Increasing the penetration of renewable energy resources in S. Vicente, Cape Verde
Introduction
Most islands depend mainly on the importation of fossil fuels for energy production and, at the same time, present a considerable potential in renewable energies. The use of this potential in the production of electricity and fresh water (usually very scarce in islands) could represent a large fraction of the total energy distribution [1].
One of the major challenges of increasing the penetration of renewable energy in a system is to integrate a high share of intermittent resources into the electricity supply system [2], [3].
The intermittent nature of some renewable energy sources as well as the small energy systems of islands introduce barriers to their penetration, like the struggle to match the demand with the supply and the problems related with the integration in the network. Hence, the integration of intermittent renewable energy sources in energy systems requires the development of energy storage technologies, energy management technologies and a greater sophistication of these systems.
The integration of renewable energy sources in energy systems of small islands presents several advantages, namely at economical level, their high technological cost is compensated by the high cost of the conventional sources of energy due to the small dimension of the energy systems and because of a very expensive security of supply. In order to achieve sustainable development, the integration of renewable energy sources for the production of electricity, together with suitable policies and regulations regarding rational use of energy, are very important. The conventional electricity production technologies are rarely adapted to the conditions of isolated areas and can seriously damage the vulnerable ecosystems and natural habitats. There is the need to develop an energy supply infrastructure that takes into consideration the seasonal variations caused by the tourist activity, without destroying the local environment or producing avoidable emissions.
Due to the several available renewable energy sources, the increasing number of technologies for their use and the different options for energy storage, the planning and modelling of energy systems are complex and demanding. Specialized models were developed to help in solving that problem [4].
To date, a number of analyses have been carried out on the feasibility of integrating renewable energy resources in islands. Many of them consider the possibility of using hydrogen as energy storage, in order to increase the penetration of intermittent renewable energy sources in the energy systems of islands [4], [5], [6], [7], [8], [9].
In many places there is already an excellent storage potential in the local water supply system. By merging the energy and water supply systems, where there is sufficient elevation difference, it is possible to use pumped hydro storage in order to increase the penetration of intermittent renewable energy sources, as for example wind, even in case where there is not much hydro potential.
In Duić et al. [10], the possibility of using pumped hydro storage in the Island of Corvo, in Azores, is analysed. In this case, with the integration of the water supply system with pumped hydro, adding storage to energy and resources systems, it is possible to significantly increase the penetration of locally available resources and thus increase the security of supply and decrease the import dependence.
A wind-powered pumped hydro system is proposed in Bueno and Carta [11] for the Island of El Hierro, Canary Islands, in order to increase the renewable sourced energy penetration of the Island grid. The results indicate that an annual renewable energy penetration of 68% can be achieved.
S. Vicente has very important and stable wind resources. The Island also has Mont Verde, a 774 m high mountain located in its centre. These features make S. Vicente suitable for the use of pumped hydro as a storage technique. As there is no fresh water available in the Island, the proposed solution considers the use of desalinated water in the pumping and hydro station to later be supplied to the population.
The installation of desalination units is a common solution throughout the world in areas with water scarcity [12]. However, desalination is a process that requires a significant amount of energy [13], thus, renewable energy driven desalination plays a vital role in the application of this technology.
There are several studies that analyse renewable energy powered desalination systems [12], [13], [14], [15], [16], [17], [18], [19], [20], [21].
The purpose of this study is to couple these two issues: the integration of renewable energy sources in the electricity supply system and the water scarcity problems of S. Vicente, using the intermittent excess to supply the desalination and the pumping units.
The innovation of this study lies on the analysis of the combination of these two supply systems (energy and water) in order to increase their efficiency.
In this article different scenarios are analysed for increasing the penetration of renewable energy in the energy system of S. Vicente Island. The tool used is H2RES, a model designed to simulate the integration of renewable energy sources and hydrogen in islands or other isolated locations.
There are two types of scenarios. Scenarios with 30% hourly intermittent energy penetration, that is considered the limit of the current conversion technology that is installed on the Island, and scenarios without this limit, where it is assumed that the conversion technologies can also provide output control and auxiliary services [8].
It is shown that is possible to have more than 30% of yearly penetration of renewable energy sources in the electricity supply system, together with more than 50% of the water supplied to the population produced from wind electricity. The penetration of renewable energy sources can reach 70%, when 100% hourly intermittent energy penetration is considered.
Section snippets
H2RES model
In Connolly et al. [22], a review is made of the different tools than can be used to analyse the integration of renewable energy. H2RES is classified as a simulation tool, as it simulates the operation of a given energy system to supply a given set of energy demands; a scenario tool, as it combines a series of years into a long term scenario; a bottom-up and an operation optimisation tool.
The H2RES model (Fig. 1) simulates the integration of renewable sources and hydrogen in the energy systems
The Island of S. Vicente
S. Vicente is the second most crowed Island in the Archipelago of Cape Verde, which is composed of 10 Islands and is situated at about 450 km of the West African coast, in the Atlantic Ocean (Fig. 2). This Island had about 74,031 inhabitants in 2005 [25], mostly concentrated in its capital, Mindelo.
S. Vicente has about 228 km2 of area and its topography is relatively uniform, having just one high point – Mont Verde – located at 774 m of altitude.
The Island is extremely dry; all of the fresh water
Conclusions and future developments
This article analyses the way to increase the penetration of renewable energy sources in the Island of S. Vicente, in Cape Verde, coupling the energy and water supply systems. Based on existing load data and meteorological data, several scenarios were built and modelled using the H2RES model. The scenarios considered wind, pumped hydro storage and desalination technologies.
The maximization of desalination from wind resulted in fractions of desalinated water produced from wind of about 57% in
Acknowledgments
The authors acknowledge the contribution of the Portuguese National Foundation of Science and Technology of the Ministry for Science and Technology by supporting Raquel Segurado with a PhD grant (SFRH/BD/31663/2006). The authors would like to thank the company ELECTRA S.A., namely Eng. Rui Paisana for its availability in supplying the necessary data for this study. Finally, the authors would also like to thank the reviewers for their valuable comments to the article.
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