Sizing standalone solar-plus-storage with nomograms


A group of scientists from Spain’s University of Murcia and Netherlands’ Eindhoven University of Technology has developed a methodology that uses modelling and simulation to apply a special kind of dimensioning charts, known as nomograms, in the sizing of off-grid PV systems linked to storage.

“The methodology can be used at any latitude in which data about irradiance is available,” research co-author Alfonso Ramallo told pv magazine.

Nomograms are diagrams in which the relationship between three variables is represented by a straight line or curve for each variable. They are usually applied to obtain reliable results quickly by drawing one or more lines.

The scientists specified that off-grid solar-plus-storage system should be sized in a way that results in a good level of storage utilization. “The design of cost-effective PV-battery systems is a challenging task because both meteorological parameters and load profiles are highly dynamic and stochastic, featuring both seasonal and diurnal variations,” they stated in the paper Nomograms for de-complexing the dimensioning of off-grid PV systems, published in Renewable Energy.

The first step of the proposed methodology consists of calculating a project’s power generation ratio, which is obtained by calculating the consumers’ average electric demand and normalizing it with the nominal power of the PV installation. The research group created an auxiliary graph to enable the calculation of this ratio without a computer.

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The second step consists of normalizing the capacity of the battery. “It is straightforward to add the same normalization to this unit, so we obtain a ratio with dimensions of time, which is highly relevant for the dimensioning of the system as time of blackout is one of the outputs of the nomograms that we want to design,” the Spanish-Dutch group specified. A specific nomogram was also obtained to calculate this value.

The resulting graphic contains information on the values calculated in the first two steps and a third value given by the number of blackout days. “Through these three parameters, the simplified sizing process of the off-grid system will be considered complete,” the academics explained. “Fixing two of the values, the chart will give the third one as output.” Battery efficiency, losses and all users’ profiles are also included as variables in the chart.

The methodology, which allows the measurement of the effect of battery performance and losses of an off-grid solar-plus-storage system, can also easily show, according to the scientists, that these parameters do not substantially change the shape of the curves. “As one could expect, the performance of the battery affects the results in a larger manner when installations with large levels of battery storage are selected; whereas the effect of the performance of the generator affects installations with all sizes of batteries as one also may expect,” they stated.

The group claim that the developed nomograms not only offer a standard calculation tool but also provide additional information about what-if scenarios with higher-level knowledge about price of modifications and site limitations.

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