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Review of different solar energy systems with backup

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This article will cover the following points:

  • Solar backup systems scenarios presentation
  • Solar backup systems diagrams presentation
  • Solar backup systems equipment selection
  • Solar backup systems sizing
  • Energy results presentation
  • Energy results discussion

Scenarios presentation

This article presents four options for off-grid solar energy systems, the goal being to familiarise the reader with the energy potentially produced by such systems. The options change in increments of 5 kW of power and are equipped with the maximum quantity of batteries that can be recharged by the system itself.

  • Scenario 1: 5 kW solar system
  • Scenario 2: 10 kW solar system
  • Scenario 3: 15 kW solar system
  • Scenario 4: 20 kW solar system

System Presentation

The figure below shows a diagram of a standard solar energy system. The figures represent the direction of the electric current and the different stages of energy conversion. The solar panels first convert the energy of the solar radiation into direct current (1), the solar inverters then transform the direct current of the solar panels into alternating current (2), the battery inverters then transform the alternating current from the inverters direct current solar (3), the batteries store electricity for later use (4) and the battery inverters ultimately transform the direct current of the batteries into alternating current when it is requested by its users (5).

Both types of inverters are necessary for two reasons. First, by converting direct current to alternating current, they make the electricity produced by solar panels usable by our electrical devices since almost all our electrical devices operate on alternating rather than direct current. Second, the inverters act as charge regulators and optimize the life of the batteries by maintaining their highest possible state of charge while protecting them from overcharging and over-discharging. They also optimize the maximum power of the system by raising or lowering the tension as needed. Inverters are therefore essential to any solar energy system, whether they are connected to the grid or off-grid.

Equipment selection

The model of solar panel used is the CS3U-380MB-AG BiKu from the Canadian manufacturer Canadian Solar. The solar panel is monocrystalline and bifacial, which means that it captures the sun’s rays from both sides (front and back). Its power is 380 watts and its efficiency is 18.94%.

The inverter model used is the SB-1SP-US-40 from the German manufacturer SMA Solar Technology. The inverter is of pure sine type and has a power which varies between 3 and 8 kW depending on the scenario. Its efficiency is 97.2%.

The inverter model used for the batteries is the Sunny Island 4548-US/6048-US from the German manufacturer SMA Solar Technology. The inverter is of pure sine type and has a power which varies between 4 and 6 kW depending on the scenario. Its efficiency is 96%.

The battery model used is the Trojan 31-AGM Deep Cycle 12V 102 Ah from the American manufacturer Trojan. The battery is an AGM battery with a capacity of 102Ah (1.22 kWh), a voltage of 12V and an efficiency of 85%.

In all three cases, the manufacturers are renowned for producing tier 1 equipment and are among our suppliers. The equipment chosen is the most popular among our customers given their excellent value for money as well as their reliability.

System Sizing 

This section explains the logic behind the sizing calculations for the four different solar energy systems as well as the autonomy they provide. In order to alleviate the explanations, the first of the four systems studied was taken as an example (5 kW of power). The same logic was applied for the following three systems. For those wishing to view the results, they are displayed in the next section.

1) Solar panels sizing

The amount of solar panels in a system depends on the total power of the system.

→ 5000 watts ÷ 380 watts/panel = 14 panels.

→ A 5 kW solar energy system has 14 solar panels.

2) Inverter sizing

The cumulative power of the inverters in a system must be at least 80% of that of the solar system and an inverter is required for every 15 panels.

→ 80% of 5000 watts = 4000 watts

→ 5000 watts ÷ 380 watts/panel = 14 panels = 1 inverter

→ A 5 kW solar energy system needs a single inverter with 4000 watts of power.

The cumulative power of battery inverters must be equal to the maximum daily power demand. The power demand is the total power required to operate a given quantity of electrical devices simultaneously. For example, a 500 watts fridge and a 1500 watts stove require a 2000 watts battery inverter so they can always operate simultaneously.

Since the power peaks constantly vary according to the quantity and the power of the electrical devices which operate at the same time, we assume that the maximum power demand is 6000 watts and this, no matter how powerful the system is.

→ For a 5 kW solar energy system, only one inverter for 6 kW batteries is required.

3) Batteries sizing

The amount of batteries in a system depends on the total power of the system and the amount of energy produced by the said system, the energy produced being the equivalent of the amount of energy storable by the batteries.

→ 5000 watts ÷ 380 watts/panel = 14 panels

→ Daily energy produced by the panels = [Number of panels * Power of the panels * Average daily hours of sunshine * Panels efficiency * Inverter efficiency] = [14 panels * 380 watts/panel * 5.6 hours of average sunshine per day * 0.1894 * 0.972] = 5.50 kWh

→ Number of batteries to be connected in parallel = Energy produced by panels (kWh) ÷ Battery capacity (kWh/battery) = 5.50 kWh ÷ 1.22 kWh/battery = 5 batteries

→ Number of batteries to be connected in series = Desired voltage of the battery bank (Volts) ÷ Battery voltage (Volts) = 24 Volts ÷ 12 volts/battery = 2 batteries

 Total number of total batteries = Number of batteries in parallel * Number of batteries in series = 5 * 2 = 10 batteries

→ A 5 kW solar energy system can therefore recharge 10 AGM batteries with a capacity of 1.22 kWh each.

N.B.1. The average amount of sunshine per day is from historical data from Environment Canada for 1981-2010 at the Pierre-Elliott Trudeau International Airport.

N.B.2. The desired voltage of the battery bank is at least 24 volts since we want to reduce the average amperage of the system to reduce costs by minimizing the diameter of the wiring required.

4) Solar energy system autonomy

The autonomy of a solar system is the final energy returned at the output of the system which can be directly used. It depends on the average sunshine as well as on the overall output of the various equipment in the system. In this case, it is the product of the energy produced by the solar panels, the efficiency of the battery inverters and the efficiency of the batteries themselves. The efficiency of the inverters must however be considered twice since the current flows there twice (see steps 3,4 and 5 on the diagram of a standard solar system shown in the previous section).

→ System autonomy = [Daily energy produced by the solar panels * Efficiency of the battery inverters * Efficiency of the batteries * Efficiency of the battery inverters] = [5.50 kWh * 0.96 * 0.85 * 0.96] = 4.31 kWh

→ The useful energy of a 5 kW solar energy system is therefore 4.31 kWh.

Results presentation

The table below shows the desired configuration of the four different solar energy systems. The amount of batteries in each system differs since a maximum amount of batteries can be charged for a given power. Obviously, the more the power of a system increases, the more inverters and batteries are needed.

The table below shows the energy data for the four different solar energy systems. In the same logic as in the previous table, the more the power of a system increases, the greater the energy produced and the greater is the energy autonomy provided by the batteries. The energy produced by the solar panels considers the efficiency of the solar panels and the inverter. The energy available in the batteries considers the efficiency of the batteries and corresponds to the useful energy of the system, which is the equivalent of the autonomy provided by the system.

Results discussion

The results of the previous section show that the energy available from the four different solar energy systems varies between 4.31 kWh and 16.33 kWh. Knowing that this quantity of energy may seem abstract to many, the goal of the following section is to explain what such quantities of energy represent.

As an indication, Hydro-Quebec claims that a 2000 square foot single-family home that does not have a pool or spa consumes an average of 25 000 kWh of electricity per year. This consumption is theoretically equivalent to 70 kWh/day. However, given that our average winter electricity consumption is two to three times greater than our average summer electricity consumption, dividing this 25 000 kWh by the number of days in the year would not be representative of average daily electricity consumption. Indeed, this division would only bear witness to an average value composed of low and large values. Suppose therefore that in winter our electrical demand is 120 kWh/day and that it fluctuates around 45 kWh/day for the rest of the year.

The previous results of 4.31 to 16.33 kWh of autonomy are therefore very little in relation to our average daily energy demand. You should therefore be aware that a solar system of 5 to 20 kW will not be able to meet all your energy demand. Depending on your level of consumption, it will meet between 10 and 60% of your demand.

To give you an even better idea of ​​what the autonomy previously described represents, the table below shows the energy required for the operation of typical household appliances.

The table above shows how easy it is to consume 13.76 kWh of energy. Indeed, knowing that the 13.76 kWh of the table does not take into account heating (which accounts for 60% of our overall energy consumption), the use of washer and dryer (both very energy-consuming) and using a multitude of other devices (vacuum cleaner, dehumidifier, fan, video game console, DVD player, sound system, coffee machine, etc.) this amount of energy is very little. It is even more important to remember that the 13.76 kWh of energy required to operate these devices can easily double if several devices are operating at the same time (Peukert effect, explained in previously published articles). The power peak is 8.32 kW. The 6 kW power peak considered in this article would therefore be too low to make all of these devices could not be in operation simultaneously. Compromises should therefore be made.

Conclusion

Ultimately, the four scenarios of solar systems presented offer daily autonomy from 4.31 to 16.33 kWh. Although they do not allow you to meet your entire energy demand, they will give you the ability to be partially energy independent while reducing your electric bills. A solar energy system is therefore an energy backup system rather than a primary energy system.

Do not hesitate to contact us for more information regarding the different solar systems offered, we will be happy to help you! Please be aware that the options presented in this article are not the only possible options and that a solar energy system is easily modifiable.