Muhammad Shahzad Aziz, Umair Saleem, Ehsan Ali, Khalid Siddiq; A review on bi-source, off-grid hybrid power generation systems based on alternative energy sources. J. Renewable Sustainable Energy 1 July 2015; 7 (4): 043142. https://doi /10.1063/1.4929703
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H. BITARAF (US), M. ROSS (CA), J. GLASSMIRE (US), H. FARAG (CA), J.C. SMITH (US), R. SEETHAPATHY (CA), M. ALBU (RO), K. ASH (AU), M. BRUCOLI (UK), P.H.R.P. GAMA (BR), T. FUNABASHI (JP), P. KOTSAMPOPOULOS (GR), K. YUKITA (JP), S. TSELEPIS (GR), A. ZOMERS† (NL)
Corresponding Members: H. FARHANGI (CA), E. SILVA (BR), N. LUIB (DE), J. ADERALDO LOPES (BR), C. MARNAY (US), P. LOPES CAVALCANTE (BR)
To meet decarbonization targets of the Paris Agreement while providing universal access to electricity calls for renewable energy solutions. It is a key strategic goal of CIGRE to provide electricity for all in a sustainable way.
The Technical Brochure shares experiences on techno-economic pre-feasibility studies. Further, the study shows the necessity of stability studies and shares examples for hybrid systems for off-grid power supply. It further presents experience from systems that have been successfully in operation for several years. It is a proof of the concept that hybrid systems for off-grid power supply are already state-of-the-art and that high shares of renewable energy resources can be achieved while keeping power quality within the prescribed limits.
As an outlook, modelling and stability studies become more and more important. A benchmark model for hybrid off-grid systems and microgrids would be useful in addition to the already existing CIGRE Benchmark. This will support informed decisions by valid comparison of different control algorithms and power conversion systems e.g. during tendering phases of projects.
UN Sustainable Development Goal Number 7 calls for universal access to sustainable energy by 2030. Initiatives such as "Sustainable Energy for All" (SE4ALL) are a global platform that empowers leaders to connect stakeholders and unlock finance to achieve universal access to sustainable energy. The purpose of this Technical Brochure is to bridge knowledge gaps on hybrid systems for electricity supplies in off-grid and remote areas. It serves as a guide for governments, entrepreneurs, technical experts and financing organizations.
The work includes five chapters and an appendix for the example case descriptions.
Hybrid systems use a mixture of distributed energy resources, including solar, wind, hydro, biofuels and fossil fuels, combined with energy storage and controlled by an energy management system. By presenting experiences related to existing hybrid systems for off-grid power supply for different applications, this study can offer broad support for the preparation of installations for remote power supply, in particular those using renewable energy.
When considering bringing modern energy services to an off-grid area, three primary modalities are typically requested:
Focus is on hybrid systems for off-grid areas. In this context, we refer to microgrids as a common technology for off-grid power supply. They may also be deployed as part of a broader grid edge approach, which is a distributed approach in contacts to traditional centralized planning and operation.
This chapter is targeted mainly towards financial institutions and management in distribution utilities and mining. The chapter discusses both technical and economic challenges for hybrid off-grid systems. It suggests a business case methodology that shows under which assumptions and in which scenarios a hybrid system provides a solid payback time, internal rate of return and net-present cost.
Appropriate modelling tools are critical to ensure the success of a particular hybrid project. Initially, a project will need to establish what is technically possible and economically viable, while complying with social and environmental needs. As the UN Energy Access Practitioner Network notes, "understanding how to best combine traditionally generated and renewable power, storage, and load management for the optimal site-specific installation, at the lowest cost, is the key for deploying efficient mini-grid solutions." [2]This will ensure that the project will likely meet the overarching goals and will improve the likelihood of funding from investors. Subsequently, additional modelling will likely be needed to finalize the design and technical assumptions.
The evaluation in the brochure considers four scenarios. Figure 1 shows the "Medium renewable" and "High renewable" scenarios as an example. Focus has been put on investment options that provide a viable payback, adequate internal rate of return (IRR) and suitable total lifecycle cost. In this example, the "high renewable" scenario represents the generation mix with the lowest levelized cost of energy (LCOE). Projects with lower carbon emissions and increased renewable usage are possible, and a similar approach may be used to screen for the lowest cost options that meet those social and environmental needs. The selected incremental investments are summarized in Table 1 [3].
With decreasing cost of PV, wind and storage and increasing robustness of control algorithms, hybrid systems have become an economically viable option for off-grid power supply. It will be important to raise awareness of this proven technology in order to deploy them more rapidly in remote sites.
Table 1 - Summary of operational benefits and economics for an island utility making hybrid investments
Sizing and application engineering request a set of studies – starting with techno-economic feasibility, then continuing with stability studies for the power system in the adequate simulation tools. The chapter shows examples of off-grid hybrid systems engineering for selected applications in mining, and examines more thoroughly dynamic stability studies for islands and remote communities.
A hybrid system can include different distributed energy resources such as generation based on Diesel or gas, Photovoltaic (PV) systems, Wind turbines or hydropower, as well as energy storage systems (ESS), and loads under one control system.
Off-grid or stand-alone systems are different from centralized power systems in terms of capacity and voltage level. The capacity of off-grid systems is less than several hundred megawatts (MW), while centralized grids are in the range of gigawatts (GW). The voltage level at off-grid systems varies by application and could be in the range of medium and low voltage levels, while centralized grids include transmission and high voltage levels. Off-grid systems are not connected to neighboring grids to balance the mismatch between generation and load, while centralized grids are connected to their neighbors via tie lines. Off-grid systems have low inertia and have frequency fluctuations, while centralized grids are stable with high inertia.
Table 2 - Island utility response to load step changes before and after BESS integration
Another example in the brochure shows how a grid impact study (or power system impact study) is used to assess the impacts of integrating renewable generation or distributed energy resources in remote communities for the creation of an off-grid hybrid system. Several studies can be performed to determine not only the specific bottleneck that can prevent a higher integration but also the specific limit of integration that allows for a responsible integration [4].
The adequacy of the system is determined through a quasi-static time series analysis to evaluate the steady-state energy balance of the system [5]. The purpose of these studies is to ensure that power balance can be met for all times during the projected lifespan of the hybrid system equipment and all possible operating conditions. Reserve margins, system voltage profiles, and operating setpoints of the generators and other controllable distributed energy resources can be evaluated to ensure that no operational thresholds are exceeded. As a general principle when integrating new resources into an existing off-grid system, it is favourable to connect at the existing diesel plant bus since:
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