Solar panels, known as photovoltaic panels or PV panels are needed to collect the sunlight during the day. It is important that the panels are never in shadow otherwise the ability of the panels to efficiently collect sunlight is dramatically reduced. The panels should be treated like a sheet of glass; they are normally mounted in aluminium frames which are then bolted to a strong steel frame. These in turn are then bolted securely to the ground or to a strong roof. Like glass, the photovoltaic panels should always be kept clean.
Cables connect the supply of energy from the solar panels to the batteries. The electricity that flows along these cables is DC. The cables need to be as thick as possible and as short as possible to minimise the resistance of the cables to the flow of electricity and thereby avoid losing the energy as heat. The supply company Solar-Wind in the UK has a calculator so you can work out what thickness cables to use.
Batteries are needed to store chemical potential energy until electricity is needed. The batteries do not want to be stored in direct sunlight (unlike the solar panels!) and a stable temperature is best for these.
Switches are needed to isolate different parts of the circuit. This is very important for safety when setting up the solar farm.
The school observatory, GJW-IN
This observatory is mounted at the same level as the roof of the school. Towards the top right of this photograph is the roof area where we mounted the solar panels. To the bottom right of the picture is a region that rarely gets direct sunlight: this was chosen as the location for the battery shed.
Protecting the batteries
We needed a structure that would protect the batteries from squirrels and so steel was used as the material for construction. The steel for the battery shed was delivered by this horse, which reversed in to allow for ease of downloading.
This consisted of a welded steel frame, clad in thin steel sheets which were painted to prevent rust. The structure was sealed against squirrel ingress by the concrete floor and by perforated aluminium mesh near the roof.
Storing the energy
We use lead acid batteries from Exide to store the energy collected by the solar panels. The batteries are arranged in four banks, each comprising twelve 2V cells thus giving a 24V battery. The capacity of the four battery banks together is 2000 Ah.
Cables to carry current
Thick cables are needed to carry the current without risk of heating and fire. If the cable have many copper threads then they are more flexible which makes them easier to handle. These thick cables are connected to fuses, switches and devices with compressed metal pieces called crimps.
For a few of our cables, we had to create our own crimps from copper tube to ensure that the diameter matched the size of our cables. We learned that the fruit of a nearby tamarind tree was sufficiently acidic that the copper became beautifully clean and perfect for our purposes.
Strong steel frames support the solar panels. It is important that they are securely fixed to the surface below, in this case the school roof, and able to withstand high winds. Holes were drilled into the roof with a hammer drill, and bolts were chemically bonded inside these.
Supporting the PV panels
The frames were oriented so the solar panels are aligned with the sun at local solar noon. The panels are securely bolted to the frames.
Conduit for the cables
Reinforced conduit protects the cables from the solar panels down to the battery shed. This protects the cables from weather and from curious local wildlife.
We hope that these monkeys will not amuse themselves anywhere near the solar farm or the observatory!
The batteries supply a voltage a little above 24V but some of our devices need be be powered at 12V, 5V or even 48V. We use Victron voltage converters to ensure each device is powered at the appropriate voltage.
Making good connections
To connect the cables to the various devices, we use crimps of different sizes to match the cable diameter. Big crimp tools are needed to ensure secure and enduring connections.
We use circuit breakers and isolators to ensure that if a part of the system needs maintenance, it can be safely disconnected.
We use fuses to protect the cables from overheating in the unlikely case of overcurrent. These fuses are mounted in strong insulating cases, one for each battery bank.
The fuses are large, and securely bolted in place to ensure that all connections are good. With the fuses all in place the circuit is complete!
After carefully testing all aspects of the system circuits, we then connected the observatory devices. These are controlled by DC IP switches made by Digital Loggers, so we can switch the devices on and off remotely, over the internet.
Connecting to Earth
A good connection to Earth (Ground) for the whole system, including the battery shed, is essential. The Earth bar on the right is connected to a thick copper rod which goes down 16-feet below the surface.
The electricity up to the observatory is via three routes, each one controlled by a circuit breaker. Each set of solar panels can be disconnected via an isolator. The controllers that govern how the panels charge the batteries are made by MorningStar.
Monitoring the battery control
An ethernet connection into the battery shed means that we can interrogate the charge controllers and monitor various diagnostics of the system health. We use Brainbox rugged industrial switches, fed by a DC input from Victron converters.
System testing and performance engineering
Rigorous system testing and checking is essential when dealing with electricity, as is on-going monitoring.
Meterological monitoring was already in place at the observatory to tell us whether the weather conditions are suitable for making astronomical observations
Network connectivity to Oxford
Thanks to the cyber-security company Sophos and their Remote Ethernet Devices this school observatory, like all the Global Jet Watch observatories, is safely connected to the University of Oxford's Department of Physics. Their generous support of our project, including the provision of the internet to this school in India, is deeply appreciated and immensely helpful in engaging the students into science, engineering and technology. Thank you Sophos!