WP 4: Activities on Rural Energy Access

Report on Technology Options October 2007

Authors: Sebastian Goelz (Fraunhofer ISE)

Werner Roth (Fraunhofer ISE) Joachim Went (Fraunhofer ISE)

0 Acknowledgements This report was compiled with the support of some colleagues from Fraunhofer ISE and

some partner companies.

The section to Technology options for Wind Power systems and PV Wind Hybrid sys-tems are a contribution from INENSUS GmbH. We thank Dipl.-Ing. Jakob Schmidt-Reindahl

a lot for this support. Please find below his contacts for detailed inqueries:

INENSUS GmbH

Dipl.-Ing. Jakob Schmidt-Reindahl

Am Stollen 19

D-38640 Goslar, Germany

URL www.inensus.com eMail js@inensus.com, Skype kuba358

Tel +49 (5321) 68 55 10-2, Mobil +49 (178) 20 98 216, Fax +49 (5321) 68 55 10-9

For the section Solar Water Pumping and Pumps benefitted from a contribution from In-

genieurbüro MAYER. We thank Priv. Doz. Dr.-Ing. Oliver Mayer a lot for this support. Please

find below his contacts for detailed inqueries:

Priv. Doz. Dr.-Ing. O. Mayer

Kössenerstr. 6b,

D-81373 München, Germany

URL www.ibom.de eMail mayer@ibom.de,

1 Content 0 Acknowledgements .........................................................................................................2 1 Content............................................................................................................................3 2 Objectives of this report for WP 4 ...................................................................................4 3 Principle design and technical options of a Multi-functional RET system .......................5

3.1 Photovoltaic hybrid system.......................................................................................5 3.1.1 Annual solar irradiance curve and annual global irradiation energy .................6 3.1.2 Compensating for the “winter solar doldrums” with an back-up generator .......7 3.1.3 Selecting a suitable back-up generator ..........................................................11

3.2 Set-up of hybrid systems ........................................................................................13 3.3 Flexible system components for PV hybrid systems ..............................................14

3.3.1 DC system featuring standardized component communication......................14 3.3.2 AC-coupled stand alone supply structure .......................................................15

3.4 Technology options for PV Wind hybrid systems ...................................................16 3.4.1 Site evaluation ................................................................................................17

3.5 Hybrid System Design ............................................................................................18 4 Solar Water Pumping ....................................................................................................21

4.1 Photovoltaic Pumping Systems (PVPS) .................................................................21 4.2 Economy of PVPS ..................................................................................................21 4.3 Description of PVP System Components and Actuating Variables ........................23

4.3.1 Drive Motors ...................................................................................................24 4.3.2 Pumps.............................................................................................................26 4.3.2.1 Centrifugal Pumps ......................................................................................26 4.3.2.2 Positive Displacement Pumps ....................................................................28 4.3.2.3 Progressive Cavity Pumps (Screw Pumps) ................................................28 4.3.2.4 Piston Pumps..............................................................................................29 4.3.2.5 Diaphragma Pumps ....................................................................................30 4.3.3 Wells ...............................................................................................................30

4.4 Sample Applications for Photovoltaic Pumping Systems .......................................32 5 References....................................................................................................................34

2 Objectives of this report for WP 4 According the available data and the rural energy needs for productive application and social infrastructure a priorization list has been developed (see WP 4: Activities on Rural Energy Access, Interim Report , July 2007). From this priorization two possible basic RET solutions have been identified to satisfy the needs with renewable energy technologies. All of these rely on solar technology for power generation. A combination with a wind power system might be suitable for the costal areas of Mozambique and the mountain areas of Swaziland. The rivers are too small and the height differences to large to install any kind of hydro sys-tem. The biomass option also seems not suitable since most of the land is already used for farming, and animals are very few in number. In this report the technical options according to up to date state of the art technology will be presented for the proposed RET:

Multi-functional RET system An option for rural energy access is the establishment of an „energy centre“, e.g. at a school. There, a larger system could be installed, supplying the school needs (e.g. water for gardening and drinking, lights, computer, with the advantage of many bene-ficiaries), the direct needs of some households nearby (e.g. irrigation and drinking wa-ter) and the needs of many people in the area (e.g. charging batteries, grind corn). The multi-functional energy system can be realised in many different designs and capacities. In the Chapter 2 this technical options and relevant conditions are de-scribed.

Mobile PV water pumping system Because the need for irrigation was by far the most urgent one, a RET water pumping system was selected as the application with highest socio-economic impact. This could be a little solar system (power dependent on use patterns) which is installed on a mobile vehicle (e.g. a little trolley). Optimally it could be equipped with different adapters to supply various applications (water pumps, battery charging). The sug-gested systems are not equipped with a storage system, which would increase the costs dramatically. This vehicle can be brought to the place of use and be shared with others. The costs for the users are relatively low, they pay per use or per time frame. However, any kind of operating institution might be required to buy, maintain, manage and rent these mobile systems. The farmers’ cooperative might be a good candidate for this job. The mobile solar pumping system consists of a solar-pump and a solar-panel. The size of the pump and the solar-panel is depending on the field acreage, which is planned for irrigation. The PV water pumping system can be realised in many dif-ferent designs and capacities. In the Chapter 3 this technical options and relevant conditions are described.

3 Principle design and technical options of a Multi-functional RET system

The modular design of photovoltaic generator allows for energy supply systems to be set up across an extraordinarily large power range. But regardless of the output of the systems, their basic design is very similar. An off-grid photovoltaic system generally consists of a photovoltaic generator, a charge regu-lator, a storage battery, and voltage regulator. If required, the photovoltaic generator can be combined with other generators. When selecting the modules, the module’s output and voltage have to be correctly dimen-sioned. Furthermore, the module has to be mechanically constructed to withstand the weather and climatic conditions in the long run. Depending on the available space and the kind of integration, the geometric dimensions and the physical properties and attachment options of the module’s frame may also play a role. To prevent the storage batteries used from being overcharged or deep discharged, a charge regulator is used between the photovoltaic generator, the battery, and the load. The charge regulator generally also contains a discharge protection diode that prevents the battery from discharging over night through the photovoltaic generator. A good charge regulator con-sumes very little power and has a low voltage disconnect that protects the storage battery from being deep discharged. The storage battery stores the energy produced by the photovoltaic generator and makes it available to the consumer during bad weather or at night. In small systems powered by photovoltaics, lead batteries are usually used. Thus, special models of car batteries with extra thick lead plates (called solar batteries) are used for mobile applications, for instance to power electric consumers in campers, boats, and weekend homes. In photovoltaic systems to power homes with permanent residents and daily charg-ing/discharging cycles, usually tubular plate (“OPzS") batteries are used. They have deep cycles and hence long service lives. Sometimes, normal car batteries are used in Solar Home Systems because they are more readily available and cheaper. For some applications, maintenance-free lead batteries are useful; their electrolyte is cap-tured in a fleece or gel. These batteries have 100 times less sulfuric acid vapors than lead batteries with fluid electrolyte, which allows them to be installed in the same housing or space as the electronics. Maintenance-free batteries do not leak and can thus be run in any position. A voltage regulator may be required to adapt the voltage of the photovoltaic system to the voltage of the consumer. For devices powered by photovoltaics, this regulator is usually a DC/DC transformer, which transforms one direct current into another. If the system voltage should be an alternating voltage, an inverter is added to the system.

3.1 Photovoltaic hybrid system

The intermittency of solar irradiation means that in most parts of the world power systems running solely on photovoltaics have to be very large and are hence expensive. To prevent

this, the PV generator can be combined with other generators. If properly designed, such PV hybrid systems ensure reliable, redundant stand-alone power, reduce the consumption of fossil fuels, are low-maintenance and have long service lives. Here, when solar irradiation is good, the solar generator covers the entire energy demand of the consumer. Excess energy is stored in batteries. At night or during bad weather, batteries cover energy demand. If the battery is about to be excessively discharged, an back-up gen-erator produces electricity and simultaneously charges the battery. Furthermore, hybrid sys-tems allow for better battery management, thus lengthening the battery’s service life. If need be, the waste heat from the back-up generator can be used to heat the battery / technology room. Photovoltaic hybrid systems are used in all power classes to attain high system availability. On the one hand, the redundancy of the generators ensures this availability; on the other, any lack of renewable energy sources (during periods of little sunshine, etc.) is compensated for. System availability is especially crucial for technical applications such as telecommunica-tions, telematics, and measurement technology. Photovoltaic hybrid systems are used to power individual consumers (e.g. homes) or micro-grids (“village grids”). In the former case, there is generally a close connection between the user and the owner of the system. In contrast, village grids have more classic energy supply structures. Hybrid systems are both used for direct current and alternating current grids. All of the hybrid systems described here have a battery1.

3.1.1 Annual solar irradiance curve and annual global irradiation energy

The annual curve of solar irradiance and the annual sum of global irradiation energy are the result of climatic and meteorological matters that depend on local and seasonal conditions. Sunny regions near the equator - such as the deserts of Africa, parts of Asia, and the south-ern US - have roughly twice the solar energy as central Europe.

1 There are also systems without batteries. However, an engine generator has to be running continu-ously in such grids. The additional renewable generator is then used as a “fuel saver”. However, such systems usually have power output above 100 kW, which are not under consideration here.

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Figure 1: Monthly averages for daily solar irradiation at the latitude of Ho Chi Minh City, Viet-

nam (left) and Freiburg, Germany (right) (Source: Fraunhofer ISE, Freiburg, Ger-many; Chart: Solarpraxis AG, Berlin Germany).

These regions also have a more balanced supply of irradiance than areas far from the equa-tor (Figure 1). For instance, the monthly average for daily solar radiation in Ho Chi Minh City, Vietnam, only varies from 4.1 kWh/m2 and 5.6 kWh/m2 /1/. In a central European town like Freiburg, Germany, however, the monthly average of the daily solar radiation in December is almost seven times less than in July. These great fluctuations in solar radiation over the year and the lack of powerful long-term storage systems mean than standalone, exclusively photovoltaic energy supply systems have to be designed for the month with the least solar radiation, which makes such systems large and in many cases uneconomical.

3.1.2 Compensating for the “winter solar doldrums” with an back-up generator

While the solar radiation fluctuates, creating “winter doldrums” far from the equator, the elec-tric demand of the consumer is roughly constant or may even be opposite to the supply of solar power, as with lighting systems. If the whole power demand of the consumers has to be covered using PV generators, the PV system has to be dimensioned to account for the “win-ter doldrums”. That makes the system expensive and inordinately large for most of the year. By integrating an back-up generator in an off-grid PV energy supply system, the lack of solar irradiation can be compensated for in the critical winter months, thus reducing the size of the PV generator and the battery. In the following, the example of two alternative system designs - an exclusively photovoltaic energy supply system (see Figure 1, section 1.1) and a photovoltaic system with an back-up generator (Figure 2) - are compared.

voltageregulator

DC load

auxiliarygenerator

chargeregulator

storagebattery

PV generator

Figure 2: Principal design of a photovoltaic system with a back-up generator (Source: Fraun-

hofer ISE, Freiburg, Germany). The following assumptions apply:

− The energy demand of the consumer to be powered is around 1 kWh per day, with power consumption constant 24 hours a day.

− The system is located in Freiburg, Germany. − The battery has the capacity to bridge 7 load days. − 100 % of the connected consumer’s power needs are to be covered.

The example is based on systematic calculations for a prototype system with standard sizes for PV generators and batteries /29/. The calculations (Figure 3) allow for useful configura-tions to be pre-selected, thus serving as the basis for designs of special systems.

Solar fraction

Figure 3: The degree of coverage for a parameterized system without an back-up generator.

PV stands for the ratio of the annual generation of electricity to the annual con-sumption; BAT stands for the battery’s storage capacity in load days. Solar fraction is the proportion of annual load supplied from solar energy /29/ (Source: Fraunhofer ISE, Freiburg, Germany).

Figure 4 shows a cross-section of the degree of coverage. Here, the size of the battery was selected to cover 7 load days as discussed above. In addition, the size of the generator is given in absolute numbers for a mean consumption of 1 kWh per day. As the example shows, a PV system without a back-up generator needs to have a capacity of at least 1.6 kWp to cover the power demand over the whole year under the prerequisites mentioned. The great uncertainty concerning the size of the solar generator required to cover power needs completely is clear here.

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Figure 4: Degree of solar coverage for a PV system without an back-up generator. A load of

1 kWh per day has to be covered; the battery is dimensioned to handle seven load days. The generator sizes apply for Freiburg, Germany /29/ (Source: Fraunhofer ISE, Freiburg, Germany; Solarpraxis AG, Berlin, Germany).

If a back-up generator is used, the PV system’s rate of solar coverage is less than 100 %. The back-up generator then supplies the difference for complete coverage of the consumer’s demand. If the system described is designed to cover 90 % of the demand with solar power, the size of the PV generator required drops to 0.55 kWp (see Figure 4) - one third of its origi-nal size (/18/, /)./30 Figure 5 compares the power produced by the PV generator with the power required by the consumer. In the system without a back-up generator, the power produced by photovoltaics surpasses demand several-fold. If a back-up generator is integrated in the system, the (not usable) excess power is reduced along with the size of the PV generator.

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Figure 5: Monthly energy balance for the example systems without (top) and with (bottom) a

back-up generator. The short columns (red) indicate the daily demand of the con-sumer (1 kWh/day), while the long columns (blue) indicate the power produced by the PV generator. The filled in section (black) of the column below represents the share of the load covered by the back-up generator; here, 10% of annual demand /2/ (Source: Fraunhofer ISE, Freiburg, Germany).

3.1.3 Selecting a suitable back-up generator

The reliability of photovoltaic systems largely depends on the kind of back-up generator used. In selecting back-up generators, the required power range, the location of the system, installation costs, the monitoring and maintenance of the system, and the service life and efficiency of the PV generator and battery used (daily storage) have to be taken into consid-eration.

If additional energy is provided from an electrochemical storage medium (long-term storage), the storage capacity has to be taken into consideration in terms of volume and weight, trans-port options, risks to the environment, and possibly the method of recharging. When fuel cells, thermophotovoltaic and thermoelectric transformers are used as generators, or when generators are connected to gas turbines or combustion engines, their environmental impact (noise and emissions) have to be taken into account along with the type, availability and price of the fuel when selecting a generator. The most important criteria for the selection of a back-up generator:

• Electric power range • Storage capacity • System costs • Maintenance requirements • Remote starting possibilities • Control and regulation possibilities • Efficiency • Environmental impact • Fuel (type, availability, price)

As discussed above, the back-up generator only provides a small part of the energy needed (some 10 % on the yearly average). The demand for back-up power is irregular, generally occurring in cold winter months with little sunlight. If an appropriate energy management sys-tem is used, it will ensure the reliable operation of photovoltaic systems that include a back-up generator with minimum consumption of back-up power and optimal management of the battery. The potential back-up generators include:

• Petrol generators • Diesel generators • Gas generators • Thermodynamic transformers with external combustion • Fuel cells • Thermoelectric generators • Thermophotovoltaic generators • Electrochemical energy storage

In addition to these controllable generators, the following can be used:

• Wind generators • Micro hydropower units • Stirling motors

3.2 Set-up of hybrid systems

Photovoltaic hybrid systems replace systems powered by classic engine-driven generators. Generally, engine-driven generators are very expensive over their life-cycle (maintenance, fuel) and usually limited to a certain number of hours a day. Hybrid systems allow for 24-hour power with great reliability. It is very hard to compare the costs for these two types of sys-tems as it is hard to put a price tag on the quality improvement of 24-hour power supply. Local wind conditions have to be checked to see if wind generators could be useful. Wind generators become more economical, the greater overall power consumption. PV generators become more economical, the lower overall power consumption is. It always makes sense to use micro hydropower systems if there is potential, but often the potential is not taken advan-tage of or is not available directly where power is needed. There are two basic types of systems that feed power to the alternating current grid. (1) In the first type, the generator, the battery, and a central inverter are coupled via a direct current rail. Engine generators feed to the DC rail via a rectifier. The central inverter then feeds the consumers (Figure 6). (2) In the second case, all of the components and consum-ers are connected to an alternating current rail.

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Figure 6: Design of a hybrid system with DC coupling of the generators (Source: Fraunhofer

ISE, Freiburg, Germany).

Here, the classic DC components (PV generator, fuel cell, thermoelectric transformer, and battery) each have an inverter / a bi-directional power converter (Figure 7).

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Figure 7: System concept with AC coupling of generators and battery (Source: Fraunhofer ISE, Freiburg, Germany).

3.3 Flexible system components for PV hybrid systems

Usually it is not possible to expand conventional PV hybrid systems without exchanging es-sential system components. At the same time, the operation strategy must be adapted. Therefore systems are developed, or rather already applied, that eliminate weaknesses with regard to flexibility and scalability, and at the same time achieve a better operation safety and comfort.

3.3.1 DC system featuring standardized component communication

There is a concept of an intelligent DC hybrid system under development which is based on a standardized interface, a systematic separation of power and information flow, decentral-ized component control units, and a universal energy management system (Figure 8). A central energy management unit collects data from all system components via a standardized protocol (UESP: Universal Energy Supply Protocol). This central unit sends control commands to the components to carry out the energy management. UESP can also be applied in AC-coupled systems. Through the standardization of the information flow and a central energy management, a “plug&play” function can be implemented, which guarantees flexibility and expandability. In the energy management unit certain operation strategies can be implemented that take tech-nical, economic, and ecological aspects into account. Thus, an optimal battery charging strategy can be implemented and optimal battery maintenance at low life cycle costs can be provided.

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Figure 8: DC system with standardized component communication (UESP system). Grafik:

STECA GmbH, Memmingen. The separation of power and information flow and the universal energy management allow for a simple scalability with regard to power without any changes in the existing system nec-essary. Above that, the available information can be used to record, control, and distribute the conditions of the components. A disadvantage however is, that all components of the system must be compatible to a standardized data bus with information protocol (UESP). /22/, /23/.

3.3.2 AC-coupled stand alone supply structure

In order to create an AC-grid as well as power supply systems for higher power output, AC-coupled stand alone grids can be designed. They allow for a simple and cost-effective grid-connection between energy suppliers and consumers on the AC-side. As the grid is modu-larly built not only the PV generator and additional power generators (e.g. Diesel generators) can be included but also wind- and water-power plants. Single- or three-phase systems are possible.

Figure 9: three-phase AC coupled stand-alone power supply structure (Grafik: SMA Tech-

nologie AG, Niestetal) Beside the PV generator and the battery unit, the three-phase plant shown in Figure 9 consists of three special stand-alone inverters, five grid inverters, a Diesel genera-tor and a communication unit. The AC coupling for off-grid power supply has been inserted in many systems worldwide since 2001. Above all, it is used for the electrification of remote off-grid areas where it allows for the establishing of flexible power supply. Due to the modularity, it has become easier to adapt the systems to the requirements of both the energy suppliers and the consumers respectively without complex installation measures /7/, /38/, /39/.

3.4 Technology options for PV Wind hybrid systems

In the existence of sufficient wind potentials the use of small wind turbines (SWT) can im-prove the overall economic efficiency of stand-alone hybrid systems for rural electrification. At good wind sites SWT generate power to overall prices way lower than PV systems. Often, solar and wind power have a negative correlation to each other. Therefore, combining wind and solar power in a hybrid system reduces the need for battery storage capacity which is one of the most expensive components of the hybrid system. SWT technology is currently being optimized by different entrepreneurs that are taking the opportunity to enter the SWT market with new, lightweight, innovative and appealing wind conversion devices. In addition, the aspect of professional development of appropriate power electronics is being addressed. These developments lead to further improvement of SWT profitability in comparison with other power generation technologies e.g. photovoltaic. The most common technical concept for SWT is the direct coupling of the rotor with a per-manently excited synchronous generator rendering superfluous the use of a gearbox and allowing a variable-speed operation. The nacelle is lightweight and requires little mainte-

nance. The generated three-phase current is converted for normal utilization (400/230/110 V, 50/60 Hz) by power electronic components. So far standard feed-in converters for photovoltaic modules are used by upgrading them with additional devices for wind power utilization. Despite these activities the power electronics are still the components with the highest failure rates. In response to this, some companies have set off to take this hurdle and to fill this gap in the market by developing integrated converter systems covering SWT operation control tasks as well as the grid connection. This approach reduces the fault liability of the entire system and simplifies the electrical installation. When connected to an AC hybrid system the wind turbine can be attached either to the AC busbar or directly to the battery as described in the chapters above. Direct coupling with the battery leads to better buffering of the fluctuating wind power thus taking care of the overall system reliability and efficiency. This requires a special power electronic device.

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PV-Modules Fig.14: Schematic diagram of an AC hybrid system containing a DC coupled wind turbine (INENSUS GmbH, Goslar)

3.4.1 Site evaluation

Apart from the technical set-up a specific economic analysis for a particular site is crucial regarding the optimal system design and profitability. Site specific economic analyses require a special site evaluation including direct measure-ments of wind speed and direction because of the fact that local obstacles such as trees or buildings are likely to have a disturbing effect on the energy yield especially for low hub heights. Rough estimations based on data from wind atlases are not accurate enough and lead to inappropriate system design. Site specific wind measurements have so far not been profit-able as the capital expenditure for equipment, transport and erection as well as data analysis went far beyond the financial scope of SWT projects. Cheaper devices have so far not pro-vided confidence in terms of accuracy, data storage and reliability.

Yet, this situation might change as new developments have come on the market. A compact and easily transportable wind monitoring tower2 with integrated sensors (for monitoring of wind speed, wind direction, brightness and temperature) and data logger automatically evaluating recorded data is now available enabling manufacturers and distributors to project more precise energy yields and calculate the profitability of their SWTs at the customers’ sites. As a result, the customer will profit from much higher planning reliability and the promises from manufacturers regarding the performance chart and energy yield of their SWTs are veri-fiable without much effort. In combination with sophisticated hybrid system design software the profitability of hybrid systems can therefore be determined more precisely and reliably.

3.5 Hybrid System Design

Rural electricity supply can be realized choosing a type of power generation from a variety of energy sources and energy conversion steps. Simple solutions might apply diesel generators but in areas where diesel fuel cannot be supplied easily, fossil fuel prices are just too high or legal constraints favour other options, renewable energy systems move into focus of atten-tion of project developers. One of the project developer’s main jobs is choosing the best sys-tem for the respective site meeting its particular requirements. For this decision-making process characteristics of an ideal renewable energy supply system can be taken as criteria for choosing the optimal system:

• Low initial expenditure • Low electricity cost price • Modularity and extensibility (ability to be extended with rising energy demand) • Easy handling and operation for users • Easy maintenance by local companies • Energy supply mainly from renewable energy sources • Quality of supply comparable to utility grid standards reducing blackouts to a mini-

mum In many cases project developers only apply technology they are familiar with such as PV systems combined with battery storage not considering other options. Too often, this results in low quality of power supply and extremely high costs of electricity. In contrast, Hybrid Power Systems also incorporating other power sources can be the preferred choice from a financial point of view thanks to balancing availability of solar, wind and hydro power re-sources. Hybrid Power Systems supply electrical consumers independently of a large central electricity grid incorporating two or more different types of power source. In many cases the electricity cost price can be reduced by 20 % and more using the Hybrid Power System ap-proach. Furthermore, modularity and quality of supply usually are increased. However, these systems need thorough design considerations utilizing expert knowledge to cover all aspects concerned and to result in a reliable alternative to common PV-only or diesel-only systems. Especially for consumers with peak loads larger than approximately 10 kW, basic design studies pay off recovering additional expenses conducting the study within a short period of time. Generally, Hybrid Power System design requires three design steps: 1st: System pre-design

2 The aeolog wind measurement system for small wind turbines. More information at:

www.inensus.com

To optimize the Hybrid Power System performance the system design has to be adapted to site specific resources (i.e. wind measurements, see above), loads and other constraints that need to be evaluated locally for each individual system. Energy demand and diurnal and an-nual load patterns of the consumers have to be measured (or estimated) and projected to future growth of consumption. The energy content of renewable resources available locally and their time-varying distribution has to be determined. Socioeconomic constraints and op-portunities concerning billing systems, the importance of continuous power supply and back-ground knowledge for operation and maintenance should be evaluated. From this information the Hybrid Power System design expert can derive a basic system de-sign concerning a general scope and size of the system that needs to be optimized and in-vestigated in more detail subsequently in the second design step. 2nd: System optimization regarding financial and technical long term performance Computer models enable the design expert to optimize the system concerning financial and technical long-term performance. In addition to input data in terms of time series evaluated in step 1 the operation of these computer models requires information about the financial framework of the rural electricity supply project. Depending on the computer software applied the computer simulation procedure results in a solution for a financially optimal combination of Hybrid Power System components, sensitivity results for critical parameters and a structure of initial expenditure and operation and mainte-nance costs. Furthermore, in some programs the battery and generator performance as well as time series with hourly power data or data with even higher resolution will be displayed for each of the components to facilitate more detailed system analysis. Of course, the accuracy of simulation results always depends on the availability of reliable input data. Many project developers only focus on reducing the initial expenditure of the energy supply system. How-ever, the electricity cost price should be of greater importance to provide sustainability of the investment. Computer models automatically optimizing the financial performance of the Hy-brid Power System are based on this approach. 3rd: System optimization regarding electrical system stability To make sure that the theoretically optimized Hybrid Power System works reliably in each mode of operation the transient behaviour should be tested comprehensively at a test stand with focus on interactions of components in terms of power oscillations among rotating gen-erators (wind and diesel). Especially destroying resonance modes have to be avoided. Criti-cal system conditions like simultaneous switching of large real and reactive power consum-ers and worst case scenarios like short circuits will be investigated. Additionally, the system reaction to failures of single components can be tested. To obtain applicable results the project developer should ensure conducting a design study of high quality covering at least the following aspects:

• Consideration of a variety of electricity generating components: PV, wind turbines, micro hydro, biomass generator, diesel generator, micro gas turbine

• Application of a grid forming module: synchronous generator and/or converter system if redundancy is necessary

• Selection of type of equipment, e.g. wind turbine: induction generator vs. synchro-nous generator, vertical axis vs. horizontal axis, stall control vs. pitch control, etc.

• Storage options (strongly depending on the size of the system): pumped storage plant, compressed air storage, battery storage (Pb, NiMH, etc.), flywheel

• AC coupling vs. DC coupling of components and loads • Load management system: Priority of loads, uninterrupted power supply for most im-

portant consumers, Dumpload, deferrable loads

• Diesel generator / biomass generator / micro gas turbine control strategy • Climatic constraints like high temperatures requiring dedicated cooling systems for

the battery compartment or dew requiring sealing of components or drains in the sys-tem

• Monitoring system for qualified trouble shooting • Concept for maintenance, repair and spares supply (financially, technically, logisti-

cally) • Training of operators and maintenance staff, local capacity building • Optional: financing models and billing system

The aspects that should be covered by a design study as well as the scope of the study vary with the size of the respective Hybrid Power System. The larger the system the more com-prehensive and detailed the design study should be carried out for the project developer to be able to benefit from available opportunities. As a result of a properly conducted design procedure the project developer will receive a suggestion for an optimized system concerning performance, economics and transient be-haviour for reliable and cost efficient Hybrid Power System operation. In most cases, the optimized Hybrid Power System will match more of the requirements mentioned above than conventional systems based on PV-only or diesel-only design. Last but not least, the project developer needs to know whom to approach for conducting the design study. This will always involve the risk of employing somebody realizing the study with the aim to favour a specific system design to serve his own interest. Thus, objective and in-dependent research institutes or companies being connected to a variety of component manufacturers not treating any of them preferentially would be most eligible. /44/

4 Solar Water Pumping

4.1 Photovoltaic Pumping Systems (PVPS)

In recent years, numerous emerging countries have begun installing photovoltaic pumping systems (Figure 10), which have an established reputation for reliability and long service life /11/.

Figure 10: Photovoltaic pumping system in Tunesia (Photo: Siemens + GTZ, Eschborn,

Germany).

4.2 Economy of PVPS

Most decisions for or against a particular type of pumping system are still being based pri-marily on economic factors, as opposed to ecological considerations and sociotechnical as-pects. The process of introducing a new technical system, or replacing an existing system with a different one, always amounts to a certain disruption of traditional structures within the culture(s) in question. Nevertheless, most technical pumping systems are still being planned with too little consideration of their impacts on the recipient social system with regard to hy-giene, education, family structure, division of labour, etc. An attempt is therefore made below to estimate the range of areas in which the use of photo-voltaic pumping systems is worthwhile.

Equivalent PV power [W]

Cos

t of e

nerg

y [€

/kW

h]

10 -2 10 2 10 410 310 -1 1 10 5 10 710 6 10 8 10 1010 90,05

0,5

5

10 1

50

500

5000Wristwatch battery, pocket calculator

Dry cell/Flashlight battery

Lead battery/Motor vehicle battery

Emergency power unit

Diesel (10kW)Diesel (100kW)

1,50€/kWh

PhotovoltaicSystems

Figure 11: Cost of energy vs. equivalent PV power rating for various types of energy storage and power generating systems (Fraunhofer ISE, 1996) [1 DM = 0,51 EUR] (Image: Fraunhofer ISE, Freiburg, Germany; Solarpraxis AG, Berlin, Germany).

Figure 11 shows the current cost of energy relevant to the equivalent photovoltaic output for various types of energy storage and various power generating systems. Up to an output of about 10 kW, photovoltaic power generation is more cost-effective than a diesel generator for general PV applications. Regarding PVPS, GTZ has made an evaluation on their economy (/26/ GTZ, 1994; /27/ GTZ 1995). With due allowance for the presented factors, Figure 12 illustrates the economically expedi-ent service range for photovoltaic pumping systems as a very general guideline (/10/ GTZ, 1991). The pump lift, or pumping head, typically ranges between 1 m and 100 m.

Daily water requirement [m3/d]1 5 10 50 100 500 1000 2500

Wat

er h

ead

1

5

10

50

100

Hand pumps

Several handpumps

Diesel pumps

PV pumps

Figure 12: Duty limits for photovoltaic pumping systems (/10/ GTZ, 1991) (Source: GTZ,

Eschborn, Germany; Image: Solarpraxis AG, Berlin, Germany).

4.3 Description of PVP System Components and Actuating Variables

Photovoltaic pumping systems can be used in diverse configurations for drawing water (/4/ Bucher, 1993). Figure 13 sketches out two application options for photovoltaic pumping sys-tems: deep-well and ground-level pumps. In deep-wells only submersible pumps can be in-stalled, whereas in ground-level different pumps types are usable (chain pumps, screw pumps, etc.).

Figure 13: PV-powered pumping system: ground-level (left) and deep-well configurations

(source: SET, Altlußheim, Germany; Image: Solarpraxis AG, Berlin, Germany)

Photovoltaic pumping systems are made up of several independent components. All such systems, however, have the same basic configuration (Figure 14).

Irradiance

Solar-generator

Inverter

WellBattery

Motor Pump

Figure 14: Block diagram of a photovoltaic pumping system (Source: IBO Mayer, München,

Germany; Image: Solarpraxis AG, Berlin, Germany). Basically, a photovoltaic pumping system comprises 6 main components: solar radiation, a solar generator, a generator/load matching module (power inverter), a drive element, a pump and a well (including all piping, of course). Depending on the case-by-base situation, such additional components as an energy store (battery) may also be included. As irradiance, solar generator and inverter functionalities have already been described in other articles, the following presentation focuses on drive motors, pumps and the well.

4.3.1 Drive Motors

The drive unit converts electrical energy into mechanical energy, and since rotational motion is preferable for use in operating pumps, electric motors are inherently well-suited for the task. For driving the pumps in a photovoltaic system, D.C. motors are the logical choice. Those populating the power range around 1.5 kW are characterized by relatively high effi-ciency (ca. 85 %). Unfortunately, however, most D.C. motors on the market use brushes for commutation. That means that they require periodical maintenance, because the brushes only last about 1,000 hours under normal operating conditions. In case of installation in a deep-well this means a scheduled dismounting of the pipe in order to reach the motor.

Figure 15: Floating pump with brush less D.C. motor (Photo: IBO Mayer, München, Ger-

many). One alternative solution is to use commutatorless D.C. motors, i.e., units with electronic commutation. Such motors are inverter-fed and wired for synchronous operation. The tech-nology calls for real-time monitoring of the rotor's angular position. Thus, if this kind of motor is installed as part of a borehole pumping system, either the sensor signals would have to be transmitted in parallel with the power conductors over a long distance to the electronic con-trol unit, or the latter would have to be integrated, together with the motor, into a waterproof capsule. Both alternatives are problematic: if the motor and the electronics are separated, line induction can cause interference, and a waterproof capsule would hardly enable mainte-nance of the electronic components. Consequently, such motors have not yet entered the wholesale manufacturing stage for high-powered underwater applications. They are, how-ever, very well suited for inclusion in ground-level pumping systems and small compressor units, as long as the motor and control system remain easily accessible. A floating pump with an electronically commutated D.C. motor is shown in Figure 15.

Figure 16: Standard asynchronous motor for borehole pumps (Photo: IBO Mayer, München,

Germany). Many photovoltaic pumping systems instead rely on asynchronous motors, some of which are available as well-proven, off-the-shelf products. Since the driving torque is generated by a rotary field, no sensors or other connections (excepting the power connections) are neces-sary. Hence, the asynchronous motor is maintenance-free, but it still has the drawback of having to be driven via an inverter. Figure 16 shows a standard-type submersible asynchro-nous motor for driving a borehole pump.

4.3.2 Pumps

The pump converts rotary motion into hydraulic output. Practice-proven centrifugal pumps are the most popular option, though displacement pumps are also suitable. Due to their low-efficiency, however, systems incorporating friction, jet or up thrust pumps must be ruled out (/13/ Herrmann, 1989).

4.3.2.1 Centrifugal Pumps

Centrifugal pumps, as kind as fluid-flow pumps, (Figure 17) have some major advantages to offer: • affordable prices (large-scale production) • easy starting • simplicity of design (modular / one design application for multiple stages)

• broad range of head- and delivery-specific applications Essentially, a centrifugal pump is a fluid flow machine. Each pumping stage comprises a cas-ing and an impeller. The pumped medium flows axially in toward the impeller, and the rotat-ing impeller accelerates the water racially outward (by centrifugal force), thus building up discharge pressure. The pump's ultimate delivery pressure can be arbitrarily increased by adding as many stages in series as desired. A multiple-stage arrangement does produce a corresponding increase in mechanical friction, however, so small-capacity pumps, usually of correspondingly small diameter, should have pumping heads of about maximum 100 m.

Figure 17: Centrifugal pump (Image: IBO Mayer, München, Germany). Since the pressure build up is a function of centrifugal force, the pump's discharge pressure is a quadratic function of speed. Photovoltaically driven centrifugal pumps start easily, because no "breakaway torque" is needed. In a solar-powered system, pronounced speed dependence is a disadvantage. The higher the head, the higher the minimum operating speed of the pump. When the incident radiation level is low (cloud cover, generator out of line with the sun, ...), the system's power output is also low, so the pump runs at low speed and is unable to develop the necessary discharge pressure. In other words, the generator's output cannot be converted into delivery capacity. Thus, the sunshine duration outstrips the time in which water actually can be pumped. This drawback can be substantially ameliorated by opting for positive displacement pumps in-stead.

4.3.2.2 Positive Displacement Pumps

Unlike centrifugal pumps, positive displacement pumps can work at relatively low speeds and, hence, are able to pump water at low insolation levels. On the other hand, to get started, most positive displacement pumps need a certain "breakaway torque", i.e., a brief burst of power to overcome their standstill friction. Basically, there are two types of positive displacement pump: • rotary displacers, e.g., rotary piston pumps, vane cell pumps and progressive cavity

pumps (screw pumps) • reciprocating displacers, e.g., reciprocating piston pumps and diaphragm pumps

4.3.2.3 Progressive Cavity Pumps (Screw Pumps)

Progressive cavity pumps ( Figure 18) act on the principle of volumetric displacement. With each turn of the pump, a certain volume of medium is moved to the discharge side. The pump is made up of a stator in the form of a hollow elastomer body, and a steel rotor (impeller) in an interference-fit con-figuration. In comparison with that of a centrifugal pump, the performance curve of a pro-gressive cavity pump is much steeper. This gives the advantage of higher efficiency in com-bination with better resistance to abrasive material entrained in the pumped medium (/43/ Zängerl, 1993).

Figure 18: Progressive cavity pump (Image: IBO Mayer, München, Germany). The interference, i.e., the tightness of fit between stator and impeller, is the decisive factor for the part-load efficiency, because it more or less precludes medium backflow, but it is also what necessitates the "breakaway torque". At first, a short surge of power (power impulse) is needed for overcoming the static friction. But photovoltaic generating systems are not readily

able to provide such an impulse. Consequently, appropriate measures must be taken to counter that effect.

4.3.2.4 Piston Pumps

Piston pumps also make an interesting water-lifting option, especially when great depths are involved (Figure 19). Their pumping head is almost completely independent of their driven speed. The number of strokes per minute is what determines the delivery rate. Such pumps have no bottom operating limit, so water can still be delivered at relatively low insolation lev-els. Like progressive cavity pumps, positive-displacement-type reciprocating piston pumps have the drawback of requiring a certain "breakaway torque". In addition, the pump's torque fluctuates in the course of a full rotation.

Figure 19: Piston pump with D.C. drive motor (Image: IBO Mayer, München, Germany). With a view to reducing the pump's starting torque requirement and torsional variation, cer-tain design modifications can be effected, e.g., tuning in the form of compensating weights, tailoring of the leverage, and incorporation of mechanical energy stores/buffers. In any con-crete case, this will require optimisation for photovoltaic deployment.

4.3.2.5 Diaphragma Pumps

Figure 20: Diaphragm pump and cutaway view of a diaphragm pump (Image: IBO Mayer,

München, Germany). Diaphragm pumps are a special kind of piston pump. For PVPS applications, they are pref-erentially employed in D.C. systems with ratings up to about 100 W. In this type of pump, a rubber diaphragm driven by a nutating disk (swash plate) takes the place of the piston. The "stroke" is relatively short and the flow rate accordingly modest. That is why diaphragm pumps tend to be installed in duplex, triplex or quadruplex arrangements. The nutating-disk drive also does away with most start up problems. Most diaphragm pumps are purchased as complete sets, i.e., with motor and pump joined together as an inseparable unit ( Figure 20). Most of their motors are of the D.C. brush type. Considering the familiar charac-teristics of D.C. motors, it is advisable to include a booster for accommodating the genera-tor's characteristic to that of the motor.

4.3.3 Wells

Most wells in developing countries are either drilled or dug (/36/ Thöle, 1988). Drilled wells with diameters ranging from 20 to 80 cm are best suited for tapping into deep-laying ground-water tables. They tend to be equipped with motor-driven pumps. Dug wells, with diameters ranging from 100 to 200 cm, are more suitable for shallower depths (2 – 20 m) and are still being sunk as dig-down wells from which water can be drawn in buckets or leather/rubber bags if the pumping system breaks down or is in need of maintenance. The main disadvan-tage of such open wells is the danger of contamination. Basically, both kinds of wells are sunk in the same manner. Figure 21 illustrates the main phases of well construction. The first step is to drill the well (

Figure 21b) according to a suitable method, e.g., standard or hydraulic percussion method.

Abutment for presses and winches

Groundwater table

Aquifer

Groundwater bed

Pump

Motor

a) b) c) d)

Gravel pill

Filter pipe

Watertank

Figure 21: Main phases of well construction (Source: Bischofsberger, München, Germany;

Image: Solarpraxis AG, Berlin, Germany). Upon completion of the drilling phase, a filter pipe is lowered into the well ( Figure 21c). Then, the space between the filter pipe and the surrounding soil is filled with gravel, after which the casing pipe / drilling fluid is removed. Finally, the top of the filter pipe, i.e., the well head, has to be sealed off with a clay packing ( Figure 21d) and the pump installed. For the majority of photovoltaic pumping systems the model for a vertical well as described in Figure 22 can be applied. This model bases on a three dimensional rotation symmetric flow, meaning that the water can flow free of pressure from all sides towards the well. With the water take out the water level will get down in the well. This draw down is dependant of the type of material surround-ing the well (permeability), the geometric dimensions on the well and the quantity of pumped water. The calculation of the draw down, also called dynamic water head, is relevant for the PVPS as the water head is varying due to changing flow rates of the pump.

Wel

l

H9

r

h

R

V

S

Figure 22: Schematic of a well (Source: Bischofsberger, München, Germany; Image: Solar-

praxis AG, Berlin, Germany). For wells with water under pressure the approach for the draw down is similar. As the major-ity of the PVPS is installed in non-pressurised wells this topic is not dealt with here. The calculation of the draw down can be made according to the following recursive formula:

f

21

f

2SS

kS3000R

rRln

kQH-HS

≈

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎠⎞

⎜⎝⎛−=

π

4.4 Sample Applications for Photovoltaic Pumping Systems

PVP systems can be used to obtain potable water for people, drinking water for domestic animals, and irrigation water for field crops. In planning such a system, one must keep in mind the fact that the pumping system is only part of a higher-order system with diverse con-trol mechanisms. Figure 23 illustrates some of the causalities that become apparent upon closer scrutiny of the overall system.

Animal husbandry

Water requirement

Groundwaterpollution

Population development

Land-arearequirement

AgriculturePVPS

Water requirement

Water requirement

Figure 23: Higher-order cross-linkage and interdependencies surrounding photo-voltaic

pumping systems (Source: IBO Mayer, München, Germany; Image: Solarpraxis AG, Berlin, Germany).

To ensure that their photovoltaic pumping systems will have sustainably positive effects, re-sponsible planners give due consideration both to these interdependencies and to pertinent ecological, hydrological and sociological relationships.

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