Energy return on investment (EROI) is measure of the net energy available to society (after accounting for energy used in energy production). So for example, if it takes 1 barrel of oil to extract 30 barrels, the net EROI ratio is 29:1. At high EROI ratios, economies have plenty of available energy to support their societies. However, at low EROI ratios, going below a 'threshold' value could seriously constrain the ability of our complex society to function. The conventional view is that fossil fuel EROI values are declining but are much higher (~30:1) than modern renewable energy technologies (RET's) (~10:1), meaning that an RET-led energy system transition may lead to dipping below a societal-level EROI threshold. In short, this translates as 'there may be a problem, but not just yet'.

However, recent research completed at the University of Leeds challenges this current wisdom. We find evidence that the level of fossil fuel EROI is declining, but crucially is much lower than previously thought. We estimated values for final energy (e.g. electricity, petrol), as opposed to conventional extraction stage (e.g. oil, coal), finding that global fossil fuel EROI values for final energy are around 5:1 and declining. Our research suggests that fossil fuels EROI may be actually lower than RET's, and we may be nearer a 'net energy cliff' than we thought, where the availability of net energy to the economy may go into a very sharp and serious decline.

To assess this, the transition to a RET energy system must be analysed with its two primary biophysical constraints in mind: 1) the energetic cost of RET production; and 2) the material cost of RET production. Assuming no other changes to socio-economic organisation, and within a Green Growth scenario, it has been found that in high RET penetration pathways the required expansion of RET infrastructure leads to increased energetic costs and a decline in global EROI from ~12:1 to ~3-5:1 by mid-century. Concurrently, given the higher material intensity of RET's, constraints on mineral resources may also limit RET deployment, increase the material intensity of the economy, whilst also limiting material use (in addition to energy) in non-energy sectors.

The PhD Topic:
Given the uncertainty surrounding the biophysical feasibility and socio-economic implications of a RET transition further and holistic research in this area is urgently required. This PhD will seek to extend our world-leading research into this area, whilst at the same time linking to the EPSRC Energy Theme priority areas "Whole Energy Systems", "Energy Efficiency", "Energy Networks", "Solar Technology", "Wind Power". Three guiding research questions are therefore proposed:
1. What are fully-temporal RET EROI values?
2. How do constraints on mineral resources limit RET deployment?
3. What is the societal-level EROI threshold and the impacts of transgression?