Technical Standards for Solar Home Systems (SHS)

To assure the quality of a photovoltaic power system and its correct functioning and guarantee costumers' satisfaction it is important that the components of the system and the system as a whole meet certain requirements.

The GTZ prepared a publication which gives an overview of different standardisation activities and existing standards that are relevant for Solar Home Systems (SHS) and  Rural Health Power Supply Systems (RHS):

GTZ, Division 44, Environmental Management, Water, Energy, Transport: Quality Standards for Solar Home Systems and Rural Health Power Supply. Photovoltaic Systems in Developing Countries, February 2000.

The following Wiki-Page is an extract of the publication mainly regarding SHS.

Introduction

PV systems for applications in developing countries have been tested, optimised and disseminated throughout the world over the last 20 years. A wide variety of demands have been made of the components and systems, partly for reasons due to countryspecific characteristics or regional availability, but also because there were no binding standards, or if there were, they were often not known.

The project activities in technical and financial co-operation at bilateral and multilateral level have moved away from the pilot phase and towards the dissemination of PV systems. Yet, secure technical standards are required for dissemination in order to minimise the need for adjustments after the fact and the related costs in the case of large unit numbers.

An international survey carried out in preparation for this publication showed that several different standardisation activities are in progress. Probably the most interesting international project is the so-called "Global Approval Program for Photovoltaics (PV GAP)", but also technical specifications such as those that have been proposed by the World Bank or the University of Madrid have already been elaborated in great detail.

The publication provides an overview of standards that are relevant for Solar Home Systems (SHS) and in Rural Health Power Supply Systems (RHS). It is intended to facilitate the selection of PV systems and components, especially in tenders, and to provide the impetus for a standardisation of PV systems on a scale that is as broad as possible. Moreover, it also identifies those components for which there is still a need for technical specifications.

This should lead, in the long term or better yet in the medium term, to binding, internationally recognised technical standards, especially for the use of photovoltaic systems in developing countries.

In preparing this publication, all of the well-known national and international institutions concerned with standardisation activities in the field of photovoltaics were contacted in writing. The existing photovoltaics projects of GTZ were also included in the survey.

In the course of the survey, information and documentation obtained from the World Bank, the World Health Organization (WHO), the international standardisation institution IEC, the European standardisation institution CENELEC, the U.S. standardisation office IEEE, as well as a series of projects, firms and experts, were compiled and evaluated.

The available PV-relevant standards were evaluated and summarised in the form of a table with a breakdown by components.

The list of standard specifications for tenders for SHS and RHS forms the largest part of the publication. Eleven different documents with specifications for PV systems and their components were evaluated for this purpose and summarised in a table. These documents varied widely in terms of quality and scope; some of them were intended for the specification of individual components, others as tender documents for whole systems.

Based on these documents, standard specifications were prepared that can be used directly as text modules for international tenders. The minimum requirements were chosen in such a way that a reliably functioning Solar Home System can be set up according to the current state-of-the-art.

Systems and components that are used for power supply to rural health stations (RHS) have to meet higher standards as a matter of principle. The available experience with PV systems in this area of application to date as well as a series of documents, especially from WHO, were evaluated and condensed. A separate list of specifications was compiled for the RHS sector.

A separate set of standard texts for tenders for Photovoltaic Pumping Systems (PVP) entitled "Proposal for Tender Documents for the Procurement of Photovoltaic Pumping Systems (PVP)" is also available from GTZ, Div. 44, Sustainable Energy Systems.

IEC Draft Standard for Small-Scale PV Systems

In June 1997, the Technical Committee TC 82 WG 3 of IEC drafted its first standard with the title "PV Stand-Alone Systems - Design Qualification and Type Approval". This draft refers to individual home application with a solar generator with a maximum of 1000 Wpeak and electrical loads like lamps, radio, TV, refrigerator and telecommunication facilities. In this draft test procedures are described that can be used to determine the electrical and technical operating characteristics of PV systems and their components.

The current draft, however, is more or less a loose collection of individual documents, and is by far not yet complete for an international standard. Important information such as minimum requirements, system layout, installation, etc., are still missing or are extremely incomplete. In the meantime, there are indications that this draft will be replaced by a new proposal of a European group of experts with representatives of the French GENEC, the Spanish CIAMAT, the German TÜV-Rheinland and the European joint research institute ISPRA.

The Global Approval Program for Photovoltaics (PV GAP)

In the interest of world-wide quality assurance and as a reaction to the lack of standards up to now, various PV producers, lending institutions (e.g. the World Bank) and governmental as well as private organisations came out in favour of a world-wide programme for quality assurance of small-scale PV systems. At the 14th PV Conference in Barcelona, Spain in July 1997, the "Global Approval Program for PV (PV GAP)" was launched. The founding members established the following mission as the goal of PV GAP:

PV GAP is a global, PV industry-driven organisation that strives to promote and maintain a set of quality standards and certification procedures for the performance of PV products and systems, to ensure high quality, reliability and durability.

PV GAP is domiciled at the Central Office of the IEC in Geneva, Switzerland, and works closely together with the IEC and its suborganisation, IEC’s Quality Assessment System for Electronic Components (IECQ). Existing IEC standards for quality approval and certification of components and systems are the basis of its work, and progress is being made on the development of new standards that are still lacking. As long as there are no binding standards for certain components, recommendations are made (Recommended Standards) for the interim, which are generally based on national or regional standards.

Furthermore, test laboratories are identified world-wide, also in the developing countries, which can carry out the type tests on the components described in the standards, reliably and reproducibly.

A Reference Manual was put together, which came out in the first edition in January 1998, and can be purchased from PV GAP for 175 US$. The manual first describes in detail the ideas, the organisation and the planned procedures for a quality assurance of PV components in the framework of PV GAP. The technical part essentially consists of a list of standards that may be relevant for PV components. A comprehensive training manual entitled "Quality Management in Photovoltaics" was published in August 1999 which contains specific quality assurance standards for PV components as well as an updated list of relevant IEC standards. It also comprises proposed standards that are currently in progress (IEC TC 82 Work in Progress). ³

Moreover, a quality seal of approval is given for PV components that were tested under PV GAP conditions. This quality seal of approval is to become established if at all possible in international tenders as the prerequisite for the approval of components and systems. Qualified and recognised producers, sales and installation companies or system integrators have the right to display the quality seal of approval for PV components and systems.

The organisational structure of PV GAP provides for the following working groups:

Organization Working Group: To develop a permanent legal entity for PV GAP and a "Seal of Quality." This group will develop a PV GAP organizational structure. This group will work on an interface with the Switzerland-based International Electrotechnical Commission Quality Assurance Program (IECQ), along with the criteria for awarding the "GAP Seal."

Standardization Working Group: PV GAP will not write standards, but will accept and promote globally the IEC standards. If no IEC standard is available, then, based on peer review, PV GAP will accept existing or future standards of other bodies as "GAP Temporary Standards (GAP TS)," and promote their use globally. These GAP TS will then be submitted to IEC TC 82 to develop them into permanent IEC standards, which, when completed, will replace the GAP TS.

Handbook Working Group: A PV GAP Handbook will be established, combining inputs from the many other organizations that already have developed a handbook or parts thereof. This PV GAP Handbook then will be promoted globally. The Handbook will incorporate all of the PV GAP-approved standards.

Testing Laboratories Working Group: This group will establish criteria and compile a list of testing laboratories to be qualified to test PV components and systems according to IEC and PV GAP Temporary Standards. Reciprocity of test results from PV GAP-qualified Testing Laboratories will be established.

PV GAP Membership: PV-related industrial and commercial organisations, their representatives, producers, system suppliers, traders/retailers, installation companies as well as supporting organisations and individuals can become members of PV GAP. They must abide by the principles of PV GAP, especially in regard to the established quality standards. The advantages of membership are primarily the better marketing of products that have the PV GAP quality seal. As far as governmental and internationally financed projects are concerned, especially in development co-operation, preference will in all likelihood be given to the use of products that meet the PV GAP specifications. Moreover, members benefit from diverse information services and discounts on purchases of IEC and PV GAP publications and standards as well as the PV Reference Manual.

Sponsors and Partnerships of PV GAP (as at 10/98): Chairman: Dr. P. Varadi, P/V Enterprises, USA Secretary: Mr. R. Kay (acting) - IECQ - Switzerland Treasurer: Mr. M. Real - Alpha Real - Switzerland

Organisations represented on the PV GAP Executive Board:

Board Members: JEMA – Japan; EPIA – Belgium; NREL – USA; SEIA – USA; Newcastle Photovoltaic Applications Centre – UK; UNDP – USA; JRC, Ispra – Italy; JQAO – Japan.

Advisory Board Members: PowerMark – USA; WIP – Germany; Fraunhofer Institute – Germany; NOVEM - The Netherlands; EDF – France; ISPMA – India; National Technical University, Athens – Greece.

It remains to be seen how PV GAP develops in the future and whether offices issuing tenders and international donor organisations refer to the quality standards made by PV GAP. As the example of the Training Manual prepared for the World Bank in 1999 shows, however, the PV GAP’s quality assurance function has already met with international interest. If these standards become established in future tender procedures, tests according to PV GAP standards will be binding on the suppliers.

³ A list of IEC standards recommended by PV GAP, Work Proposals and recommended standards is provided in Annex 4. (To read the Annex of the publication download the whole text.)

The Universal Technical Standard for Solar Home Systems

The Instituto de Energía Solar at the University of Madrid has drawn up a proposal for SHS systems in co-operation with the Joint Research Centre, Ispra, the German WIPConsult and LTV-Genec within the framework of the European THERMIE-B research programme (SUP-995-96).

On the basis of 15 different documents, tenders, specifications, project reports, test requirements, etc., a set of criteria was developed for all SHS components as well as for the SHS system as a whole. Corresponding explanations that are plausible and easy to understand are provided on all of the specifications. The proposal for a new layout and sizing approach, however, does not seem very practicable.

Some of the formulations of specifications in this document have been taken over from the World Bank tenders for Sri Lanka and Indonesia.

Various PV experts commented on the draft of this standardisation proposal (it does not make any claim to being an international standard), which was distributed in June 1997, and the suggested revisions were incorporated in the final version which was released in Spring 1998. A detailed commentary by the author in behalf of GTZ on the draft study was sent to the Instituto de Energía Solar in early November 1997 and the revisions suggested in the commentary were largely taken into account in the final version. The current, revised version of this GTZ publication now contains the proposed specifications, if they did not conflict with statements from other sources or the opinion of the author.

CENELEC Draft Standards: Test Procedures for Charge Regulators and Lighting Systems in Solar Home Systems

The TÜV-Rheinland, the Fraunhofer Institut für Solare Energiesysteme and the Energy Technology Laboratory of BBPT in Indonesia have elaborated two detailed proposals for standards for charge regulators and lamps (with electronic ballasts) which have been brought into the CENELEC and IEC working groups and are currently under discussion. A panel of experts with representatives of the French GENEC, the Spanish CIAMAT, the German TÜV-Rheinland and the European joint research centre in ISPRA (Italy) is pursuing further work on these draft standards and is also commissioned to develop test procedures for both laboratory and field tests on the other system components and on the SHS system as a whole. These drafts are to be completed by the end of the year 2000 if at all possible.

In the draft standards for charge regulators and lighting systems, minimum requirements and test procedures for type tests of these two components are outlined in detail and subdivided into Part 1: Safety Tests, Part 2: EMC Tests and Part 3: Performance Tests. The revised criteria (as of 07/98) have been included in the specifications in Chapter 5 over other specifications, since it can be assumed that these draft standards are essentially accepted by the standardisation institutions (IEC, EN and DIN). Three further draft standards of this CENELEC group (PV modules, batteries and Solar Home Systems) are still in the early draft stage and still have to be properly completed and elaborated. They were incorporated in chapter 5 as a supplement to the more detailed specifications from other sources.

Initial concerns that these comprehensive type tests (a total of 30 individual tests and 13 function tests for charge regulators) could be too high of a cost factor for most producers of an electronic device in the price range of 40 to 100 Euro have been put to rest in the meantime by statements of various experts and representatives of the producers. After consulting with PV companies at an information event of GTZ and DFS, in early December 1997, most of them seem prepared to meet the high quality standards and to allow their products to be tested accordingly.

In the opinion of a leading producer of charge regulators, the high quality standards are justified, because these units have to operate reliably under the widest variety of climatic, environmental and application conditions world-wide. Comprehensive climate tests, for example, are indispensable and the widest variety of tests on electromagnetic compatibility (EMC) do not just prevent the device's interference with radio reception but also enhance the operating safety in the event of outside disturbances like surge voltages from lightning, for example.

Aside from the costs for the tests, which necessarily have to be taken into account in the product price, the process has nothing but advantages for the users, because the operating safety of the whole system is improved. Fears that only a small number of highly qualified and best-equipped laboratories in industrialised countries will be able to carry out these tests should be dispelled by qualifying and accrediting smaller laboratories in developing countries, for example in the framework of PV GAP. The world-wide networking should make it possible for every producer to have his products tested and certified in any accredited laboratory anywhere on the world.

Overview of Specifications for Solar Home Systems and Rural Health Power Supply

In the framework of the international survey, various documents with specifications for Solar Home Systems and their components were evaluated and summarised in the form of a table. The specifications in the table were subdivided into the following categories:

• PV generator
• Support structure
• Battery
• Charge regulator
• Lamp, ballast
• Wiring, installation
• Documentation

The following 11 documents were evaluated. Not all of the documents did include all components.

[ 1] Madrid “Universal Technical Standard for Solar Home Systems”, Instituto de Energía Solar, Universidad Politécnica de Sdrid, European Commission, Thermie B: SUP-995-96, EC-DGXVII,1998 ⁴

[ 2] FHG-ISE ´94 “Ladereglertest”, Fraunhofer Institut für Solare Energiesysteme, for GTZ OE 4150, Energie und Transport, 1994

[ 3] GTZ ´93 “Standards für SHS-Laderegler” und “Vorläufige Grundanforderungen an elektronische Vorschaltgeräte”, GTZ, 1993

[ 4] PSE Tunisia “Lastenheft Laderegler und elektronische Vorschaltgeräte für Photovoltaische Kleinsysteme”, c. 1994

[ 5] Steca Midi “Datenblatt, Solarix Midi & Mini”, Steca Solarelektronik, c. 1993

[ 6] SEP Marocco “Proposition d’un standard technique pour les systémes photovoltaiques familiaux”, CDER, Morocco, 1997

[ 7] Namibia Health ”Tender: Okavango Clinics: Photovoltaic Systems”, GTZ, Department of Works, Namibia, 1997

[ 8] Namibia SHS “Tender Annex B: Specifications for Solar Home Systems (50Wp)”, GTZ, Ministry of Mines and Energy Namibia, 1997

[ 9] TÜV/CENELEC Standard Proposals: “Test Procedures for Charge Regulators and Lighting Systems in Solar Home Systems”, CENELEC CLC BTTF 86-2, 1998

[10] Philippines ‘94 “Material Specification for Solar Home Systems”, GTZ SEP Philippinen, 1994

[11] World Bank “Indonesia: Solar Home Systems Project, Specifications”, World Bank, 1996 The results of the evaluation are listed in Annex A2. There is a separate table for each component. ⁵

The specifications table can be used to get an initial overview of which criteria and corresponding components are mentioned in the respective documents.

The table for charge regulators is the most comprehensive; charge regulators are included in all 11 documents. Due to the variety of requirements, local conditions, personal preferences and, last but not least, the different purposes for which the documents are used, a total of 91 criteria were identified for charge regulators, some of which complement each other, or also conflict with one another, and in many cases can be summarised into more general criteria.

At the same time, however, this variety of criteria also shows that there is an urgent need for standardisation, especially of the main components charge regulator and lamp/ballast. On the other hand, if one considers the table for PV generators, for example, one finds that many criteria are already covered by the reference “Qualified according to IEC 61215”.

⁴ The specifications from Madrid University additionally contain a three-tier classification of the criteria according to compulsory (C), recommended (R) and suggested (S).

⁵ The table can be made available by e-mail as an Excel file upon request.

Specifications for Tenders of SHS and RHS

The most up-to-date, comprehensive and best elaborated documents from the previous chapter 4 have been used as the basis for the proposed specifications of SHS and RHS.

Specifically, these are:

• the tender documents of the World Bank for 200,000 SHS in Indonesia (and similarly 30,000 SHS in Sri Lanka) [11]

• two tender documents for SHS [8]and RHS [ 7] in Namibia (which are partly based on the World Bank specifications)

• the proposal by the University of Madrid for a “Universal Standard” [ 1]
• the CENELEC draft standards for charge regulators and lamps/ballasts of the TÜVRheinland and FHG-ISE [ 9]

The specifications which, in the author's opinion, gave the best technical description were selected from these documents, revised and compiled according to component and topic. These can be used directly as text modules for international tenders. The minimum requirements in each case were selected in such a way that a reliably functioning system can be set up according to the technical state of the art.⁶

⁶ A separate compilation of tender documents for photovoltaic pumping systems (PVP) entitled “Proposal for Tender Documents for the Procurement of Photovoltaic Pumping-Systems (PVP)” is available from GTZ, Div. 44.

Tender Specifications for Solar Home Systems and Rural Health Power Supply Systems

Some of the texts proposed for the specifications presented here include additional notes marked “Optional”, “Health” and/or “Comment”:

Optional: Optional specifications for higher requirements, alternative equipment or special environmental conditions

Health: Additional or alternative specifications for Rural Health Power Supply Systems (RHS) with stricter requirements

Comment: Explanation of the reason for choosing a certain specification or a personal opinion of the author

Photovoltaic Generator (PV Modules)

The PV array shall consist of one or more mono- or polycrystalline photovoltaic solar module(s).

Crystalline PV modules must have been tested for qualification in compliance with IEC 61215, “Crystalline Silicon Terrestrial Photovoltaic Modules; Design Qualification and Type Approval”.⁷

The PV module(s) should have a rated peak power output of at least 45 Wpeak (with an allowable tolerance of -2.5 W_peak (-5%), alternatively -5 W_{peak</sub (-10%)), under Standard Test Conditions (STC) as defined in IEC 61215 and IEC 60904-3.}

Comment: To date the efficiency and reliability of “thin-film PV-modules” does not yet measure up to the standards of crystalline PV cells, and the price reduction is also still not convincing. In spite of that fact, this option might be feasible for future applications, especially for low-cost Solar Home Systems.
The World Bank has already allowed the use of thin-film PV modules in their Indonesian SHS project tender, but only one company (Canon, Japan) offered these kinds of modules in their quotation.

Optional: If thin-film photovoltaic modules are used, they must be product-tested and certified in accordance with IEC 61646. The peak power output for thin-film modules should be the value after light soaking.

The minimum acceptable operating voltage at MPP (Maximum Power Point) of the PV module shall be no less than 16 Vdc at a cell-operating temperature of 60° Celsius.

Optional: Each module shall comprise not less than 36 series-connected single- or polycrystalline silicon solar cells.

Comment: In order to allow a full charge of a 12 V battery under “controlled gassing” conditions, a voltage of 14.5 to 15 V must be available at the battery terminals. Including voltage losses via cables (0.5 to 1.0 V) and blocking diodes (0.4 V/Schottky diode), the PV generator voltage should be at least 1.0 to 1.5 V above that maximum battery voltage. Under certain conditions, e.g. with the use of sealed batteries (no gassing allowed), very low system losses or permanently low ambient temperatures, this value might be lower, and even PV modules with less than 36 cells might be used. But a PVgenerator voltage of 14 - 14.5 V as recommended in the “Universal Standard” Proposal [1] will definitely be too low for SHS applications in tropical countries.

In a PV array all modules should be of the same type and be interchangeable. The cabling and protection diodes must also be uniform. However, if there are sub-arrays which power separate loads or batteries, then different types of modules may be used in each sub-array if necessary.

The module(s) shall be equipped with a sealable waterproof (international protection code IP54) terminal (junction) box. The poles inside shall be clearly marked. A strain relief for the cables must be provided.

Optional: The junction box must have outlets that allow for attachment of flexible conduit pipes. If the module does not have a junction box which allows for a direct conduit connection, a weather-resistant junction/combiner box must be attached to the support structure.

Optional, for system voltage greater than 50 V: The PV modules must have bypass diodes to offer protection against hot spots in case of partial shading. The PV modules shall have a frame of non-corrosive material, e.g. anodized aluminium or stainless steel. The frame shall ensure that the module is resistant to torsion during handling and extreme weather conditions.

Comment: Only solid, framed modules are applied for SHS and RHS. At certain sites and for certain applications flexible, unframed modules may be preferred (e.g. tents of nomadic tribes). However, up to now no experience for this module type under rural conditions is available.

Each module must be clearly and permanently labelled according to DIN 40025 “Datasheets and Labels of PV Modules”, indicating: Name of Manufacturer, Model Type or Number, Serial Number, IP-Protection Code, Maximum System Voltage, Power (Watt Peak) Rating (P_max), ± Manufacturing Tolerances, Short-Circuit Current (I_SC), Open Circuit Voltage (U_OC), Maximum Power Point Voltage (U_MPP), all at Standard Test Conditions.

The PV module manufacturer, or module supplier, shall provide a minimum 10-year warranty for the replacement of any modules which:

• show defects, in terms of the qualification test stipulations of lEC-61215
• show power degradation greater than 10% below the rated power specification

(unless damaged by abuse or extreme conditions like lightning, exceptional hail, etc., which are not covered by the qualification test conditions).

For the purpose of this warranty, the rated power specification shall be a fixed value and not a range, and to effect the warranty any tests of power degradation shall conform to the international procedures for testing and referencing PV module power output.

Health, Optional: Each solar module has to have the individual clinic name sandblasted onto the bottom right-hand corner. The names are to be a minimum of 10 mm in height and are to be positioned so that the marking in no way impairs the functioning of the module or negates the guarantee issued with the module.

Support Structure

• Support structures should be able to resist, at least, 10 years of outdoor exposure without appreciable corrosion or fatigue. (C)
• Support structures must withstand winds of 120 km/h. (R)

Several materials can be used for support structures, including stainless steel, aluminium, galvanised iron with a protective layer of about 30 μm, treated wood, etc.

• In the case of framed PV modules, only stainless steel fasteners (screws, nuts, rings, etc.) may be used for attaching them to support. (C)

It is worth mentioning that frameless PV modules bonded to a frame with suitable adhesive products, while today scarcely used onto SHS market, are performing well in general PV applications and can also be accepted.

• Tilt angle should be selected to optimise the energy collection during the worst month, i.e., the month with the lowest ratio of monthly mean daily irradiation to the monthly mean daily load. Generally, constant user load can be assumed. Then, the following formula can be used
  Tilt (°) = max {|Φ|} + 10°}
  where Φ is the latitude of the installation. (R)

This formula leads to a minimum tilt angle of 10°, which is sufficient to allow rainwater to drain off the surface. It may also be useful to note that slight azimuth deviations from south/north (+/- 30°) and in the tilt angle (+/- 10°) have a relatively small influence on the energy output of a PV array.

Most of the consulted experts are opposed to manual tracking because it implies a risk of damage to the modules, and a risk of energy lost through poor or no adjustment. However, it has been used in some places with positive results not only in terms of energy gain, but also in terms of user participation. Naturally, adequate training is needed and the tracking features, including any hinges and other coupling devices needed to allow the modules to be moved, must also meet the requirements specified above. Hence:

• Static support structures are generally preferable to tracking-ones (R)
• In the case where manual tracking (2 or 3 positions per day, moving from East to West) is used, all of its features must meet the support structure requirements specified above (C).

 

Battery

For the battery, the most important feature of its operation in SHSs is cycling. During the daily cycle, the battery is charged during the day and 11 discharged by the night-time load. Superimposed onto the daily cycle is the seasonal cycle, which is associated with periods of reduced radiation availability. This, together with other operating parameters (ambient temperature, current, voltages, etc.) affects the battery life and maintenance requirements. In order to maximise the lifetime of lead acid batteries, the following operating conditions must be avoided⁸:

• High voltages during charging (to prevent against corrosion and loss of water)
• Low voltages during discharge (corrosion)
• Deep discharge (sulphation, growth of dentrites)
• Extended periods without a fully charging (sulphation)
• High battery temperatures (all ageing processes are accelerated)
• Stratification of the electrolyte (sulphation)
• Very low charge currents (sulphation)

These rules lead to specifications for sizing (both battery and PV generator) and for battery protection procedures (charge regulator). However, it must be pointed out that some of the rules are generally in contradiction with each other (e.g. full charging needs high voltages but high voltages accelerate corrosion), so compromises must be found taking into account the particular local conditions: solar radiation, PV module and battery prices duties and taxes, local manufacturing, recycling infrastructure, etc. Perhaps this explains the lack of consensus on this issue among the different information sources (standards, experts, etc.) that have been consulted during the preparation of this standard, and the requirements given below should therefore be adapted to suit the local circumstances.

The need to prevent excessive discharge leads to the need to limit the maximum depth of discharge to a certain value, PD_MAX, which usually ranges from 0.3 to 0.6, but can approach 0.8 according to the type of battery. The supply to the load must be cut when this limit is reached. The available or useful capacity, C_U, is therefore less than the nominal capacity, C_B, (which refers to the whole charge that could be extracted from the battery if no particular limitations were imposed) and equal to the product C_B x PD_MAX, , such that:

[8] G. Bopp et al., "Energy Storage in Photovoltaic stand alone energy supply systems". Progress in Photovoltaics (to be published).

The wiring

Source: Universal technical standard for solar home systems, Version 2, Thermie B SUP 995-96, EC-DGXVII, 1998, updated in 2001.

  ⇒ Back to Solar Section