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Surrey research could lead to more efficient, cheaper solar energy

Chemical Engineering researchers at the University, collaborating with colleagues in France, Spain, Belgium and Switzerland, have discovered a way of improving indirect solar energy plants.

Photo: Torresol Energy

The research has been carried out by academics from the Department of Chemical and Process Engineering and is a key finding in the EU PF7 ‘Concentrated Solar Power in Particles’ project. The project was led by the French national research centre CNRS (Centre national de la Recherche Scientifique) with the Surrey contribution being carried out by Professor Jonathan Seville, Dean of the Faculty of Engineering and Physical Sciences, and Dr Pablo Garcia-Trinanes.

The research team has discovered that carrier particles, which transfer and transport energy from the heated walls of solar receivers, move and behave in a distinctive way. This has enabled them to pinpoint the optimum conditions for transferring heat, making the process of utilising solar energy faster and more efficient.

In indirect solar receiver systems, the sun’s rays are collected by arrays of multiple mirrors and focused into a solar receiver where they heat up a heat transfer fluid (HTF), which can in turn exchange its heat to produce steam to drive turbines for electricity generation. Large indirect solar plants already exist in Spain, Southern France and North Africa, with mega plants of 580MW capacity currently being built Morocco.

However conventional HTFs, which are based on molten salt, only operate in a limited temperature range and have problems of degradation and corrosion. The main aim of the EU project has therefore been to develop a specialised HTF which overcomes this limitation, in the form of a dense gas-particle suspension. The contribution of the Surrey researchers has been to use a positron-emission tracking technique to find out how the carrier particles behave within the arrays of parallel tubes which make up the solar receiver.

“We’ve been able to successfully determine the particle motion inside tubular solar receivers,” said Dr Garcia-Trinanes. “Gas/particle suspensions are notoriously unstable. Using this unique radiation-based tracking technique we can see exactly how individual particles behave and so optimise the operating parameters for the system. For example, increasing the suspension gas flow causes more particle motion and so increases the heat transfer from the walls, but too much gas flow ‘blankets’ the walls and reduces the amount of heat transfer which can be achieved.”

“I’m thrilled that we’ve been able to clarify the particle motion, mass transfer rate and time close to the wall region inside solar receivers. We hope that our findings will enable more and more companies to ensure that their carrier particles transfer more energy and speed up the heat transfer process and, of course, the end-user to benefit from cost-effective solar energy”.

Professor Seville commented, “We are proud to have been able to contribute our knowledge and expertise in heat transfer technology to the FP7 project, and to have worked with our partners to achieve this very important breakthrough.”

The project partners include eight highly ranked public research organisations and universities, including the University of Birmingham’s Positron Imaging Centre, and three private companies.

The EU FP7 ‘Concentrated Solar Power in Particles’ European Project, which began in 2012, will conclude on 19th November 2015.

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