How much electricity in a hybrid solar-fossil power plant is really renewable? Old problem, novel approach.


Globally, legislative measures and policies are being tabled and implemented with the aim to curb greenhouse gas emissions. The determination to combat and phase out fossil fuels has led to various approaches being devised. One notable approach is the adoption of hybrid power plants. Such systems operate by integrating renewable energy resources such as solar radiation and geothermal among others, into existing fossil powered systems. Currently, in power generation interest in such systems is rapidly rising owing to fact that they can provide more economic, sustainable and reliable power under all load-demand conditions, as compared to renewable systems only.

As of now, the greatest concern is what fraction of the electricity produced in such facilities can be considered as generated from the renewable resources. This alone creates a lot of arbitrariness in evaluating the share and the fraction of the power that qualifies for subsidies granted to renewable electricity, as normally prescribed by most of the policies that promote the applications of renewable primary energy resources. So far, various techniques have been proposed, however, a common limit to all the classic allocation methods, is that the impact of the actual conversion efficiency process of the two resources within the power plant is not considered.

In this view, Italian researchers at University of Brescia Dr. Paolo Iora and Dr. Gian Paolo Beretta together with Professor Ahmed Ghoniem at Massachusetts Institute of Technology developed a novel Exergy Loss based (EL) allocation method for the electricity produced in hybrid renewable-fossil power plants. The rationale behind this approach is that the electricity allocated to the fossil and renewable resources are obtained by subtracting from the respective source input exergies, the corresponding exergy losses, that are identified by dividing the plant into three parts; namely: the renewable, fossil and hybrid sections. The advantage of this approach is that the allocation is based only on the performance of the power plant given by its internal exergy balances, and hence the results are independent from any external arbitrary assumptions on the reference conversion efficiencies of the two resources, as it is typical of classical methods. Their work is currently published in the research journal, Energy.

In brief, the research team started by introducing the novel approach and justifying the rationale of their proposed Exergy Loss based allocation method. Cautiously, they introduced and analyzed the ISCC plant they had chosen as their case study. Eventually, they presented results and compared them with those of other classical approaches.

The authors reported that, in contrast to other classical allocation methods – mainly based on a black-box approach and require reasonable assumptions for the efficiencies of single-resource reference production methods – their Exergy Loss based allocation method based its allocation on a reasoned analysis of the plant layout details and an exergy analysis of the main subprocesses. In addition, compared to separate production reference (SPR) allocation method, the scholars noted that the Exergy Loss technique was advantageous such that it required no arbitrary specification of reference efficiencies by some external authority.

In summary, the study presented a new and novel technique for allocating electricity produced in a hybrid power plant among the fossil and renewable energy input resources it consumes. Overall, the approach was shown to take into account the efficiency of the processes with which the fossil and renewable input sources are converted within the power plant itself, thereby providing a more consistent and fair allocation of the two resources into the produced electricity and overcoming most of the contradictions observed in the classical methods.

About the author

Paolo Iora is Full professor of “Energy and Environmental Systems” at the Department of Mechanical and Industrial Engineering, University of Brescia since June 2018. He graduated in Mechanical Engineering at the University of Brescia (2000) and worked as Postgraduate Student from December 2003 to April 2004 at the Department of Chemical Engineering and Chemical Technology of Imperial College (London), focusing on the development of dynamic simulation models for high temperature fuel cells. In 2005 he took his doctorate in Energetics, at Politecnico di Milano.

From January to April 2010 he was visiting professor at the Mechanical Engineering Department of Massachusetts Institute of Technology (MIT), working on the development of advanced hydrogen storage systems for PEMs. In September 2012 he was visiting professor in the same Department, collaborating on energy policy for cogeneration plants and fossil-renewable hybrid plants. His research activities mainly focus on advanced systems for energy conversion and storage. He is author of 37 papers published on peer review international journals, and numerous contribution presented in national and international conferences.


Paolo Iora, Gian Paolo Beretta, Ahmed F. Ghoniem. Exergy loss-based allocation method for hybrid renewable-fossil power plants applied to an integrated solar combined cycle. Energy, volume 173 (2019) page 893-901.

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