Technical Standards for Solar Home Systems (SHS)

The "Instituto de Energía Solar" at the "Universidad Politécnica de Madrid" prepared a report that is designed to form the basis for a Universal Standard for SHS. This Wiki-Page gives an overview of the standards proposed for the different parts of a typical SHS.

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

Introduction

Field experience with PV rural electrification has shown that the performance of solar home systems SHSs is not always entirely satisfactory. However, in-depth studies of the problems encountered in existing installations have revealed that the pure solar part, i.e. the PV generator, rarely fails. The PV system is often initially blamed for the failure but, when things go wrong, it is usually the other PV system components or the appliances which are powered by the PV generator which are found to have failed. This is mainly because, while PV modules are highly standardised and certified using internationally validated procedures, there are no equivalent standards and procedures available for balance-of-system components, component matching or installation quality, even though the quality of these components has a dramatic influence on user satisfaction and operating costs.

This report results from work which has been funded by the European Commission under Thermie B contract (ref SUP 995 96), and is designed to form the basis for a Universal Standard for Solar Home Systems. It is based on a world-wide review of existing technical standards for SHSs (see Annex 1), which has revealed a large number of inconsistencies¹ between these standards. In particular, different approaches have been found for system sizing, and for specifying types of PV modules, the number of cells in PV modules, types of battery, charge regulation voltage set points, operational information for users, voltage drops, safety measurements and ballast, cables and connectors requirements.

In preparing this report, each of the different approaches has been assessed using scientific reasoning, empirical evidence and the personal experience of the authors. To a large extent, the standard proposed here can therefore be considered as Universal, because each of the existing standards has provided extremely valuable inputs. Moreover, a first draft version has been circulated amongst a wide number of experts from different countries (see Annex 2), and their invaluable comments have also been taken into account.

In parallel with the above mentioned review of existing standards, an enquiry was carried out to identify the concerns of key persons involved in PV rural electrification programmes, and to seek their views on the usefulness and implementation possibilities of a Universal Standard for SHSs. The need for flexibility, which would allow it to be adapted to the particular conditions of each country (climate, local manufacture, internal market, indigenous capabilities, etc.), has been the most outstanding demand. In order to meet this demand, the requirements presented in this standard have been classified into three categories: Compulsory, Recommended and Suggested.

Compulsory requirements (C) are those which could directly affect safety or reliability. Failure to meet these requirements could lead to personal injuries or to SHS failure, and they are therefore intended to constitute a minimum core of requirements which must be fulfilled anywhere in the world.

Recommended requirements (R) are those which would normally lead to system optimisation. Most of these requirements are universally applicable, and failure to meet them would normally lead to a cost increase. However, because economic considerations can depend on local conditions, the application of these requirements must be reviewed for each particular case.

Suggested requirements (S) are those which might be expected to produce a sound installation. However, it should be noted that any judgement of soundness is essentially subjective, so the suggested requirements given here may have been influenced by the personal experience of the authors, and their applicability should also be reviewed for each particular case.

[1] "Systematic comparison of SHSs existing standards". IES Internal Report. 1997

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, 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

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