To the question of energy return on investment (ROI) of solar energy

Why? 

Because everything is clear even without energy return on investment (EROI). The “energy yield” of photovoltaic solar energy is high and will continue to grow as the energy efficiency of production increases and the material consumption of devices decreases.

EROI — energy return on investment or energy returned on energy invested  — the ratio of energy received to energy spent, “energy profitability” (EROI = Lifetime energy output / Energy input). The indicator appeared and began to be used in the scientific literature in the 1970s and 80s. 

Why?

The exhaustibility of natural (energy) resources and the realization that the development of new, increasingly difficult deposits is associated with increased energy costs, gave rise to concern that new units of extracted energy may not justify the energy costs for this extraction. Appropriate metrics and methodologies have been developed to measure the relationship.

The EROI of major commodities (oil and gas) is known to decrease over time:

This chart shows trends from “past years” but is taken from a new 2020 scientific paper on the outlook for LNG and conveys direction.

Somewhere in the early 2000s, and maybe at the end of the 1990s, when the cost (unit) of solar energy was extremely high and, at the same time, the introduction of “alternative” energy technologies began to be massively supported by Western countries, naturally arose a question: are these technologies viable in terms of energy efficiency? Are we investing in an “energy impasse”? The EROI metric has been applied to both solar and wind energy, and a great many relevant scientific papers have been published.

In recent years, interest in the topic has subsided, and only some bloggers, scooping scraps of information “from forgotten newspapers” and from “experts” (other ignoramuses) like them, now and then thoughtfully, as if revealing the sacred, say: “EROI!”. Sounds smart, no one understands, and in some circles, you can pass for a connoisseur. Sometimes they also add something about the law of conservation of energy, but this is a completely mysterious train of thought.

Of course, “by inertia” scientific papers on the topic come out. Here is a 2020 article (conclusion: EROIs of wind and solar technologies are generally high and rising). But again, this is not a “new revelation”, but something in the wake of “past battles”. Anything can be found, even the “Life Cycle Assessment Model to Quantify the Environmental Impact of a 3.6kW PV System in Bangladesh” (2019 research paper), which also calculates EROI, more specifically the Energy Payback Time (EPBT).

These works are of interest only to the authors themselves as another line in the list of their publications.

What is the reason for the lack of interest?

It’s all about the economy.

When you have an (unsubsidized, of course) cost of solar electricity of 1 cent per kilowatt-hour or even less than 1 cent per kilowatt-hour, and you can fix such a one-part price for 25 years, who cares about the theoretical, rather complicated, inaccurate, and ill-suited for cross-industry comparisons metric like EROI? If you like math, you can try to deduce EROI from this price, since most of the data for such a calculation is available, and the energy cost of the object’s life cycle is included in it. Well, you get 50:1 or 100:1, how will this affect the structure of the world?

Estimating the volume of energy costs during the life cycle of a solar energy facility (as well as wind energy) is relatively simple, already definitely simpler (and more accurate) than for energy raw materials. Take, for example, EROI oil. It’s not at all clear what it is, a complete abstraction. Of course, over the history of the development of science, appropriate tools have been developed, System boundaries have been defined, and appropriate methods for estimating EROI exist for anything. At the same time, there is a huge spread in EROI estimates for the same energy products, which indicates an incredibly low scientific and practical (yes, whatever) value of the indicator. See, for example, an article in Scientific American for some funny examples. Here is what is said about the EROI of nuclear power: “Some argue that the EROI is less than one…while others…estimate that the EROI is much higher than perhaps any other energy source.”

As stated at the beginning of the previous paragraph, the calculation of energy costs for photovoltaic solar energy is much simpler and more accurate than for energy feedstocks (and for the corresponding sectors of the electricity industry based on burning hydrocarbons), as well as nuclear energy. The reason for this is simple: the bulk of the energy costs associated with the life cycle of a solar power plant falls on the stage of standardized, high-tech industrial production. Roughly speaking, the main energy costs are made at the plant. The plant is connected to the relevant energy supply systems with the appropriate energy accounting systems.

What is included in the life cycle of a photovoltaic solar generation facility? 

Everything. From the extraction of raw materials to the disposal of the remains. There are methods. The Photovoltaic Program of the International Energy Agency (IEA PVPS) updated its guidelines (see 3.2.3. System boundaries).

The production stage, where the main energy costs arise, consists of the following main stages (we are considering solar modules from crystalline silicon ~95% of the world market here):

• Production of polysilicon

• Bullion production

• Cutting silicon wafers

• Production of elements (cells)

• Production (“assembly”) of modules

The contribution to the total energy consumption of the last three stages is relatively small, the first two stages — the production of polysilicon and ingots from it — are very energy-intensive, they account for the bulk of the energy costs of the life cycle of a solar energy facility (for this reason, the EROI of thin-film modules, such as CdTe, above — there are no silicon smelting processes).

If in the mid-2000s the production of solar modules required 13–14 grams of polysilicon per watt, today the specific consumption is approaching 3 grams per watt.

Anyone with a serious interest in solar energy should read Facts about Photovoltaics, a report by the German Institute for Solar Energy Systems (Fraunhofer ISE), which comes out a couple of times a year. This book is structured in the form of answers to numerous questions concerning the industry.

There is also an answer to the question: “Does the production of photovoltaic modules consume more energy than they can produce?”

Answer: No. EROI depends on the technology and installation location. The authors refer to a “recent study” (2017) that found the EROI of 25-year silicon solar installations located in Switzerland to be 9–10, respectively, with an energy payback period of 2.5–2.8 years. “Wind farms have a significantly shorter energy payback period, typically less than a year,” the authors note. [In this article we talk about solar energy, but still, we recall that the issue of the energy payback of wind energy has been studied thoroughly. The EROI of wind turbines is 35–45, and as the power of the installations grows and the consumption of materials decreases, it will grow.

Vestas publishes a Life Cycle Assessment (LCA) for each wind turbine model. These are more than 100 pages of reports, certified by independent scientific bodies, which take into account everything, down to the nail, including all energy costs at all stages of the life cycle. EPBT calculation included in the documentation].

Yes, Energy Payback Time (EPBT) — the time for which the energy costs that are incurred / will be incurred during the life cycle of an object are recouped — this is a metric that is used in science and professional literature more often than EROI, but from which EROI is easily derived (EROI = T/EPBT, where T is the life cycle of the object).

The Fraunhofer authors understand, of course, that the 2017 work, which uses the original data from previous years (2015–2016), is now out of date. However, they link to this rather well-known article with absolutely no reflection. The fact is, as we noted earlier, the issue of EROI about solar (and wind) energy is secondary, representing only limited theoretical interest.

Over the past few years, there have been tremendous changes in increasing efficiency in the entire chain of production of solar modules, both the overall scale of production and the size of individual factories have grown radically, many times over. Today, factories producing under one roof 5 or more GW of products (plates, elements, modules) per year have become the norm. Five years ago, there were no such large factories in principle.

The scale effect leads to a significant reduction in specific (per watt of output) energy costs.

Back in 2016, the scientific work “Energy learning curves of PV systems” was published, in which, by analogy with the financial function (output volume — cost), the dependence of specific energy costs on the output volumes of solar modules was calculated. The learning rate for modules turned out to be 17% and for photovoltaic systems 14%.

The graph from the article shows that the EROI values ​​that I mentioned above (“50:1 or 100:1”) are not a figure of speech. According to the calculations of the authors, something similar is obtained in some regions of the world already in 2020.

In May 2021, the German Environmental Protection Bureau (Umweltbundesamt) published a large (392 pages) report “Updating and assessing the environmental balances of wind and photovoltaic systems, taking into account current technological developments”. For German conditions, the EPBT of solar installations with single-crystal silicon modules turned out to be 2.1 years (for thin-film CdTe modules — 0.9 years), that is, taking into account the 30-year service life considered in the report, EROI for silicon modules exceeds 14 (in unfavorable conditions plan for the development of solar energy natural conditions). For thin-film CdTe modules, EROI exceeds 33.

Conclusion

The EROI metric is poorly suited for cross-industry comparisons due to the difficulty of establishing comparable system boundaries and the lack of a unified calculation methodology. This is also evidenced by the cardinal differences in the scientific literature on EROI estimates for the same energy carriers (or energy sectors).

EROI is a “minor” indicator of dubious scientific and practical value.

Unlike many other industries, calculating the EROI of solar energy (in methodological terms) is relatively simple, since the boundaries of the system are clearly defined, and energy costs are recorded by appropriate metering devices. At the same time, of course, obtaining these initial data for calculation is a separate task.

The EROI of solar energy is high and will grow further due to the increase in the energy efficiency of production and the reduction in material consumption per unit of output.

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