.2. The ceiling to desk height is 2 meters.3. The area is to be illuminated to a general level of 250 lux using twin lamp 32 watt CFL luminaires with

a SHR of 1.25.4. Each lamp has an initial output (Efficiency ) of 85 lumen per watt.5. The lamps Maintenance factor (MF) is 0.63 ,Utilization Factor is 0.69 and space height ratio (SHR)

is 1.25.

Calculation

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1. Total wattage of fixtures:

Total wattage of fixtures = Number of lamps x each lamp’s watt.

Total wattage of fixtures = 2 × 32 = 64 Watt.

2. Lumen per fixtures

Lumen per fixtures = Lumen efficiency (Lumen per Watt) x each fixture’s watt

Lumen per fixtures = 85 x 64 = 5440 Lumen

3. Number of fixturesRequired number of fixtures = Required Lux x Room area / MF x UF x Lumen per fixture

Required number of fixtures = (250 x 20 x 10) / (0.63 × 0.69 × 5440)

We will need 21 fixtures

4. Minimum spacing between each fixtureThe ceiling to desk height is 2 meters and space height ratio is 1.25, so:

Maximum spacing between fixtures = 2 × 1.25 = 2.25 meter.

5. Number of required rows of fixtures along with width of roomNumber of rows required = Width of room / Max. spacing = 10 / 2.25

Number of rows required is therefore = 4.

6. Number of fixtures required in each rowNumber of fixtures required in each row = Total Fixtures / Number of rows = 21 / 4

Therefore, we have 5 fixtures in each row.

7. Axial spacing between each fixture:

Axial spacing between fixtures = Length of room / Number of fixtures in each row

S.K.B.P. EE … and that would be: 20 / 5 = 4 Meter

8. Transverse spacing between each fixture:Transverse spacing between fixtures = Width of room / Number of fixtures in row

… and that would be: 10 / 4 = 2.5 Meter.

Number of rows with lighting fixtures = 4 Number of lighting fixtures in each row = 5 Axial spacing between fixtures = 4.0 meter Transverse spacing between fixtures = 2.5 meter Required total number of fixtures = 21

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