8. Evaluate Quotations and Assess Quality
On the submission date, the quotations / offers by different suppliers are to be opened and need to be evaluated with regard to technical and financial aspects. The following factors should be taken into consideration:
- For the comparison and evaluation of the different quotations / offers, it is advisable to prepare an Excel worksheet in which features and prices of the individual system components and services are entered.
- The definition of evaluation criteria and weighting of technical and financial aspects facilitate the assessment.
The following aspects help to assess the quality of system components offered:
It is important to assess the warranty period, which is often limited to 5 years. Individual system components, such as solar panels, usually come with a 10 year product guarantee and a linear performance warranty which underwrites at least 80% power output by the end of the 25th year.
Solar generator/quality of solar panels
Installed under harsh environmental conditions, solar panels are constantly exposed to high temperatures and UV irradiance, dust, humidity and rain. This puts a lot of stress on embedding materials and electrical connections. Therefore, only high-quality products should be offered that meet the standards of the International Electrotechnical Commission (IEC).
For the electrical installation of a photovoltaic system, wiring and cabling should be used that meet the requirements for this application. For DC connections, single-wire cables with double insulation are a practicable and reliable solution. They should be UV and weather resistant and suitable for a wide temperature range.
PV array combiner box
The combiner box should be made to Protection Class II and demonstrate a clear separation of the positive and negative sides within the box. If mounted externally, it should be protected to at least IP54 Ingress Protection rating or higher).
| Note: The protection class from EN60529 is indicated by short symbols that consist of the two code letters IP and a code numeral specifying the protection degree. The first digit represents limited protection against dust ingress (no harmful deposits). The second digit represents protection against splash water from any direction.
In most Solar Powered Irrigation Systems, PV panels are installed in the open field and therefore require a sturdy and weather-resistant mounting structure. Quality mounting systems consist of galvanized steel or aluminum profiles. When mounting PV panels and profiles, specially developed brackets, screws, washers and nuts should be used (this also contributes to reducing the risk of theft, which should be part of the evaluation criteria). To avoid galvanic corrosion, it is important to select materials with similar corrosion potentials or to break the electrical connection by insulating the two metals from each other.
Pump controller/ inverter
Modern controllers must incorporate high-efficiency power electronics and utilize Maximum Power Point Tracking (MPPT) technology to maximize power use from the PV generator. Additional features to increase system reliability should include over- and under-voltage protection as well as protection against reverse polarity, overload and over-temperature.
Solar water pumps must be constructed from non-corrosive stainless steel. Since DC motors tend to have higher overall efficiency levels than AC motors of a similar size, they are often the first choice of quality solar pump manufacturers. Some solar pumps are still equipped with comparably cheap brushed DC motors. The main disadvantage of brushed motors is that brushes are subject to wear and tear and need to be replaced at regular intervals (approximately every two years). Therefore, in terms of system reliability, the use of brushed DC motors is not recommended as regular maintenance cannot be assured in remote areas of developing countries.
Water distribution system
Water-saving irrigation technologies working at comparably low operating pressures are the preferred option in connection with PV pumps. To assess the suitability of the distribution systems, it is important to know the hydraulic characteristic. Details should be provided by the supplier / system integrator. The performance under low operating pressures (e.g. in the early morning and late evening) and the uniformity of water distribution across the field is of particular interest.
After a first technical evaluation
- Results should be discussed with other technical experts (agricultural advisors, research institutes etc.).
- Quoted prices of suppliers and related services offering similar products need to be compared.
- The providers with the best quotations / offers should be invited for individual presentation and negotiation.
- Structured comparison of qualified quotations / offers;
- Ranking of quotations / offers;
- Invitation of potential suppliers / system integrators for presentation and negotiation.
- Quotations / offers including technical and financial parts;
- Unit price listing;
- Quality and safety certificates;
- Technical data sheets of system components;
- hydraulic characteristic of irrigation system;
- information on warranty and after- sale services.
- Agricultural service providers;
- Suppliers / system integrators.
- High-quality systems that are good value for money should always be given preference.
- Costs should never be reduced by compromising on system quality or by decreasing support services.
- A conclusion of maintenance contracts between the producer and the service provider is recommended but not very common.
- System integration in the form of turn-key solutions is preferable, yet still very hard to find.