Getting Started with Off Grid Power Systems

 

Working examples of an off-grid system

What is the simplest form of off-grid power system?

The simplest form is a solar panel, or wind turbine, connected directly to a small fan or pump.

When the sun shines brighter or the wind blows, the fan or motor turns faster!  If you don't care about the volume of air moved or water pumped then the design process is really trivial. On the other hand, if the amount of air moved or water pumped must meet a minimum requirement, then it is absolutely essential that the design process is considered in more detail.

Given the above then when the sun shines brighter, the solar PV panel will produce more current and voltage, thus the fan or pump will produce more power, and as it does so it will move more air and water.

To really understand the relationship between the solar panel and the fun or pump we have to examine the operating characteristics of both the solar panel and the fan or pump.

To do that we have to examine the V/I (output) curve of the solar panel and superimpose this onto the operating characteristics of the fan or pump.  There may be a few surprises in store. We'll come back to this later, but needless to say a solar panel does not operate anywhere near its maximum power at low light levels and yet at high illumination levels the solar panel only delivers a fraction of its maximum power capability.  Thus, all designers need to decide how much air or water movement is needed at the various irradiance levels, and choose the appropriate solar module with discretion.

It must be borne in mind that regardless of the choice of solar panel, there will be a portion of the fan or water pump or PV characteristic where maximum power will be not be transferred.  Therefore, if it is required to operate the fan or pump at all levels of irradiance (illumination levels) then a maximum power point tracker (MPPT) will need to be incorporated into the design.  MPPT trackers are very useful at illumination levels where it is required to start and stop the fan or pump at low levels of irradiance and therefore benefit from greater air or water flow at irradiance levels between start and stop.

The big question now: is it cheaper to include an MPPT tracker with a smaller PV module or go for broke and deploy a larger solar panel to obtain the same performance?

Water pumping in more detail

Let's now examine the water pumping application in more detail.

Water pumping applications do not normally require a battery unless the water source will not meet the needs of demand during the period of PEAK SUN.  Batteries should be used if water is required to be pumped over long distances, perhaps at a slower rate, using a smaller pump.

It is common practice to charge a battery so that the pump can meet its demand and run for extended periods if required. It is also common practice, and it may be more cost-effective, to pump all the water that is required and store the excess in a storage tank.  If we really want to be smart then it is possible that that the water stored in the tank could be used to turn a generator to provide additional electricity when required.

Basic steps required

We need to understand what is required to build a water pumping system.  First of all, we need to determine the daily water need; second, we need to know the volume of water available, the height the water is to be pumped (HEAD of water) and the number of hours per day the pump is to be run and thus we can determine the RATE at which the pump is utilised.

Pump power = pumping rate + pumping height

Once the size of the pump is known, then the Ah requirement of the pump motor can be determined and then the size of the solar panel(s) can be determined to provide the required Ah to run the pump.  An MPPT controller will enable the useful pumping time to be extended so a smaller pump motor and solar PV panel can be deployed.

The Design Approach

If you can obtain the all the available pumping data from the pump manufacturer and then all you need to know is the daily amount of water needed and the overall pumping height.

Let's assume that 1000 gallons per day are required to be pumped, the pumping height is 100 ft and the hours of sunlight are 4 hrs per day. What this means is that 1000 gallons must be pumped in 4 hours if no batteries are used, or put another way, 1000 / 4 / 60 = number of gallons per minute and with a height of 100ft, including 5% losses then 100 x 1.05 = 105 ft.

The solar PV module is usually sized to be 125% of the product of the pump volts and amps.

SOLAR POWERED PARKING LIGHTS

In this scenario, it must be obvious that a battery will be needed to power the lights. In all cases the first step is to determine the lighting load, then select the battery according to the load and then the number and type of solar PV modules required. In summary:

Step 1 - determine the lighting load;

Step 2 - determine the number of batteries required;

Step 3 - determine the number and type of solar modules required.

Let's take another look at this. In determining the lighting load in WATTS for step 1 we need to determine the amount of light that is required and the area over which it falls - the illumination. We really do need to know the level of illumination before we can go any further.

Now, you may think, and quite reasonably, that illumination is measured in Watts/m2 or if you like in foot-candles or in other words the amount of light emitted in lumens, this is simplified and I have cut corners but you get the point.

Examine the table below

 

Luminous efficacy (column 2 in above table) is a measure of the efficiency with which electrical energy is converted to light energy. This is measured in lumens per watt.  The table above shows the luminous efficacy of several light sources.

Working out the wattage of a light will therefore depend upon the required illumination level and the area to be lit as well as the luminous efficacy of the source of the light. Loads of other factors also come into play, such as the whether the light source is directed in one direction only or whether some of the light will be absorbed by walls or something else.

For reasons of this example we will not concern ourselves with temperature and colour at this stage though they are important factors.

For most solar PV applications, the most popular and efficient light sources are LED, metal halide and sodium lamps.

Step 1 - work out the total lumens required.

The formula: Lumens = (FC) x (area) + CU) / (MF) / (RCR)

Where:

FC = foot candles - ft2

CU = coefficient of utilisation

MF = maintenance factor

RCR = room cavity ratio

NOTE:

Good light sources will have a CU of 0.8 or more and an MF of 0.9 or more

RCR is generally assessed to be 1.0 for outdoors and <1 for indoors.

For a car park light in London of 100W and for the lights to be on for 10hrs per evening, then the maximum amount of time the lights will operate is 10 hours during the worst month of December. During the summer months the lights will only need to operate 6hrs per night.

Thus, (winter worst case) = 100 x 10 = 1000Wh per day

Summer (best case) = 100 x 6 = 600Wh per day.

What are the battery storage requirements?

How many days of backup (storage capacity) will be required? For the UK, it is normal to design a battery bank with at least 5 days of backup.

For a 24V DC system, this means that the winter daily connected load will be 1000 / 24 = 41.67Ah, which must be supplied by the batteries. Allowing for wiring losses and an 50% depth of discharge, the batteries will need to have a 24Hr discharge rate of:

41.67(Ah) x  5 (days of storage) / 0.90 (losses) / 0.5 (depth of discharge) =463Ah @ 24V.

NOTE: the discharge cycle is over a 5 day period and the batteries will be discharging for 4.8Hrs each day.

What is the size of the solar array required?

This must be done carefully. Step 1 is to calculate the energy that must be supplied to the batteries by the solar array.

Remember the connected (winter load) load is 1000Wh.:

We will, for the purposes of this example, assume that an MPPT solar controller connects the array to the batteries. If we also assume that the wiring from the array to charge controller and to the batteries has an efficiency of approx 95%, then the solar array must generate 1234 / 0.95 = 1299Wh per day.

We must not forget that the solar array itself will experience around 14% losses (such as temperature, elevation, etc), then the array must be sized to compensate. Thus, the solar array size will be 1299 / 0.86 = 1511 Wh/day at STC.

So far so good. However, rather than rushing out to buy a solar panel that can offer the required output, let's pause for a moment as ask the question is it possible to optimise the array further? Good question and the answer is YES!

By taking into account the ANGLE OF TILT and LATITUDE it is possible to optimise the performance of the solar array for both winter and summer use.  If we really do know the angle of tilt for the modules as well as the average, daily PEAK SUN HOURS, then we can determine the size of the solar array even more accurately.

Thus, the solar array size is could be: 1511Wh/day / 2Hrs (PEAK SUN) or 755W.

We can achieve this by using a number of different solar panels. For example, we could use 3 x 215W solar modules connected in series. This is not exact, but it is close. On the other hand we could use, 4 x 185W solar modules. It is important to check the Voc of the chosen solar module to ensure that the maximum solar controller input voltage is not exceeded.

 

NOTE: The above simple example is provided for guidance only.  Bright Green Energy Ltd will not accept responsibility for systems designed using the examples shown.  Designing off grid systems can be complex and systems must be correctly designed.  Seek professional help if you require assistance.

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