Anatomy of a photovoltaic battery system – Part 2
In our first article, we examined the components in a battery based photovoltaic system with some pictures of systems we have built. In this article, we will look at the system at Main Building at Maya Mountain Research Farm, that powers the kitchen, the common area/classroom, our library and office, and where Celini and I live. We will look at the choices we made for two systems, the original 12 volt system built around a power centre I custom built, and a 24 volt system built around a preassembled OutBack power centre.
Our system originally was based on four 6 volt batteries wired in series parallel. While it had an inverter, allowing us to run computers, drills, recharge flashlights, and things requiring 110 VAC, all the lights in the house were 12vdc lights. We had, and still have, 110 VAC running to the kitchen, and available in the library, the common area downstairs, and for the washing machine.
The system on the main building was comprised of a mix of six 75 watt panels, one 80 watt panel, two 60 watt panels and two 125 watt panels, all of varied pedigree. Some were panels we obtained second hand, and others were extra panels we obtained through installations we have done. 900 watts was the maximum amount we could route through the original MX60 amp charge controller.
Each panel was on its own circuit breaker in an Outback PV combiner box, and then fed as through another circuit breaker to a Xantrex C60 charge controller.
The original system was built around an OutBack PSDC2 custom power centre with an OutBack MX60 charge controller that I built. Many of the systems I have done have been built around Outback power centres. I like the neat and orderly (and safe) wiring it makes possible. A Xantrex Prosine 1800 watt inverter converted the 12vdc to 110 AC.
The original system also had an Air X wind turbine. I have installed 6 of them in marine protected areas, but most of them are no longer working. I liked the appeal of a hybrid wind/solar system. This produced power, but not that much. Part of the problem was the siting. When we installed it, the trees to the east did not interfere with the panels. As the years passed, the trees grew quite tall, and made a significant barrier, with a lot of turbulence. Even long after the trees had grown taller than the building, on occasion, we would get useful energy from the turbine, but mostly in severe winds and stormy weather.
For wind, taller and bigger is better! If I were to replace this wind turbine, I would consider a larger wind turbine, a 3-5kw turbine on an 80 foot tower, placed over in the future sheep pasture to the east where there is little interference from tall trees, and run wire from there to the main building. For now, we are running Main Building 100% from our photovoltaic system.
When we changed over to 24vdc, we opted to not replace the AirX wind turbine.
The original system was four L16 batteries, in series parallel for 800 amp hours at 24vdc. The system had worked well for us for 8 years, but with students traveling with laptop computers more often, and with between 2 and 10 interns at anytime, we desired to increase our generating capacity. After much thought, we decided to expand to a 24vdc system.
MMRF has a long history of working with both OutBack Power and BP Solar. In 2007, Outback Power donated an inverter and a charge controller for MMRFs classroom, neatly matching a donation we received from BP Solar, who donated five 125 watt panels for our classroom, and a 175 panel for our panel direct water pumping system, which still runs a SunPump SDS submersible pump installed into a spring. The third article in this series will discuss photovoltaic water pumping.
Since then, both OutBack and BP solar have assisted MMRF in work we do in the communities adjacent to the Maya Mountains in southern Belize. With a request from MMRF, Outback assisted Tumul Kin School of Learning to power their boys dormitory. This system was partially funded by IRISHAID.
BP Solar has assisted MMRF and Ya’axche Trust, an NGO that we work with, to build a water system for the community of Medina Bank, with the pump funded by Cara Huddleston, and assistance from UNDP and Ya’axche Conservation Trust at the village of Medina Bank. BP Solar also worked with MMRF to serve the community of Aguacate, whose water system was built by MMRF, Plenty Belize with assistance from UNICEF.
We approached the engineering department at BP Solar in early 2011 about the possibility of receiving another donation from them for the purpose of replacing our existing photovoltaic system. BP donated eight 160 watt panels, BP3160B, for a total of 1280 watts of solar.
BP Solar, a subsidiary of the oil company BP, was one of the largest producer of photovoltaic panels in the world. They generated lots of engineering samples, which they donated to projects around the world until late 2011, when the market had become flooded with cheap Chinese panels. BP Solar ceased operations. This was a sad development as BP Solar, previously Solarex, had been a major producer of panels for 40 years. Their program to distribute engineering samples of their panels has benefitted many people in rural Toledo District of Belize, as well as many other places. Thanks to the engineers, Jay Miller and Pete Resler, for their support.
We once again approached our friends at OutBack and asked for a power centre to match the photovoltaic panels donated by BP Solar. OutBack donated a Flexpower One power centre for our main building. It is a beautiful piece of engineering, very neatly put together. It is equipped with excellent monitoring equipment. Ours has an 80 amp charge controller, a 3kw inverter and operates at 24vdc. The inverter is sealed, which is a good thing in this sometimes buggy and perpetually humid environment. Marty Spence, John Webber, Glenn Baker and Ed Wold were all a huge help. A big thank you from us to them for their continued support.
Since then we have installed over 20 systems built around the OutBack Flexpower One.
Wiring the eight panels into two strings of four panels in series was our initial plan, but configured this way, the arrays sometimes put out a bit over 150vdc, and the MX80 would stop accepting power. We ended up rewiring four arrays of two panels each, which sends the voltage in at 72-76 volts. At the controller this is stepped down through Maximum Power Point Tracking (MPPT) to match the voltage of the battery. While doing that, the wattage remains the same, so the amperage is boosted.
Like all of the systems we build, the panels are mounted on 2 inch galvalume square tubing to make a rigid mount for the panels, which keeps them elevated so that air can circulate below and around them to reduce the temperature. Galvalume is widely available in Belize. Because MMRF is a farm devoted to agroforestry, ground mounting would result in a being shaded for parts of the day, and we opted to roof mount this and all the other systems we have.
We designed the rack so that each array of four panels can be removed by unbolting the array from the support structure. In the event of a hurricane threat, we can remove the panels. I would redo this as four arrays of two panels. Loading four panels onto the roof at one time is no easy feat.
From the roof, the power runs on 8 gauge wire into the PV combiner box. This is another great Outback product, also generously donated to MMRF by Outback. This allows each panel, or series of panels, to run through a circuit breaker. Unlike the previous system, where we wired each panel, individually, in parallel, with the new system we wired the panels in series. Taking advantage of the MPPT technology of the Outback MX80 charge controller, we ran each of the four strings of two panels through their own 10m amp circuit breaker.
From there the power goes through a ground fault circuit breaker, and then to the charge controller in the Flexpower One.
One component on the Flexpower One is the power indicator Flexnet meter, which is a series of LED’s. This gives you an idea, at a glance, how the batteries are doing. Happily, we seldom draw the batteries down below 70%.
Many of our original circuits, lights and fans, were on DC power at 12vdc. To retain those DC circuits in the main building with battery voltage at 24vdc, we run DC though a Samlex 20 amp DC-DC converter, which coverts 24vdc to 12vdc. The Samlex converter puts out a steady voltage of 13.8, which makes the DC lights bright, and keeps the fans at a high speed.
I like the simplicity of having some DC circuits. I turn off the inverter when we have lightning. Having some circuits on DC gives us light in a lightning storm.
In 2017, we replaced the tired lead acid batteries with a 500 amp hour NiFe battery bank obtained through Iron Edison, which was generously made available through funding from LUSH fund, of LUSH Cosmetics. LUSH Fund has generously supported training for over 80 Belizeans at Maya Mountain Research Farm in permaculture and inga alley cropping over the years.
The NiFe batteries are slightly less efficient at energy storage, but can be discharged heavily, down to 10% without damage. Instead of using a sulfuric acid as the electrolyte like lead acid batteries, NiFe batteries use an electrolyte of potassium hydroxide, which is caustic, but not toxic. At four years of age, the NiFe batteries show no sign of loss of capacity, and we expect them to last for another 20 years. As I explained in the first article, short of mechanical destruction, nothing can damage these batteries that cannot be reversed chemically or electrically.
The system at Main Building is robust. When the sun is out, we are able to run our heavy loads. Even when the sky is overcast or cloudy, we generate useful amounts of energy. Most days our battery bank is fully charged by 9-10 in the morning, and we look for ways to use the energy we cannot store in our battery.
We have been able to run power saws, drills, as well as the blender and coffee grinder we use in the kitchen. A hot air popcorn maker put no significant strain on the system. The new system allows us to enjoy a washing machine, freezer, electric lights, blenders, fans, computers, oil press, internet, power tools, projectors, coffee grinders, pretty much whenever we need it.
Having photovoltaic systems enables people in remote areas to have lights provided by electricity. This reduces or eliminates the use of kerosene laps and candles, and their associated respiratory health risk, as well as fire risk, reduces generator use and lets people enjoy lights, fans, radio, refrigeration and access to information.
The equipment donated for this system makes the work we do more effective. I would like to thank Outback Power, BP Solar and Lush Fund for their generous support in this project and in other projects in the past. The components they donated to MMRF for this project have been used by many hundreds of people since we installed them.
In our next article we will look at photovoltaic water pumping, using one installation as a case study.
Support for this was made available by the generosity of:
Unit 21-22 Dolphin Quays
1628 W Williams Drive
Phoenix, AZ 85027