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MamboViewPoint is a new ecolodge in the Usmabara Mountains in Tanzania, where the use of renewable energy is very important. Using solar energy is not rocket science but a good design can be difficult.

The MamboViewPoint solar installation design is explained below in detail so as to freely transfer our knowledge to anyone who who wants to use solar energy. If you design it well you can both profit and have a small environmental footprint. The article is a bit technical so if you are not a technician, you just can read the yellow “what we have learned” boxes and believe us that it’s the truth...

Solar energy is feasible (but not very profitable) in countries with a lot of sunshine like Tanzania, however few solar systems are found in Africa compared to Europe, where solar systems are profitable because of heavy grants. In Africa they can not compete with the regular electricity provider rates but stll they offer you a far more reliable and
CO2 neutral system.

Energy load

The first most important thing to consider is the energy load required. This involves two separate issues:

1. Try to reduce the load as much as possible
2. Try not to cover all your energy with solar but only a fair part.

Reducing the load

22The most important method is to use LED lightning. Nowadays there are good LED lamps available which have a warm-white colour, fit in the regular fittings and operate with the regular voltage (230V). Also tube lights are available in LED. You just take out the starter from the regular tube light fittings and there you go. Also it is not always necessary to have that much light. A well considered plan for spotlights is better than some big lights for the whole place. Unfortunately in Tanzania there are hardly any reliable LED lights available.

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Fridges take a lot of energy. Therefore you should use special "solar" fridges which are very economical, for example 30W for a 60 liter cooling box. They have an upper-door which is important because the cold stays inside when you open the fridge. They are also adaptable for use as a freezer.

Switching the lights on and off: leaving it up to human beings is not good enough because they will forget to switch it off. There are affordable sensors which you can connect to lights in toilets, bathrooms and other rooms with temporary use only.

33Use timers to switch lights on in places that need only light during the evening or the night. There are also light sensors but those strangely enough are not easy to find and when found they are most of the times expensive.

Use netbook computers only. Old, second hand computers and monitors use a lot of power and are slower than modern, affordable netbooks. Modern netbooks and LCD screens use less than 30Watts which saves 90% compared to old computer-monsters.

Outdoors you can use outdoor solar lamps that have their own solar panels. The small ones are cheap and available at least in rich countries. But the best are those who have a sensor. In the dark they burn small and when you pass they go bright.

Cover only a fair part of energy: solar panels are still expensive. If you try to cover all your needs with solar the last 10 or 20% will make your installation extremely expensive because you have to calculate for the maximum use and those big users like water pumps and ovens only are used for a small part of the time. It’s better to get power for those big short period consumers from another source like the public grid or a generator. As long as solar panels and
batteries are expensive this is the best solution. Unfortunately most solar suppliers are calculating with the maximum use, also because its in their interest to sell as many panels and batteries as possible. It is the same during the rainy season. If you cover one week without sun this is fair enough. You are left with some generator-days a year, which is acceptable.

44

What we learned about the load:

• Use LED lights only.
• Use netbook computers only.
• Don’t try to power the last 20% also with solar.
• Use solar fridges only.
• Use timers and sensors to switch the lights.
• Don’t design for the maximum.

Voltage

55Nowadays worldwide electricity is 230 Volt AC. (high voltage alternating current). Car electronics are 12V DC (low voltage direct current). This is an important detail because for both currents (230V AC and 12V DC) there are a numerous number of equipment available. If you choose any other voltage you will have trouble connecting and you will need converters. If you use 12V, the equipment has to be close to the battery because you will lose a lot of energy in the lines and the lines havr to be thick and are expensive. On a huge terrain like MamboViewPoint it would even be impossible to get the 12V to the other side of the terrain. For small installations 12 Volt DC will do and you can connect direct LED lights to the battery. But for a more serious installation you will need an inverter which inverts the 12Volt DC from the batteries to 230V AC for the consuming equipment.

For big installations the problem with 12V is that you need a lot of current. For example a 1-Kilo-watt Load (which can be normal for a household) will give about 4 Amperes if you have a 230V installation but it will need about 80 Amperes when you have a 12V DC installation. This is one of the reasons that solar panels sometimes are joined together to
make it a 48V or even 240V DC installation. The disadvantage however is that if you do so, you need special chargers and also can’t use the batteries for 12Volt loads anymore. Another thing is that the panels need to be all from the same wattage and if only one small part of one of the solar panels in is the shade or broken the rest of the panels will not deliver the maximum current anymore. If you stick to 12 Volt this risk is far smaller since every solar panel works at its own and you can couple as many solar panels from different wattage as you want.

What we learned about the voltage:

• Use 12 Volt solar panels and batteries only.
• Only for small installations you can use 12V DC
else you need an inverter to 230V AC.
• In case of a 12V installation the consumers must
be close to the batteries.

Solar panels

66There are many different types of solar panels. Which ones to use and how to install them? Basically it does not matter which ones you choose, as long as they are of the same wattage. However there is no need for this in a 12V installation, it is handy because you can later change if desired and for making an array its nicer if they are all the same. It is important to know that solar panels work best if they are put square to the sun. Since the sun is moving from east to west everyday and from north to south and the other way around once a year you gain the most if you can turn them in both directions. This will give you at least 50% more energy than if you mount them in a fixed direction. You may either do this by automatic systems, which unfortunately are a bit fragile and expensive so they can eat the profit totally, or turn them by hand.

Earlier we already stated that a little bit of shade will reduce the power dramatically. Therefore it is important that all your panels are in the sun as much as possible. There is no use to put an extra panel when only a piece of it is in the shade. Unfortunately the panels do not work during the night or when it is cloudy, in contrast to what you often read that panels deliver energy even when it is cloudy - forget it!

What we learned about the solar panels:

• Use solar panels of the same size and wattage.
• Don’t put any part of the panels in the shade.
• If possible make them rotating in 2 directions.
• Afterwards you always can add more solar panels.
• Don’t believe stories that solor panels deliver
energy during cloudy weather, they don’t.

 

Batteries

77The batteries for a solar installation are of a special type. This is because the charge and load is in contrast with a car battery and kind of steady (if you use a car battery like that, it will not last long). If you use more batteries (what you most the time have to do with big installations) they must be all of the same type (Ampere-hour). Remember that it is unwise to add more batteries later because the new ones will not have the same specs like the old ones. Very important is never to unload batteries completely. If you do so it will be very difficult to charge it again and it can even damage the battery. The equipment on the load side has to be designed that it switches off when the voltage is too low. Unfortunately batteries do not have a very long lifetime. This means that you have to replace them 2 or even 3 times within the lifetime of the solar panels. To calculate the number of batteries it is important to know that the loss of the batteries depends on the time within which they are charged or discharged. The loss is between
10% with currents till 5% of the capacity up to over 50% if you use currents over 30% of the capacity. So keep in mind that you do not get the same power out of a battery as the one you put into it.

What we learned about the batteries:

• Always use special solar batteries. Car batteries
can work but will not last for long.
• Design the installation in a way that you do not
need more batteries later.
• Never unload batteries completely and use
equipment to protect total unload.
• Charging and discharging always gives a loss.
This loss depends on the time within which you
charge or discharge. The faster you do the higher
the loss. The loss can go over 50% (!)

Chargers

To charge the batteries from the solar panels you need a charger between the panels and the battery. This can be one or more. To lower the current and also to spread the risk in case of damage it is better to make groups of solar panels all with their own charger. For small installations you can use a charger which is combined with a prevention for
total unloading.

Inverter

To make 230 V AC out of 12V DC you need an inverter. This is an important part of the installation. There are different kinds of inverters. The cheapest ones do not produce a nice alternating current shape like the one you will get from the public grid. In that case you can get difficulties with some equipment which needs nice AC-shapes. The ultimate converter is the one which combines different functions:
• Converting 12V DC from the battery to 230V AC.
• Combining current from other sources like a generator or the public grid with that from the solar installation.
• Charging the batteries from another source in case the solar panels are not enough (f.e. during longer periods of cloudy sky).

Our Victron Phoenix converter meets the above requirements and turned out to be very reliable. Other brands will probably offer similar equipment.

88

The combination of solar power and an external source is a very sophisticated and not obvious excercise, but at the same time it is a “killing application”.  You have to realize that the solar power has to synchronize with the external source before you can combine it. This has to happen without a current disconnection. With the Victron Phoenix it is even possible to limit the power which is provided from the external source which can be helpful if the external power source has a limit or if you don’t want to use external power because you want to favour the solar power. If you use a combination like that you have to regulate the installation carefully because of the following reasons:
• If the external power is always available, the battery will be charged from there and you will not use the solar panels anymore even if it’s sunny.
• During daytime the battery can be charged from both the solar panels and from the external source. The total can exceed the maximum acceptable current.

The best choice is to regulate the system in a way that the batteries are only charged from the external source if they are less than half full.

What we learned about the inverter:

• There are two types of inverters. The cheapest one
does not make a nice current-shape which can be
insufficient for some equipment.
• The ultimate inverter combines charging and a feed
from the solar installation with another source like a
generator.
• It is important to adjust the inverter carfully to get the
maximum profit.

 

How to calculate the capacity from the solar panels and the batteries

99In annex 1 you find the calculation of MamboViewPoint. First the daily consumption is calculated which is in our case about 2Kwh. Then we calculate how many batteries are necessary to be able to survive for 6 days if the sun is not shining. This is about 12kWh which means there is a need of total 1 kAh batteries. (10 batteries of 200 Ah each, 1 kAh = 12 kWh with 12V). For one day consumption we need 6 panels of 65W. (this supposes the sun is fully shining for 6 hours and the efficiency of the panels is 65%). If we want to reload after a period of clouds in 6 days this needs another 7 panels.

Remarks:
• Full sunny days are not many so recovering in general will take more than 7 days.
• On the other hand full cloudy days are also rare so it will take over 6 days till fully empty in general.
• If the batteries are full there is a not-used over capacity of 100%(!).
• The over capacity will be reached after 8 full sunny days (6 for reloading 2 for the daily use) in general it will take more days and in case of half-clouds even not be reached.
• Since we need the generator for the water pumps and the oven, it is running every day for about 2 hours. In periods of cloudy weather it can charge the batteries with 960Wh which extends the period till empty from 6 days till 9 days. In case of fully empty the generator has to run for 2 hours a day more which is still acceptable.

What we learned about designing:

• The capacity of the batteries is depending of the consumption and how many days without sun you want to survive.
• The number of panels is depending of in how many days you want to recover after cloudy days. If you choose too much panels you create an unused overcapacity.
• To reduce costs and over capacity it is fair to support with an external power source in case of more cloudy days on a row.

 

Financial profits and losses

If you buy a solar installation because you think it is cheaper, forget it, it is not. Even in good conditions you will not gain enough energy to make it profitable. Unfortunately the solar panels are way too expensive. In western countries only because of heavy grants people can turn a small profit, but if you do not have grants the public grid is cheaper.
Below you find the calculation which shows that solar energy is about twice as expensive as the public grid.

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Why should you us solar power if it is so expensive?
If you are in a remote area without public facilities, there is no choice and it is cheaper (and way more silent) than a generator. If there is a public source, you will notice that the public grid in development countries often is not reliable, and if they have power it can be of a very bad quality. Especially the combination of both can make your installation very reliable with a constant voltage. If you have a public grid you even can reduce the capacity of the solar system just to make it reliable.

What we learned about the costs:

• Solar energy is more expensive as energy
from the public grid.
• In developing countries solar energy is usually far more reliable than the public grid, especially when you combine it with the public grid.
• If you do not have a connection to the public grid there is no choice.

 

Experiences with the MamboViewPoint solar installation

1010The installation like it is described in this paper showed to be very reliable. After one year use the only power interruptions have occured because of maintenance and extensions. This has even made people who have to
face the constant power interruptions of the local power provider jealous. Please note that in Mambo there is no public electricity at all, but the economics in small towns like Lushoto often weigh heavily because there is no
electricity during parts of the day or sometimes even days in a row or months in a row as recently happened in Zanzibar.

Keeping an eye on the system

It is very important to keep an eye on the system, what is your use and what are your profits. Also you can prevent a power cut if the batteries are nearly empty by charging from the generator in time. For this you need the following additional equipment:
• A kilo-Watt-hour meter
• A battery monitor for the left capacity and the momentary load or charge
• On the chargers a display for the delivered power and the momentary profit.
• A mobile powermeter to measure the use of the individual lamps or other users.

What we learned about the experience:

• Solar energy can be very reliable.
• You need some measure equipment to keep
track of the behaviour.
• It needs daily attention to turn the panels in
the sun and to see if extra charging is needed.

 

Postscript:

Algthough we tried to be complete, some things in this guide can be wrong or incomplete. We appreciate your comments so we can improve this document. Please feel free to add comments here and/or to email info [at] mamboviewpoint.org

Annex 1
Calculation of the needed number of batteries and solar panels for MamboViewPoint

annex1annex1

Annex 2 Block diagrams

Below the block diagrams of the MamboViewPoint solar installation and a small installation
(which we use for the luxury tents)

annex2aannex2a

annex2bannex2b

Annex 3: Equipment

annex3annex3

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