Off-grid power to remote locations: a balanced mix of renewable and fossil sources

There are articles around the web assuming that off-grid power can be supplied by 100% renewable sources coupled with storage facilities. In most actual situations that’s not the case.

When approaching the sizing of a power supply system two parameters are essential: [1] the availability rate and [2] the variations of the power source over the year and over the day.

The reason is that you cannot ensure 100% availability of a source which is itself not 100% available, unless you increase the size of the storage to a degree which makes the system either not financially sustainable or technically impossible to be operated efficiently.

The availability rate differs widely depending on the users served by the power system. Typically, the required availability is around 95-98%. In some cases (e.g. oil pumping systems, mobile telecom infrastructures) the rate must be close to 100%. When talking about solar power, data sources provide the typical reverse-bell daily irradiation diagram, but no clues are available about the variations (actually the mean square variation) around the average value.

You must then make assumptions according to historical weather data to define the number of continuous days of full clouds your system would be required to face in order to grant the users with the required availability rate.

Another point of concern is the seasonal variation of the irradiation.

If the PV generator is sized to supply enough energy during the winter (or low-sun) season, you must expect to “waste” energy during the summer – unless the user has seasonal power-loads consistent with the seasonal variations of the solar irradiation (like e.g. seasonal farming).

It won’t be even thinkable to have batteries sized to store months of energy for only a 2-cycle duty per year (unless you are capable to find other means to store the energy locally).

There is also a relationship between the size of the solar generator and the storage size. The batteries provide for two services: [1] to store the energy produced by the PV plant in excess of the energy supplied instantaneously to the user and [2] to supply the loads with energy as the PV plant is not producing. The point is that the lifespan of electrochemical batteries is affected by the number of charge-discharge cycles as well as by the completeness of each cycle. The more the cycles are complete (according to the set-point provided by manufacturers) the more the batteries last. So the number of cycles has also to be factored in the economic simulation.

What it turns out in most of the cases, is that the optimal system’s solution requires a programmable source of power – typically a diesel or LNG genset.

The genset has two functions: to recharge the batteries during low-sun periods (and/or to feed power directly to the loads) and to contribute to reach the required availability rate. The good point is that given that the batteries may supply (for short periods) a power capacity higher than the generator’s peak power, the generator’s size does not have to bear the full loads’ capacity.

Simulations with a spreadsheet help come to the optimal mix of solar power and fuel-powered generator in a proportion generally around 80/20 of the total energy supplied.

Which is indeed by far cheaper – and environmental welcoming – than a 100% diesel-fuelled power system.

One comment

  1. True enough. Our household runs on 100% solar when the sun is shining, but diesel for the odd cloudy day, or for high power draw like welding etc. Also 100% reliance on one power source doesn’t allow for any redundancy or maintenance downtime.

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