Maya Mountain Research Farm was founded in 1988 in a remote area that had no electricity. In 1994 we built our first small stand alone 12 volt system which ran two 12volt lights and 12 volt fan. That was a huge quality of life enhancer. Since then we have built larger systems for home use, and dozens of small battery based photovoltaic lighting systems in rural households, battery based lighting systems in 15 schools, one clinic and 12 ranger stations in protected areas, and two village level photovoltaic water pumping systems.
In this article we will look at the anatomy of a battery based off grid photovoltaic system. In following articles we will examine the system we have at the Main Building at MMRF, and photovoltaic water pumping.
A battery based photovoltaic system is stand alone, and not tied to the grid. Generally it is comprised of solar panels, to make the energy, a charge controller, to manage the amount of energy coming into the batteries, batteries to store the power, an inverter to take the Direct Current (DC) power of the batteries and convert it to the Alternating Current (AC) of lights and fixtures that are commonly available. Some systems may have input from a generator, or from a wind turbine, or from a hydro plant.
Solar Panels
Photovoltaic panels convert DC electricity from sunlight. Solar panels are generally either monocrystalline silicon or polycrystalline silicon. Monocrystalline panels are more common, and are slightly more efficient than polycrystalline panels at up to 20% efficiency. Polycrystalline panels tend to be a little bit cheaper and are a little less efficient, at 12-15% efficiency. Regardless of which type of panels, most panels ship with a 10-25 year warranty. They typically ship in panels with 36 cells, 60 cells or 72 cells. For the most part we try to work with 72 cell panels, which is an industry standard voltage and originally designed for 24 volt batteries.
There are other types of solar panels, such as thin film panels, which are Cadmium telluride (CdTe) Amorphous silicon (a-Si), and Copper indium gallium selenide (CIGS). These are less common and less efficient, at 5-10% efficient, but they are not often used for home applications, and my experience with them is limited.
Solar panels need to be mounted in locations that have unobstructed access to sunlight, especially between the hours of 9AM and 3PM, the “peak hours” of possible power generation. In some locations, solar panels can be ground mounted, and in other locations they will need to be roof mounted. Ground mounted panels can also be mounted on tracking arrays that keep the panels pointed at the sun, obtaining more power over the course of the day. These are more common in water pumping systems.
Ground mounted panels have some distinct advantages. They are easier to service, cleaning the glass of dust, bird droppings or leaf litter, and easier to install. The main disadvantage of ground mounting is that the panels are easier to steal. Roof mounted panels are less prone to theft, but are harder to install and harder to service. Our panels are roof mounted.
Panels can be wired individually, or in strings of panels wired in series, or in parallel, or in series parallel.
The DC power coming from the panels to the battery bank passes through a charge controller that protects the batteries from overcharging. I use a PV combiner between the solar panels and the charge controller for our systems so that each panel or string of panels is able to be disconnected for servicing and for over circuit protection.
Charge controllers
Charge controllers moderate the power coming from the panels to the battery bank and protect the bank from overcharging. There are two main technologies commonly used in photovoltaic battery charging. They are Pulse Width Modulation (PWM), an older technology that lends themselves to small systems of 12 or 24 volt, and Maximum Power Point Tracking (MPPT), with MPPT now becoming the industry standard.
MPPT controllers are like automatic transmission for electricity. They are rated for a certain amperage, such as the Outback MX60, which handles up to 60 amps, or the Outback MX80, which handles up to 80 amps at whatever voltage the battery bank is set at. They sense the battery voltage and drop the voltage coming off of the array to meet the voltage of the battery bank. Wattage remains constant, and dropping the voltage from the battery increases the amperage. This makes them very efficient.
The MX80 charge controller can accommodate 1000 watts of panels at 12vdc, 2000 watts at 24vdc or 4000 watts at 48vdc.
There are several companies that make high end MPPT charge controllers, and we have used others, but we prefer to use Outback Power components. Outback Power makes high quality power conditioning and logging equipment. We have a long relationship with Outback Power. They donated components for our Main Building, which we will describe in a follow up article, and also donated components for one of our other buildings, and for a few school systems we have built.
One feature we love on the MX series of charge controllers is the “Log”, which can give us data up to 180 days back on maximum and minimum voltage, which is useful to see how the batteries have been maintained.
Batteries
Electricity comes from photovoltaic array to the charge controller and on to the batteries, which store the power for use when the sun is not shining. Batteries are an important part of the system, and one part of the system that can be damaged if not maintained.
There are several kinds of batteries, lead acid batteries, the most common, lithium ion batteries (Li-ion), which are often used in electric cars, nickel iron batteries (NiFe), which are bulky and well suited for stand alone battery based photovoltaic systems, and nickel cadmium batteries, which are toxic and have fallen out of favour for renewable energy systems.
Battery voltage for off grid battery based systems can be 12, 24 or 48 volts. Some systems can be designed using 36 volts, but these are uncommon.
Lead acid
Lead acid batteries are the most common batteries. Their chemistry is like the starting batteries found in a car. There are some differences between car starting batteries and deep cycle batteries. Starting batteries are designed to give up a lot of power, fast, to kick the engine over, and then take a lot of energy back through the alternator, fast. They typically only give a small percentage of their power, for a short time, and are not cycled deeply. Deep cycle batteries are designed to take in energy during the day and then use the energy later.
The most common lead acid batteries are flooded batteries, where the electrolyte is accessed through the top of the battery and is topped up with distilled water. These batteries lose water when being charged and being discharged through a process called electrolysis, where the water molecules are broken down into their constituent parts of hydrogen and oxygen. The resulting hydrogen and oxygen gasses are flammable. The batteries are refilled with distilled water.
Sealed lead acid batteries, notably gel cells and absorbed glass matt batteries, have one single advantage over floating batteries in that they do not off gas, but tend to be short lived and easy to damage. They are very unforgiving of mistakes.
Strings of deep cycle lead acid batteries can be wired in series to increase voltage, or in parallel to create more ampacity at the same voltage. Three parallel strings is the maximum advised for lead acid batteries.
A deep cycle lead acid battery is like a wooden water vat. If a wooden water vat remains without water for a lengthy period, it can dry out, the wood will shrink, and then it will not hold water as well as it used to. Lead acid batteries are like that, too.
Deep cycle lead acid batteries are damaged by being discharged below 50% of their capacity, or if the electrolyte goes below the top of the plates in the battery, exposing them to air. If well maintained, lead acid batteries can last for 7-15 years, depending on the battery, but lead acid batteries have a finite lifespan, no matter how well you maintain them.
One good thing about lead acid batteries is that lead is common and relatively inexpensive, and they are close to infinitely recyclable.
Lithium
Lithium batteries are common in phones, usually lithium cobalt, and in cars, lithium ion. They are less commonly used in home systems, although that has changed in the last few years, notably with the introduction of Tesla’s Powerwall.
Li-on batteries have some advantages over lead acid batteries. They tend to ship with a 10 year warranty, and are expected to last up to 20 years. They are sealed and present little or no maintenance issues. Unlike lead acid batteries, Li-ion batteries can handle up to 10 parallel strings. Li-ion batteries main advantage over lead acid is their energy density. They are lighter than lead acid batteries. This is part of why they are preferred for electric cars.
Li-ion batteries are recyclable, but recycling is more expensive than mining new ore. They are less toxic at the end of their life than lead acid or nickel cadmium batteries.
There are multiple disadvantages to li-ion batteries, however. Li-ion batteries are potentially flammable. The conditions leading to a fire are mechanical breakage, thermal runaway, when the batteries are not cooled, or a short circuit. The gasses from a compromised battery are both toxic and flammable.
Additionally, Li-ion batteries are energy intensive to make, involve mining, often in the developing world which sees little benefit from the mining, with serious environmental and human health consequences. Accusations of poor waste disposal and child labour in cobalt mining are persistent and well documented.
For the above reasons, we do not use Li-ion batteries.
Nickel Iron batteries
Nickel Iron batteries are the Cadillac of batteries. They are an old battery technology from the early 1900s. They are uncommon in home based systems because of their price. They are mostly made in China. Their price, however, spread over their life span of 30 years or more, make them the least expensive option.
NiFe batteries use an electrolyte, like lead acid batteries do, but the electrolyte is alkaline, using potassium hydroxide and lithium hydroxide and water. Like lead acid batteries they lose some of their water from off gassing, evaporation and electrolysis, and care must be taken in placement to avoid the accumulation of explosive hydrogen gasses.
Unlike lead acid and Li-ion batteries, there is little or no degradation of the cathodes and positive plates in their battery, which makes them extremely long lived.
While home usage in battery based renewable energy systems is relatively minor, NiFe batteries are still used in electric trains in the NYC and London subway systems.
The original NiFe batteries from the early 20th century can still be used if you can find them. NiFe batteries are very robust and not easy to damage. Short of mechanical damage that compromises their structural integrity, or allowing the electrolyte to evaporate below the positive plates top, nothing can be done to damage a NiFe battery that is not reversible chemically or electrically. A change of electrolyte, a discharge and/or controlled overcharge should return the battery to its original capacity.
Though the upfront cost of NiFe batteries are high, the value of these batteries is clear over other battery technologies when considering it’s lack of toxicity at the end of it’s life cycle, and its lifetime cost as expressed in the following formula:
[Purchase Price ÷ (Usable Capacity × Cycle Life)]
Inverters
The power coming from the photovoltaic panels through the charge controllers and stored in the batteries is direct current (DC) power. DC power is great for running freezers and fans and lights that are specifically designed to run off a DC power source, but these items tend to be more expensive and are not commonly available. DC freezers tend to be much more efficient, but the initial cost is significantly higher than a typical AC freezer.
The electrical grid the grid that feeds into houses is alternating current (AC) power. Many things we want to use are designed to use AC power, such as washing machines, fans, blenders, computers and modems, stereos, TVs, fridges and freezers. Depending on what country we are in, the voltage coming into our homes can be 100 to 240 volts, but tend to be 110 or 220 volts, at either 50 or 60 hertz.
To step up the lower voltage DC from the 12, 24 or 48 volt batteries to the higher 220 or 110 AC volts our appliances need, is done by something called an inverter. Inverters produce different forms of electricity. Older inverters made square waves, where the output was a simplified wave form of square waves to imitate the sinusoidal wave form off the grid. Later came “modified sine wave” inverters, which are really more closely related to square wave inverters. Lastly came “sine wave” inverters, which produce a form of electricity like the grid, but often a much cleaner form of AC power than the grid. Most inverters now are sine wave inverters, and apart from cost, there is no reason to buy a non sine wave inverter.
Inverters are designed around specific battery voltages, and unlike panels and MPPT charge controllers, are locked in at the voltage selected, so careful considerations of future energy usage should be considered when selecting a battery voltage.
Inverters are an important part of the system as most of the energy being used is going to be AC power. It is a good idea to have some things on DC power if the inverter fails, and those are often DC lights, DC fridges and freezers, and DC fans.
Over circuit protection
Circuit breakers or fuses are required to make our systems safe in the event of a short circuit. Every single wire connected to the positive post of a battery needs to be fused. In cars the fuses are usually blade fuses or cylinder fuses, which trip or burn out if a short circuit occurs. In small battery based photovoltaic systems, car fuses are adequate, but for larger systems, we need DC rated circuit breakers so that if a short circuit occurs, the fuse blows, or the circuit breaker trips. If not for these, the wire in the short will get very, very hot, very quickly, the insulation may catch fire. Every single wire coming from the positive terminal on the battery or battery bank needs to have a circuit breaker or fuse attached to it.
Well designed photovoltaic systems have redundant over circuit protection.
Repeat this mantra until internalised: “Unfused wires cause fires. Unfused wires cause fires. Unfused wires cause fires.”
In our next article, we will look at the system here at MMRF, and in the following article we will look at photovoltaic water pumping, which represents best use for photovoltaic panels. .
Extremely useful information, thanks for writing this. We’re currently looking at various solar options here in eastern Oregon and found this article extremely helpful.