Technical
As with most of the southern hemisphere, South Africa is in the fortunate position that it has a very high solar potential, making this renewable energy source an attractive possibility.
How does it work/what do I need?
Photovoltaics (PV) is best known as a method for generating electric power by using semiconducting silicone solar cells to convert energy from the sun into a flow of electrons. In other words, PV systems convert sunlight directly into electricity.
Nice to know: The term “Photovoltaic” comes from the Greek φῶς (phōs) meaning “light”, and from “Volt”, the unit of electro-motive force.
World Solar Map
A typical PV system consists of an arrangement of several components, including solar panels to absorb and directly convert sunlight into electricity (DC power). A charge controller, not to only charge the connected batteries but also to prevent under- and overcharging to increase battery life and performance. A solar inverter to change (invert) the electrical current from DC to AC (AC is the type of power our normal household loads use). An integrated battery solution to store the electrical power. Lastly we need mounting, cabling, protection and other electrical accessories to set-up a working system.
Grid-Tied PV System:
A Grid-Tied PV system allows consumers to use their own solar generated energy in conjunction with electricity purchased from the utility i.e. Eskom. Therefore, it does not need to produce all of the electricity demand. Basically, a grid-tied PV power plant generates electrical energy from the sun during the day for self-consumption. These systems require no batteries and generate electricity typically for a business that primarily needs power when solar power is available, i.e. during the day. When this business however consumes more energy than what the grid-tied system can generate at that point in time, electrical power from the utility is used to supplement electricity obtained from the PV system.
In the instance where a grid-tied system generates more power than what is required by the consumer, and if allowed, this excess power can be exported back into the utility grid for compensation from the utility company or municipality. In South Africa this is mostly not allowed and therefore a limiting device needs to be installed to prevent excess power from being generated and exported by the PV array back into the grid.
Where feeding back into the grid is allowed, the grid in many ways acts as a virtual battery but without the need for maintenance or replacement and with a much higher efficiency. Therefore this type of PV system costs significantly less than a conventional PV and battery power plant system. In addition, a grid-tied system offers other financial benefits like an immediate reduction in energy costs, long term energy savings on grid power, fixed energy costs thereby mitigating utility energy tariff increases and reduced dependence on grid power.
It is important to note that a grid-tied system cannot provide backup or protection from power outages due to it not having a backup component i.e. a battery. Therefore, when the grid fails, a grid-tied systems will shut down and will seize to operate.
Hybrid PV System:
A Hybrid system combines two or more modes of electricity generation together. Typically when considering renewable energy sources, photovoltaics and wind turbines can be used in conjunction. However more commonly used together are photovoltaics and utility power (or generators for off-grid applications).
A good application to use this technology is if the power generated by a PV array is insufficient to supply the daytime loads and charge the installed batteries. For this a hybrid system is a good option as it can utilize utility power to recharge the batteries from the grid when a lower off-peak electricity rate applies.
This being said, South Africa implements a tariff structure that enforces different kWh rates as your usage increases as well as different rates for different times of the day and days of the week. A hybrid system can be useful to draw stored power from the battery when the utility rate is excessive. Therefore hybrid systems can provide peak tariff shifting/arbitrage where stored power can be drawn from the battery in peak tariff times to assist in balancing your total power needs.
Another use is if your grid connection capacity is insufficient to supply a required load. A hybrid system can supplement any extra power needed and as a result allow you to avoid a costly mains upgrade to your property.
Therefore a hybrid solar system empowers the user with energy security and self-sufficiency. It locks in future electricity cost and empowers the user when using solar power as well as providing the user with an uninterruptable power supply.
Off-Grid PV System:
Such a system refers to not being connected to the utility power grid. In other words a stand-alone system resulting in its users being self-sufficient. Normally a PV system like this will have a backup generator supporting the primary PV power source in case of system failure or in the event of persisting periods without sufficient solar irradiation. When considering an off-grid PV system, it is advisable to mitigate the possible threats by opting for high quality equipment with local support and also a larger than usual battery backup.
In the current South African era with unreliable utility power and inflated price increases, it is a great opportunity for a remote area without services available to install an off-grid PV system as it is often cheaper than extending power lines into such areas.
Mini-Grid PV System:
The mini-grid or micro-grid system types are mainly used where decentralized PV plant installations are required. As per the name a small grid network is built up that is independent of the main utility grid supply. This small or mini electrical grid network is setup to link various internal connection points within a specific site. These connection points could be various roof structures that are scattered around the specific geographical area. Each independent roof would have their own PV array and grid-tied PV plant installed; they are then interconnected into the mini-grid reticulation system.
There is also a grid-forming central hybrid battery inverter installed on the network to create the base load power and grid reference that will be used to start and maintain the mini-grid network.
Each independently installed PV plant on these various roofs within the site would all contribute to producing and supplying PV power into the entire mini-grid network. Sites with higher energy consumption requirements will use power generated by these other roof spaces with the network, the central Hybrid battery inverter will also use the network’s excess power generated to charge the batteries for night-time use or run the daytime loads.
Monocrystalline:
This technology has the best conversion efficiency but is also the most expensive. The cells use a very pure silicone crystal which are grown from a complicated crystal growth process. Silicon wafers are cut from these crystals and converted into solar cells which in turn are assembled into different size solar panels. Because mono PV panels are more efficient, the same wattage panel is smaller than its counterparts and is therefore the preferred choice when limited space is available.
Polycrystalline:
Also known as Multicrystalline, these modules are less expensive and slightly less efficient. They are not grown in a single crystal but in a large block of multiple crystals. This gives them that striking shattered glass appearance. The remaining process to create solar panels are identical to that of mono panels. It is by far the most commonly installed PV panel in South Africa.
Amorphous:
Not really crystals, but a thin layer of silicon deposited on a base material such as metal or glass to create the solar panel. It is a lot cheaper but the efficiency is much less as well. Therefore, more square footage is required to produce the same amount of power as the other two types.
Solar panels are categorised into three tier classifications. Tier 1 is the highest and tier 3 the lowest “quality” product.
The tier ranking scale is orchestrated by Bloomberg New Energy Finance Corporation and is used to rank solar panel manufacturers in terms of their bank-ability or financial stability. Tier rankings are about the panel manufacturers and not the panels themselves. Therefore it is not a direct guarantee but rather a very good indicator of quality. So when it comes to applying tier rankings, the reasoning goes something like this: “If these panels are good enough to get financing to be installed on gigantic solar farms, surely they’ll be good enough for me.”
According to the above indication, Tier 1 manufacturers represent only a 2% share of the total number of solar panel manufacturers. What sets them apart from the rest are that they are vertically integrated, meaning they manufacture their panels from the ground up. These manufacturers control the complete production process, everything from the silicon cells to the panel frames and ultimately assembly. They are highly innovative and use automated manufacturing techniques that are contributing to a higher level of quality, while reducing manufacturing costs at the same time. Tier 1 manufacturers invest heavily in research & development and finally have been producing solar panels for at least 5 years.
Tier 2 manufacturers comprise of about 8% of the market and have been manufacturing PV modules for between 2 to 5 years. Compared to Tier 1 manufacturers they invest relatively little money in R&D and robotic automation plays a small role in their production lines.
Tier 3 manufacturers comprises of the majority of the PV manufacturing market with a whopping 90% market share. They however have the least experience and their operations are limited to assembling by utilising components from other companies. They do not invest in R&D and they are highly reliant on manual labour.
An inverter is a power electronic device that is appropriately named after its primary function namely to invert DC (Direct Current) power into AC (Alternating Current) power. The opposite of this namely changing AC into DC power is called rectification. The piece of power electronic equipment responsible for this function is called a rectifier.
DC power is the unidirectional flow of a DC charge. Because it has only one polarity of voltage and current, either positive or negative, it means that it has a zero frequency. DC distinguishes itself from AC in the sense that the electric current in a DC circuit flows in a constant direction compared to that of an AC circuit where the current continuously reverses direction. This continues change in direction or polarity happens at different rates. In South Africa this happens at 50Hz meaning that the power constantly change between +230V and -230V at a rate of 50 times per second. This calculates to a period (T) of 0.02 seconds or 20ms per cycle.
An inverter therefore does not produce power but converts it from one form to another. The power generated from PV modules and that stored in batteries is DC. It is therefore required to change (invert) this DC power generated by the PV plant into usable AC power as this is what most of our household appliances use.
There are a few different types of inverter technologies available namely square sine wave, modified sine wave and pure sine wave inverters.
Square Sine Wave Inverter
Square sine wave inverters are the least expensive and produces a very inefficient square wave output. This type of output is accomplished by using a positive DC current and changing the polarity to negative, back to positive and so on. This continues reversal of polarities at a required frequency produces a square sine wave as shown below:
This output is suitable for low sensitivity loads like incandescent lighting, kettles and electric heating. On the other hand these sudden power reversals are quite brutal on some electronic devices.
Modified Sine Wave Inverter
This is a very common and popular technology as it works well with most AC loads and is also relatively well priced. It however causes problems with delicate electronic equipment like laptops, digital clocks, smart home devices and medical equipment.
A modified sine wave inverter produces an AC waveform somewhere between a square sine wave and a pure sine wave inverter. The wave produced is a kind of rounded-off square sine wave and differs from a pure sine wave in the sense that it produces an instantaneous peak voltage for a few milliseconds and then drops back down to zero for a few milliseconds before it cycles down into the valley. The end result looking somewhat like this:
Pure Sine Wave Inverter
A pure sine wave inverter produces the best and most efficient AC power output of all three mentioned inverter technologies. In many cases its output is even better than that of the utility power supplying our homes. When looking at the graph below, it can be seen that the current gradually swaps from one direction to the other in a sine wave pattern. Therefore this technology is suitable to supply power to any AC load but it is also the most expensive of the three technologies.
When comparing the rounded-off square waves of the modified sine wave inverter to that of the pure sine wave inverter, it can clearly be seen that the modified sine wave inverter is delivering more power to the appliance overall than a pure sine wave inverter. This is because there is more area under the rounded-off square curve compared to that of the pure sine wave curve. This makes modified sine wave inverters less efficient than its pure sine wave counterparts. This wasted power is dissipated as heat and may result in the appliance overheating.
For the energy harvested from the sun via the PV modules to be either stored in the battery or for it to be supplied to the inverter for conversion, it first needs to go through a charger. The function of this charger is to convert the PV energy to supply the batteries with the correct rate of charge for the different charging stages of the battery connected. For this to happen a charge controller is required. For PV generation plants there are mainly two options available namely a PWM (Pulse Width Modulation) or a MPPT (Maximum Power Point Tracker) charge controller.
PWM Charge Controller:
A PWM charge controller is the more basic and cheaper of the two charging technologies. When a conventional charger is charging discharged batteries, it simply connects the PV modules directly to the batteries. This forces the PV modules to operate at the lower battery voltage. According to the below formula, Power (Watt) is equal to Voltage (Volt) times Current (Ampere).
P = V x I.
If now the PV module’s voltage is “pulled” down to that of the discharged battery, this will also limit the power output of the PV module.
Let’s consider a basic example using a solar panel with the following specifications:
- Power rating: 80Wp
- VMP: 18V
- IMP: 45A
Therefore according to the power formula:
P = V x I
= 18V x 4.45A
= 80.1W
But if this PV module now gets connected to a battery sitting at 12V:
P = V x I
= 12V x 4.45A
= 53.4W
Therefore even though the PV module is rated at 80Wp, it will only deliver 53.4W due to the lower battery voltage. Therefore for this example, using a basic charger degrades your PV array capacity by more than 33%.
MPPT Charge Controller:
A MPPT charge controller is a much more complicated and therefore a more expensive option in comparison to its PWM counterpart. Unlike a conventional charger, a MPPT varies the electrical operating point of the PV module in such a manner that it is able to produce all the power it is capable of. But because the battery voltage sits on 12V, the output of the MPPT must also be 12V. This is accomplished through a high efficiency DC-to-DC power converter converting the 18V PV module voltage at the MPPT input to the battery voltage at the output. The MPPT will compensate for this lower battery voltage by delivering 6.67A into battery maintaining the full 80W power of the PV module.
P = V x I
= 12V x 6.67A
= 80W
To visually compare this operational difference between these two technologies, have a look at the graph below.
Because the conventional charge controller simply connects the PV module to the battery, it is forced it to operate at the batteries’ 12V. The result is that the PV module only produces 53.4W. When a MPPT charge controller is being used, it continuously calculates the voltage at which the PV module is able to produce its maximum power. In this example the PV module’s VMP (Maximum Power Voltage) is 18V under standard test conditions (STC). At this voltage the MPPT can extract the full 80Wp the PV module is able to produce regardless of the battery voltage.
MPPT technology therefore makes it possible to harvest additional power from the PV modules and makes it available as increased battery charging current.
Even though the MPPT charge controller is quite a bit more expensive than a PWM, the extra power output produced by the same size of PV array more than makes up this difference.
The most cost effective and efficient way is for electricity to be used as it is generated. In the case of PV generation plants this is not really practical for “normal” households because the occupants are not present most of the time when the electrical energy is being generated i.e. daytime. Therefore a means to store this energy for later usage is needed. This can be accomplished by temporarily storing this generated power as chemical energy i.e. batteries. Unfortunately energy storage typically comes at a significant cost.
Over the years much development in this area has taken place. Over the last few decades different varieties of deep cycle lead acid batteries like VRLA (Valve Regulated Lead Acid), AGM (Absorbent Glass Mat) and Gel were the choice of most. In recent years, however, another option has made its presence felt, namely lithium. Different lithium based energy storage technologies also exist, each with its own pros and cons. Common chemistries using lithium are:
- Lithium Cobalt Oxide (LiCoO2) – LCO
- Lithium Titanate (Li2TiO3) – LTO
- Lithium Manganese Oxide (LiMn2O4) – LMO
- Nissan Leaf;
- BMW i3.
- Lithium Nickel Manganese Cobalt Oxide (LiNiMnCoO2) – NMC
- Emerging big brand EV’s;
- LG Chem;
- Samsung
- Lithium Nickel Cobalt Aluminium Oxide (LiNiCoAlO2) – NCA
- Tesla;
- Panasonic
- Lithium Iron Phosphate (LiFePO4) – LFP
- Freedom Won.
LiFePO4 stands out as the leading chemistry for stationary storage applications as is the case with PV generated power plants. Some advantages of LiFePO4 include:
- Very high thermal stability – very safe;
- Highest efficiency of all lithium cells – 96% round trip;
- Longest life (cycles and years);
- Fast charge and discharge capability (for PV systems a quick and efficient charge is essential);
- Widest safe operational temperature range;
- Resistant to high ambient temperatures;
- Partial or incomplete charges and micro cycling presents no problem;
- Extremely low self-discharge and long shelf life;
- Extensive real world performance references;
- Mature and understood technology.
When comparing deep cycle lead acid batteries with LiFePO4 technologies, it becomes clear why LiFePO4 took the lead in becoming the choice of preference when installing a battery backed up PV system.
Comparisons between Lead Acid and LiFePO4 batteries:
Subject | Lead Acid | LiFePO4 |
|---|---|---|
| Service life | Varies wildly but typically 500 to 2000 cycles, 2-8 years | 5000 to 7000 Cycles, 15-20 years |
| Depth of Discharge (DoD) | 50% Maximum practical, 30% ideal | 90% Maximum, 80% daily ideal |
| De-rating from nameplate capacity | De-rate capacity to 65-75% of 10hr (C10) discharge rating | No de-rating |
| Overall energy efficiency | Round trip efficiency 55-65% | Round trip efficiency = 96% Charge and discharge = 98,5% |
| Self-discharge | Approximately 5% per month. This increases with age, cycling and elevated temperature | About 5% in the first 24 hours and then ±1-2% per month |
| Charge and discharge | With higher charge currents and multi-stage charge methods, charge times can be reduced to 8 - 10 hours | Can accept fast charging and discharging – 1 Hour standard, 30 minutes quick |
| Nominal voltages (P = V x I) | 2V per cell, Therefore 6 cells = 12V for a typical 12V battery | 3.2V per cell Therefore 4 cells = 12.8V for a typical 12V battery (Higher voltage = more power) |
| Communication | Non intelligent battery – No internal communication capability | Has a built in BMS (Battery Management System) which controls the individual cells’ balancing, calculates SoC, communicates with the system controller & other batteries on the same DC bus through CAN communication protocol, records historical data etc. |
| Weight and space saving | Very heavy and space occupying technology | Weighs approximately 3 to 4 times less and occupying up to 70% less space than its lead acid counterpart |
| Temperature resilience | Ideal 25°C
| Ideal maximum 45°C
|
| Maintenance | Mostly required | Zero maintenance |
| Safety | Most causes gassing – Needs to consider safety area classifications | Zero gassing (100% sealed) - Maximum safety, can therefore put in same area as other equipment |
| Upfront cost | One of the cheapest energy storage options available | Costs 10-50% more than lead acid depending on application and size |
| Life cycle cost | R5-R6 per kWh | 25% of Lead acid |
| Capacity indication | Ah | kWh – More practical as power is indicated in Watts, not Amps |
A primary benefit of LiFePO4 is that it’s Ah (Ampere Hour) rating only needs to be 35-50% of that of lead acid. There are mainly two reasons for this:
- Lead acid batteries can be discharged to a maximum level of 50% whereas LiFePO4 can be discharged to 90-100%;
- LiFePO4 doesn’t have a 3-stage charging cycle as is the case with lead acid. These three stages are Boost, Absorb and Float. During the Absorb stage the batteries need to be supplied with DC energy for up to two hours for the lead acid battery to maintain its charge. This is not the case for LiFePO4 and therefore the charge needed during this phase can be either used elsewhere when LiFePO4 is installed or the PV system (solar panel array) can become smaller.
A disadvantage however is that LiFePO4 has a very flat voltage curve and this makes it difficult to accurately measure the SoC (State of Charge) of the battery making use of its voltage. Below is a typical Voltage vs SoC Curve.
