In this article, we are going to discuss in detail setting up a complete DIY energy system. We cover the components needed, the sizing process, and useful design tools. This article is therefore meant as a guide and us at @chargedengineering hope that this helps you in designing your off-grid power system whether it be for camping, RV, or just for extra energy storage in your household.

The main motivation behind us taking up this project is because of JAG35’s 3.7Kwh custom steel enclosure project designed to be used with the high-quality Japanese LEV60 lithium-iron-phosphate cells. Jehu Garcia discusses further setup and assembly of this project at length here: https://www.youtube.com/watch?v=IAFsVAk_Eq0

 

Project Layout

Let’s start with a simplified general layout of the project.

 

1. General layout of the project

Design Process

a) Energy Requirements

As mentioned earlier, the motivation behind this piece was the 3.7Kwh steel enclosure project by JAG35, therefore our sizing process is going to revolve around this. This is the most crucial stage of our process as it has an effect on all the other stages of the design as well as the components that will be required to successfully complete this project.

Determine your daily energy needs by considering all the electrical appliances that you use, their energy ratings, and how long each is powered on. Appliances are usually rated in Watts, which gives the energy requirement it needs in a single hour. 

Multiplying that rating by the number of hours you power it on will give a Watt-hour (wh) rating.

After determining the watt-hour rating of all the appliances add them up and that will give you the total energy you use daily. For more precise results, you can also use digital watt-hour meters or billing data from your utility company in case you are designing for a household that is already connected to the grid.

Project Specifics

1. System size - We will consider our energy needs as 2 kWh per day, which is just slightly lower than the average household energy needed for a household in the United States. That is equal to having a 400 W rating of appliances running continuously for 5 hours.
2. Voltage & Autonomy  - We are going to design it as a 48V system allowing for up to 3 days of autonomy. 
3. Solar Insolation  -  This is another important aspect to consider in design. This is the amount of solar radiation received on a horizontal surface over the course of a day. This has an impact on how much energy solar panels can produce. You can make use of this tool from the National Solar Radiation Database to help you, you just need the latitudes and longitudes of your location  - https://nsrdb.nrel.gov/data-viewer

We use 3.5 kWh/m²/Day

 

b) Battery Sizing

The batteries in use are the LEV60F LiFePO4 batteries. To make a 48V system, we will require 16 of these in the custom steel enclosure. These batteries are selected due to their quality and the safety ratings of Lithium Ion Phosphate batteries. This safety is further enhanced by design with the addition of other components such as the Battery Management system and temperature monitoring sensors.

In its sizing, we further consider the inefficiency of the conversion process from DC to AC happening in the Inverter and the depth of discharge.

Special considerations

1. Depth of discharge  (DOD) -  Determines the level up to which the battery bank can be discharged. Recommended range DOD for LiFePO4 batteries is between 50 - 80 %. The higher the level of discharge in batteries the less the lifecycle of the battery. We use a DOD factor of 60 %. Feel free to use other figures within the range.
2. Efficiency of Conversion Process -  We make the assumption that during the DC to AC conversion process in the Inverter, 8 % of power is lost or consumed in the process. This leaves us with 92% efficiency.

 

 

c) PV Array 

Next on the design pipeline is the sizing of Solar modules. Here, we take into consideration the insolation and our energy needs in kWh. An efficiency factor is also considered, solar modules do not have the capacity to harvest 100% of the energy it is rated for, partly due to direct recombination and other external factors such as dirt on solar panels or the direction the panels are placed relative to the sun’s rays.

This Oriental calculator will help you select the optimum orientation of your solar panel  -  https://footprinthero.com/solar-panel-tilt-angle-calculator

 

 

Monocrystalline solar panels are made from a single pure silicon crystal and this is the one we will select. Now, we need to get 816 Watts of solar panels. It might be difficult to get exact, hence we are looking for a 1000-watt solar panel to be utilized in a 48V system.

d) Charge Controllers

There are two types of charge controllers;

1. Pulse Width Modulation Charge controllers -Works by controlling power going to the battery. When the battery approaches capacity, the ON time of the pulses changes to reduce charge power. The voltage produced by solar arrays should match that of the battery in this setup. These controllers are ideal for small-sized setups.
2. Maximum Power-Point tracking controllers - A bit much more complicated and used for large systems. It determines the maximum voltage produced by the solar array and downsizes that to the voltage at the terminals of the battery. In doing this, it ensures proper utilization of power from the panels, as the drop in panel voltage enables the battery to be charged under higher currents. In the case of MPPT controllers, the voltage produced by PV arrays should not necessarily match that of the battery bank.

We now get into the nitty-gritty of solar controller sizing. There are a few adjustments that need to be made to factor in the voltage given off by crystalline and multi-crystalline solar panels during cold weather. The panel open circuit voltage given by the manufacturer is rated at the standard or reference temperature 25℃. During low temperatures, solar panels give even higher voltages. 

A correction factor is applied to the open circuit voltage to account for this. You can find the correction factors in the NEC table 690.7(A).

For our design, we make design for the worst possible scenario in this case, we expect our panels to face temperatures of up to -40℃. According to the NEC table, we apply a temperature coefficient factor of 1.25. You can adjust these as per your design considerations.

We chose to use the 200W, RSM-M200D solar panels with an open circuit voltage of 45.4V and short circuit current of 5.83A.

We need 6 of these panels arranged in a 2s3p configuration to give a Voc higher than that of the battery. 

2s because, as a rule of thumb, a voltage that is 20% higher than the battery voltage suffices.3p to enable us to match our value obtained earlier of a PV array size of greater than 800 Watts.

Our total open circuit voltage considering 2s configuration;

 

 

113.5V is the temperature-compensated voltage, this is the highest voltage that our PV array can produce in sub-zero temperatures.

The array short circuit amperage is (5.83 * 3 ),  which gives around 17.5 amps

Next is determining the amperage that each of the solar panels can give. The 6 solar panels effectively make our system a 1200W system. The maximum typical charge voltage of 48V cells is around 58V. For 16 LEV60F, the max charge is (3.5V*16), and its charge voltage is 56V.

The maximum current that can be delivered at any point in time by our system by the controller is;

 

These two figures the temp compensated voltage and maximum current help us in deciding the size of the charge controller that we should select for our system.  According to this, we need the Victron Smart Solar MPPT 150|35. This indicates that our system can handle voltages of up to 150V and 35 amps of current.

 

e) Inverter Sizing

For this project, we are going to use a string or a central Inverter. This is whereby a large inverter is located at a central location unlike in micro inverters. For more information on this, check our past article on Inverters and Inverter sizing.

1. Simplified representation of a central inverter setup

An important factor we have to take into consideration is the startup requirements of appliances such as the fan etc. These are appliances that draw a huge amount of current during startup. The majority of Inverters are sized in VA, that is volt-amperes. We get this factor by factoring in the power factor of our appliances. The power factor is a measure of the efficiency of how these appliances utilize power. Let us consider an average power factor of 0.85.

 

We need a  0.47 kVA Inverter, considering the safety factor/ surge factor.

We need to select an inverter rated more than 0.47 kVA, and we also need an inverter that can handle the 1200-watt solar input. For this, we select the XYZ INVT 2000 Watt Pure Sine Wave Inverter (48V DC to AC 110V 120V).

 

Conclusion

This article offers a comprehensive guide to designing an off-grid solar energy system, catering to a range of applications from camping to household energy storage. The design process covers essential components, sizing considerations, and the use of design tools, providing a valuable resource for those embarking on similar off-grid projects. Make sure to check out the great off-grid power options on http://jag35.com . Keep on building!

Article contributed to JAG35, written by Japhet K, Electrical Engineer at http://charged-engineering.com