Difference between revisions of "Technical Standards for Solar Home Systems (SHS)"
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=== Battery === | === Battery === | ||
| − | Comment: The storage batteries are still the weakest, most vulnerable component in a | + | <u>''Comment''</u>'': The storage batteries are still the weakest, most vulnerable component in a photovoltaic power supply system. This might also be the reason why different types of batteries, ranging from automotive starter batteries and so-called “Solar Batteries”, all the way to high-quality industrial tubular plate (OPZS) batteries, and also sealed maintenance-free batteries, are used in PV systems.'' |
| − | photovoltaic power supply system. This might also be the reason why different types of | ||
| − | batteries, ranging from automotive starter batteries and so-called “Solar Batteries”, all | ||
| − | the way to high-quality industrial tubular plate (OPZS) batteries, and also sealed | ||
| − | maintenance-free batteries, are used in PV systems. | ||
| − | The “Universal Standard for SHS” [1] gives a brief overview of the various aspects, | + | ''The “Universal Standard for SHS” [1] gives a brief overview of the various aspects, advantages and disadvantages of the different battery types and their useful application in SHS. Some of the following observations may serve as an introduction for planners of subsequent specifications:'' |
| − | advantages and disadvantages of the different battery types and their useful application | ||
| − | in SHS. Some of the following observations may serve as an introduction for planners of | ||
| − | subsequent specifications: | ||
| − | The most important feature of battery operation in SHSs is cycling. During the daily | + | ''The most important feature of battery operation in SHSs is cycling. During the daily cycle, the battery is charged over the day and 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, voltage, etc.), affects the battery life and maintenance requirements. In order to maximise the lifetime of lead-acid batteries, the following operating conditions must be avoided:'' |
| − | cycle, the battery is charged over the day and 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, voltage, etc.), affects the battery life and maintenance | ||
| − | requirements. In order to maximise the lifetime of lead-acid batteries, the following | ||
| − | operating conditions must be avoided: | ||
| − | *High voltage during charging (to prevent corrosion and loss of water) | + | *''High voltage during charging (to prevent corrosion and loss of water)'' |
| − | *Low voltage during discharge (corrosion) | + | *''Low voltage during discharge (corrosion)'' |
| − | *Deep discharge (sulphation, growth of dentrites) | + | *''Deep discharge (sulphation, growth of dentrites)'' |
| − | *Extended periods without full charging (sulphation) | + | *''Extended periods without full charging (sulphation)'' |
| − | *High battery temperature (all aging processes are accelerated) | + | *''High battery temperature (all aging processes are accelerated)'' |
| − | *Stratification of the electrolyte (sulphation) | + | *''Stratification of the electrolyte (sulphation)'' |
| − | *Very low charge current (sulphation) | + | *''Very low charge current (sulphation)'' |
| − | These rules lead to specifications for sizing (both battery and PV generator) and for | + | ''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 generally contradict each other (e.g. full charging requires high voltages but high voltages accelerate corrosion), so compromises must be found that take the particular local conditions into account: 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 various sources of information (standards, experts, etc.) that have been consulted during the preparation of this standard; therefore, the requirements given below should be adapted to suit the local circumstances.'' |
| − | battery protection procedures (charge regulator). However, it must be pointed out that | ||
| − | some of the rules generally contradict each other (e.g. full charging requires high | ||
| − | voltages but high voltages accelerate corrosion), so compromises must be found that | ||
| − | take the particular local conditions into account: 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 various sources of information | ||
| − | (standards, experts, etc.) that have been consulted during the preparation of this | ||
| − | standard; therefore, the requirements given below should be adapted to suit the local | ||
| − | circumstances. | ||
| − | The need to prevent excessive discharge leads to the need to limit the maximum depth | + | ''The need to prevent excessive discharge leads to the need to limit the maximum depth of discharge to a certain value, PD<sub>MAX</sub>, which usually ranges from 0.3 to 0.6, but can approach 0.8, depending on the type of battery. The supply to the load must be cut off when this limit is reached. The available or useful capacity, C<sub>U</sub>, is therefore less than the nominal capacity, C<sub>B</sub> (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<sub>B</sub> * PD<sub>MAX</sub>, such that:'' |
| − | of discharge to a certain value, PD<sub>MAX</sub>, which usually ranges from 0.3 to 0.6, but can | ||
| − | approach 0.8, depending on the type of battery. The supply to the load must be cut off | ||
| − | when this limit is reached. The available or useful capacity, C<sub>U</sub>, is therefore less than the | ||
| − | nominal capacity, C<sub>B</sub> (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<sub>B</sub> * PD<sub>MAX</sub>, | ||
| − | such that: | ||
| − | C<sub>U</sub> = C<sub>B</sub> * PD<sub>MAX</sub> | + | ''C<sub>U</sub> = C<sub>B</sub> * PD<sub>MAX</sub>'' |
| + | |||
| + | ''A good compromise between cost and reliability is typically obtained with a battery whose useful capacity ranges from three (in regions where extended cloudy periods are not expected) to five (in regions where cloudy periods are expected) times the total daily energy consumption in the house, so that the depth of discharge in the daily cycle, PD<sub>d</sub> ranges from 0.06 to 0.2. The selection of a particular capacity mainly depends on the battery type. High-quality batteries are better able to resist deeper cycling than low-quality batteries. Hence, for the same application, high-quality batteries can be smaller than low-quality batteries, in terms of nominal capacity.'' | ||
| + | |||
| + | ''The highest-quality PV batteries are made with tubular plates and grids with low Sb-Se content. More than 8 years life, with PD<sub>d</sub> = 0.2 and a maintenance period of 1 or 2 times per year, are attainable with such batteries. A particular disadvantage of tubular batteries for SHSs is that they do not readily accept low rates of charge. They are also expensive and are rarely available in the current markets in developing countries. Nevertheless, they should not be excluded from SHS programmes. On the contrary, it is recommended that large rural electrification programmes consider encouraging manufacturers to put these products on the market.'' | ||
| + | |||
| + | In contrast, automotive batteries, usually referred to as SLI, have a number of | ||
| + | advantages. They are usually the cheapest batteries when compared in terms of nominal | ||
| + | capacity (the difference in cost can be 4 or 5 fold), they are often locally produced and | ||
| + | are widely available. Local production is not only convenient for economic and social | ||
| + | reasons, but also because it represents the best means of recycling old batteries and | ||
| + | avoiding environmental damage. Their main drawback lies in their relatively short | ||
| + | lifetime. Because their cell design is optimised to deliver heavy currents during short | ||
| + | periods of time, they have large areas of thin plates, and are poorly suited to supplying | ||
| + | smaller currents for many hours before being recharged, as is required by SHS. It is | ||
| + | therefore necessary to use larger battery capacities leading to PD<sub>d</sub> £ 0.1, and a density of | ||
| + | electrolyte which is lower than would normally be used in this type of battery (for | ||
| + | example, 1.24 instead of 1.28 g/cl). This is necessary to reduce grid corrosion and hence | ||
| + | to lengthen battery life. The associated increase in internal resistance in the battery does | ||
| + | not pose any problems in SHS, because the charge and discharge currents are relatively | ||
| + | low by comparison to conventional battery charge and discharge regimes. Classical SLI | ||
| + | batteries use lead grids alloyed with antimony and require periodic topping up with water. | ||
| + | |||
| + | The short lifetimes of automotive batteries can also be compensated to some extent by | ||
| + | introducing relatively simple modifications to the battery design but not to its technology. | ||
| + | The most common modifications are thicker electrode plates and a larger quantity of acid | ||
| + | solution in the space above the plates. Such modified SLI batteries are sometimes | ||
| + | marketed as "solar" batteries and represent a promising alternative for the future of | ||
| + | SHSs. Wherever possible, modified SLI batteries should be selected (and local | ||
| + | manufacturers should be encouraged to make them) in preference to conventional SLI | ||
| + | batteries. Certain conditions must be met in order for a battery to be categorised as | ||
| + | "modified SLI", as follows: | ||
| + | |||
| + | *The thickness of each plate must exceed 2 mm. | ||
| + | *The amount of electrolyte must exceed 1.15 l per 100 Ah of 20-hour nominal capacity and per cell. | ||
| + | *The separator must be made of microporous polyethylene. | ||
| + | *The density of electrolyte must not exceed 1.25 g/cl. | ||
| + | |||
| + | "Low-Maintenance" SLI batteries, sometimes marketed as maintenance-free batteries, | ||
| + | often employ grids containing calcium alloys. The calcium increases the voltage at which | ||
| + | gassing begins but reduces the cohesion of the active material to the grids. Hence, it | ||
| + | cuts down the loss of water but also reduces the cycle life. Such batteries are particularly | ||
| + | vulnerable to damage from deep discharge. In addition, they are also liable to be | ||
| + | damaged by high temperature variations. Hence, many PV system designers strongly | ||
| + | recommend that they not be used in PV applications in hot countries. However, the | ||
| + | maintenance-free feature is still attractive, and extensive use has been made of these | ||
| + | batteries in some countries like Brazil. | ||
| + | |||
| + | "No-maintenance" batteries of various kinds are also made for professional applications | ||
| + | by using a semi-solid electrolyte (gel or malting). Such batteries, referred to as VRLA | ||
| + | (valve-regulated lead acid), are more often resistant to deep discharges, but they are | ||
| + | usually very expensive for SHSs, and they require specific recycling facilities. They are | ||
| + | not considered in the present standard although they represent a legitimate technology | ||
| + | choice in some cases. The same is valid for NiCd batteries. | ||
| + | The 20-hour nominal battery capacity in amp-hours (measured at 20 W and up to a | ||
| + | voltage of 1.8 V/cell) should not exceed CR times the PV generator short-circuit current | ||
| + | in amps (measured at Standard Test Conditions). CR values are proposed for each type | ||
| + | of battery in the table below: | ||
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Revision as of 14:57, 20 July 2009
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):
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.
Current Status of Standardisation Activities for Solar Home Systems
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 (To view the tables downolad the whole text.)
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 Wpeak (-5%), alternatively -5 Wpeak (-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 (Pmax), ± Manufacturing Tolerances, Short-Circuit Current (ISC), Open Circuit Voltage (UOC), Maximum Power Point Voltage (UMPP), 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.
Module Support Structure
The module support (array mounting) structure shall hold the PV module(s).
The module(s) shall be mounted either on the rooftop of the house or on a metal pole that can be fixed to the wall of the house or separately in the ground, with the module(s) at least 3 (4) meters off the ground.
Roof-mounting: Minimum clearance between the PV module(s) and the roofing material must be at least 10 cm. It is recommended that the module mounting structure be supported on top of a pole at least 50 cm long or fixed with supporting angles at four positions. The mounting structure must be anchored to the building or to the under-roof beam structure and not to the roofing material.
Wall-mounting: A metal pole must be fixed to the outer wall of a house by appropriate clamps and fixing material (screws and wall plugs in solid walls or screws in wooden beams) in at least two positions at a reasonable distance. If the pole is not higher than the top of the house, the problem of shading from house-walls or roof-parts must be taken into consideration.
Ground-mounting: A metal pole at least 2“ (50 mm) in diameter must be used with the modules attached at the top of the pole. The pole must be anchored in concrete at least one meter deep in the ground.
The pole and mounting structure shall be sufficiently rigid to prevent twisting in the wind or if large birds alight on the array. The support structure shall be able to withstand winds up to 120 km/h (150 km/h in windy areas).
All metal parts shall be made of non-corroding materials (aluminium, stainless steel) or adequately protected against corrosion by galvanisation (layer approx. 30mm). The support structure should be able to withstand at least 10 years of outdoor exposure without appreciable corrosion or fatigue.
The structure shall incorporate galvanised steel or stainless steel hardware (bolts, nuts, washers, etc.) for all external connections. These include the modules-to-structure, structure-to-pole and pole-to-building attachments. Particular attention shall be given to protection against galvanic corrosion if different metals are in contact. Different kinds of metal have to be kept separate. Under corrosive environmental conditions (high humidity, high salt content), only stainless steel hardware is allowed.
Optional: The use of rivets or tamper-proof (non-removable) screws for theft protection is recommended.
No objects (trees, buildings, etc.) shall shade any part of the PV modules at any time of the year between 90 minutes after sunrise and 90 minutes before sunset. It should be noted that shading of even a small part of a module or array could cause a considerable reduction in power output. In situations where partial shading is unavoidable, this must be compensated in the system sizing calculations.
The PV modules shall be mounted in a position which allows safe, controlled access for inspection and cleaning. However, security from possible theft and damage may also be important considerations. Where necessary, suitable measures shall be taken to reduce the risk of theft or damage (e.g. from flying stones). It should be possible, however, to remove modules for service, using appropriate tools.
The module(s) shall be installed facing towards the equator (south in the northern hemisphere and north in the southern hemisphere).
The tilt angle should be selected by computer simulation to optimise the energy collection during the month with the lowest mean daily irradiation. To guarantee a selfcleaning effect of the modules by rainwater, the modules shall not be installed at a flatter angle than a minimum tilt angle of 15°.
Optional: If computerised calculation is not available, as a general rule (in areas up to a latitude of 40° around the equator), a tilt angle to the horizontal plane equal to the latitude +10° can be assumed as a good approximation.
Optional: Where necessary, deviations ±5° from the orientation to the equator shall be acceptable, unless otherwise specified. Where necessary, deviations of ±5° from the optimum tilt angle shall be acceptable.
Optional: Facilities for users to adjust the tilt angle in different seasons or to perform a manual tracking throughout the course of the day shall be acceptable, provided the users are well informed and wish to do so. In this case, the strength of mounting (e.g. resistance to wind-loading) shall remain sufficient.
Optional: Passive tracking systems for the PV generator shall be acceptable, if the additional gain of energy justifies the additional cost, provided that the tracking system can withstand wind-loading requirements and is of proven reliability. As the advantage of a tracking system is only valid during direct sunshine periods, the energy gained by the tracker can only be considered for system calculations if detailed irradiation statistics, including measurement of direct and diffuse insolation, are available for the site. Active trackers (requiring electrical power) are not allowed.
Battery
Comment: The storage batteries are still the weakest, most vulnerable component in a photovoltaic power supply system. This might also be the reason why different types of batteries, ranging from automotive starter batteries and so-called “Solar Batteries”, all the way to high-quality industrial tubular plate (OPZS) batteries, and also sealed maintenance-free batteries, are used in PV systems.
The “Universal Standard for SHS” [1] gives a brief overview of the various aspects, advantages and disadvantages of the different battery types and their useful application in SHS. Some of the following observations may serve as an introduction for planners of subsequent specifications:
The most important feature of battery operation in SHSs is cycling. During the daily cycle, the battery is charged over the day and 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, voltage, etc.), affects the battery life and maintenance requirements. In order to maximise the lifetime of lead-acid batteries, the following operating conditions must be avoided:
- High voltage during charging (to prevent corrosion and loss of water)
- Low voltage during discharge (corrosion)
- Deep discharge (sulphation, growth of dentrites)
- Extended periods without full charging (sulphation)
- High battery temperature (all aging processes are accelerated)
- Stratification of the electrolyte (sulphation)
- Very low charge current (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 generally contradict each other (e.g. full charging requires high voltages but high voltages accelerate corrosion), so compromises must be found that take the particular local conditions into account: 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 various sources of information (standards, experts, etc.) that have been consulted during the preparation of this standard; therefore, the requirements given below should 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, PDMAX, which usually ranges from 0.3 to 0.6, but can approach 0.8, depending on the type of battery. The supply to the load must be cut off when this limit is reached. The available or useful capacity, CU, is therefore less than the nominal capacity, CB (which refers to the whole charge that could be extracted from the battery if no particular limitations were imposed) and equal to the product CB * PDMAX, such that:
CU = CB * PDMAX
A good compromise between cost and reliability is typically obtained with a battery whose useful capacity ranges from three (in regions where extended cloudy periods are not expected) to five (in regions where cloudy periods are expected) times the total daily energy consumption in the house, so that the depth of discharge in the daily cycle, PDd ranges from 0.06 to 0.2. The selection of a particular capacity mainly depends on the battery type. High-quality batteries are better able to resist deeper cycling than low-quality batteries. Hence, for the same application, high-quality batteries can be smaller than low-quality batteries, in terms of nominal capacity.
The highest-quality PV batteries are made with tubular plates and grids with low Sb-Se content. More than 8 years life, with PDd = 0.2 and a maintenance period of 1 or 2 times per year, are attainable with such batteries. A particular disadvantage of tubular batteries for SHSs is that they do not readily accept low rates of charge. They are also expensive and are rarely available in the current markets in developing countries. Nevertheless, they should not be excluded from SHS programmes. On the contrary, it is recommended that large rural electrification programmes consider encouraging manufacturers to put these products on the market.
In contrast, automotive batteries, usually referred to as SLI, have a number of advantages. They are usually the cheapest batteries when compared in terms of nominal capacity (the difference in cost can be 4 or 5 fold), they are often locally produced and are widely available. Local production is not only convenient for economic and social reasons, but also because it represents the best means of recycling old batteries and avoiding environmental damage. Their main drawback lies in their relatively short lifetime. Because their cell design is optimised to deliver heavy currents during short periods of time, they have large areas of thin plates, and are poorly suited to supplying smaller currents for many hours before being recharged, as is required by SHS. It is therefore necessary to use larger battery capacities leading to PDd £ 0.1, and a density of electrolyte which is lower than would normally be used in this type of battery (for example, 1.24 instead of 1.28 g/cl). This is necessary to reduce grid corrosion and hence to lengthen battery life. The associated increase in internal resistance in the battery does not pose any problems in SHS, because the charge and discharge currents are relatively low by comparison to conventional battery charge and discharge regimes. Classical SLI batteries use lead grids alloyed with antimony and require periodic topping up with water.
The short lifetimes of automotive batteries can also be compensated to some extent by introducing relatively simple modifications to the battery design but not to its technology. The most common modifications are thicker electrode plates and a larger quantity of acid solution in the space above the plates. Such modified SLI batteries are sometimes marketed as "solar" batteries and represent a promising alternative for the future of SHSs. Wherever possible, modified SLI batteries should be selected (and local manufacturers should be encouraged to make them) in preference to conventional SLI batteries. Certain conditions must be met in order for a battery to be categorised as "modified SLI", as follows:
- The thickness of each plate must exceed 2 mm.
- The amount of electrolyte must exceed 1.15 l per 100 Ah of 20-hour nominal capacity and per cell.
- The separator must be made of microporous polyethylene.
- The density of electrolyte must not exceed 1.25 g/cl.
"Low-Maintenance" SLI batteries, sometimes marketed as maintenance-free batteries, often employ grids containing calcium alloys. The calcium increases the voltage at which gassing begins but reduces the cohesion of the active material to the grids. Hence, it cuts down the loss of water but also reduces the cycle life. Such batteries are particularly vulnerable to damage from deep discharge. In addition, they are also liable to be damaged by high temperature variations. Hence, many PV system designers strongly recommend that they not be used in PV applications in hot countries. However, the maintenance-free feature is still attractive, and extensive use has been made of these batteries in some countries like Brazil.
"No-maintenance" batteries of various kinds are also made for professional applications by using a semi-solid electrolyte (gel or malting). Such batteries, referred to as VRLA (valve-regulated lead acid), are more often resistant to deep discharges, but they are usually very expensive for SHSs, and they require specific recycling facilities. They are not considered in the present standard although they represent a legitimate technology choice in some cases. The same is valid for NiCd batteries. The 20-hour nominal battery capacity in amp-hours (measured at 20 W and up to a voltage of 1.8 V/cell) should not exceed CR times the PV generator short-circuit current in amps (measured at Standard Test Conditions). CR values are proposed for each type of battery in the table below:
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