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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 different of standardisation activities and existing standards that are relevant for Solar Home Systems (SHS) and Rural Health Power Supply Systems (RHS).
The following Wiki-Page
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.
Reliability
SHS reliability, in the sense of lack of failures, depends not only on the reliability of the components, but also on some other features of the system which can directly affect the lifetime of batteries and lamps, such as size, the voltage thresholds of the charge regulator, the quality of installation, etc. Each component of the system must fulfil similar quality and reliability requirements because, if there is only one bad component in an otherwise perfect system, this will limit the quality of the whole.
PV Generator
- PV modules certified according to the international standard IEC-61215 or to the national standard of PV modules used by the relevant country. (R)
This requirement currently excludes thin-film PV modules, although certification procedures for such modules are also available (IEC-61646, SERI/TR-213-3624). Thin film modules are permitted in some projects supported by the World Bank and promising new modules are emerging onto international markets but until now the field experience with commercially available thin-film modules has been rather discouraging 6,7. Their use in largescale programmes is therefore still considered to be extremely risky and it is recommended that they should only be accepted if supported by comprehensive long term guarantees.
[6] M.J.Manimala. "Solar Photovoltaic Lanterns in rural India: a socio-economic evaluation of the schema as implemented in the state of Maharashtra in India". 12th EC PV Solar Energy Conference. Amsterdam. 1994
[7] E. Dunlop et al. "Electrical Characterisation and Analysis of Operating Conditions of Amorphous Silicon Building Integrated Photovoltaic Modules". 14th EC PV Conference. Barcelona. 1997
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, PDMAX, 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, 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 x PDMAX, , such that:
[8] G. Bopp et al., "Energy Storage in Photovoltaic stand alone energy supply systems". Progress in Photovoltaics (to be published).
The charge regulator
The loads (mainly lighting)
The wiring
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