Energy Returned on Energy Invested: Part 4 – Renewables


Welcome back to our series on EROEI, or Energy Returned on Energy Invested.

Our previous articles established how much energy is lost or consumed in moving fossil fuels from their production point to their use point. This must happen before they are processed into a convenient form that we can store and use as fuel.

Now, we need to look at renewables.

Renewables are different as they are typically set up to deliver electricity as an output. As we’ve seen, with fossil fuels, an extra step is involved before we produce electricity.

Firstly: The transmission and distribution of electricity through an AC network involves more losses than you might think.  

AC transformer losses associated with stepping up and down the transmission voltage range from 1-3%, and losses over typical grid distances are anywhere from 2-4%.  

We can lose a further 4-6% in the final lower voltage distribution to the end user. Put all this together, and typical losses between a generator and a consumer are 7-13%. We will assume a global average of 10% here. 

Note: Historically, electricity has not been transmitted over ultra-long distances due to the high losses making it uneconomic. However, High Voltage Direct Current (HVDC) technology, which has been around for a surprisingly long time, is changing that with transformer losses of <1% and transmission losses of 3-4% per 1000 km.  China has an HVDC line over 3,000 km long with ~12 GW capacity!

Curtailment is another source of loss for renewables.  

And, as more renewables enter the grid, they suffer from it. Typical levels of curtailment in most areas are in the low single-digit percentages. Still, it’s not hard to envision a future where 50% of generation potential is curtailed, so looking at the energy system of the future, we will assume 50% curtailment.

Note: The EROEI calculation for renewables is different from that for fossil fuels as, once constructed, a renewable asset needs no fuel, and none of the energy it delivers is consumed in the delivery process. Some of that energy, however, will be lost through curtailment and transmission. Therefore, the EROEI equation for a renewable asset with 50% curtailment, 10% grid losses and a baseline EROEI of 20 becomes 20 x 0.5 x 0.9 = 9.  

This is very different from the fossil fuel system, where energy consumption is essential to delivering energy (See EROEI part 2 for details).

Bringing all this together, what happens to our EROEI for renewables?

Solar drops from a range of 5-20 to ~2.3-9.0, and wind comes in at ~6.8-16. 

But let’s not forget we have assumed 50% curtailment.  See the below chart for a perspective on the impact of different degrees of curtailment on EROEI

Conclusion:  The EROEI of renewables looks even better compared with fossil fuels when you look at energy delivered to the point of consumption.

Renewables also have an ace up their sleeve:  they deliver pure Exergy.  Exergy is the useful work content of energy, and electricity is effectively pure Exergy.  

The implications of this for EROEI will be covered in our next article.

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