Generators for off grid systems

Wednesday, November 18, 2020
Training

This blog will briefly look at generators and their contribution in an off grid renewable energy system.

Off Grid generators, why?

In most cases when designing an off grid system a generator is required due to a variety of reasons: large loads for sustained periods may be better off serviced by the generator; during cloudy conditions, batteries may be put under undue strain hence generator input and if solar “real estate” is limited due to space constraints the generator shares the workload.

All the above is based on the relative costs of the inputs, solar and generator and the storage component, the batteries with the amount of energy consumed and when during a specified time period.


Off Grid generators, options

Generators come in a variety of sizes and fuel options:

  • Diesel
  • Petrol
  • LPG and natural gas


Diesel generators tend to be offered in 1500 rpm and 3000 rpm, and can be either water cooled or air cooled, with petrol and gas units usually being 3000 RPM and air cooled.

Selection criteria based on loads

If the generator is required daily, the best option is usually a 1500-RPM water cooled diesel followed by 1500-RPM air cooled diesel and for less frequent operation and smaller loads, the  petrol and gas units may suffice.

Standby, continuous, and prime power ratings?

Generators have different ratings depending on the specific application.

Standby rating 

The generator is required for emergency situations of a short duration.

Continuous rating

If supplying power continuously to a constant load, but not being subjected to overloads.

Prime power rating

Can cope with unlimited run time, heavy loads and is the primary power source.

Generator power factor

This is the ratio between kilowatts (kW) and kilovolt amps (kVA) that is drawn from an electrical load.

An approximate value for generators is (typically 0.8). The higher the power factor, the more efficient the transfer of energy.

Generator power factor to kW:

  • 6.40 kW = 0.8 (pf) x 8 (kVA)    
  • 8.00 kW = 0.8 (pf) x 10 (kVA)
  • 9.60 kW =  0.8 (pf) x 12 kVA)     

Generator optimum loading

A generator is designed around optimum loading which means it is operating at its most efficient point. With diesel generators this is ideally around 80 – 85% of full load.

If we have an 8 kVA generator with a pf of 0.8, the output is 6.4 kW. Now, if we load up this generator 85% we are talking about 5.44 kW!! 

So an 8 kVA genset should ideally output 5.44 kW when servicing loads for most of the time.

Off Grid generators, their role 

The generator has to service the loads,  charge the batteries and a lot of the time, do both so the off grid designer has to allow for this when selecting the best generator for the job and their decision must  also take into consideration  appliance load surges. 

Generator load surges

So what about appliance surges? Things like washing machines, pumps, and some refrigerators have a start up surge that the generator must be able to cope with, so we have to look at the generator’s alternator rating in regards to be able to address these inductive load start up surges.

Conclusion

With off grid systems incorporating energy storage and renewable energy a fossil fuel component is usually part of the picture. This genset must operate in conjunction with the other  components, all determined by the consumption load profile of the business or residence. Changing any of the key components affects the others ability to correctly service the loads. If there is minimal renewable input and energy storage capacity the generator must be capable of continuous or near continuous performance and therefore the correct rating must be applied.












 







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