Energy Systems — by Chris McLeod October 12, 2011
This is the concluding article in the series A Solar Powered Life which looks at the various issues, compromises and components of an off-grid (standalone) solar power system. In previous articles I have written about the individual components in this system, but in this article I’ll explain how all of these components are connected. It’s also worth reflecting on the question of why a standalone solar power system would be of interest to permaculturalists, so we’ll take a look at that also.
Despite what some people may say, connecting up a solar power system is quite straightforward. Although each component is highly complex, they are modular and so they are relatively easy to connect into a complete system.
The various components of my off-grid solar power system are connected up as shown in the following diagram:
The above is a diagram of my own off grid power system, however most off-grid solar power systems are similar, from the most simple to the most complex.
Below is a brief discussion of each of the individual components in the system.
Solar Panels: Solar panels were discussed in detail in Parts II and III of the article series so you should have an idea of how many solar panels you would require to cover your desired electricity usage.
Wire: Each component in an off-grid solar power system will have both a positive and negative wire.
A general rule to consider with wiring is that positive wires are red and negative wires are black. Sometimes you may get different colours, or two colours that are the same. In this instance, unless the cables are specifically marked as positive and negative, you will have to utilise the services of a tool called a multimeter to figure it out. Also, it is most important to remember that the more current (amps) that the wire is expected to carry, the thicker the copper in the cable will be (and the more expensive that cable will be).
Bus: This doesn’t refer to a large vehicle designed to carry 48 passengers! It is actually the technical term for a block of metal (usually copper or brass) that has many connecters so that you can connect up lots of wires together. You would usually require two of these, one for the positive cables and the other for the negative cables. You use a bus because it is easier than trying to connect up 12 different cables together any other way.
Fuse: Fuses are the security guards in any off-grid solar power system. They protect the wires from being overloaded, melting and catching fire as discussed in the previous article – Part VIII — Wire and Myth Busters.
Regulator: Regulators (also known as solar controllers) are the brains in an off-grid solar power system. They control the flow of energy from the solar panels into the batteries. This is an important job because batteries are easily damaged by being over-charged. Regulators were discussed in detail in Part V– Living Within Your Means.
It is important to note that regulators are specified with a maximum current (amp) value that they can handle at a certain voltage. This maximum limit should not be exceeded for the same reasons that undersized wire should not be used (they will overload and catch fire). The higher end regulators are generally specified to handle between 60A and 80A at various voltages.
A good rule of thumb is that with regulators, the more you spend the better the quality and the higher the capacity that they can safely handle.
Batteries: It goes without saying that without a battery you could not store power for later use (eg. powering lights at night). All batteries are different and for an off grid solar power system you require deep cycle batteries which will hold more power and last a lot longer than other types. In an emergency, you could use a motor vehicle battery but it won’t last long unless it is used very sparingly.
Lead acid batteries are generally 2v each. This means that to get a 12v battery, you can wire 6 x 2v batteries together in series. It’s good practice to use all of the same type and size of batteries in any system. Even 12v car batteries are actually made up of 6 x 2v batteries wired together in series (see below) – they’re just in the same box.
Some people wire several large batteries together in parallel (see below) so as to keep the voltage the same but have a larger number of amps available. For example, a person may have 4 x 12v @200 Ah batteries wired together in parallel (see below) to give 12v @ 800Ah. This could be because it is cheaper to buy 4 smaller batteries than one large battery. There is much debate within the renewable energy community about this practice.
Shunt: This doesn’t refer to a vehicle accident! A shunt is a block of copper that sits in the wiring between the batteries and the inverter on either the positive or the negative wire – it doesn’t matter which. Connected to this block are an additional two small sensor wires which are connected into the regulator. These wires allow the regulator to measure the flow of current (amps) from the battery to the invertor. This information will then be shown on the regulator display. You don’t have to have a shunt in a system, but I included one so that my regulator could record and monitor the number of amps that the inverter takes out of the battery every day. If you look at the above diagram, you will see that the inverter is connected directly to the battery bypassing the regulator, so having a shunt is the only way you will know how much energy is being used versus how much is being generated within the system. Without the shunt you are flying blind and will only be able to take a rough guess at the level of charge in the batteries by looking at the battery voltage.
Inverter: The inverter is a transformer which takes extra low voltage DC electricity from the batteries and converts it into useful mains AC electricity. Inverters vary in capacity and efficiency so it is worthwhile doing a bit of research into these before purchasing. A general rule of thumb is that the less you pay for an inverter, the lower capacity and more inefficient that inverter will be. Some inverters can use large amounts of battery power whilst switched on and doing virtually nothing, so beware. Inverters were covered in more detail in Part VII – To Invert or Not to Invert.
That is all of the components in the entire system from start to finish. There is a lot of variability with each off-grid solar power systems in terms of capacity and quality of components, but essentially they all operate with the same components.
There are a couple of additional technical issues to consider before finishing the discussion about solar power systems:
Choosing a Voltage: Before any component in an off-grid solar power system is either purchased or connected up, you need to determine what voltage the overall system will operate at. This is particularly important because all of the above components must operate with a consistent voltage. Usually, these types of systems operate at either 12v, 24v or 48v. There are benefits to be had in wiring a solar power system at a 48v rather than the lower voltage of 12v. The benefits relate to cost savings on copper wire and they are discussed below under the heading “series wiring versus parallel wiring”. In choosing a voltage however, the most difficult component in the system are the solar panels.
Voltage output of a solar panel: Most large solar panels are manufactured to provide an output that is suitable for either a 12v or 24v system. You can quickly and easily tell which is which without any tools, because 12v solar panels have 36 individual solar cells, whilst a 24v solar panel has 72 individual cells. If you look at the solar panel you should be able to quickly count the number of individual solar cells and tell the suitable voltage without any tools.
Series wiring versus Parallel wiring: You have a choice in how you wire solar panels (or batteries) together. The technical term for this choice is series wiring or parallel wiring. It’s simple because series wiring adds the voltage together, whilst parallel wiring keeps the voltage the same, but adds the current (amps) together.
With series wiring this means that two identical 12v solar panels can be wired together to produce an output suitable for a 24v system at the original current (amps).
With parallel wiring this means that two identical 12v solar panels can be wired together to produce an output suitable for a 12v system but at double the current (amps).
So why would you choose parallel over series wiring?
Wiring many solar panels in series to produce a low current (amps) at a higher voltage (volts) means that you can use thinner (and therefore much cheaper) cables. The copper in cables is really expensive and the higher the current (amps) that the cable is required to carry, the thicker that the copper in that cable will have to be (this was covered in the previous article Part VIII — Wire and Myth Busters).
Most grid connected solar power systems keep the current (amps) low and the volts high to be able to utilise thinner (and therefore cheaper cables) which keeps overall prices lower by wiring those panels in series. The inverters in those grid connect systems are designed to convert high voltage DC electricity into mains voltage AC. Because the grid connect inverters are wired up directly to the solar panels, there is no requirement for batteries. Because there are no batteries, there is no requirement to produce extra low voltage.
However, with off-grid solar power systems, most components are designed to keep the voltage extra low so you can store that energy in an extra low voltage battery. The difference between this type of system and a grid-connected system is that the batteries sit between the solar panels and the inverter/regulator. This means that you have to maintain an extra low voltage so that the energy from the solar panels can be fed into those batteries. The problem is that keeping the voltage extra low and the current (amps) high, which requires parallel wiring, means that you have to use thicker and therefore more expensive cables. The use of appropriate wire in these off grid systems is non-negotiable or they will overload and catch fire.
The method of wiring in series versus parallel is shown in the diagram at right, reproduced from the book Energy from Nature, compiled by Peter Pedals and published by the Rainbow Power Company in Nimbin, Australia.
As you can gather from the article series there is no perfect electricity system — solar or otherwise. All such systems involve compromise which usually gets down to the question, “how much can I afford to spend on this system and what sort of compromises can I live with?”.
Why would a standalone solar power system be of interest to Permaculturalists?
Some people have commented that a Permaculture website is a funny place to discuss a standalone (off-grid) solar power system and over the course of the series I have been quite surprised by the passion of some of the comments. Therefore, given that this is the concluding article in the series, it’s worth reflecting on some of the reasons below:
Diversity = Resilience: Our electricity supply systems lack diversity as the majority of the population rely upon on massive generators (coal, gas, hydro and/or nuclear power plants) in distant locations with delivery through vast highly complex distribution systems. A system that is not diverse, is not resilient and minor failures can have a great impact on the whole system. This can be contrasted with a Permaculture system such as a food forest which is resilient system because of the diversity of plants within it. That diversity allows a food forest to absorb shocks and still produce at least some output.
Knowledge: A solar power system is an energy supply system and Permaculture is all about systems and design. Because solar power is a small, local and independent system, it forces you to be involved with its operation and state of health. This differs, for example, from simply just switching on an air-conditioner when you are connected to the electricity grid and not knowing or caring what the impacts of that decision would be.
Independence: A well designed standalone solar power system will give you independence. Isn’t this really the end point of Permaculture? Taking responsibility or ownership for your food supply is a long way along the road to achieving the spirit of the Permaculture ethics. When you consider it, a food supply system, is really another type of solar power system in that it harvests and stores solar energy for later use.
Local Resources: Once installed, a solar power system makes it possible for the owner to harvest sunlight and convert it into electricity. This allows people to utilise a locally available resource.
Passivity: If the solar power system is well designed and sited it is essentially a passive system in that it provides an output without further ongoing inputs. There is really very little ongoing maintenance if it is well set up and you monitor the system regularly. Is this that different from planting a food forest where after the initial establishment, the effort involved becomes less as time goes on?
Electricity in remote and unreliable places: There are plenty of places in the world where access to electricity is difficult, intermittent or non-existent. These can vary from a shed in a paddock to a remote developing world location. Solar power has so much to offer for these locations where otherwise fossil fuel generators may have to be utilised to produce electricity.
Resilience to System shocks: Unexpected things happen. The other day I found aphids floating around in a mint tea served by my neighbour and I wondered about how many I’d consumed before noticing. Most of time unexpected things are easily dealt with, but occasionally they’re not. Disasters can be an unexpected event and they can also occur quickly and with little notice.
Quite often when a disaster hits, the electricity supply is either switched off because of safety concerns or because it’s been cut as a direct result of the disaster. It’s at this point that you become aware of all of the things that electricity is used for. Consider the following items when electricity is not available:
- Electric pumps are generally used to move water or sewage. They are also used to move liquid fuels such as petrol. These uses are an invaluable service.
- Refrigeration also poses a problem because some food and medicines will quickly spoil if unrefrigerated.
I’ve noticed that in rural areas plenty of people have small generators. These can supply electricity for useful things such as lights, pumps and refrigerators. However, the risk with these is that it can take a lot of fuel to keep a generator operating. This can vary between 0.5 litre and up to 3.0 litres of fuel per hour. If a generator had to run for a week or several weeks it would take a lot of fuel and very few people have large supplies of fuel on their properties. In addition to this, my experience with small generators is that they are not particularly reliable or capable of running continuously 24 hours per day. A solar power system can provide a measure of resilience by providing a continuity of supply in these conditions.Comments (10)