Solar 101

Shane has been running night classes for local workers and the technicians who will be responsible for managing the system when we leave. It’s been a chance for even his team to further grow their understanding of what we are building here; how the component elements fit together like building blocks to efficiently deliver the energy capacity that is needed here. We’ve been asked by a few people to put it in writing so this knowledge can be shared. So here goes the not-too-technical guide to off-grid solar. Or the long answer to the question “so what exactly are you doing out there?”

Electricity flows like water through a system that allows you to store reserves and pump it in different directions to manage availability and meet demand. This is the starting concept for class #1.

 

Reusing panel boxes as educational supplies

Just as rainwater is captured, pumped into tanks when it’s raining and then pumped in the other direction when it is dry, so energy can be transformed and moved to where it is needed.

In our system, solar panels “catch” the sun’s energy like buckets in the rain. This energy travels along underground cables to the solar inverters that convert the energy from direct current (DC) to alternating current (AC) that we use in our homes. All this energy then goes to the multi-cluster box (MC Box) which acts like a big controlling valve at the heart of the system.  If the MC box is the heart the system the battery inverters are the brain. And one battery inverter rules them all. The master battery inverter decides where this energy goes. Direct to the village? Or if the sun is blaring and more is being produced than used, excess energy can be sent to the battery inverters that transform it back into DC, charging the batteries, which act as tanks.

At night when the sun goes down and everyone turns on their lights and start watching Masterchef Tuvalu, the master battery inverter changes the direction of flow and starts drawing that excess stored energy from the batteries and using it to power the village.

And if the Tuvaluans are unlucky enough to have a few days of torrential rain with no sunlight, the master battery inverter is clever enough to send a message to start the generator, which acts like a pump drawing another source of energy into the system.

How your Solar electricity plant works

So how will having this system change energy use on Vaitupu?

Vaitupu is currently powered by a 110kW generator, which operates 18 hours a day (from 6am to 12 midnight). Over this 18-hour period, this hungry beast consumes 250L of diesel. Over a year that’s enough for me to drive my car 1.2 million kilometres (or 60 people to drive their car 20,000kms)! And that doesn’t include the energy used to transport the diesel to this tiny remote island.

And while the village load rarely exceeds 50kW at any time, a generator this big is needed to manage peak demand in the morning when everyone’s freezers go into over-drive and to provide some redundancy. So there is a lot of diesel being burned and not all of the energy that is being produced, can be used.

The current energy load profile for Vaitupu is hinted at in paragraph above. At 6am every morning when the generator comes on, all the fridges and freezers that have slowly been thawing over the past 6 hours, turn back on. And they have to work very hard for the next few hours to cool everything back down again. Things start to settle around 11am and then there is another peak in the evening when everyone gets home, starts cooking dinner and turns the lights on. The current load profile from an average day is plotted below.

Average daily Vaitupu village energy use

Once the solar system is turned on, it is expected that the load profile will level out. There will still be a bump in the evenings but no more morning peak and less work for all those exhausted refrigeration motors; fridges will now come on for a few minutes every hour; food will stay frozen; fans will keep running overnight keeping mosquito’s at bay and visiting Kiwi’s will be able to sleep.

Back at the powerhouse, the battery state of charge will slowly go down overnight. And then when the sun comes out in the morning, the solar array will start pumping energy into the system fuelling the village and simultaneously recharging the batteries.

For the geeky geeks, here is a graph from a day last week in Pukapuka (Cook Islands); a system that Dean installed December last year. The blue line is the village load. Red line is the solar energy utilised by the system. Light blue line indicates what potential solar energy was available to harness on that day. But batteries were already charged by 11am and the system was clever enough to only draw in what was needed for the rest of the day to cover village load and keep topping the batteries up ready for the next overnight drain.

And here’s another cool graph.

Worked it out yet?

This is how energy flows through the system. Overnight, energy is being drawn from the system at a relatively constant rate and the battery state of charge slowly drops. When the sun comes up, suddenly there is a lot more energy coming into the system than going out. The batteries are charged by 11am and the solar inverters slow down the draw from the array. Throughout the remainder of the day the system draws enough energy to power the village and keep topping up the batteries. Then the sun goes down and it all happens again.

Note: each line represents a different cluster of batteries and their different rate of charge. SMA charge algorithms were charging the batteries at different rates this day.

So that was class #1. Still following? Time for class #2.

I said before that we are harnessing energy using component building blocks which allow us to package it up into the most efficiently transportable resource so that we can distribute it where it is needed or store it up to use later.

Power = voltage x current – these are our electrical basics.

Simply put, voltage is like pressure and current like flow. You can have a skinny pipe with high pressure but low flow sending the same volume of water down the line as a fat pipe with low pressure. The same applies with electricity cables. The voltage at which you choose to move electricity around has a lot to do with the cost and practicality of cable thickness.

Even working with relatively thin cables is hard work

When solar panels are connected in series, the voltage of each panel is summed together. If each panel has is rated to 34 Volts and 8 Amps, a string of 10 panels connected in series will deliver 340 Volts at 8 Amps. Connect two of these strings in parallel and you have 340 Volts at 16 Amps.

At the front end of the system on Vaitupu is the solar array comprising 1,608 solar panels. Each solar panel has a voltage of 38V and can produce a current of 8 amps at full power. The electrical configuration of the array comprises strings of 24 panels connected in series delivering 912V-open circuit voltage at 8A. These are then paralleled going into the inverter to raise the current to 32A resulting in a total solar inverter capacity of 410kW across 15 inverters.

The solar panels are connected to solar inverters. These inverters change the direct current (DC) energy from the solar array to alternating (AC) energy that can be used to power homes.

At the other end of the system is the battery bank comprising 576 2V batteries. 24 of these batteries are connected in series to produce the equivalent of a single 48V battery with a current capacity of 3,670 Ah. Two of these banks are then connected in parallel to produce a 48V 7,040 Ah battery cluster weighing just under 10T. Vaitupu has 12 of these battery clusters and is the biggest system we will build in Tuvalu.

This is what 120 tonnes of electrical storage looks like

The batteries are connected to 36 battery inverters. These inverters change the DC energy stored in the batteries into AC energy that can be used to power homes. These inverters have the technology move energy bi-directionally. They can charge the batteries when the sun is shining, or draw energy from the batteries at night to power the village.

Everything is connected to the multi-cluster box, which runs the show.

The whole system is based on series and parallel. The solar panels are connected in series and parallel to raise both the voltage and current, the solar inverters are connected in parallel. The batteries are connected in series, and then in parallel. The battery inverters are connected in parallel.

Why? Series allows us to build the voltage up high enough that transmission of the energy becomes practical; we would need a cable as thick as my leg and as heavy as a tractor to transmit 38V (and very high current) electricity from the array. And having many of the same unit running in parallel creates redundancy in the system. This redundancy means that in the event of a failure in any part of the system the system as a whole will continue to operate. It also means that this section of the system can be shut down and isolated for maintenance without having to turn the power off completely.

Just like Lego after all.

On Friday this week we have the 6^th and 7^th form (Year 11 and 12) physics students visiting the site. We’re excited to open the doors and let people see what is being built here. Hadley will be fielding the hard questions.