Making energy green – a method for quantifying the ecosystem maintenance energy and the green energy return on energy invested

Limiting the impact of human use of energy on the environment is necessary to maintain the Earth's ecosystems such that they can continue to provide the ecosystem services on which humans and all other living organisms depend. Because the methods available to limit the impact on the environment of energy production and use require energy, energy used for this purpose—termed ecosystem maintenance energy (ESME)—reduces the net energy available. A capacity to estimate the energy that can be produced while sustaining ecosystem services, termed green energy, is critical to future energy use. This paper sets out a method to determine ESME by assigning energy consumption to the processes needed to remove the source of the impact on the ecosystem that results from the energy system lifecycle. We illustrate the method by the use of a case study based on a concentrated solar power plant (CSP) with natural gas back up. Green energy production is assessed in terms of the green energy return on energy invested, EROIg. CSP ecosystem impact is determined via a lifecycle assessment of the plant's operations for varying degrees of natural gas back-up. ESME is then determined by calculating the energy consumption of the impact source removal (ISR) process needed to remove the ecosystem impact. We show that, although the CSP plant produces increasing amounts of net energy with increasing natural gas back-up, the capacity to produce net green energy reduces with increasing natural gas back-up once ESME is included. For the CSP plant examined, EROI increases, while EROIg decreases for increasing use of natural gas. This indicates the large impact of ESME on the plant's energy return and the necessity of its inclusion in energy assessments.