The Environmental Impact of Distributed Energy Resources (DERs) in Modern Electric Power Systems

By Daniel Akinyele and Yoash Levron

The role of distributed energy resources (DERs) in realizing a smart electricity grid cannot be overemphasised. DERs, such as solar PV, wind, and biomass, are small-scale energy production units that can be operated in either a grid-integrated mode and/or a grid-independent (islanded) mode. These sources help to improve the electricity system efficiency and reliability by supporting peak power demands, and providing active generation and ancillary services, such as stabilizing the frequency and voltage profile.

The increasing integration of distributed energy resources (DERs) in modern power grids poses many challenges, several of these are purely technical, and several are associated with other factors such as social impact, environmental impact, and policy. These factors should be taken into account when designing, planning, and operating power grids and distributed sources. To address these challenges, a STEEP (S- Social, Technical, E-Economic, E- Environmental, P- Policy) framework can provide a comprehensive analysis of DERs in future electrical systems.

In this light, this article focuses on the environmental impact of DERs in electric power systems, with emphasis on renewable energy technologies. A common assumption is that renewable energy-based DERs are eco-friendly, however, it is currently unclear to what extent they are. Finding an answer to this question can serve as a basis for comparing the environmental performance of these systems with the conventional energy option based on fossil fuels. This can be achieved through a Life Cycle Analysis (LCA), which guides designers, developers and policy makers in selecting the best energy resource or technology.

The LCA approach helps to assess the environmental impact of various technologies, and provides an opportunity for ascertaining the environmental sustainability of different energy resources by examining both energy and emissions-related aspects. Certain indicators are computed when LCA is being conducted, such as the cumulative energy demand (CED), energy payback time (EPBT), net energy ratio (NER), the life cycle emission rate (LCEr) and/or the global warming potential (GWP). While the first three factors are part of the energy related aspect of the LCA, the last two represent emissions-related aspects.

The CED represents the total primary energy required for manufacturing, transporting, installing, operating and decommissioning an energy technology, and is measured in MJ-eq. The EPBT indicates how long an energy technology is required to operate to recover the primary energy requirement over its life cycle, measured in years. The NER, also referred to the energy return on investment (EROI), shows how many times the energy technology is able to produce the primary energy requirement over its life cycle. The LCEr represents the life cycle emissions per unit energy generated by the technology (gCO2-eq/kWh), while the GWP is defined as the life cycle greenhouse gases (GHGs) measured in (kgCO2-eq). The emissions are evaluated as the equivalent quantity of CO2 that has equal GWP computed over an integrated time of 100 years, considering the most recent GWP factors published by the IPCC for carbon dioxide (CO2), methane (CH4), dinitrogen oxide (N2O) and chlorofluorocarbons (CFC). The greatest percentages of primary energy requirements and the life cycle emissions for renewable energy technologies are usually obtained during manufacturing (e.g. ~90% for solar PV technology).

Energy generation technologies are required to fulfill two major requirements: meeting acceptable techno-economic and environmental performance levels, and achieving a net energy yield that is greater than zero. A positive energy yield implies that the energy output is greater than the energy input (i.e. CED) over the system’s life cycle, meaning that its NER > 1. On the other hand, a negative energy yield practically signifies that the technology consumes more energy than it is able to produce over its lifetime. This means that it is not a renewable energy system because its NER< 1. This explains why one energy technology cannot be sustainable for all geographical locations, as a result of disparities in climate, resource and material availability, social and local conditions etc. Therefore, it is crucial to consider the environmental impact of energy technologies from the perspective of LCA, to also ensure that the expected energy yields over their life cycle are carefully evaluated.

The LCA, as defined by ISO 14040, compiles and assesses the environmental burden or consequences of an energy technology at all stages over its life cycle, i.e. cradle-to-end-of-life analysis. Practically, such an analysis should consider all the components of the technology. For instance, in a solar PV technology, the environmental impact analysis should consider both the PV array and the balance of system (BOS). The BOS includes the battery, inverter, cable, circuit breakers etc. This way, a better analysis of the DERs’ environmental impacts is achieved. The comparison of the CED, EPBT, NER, LCEr and GWP measures for renewable energies to those of fossil-fuel options will also be useful with compliance to standards, e.g. IEA methodology guidelines on life cycle assessment of photovoltaic electricity.

Achieving detailed environmental impact analysis of DERs in power systems requires new ways of thinking. For instance, the carbon emissions saved/avoided by a renewable energy technology is assumed in several research works to be equal to the emissions that would have been produced if a diesel or petrol plant were used. While the total life cycle impact of the grid-independent technology needs to be considered as the sum of the impacts of its individual components, the total life cycle environmental impact of the grid-integrated system will be a sum of the impact of the existing grid and the impact of the connected DERs. This point of view can assist researchers, developers and decision-makers around the world, in planning DERs for grid-connected and off-grid applications, and encourages a careful examination of the environmental sustainability of the entire electricity infrastructure.

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