Solar Water Pumping for Irrigation: Case Study of the Kilimanjaro Region

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BA

CHELOR

THESIS

Energy Engineering - Renewable Energy, 180 credits

Solar Water Pumping for Irrigation

Case Study of the Kilimanjaro Region in Tanzania

Niclas Bengtsson, Johan Nilsson

Bachelor Thesis in Energy Technology, 15 credits

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Abstract

This study has been conducted as a Minor Field Study (MFS). It focuses on solar water pumping for small-‐scale farmers in the Kilimanjaro Region of Tanzania. The purpose is to investigate the possibilities for rural farmers to operate their irrigation with solar power instead of their current option: fossil fuels, primarily petrol. The study was conducted in three phases, starting with pre-‐study in Sweden, followed by field study in Tanzania from January to March 2015 and finishing with summarizing and calculating in Sweden. Fuel powered water pumping has a cheap capital cost; however, it is expensive and problematic to maintain and operate. Solar powered water pumping is almost completely opposite. It has a higher initial cost; however, it is considerably cheaper to run. The results indicate that the investment in solar power might be too expensive for the farmers, as long as they do not receive external financial and educational support. Assuming that the farmers are able to obtain a solar water pumping system, results show that they will benefit and save a considerably amount of money over a long period of time. Also, solar water pumping is environmentally friendly compared to the systems in Tanzania today.

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Sammanfattning

Den här studien har blivit genomförd som en Minor Field Study (MFS). Den fokuserar på solcellsdriven vattenpumpning för småskaliga bönder i Kilimanjaroregionen i Tanzania. Syftet är att undersöka möjligheterna för bönder i landsbygden att driva deras bevattningssystem med solenergi istället för deras nuvarande alternativ, fossila bränslen (främst bensin). Studien blev genomförd i tre faser: först förstudie i Sverige, följt av fältstudie i Tanzania från januari till mars 2015 och den avslutades med summering och kalkylering i Sverige. Bränsledriven vattenpumpning har en billig kapitalkostnad, men är dyr och problematisk att underhålla och driva. Solcellsdriven vattenpumpning är tvärtom. Den har en högre initial kostnad, men är avsevärt billigare att driva. Resultaten indikerar att investeringen i solenergi kan vara för dyr för bönderna om de inte får extern support med finansiering och utbildning. Resultatet visar att om bönderna kan erhålla ett solenergidrivet vattenpumpningssystem, så kommer de ha stor nytta av det och spara en avsevärd summa pengar över en lång tid. Dessutom är solcellsdriven vattenpumpning miljövänligt jämfört med systemen i dagens Tanzania.

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This study has been carried out within the framework of the Minor Field Studies Scholarship Programme, MSF, which is funded by the Swedish International Development Cooperation Agency, SIDA and administrated by The Swedish Council for Higher Education. The Swedish Council for Higher Education is a Swedish government agency with many different tasks in the education sector. One of them is to provide organizations and individuals in Sweden with the opportunity to participate in international exchanges and partnerships. Minor Field Studies (MFS) gives students at Swedish higher education

institutions the opportunity to conduct field studies in low and middle-‐income countries for the purpose of gathering material for an essay or degree project at Bachelor's or Master's level. The higher education institutions apply for funding from the Swedish Council for Higher Education, and the students then apply for scholarships via their higher education institution.

The Student Affairs Department administrates the MFS programme at Halmstad University. Rebecca Rosenberg International Coordinator

Program Officer MFS Programme Student Affairs Department Halmstad University

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Acknowledgements

This bachelor thesis was made possible with the help of numerous people and organizations. Without them, this study would not have been accomplished. Therefore, we would like to give our acknowledgement to the following:

• SIDA: This study was conducted as a Minor Field Study (MFS) with financial support from the Swedish International Development Cooperation Agency (SIDA). Without this support, the field study in Tanzania would not have been possible. We would therefore like to thank SIDA for their trust in giving us this opportunity.

• Ingemar Josefsson, supervisor: For introducing us to MFS and

encouraging us to write this kind of thesis. We would also like to thank you for your support and advice throughout the project.

• Giliard Mollel, TAHA: For taking us with you in your daily work and helping us collect the information we searched for. You have been an incredible support for us and without your help we would not have succeeded with this project. Thank you for your time and all your help.

• Adam Mark, RVTSC Moshi: For introducing the idea about combining solar water pumping with irrigation and for helping us get in contact with farmers in the area.

• Farmers in the Kilimanjaro Region: Thank you for taking your time and answering our questions. We would also like to thank you for showing interest in our project.

• Jan Kleinhans: For letting us stay in your apartment during our stay in Moshi. We would also like to thank you for sharing your experience about living in Tanzania and giving us advice during our stay.

• Deogratius Matemu: For helping us meet with farmers and taking the time to help us during our stay. We would also like to thank you for lending us your camera charger when ours broke.

• Laurence Eliya: For being an excellent guide during our three-‐day safari trip. We would also like to thank you for the email conversations and advices before our trip.

• Ulla Utterfors: For all your tips and advice about Tanzania before our departure.

• Nicholas Lloyd-‐Pugh: For helping us with the English language in our thesis.

• Families, friends and other contributing people: Last but not least, we would like to thank you for supporting us throughout the whole project.

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8. References ... 39 9. Appendix ... A.1 Appendix A Questionnaire ... A.1 Appendix B Survey ... B.1 Appendix C Friction loss diagram for PVC pipe ... C.1 Appendix D Prices on Solar Water Pumping Systems ... D.1 Appendix E Irrigation Method Comparison ... E.1 Appendix F Life Cycle Cost ... F.1

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Figures

Figure 1: Map over Tanzania. ... 1

Figure 2: Map over Tanzania, showing the Kilimanjaro Region in red. ... 2

Figure 3: Example of a centrifugal pump. ... 7

Figure 4: Example of a helical rotor pump. ... 8

Figure 5: Basin irrigation. ... 16

Figure 6: Drip irrigation scheme. ... 16

Figure 7: Pumping water with centrifugal pumps from the river in Moshi rural. 17 Figure 8: Education level among the farmers. ... 19

Figure 9: The most common field sizes, in acres, among the farmers. ... 20

Figure 10: The most common water source among the farmers. ... 20

Figure 11: Most common irrigation method used by the farmers. ... 21

Figure 12: The most common energy source used by the farmers. ... 21

Figure 13: The figur shows that drip irrigation is more beneficial than basin. ... 22

Figure 14: Solar panels mounted on top of a roof. ... 23

Figure 15: Water storage tanks on top of a roof. ... 23

Figure 16: System layout of the designed SWPS. ... 27

Figure 17: Pump performance curve for Lorentz PS150 C-‐SJ58. ... 29

Figure 18: Diagram showing the break-‐even point between petrol and solar options. ... 33

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Tables

Table 1: Steps for designing a solar water pumping system. ... 11

Table 2: Monthly insolation in Moshi, Tanzania. ... 12

Table 3: The cost of the farmers SWPS ... 24

Table 4: Monthly insolation during dry season in Moshi, Tanzania. ... 27

Table 5: Requirements for the pump. ... 30

Table 6: Suitable panel selected. ... 30

Table 7: Summary of the main components in the SWPS. ... 31

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Abbreviations

Abbreviation Definition

GDP Gross Domestic Product

GNP Gross National Product

TAHA Tanzania Horticultural Association

SWPS Solar Water Pumping System

PWPS Petrol Water Pumping System

LCC Life Cycle Cost

BEP Break-‐Even Point

DC Direct Current

AC Alternating Current

TDH Total Dynamic Head

kWh Kilowatt hour

kPa Kilopascal

$ United States dollars

VAT Value Added Tax

CO2 Carbon Dioxide

MFS Minor Field Study

SIDA Swedish International Development Cooperation Agency

USAID United States Agency for International Development

ha Hectare (10,000 m2₎

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Definitions

Solar radiation The amount of solar energy that reach the earth. The outer atmosphere receives a nearly constant value of 1,36 kW/m2_{.
However,
the
maximum
amount
of} solar radiation that reaches the surface of the earth is approximately 1 kW/m2_{because
the
radiation
has} to travel through the atmosphere.

Solar irradiance The amount of solar energy received on a specific surface. Usually expressed in kW/m2_.

Insolation The amount of solar irradiance measured over a specific period of time. Average daily insolation in a month is often used when designing a solar system. Usually expressed in kWh/m2_.

Peak sun hours The average daily insolation, expressed in

kWh/m2_{.day,
can
also
be
expressed
in
the
term
peak} sun hours. This is a term that shows the amount of “full sun hours” in a day. To achieve the peak sun hours, the total insolation is divided by 1 kW/m2 (the approximately maximum value of solar irradiance on the surface of earth).

Crop rotation A technique where different crops are grown in cycles to increase nutrition in the soil and minimize spread of pest.

Power output warranty If the warranty is 25 years, the solar panels power

output efficiency should be at least 80 % after this period of time.

Acre An American or English unit of area that is equivalent to 4,047 m2_{or
0.405
ha.}

Azimuth One of the coordinates for a celestial body (e.g. the sun). The azimuth is along the horizon where 0° is north, 90° is east, 180° is south and 270° is west.

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1. Introduction

1.1 Background

Tanzania is located in eastern Africa. With its 945,000 km2_{it
is
more} than twice the size of Sweden. There are over 50 million inhabitants and the majority speaks the official language of Swahili; although, there are many people who speak English as well as local tribe languages. Tanzania’s largest neighbors are: Kenya to the north, the Democratic Republic of Congo to the west, Mozambique to the south and the Indian Ocean to the east. Africa’s highest mountain,

Kilimanjaro, is located in the northeast and Lake Victoria in the northwest (Nationalencyklopedin).

Tanzania is one of the poorest countries in the world with a 2013-‐year’s GDP per capita of $695 compared to Sweden’s which was $58,164. Agriculture is a large part of the trade and industry; it accounts for 28 % of the nations GNP.

Approximately 80 % of the population earns their living from agriculture (Nationalencyklopedin).

There are many small-‐scale farmers who work all day long to provide food for their own families and if there is any surplus, they sell their produce on a very fluctuating market. In addition to the uncertain market prices, many of the farmers also struggle to obtain enough water to irrigate the crops. Even if there is a water source nearby the farm, they still have the problem of water collection and transportation. Most farmers use fuel powered pumps, which is very

expensive due to the high fuel price. These pumps are not only expensive to run, maintain and repair, but they also have a large environmental impact.

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1.1.1 The Kilimanjaro Region

Mt Kilimanjaro is 5,896 m high and close to the Kenyan border. It is the world’s highest freestanding

mountain. Every year around 25,000 trekkers climb the mountain, this because it is a relatively easy climb, which can be done without ropes and experience (Lonely Planet, 2015). The Kilimanjaro Region, marked red in Figure 2, is the area around Mt

Kilimanjaro. It is 13,250 km2_{large
and} has a population of 1.6 million people (National Bureau of Statistics,

Tanzania, 2013) and over 240,000 agricultural households (Ministry of Agriculture, 2012).

Moshi is the most densely populated district and also the capital city of the region. The other districts are Siha, Hai, Rombo, Mwanga and Same. Many farmers in these areas collect their water from the mountain. Those who live on the slopes or nearby irrigate using gravity and the farmers who live further away use pumps to collect the water from rivers, streams, boreholes or wells.

The region has two rainy seasons, the short rain season is from November to December and the longer rain season is from March to May. The annual rainfall is 700-‐1,200 mm and the average temperature over the year is 20-‐26 °C (The World Bank Group, 2015). During the rainy seasons, most farmers cultivate maize and during the dry seasons they mostly cultivate: green peppers, beans, onions, tomatoes and watermelons.

1.1.2 Tanzania Horticultural Association

TAHA is a membership-‐based farmers organization that was founded in 2004. Since then, they have brought together large-‐scale professional operations and small-‐scale farmers to try and help everyone develop their business. TAHA is the fastest growing farmer organization in the region and they have support from the government of Tanzania and other partners such as USAID. By bringing all parts of the horticultural sector (producers, traders, exporters and processors) together, everyone can cooperate to help contribute to economic growth and the eradication of poverty (TAHA).

Small-‐scale farmers who are members in TAHA receive advice and help on how to make their farming more profitable. TAHA also help farmers to invest in equipment and to get in touch with other people in the same business. Together with their partners, they arrange field days where members can visit

demonstration farms to obtain more knowledge about new technology and how to produce better yields and bigger quantities of crops.

Figure 2: Map over Tanzania, showing the Kilimanjaro Region in red (Sémhur, 2009).

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1.2 Purpose

The purpose of this project is to investigate the feasibility of a more sustainable solution for the small-‐scale farmers’ irrigation systems in Tanzania, with focus on the Kilimanjaro Region. The study will focus on a system using solar power, due to the low operating cost and environmental benefits. Solar power is also especially interesting considering the reducing cost of solar components and the great yearly insolation in Tanzania. The focus will be on comparing the systems that are in use today with a solar powered option to give a picture of the

advantages and disadvantages of the different systems. The study will also

investigate what kind of irrigation methods that is in use in the area and which of the methods that would fit a solar powered system best. Besides the economic aspect, it is also important to consider the social aspect of the farmers. For example what problems the farmers encounter in their daily work, what the farmers knowledge about solar power are and what their thoughts are on changing to a new type of system. In the end, the report will show which type of solar solution that would suit the small-‐scale farmers of the Kilimanjaro Region best both from a technical and economic aspect.

1.3 Objectives

The overall objective of this study is to investigate the feasibility of using SWPS for small-‐scale irrigation instead of the PWPS that are the most common today. The specific objectives are:

• Investigate the farmers’ present situation through interviews.

• Investigate what kind of irrigation methods that is in use in the area and decide which one that would fit a SWPS best.

• Make an economic comparison between SWPS and PWPS. • Design a SWPS that could be built in the area.

• Investigate the difficulties of changing to SWPS and give recommendations on how to solve them.

• Investigate if there is any SWPS in use in the area today.

• Investigate the environmental benefits of using a SWPS instead of a fuel powered pumping system.

1.4 Earlier Feasibility Study

The technology for using solar water pumping has already been implemented in several projects around the world. The following case study is an example of this.

“Irrigation schemes using solar energy: A case study in Togblo” (Noumon,

2008)

A civil engineering student from the University of Karlsruhe in Germany investigated the possibility of introducing SWPS in Benin. The field study was performed during 2008 in the southern part of Benin where there is a relative abundance of water. Despite this, the farmers in the area faced many problems. Some of the main problems were:

• High running cost for farmers using fuel powered pumps due to high fuel and maintenance cost.

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• High running cost for famers using electric powered pumps. This was both due high cost and unreliable supply of electricity. The unreliable supply forced the farmers who used this type of system to couple the electric pump with a fuel powered pump for reliability.

• Need for large amount of manpower for farmers who used manual irrigation.

• Problems to receive financial support and loans.

The main objective of the project was to find out if solar water pumping could be cost-‐competitive compared to other ongoing pumping systems in the area. The study also investigates what type of irrigation method that suits a SWPS best and what type of crops that gives the highest profit.

The irrigation method that was chosen in the study was drip irrigation with a water reservoir. This was primarily for the reason that drip irrigation needs a relatively low daily demand of water, is quite simple to run and maintain as well as it needs less manpower than manual irrigation. The low water demand limits the size of the reservoir, pump and the required amount of solar panels. The main crop used in the study was tomato. This was due to the high price for tomatoes during dry season and that tomato is a fast growing crop with modest water requirements.

The conclusion of the study was that the solar system combined with a drip irrigation system is possible for the farmers to afford with technical and financial assistance. The study shows that both technical and financial institutions in the area showed a great interest in the solar technology. This would make it possible for farmers to receive the needed assistance. The economic analysis show that smaller plots of 0.5-‐1 ha rather than larger plots are more economically feasible and affordable by farmers when installing a solar powered system. The crop selection was also of big importance and a crop of high economical value should be chosen when investing in a more expensive irrigation system. In the economic comparison between the solar powered, fuel powered and electric powered system, there are very big differences in investment and running cost between the systems. The solar powered system had many times higher investment cost compared to the other two systems. However, it could still be justified when compared to the fuel powered system, seeing to high running and maintenance cost as well as very short lifetime (approximately three months) of the fuel pump. When compared to the electric pump it was harder to justify the

economical benefit of the solar system. However, as a result of unreliable supply of electricity in Benin, solar system is a good option, if the investment is possible to afford for the farmer. Worth mentioning is that all components for the SWPS was imported from USA. Prices for the components may have been different if the components were bought locally.

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The author of this study believed that solar powered systems in Benin could have a bright future if there is a continuous interest from the government, financial and technical institutions and the farmers themselves. The author also believed that solar systems would be more economically feasible in the future with a continuous increasing oil price and continuous decreasing price on the solar components.

(Noumon, 2008)

1.5 Hypothesis

The hypothesis this study is based on is:

• A SWPS is economically competitive compared to a PWPS for small-‐scale irrigation in the Kilimanjaro Region, Tanzania.

1.6 Delimitations

This project has the following limits:

• The project will only consider small-‐scale farmers in the Kilimanjaro Region that have access to a water source and are using pumps to collect water.

• No actual SWPS will be built. This is a feasibility study for the implementation of SWPS for irrigation in the Kilimanjaro Region.

• The study will target smaller off-‐grid systems and does not consider any grid-‐connected systems.

• Only components sold in Tanzania will be investigated.

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2. Methodology

In order to reach the objectives of the study, the process was divided into three phases: pre-‐study in Sweden, field research in Tanzania and summarizing in Sweden.

2.1 Pre-‐study in Sweden

To gain more knowledge about solar power and water pumping, a pre-‐study was conducted. Information was gathered through Internet searches and literature review.

There were also dialogues with contacts in Tanzania through e-‐mail. The

purpose of this was to obtain a better understanding of farmers’ situation and to be as prepared as possible before arrival.

2.2 Field Research in Tanzania

The field research was conducted by interviewing farmers and by observing their fields and irrigation systems. Questionnaires were created in order to facilitate the survey. By dividing the questionnaires into different areas, the answers gave a general picture of the farmer’s daily challenges and problems. The interviews were conducted in a semi-‐structured way, allowing

supplementary questions to clarify diffuse answers. All farmers were interviewed at their farm. As a result of this, it was easier to observe their situation and take additional notes if needed.

An important part of the field research was the help from TAHA. From a contact in this organization, Mr. Mollel, a lot of additional information that the

questionnaires did not answer could be gathered. Through Mr. Mollel, a working SWPS was located and the owner of this system was interviewed.

2.3 Summarizing in Sweden

Back in Sweden, the first priority was to summarize all gathered information. The questionnaires’ answers were structured in an Excel sheet and all other information was compiled and analyzed. In this way, it was easy to see if there was anything more that needed to be obtained. Through e-‐mail contact with Mr. Mollel at TAHA and solar companies in Tanzania, supplementary data was collected.

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3. Literature Review

3.1 Water Pumping for Irrigation

The main function of a pump is to transfer energy from a power source to a fluid, in order to create a pressure on the fluid to transport it from one location to another. In an irrigation system, the pump is usually used to lift water from one level to a higher level or add pressure in order to obtain the required working pressure of the system. A pump might for example move water from a well or a river to an irrigation system or up to a tank for later use (Ali, 2014).

3.1.1 Pumps

There are a large amount of pumps out on the market for different uses and areas. By investigating two large solar pump manufacturers, Grundfos (Grundfos, 2015) and Lorentz (Lorentz, 2015), two types of pumps are most commonly found for SWPS application. Those pump types are:

• Centrifugal pump

• Helical rotor pump (a type of positive displacement pump)

Centrifugal pump

The centrifugal pump, seen in Figure 3; use centrifugal force to increase the velocity of water. When water enters the pump it moves through an impeller, similar to a propeller, this causes the water to start spinning. The spinning action forces the water to be pushed against the pump walls. This happens through the means of the centrifugal force. When this happens the water picks up speed, which later on becomes pressure when the water leaves the pump (Ali, 2014). A centrifugal pump can be either single-‐stage or multi-‐stage. A multi-‐stage

centrifugal pump has several stages with casings and impellers. This means that the water moves from one casing to another, which makes the pressure increase at every stage. The advantage of a multi-‐stage pump is that it can achieve higher pressures without increasing the diameter of the impeller because of the multiple stages. Compared to a single-‐stage pump, the multi-‐stage pump also achieves higher efficiency at same capacity and head (Volk, 2005).

From the selection at Lorentz

(Lorentz, 2015) and Grundfos (Grundfos, 2015), the centrifugal pump is divided into two groups:

• Surface pump • Submersible pump

Figure 3: Example of a single stage centrifugal pump (Fantagu, 2008).

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Centrifugal surface pump

This pump is the most popular choice where the water source is shallow or located above the pump. The maximum suction lift is limited by the atmospheric pressure. This is because the driving force that forces the water up into the impeller is the absolute pressure of the water at the water source (Volk, 2005). At sea level the absolute pressure of the water is approximately 101 kPa

(Nationalencyklopedin). This means theoretically, if the impeller could create a perfect vacuum, the maximum suction height would be 10.3 m (1 kPa equals 0.102 m of head). In reality, the height of suction is shorter due to friction losses in the inlet pipe and lack of perfect vacuum from the impeller. The maximum height of suction at sea level is usually not more than 8 m. This limit decreases at higher elevations due to lower atmospheric pressure. Worth mentioning is also that the suction head should be kept as low as possible to maximize the

efficiency of the pump (Ali, 2014). The popularity of the surface pump is mainly because of its simplicity, compactness and price. It is the cheapest configuration of a single-‐stage pump and is the most common solution for portable pumps (Volk, 2005).

Submersible pump

The submersible pump is installed completely under water where the motor and the pump are connected as one single unit. The typical pump used in wells and boreholes are often shaped as a long, narrow cylinder, which is installed vertically in the well. The submersible pump has the big advantage that it does not rely on the external air pressure, which makes it a good choice where the water source is below the suction limit and high heads are needed. One of the disadvantages compared to a surface pump is when there is a problem or when the unit needs maintenance. Usually the pump needs to be brought up to the surface, which can be a large operation if the well or borehole is deep. Therefore, it is very important that the pump is designed and installed correctly to minimize these problems (Volk, 2005).

Helical rotor pump

The helical rotor pump, seen in Figure 4, is a type of positive displacement pump. In a positive displacement pump, a forced flow of the fluid occurs when the chamber, that is enclosing the fluid, changes its volume (Volk, 2005). A helical rotor pump uses a helix

rotor, rotating in a double-‐helix stator.

Between the rotor and the stator, occur fixed-‐size cavities. When the rotor rotates, the cavities moves along with it; however, the volume and shape of the cavities stays the same. These cavities move the water through the pump. The shape of a

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often long, thin and cylindrical, which makes it easy to fit inside boreholes with a small diameter. The working mechanism of the pump also makes it a good option where the head for the pump will change over time, for example due to seasonal changes in water level. This is because the flow varies very little with change in head, so the output from the pump will not change (Fraenkel & Thake, 2006). In the selection at Lorentz and Grundfos, the helical rotor pump is only used for submersible applications.

Summarizing

Which pump selected for the specific system is not always clear. Several pump types can fit the system configuration. The characteristics for the pumps have to be investigated in the pump performance curves and a consideration has to be done for every specific system. However, a rule of thumb is that:

• Surface centrifugal pump is used when the suction limit for the specific location is not exceeded.

• Submersible centrifugal pump is used for low heads and high flow rates. • Submersible helical rotor pump is used for high heads and low flow rates. (Morales & Busch, 2010)

3.1.2 Solar Water Pumping System

A SWPS for irrigation usually consists of four main components:

• Solar panel

• Electrical controller • Electric-‐powered pump • Water tank and/or batteries

Solar Panel

A solar panel consists of multiple solar cells built in series. Each cell is built up of two or more layers of a semiconductor material that produces DC power when exposed to sunlight. The most common type of solar cells on the market consists of layers of crystalline silicon. They generally perform an efficiency of around 15 %. The solar panels come in different sizes with different power outputs. They can be arranged in series, parallel or both depending on the voltage and current requirements of the components in the system. When solar panels are arranged in series the voltage output will be the sum of all the single panels voltage output while the current stays the same. When arranged in parallel it is the opposite. The power output decreases over time but should not decrease more than 10 % over a period of 10 years (Morales & Busch, 2010). Also worth mentioning is that prices on solar panels have been divided by five between 2008 and 2014 (IEA, 2014).

Electric controller

An electrical controller is an important component of a SWPS. The electrical controller’s duty is to control the electrical power from panels to pump and provide necessary protection in the system. The component usually has a main switch which makes it possible to disconnect the solar panels from the system. The controller can also switch off the pump when there is not sufficient power

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produced from the panels to reach the minimum power required by the pump. Furthermore, the controller can also limit the power to the pump when the panels produce too much power to limit the pump from exceeding its maximum speed rate. A safety device for preventing the pump from running dry under low-‐ water conditions can also be included in the controller (Morales & Busch, 2010).

Electric powered pump

The pumps used in a SWPS are usually powered by DC electric motors. This is primarily because solar panels provide DC output. If an AC motor is used, a more complex control system with an inverter is required. This provides extra costs to the system and power losses due to the efficiency of the inverter. Use of an AC motor can be justified if the solar system is used for more than water pumping and an inverter is required anyway; for example household electricity. If not, a DC pump should be the recommended choice. The type of pumps used for a specific system mostly depends on the water requirements and type of water source (Morales & Busch, 2010).

Water storage tank and batteries

Normally, there are two ways to store energy in a SWPS: water storage tanks and batteries. Water tanks are more effective and less expensive than batteries. If properly maintained, tanks are a longer lasting and less complex option (Morales & Busch, 2010).

“Remember, the first goal of a solar-‐powered water pump system is to store water, not electricity.” (Morales & Busch, 2010)

A water storage tank stores potential energy and is often the best way to store energy produced by solar panels. The tank is used to store water from the sunny days, when the energy production is high, to days where the energy production is low; for example, cloudy days or days of system malfunction. Preferable, the amount of stored water should be enough to satisfy the water needs for at least three days, depending on the location’s climate. The tank shall be elevated to provide enough gravity-‐induced pressure to distribute the water to the field. The material and structure should be UV-‐resistant in order to maximize its life span (Morales & Busch, 2010).

Batteries can be used to store energy from periods of high insolation to periods of lower insolation, when the produced energy from the solar panels is not enough to power the pump. A controller unit must regulate the battery charge and discharge to prevent battery failure (Morales & Busch, 2010).

3.1.3 Designing a Small-‐Scale Solar Water Pumping System

When designing a small-‐scale SWPS there are several aspects to consider. These aspects can be divided into 11 steps to simplify the understanding of the design. The 11 steps can be seen in Table 1.

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Table 1: Steps for designing a solar water pumping system (Morales & Busch, 2010). Step 1 Water requirement

Step 2 Type of water source

Step 3 Water storage

Step 4 System layout

Step 5 Insolation and solar panel location

Step 6 Designed flow rate for the pump

Step 7 Total dynamic head for the pump

Step 8 Pump selection

Step 9 Solar panel selection and array layout

Step 10 Delivery point pressure

Step 11 Summary of the system

Step 1 – Water requirements

The first thing to consider when designing a SWPS is to determine the water requirement. When designing a system for irrigation, the average water requirement for the crops grown on the field can be used to determine this factor. This requirement differs depending on the location. Therefore, the crop water requirement should be investigated for the specific location where the system is being installed. In addition, when the implementation of crop rotation is used, the crop with the highest water demand should be used for the design (Morales & Busch, 2010).

Step 2 – Type of water source

This step is important to determine what type of pump that fits the system best and to find out if the source can handle the water requirement. The first thing to determine is what type of water source that will be used in the system. Common sources are: well, borehole, river and spring. The next step is to determine the properties of the source. Important factors to consider are: static water level, dynamic water level and quality of water (Morales & Busch, 2010).

• Static water level:

The distance from the surface of the water to the top of the well. This can be measured when there is no pumping from the source and the water is given time to be refilled. The static water level can change over time and depending on season (Morales & Busch, 2010).

• Dynamic water level:

The distance from the top of the well to the surface of the water when the pump is running. The water level will typically descend when the pump is running. Depending on the size and refilling rate of the source, the water can descend several meters. If there is question about the capacity of the source, a test of the dynamic water level should be performed. This can be done through a pumping test. The test is usually performed by pumping

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water from the source with several pumping rates. From the test, the capacity of the source can be determined. It is important to not exceed the capacity of the source. This can lead to the pump running dry and risking severe damage (Ali, 2014).

• Quality of water:

The quality of water in the source. This is more important when water is used for human consumption rather than irrigation purposes. However, if there is suspicions that the water is contaminated, a water quality test should be performed to determine the content in the water (Morales & Busch, 2010).

Step 3 – Water storage

In a small-‐scale SWPS, water is usually stored in a water tank. A tank is, as previously mentioned, often the most economical and simplest way to store water in a small-‐scale SWPS. The tanks volume should be able to hold at least three days of the daily water requirement. If the daily water requirement is large, multiple tanks can be connected together for storage.

Step 4 – System layout

Designing a sketch of the system layout is important to get a picture of the system. The sketch should include where the different components should be located. It should also include the elevations and distance between the

components. The components typically included are: • Water source • Pump • Solar panel • Storage tank • Pipelines

(Morales & Busch, 2010)

Step 5 – Insolation and solar panel location

Data that shows the available insolation should be collected for the location. This can be done through several insolation data programs on the Internet. These programs usually show the daily and monthly insolation from different

measuring stations around the world. One example of an insolation program is PVGIS, a free program made by the Joint Research Center from the European commission (Joint Research Centre, 2014). Another option is to contact a qualified expert in the area, who can measure the insolation on site, to get the exact values for the selected location (Morales & Busch, 2010). The average monthly insolation used in this study can be seen in Table 2.

Table 2: Monthly insolation in Moshi, Tanzania (Joint Research Centre, 2015).

Optimal azimuth = 327°, Optimal angle = 1°

Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Peak sun hours

6.5 6.5 6.7 5.2 4.5 4.1 4.4 5.0 6.0 6.3 6.1 6.1

(24)

The angle and direction of the panels should also be considered in order to maximize their production. If the panels are fixed and used for year round applications, the angle and direction should be a mean value of the yearly

direction of the maximum solar irradiance. Furthermore, it is important that the location of the solar panels does not have any significant shadowing during the day in order to get full sun exposure (Morales & Busch, 2010).

Step 6 – Designed flow rate for the pump

The flow rate, which the pump is designed for, is based on the peak sun hours for the chosen design month and the daily water demand. There are several ways to choose the design month for the system. However, the most common way is to design after the month with the least amount of insolation. The reason is to ensure that the system is not undersized for any month of the year (Morales & Busch, 2010).

The designed flow rate for the pump can be calculated from Equation 1.

_{𝐷𝑒𝑠𝑖𝑔𝑛𝑒𝑑 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 𝑓𝑜𝑟 𝑝𝑢𝑚𝑝 =}𝐷𝑎𝑖𝑙𝑦 𝑤𝑎𝑡𝑒𝑟 𝑑𝑒𝑚𝑎𝑛𝑑

𝑃𝑒𝑎𝑘 𝑠𝑢𝑛 ℎ𝑜𝑢𝑟𝑠

(1)

Step 7 – Total dynamic head for the pump (TDH)

TDH for a pump is the sum of the vertical lift, pressure head and friction losses.

• Vertical lift:

The vertical distance from water surface at the source and water surface in the tank. As previously stated, the water level in the source can change depending on season and usually change when the pump is running. The vertical lift should therefore be designed for the time when the water level in the source is at its minimum.

• Pressure head:

The pressure at the delivery point in the tank. If the delivery point is on the top of the tank, this parameter can be set to 0.

• Friction loss:

Pressure losses due to friction in the pipes. This parameter is determined through four factors: inner diameter, length and roughness of pipe as well as flow rate of the water. The friction losses can be obtained from a

friction loss chart/table for the selected pipe. (Morales & Busch, 2010)

Vertical lift can be calculated from Equation 2:

𝑉𝑒𝑟𝑡𝑖𝑐𝑎𝑙 𝑙𝑖𝑓𝑡 = 𝐷𝑦𝑛𝑎𝑚𝑖𝑐 𝐻𝑒𝑎𝑑 + 𝑇𝑎𝑛𝑘 𝐸𝑙𝑒𝑣𝑎𝑡𝑖𝑜𝑛 + 𝑇𝑎𝑛𝑘 𝐻𝑒𝑖𝑔ℎ𝑡 (2)

When using a friction loss table, seen in Appendix C, length of pipe and flow rate have to be known. Length of pipe is calculated from Equation 3. A preferable diameter of the pipe is selected from the table. A smaller diameter equals more