Feasibility study on small-scale Pumped Hydro Storage for isolated mini-grids in a low-resource setting

Abstract

A large portion of the global population, mostly in the Sub-Saharan African region, still has no access to electricity. Due to the remoteness and limited accessibility of some communities, off-grid solutions such as mini-grids are required to electrify these regions. An essential part of such an isolated mini-grid is the energy storage system. DC Opportunities aims to establish these mini-grids as a third step in the electrification process. As a means of energy storage, the small-scale employment of the mature technology of pumped hydro storage (PHS) should be explored, making use of height differences in terrain often present in these areas. While PHS is the most common energy storage technology, it has thus far only be employed on a large scale and only recently research on its small scale applicability has been gaining ground.
In order to conduct a feasibility study on the use of PHS as a means of energy storage for isolated mini-grids in low-resource settings, such as those in Sub-Saharan Africa, local challenges are identified. These include a suitable operating range, quality of supply and power issues, limited component and material availability, potential arrival of the main grid, funding, low tariffs and revenue, a low-trust society, limited technical skills, theft and vandalism, political vertical networks and corruption, regulatory issues, secondary water use and environmental impact. Based on these challenges, a program of requirements is presented, which consider the technical, economic and socio-cultural requirements.
Subsequently, the available design options are discussed, and based on the program of requirements a technical design synthesis leads to a provisional technical design. This exploration has led to the conclusion that earth dam reservoirs should be used for the water storage system, Glass Reinforced Plastic or Mild Steel pipes for the water conveyance system and a binary unit utilizing a Pump-as-Turbine or a ternary unit employing a Pelton turbine for the powerhouse configuration; this configuration can be either designed for an one-time installation or scalable installation. Based on the provisional technical design and the local challenges, an organizational structure, using the existing local power structure, ensures the PHS plant embeds itself in the community through community involvement and participation, as well as local capacity building by the project facilitator. The capacity requirements of the reservoir, as well as the diameter and material of the penstock, and the powerhouse configuration appear to be highly dependent on site-specific conditions. Therefore an energy flow model was developed, which in combination with a penstock selection tool and a generated demand profile is able to determine the requirements of a Hybrid Energy Storage System (HESS) utilising PHS as its main component for an isolated mini-grid. This model has been deployed for a case study in Adi Araha, Ethiopia, yielding positive results. Further exploration into the economic implications of the modeled system, and socio-cultural factors to be considered for the case study, led to the conclusion that for the case study a PHS plant could prove to be a technically, economically and socially feasible solution. Further research is however warranted, specifically on site.
The developed model was further employed to attain a conclusion on the general economic feasibility. While the socio-cultural factors are very site specific and can not easily be generalized, the results exhibit that for a situation where the gross head is at least 250 meter, and preferably around 300 meter, and the maximum power output requirements of the PHS plant is at least 150 kilowatt, technical and economic feasibility is attainable. Isolated mini-grids requiring a storage solutions, adhering to these conditions, should thus be further investigated to more accurately determine the feasibility of a small pumped hydro storage plant.