Thursday, 26 May 2016

If renewables have low energy return on energy investment – so what?

Energy return on energy investment (ERoEI) is a measure of how much energy we get out from a generating system compared to how much energy we put in. Self evidently (you might think) ERoIE has to be greater than one to be worthwhile. After all, if you only get 0.5 units out for every 1.0 unit in, what would be the point? Furthermore, some commentators insist that in a modern society, using energy at the rate we do, we need ERoEI to be at least 5 - and that means most forms of renewable energy, especially solar PV panels, are pointless [1].

"The greatest risk to human society today is the notion that we can somehow replace high ERoEI fossil fuels with new renewable energies like solar PV and biofuels. These exist within the energy web because they are subsidised by the co-existing high ERoEI fossil fuels." [1]


ERoEI for a power station is 0.4 but it is still useful.
The first problem with this simplistic analysis is that all kinds of energy are not the same. Consider a power station. You put in 100 kWh of gas energy and you only get 40 kWh of electricity out. So what was the point of that? Actually it was very useful because electricity is far more versatile than gas. For example you can use it to run computers and washing machines.

ERoEI for pumped storage is 0.75 but it is still useful
Even the same kind of energy can be more or less valuable depending on when it is available. Our hydro-electric pumped storage systems may only be 75% efficient but they can deliver power fast on demand. It is definitely worth putting in 100 kWh of energy when it is cheap so that you can deliver 75 kWh of energy when someone really needs it and is prepared to pay more for it.

Wind and PV deliver electricity and that is much more versatile than coal or gas. On the other hand they are intermittent which makes them less valuable. Obviously things are a lot more complicated than the simple ERoEI model allows.

ERoEI does not matter as long as it is > 1 on average and we have enough throughput
We do need ERoEI to be greater than 1 on average – but claiming that modern society needs a minimum value of 5-7 is nonsensical. Granted, developed societies use a lot of energy. But that does not mean we need a high ERoEI. Suppose society needs 1,000,000 kWh. That can come from 100,000 kWh with an ERoEI of 10, or 200,000 kWh with an ERoEI of 5, or even, 9,000,000 kWh with an ERoEI of 1.1. It doesn’t matter, as long as ERoEI is more than 1 (on average) and we have enough energy throughput.

In practice there are limits to throughput.

The limiting factor for fossil fuels is climate change
Until recently, the limiting factor for our use of fossil fuels was how fast we can get it out of the ground. Now, however we realise that there is another limit to our use of fossil fuels - how much we can use without making life difficult for ourselves because of the changing climate.

The limiting factor for renewable energy is land.
The most important limiting factor on renewable energy is land. This is especially true for bio-crops that compete with food crops. PV panels are 10-40 times as efficient as bio-crops in terms of energy conversion and wind turbines are pretty good too as you only need a small amount of dedicated space for the base. You can use the land between the turbines for other things. However, biomass energy is storable and that gives it extra value.

What is the ERoEI for solar PV? That depends on who you ask.
So what is the ERoEI for solar PV panels? This metric seems to be favoured mainly by groups that seek to show that renewables are a waste of time and we need fossil fuels. Various groups have come up with values between about 2.4 and 5.8 [2].

A particularly nonsensical feature of some of these analysis is the way they calculate the energy costs of human labour. The usual approach is based on the energy intensity of the economy and the price of labour. Suppose, taken as a whole, the UK uses 4 kWh for every £GDP, and the labour costs for PV are £100. Then the energy requirements for labour are 4 x 100 = 400 kWh.


If we fly to New York for a weekend holiday, that is not claimable on expenses 
This methodology assumes that all of the energy used by a worker is due to the system they are working on whereas in fact most of the energy we use is completely unrelated. We might decide to fly to New York for a weekend holiday - but we would not claim it on expenses. By the same logic we should not charge the energy use for the flight against the products we make when we are working.

The IEA reckons PV energy payback time is 0.8 to 1.7 years - good enough for anyone.
I have more faith in an analysis from the International Energy Agency. This is based mainly on energy to extract materials and do the manufacturing. It includes supporting frames and balance of system equipment as well as the panels but it does not include labour. They reckon on energy payback time of about 0.8 to 1.7 years, for rooftop solar in Southern Europe [3]. Assuming a lifetime of 20 years this gives an ERoEI of 12 to 24. I should think anyone would be happy with that.

I have written this post in response to Euan Mearns' blog from yesterday [1]. It also relates to research I have been doing recently (for Transition Cambridge Energy group) on how to calculate embodied energy - the energy used to make things. There are all sorts of interesting issues, such as how to account for by-products and recycling rates. See

[1] ERoEI for Beginners (Euan Mearns, Energy Matters) May 2016
[2] Energy Return on Energy Investment – a quintessential but Possibly Inadequate Metric for Sustainability in Solar-Powered World? (IEEE) Aug 2014
[3] (Life cycle inventories and life cycle assessments of Photovoltaic systems (IEA) Jan 2015.

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