On June 17th, we held a presentation in the offices of Silba cooperative of a pre-investment study for the photovoltaic desalinator construction. Prepared in cooperation with the Faculty of Mechanical Engineering and Naval Architecture and the Island Movement, the presentation took place in a pleasant atmosphere. After an extensive debate, the locals supported the presented solution to their island’s water supply issue.
The study is part of the PROSEU project funded by the European Commission within the Horizon 2020 program. It aims to promote renewable energy sources within the European Energy Union through a prosumer model. A prosumer is basically an active user of energy from renewable sources, which he produces and consumes. The Croatian partner in the PROSEU project, and authors of the study, was the Department of Power Facilities, Energy and Environment research group from the Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb (UNIZAG FSB). Members of the Island Movement, a network of islanders operating in Croatia, also took part in the study development. If you were unable to attend, you could view the full clip at this link.
Water problem
Silba is a small island with about 300 permanent residents administered by the City of Zadar municipality. One of their major problems is the lack of a continuous and secure water supply. In 2020, there were no potable water sources on the island, and the water carrier delivered water at the cost of approx. 7,72 HRK per 1m3. However, the actual water price is 75.72$ due to the high cost of the water carrier transport. The difference is paid from the state budget for island residents, while non-residents and legal entities are obliged to pay the full price. In addition to the high price of water covered by all taxpayers, additional costs come from water losses that reach approximately 20% in the secondary water supply network, given the network in Silba is dilapidated. For example, the island has four large water tanks (reservoirs) with a total volume of 950 m3 connected to a secondary network delivering water to privately owned smaller tanks (cisterns). A big problem is the seasonal nature of tourism resulting in a large oscillation in water consumption on a tourist-oriented Silba. According to the available data, Silba has a population that averages around 5,000 people during the summer season, increasing the number of water consumers by about 1600%!
Since water carriers supply other islands in the Zadar archipelago and given the limited capacity of reservoirs on the island, Silba is often without sufficient water. Consequently, this greatly influences the continuity and security of the water supply. In a conversation with the local population and a representative of the Vodovod Zadar Ltd., we estimated that in the period from January 1 to May 1, approx. 700 -1,000 m3 of drinking water is delivered, and from May 1 to October 1, approx. 6,000 – 8,000 m3 of drinking water (depending on the weather and the amount of precipitation in that period).
The aim of the study
The main goal of the study is to propose scenarios for a drinking water self-sufficient island. Islanders of Silba would thus use the water for their own needs, the needs of the tourism sector, and for the production of their own electricity. The Silba study proposes a desalinator with a photovoltaic power plant as a concrete and sustainable solution. The power plant would supply energy to the desalinator and sell excess electricity to the grid or accumulate it in batteries. In addition to setting up the facility, the plan is to build an additional water tank and restore the secondary water supply network. There is already a 21,060 m2 building plot on the southeast side of the island assigned for desalination in the City of Zadar spatial plan.
What is desalination?
Desalination is the process of removing salt and minerals from sea, brackish, or river water. There are two processing methods, and in the case of small and remote environments, such as Silba, membrane desalination technology is suitable. In short layman’s terms, water is pumped and pressurized through half-membranes separating salt/minerals and water, thus leaving salt on one side while pure water passes into the next processes after which it is ready for use. After desalination, the water is filtered, purified from bacteria, microorganisms, salt, and has the appropriate pH index.
The study lists several scenarios and combinations to solve the problem.
1. Scenario A – off-grid photovoltaic power plant and desalinator
The first solution proposes photovoltaic electricity production with its own battery for storing excess electricity, accompanied by the installation of a desalinator for potable water production and possible replacement of the secondary water supply network. According to detailed electricity production calculations based on meteorological conditions in previous years and data on water consumption and rainfall, we concluded that more water could be produced than needed per capita. According to national figures, the average person consumes 4.5 m3 of water per month, while on Silba it will be possible to produce 4.7 m3 per person in the off-season period through a desalinator. Taking into account currently available data, it follows that the monthly consumption of drinking water brought by the water carrier on Silba in the period January-April and October-December is about 0.40 m3 per capita, which is far below the average. In the autumn-winter-spring period, islanders collect rainwater in cisterns and thus do not depend on water from the carrier. With this solution, it will be possible to produce more water during the season due to the increased sunny hours. For the summer period of increased water consumption, the plan is to additionally install a 500 m3 tank to meet the required amount of water.
2. Scenario B – on-grid photovoltaic power plant and desalinator
Another proposed solution is an on-grid photovoltaic power plant with water production in the desalinator. The excess electricity would not be stored in a battery but handed over to the local power grid. The maximum of electricity delivered to the electricity grid is reached in the spring and autumn months when there is no significant need for potable water, and at the same time, the irradiated solar energy is relatively high. In the summer months, there would be a negligible transmission of electricity to the grid. The photovoltaic power plant would operate to produce a sufficient amount of potable water.
Both solutions have been proposed with and without replacement of the existing secondary water supply network. However, with the new secondary network, islanders would pay 30% cheaper for the desalinated water. Replacing the water supply network and introducing new pumps for continuous water pumping would eliminate the existing 20% water losses. The realization of one of the proposed scenarios would significantly increase the quality of life in Silba.
Cost-effectiveness
Both scenarios are cost-effective, but the on-grid option (with and without water supply replacement) is slightly more favorable because it does not involve an initial investment to purchase and install a battery to store excess electricity. The price of desalinated water would be competitive with the water carrier. It would always be available in larger quantities, and the cut in state subsidies for potable water would be significant.
The high investment cost (desalination plant, photovoltaic power plant, additional tank construction, and water supply network renovation) is the main risk of the project implementation. To overcome this risk, the Island Movement, in cooperation with the Cooperative for Ethical Financing, proposes the so-called crowdfunding business model.
A water supply from a desalination plant on the island of Silba would give it the status of self-sustainability and environmental awareness due to the carbon dioxide emissions reduction. In addition to the direct benefits of using water for private and tourist purposes, water production brings new possibilities and opportunities. Water self-sufficiency would ensure greater flexibility with accommodation booking, extend the tourist season, and generate higher income. Innovation is an important factor in modern tourism, and integrating renewable energy sources with water and electricity production would brand Silba as a community that promotes sustainable development and turns to smart, green solutions. In addition to tourism, islanders could dedicate more time to agriculture. Its indigenous varieties and controlled production would make Silba an even more attractive destination while increasing the food supply security.
You may find a complete study with all calculations, graphs, and detailed explanations HERE.
Author: Mirna Dalić
Photo: www.pixabay.com