Solar Heat could Provide Namibia with Power and Water, Study Finds

PUBLISHED 17 MAY 2018   

Countries around the world are rapidly turning to solar power to  supply their energy needs as it is both green and renewable. Now a new study conducted by researchers from Stellenbosch University has found that energy from the sun can be harnessed to make water too.  In arid countries, where water is scarce and sunshine plentiful, this could help solve water scarcity while at the same time providing the country with energy.

By Mansouraboud68 - Own work, CC BY-SA 4.0, Link

The study, which was recently published in the scientific journal  Renewable Energy, shows that by utilizing a number of desalination technologies, seawater desalination can be integrated into a concentrated solar thermal energy plant and would be financially viable at the proposed Arandis solar park in Namibia.

Concentrated solar power (CSP) uses thermal energy from the sun to power a conventional power block that is typically driven by fossil fuels.

According to the study, the proposed 100 Megawatt CSP+desalination plant would cost more to construct than the desalination plant situated to the north of Swakopmund, which operates on grid-powered reverse osmosis. But generating electricity and water at the same plant would offer additional advantages that the current grid-powered reverse osmosis plant cannot offer. For example, it would simultaneously generate electricity while desalinating water, while the current desalination plant consumes electricity — a scarce commodity in Namibia.

Furthermore, it will have the capacity to store thermal energy which will enable it to supply 15% of Namibia's peak energy demand in the evenings, making the country less dependent on its neighbours for its energy supply. This will also help reduce the country's carbon footprint and help it to meet the 70% renewable energy target set for 2030. 

"In 2017, Namibia imported approximately 60% of its annual electricity from surrounding countries. Most is from South Africa's power utility, Eskom, and that's mostly coal," said the research study's co-author, Ernest Dall, who received a bursary from NamPower to complete his Masters in Mechanical Engineering at Stellenbosch University and is currently employed as an engineer in the national power utility's Power Systems Development division.

"I luckily had the option to choose my research topic and I wanted to do something looking at future issues that do not just affect Namibia, but issues that are global, and that is basically electricity and water scarcity, which if you think about it, go hand-in-hand in desalination," he related. "There is a synergy between power generation and desalination of seawater," explained Dall. "The desalination plant effectively replaces the condenser in the Rankine cycle, whilst the Rankine cycle in turn provides a ready source of low grade heat for desalination.The concept hinges on utilizing that waste heat from your CSP plant and trying to use that as a heating source."

The profitability of co-generating both electricity and water is equivalent to that of a CSP plant only generating power.The added cost is minor, about 13%. The solar field would be oversized in relation to the power block, and both operations would be simultaneous.

"You can do both at once, generate electricity and desalinate water," said Dall. "Because the desalination plant functions as the dry cooled condenser - the one is dependent on the other. One in fact cannot function without the other one unless there's a redundancy added in the system."

According to Dall, revenue from water sales would subsidize the cost of electricity production, as water — which in Namibia is a scarce commodity that is high in demand —  fetches relatively high prices. 

"Water is considered a precious commodity in Namibia with prices seen to rise in the past resent years. We experienced a drought now for a few years and I don't believe that water prices will decline anytime soon." CSP+desalination could be attractive in similar water-stressed nations, Dall said.

"Feasibility would depend on various boundary conditions like local water and electricity tariffs, solar resource, demand, etc," said Dall. "Whether water is more or less expensive relative to the price of electricity. This would affect how you would optimize the CSP+desalination plant."

The Namibian feasibility study estimates it would cost 50-100% more to desalinate water at the inland CSP+thermal desalination than it would at the coastal grid-powered desalination plant, with the 48km distance from the sea and the height above sea level being a big factor due to electricity consumption associated with pumping costs.

"The pumping is quite significant, because of the distance and height difference. It is 12 MW, which represents quite a lot of the electricity generated from a 100 MW CSP plant," Dall said.

However, solar power generated by low cost solar PV panels could run the pumps at minimal cost.

"You could oversize your pumping facility so that you could just pump during the day with PV. I think that could potentially bring the cost down but I've not looked into this," he said, adding, "maybe that is something that someone else can do." 

Journal Reference
J.E. Hoffmann, E.P. Dall, Integrating desalination with concentrating solar thermal power: A Namibian case study, published at Renewable Energy (Jan 2018).