Model performance for sustainable agriculture company

Frazer-Nash Consultancy’s Anthony Kwong describes how modelling and analysis helped sustainable agriculture company Sundrop Farms to justify its investment in energy-storage equipment.

Taking investment decisions about expensive new equipment is never an easy task — even when you’re buying existing technology. Thermocline tanks, for example, which store heat energy through the stratification of less dense hot fluid above denser cold fluid, can represent an attractive solution for the renewable sector. However, they do so at a hefty price: in the region of 1 million euros (approximately AU$1.48 million) for a 10,000 m³ tank, according to one study^[i] of typical water tanks intended for large-scale thermal energy storage.

The advantages that thermocline tanks can offer to renewable businesses, however, are manifold. They can be used to satisfy variations in energy demand across the day, reducing energy bills; and, as solar thermal technology becomes more affordable, can provide long-term, seasonal heat storage in a variety of situations — from supplying domestic hot water to heating huge commercial greenhouses. They are also cheaper than more basic, two-tank solutions.

But while thermocline tanks have many advantages, the high capital outlay required for their purchase means that business lenders can view investment in the tanks as a source of risk.

Supporting Sundrop Farms’ sustainability goals

In its recent role as owner’s engineer to Sundrop Farms, Frazer-Nash Consultancy helped the sustainable agriculture company to assure the potential performance of its thermocline tank at the design stage, through computational fluid dynamics (CFD) modelling.

Sundrop Farms grows fresh fruits and vegetables hydroponically, using mainly renewable inputs, minimising waste and building its climate and irrigation-controlled greenhouses in inhospitable areas where traditional horticulture isn’t feasible. With global food demand predicted to increase by 50% by 2050, plus increasing climate change and water scarcity, Sundrop believes that existing farming practices are unsustainable and thus that sustainable production methods offer agriculture’s future. The company grows more than 17,000 tonnes of tomatoes each year, 15% of the total Australian tomato market, and delivers year-round yields that are claimed to be 15–30 times higher than conventional field production.^[ii]

Sundrop’s 20 ha greenhouse at Port Augusta, South Australia, uses concentrated solar power to desalinate seawater for irrigation, to generate electricity and to provide greenhouse heating. Its solar thermal infrastructure includes more than 23,000 ground-based heliostats, which collect and reflect the sun’s rays onto the top of a 115-metre-high solar tower. The heat generated from the solar tower is stored in a 22,000-cubic-metre thermocline tank next to the tower. Closed loop internal piping systems circulate this water to either heat the greenhouses or to drive a multi-effect distillation plant to generate fresh water from a seawater supply. The excess steam generated by the solar thermal system can also be used to spin a turbine, powering a generator that can produce up to 1.5 MW of electricity to run the site, to complement the peak heat production rate of 39 MW^[iii] from the solar thermal plant and avoiding the emission of up to 16,000 tons of CO² annually^[iv]. Sundrop required independent assurance that the single 22,000 m³ thermocline tank proposed by the plant designers would meet its operational needs — a more technologically advanced heat storage solution than the traditional, but more expensive, option with separate hot and cold tanks — as this type of tank had not previously been used in this way.

A model solution

Frazer-Nash’s CFD model simulated the planned operations of Sundrop’s thermocline tank, including the injection of hot and cold water into the tank through radial diffuser tubes, and looked at the effect this had on the thermocline layer — the layer above and below which the water is at different temperatures. With around five million cells, the numerical model was refined around the inlet and outlet tubes to ensure that the effects of the high-velocity flow near the holes would be captured sufficiently without compromising on the runtime required for the simulation. The expected temperatures over a four-hour charging period were predicted using the model.

Through examining and assessing the tank’s performance during the design stage a number of aspects of the design could be evaluated — for example, whether the injection system minimised the potential convective and turbulent mixing effects. The CFD modelling was able to capture the complex geometries and flow features that define tank performance. It showed that near the top of the tank (above the injection holes), the water temperature would rise gradually over time, and that as the hot water filled the tank, the thermocline layer would develop to a thickness of about 2 m. After two hours, the shape and thickness of the thermocline would be largely ‘frozen’ as it descended further into the tank. The simulation predicted that strong mixing could occur near the injection holes, partly caused by convection and turbulence due to the water ‘jets’ and partly by the unstable stratification that resulted from the injection of hot fluid. As the layer of hot water penetrated down the tank and the thermocline layer moved further away from the injection zone, the mixing effect was shown to reduce, with the heat transfer mainly limited to conduction.

The CFD modelling took about three days to compute a four-hour charging cycle on a high-performance computer, showing the tank operation simulated would perform satisfactorily and meet Sundrop’s needs. Even a high-fidelity CFD model would have taken nine years to simulate a six-month operation, so an analytical model was then created — based on the CFD results — and without loss of generality, was able to quickly and reliably predict the long-term performance of the tank over a six-month operation period. Finally, the models’ predictions were validated, by being compared with Sundrop Farms’ commissioning data — the measurements from the tank after it had been purchased and was in operation. This data showed a good agreement with the analytical model, with the tank actually performing slightly better than predicted. As well as validating the modelling, this increased confidence in the predictions it had made about long-term tank performance.

CFD could also be used to model a range of fault scenarios: for example, if temperature inversions within the tank were likely to cause rapid mixing of the surrounding fluid. It would be possible to examine, on a case-by-case basis, the effects of unavoidable temperature changes during operation and how this would impact on the tank’s performance. In addition, the method developed could be applied generally to any tank and injection mechanism, and on other fluids including molten salt.

Using modelling and simulation helped Sundrop Farms reduce the risk of its investment decision and confirm that its thermocline tank purchase would meet its needs. By validating that a single tank would perform the necessary operation, the company was able to avoid having to use a more expensive two-tank solution, saving it money. Modelling offers an efficient way to realistically quantify the performance of a thermocline tank — or, indeed, a range of other equipment used in the renewable sector — without having to resort to gazing into a crystal ball.

By providing a useful evaluation of potential performance during the design stage — when changes can be made to mitigate design issues that might affect performance — modelling and simulation help companies avoid expensive reworking or modifications in the later stages of their projects.

Dr Anthony Kwong is a Principal Consultant at Frazer-Nash Consultancy’s Adelaide office, and a Fellow of Engineers Australia. He completed a PhD at Cambridge University prior to joining Frazer-Nash, and his main areas of interest include thermal/fluid dynamics and acoustics. Over his 20-year career with Frazer-Nash, Anthony has worked closely with the UK nuclear industry. Since moving to Australia in 2013, he has also worked on a number of renewable energy projects, including Sundrop Farms.

[i] IEA-ETSAP/IRENA (2013) mentioned a number of systems in Germany, most of them consist of a 5000 to 10,000 m³ water storage tank, at an investment cost between 50–200 euros/m³.

[ii] Figures from www.sundropfarms.com.

[iii] ‘Sundrop Farms pioneering solar-powered greenhouse to grow food without fresh water’ from www.abc.net.au/news/2016-10-01/sundrop-farms-opens-solar-greenhouse-using-no-fresh-water/7892866 (interview with Sundrop Farms’ head grower Adrian Simkins).

[iv] Figure quoted in www.greenindustries.sa.gov.au/sa-companies-already-working-towards-a-circular-economy.