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Best Practice Guidelines for Hybrid Systems

Hybrid Systems Comprising

Solar PV and Fuel Fired Generators

Best Practice Guidelines

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Best Practice Guidelines for Hybrid PV Systems

Table of ContentsGlossaryiiIntroduction1Part 1: System Design21.1ENERGY SOURCE MATCHING21.2ENERGY EFFICIENCY21.3STANDARDS FOR DESIGN21.4 OVERVIEW OF SYSTEM DESIGN51.5 LOAD (ENERGY) ASSESSMENT51.6HYBRID SYSTEM DESIGN METHODOLOGY61.6.1Load Assessment and Battery determination61.6.2PV ARRAY SIZING: d.c. Bus - Standard Switched Controllers101.6.3 PV ARRAY SIZING: d.c. BUS - MPPT141.6.4PV Array Sizing: a.c. Bus - Loads Supplied Directly by Array181.6.5PV Array Sizing: a.c. BUS - Loads Supplied by Battery191.6.6PV ARRAY SIZING: A.C. BUS201.6.7 Battery Inverter Selection211.6.8Grid Connect Inverter for a.c. Bus system221.6.8 Battery Charger261.6.8 Selecting and Sizing Fuel fired Generator271.6.9 Fuel Generator Control Strategies321.6.10 Summary of Losses in a Hybrid System331.7 CABLE SELECTION-ARRAY361.7.1 Sub-array371.8 CABLE PROTECTION: Array371.9 CABLE SELECTION-Voltage Drop391.10 Main Battery Cable: Selection and Protection421.11 PLUGS and SOCKETS431.12DC Switch disconnector at controller432.1STANDARDS for INSTALLATION452.2DOCUMENTATION472.3 PV MODULES472.4 PV ARRAY482.5 OUTDOOR MOUNTED COMBINER BOXES502.6 PV ARRAY SWITCH DISCONNECTOR512.7CONTROLLER INSTALLATION512.8INVERTER INSTALLATION512.9 BATTERY INSTALLATION512.10VENTILATION OF BATTERIES542.11PREVENTING SPARK IGNITION SOURCES NEAR BATTERIES552.12PREVENTING EXCESSIVE CURRENT FROM BATTERIES562.13 BATTERY SAFETY AND WARNING SIGNS562.14 GENERATOR INSTALLATION562.15CABLE INSTALLATION572.16WIRING OF ARRAYS WITH VOLTAGE ABOVE DVC-A582.17 WIRING FROM ARRAYS WITH VOLTAGES ABOVE DVC-A TO PV ARRAY SWITCH-DISCONNECTOR NEAR CONTROLLER592.18EARTHING OF ARRAY FRAMES (PROTECTIVE EARTH/GROUND)592.19 SIGNAGE612.20 COMMISSIONING62Part 3: System Maintenance633.1 PV Array Maintenance63Visual Inspection63Mounting System Inspection63Module Cleaning633.2 Controller Maintenance643.3 Battery Bank643.4 Inverter Maintenance (Battery and Grid Connected)653.5 Balance of System Maintenance653.6 Fuel Generator maintenance66Annex 1: System Installation Checklist67Annex 2: Testing and Commissioning Checklist69Annex 3: Table showing Peak Sun hours for various sites and tilt angles70Annex 4: Tilt and Orientation diagram for 1N.71Annex 5: System Design72DC bus system76AC bus system design79

Glossary

a.c. bus system

A PV system, ( stand alone or hybrid, where the PV array is connected to the a.c. side of the system, i.e via a grid connect inverter

Alternating Current (AC)

The form of electricity in which the polarity of the current is periodically reversed.

Altitude

The height of the Sun from the horizon.

Ampere-hours

The unit used to measure electrical energy and to indicate the storage capacity in batteries. Symbol, Ah. To convert Ah to kWh multiply the Ah by the voltage then divide by 1000.

Azimuth

The eastwest position of the Sun. The solar industry standard is to express azimuth clockwise from true north (0360); however it can also be quoted with a direction, east or west (i.e.0180E or 0180W).

Battery

Electrical energy storage device. Two or more electrochemical cells electrically connected together in a series or parallel combination to provide the required operating voltage and current levels.

Battery Capacity

The maximum total electrical charge, expressed in ampere-hours (Ah) that a battery could deliver to a load under a specific set of conditions. The battery capacity depends on the rate at which the charge is given up by the battery.

Battery Charging Voltage

The voltage specified as that required to charge batteries. For 12V batteries the final battery charging voltage is around 14V and photovoltaic modules are often rated at 14V so that the charging current is known at that voltage.

Charging Rate

The current applied to a battery or cell to restore its capacity. The charging rate is specified by the CX where x is the number of hours to bring the battery to full charge, e.g. if a battery with a capacity of 100 Ah is charged at the C5 rate, it is brought to full charge in 5 hours and 20 Amperes would be the charging current.

Central inverter

Usually a collection of small inverters centrally located in one housing.

Circuit

A circuit is the path that current flows from one charged point to another.

Circuit breaker

A mechanical device which will open a circuit under fault conditions. When too much current passes through, the device will open and prevent current flow. The circuit breaker can then be manually operated to close the circuit.

Combiner box

An electrical component for combining and housing the wiring from the PV array.

Current

The rate of flow of electrical charge is the net transfer of electrical charge per unit time. The unit of current is the Ampere (A). In electric circuits the current is referred to by the symbol (I).

Daily Demand or Daily Load

This is the energy requirement calculated on a daily basis. This load varies from Day to day and at different times of the year. In sizing a system the average daily demand over the whole of the year is often used. Units can be Wh, kWh, or Ah.

Days of Autonomy

The number of days, without input from the energy source, for which the battery storage system must supply electrical energy.

d.c. bus system

A PV system (stand alone or hybrid) where the PV array is connected to the d.c. side of the system, i.e. batteries via a controller

Decisive voltage classification (DVC)

A classification system for different voltage ranges.

Depth of Discharge (DOD)

The Ahs removed from a cell or battery expressed as a percentage of the rated capacity, e.g. the removal of 25 Ah from a fully charged 100 Ah rated battery results in a 25% depth of discharge.

Direct Current (DC)

Electricity in which the current always moves in the same direction.

Discharge Rate

The current removed from a cell or battery. Battery manufacturers refer to the rate of discharge not by Amps but by the time it would take to completely discharge the battery. For example, a battery which has a rated capacity of 100 Ah is subjected to a current withdrawal of 5 A, it would take 20 hours to completely discharge the battery. In this case, we say that the battery is discharging at the 20-hour rate or at C20.

Disconnector

A mechanical switching device which provides, in the open position, an isolating distance in accordance with specified requirements.

Efficiency

The ratio of energy (power) produced by a device to the energy (power) consumed by the same device: this is a number less than or equal to unity.

Electrolyte

A non-metallic electrical conductor in which current is carried by the movement of ions. In lead acid batteries, the electrolyte is an aqueous solution of sulphuric acid.

Energy

The amount of electric energy transferred: a product of power and time. Energy is measured in watt hours (Wh) and is calculated using: E = P x t

Equalisation

The process of restoring all cells in a battery to an equal state of charge. In a lead acid battery, this is done by using a charging voltage of around 2.5 Volts per cell.

Equipotential bonding

Equipotential bonding (or protective earthing) involves electrically connecting earthed, conductive metalwork so that it is at the same voltage (potential) as earth throughout. This is required for safety reasons to protect people from electric shocks.

Fuel Generator

A device that converts mechanical energy into electrical energy to supplement the renewable energy source in a Hybrid system. The generator can be powered by petrol, diesel, LP gas or steam.

Functional earthing

Functional earthing is designed to ensure optimal performance of the PV array, but it is required only if specified by the manufacturer.

Fuse

A device that protects conductors from excessive current. The fuse is rated to carry a certain current, and when this current is exceeded the fuse will open the circuit (by melting).

Hybrid System

Technically any system that includes two types of generators is a hybrid system. Throughout this document, the term Hybrid Systems refers to a system comprising solar PV and fuel fired generator.

Irradiance

The total amount of solar radiation available at any point in time per unit area and is measured in W/m2 or kW/m2. It is a measure of power.

Irradiation

The total amount of solar radiation available per unit area over a specified time period such as one day. It is the sum of irradiance values over a time period and is often measured in kWh/m2/year or MJ/m2/day. It is a measure of energy.

Junction box

A box containing a junction of electric wires or cables.

Kilowatt peak (kWp)

This is a non-SI unit used in the solar industry to describe the nominal power of a solar PV system: it refers to the peak output under standard test conditions.

Low Voltage (LV)

Electrical systems that operate over 120 V DC (ripple free) or 50 V AC. Systems of these voltages require an electrical license to operate or install.

Maximum Power Point (MPP)

The MPP is thepoint on the IV curve that gives the maximum power. It occurs when the load resistance is equal to the internal resistance of thePV cell.

Maximum Power Point Tracker (MPPT)

An electronic device included within the inverter that alters the PV arrays electrical output so that it is performing at the maximum power possible at any given time.

Module inverter

An inverter designed to be mounted on the back of a module.

Module efficiency

The amount of electrical power produced by the module per amount of light energy hitting the module. This is typically lower than cell efficiency due to losses from reflection from glass etc.

Monocrystalline solar cells

The most efficient and most expensive solar cells. They have a smooth monochromatic appearance.

Multicrystalline solar cells (polycrystalline)

Solar cells, less efficient than monocrystalline, but cheaper to make and buy. They have a glittering effect when in sunlight.

Multi-string inverter

An inverter with multiple MPPTs (e.g. one MPPT per string).

Nominal Operating Cell Temperature (NOCT)

The photovoltaic cell junction temperature corresponding to nominal operating conditions in a standard reference environment of 800 W/m2 irradiance, 20 C ambient air temperature, 1 m/s wind speed and electrically open circuit.

Open Circuit

An open circuit is where the current path is broken so that the current is equal to 0.

Peak Sun Hours (PSH)

A unit of energy used in the solar industry when measuring irradiation. 1PSH = 1kW of solar energy falling on a surface of 1m2 for 1hour.

Photovoltaic (PV)

A device that creates electricity when sunlight hits its surface.

Power

Power is the rate at which electrical energy is transferred. Power is measured in watts (W), and is calculated using P = V x I

PV array

Strings of PV modules are electrically connected in parallel to form an array. Also called a solar array.

PV cell

A single PV device. Also called a solar cell.

PV module

PV cells are physically and electrically connected to form a PV module. These cells are held together by a frame and covered by a protective substance such as glass. Also called a solar module.

PV string

When PV modules are connected in series they form a string.

PV sub-array

Very large PV arrays are often made up of many smaller PV arrays known as sub-arrays.

PV system

The PV array and all associated equipment required for it to work. Also called a solar electric system.

Regulators

Devices used to control the charging current to a battery to make sure that the battery is not overcharged.

Resistance

The opposition to current and is measured in ohms ().

Root mean square (RMS)

RMS is how AC power is usually quoted; for example: VRMS = 0.707 VP ... IRMS = 0.707 IP

Self Discharge Rate

The rate at which a battery discharges when it is idle. As a battery ages its self-discharge rate generally increases.

Short Circuit

A short circuit is where the current is flowing in a closed path across the source terminals.

Solar Altitude (Elevation) Angle

The angle between the sun and the horizon. This angle is always between 0 and 90.

Solar Azimuth Angle

The angle between north and the point on the compass where the sun is positioned on a horizontal plane. The azimuth angle varies as the sun moves from east to west across the sky throughout the day. In general, the azimuth is measured clockwise going from 0 (true north) to 359.

Solar Cell

A small photovoltaic unit that generates an electrical current when hit by sunlight.

Solar Modules

See PV module.

Solar radiation

Energy coming from the Sun.

Specific Gravity

This is the ratio of the density of the battery electrolyte with respect to water. Hydrometers, which sample the electrolyte, are used to monitor the state of charge of the batteries.

Standard Test Conditions (STC)

Standardised test conditions that make it possible to conduct uniform comparisons of PV modules by different manufacturers.

State of Charge

The available capacity in a cell or battery expressed as a percentage of rated capacity.

String inverter

An inverter with only one MPPT.

Surge capacity

The capacity of an inverter to deliver power at a higher rate than its rated power output for given short periods (measured in seconds) of time.

Switch-disconnector

A mechanical switching device capable of making, carrying and breaking currents in normal circuit conditions and, when specified in given operating overload conditions. In addition, it is able to carry, for a specified time, currents under specified abnormal circuit conditions, such as short-circuit conditions. Moreover, it complies with the requirements for a disconnector.

Temperature Correction Factor (Batteries)

The amount by which the capacity of a battery should be adjusted to account for changes in temperature. Battery capacities are usually given at a reference temperature of 25 C.

Temperature Correction Factor (Solar Cells)

The amount by which the voltage, current or power from a solar cell will change with changes in the temperature of the cell.

Thin film solar cells

Made from materials that are suitable for deposition on large surfaces such as glass. Very thin in comparison to monocrystalline and multicrystalline solar cells. Least efficient technology, but the cheapest to manufacture.

Voltage

Tthe potential difference between two points. Voltage is measured in volts (V).

Best Practice Guidelines for Hybrid Systems

vii

Introduction

This set of guidelines sets out best practice for the design, installation andmaintenance of Hybrid Systems comprising solar PV and fuel fired generators.. These systems are typically installed in areas where the grid is not available however they be installed in conjunction with fuel generator based mini-grids e.g. in remote villages. However these guidelines could also be applied to systems that are used as a back-up system for a building, which is connected to the grid and the client wants a fuel generator available in periods of long grid outages.. The output of such a system either feeds directly to circuits, which are not connected to the grid or connects to circuits during a grid failure via a changeover switch.

The guidelines cover following system types:

Hybrid systems comprising solar PV (d.c. bus) and fuel fired generators.

Hybrid systems comprising solar PV (a.c. bus) and fuel fired generators

The guidelines are divided into three parts as follows:

Part 1: System Design

Part 2: System Installation

Part 3: System Maintenance

System Design outlines the best practice processes undertaken when determining why a system is required and then selecting and matching the equipment that will be part of a hybrid system comprising solar PV and fuel fired generator to meet specified design requirements. It provides the formulas used to match components and for determining the energy output of the system. Once the modules, controllers, batteries, inverter and fuel generator have been determined, international and local standards are applied to determine and select the balance of system requirements such as cables, isolation and protection devices.

System Installation outlines the best practices when following international and local standards to install the hybrid system comprising solar PV and fuel fired generator

System Maintenance outlines the best practices that should be employed on maintaining the hybrid system comprising solar PV and fuel fired generator to prolong the lifetime of the system as well as to reduce system losses.

Note:

1. Throughout the rest of these guidelines, the term Hybrid Systems will be used and it refers to a system comprising solar PV with fuel fired generator.

2. Lithium Ion batteries are not covered in this guideline because currently there are international standards with respect to their installation

Part 1: System Design

This set of guidelines provides the minimum knowledge required to design a Hybrid system using the d.c. and a.c. bus configurations. A hybrid system will be designed:

Either as a d.c. bus system with fuel generator supplying a.c. loads.;

Or as an a.c. bus system with fuel generator supplying a.c. loads. .

or as a combination of both

The design of any Hybrid system should consider the electrical load as well as a number of criteria, including:

Budget

Power quality

Environmental impact

Aesthetics

Site accessibility

fuel availability

Having applied the comprehensive design criteria, a designer shall use this information to:

Determine the size of the PV array (kWp) to meet the daily energy requirements;

Select the appropriate system d.c. voltage;

Determine the controller size and type based on the size of the array;

Determine the battery capacity;

Determine the fuel generator capacity and its run times.

Determine all the balance of equipment

1.1ENERGY SOURCE MATCHING

Heating and lighting should be supplied from the most appropriate source. For example

Cooking - gas or wood burning stove or wood burning/charcoal etc.

Water heating - solar water heating with gas or wood backup

Lighting - electrical lighting most often used but natural light (day lighting) should be considered.

1.2ENERGY EFFICIENCY

All appliances should be chosen for the lowest possible energy consumption for each desired outcome, such as

High efficiency lighting

Energy efficient refrigeration

1.3STANDARDS FOR DESIGN

System designs should follow any standards that are typically applied in the country or region where the solar installation will occur. The following are the relevant standards for Kenya. These standards are often updated and amended so the latest version should always be applied.

The following Kenyan standards are applicable:

KS 1672Glossary of terms and symbols for solar photovoltaic power generation

KS 1673-1:2003Solar photovoltaic power systems-design, installation, operation, monitoring and maintenance-code of practice .Part 1:General PV

KS 1673-2Generic specification for solar photovoltaic systems system design, installation, operations, monitoring and maintenance

KS 1674Crystalline silicon terrestrial photovoltaic (PV) modules design qualification and type approval

KS 1675 Thin-film terrestrial photovoltaic (PV) modules design qualification and type approval

KS 1676 Terrestrial photovoltaic (:PV) power generating systems general and guide

KS 1677Procedures for temperature and irradiance corrections to measured I-V characteristics of crystalline silicon photovoltaic devices

KS 1678Photovoltaic devices

KS 1679 UV test for photovoltaic (PV) modules

KS 1680 Overvoltage protection for photovoltaic (PV) power generating systems guide

KS 1681Characteristic parameters of hybrid photovoltaic systems

KS 1682 Salt mist corrosion testing of photovoltaic (PV) modules

KS 1683Rating of direct coupled photovoltaic (PV) pumping systems

KS 1684 Susceptibility of a photovoltaic (PV) module to accidental impact damage (resistance to impact test)

KS 1685Photovoltaic system performance monitoring guidelines for measurement, data exchange and analysis

KS 1686 Analytical expression for daily solar profiles

KS 1709-1:2009Batteries for use in photovoltaic power systems - Specification Part 1: General requirements.

KS 1709-2:2009Batteries for use in photovoltaic power systems - Specification Part 2: Modified lead acid batteries

KS 1709-4:2009Batteries for use in photovoltaic power systems - Specification Part 4: Recommended practice for sizing lead acid batteries for photovoltaic (PV) systems.

KS IEC TS 62257-8-1:2007 Recommendations for small renewable energy and hybrid systems for rural electrification - Part 8-1: Selection of batteries and battery management systems for stand-alone electrification systems - Specific case of automotive flooded lead-acid batteries available in developing countries

KS IEC 61727 Photovoltaic (PV) systems - Characteristics of the utility interface

KS IEC 62446:2009 Grid connected photovoltaic systems - Minimum requirements for system documentation, commissioning tests and inspection

KS IEC 60904-1 Photovoltaic devices - Part 1: Measurement of photovoltaic current-voltage characteristics

KS IEC 62093:2005 Balance-of-system components for photovoltaic systems - Design qualification natural environments.

KS IEC 62124:2004 Photovoltaic (PV) stand-alone systems - Design verification.

KS IEC 62116:2008 Test procedure of islanding prevention measures for utility-interconnected photovoltaic inverters

KS IEC 61683:1999 Photovoltaic systems - Power conditioners - Procedure for measuring efficiency

KS IEC 62109-1:2010 Safety of power converters for use in photovoltaic power systems Part 1: General requirements

KS IEC 62109-2:2011 Safety of power converters for use in photovoltaic power systems Part 2: Particular requirements for inverters

*being considered for adoption

The following international standards (IEC) are applicable:

IEC 60364-5Wiring rules

IEC 62548Photovoltaic (PV) arrays design requirements

IEC 62305Protection against lightning

IEC 61730.1Photovoltaic module safety qualification: requirements for construction

IEC 61730.2Photovoltaic module safety qualification: requirements for testing

IEC 61215 Crystalline silicon terrestrial photovoltaic (PV) modules Design qualification and type approval

IEC 61646 Thin-film terrestrial photovoltaic (PV) modules - Design qualification and type approval

IEC 61427-1* Secondary cells and batteries for renewable energy storage - General requirements and methods of test - Part 1: Photovoltaic off-grid application

IEC 61427-2* Secondary cells and batteries for Renewable Energy Storage - General Requirements and methods of test - Part 2: On-grid application.

IEC 61724* Photovoltaic system performance monitoring - Guidelines for measurement, data exchange and analysis

IEC 61702* Rating of direct coupled photovoltaic (PV) pumping systems application.

IEC/TS 62548Photovoltaic (PV) arrays design requirements

IEC 62894* Photovoltaic inverters - Data sheet and name plate

*Being considered for adoption

1.4 OVERVIEW OF SYSTEM DESIGN

Four major issues arise when designing a system:

1. The load (power) required to be supplied by the system is not constant over

the period of one day;

2. The daily energy usage varies over the year;

3.The energy available from the PV array may vary from time to time during the day;

4.The energy available from the PV array will vary from day to day during the year.

Since the system is based on photovoltaic modules, then a comparison should be undertaken between the available energy from the sun and the actual energy demands. The worst month is when the ratio between solar energy available and energy demand is smallest.

The design of a hybrid system requires a number of steps. A basic design method follows:

1. Determination of the energy usage that the system must supply.

2. Determining how the system provides the total required daily energy. This could be completely by the PV array with the generator only used as a back up or where the generator is used than the following must be determined :

a. load energy provided directly from fuel fired

b. load energy provided directly from solar array

c. load energy provided by batteries being charged by the fuel fired generator

d. load energy provided by batteries being

where a+b+c+d= Total Required Daily Energy

3. Determination of the battery storage required.

4. Determination of the energy input required from the PV array.

5. Determination of Fuel generator size and its run times.

6. Selection of the remainder of system components.

1.5 LOAD (ENERGY) ASSESSMENT

Electrical power is supplied from the batteries (d.c.) or via an inverter to produce 240 volts a.c.. Electrical energy usage is normally expressed in watt hours (Wh) or kilowatt hours (kWh).

To determine the daily energy usage for an appliance, multiply the power (in Watts) of the appliance by the number of hours per day it will operate. The result is the energy (Wh) consumed by that appliance per day.

An energy assessment should be undertaken for the loads present in the system. For hybrid systems the loads typically are all a.c. however there can at times be some small d.c. loads. The Stand-alone Guideline has examples of energy assessments for a.c. loads and d.c. loads. Example of a large daily energy usage being supplied by a hybrid system is shown in annexure 5.

You need to calculate the electrical energy usage with the customer. Many systems have failed over the years not because the equipment has failed or the system was installed incorrectly, BUT BECAUSE THE CUSTOMER BELIEVED THEY COULD GET MORE ENERGY FROM THEIR SYSTEM THAN THE SYSTEM COULD DELIVER. It failed because the customer was unaware of the power/energy limitations of the system.

The problem is that the customer may not want to spend the time determining their realistic power and energy needs which are required to successfully complete a load assessment form. They just want to know: How much for a system to power my lights and TV?

A system designer can only design a system to meet the power and energy needs of the customer. The system designer must therefore use this process to understand the needs of the customer and at the same time educate the customer. Completing a load assessment form correctly (Refer to annexure 5 to complete the load assessment forms and learn about the design process) does take time; you may need to spend 1 to 2 hours or more with the potential customer completing the tables. It is during this process that you will discuss all the potential sources of energy that can meet their energy needs and you can educate the customer on energy efficiency.

For large loads such as villages than if possible it is best to data log the energy profile over a period of time to obtain an hourly load profile.

1.6HYBRID SYSTEM DESIGN METHODOLOGY

Annexure 5 of this document contains a worked example for both d.c and a.c bus systems

As discussed above, hybrid systems can be one of three types:

d.c. bus systems supplying a.c. loads.

a.c. bus systems supplying a.c. loads.

or a combination of the both.

There are a lot of design steps that are common to the both system possibilities and we will be discussing them first in the below sections.

1.6.1Load Assessment and Battery determination

1. Load Assessment

The most important step is to perform a Load assessment before designing any hybrid system.

The steps involved in estimating the load are:

1.List all the items in the household, village or site in question that will draw electricity from the system

2.Determine the rated power of each of these items.

3.Estimate the number of hours per day that the item would be used.

4. Multiply the power (in watts) by the number of hours to determine the energy used by each in a unit called watt-hours (watt-hrs or Wh) item.

5.Add up the answers.

We also calculate the continuous VA demand and the surge demand to make sure that the inverter can handle the a.c. loads safely.

For some systems where the generator is being provided for back-up power, the PV system will be required to meet all the regular loads. Prior to a.c. bus systems, the conservative approach was to assume that none of the load was being provided directly by the array during the day. However in reality, unless the loads were only nigh-time lights, then some of the load would be supplied directly by the solar. This is still a good conservative approach and it is recommended that it still be applied to systems where the generator is used only for backup .

For systems where the generator is operating daily, the time that the generator operates should be determined and then based on that information, the following would be determined:

a. load energy provided directly from fuel fired generator

b. load energy provided directly from solar array

c. load energy provided by batteries being charged by the fuel fired generator

d. load energy provided by batteries being

2. Battery Inverter Efficiency

For a.c. systems, the efficiency of the inverter must be considered. Typically the peak efficiency of the inverter may be over 90%, but in many systems, the inverter will sometimes be running when there is very little load on the inverter, so the average efficiency is about 85% to 90%.

The total amount of a.c. energy being supplied through the inverter must be divided by this efficiency figure to obtain the energy to be supplied to the inverter from the battery bank.

For hybrid systems with generator as back up, it is recommended that all the daily energy required be used.

For hybrid systems, where the generator operates daily than it is recommended that:

For d.c. bus systems, the energy value used is the total daily energy less the energy supplied directly by generator;

For a.c. bus systems, the energy value used is total daily energy minus the energy supplied directly by generator and minus the energy supplied directly by the array through the inverters (grid connect type) connected directly to the array.

3. Battery Selection

System voltages are generally 12, 24 or 48 Volts. The actual voltage is determined by the requirements of the system. In larger systems, 120V, 240V d.c. or higher could be used, but these are not typical residential or small commercial systems.

As a general rule, the recommended system voltage increases as the total load increases. For small daily loads, a 12V system voltage can be used. For intermediate daily loads, 24V is used and for larger loads 48V is used.

Figure 1: Choosing the system voltage

1 kWh

3-4 kWh

Use 12 Volt

system voltage

Use 24 Volt

system voltage

Use 48 Volt

system voltage

The system voltage change-over points are roughly at daily loads of 1 kWh and 3-4 kW, but this will also be dependent on the actual power profile.

One of the general limitations is that maximum continuous current being drawn from the battery to the battery inverter should not be greater than 150Ad.c. .

To convert Watt-hours (Wh) to Amp-hours (Ah) you need to divide by the battery system voltage.

Battery capacity is determined by whichever is the greater of the following two requirements:

1. The ability of the battery to meet the load energy being supplied by the battery bank, for some systems this could be for a few days, sometimes specified as days of autonomy of the system;

OR

2. The ability of the battery to supply peak power demand.

The critical design parameters include:

Parameters relating to the energy requirements of the battery:

a) Load energy being supplied by the battery

b) Daily and maximum depth of discharge

c) Number of days of autonomy

Parameters relating to the discharge power (current) of the battery:

a) Maximum power demand

b) Surge demand

Parameters relating to the charging of the battery:

Maximum Charging Current

Based on these parameters there are a number of factors that will increase the battery capacity in order to provide satisfactory performance. These correction factors must be considered.

4. Days of Autonomy

If the fuel generator is being is used just for back up, extra capacity is necessary when the loads require power during periods of reduced input. The battery bank is often sized to provide for a number of days autonomy. However for hybrid systems where the generator operates daily, the days of autonomy could be less than 1 day.

It is recommended that from hybrid systems:

With the generator being used as back up, the days of autonomy are between 3 and 5 days.

With the generator being operated daily, a minimum of 1-day autonomy is applied.

5. Maximum Depth of Discharge

The battery manufacturers will specify the maximum allowable depth of discharge. That is the depth to which the battery can be taken before the battery will be damaged. This figure therefore provides the capacity that can be taken out of the battery to supply the loads before this point is reached. The manufacturers depth of discharge can vary in the range 0.5 - 0.8 (50 to 80%).

6. Battery Discharge Rate

The actual discharge rate selected is highly dependent on the power usage rates of the connected loads. Many appliances operate for short periods only, drawing power for minutes rather than hours. This affects the battery selected, as battery capacity varies with discharge rate. Relevant information, such as a power usage profile over the course of an average day, is required for an estimate of the appropriate discharge rate.

For small systems, this power profile information is often impractical.

The C100 (100hr discharge rate) capacity rating of the battery could be used for hybrid systems where 5 days autonomy has been used in determining the battery capacity however for hybrid systems where the generator is required daily, the 10hr (C10) rate. is recommended.

7. Battery Temperature derating

Battery capacity is affected by temperature. As the temperature goes down, the battery capacity reduces. In Figure 2, the graph d.c. shows the battery correction factor for low temperature operation. Note that the temperature correction factor is 1 at 25C as this is the temperature at which the battery capacity has been specified.

In Kenya, the night-time temperature varies greatly between the regions at altitude to those regions located in the hotter lower altitudes, so it is important that the lowest temperature for the actual location of the selected site is used. If the system is being installed in regions where the temperature can get very cold, the temperature derating must be determined from figure 2.

Figure 2: Effect of Temperature on battery capacity

8. Battery Selection

Deep discharge type batteries / cells should be selected for the required system voltage and capacity in a single series string of battery cells.

Parallel strings of batteries are not recommended, but if it is unavoidable the number of parallel strings should be kept to a minimum. (Note: Battery manufacturers typically only allow a maximum of 3 or 4 batteries in parallel). Where parallel strings are necessary, each string must be separately fused.

1.6.2PV ARRAY SIZING: d.c. Bus - Standard Switched Controllers

The calculation for determining the size of the PV array is dependent on the type of controller used. Historically, standard switched controllers were the most common controllers used. In recent years, a number of maximum power point trackers (MPPT) have become available. This section determines how to size the PV array using switched controllers based on the PV array meeting the daily load requirements all year. Section 1.6.3 details how to size a PV array using a MPPT.

The size of the solar array should be selected to take account of:

Seasonal solar radiation data for selected tilt angle and orientation, taking shading into account

Variation of daily/seasonal energy usage

Battery efficiency

Manufacturing tolerance of modules

Temperature effects on the modules

Effects of dirt on the modules

System losses (e.g. power loss in cables)

Inverter efficiency

Solar irradiation data is available from various sources; some countries have data available from their respective meteorological department. One source for solar irradiation data is the NASA website: http:/eosweb.larc.nasa.gov/sse/. RETSCREEN, a program available from Canada that incorporates the NASA data, is easier to use. Please note that the NASA data has, in some instances, had higher irradiation figures than that recorded by ground collection data in some countries. However, if there is no other data available, this data can be used.

Solar irradiation is typically provided as kWh/m2 . However it can be stated as daily peak Sunhrs (PSH). This is the equivalent number of hours of solar irradiance of 1kW/m2.

Annexure 1 provides PSH data on the following sites:

Mombasa (Latitude 0403S Longitude 3940E)

Nairobi (Latitude 0117' S' Longitude 3649' E)

Wajir (Latitude 01 45' N Longitude 4003 E)

Lodwar (Latitude 0307'N, Longitude 3536'E)

The variation of both the solar irradiation and the load energy requirement should be considered. If there is no variation in daily load between the various times of the year, the system should be designed for the month having the lowest irradiation, i.e. peak sun hours (PSH).

Note: PV systems can be mounted on the roof of a building. The roof might not be facing true north (southern hemisphere) or south (northern hemisphere) or at the optimum tilt angle. The irradiation data for the roof orientation (azimuth) and pitch (tilt angle) will be used when undertaking the design. Please see the following discussion on tilt and orientation for determining peak sun hours for sites not facing the ideal direction.

EFFECT OF ORIENTATION AND TILT

When the roof is not oriented to true north (southern hemisphere) or south (northern hemisphere) and/or not at the optimum inclination, the output from the array will be less than the maximum possible.

Annex 4 provides a diagram showing estimated tilt and orientation losses for a location with a latitude of 1N.

The table provides values for orientations at 5 intervals (azimuths) and inclination angles at 10 intervals.

Using this figure will provide the system designer/installer with information on the expected output of a system (with respect to the maximum possible output) when it is located on a roof that is not facing true north (for the southern hemisphere) or south (for the northern hemisphere), or at an inclination equal to the latitude angle. The designer can then use the peak sun hour data for their particular location to determine the expected peak sun hours at the orientation and tilt angles for the system to be installed.

1. Daily Energy Requirement from the PV Array

In order to determine the energy required from the PV array, it is necessary to increase the energy from the battery bank to account for battery efficiency.

The average columbic efficiency (in terms of Ah) of a new battery is 90% (variations in battery voltage are not considered).

For hybrid systems having a generator as back up, it is recommended that the total daily energy required is used to determine the daily energy requirement from the PV array and conservatively it is assumed that all the energy is provided via the battery.

For hybrid systems where generator operates daily, it is recommended that the energy value used to determine the daily energy requirement from the PV array is:

Total Energy required

minus

the energy provided directly by the generator

minus

the energy supplied by the batteries charged by the generator

For hybrid systems where the generator is operating daily, a specified portion of the daily load energy requirement will be agreed to be met by the array during the day, and the daily energy requirement from the array calculated above will be divided into 2 parts:

First part: that portion to be supplied by the PV array during the day. This will not need to be increased to take the battery efficiency into account.

Second part: that portion of the load provided by the battery bank as charged by the PV array this figure will need to be increased to take the battery efficiency into account.

2. Oversize Factor

As the hybrid system includes a fuel generator, the oversize factor of 10% (which is the recommended oversize factor for Kenya in the absence of a fuel generator) need not be used. The fuel generator would be sufficient to provide the equalisation charge to the battery bank.

3. Derating Module Performance

The PV array will be de-rated due to:

a. Manufacturers Output Tolerance

The output of a PV module is specified in watts and with a manufacturing tolerance based on a cell temperature of 25 degrees Celsius. Historically this has been 5%, but in recent years typical figures have been -0% and +5%. When designing a system it is important to incorporate the actual figure for the selected module.

b. Derating Due to Dirt

The output of a PV module can be reduced as a result of a build-up of dirt on the surface of the module. The actual value of this derating will be dependent on the actual location; some city locations might have a high dirt derating due to car pollution, some coastal locations might have a high dirt derating due to salt build up and some locations might have long periods with no rain to naturally wash the modules.

If in doubt, an acceptable dirt derating value would be 5%.

c. Derating Due to Temperature

A solar modules output power decreases with temperatures above 25C and increases with temperatures below 25C. The average cell temperature will be higher than the ambient temperature because of the glass on the front of the module and the fact that the module absorbs some heat from the sun. The output power and/or current of the module must be based on the effective temperature of the cell. This is determined by the following formula:

Tcell-eff = Ta.day + 25C

Where:

Tcell-eff = the average daily effective cell temperature in degrees Celsius (C)

Ta.day = the daytime average ambient temperature for the month that the sizing is being undertaken.

Since the modules are used for battery charging, the current at the charging voltage at the effective cell temperature should be used in calculations. For a 12V battery, a charging voltage of 14V is appropriate. If curves are unavailable to determine the current at effective cell temperature, the Normal Operating Cell temperature (NOCT) provided by the manufacturers can be used.

Therefore the derated module output current is calculated as follows:

The Current of the module at 14V and effective cell temperature (or NOCT current)

multiplied by derating due to manufacturers tolerance

multiplied by derating due to dirt

I (NOCT) x fman x fdirt

If a module has a 3% (0.03) manufacturers tolerance, the modules current is derated by multiplying by 0.97 (1-0.03).

If a module has a 5% (0.05) loss due to dirt, the modules current is derated by multiplying by 0.95 (1-0.05).

4. Number of Modules required in the Array

I. Determine the number of modules in series: to do this, divide the system voltage by the nominal operating voltage of each module.

II. To determine the number of strings in parallel, the PV array output current required (in A) is divided by the output of each module (in A). Then round up to the next whole number.

III. To determine the number of strings in parallel, the PV array output current required (in A) is divided by the output of each module (in A). Then round up to the next whole number.

5 CONTROLLERS: Standard Switched Controller

PV controllers on the market range from simple switched units that only prevent the overcharge (and discharge) of connected batteries to microprocessor based units that incorporate many additional features such as:

PWM and equalisation charge modes

DC Load control

Voltage and current metering

Amp-hour logging

Generator start/stop control

Unless the controller is a model that is current limited, these should be sized so that they are capable of carrying 125% of the arrays short circuit current and withstanding the open circuit voltage of the array. If there is a possibility that the array could be increased in the future, the controller should be oversized to cater for the future growth.

(Note: sometimes the controller is called a regulator)

1.6.3 PV ARRAY SIZING: d.c. BUS - MPPT

Please refer to start of section 1.6.2 for information on solar irradiation for Kenya.


The size of the PV array should be selected to take account of:

Seasonal variation of solar irradiation

Seasonal variation of the daily energy usage

Battery efficiency (Wh)

Cable losses

MPPT efficiency


Dirt

Temperature of array (the effective cell temperature)

With the standard controller, the only sub-system loss was the battery efficiency and the calculations are undertaken using Ah. When using a MPPT, the calculations are in Wh and the sub-system losses in the system include:

Battery efficiency (watt-hr)

Cable losses

MPPT efficiency

In order to determine the energy required from the PV array, it is necessary to account for all the sub-system losses. Depending on how the hybrid system operates, the energy required at the battery in Wh is then divided either by all these three losses or just two losses (cable loss and MPPT efficiency) to determine the required energy to be provided by the array.




minus


minus





2. Oversize Factor

As the hybrid system includes a fuel generator, the oversize factor of 10% (which is the recommended oversize factor for Kenya in the absence of a fuel generator) need not be included. The fuel generator would be sufficient to provide the equalisation charge to the battery bank.


The PV array will be de-rated due to:

a. Manufacturers Output Tolerance

The output of a PV module is specified in watts and with a manufacturing tolerance based on a cell temperature of 25 degrees Celsius. Historically this has been 5%, but in recent years typical figures have been -0% and +5%. When designing a system it is important to incorporate the actual figure for the selected module.

b. Derating Due to Dirt

The output of a PV module can be reduced as a result of a build-up of dirt on the surface of the module. The actual value of this derating will be dependent on the actual location; some city locations might have a high dirt derating due to car pollution; some coastal locations might have a high dirt derating due to salt build up; and some locations might have long periods with no rain to naturally wash the modules.

If in doubt, an acceptable dirt derating value would be 5%.

c. Derating Due to Temperature

A solar modules output power decreases with temperature above 25C and increases with temperatures below 25C. The average cell temperature will be higher than the ambient temperature because of the glass on the front of the module and the fact that the module absorbs some heat from the sun. The output power and/or current of the module must be based on the effective temperature of the cell. This is determined by the following formula:

Tcell-eff = Ta.day + 25C

Where:

Tcell-eff = the average daily effective cell temperature in degrees Celsius (C)

Ta.day = the daytime average ambient temperature for the month that the sizing is being undertaken.

The three different solar modules available on the market each have different temperature coefficients. These are:

A) Monocrystalline Modules

Monocrystalline Modules typically have a temperature coefficient of 0.45%/oC. That is for every degree above 25oC the output power is derated by 0.45%.

B) Polycrystalline Modules

Polycrystalline Modules typically have a temperature coefficient of 0.5%/oC.

C) Thin Film Modules

Thin film Modules have a different temperature characteristic resulting in a lower co-efficient typically around 0%/C to -0.25%/C, but remember to check with the manufacturer.

The derating of the array due to temperature will be dependent on the type of module installed and the average ambient maximum temperature for the location.

The typical ambient daytime temperature in many parts of Kenya is between 25 and 40oC during some parts of the year. So it would not be uncommon to have module cell temperatures of 65oC or higher.

With switched controllers, the temperature effect was used to determine the operating current of the module/array. With MPPTs, the temperature derating power factor must be calculated.

Therefore the derated module output power (Pmod) is calculated as follows:

The Power of the module at STC

multiplied by derating due to manufacturers tolerance

multiplied by derating due to dirt

multiplied by derating due to temperature

Pstc x fman x fdirt x ftemp

If a module has a 3% (0.03) manufacturers tolerance, the module current is derated by multiplying by 0.97 (1-0.03).

If a module has a 5% (0.05) loss due to dirt, the module current is derated by multiplying by 0.95 (1-0.05).

If a module is operating at an ambient temperature of 30 degrees and a temperature coefficient of -0.5%, it has a 15% (0.15) loss due to temperature (0.5%x(30+25-25)). The module current is derated by multiplying by 0.85 (1-0.15).

4. Number Of Modules Required In Array

To calculate the required number of modules in the array, divide the required array power by the adjusted (i.e. derated) module power.

The exact final number required will depend on the MPPT selected, and then matching the array to the MPPT voltage operating windows.

5. Selecting MPPT

The output voltage of the MPPT shall match the battery voltage selected.

The maximum input power rating of the MPPT shall be equal to or greater than the rated power of the array.

The maximum input current rating of the MPPT must be equal to or greater than the rated current of the array. Note that the current of the array will be dependent on the number of parallel strings, while the number of parallel strings will be dependent on the number of modules in series in each string. The number of modules in series must match the operating voltage window of the MPPT as detailed below.

Matching the PV Array to the Voltage Specifications of the MPPT

The MPPT typically will have a recommended minimum nominal array voltage and a maximum voltage. In the case where a maximum input voltage is specified and the array voltage is above the maximum specified, the MPPT could be damaged.

Some MPPT controllers might allow the minimum array nominal voltage to be the same as that of the battery bank. However the MPPT will work better when the minimum nominal array voltage is higher than the nominal voltage of the battery. Please check with the MPPT manufacturer because these could vary.

It is important that the output voltage of the string is matched to the operating voltages of the MPPT and that the maximum voltage of the MPPT is never reached.

The output voltage of a module is affected by cell temperature changes in a similar way to the output power. The manufacturers will provide a voltage temperature coefficient. It is generally specified in V/C (or mV/C) but it can also be expressed as a %.

To ensure that the Voc of the array does not reach the maximum allowable voltage of the MPPT, the minimum day-time temperatures for that specific site are required.

In early morning at first light, the cell temperature will be very similar to the ambient temperature because the sun has not had time to heat up the module. In Kenya, the average minimum temperature is 200C (this could be lower in some mountain areas) and it is recommended that this temperature is used to determine the maximum Voc. (Note: If installing systems in the mountains use the appropriate minimum temperature. Many people also use 0C, if appropriate for the area). The maximum open circuit voltage is determined similarly to the temperature derating factor for the power.

1.6.4PV Array Sizing: a.c. Bus - Loads Supplied Directly by Array





Seasonal variation of the daily energy usage being supplied directly by the array

Cable losses (d.c and a.c)

Grid connect inverter efficiency


Dirt


The sub-system losses in the system include:

Cable losses, d.c. and a.c.

Grid Connect Inverter Efficiency

In order to determine the energy required from the PV array, it is necessary to account for all the sub-system losses. The a.c. energy that is being supplied directly during the day is increased by dividing by the d.c. cable losses, the grid connect inverter efficiency and the a.c. cables losses. (Note: the a.c. cable losses are those between the grid connect inverter and the a.c. point of attachment, which is usually the main switchboard or nearest distribution board)

2. Oversize Factor

As the hybrid system includes a fuel generator, we dont have to consider an oversize factor.


Please refer to derating module performance section in section 1.6.3 for determining the derating of the module performance.


To determine the PV array output power required (in W) the daily energy requirement from the array (in Wh) is divided by the selected daily irradiation value (that is, the PSH). Since there is a generator in the system, the PSH could be the yearly average or, if the generator run time needs to be reduced, it might be based on the worst month with respect to load energy and solar resource energy.

To calculate the required number of modules in the array, divide the required array power by the adjusted (derated) module power.

The exact final number required will depend on the grid connect inverter selected and then matching the array to the grid connect inverters voltage operating windows.

Section 1.6.8 provides in detail how the select the grid connect inverter and matching it with the array.

1.6.5PV Array Sizing: a.c. BUS - Loads Supplied by Battery





Seasonal variation of the daily energy usage being supplied directly by the array

d.c Cable losses between array and grid connect inverter


A.c cables losses between the grid connect inverter and the battery inverter

Battery inverter efficiency when operating as a battery charger

d.c. cable losses between the battery inverter and battery and back to the battery inverter

Watt-hr efficiency of the battery bank

Battery inverter efficiency when providing a.c. power from the battery bank


Dirt


The sub-system losses in the system include:

Cable losses- a.c and d.c.


Battery inverter efficiency both when charging batteries and when providing a.c. power from the batteries


In order to determine the energy required from the PV array, it is necessary to account for all the sub-system losses. The a.c. energy that is being supplied to the loads via the battery bank and charged by the PV array by dividing by the:

d.c. Cable losses between array and grid connect inverter


a.c cables losses between the grid connect inverter and the battery inverter

Battery inverter efficiency when operating as a battery charger

d.c. cable losses between the battery inverter and battery


d.c. cable losses between the battery and battery inverter

Battery inverter efficiency when providing a.c. power from the battery bank

2. Oversize Factor

As the hybrid system includes a fuel generator, we dont have to consider an oversize factor.


Please refer to derating module performance section in section 1.6.3 for determining the derating of the module performance.


To determine the PV array output power required (in W), the daily energy requirement from the array (in Wh) is divided by the selected daily irradiation value (that is the PSH). Since there is a generator in the system, the PSH could be the yearly average or if generator run time wants to be reduced it might be based on the worst month with respect to load energy and solar resource energy.

To calculate the required number of modules in the array, divide the required array power by the adjusted (derated) module power.

The exact final number required will depend on the grid connect inverter selected, followed by matching the array to the grid connect inverters voltage operating windows.

Section 1.6.8 provides in detail how the select the grid connect inverter and matching it with the array.

1.6.6PV ARRAY SIZING: A.C. BUS

Sections 1.6.4 and 1.6.5 detail how to determine the size of an array in an a.c. bus system when loads were being supplied direct (1.6.4) and when loads were being supplied via the battery (1.6.5). In reality in an a.c. bus system the loads could be supplied by both and therefore determining the array will involve undertaking the two calculations, that is determining size of array to meet loads directly and the size of array meeting loads by battery and adding the two array sizes to get the total array size required.

For hybrid systems with generator as back up then it is recommended that all the daily energy required is used to determine the daily energy requirement from the PV array and for conservatism it is assumed that all the energy is provided via the battery.




MINUS


MINUS





1.6.7 Battery Inverter Selection

The type of battery inverter selected for the installation depends on factors such as cost, surge requirements, power quality and for inverter/chargers, a reduction of the number of system components required.

Inverters are available in 2 basic output types (topologies): modified square (or sine) wave and sine wave.

Modified square (sine) wave inverters generally have good surge and continuous capability and are usually cheaper than sine wave types. However, some appliances, such as audio equipment, television and fans can suffer because of the output wave shape.

Sine wave inverters often provide a better quality power than the 240V grid supply.

If affordable to the end-user, it is recommended that sine wave inverters be used.

There are many types of battery inverters available:

Inverter only (modified square wave or sine wave)

Inverter/Charger: this converts to being a battery charger when there is another a.c. power source available e.g. fuel fired generator (modified square wave or sine wave)

Interactive inverters: These act as inverter chargers, but the inverter can synchronise with the fuel generator. (Sine wave only)

Multimode inverters: these are similar to the interactive inverter, but having an extra feature whereby they can provide the a.c. supply for the grid connect inverters to operate even while the multimode inverters is operating in battery charger mode.

The selected inverter should be capable of supplying continuous power to all AC loads

AND

Capable of providing sufficient surge capability to start any loads that may surge when turned on and particularly if they turn on at the same time.

Where an inverter cannot meet the above requirements, attention needs to be given to load control and prioritisation strategies.

However in some hybrid systems, where the generator operates daily, the inverter might be sized based on its battery charging capability, meaning it has a larger VA rating than that required to meet the continuous power requirements.

For a.c bus systems, the battery inverter also needs to be sized according to the size of the solar array. The battery inverter must be sized so that all the solar power in Kw required for charging the battery bank can be provided through the battery inverter.

A conservative approach is that the battery inverter kVA rating is 80% of the grid- connect inverter rating. However this figure could be reduced based on the amount of power that is supplied directly to the loads and the fact that solar power does vary throughout the day. However it is important that solar power is not wasted, meaning the solar might not be able to charge the batteries because of the capacity of the battery inverter.

1.6.8Grid Connect Inverter for a.c. Bus system

1. Inverter Size

The selection of the inverter for the a.c. bus system will depend on:

The power output of the array

Matching the allowable inverter string configurations with the size of the array in kW and the size of the individual modules within that array

Inverters are typically rated for:

Maximum d.c. input power, i.e. the size of the array in peak watts;

Maximum d.c. input current; and

Maximum specified output power, i.e. the a.c power they can provide to the grid;

The maximum power of the array is calculated by the following formula:

Array Peak Power =

Number of modules in the array X the rated maximum power (Pmp) of the selected module at STC.

The designer must follow the manufacturers recommendation when matching the peak power rating of the array to that of the inverter. If the manufacturer does not have specific recommendations, the designer should follow the following guidelines for specifying the rating of the inverter.

Worked Example:

Using the information from the previous examples, what is the maximum d.c. input power of the array?

Answer:

As previously calculated, the peak power of the array is 4kWp.

Does that mean the inverter should be rated at 4kW?

Many inverter manufacturers provide the maximum rating of a solar array in peak power for a specific size inverter. Designers shall follow the recommendations of the manufacturer.

If the manufacturer does not provide any recommendations, the designer could match the array to the inverter allowing for the derating of the array.

In the section on Derating Module performance, the typical PV array output in watts is derated due to:

Manufacturers tolerance of the modules

Dirt

Temperature

Inverter with Crystalline Modules

Based on figures of:

1 for manufacturers' tolerance,

0.95 for dirt derating, and

0.85 for temperature derating (Based on ambient temperature of 30C )

The derating of the array is: 0.1 x 0.95 x 0.85 = 0.80

As a result of this method of derating being experienced in the field, the inverter can easily be rated at 80% of the peak power of the array and possibly even less. However, if possible, confirm with your inverter supplier.

Inverter with Thin Film Module

The temperature effect on thin film modules is less than that on crystalline modules. Assuming the temperature coefficient is only -0.1%/C, the temperature derating at ambient temperature of 30C is 0.97.

Based on figures of:

1 for manufacturers tolerance,

0.95 for dirt derating, and

0.97 for temperature derating (Based on ambient temperature of 30C)

The derating is: 1 x 0.95 x 0.97 = 0.915

As a result of this method of derating being experienced in the field, the inverter can easily be rated at 91% of the peak power of the array and possibly even less. However if possible confirm with your inverter supplier.

Worked Example:

What would the maximum input power rating of an inverter be if the 4kWp system were connected to a) crystalline modules, b) thin film modules?

Answer:

The array peak power is 4kWp .

a) With crystalline modules, this array can be connected to an inverter with an output rating of:

0.8x 4kW = 3.2kW (for crystalline modules)

b) With thin film modules, this array can be connected to an inverter with an output rating of:

0.91x 4kW = 3.64kW (for thin film modules)

2. Matching Array Voltage To Inverter Operating Voltages

The output power of a solar module is affected by the modules cell temperature. As shown in previous sections for multicrystalline PV modules, this effect can be as much as 0.5% for every degree variation in temperature.

This variation in power due to temperature is also reflected in a variation in the open circuit voltage and maximum power point voltage.

Most inverters will have an operating voltage window. If the solar array voltage is outside this window, the inverter will either not operate or the output power of the system will be greatly reduced.

Most inverters will also have a minimum and maximum input voltage, which will be specified by the manufacturer. If the maximum input voltage is exceeded, the inverter could be damaged. Some inverters will nominate a voltage window within which they will operate, and then also include a maximum voltage, higher than the maximum operating voltage of the window: this figure is the voltage where the inverter could be damaged.

For best performance of the system, the output voltage of the solar array should be matched to the operating voltages of the inverter. To minimise the risk of damage to the inverter, the maximum voltage of the inverter shall never be reached.

As stated earlier, the output voltage of a module is also affected by changes in cell temperature. As cell temperature increases, output voltage decreases. As cell temperature decreases, output voltage increases. The PV module manufacturers will provide a voltage temperature co-efficient. It is generally specified in V/C (or mV/C), but it can be expressed in %/C .

To design systems where the output voltages of the array do not fall outside the range of the inverters d.c. operating voltages and maximum voltage (if different), the minimum and maximum day time temperatures for that specific site are required.

The following sections detail how to determine the minimum and maximum number of solar modules allowed to be connected in series to match the operating voltage window of an inverter. Many of the inverter manufacturers have software programs for doing these calculations. Ensure that the temperature information for each design is site-specific.

MINIMUM VOLTAGE WINDOW

When the temperature is at a maximum, the Maximum Power Point (MPP) voltage (Vmp) of the array should never fall below the minimum operating voltage of the inverter. The actual voltage at the input of the inverter is not just the Vmp of the array, the voltage drop in the d.c. cabling must also be included when determining the actual inverter input voltage.

Since the array is typically operating with irradiance levels of less than 1kW/m, the actual MPP voltage can be reduced further. It is recommended that a safety margin of 10% on the lower inverter window is included.

Worked Example:

Using the information from the previous examples, what is the minimum number of modules that can be connected in a string to an inverter with a minimum input voltage of 150V? Assume that the module has a Vmp of 36.0V and that the MPP voltage co-efficient is 0.18V/C. Assume a voltage drop of 3% and that the maximum cell temperature experienced is 75C.

Note: The voltage coefficient for Vmp and Voc are slightly different. If the coefficient for each is provided, use them accordingly. If only the voltage coefficient for Voc is provided, it can be used for Vmp calculations.

Answer:

The Vmp of a module is 36.0V.

The difference between the cell temperature and STC is (75C -25C) = 50C

For every degree Celsius, the voltage is reduced by 0.18V. Therefore, the reduced Vmp of the module is 36.0 (50 x 0.18) = 27.0V.

Assuming the voltage drop is 3%, the further reduced Vmp of a module is (28.0 x 0.97) = 26.19V.

The minimum input voltage of the inverter is 150V. After adding the 10% safety margin, the new minimum input voltage is (150V x 1.1) = 165V.

The minimum number of modules that can be connected in a string is then (165V / 27.16V) = 6.3. This number must be rounded up, therefore the minimum number of modules that can be connected in a string to this inverter = 7.

MAXIMUM VOLTAGE WINDOW

At the coldest daytime temperature, the open circuit voltage of the array shall never be greater than the maximum allowed input voltage for the inverter. The Open Circuit voltage (Voc) is used because this figure is greater than the MPP voltage and it is the applied voltage when the system is first connected prior to the inverter starting to operate and connecting to the grid.

Note: Some inverters provide a maximum voltage for operation and a higher voltage as the maximum allowed voltage. In this situation, the MPP Voltage is used for the operation window and the open circuit voltage for the maximum allowed voltage.

In early morning, at first light, the cell temperature will be very close to the ambient temperature because the sun has not had time to heat up the module. Therefore the lowest daytime temperature for the area where the system is installed shall be used to determine the maximum Voc.

It is recommended that a safety margin of 5% on the maximum input voltage is included.

Worked Example:

Using the information from the previous examples, what is the maximum number of modules that can be connected in a string to an inverter with a maximum input voltage of 600V? Assume that the module has a Vmp of 41.0V and that the Voc co-efficient is 0.16V/C. Assume that the minimum cell temperature experienced is 0C.

Answer:

The Voc of a module is 41.0V.

The difference between the cell temperature and STC is (0C -25C) = -25C

For every degree Celsius, the voltage is increased by 0.16V. Therefore, the increased Voc of the module is 41.0 (-25 x 0.16) = 45.0V.

The maximum input voltage of the inverter is 600V. After subtracting the 5% safety margin, the new maximum input voltage is (600V x 0.95) = 570V.

The maximum number of modules that can be connected in a string is then (570V / 45V) = 12.7. This number must be rounded down, therefore the maximum number of modules that can be connected in a string to this inverter = 12.

In this example, each string must consist of between 7 -12 modules only. As we required 16 modules, we could have two parallel strings of 8 modules.

It is important that the number of modules in a string is selected to ensure that the output voltage of the array is always within the voltage operating window of the inverter.

1.6.8 Battery Charger

The output of a fuel-fired generator is usually a.c. and must be converted to d.c. for the purpose of battery charging. The device that does this rectification is called a battery charger. The battery charger should be selected such that it converts the 240-volt, 50 Hz a.c. to d.c. at the required voltage for the battery storage bank. It should be able to provide a continuous direct current up to the maximum allowable charge rate of the batteries.

The efficiency of the battery charger also needs to be considered. If it is not specified, the average efficiency can be calculated by dividing the average d.c. output power by the average a.c. input power to the battery charger.

When the battery charger is selected, the battery chargers maximum and continuous AC requirement, and maximum and continuous DC output can be obtained from the specifications. For transformer type chargers, the charging rate at low battery voltages is determined by the maximum output capability of the charger and will reduce at higher battery voltages. For solid-state chargers, the charging current is fixed until the battery bank nears full charge, at which point the charging current is reduced to a 'float' level.

The battery manufacturer must specify the maximum rate of charging current from the battery charger. This is generally rated at the 10h rate.

The maximum charge rate is:

That is, it is 10% of the C10 capacity of the battery.

A separate battery charger is typically used when the inverter is just an inverter. In many hybrid systems, the inverter acts as a charger as well as being an inverter.

1.6.8 Selecting and Sizing Fuel fired Generator

Fuel fired generators are energy sources that serve the purpose of supplying energy to a load in the event that the main energy source is not available or underperforms or when the load demand is larger than the main energy source can provide. They are run intermittently and aid in demand management. Although having a comparatively higher fuel cost than any renewable energy source, as well as their fuel base being of a fossil nature, they do provide assistance at times when a constant and quality supply of electricity cannot be provided.

Fuel generators are very useful in the context of Hybrid applications in the case that a solar power system is not able to provide enough energy to satisfy a particular load demand. They are also useful in the purpose of charging batteries or even being directly connected to the load demand so as to provide a continuous supply of electricity to the Hybrid system, depending on the particular set-up of the system.

The critical factors for selecting a fuel generator are-:

Whether the fuel generator can meet all the power requirements of the appliances that need to be operated when the fuel generator is operating. The total of these requirements is the apparent power (VA) of the appliances. In general the battery charger must be considered a load.

Not to oversize the fuel generator, because operating fuel generators at light loading will lead to increased wear and tear and greater maintenance requirements.

Possible configurations for generator in a Hybrid system are:-

Switched: which can be either with a separate inverter and charger as shown in Figure 3 or similar to that shown in Figure 4, where the interactive inverter shown is actually an inverter/charger and the switching for the loads from the generator to the inverter is internal. These types of systems are always configured as d.c. bus.

Parallel: where the inverter is interactive and synchronises with the generator. These can be configured d.c. bus and a.c. bus depending on the type of inverter.

Switched Configuration - The key feature of a switched configuration is that the output from the fuel generator may supply both the battery charger and the a.c loads directly (refer to figure 3). When the fuel generator is not running, the a.c loads will be switched to the inverter.

A changeover switch may do the switching between the fuel generator and inverter automatically, or this can be done manually. In either case, it is essential that the changeover switch or contactor shall have a break-before-make action. Where a manual changeover switch is used, a switch with a centre-OFF position is recommended. The battery charger, inverter and automatic changeover contactor may be incorporated into a single unit (inverter-charger).

If the inverter output is not synchronised before changeover, a break in supply of at least 500ms should ideally be maintained. This will prevent arcing of the contacts and increase the life of the contactor. The length of time required of the break in supply depends on the type of loads being supplied. Large inductive loads may require 500ms or more, while 30ms may be adequate for standard household loads.

A switched configuration provides reasonably efficient utilisation of the fuel generator power, but does require that a.c loads are tolerant of short breaks in supply. Desktop computers operating on this kind of system will require their own mini-UPS system, or extremely careful co-ordination between computer usage and changeover time. Alternatively, a dedicated inverter (sized specifically to suit the sensitive a.c equipment), and separate wiring could be used for loads requiring no-break power. In this case, the low volt cut-out setting on the no-break inverter should be set to minimum in order to avoid drop out due to battery voltage dips caused by load surges in the main inverter.

A parallel system, by contrast provides no-break power but it is not ideal in every situation. An intermediate form is a switched system using a very fast changeover. This requires that the inverter synchronises first, before changing over, and that the changeover contactor has a very fast action, operating in less than one cycle. This may be short enough not to affect the operation of sensitive loads.

The availability of trained personnel may be another factor in the decision whether to use a manual switched system.

Figure 3: Switched Configuration

Generator Sizing for Switched Configuration

The fuel generator must be sized to meet the AC demand when the fuel generator is operating, as well as the demand of the battery charger. The fuel generator must also be capable of meeting the surge demand. The fuel generator should therefore be sized to meet the following two formulae:

Sgen = (Sbc + Smax chg.) x fgo

Where:

Sgen = Minimum apparent power rating of the fuel generator. (VA)

Sbc = max apparent power consumed by the battery charger under conditions of max output curent and typical max charge voltage. (VA)

Smax.chg = max a.c. demand during battery charging (VA)

fgo = Fuel generator oversize factor.

And,

Sgen= (Sbc + Ssur.chg) fgo Alt Surge Ratio

Where:


Sbc = max apparent power consumed by the battery charger under conditions of max output curent and typical max charge voltage. (VA)

Ssur.chg = max a.c. surge demand during battery charging (VA)


Parallel Configuration - The key feature of a parallel configuration is the use of an interactive inverter i.e. one which allows bi-directional power flow. An interactive inverter works as an inverter or battery charger, and is capable of synchronising with, and supplying a.c power in parallel with, a fuel generator (refer to figure 4).

A contactor (or contactors) under the control of the inverter connects a.c. loads to the inverter or the fuel generator or both. When operating in parallel with the fuel generator, the direction and magnitude of power flow through the inverter at any instant is controlled to ensure optimal (or near optimal) loading of the fuel generator and battery charging at the highest possible rate, while at all times meeting the load demand.

This configuration provides the best use of the fuel generator, and the highest quality of output power (a pure sine wave, without breaks in the supply voltage). Some inverters even clean up the fuel generator output waveform. A parallel configuration may also allow the battery size to be greatly reduced. An important practical consideration is that the inverters allowable voltage and frequency window must be wide enough to cope with the range of operation of the fuel generator.

Switched systems are preferable to parallel systems where the quality of fuel generator power is poor, such as with an old fuel generator supplied by the client. The issue here is that a fuel generator with poor governing and/or voltage regulation may cause difficulties for an interactive inverter trying to maintain synchronism. A switched system may also be considered when the charging capacity of interactive inverters is inadequate for the battery and system operating conditions.

An important concept in parallel systems is that of synchronisation. For two a.c. power sources to operate in parallel, they must be synchronised before being connected together; the voltage from each source must be identically matched in frequency and phase, as well as amplitude. This also means that the inverter must produce a pure sine wave output.

It is much simpler to control the inverter output to achieve synchronisation, than to control the fuel generator. The inverter already has all the internal circuitry to completely control its output - whereas a fuel generator requires additional synchronising gear. For an alternator, synchronisation means physically synchronising the rotation of the machine with the voltage waveform of the supply that it is synchronising with. The cost of this scale of equipment is such that it is usually only considered for fuel generators around 100kVA or more, and would be used where two or more fuel generators are required to operate in parallel, i.e. only used with large capacity (high cost) systems.

Figure 4: Parallel System

Generator Sizing for Parallel Configuration

In parallel systems, the fuel generator and inverter sizing are interrelated. Therefore the combined ratings of the fuel generator and inverter can be sized to meet the maximum demand and surge demand of the loads.

The following equations may be used to determine the minimum fuel generator sizing:

Sgen = (SmaxSinv30min) fgo

Where:


Smax = max apparent a.c. power demand (VA)

Sinv30min= 30 min apparent power rating of the inverter (VA)


And,

Sgen= (Ssur Sinv.sur) fgo Alt Surge Ratio

Where:


Ssur = surge rating of the loads (VA)

Sinv.sur = surge rating of the inverter (VA)


However, in practice the fuel generator will be larger because many other factors must be considered. These include:

Whether the maximum demand is required for long periods during the day, when the customer might not want the fuel generator operating for that length of time.

Whether the fuel generator using the inverter to charge the batteries forms part of the design of the hybrid system. If the inverter is acting as an inverter for long periods of time when the fuel generator is operating, the batteries are not being charged. This could result in longer fuel generator running time than originally planned.

Sometimes a parallel inverter is selected just for its ability to change between fuel generator and inverter without having any break in power supply and therefore the paralleling feature is not required in the system sizing.

If the fuel generator is only required to operate every 2nd or 3rd day to meet the requirements to charge the batteries, the inverter must be sized to meet the maximum demand and the fuel generator sized to meet demand plus battery charging as per the formulas for switched systems.

1.6.9 Fuel Generator Control Strategies

In addition, the actual operation of the fuel generator will depend on how the fuel generator is controlled, which could be:

I. if the fuel generator starts at a pre-determined time each day, the batterys depth of discharge will vary and the fuel generator run-time will vary;

II.

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