Guest Blog by RayGen with contributions from Derek Scott – Engineering Manager, Neil Faragher – Senior Product Development Engineer and Zoran Lasich – Process Development Engineer.
RayGen’s vision is to accelerate the global transition to renewable energy with innovative solar infrastructure projects that provide electricity, heat, cooling and desalinated water - all day and night.
RayGen’s Solar Power Plant technology utilises electro-thermal energy storage (ETES) combined with RayGen’s unique ‘PV Ultra’ concentrated PV technology that enables cost-effective solar power-plus-storage. This technology co-generates electricity and heat, feeding energy from sunlight into an electro-thermal storage at an energy efficiency of nearly 90% (one-third electricity and two-thirds heat). By using by-product heat, energy can be stored with an electrical round-trip efficiency of 70% – delivering the energy efficiency of pumped hydro energy storage with far fewer siting constraints.
RayGen recently received funding from a mix of private investors and public funding from Australian Renewable Energy Agency (ARENA) to build, commission, and operate the RayGen Power Plant Carwarp (RPPC), which will add 4MW of solar and 3MW / 50MWh (17 hours) of storage to the West Murray grid. The RPPC project will save 10,000 tonnes CO2 emissions per year and provide renewable day/night electricity to approximately 1,000 homes. It also acts to demonstrate the PV Ultra and Thermal Hydro storage technologies for larger projects in the future.
What makes this technology unique and what challenges does it provide?
This project will be the only power plant of its kind in the world to use concentrated light focused on a central receiver to generate electricity using photovoltaic (PV) cells, and collect thermal energy from cooling the PV modules to create dispatchable power. This provides renewable energy that is available on demand, when the sun's not shining or the wind's not blowing.
Components of the technology such as the Organic Rankine Cycle (ORC) engine and thermal energy storage are proven technologies used around the globe for other applications such as geothermal power systems and European district heating systems. The primary challenge currently facing the engineering team is to integrate these technologies with RayGen’s PV Ultra system.
A specific area that required development involves the heat storage system. This is an insulated pit dug into the ground where it is essential to maintain stratification of the hot water stored, with the buoyancy forces driving the hottest water to the top while the cooler water remains at the bottom of the pit. One of the engineering challenges is in how to design the diffusers for pit inlets and outlets to help maintain a stable temperature difference within such a large thermal storage pit (with a vertical separation of 11 metres from top to bottom between the diffusers).
An initial assessment was done using the Froude number, a ratio between buoyancy and inertial forces, to arrive at an overall size for the diffuser. From this baseline, Ansys CFD was used to investigate the finer details and optimise the design to prevent disturbance of the boundary zone between the different water temperatures within the pit (known as a thermocline) and maintain thermal stratification.
The simulations involved prescribed flow rates of 300 kg / sec and a zero gauge pressure at the boundary. Parametric changes to the diffuser design were investigated, relating to various detailed concepts including baffles and flow screens.
Post-processing of the CFD results was done to compare the velocity profiles at the boundary of the diffuser itself, as shown in the image below. Each diffuser design was assessed for impacts on the uniformity of the fluid flow further out into the pit, aiming to reduce turbulence and avoid generating certain flow features such as reversed flow across this boundary. Through many design iterations the turbulence has been reduced and the peak velocities kept low at the diffuser outlet.
Initial design (top) and optimised design (bottom) showing flow velocity vectors at diffuser boundary.
The upper image in the figure below shows an early design candidate where there are turbulent flow paths and reverse flow around the bottom edge. Below that is a near final design showing significantly reduced the peaks and more uniform flow.
Flow paths for initial design (top) and optimised design (bottom).
We’re very happy that CFD has enabled us to go from our initial baseline design, which tended to produce a strong jet of water through one side and produced unwanted reversed flow across parts of the diffuser area, to our current design which we’ve delivered in just 4 weeks of simulation work and has an improvement in flow uniformity of 30% and no reverse flow.
Simulation has proven to be an essential tool because to build something of this size just to test it, in a tank or even a scale model, would be time and cost prohibitive. Physical prototyping at the scale of this device is difficult and would likely take many months for even a single design as the diffuser designs are over four meters in diameter, so we have really had to rely on the use of simulation during this engineering design stage. In addition, the post-processing of CFD simulation results provides a wealth of data and extra insights that you may not be able to obtain using a physical test.
Our team also continues to use Ansys simulations for both FEA and CFD analysis of the thermal and stress distributions with the PV module.
Prior to starting this project, the team had very limited experience using Ansys. We completed a full week of training with LEAP to help us get started, followed by online sessions during COVID to keep us on track as our engineering priorities changed.
Our future plans involve much larger scale deployment of this technology, which is the ultimate goal of RayGen. This first four-megawatt project that we're building currently is the stepping stone to much larger projects in the 100 MW plus scale. At this scale, we also intend to use simulation to help define the sweet-spot combination of larger and more numerous pits and whether single or multiple diffusers perform optimally.
“LEAP’s engineers have been very helpful. Their response time to any technical questions is really quick and we’ve valued their advice as our use of simulation has grown on this and other projects”. Zoran Lasich, Process Development Engineer, Raygen.
Find out more about RayGen’s exciting Solar Power Plant technology at https://raygen.com/
