Solar energy for desalination

Solar Energy for desalination – Cranfield’s contributions cover both small-scale and large-scale systems

The Concentrating Solar Power (CSP) sub-team within the Centre for Renewable Energy Systems at Cranfield is working on seawater desalination using strong direct sunlight.

According to the UN, water scarcity affects more than 40 per cent of the global population and is projected to rise. 800 million people do not have access to clean water and over 1.7 billion people are currently living in river basins where water use exceeds recharge. One child every two minutes dies due to a lack of safe drinking water (Water Aid).

Current desalination plants use fossil fuels to create electrical power to pump seawater through membranes to extract the salt in a process known as reverse-osmosis. Cranfield’s designs use heat from strong direct sunlight to evaporate and then distil seawater. A schematic of a small-scale system (up to 100L/day) is shown in Fig 1, part of a British Council funded project to provide drinking water in Palestine and Jordan, primarily aimed at refugee camps and disaster zones.  The systems uses plastic linear Fresnel lenses as solar concentrators and a blackened steel tube absorber. These track the sun around two rotational axes. The heat is transferred via a fluid into the evaporation chamber where the seawater evaporates. Distillation takes place in an adjacent chamber. A thermal storage option is included so that the unit can function in the absence of sunlight.

Fig 1: Small-scale desalination unit for emergency survival situations

Cranfield is also working with the UK Company “Solar Water” on a much larger scale desalination system, utilising staff from both the Centre for Renewable Energy Systems and the Centre for Thermal Energy Systems and Materials. This project began life as an MSc Group Project, as shown in Figure 2a, where the student’s work accelerated the progress towards the design of a prototype, shown on this YouTube video.

This work was part-funded by the EU Horizon 2020 programme, and partly by Solar Water themselves, with Cranfield University leading the technical design work. In this system, designed to produce up to a million litres of fresh water each day, saltwater is evaporated in a large (up to 80m in diameter) geodesic dome constructed primarily from glass and steel. The evaporation rate is accelerated by the addition of concentrating solar radiation. This can be provided both by heliostats which redirect sunlight towards the dome, but also from a concentrating solar field adjacent to the dome. The heliostats have 2-axis tracking with control systems to ensure that they are always in a position to reflect sunlight towards an appropriate section of the dome. The additional solar field also collects sunlight, using solar collecting silvered-glass mirrors which concentrate solar energy onto an absorber and subsequently into a heat transfer fluid (oil or molten salts are commonly used). This heat is then piped into the seawater within the dome to provide additional heating. In order to maximise the production, it is also advisable to separate the evaporation of seawater from the distillation of the potable water. In this case, a separate condenser section is connected to the geodesic dome. A prototype of this system is planned to be built early next year. If the system takes off commercially, these large desalination domes could become a familiar feature in the hot and sunny water-stressed regions of the world.