Off-Grid Photovoltaic System for Water and Electricity Supply in a primary school of Thmor Keo, Cambodja
Introduction
In 2015, our consultant Federico Brinchilin travelled, together with Raf Van Gorp, Hans Wuyts, Amadeus Maes and Alexander Papen to Thmor Keo in Cambodja in collaboration with Humasol, a non-profit organization whose goal is to make renewable energy, water and sustainable technology accessible to everyone.
During the project they installed an off-grid photovoltaic system for water and electricity supply.
Throughout the scope of the project, Humasol was working together closely with local partner Banteay Mean Chey, founded by Chay Nowé, the Consulate General of Cambodia.
A perfect match
Due to a turbulent 20th century, Cambodia is less prosperous than its neighbouring countries.
Most of the intellectual people have been killed by the Khmer Rouge regime, causing the level of education to drop drastically.
Therefore, it is important to improve the conditions at schools, giving students an incentive to attend. Especially in rural Cambodia, the conditions are very poor as there is neither an electricity grid nor streaming water.
On the upside, throughout the previous years, the solar industry has been growing in Cambodia.
Especially the region we operate is not only very appropriate for solar radiation purposes, but solar energy also creates jobs and contributes to sustainable development.
This exactly coincides with the vision of Humasol. Contributing to the development of a less prosperous region, in a sustainable way, by technical and financial means. Discover more related projects on their website.
Furthermore, solar energy is an ideal solution to the increasing electricity demand in rural Cambodia, as it is impossible for electricity companies to meet this demand in the nearby future.
A perfect match, really.
About the project
Our project consists of the design and implementation of two separate systems.
First of all, we will install an AC network in the two school buildings. This will provide electricity for basic appliances such as lights, laptops and a printer.
Independent of this battery-based system a water pump needs to provide water for faucets and toilets.
The dimensioning and technical details are given later in the report.
Expectations
On one hand, the local partners expect us to build a durable system. In order to achieve the best possible result, a lot of preparatory work has to be done beforehand. The director of the school in Thmor Keo is the contact person in the village and will monitor the progress of the project. The partner expects us to ask for help whenever this is necessary and will provide logistical support.
On the other hand, I and the other students hope to be able to satisfy the expectations of the local partner by improving the conditions at the school.
During the scope of the project, it is essential to involve local people as much as possible in the construction process of the system. By doing so, we want to guarantee that our knowledge is transferred to the locals and make them aware of the potential hazards. We hope the director of the school can help us with the transportation of the components from Phnom Penh to Thmor Keo.
Last but not least we expect this summer to have a wonderful impact on our lives, in whatever way this will consolidate itself.
Planning
In December 2014 we started off by writing an elaborate literature study in cooperation with the other Humasol teams. This literature study aims at bringing together all information necessary to build an off-grid solar energy system. Future project students will be able to use our literature study in order to make a lot of progression in a short amount of time. It gives an in-depth technical explanation of the different components together with their function within the system.
The actual design of the electrical system started in February. This design is highly variable as it completely depends on the availability of components in Cambodia. Whenever new information from suppliers arrives the design has to be reviewed. The design of the pumping system started in March.
Goals
- Install solar panels generating electricity for the school
- Install the AC net, since there is none
- Install a solar pump to get water in tanks on a tower
- Build the tower on which the water tanks will be put
- Make a piping network to distribute the water from tanks to utilities
Technical specifications
Dimensioning of PV installation
This chapter describes the dimensioning of the different components of the electrical installation, we make a choice based on the demand, availability and technical specifications.
We start off by determining the loads the system has to be able to support. When the loads are fixed, all other components follow in a natural way. This method will thus allow us to design a battery-based system that can carry all the loads. The danger of this method is that the costs can increase considerably. Sometimes it is more efficient to undersize the system forcing the user to consume the available energy in a more economical manner.
The main building with the complete electrical system, including the pump at the left. There are two grounding rods in green (AC and DC).
Determining the loads
| Load | Number | Power/piece | Total power | Hours/day |
| LED-tube | 45 | 20 W | 900 W | 4 h |
| Laptop | 8 | 80 W | 640 W | 5 h |
| Printer | 1 | 50 W | 50 W | 1 h |
This table allows us to calculate two important quantities. The maximal continuous power usage is Pmax = 1590W, and the total energy used on a daily basis is Eday = 6850 Wh.
However, when the weather is bad for some days, the local people should be aware of the fact that they should spend less energy.
Batteries
The batteries should be able to store the complete energy consumption of one day. In other words, this means that there will be one day of autonomy. For safety reasons we limit the voltage of the battery bank to Ubank = 24 V. The DOD is the depth of discharge. We start by taking a DOD of 0,6. ? is the efficiency of the inverter, which is more or less equal to 0,9. ? is the efficiency of the battery bank, which we also take to be about 0,9. The general formula to calculate the capacity needed is the following:
Capacity = Eday/ Ubank ∙ ? ∙ ? ∙ DOD
-> Capacity = 590 Aℎ
It is difficult to determine how many cycles will occur in one year, as the batteries are being charged and discharged at the same time. This is why the discharging rate will be low during the day. There will only be a significant discharge in the evening, the early morning or during cloudy periods. The capacity needed with a DOD of up to 40% is about 850 Ah. Four parallel strings of 200 Ah seem to be the best solution.
The battery study conducted by last year’s Humasol project students led to an important conclusion. It is not because batteries should theoretically have a longer lifetime, which this lifetime will be reached in reality. It all depends on how the batteries are treated. If the user doesn’t maintain the battery bank in a disciplined manner, the negative effect on these batteries will increase. Highly fluctuating charging and discharging will decrease the lifetime considerably. Because a lead-acid battery is somehow robust, it can deal with a chaotic environment much better than other types of batteries. Even though the school will have fixed charging and discharging patterns, we should account for possible random fluctuations and bad maintenance by the local people.
During the installation of the batteries, they should be shielded away from the sun as much as possible. Increased temperature will result in a reduction of the lifetime. This applies to every type of battery. The batteries should definitely not be placed on the ground, as moisture and other filthy particles could damage them.
Detail of the power electronics
Inverter
The inverter should always be able to deliver the maximum continuous power. We use the Phoenix Inverter 24/3000 with a maximum continuous power of 2500 W at 25°C seems to be the best option.
Solar panels
A solar panel has a certain peak power, that shows the monthly peak sun hour for the Phnom Penh region. This is measured in kWh/m2/day onto a horizontal surface, facing directly to the south.
Peak sun hours Phnom Penh region
The general rule of thumb used to determine the optimal angle of the solar panel array is that it should be equal to the latitude of the location.
In Cambodia, this is about 15°.
In order to calculate the number of panels necessary, we use a minimal peak sun hours (SHmin = 4,5). There should also be a safety factor that accounts for the shading of the panels and the efficiency of the whole system, including the Joule losses in the electrical wiring. We take this factor µ to be between 0,75 and 0,80. The number of panels can then be determined by the following formula.
Number of Panels = Eday / Ppeak ∙ SHmin ∙ ?
We use 240 Wp Yinlgi Solar panels that yield a voltage of about 35 Voc. There can only be two of these in series as the input voltage of the charge controller is limited to 100V. Using the formula above gives a number of solar panels equal to 8. This is split up into 4 parallel chains of each 2 panels in series.
The solar panels, grounding is green and goes to the DC grounding rod, three panels in series
Charge controller
The charge controller should be able to handle two things. The voltage from the solar panels applied at the input and the charging current is drawn by the batteries at the output. Standard charge controllers have a maximal output current of 50 A. This implies a power of 1200 W.
This is however not enough to handle all the power coming from the solar panels when they are working at their fullest potential. This implies the need for a second charge controller. The BlueSolar MPPT 100/50 (12/24V-50A) is available at Kamworks and has a maximum input voltage of 100V. This allows us to place solar panels in series as long as the Voc stays under the threshold of 100V. With the chosen 240 Wp panels, we are able to make strings of two panels resulting in a Voc of about 70V.
DC cable
This section describes the cables from the charge controller to the batteries, the connections in- between the batteries and from the batteries to the inverter. For the DC cables from the panels to the charge controller, we need special ultraviolet resistant cables, which are more expensive.
The biggest DC current will flow from the battery bank to the inverter. We chose the inverter to be one capable of handling 3000VA at 25°C. This corresponds to a maximum current of about 125A. The cable area can easily be calculated by the following formula.
A = ? ∙ l ∙ Imax / ? ∙ U
? is the resistivity of copper, l is the length of cable between the batteries and the inverter, Imax is the maximum current, U is the voltage of the battery bank (24 V) and ? is the voltage drop due to losses in the wire, which is chosen to be about 0,01 to 0,02. The numerical result should be rounded up to standard wire size. It seems sensible to use the same area for all DC wiring. By doing so we do not need to buy different areas giving us more flexibility.
Once the maximum current stays fixed, the remaining factors are the voltage drop and the length of the cable. Let’s say that in the worst-case scenario we have a length of about three meters. The voltage drop can be chosen freely, as long as it remains under 3%. Because the length of cable for the DC wiring is small compared to the AC wiring lengths, we can easily neglect the surplus price of DC wiring coming from the difference between 16 and 35 mm2 wires. The price of armoured cable and submergible cable also outweighs the price of the DC cable. Keeping this in mind, we choose an area of 35 mm2. This corresponds to a voltage drop of 0,8 % in the worst case. With a cable area of 16 mm2 there would be a voltage drop of 1,7 % in the worst case.
The next table collects the results. The metric used to determine the price is $0,12/mm2/m. We estimate the total length of the DC cable to be 20 meters. This includes connecting the two charge controllers to the battery bank, connecting the batteries with each other and connecting the batteries with the inverter.
| Area [mm2] | Voltage drop [%] |
| 16 | 1,7 |
| 35 | 0,8 |
As mentioned before, we take the same area for the DC cables throughout the whole DC system, apart from the interconnection between the solar panels and the charge controllers. Problems could arise if the cable is too thick to match the fittings of the charge controller or the inverter. Therefore, it is very handy to take specific tools with us to Cambodia in order to make small adjustments to cables. Stripping tools and connected devices such as WAGO connectors, wiring nuts and heat shrinks are essential and can spare a lot of our time.
AC cable
The area of the AC cable is 2,5 mm2. This is a standard AC cable size. There are two big buildings with a total of 15 classrooms that need to be wired. As we dimensioned the system for 45 lamps, we are going to divide the 45 lamps uniformly over the 15 rooms, which gives three lamps in every room. The eight laptops are mobile. In this dimensioning, we choose to confine the outlets to the teachers’ room. We drew four outlets in the electrical scheme. Because the outlets have to be grounded, we use 3×2,5 mm2 cables for this. We don’t know where the teachers’ room is located, but we estimate a length of 30 meters is necessary. During the project, we will off by installing lamps in the main building. If we have enough time, and the budget for the armoured cable, we will also install lights in the second building.
Detail of the teachers’ room, only two lamps drawn for simplicity
The lamps do not need grounding. Cables of 2×2,5 mm2 are therefore in order. The AREI states that there may be 8 lamps in one circuit in parallel. When we place three lamps in every room, this means there will be four circuits in the main building and two in the other building. A combined length of 220 meters is estimated. The transition of the main building to the other building has to be made under the ground with armoured cable. This needs to be 4×2,5 mm2 armoured cable to minimize costs.
Both the DC and AC cable connections should be taped off with (black) electrical tape.
PV Cable
The cable from the solar panels to the inside of the building should have stronger insulation than normal cables. Cables that are exposed to the sun should be insulated this way making them UV and weather resistant. Because these PV cables are expensive, making smart routing can reduce the price significantly. The maximal current through these cables is equal to twice the short circuit current given in the datasheets of the solar panels, as there are two parallel chains for every charge controller: Imax = 17,5 A. Taking a maximal length of 10 meters, the previous formula from the section of DC cables yields an area of 6 mm2.
Fuses
Every AC circuit requires a fuse with a nominal current of 20A. In an AC circuit, it doesn’t matter on which side the fuse is installed as the current constantly changes direction. DC cables with a surface area of 16 mm2 require fuses with a nominal current of 80A and 35 mm2 DC cables need 100A fuses. The previous information can be found in the literature study. Although both the charge controllers and the inverter have built-in security, there still has to be the possibility to decouple these components. This is why there should be a fuse between the solar panels and every charge controller and a fuse between the batteries and the charge controllers. In the DC circuits, it is obvious that there should always be a fuse on both the + and the – side of the circuit.
Grounding
It is important to keep the AC and DC grounding separate, by using two different grounding rods. It seems practical to divide the AC grounding into two parts. One grounding device is used for the main building and another one for the smaller building. The price of three grounding rods with a length of 3m is about $85. The rods need to be installed as explained in the literature study.
As we saw in the literature study, a good grounding has to have a resistance of no more than 50Ω, if possible less than 30Ω. The following formula calculates the area of the grounding wire, and this is applicable for both DC and AC. In this formula, l is the length of the cable, which is unknown at this moment. R is the resistance and ? is the resistivity of copper as used in the previous formula.
A = ? ∙ ? / ?
If you fill in some values you can see that it is possible to reach a very long distance, up to one kilometre, with the standard AC area of 2,5 mm2 while remaining under 20 Ω. However, making the length of the grounding cable longer than necessary, will increase the cost and the resistivity.
Hence, it is wise to minimize the amount of grounding cable in order to protect the system from surges as much as possible.
Every grounding cable should be isolated in a green/yellow colour. It is also smart to tape off the electrical connections with the ground by use of a green/yellow electrical tape.
DC grounding
It is crucial to ground the frame of the solar panels carefully by interconnecting the entire frame electrically and bringing the grounding wire to the grounding rod. Because this wire is running on top of the roof, it has to be PV cable.
The charge controllers don’t need to be grounded, neither should the batteries. When the batteries are stored on a conducting surface, this surface has to be grounded. Therefore, it is better to use a non-conducting surface. This surface should also be dry and thermally insulate the batteries.
AC grounding
The inverter has to be grounded on the AC side. The lamps do not have a grounding pin. Outlets, on the other hand, do need to be grounded.
Lightning protection
The lightning can destroy the entire system in a split second. Ideally, one would install a rod on the top of the roof, and connect this rod with the ground. But this has to be done decently and we have not examined this thoroughly. When it is not installed properly, the roof can be set on fire or heavily damaged. As we are no experts in this field, a rod on the roof is not a good idea unless we can find an expert in Cambodia. The fuses incorporated throughout the entire system will offer the necessary protection from surges due to lightning.
Dimensioning pump installation
It has been decided not to use batteries within the solar pump system. The reason for this is to avoid the use of a component that is expensive, not eco-sustainable and that requires a lot of maintenance.
In order to have water during the night or when the radiation of the sun is insufficient, the water is stored inside a tank. In this manner, the tank is equivalent to the batteries in the electrical system. The tank needs to be elevated up to a certain height in order to establish a reasonable flow rate at the taps. In the drawing below a simple scheme of a possible hydraulic system is shown. The hydraulic system can be divided into two sub-systems that are dimensioned separately. The su