Combining the economics of hydropower with the flexibility of batteries

RayGen started life in a Melbourne garage back in 2010. It was the brainchild of Dr John Lasich, who by that point had already launched, grown and sold one renewable energy company after completing a PhD on concentrated solar systems. 

For the first eight years of its life, RayGen was an innovative but relatively conventional solar energy start-up. Dr Lasich and his ever-growing team leveraged their sophisticated engineering and manufacturing expertise to create incredibly compact and efficient photovoltaic modules. Modules designed to be capable of converting concentrated beams of sunlight - beams powerful enough to melt steel - into electricity. The other proprietary technology RayGen's engineers developed was control software that allowed mirrors (heliostats) to autonomously track the sun then shine a concentrated beam of sunlight onto the aforementioned modules. These modules are placed in a panel - one that is 4000 times more powerful than a standard solar panel - mounted on top of a tower. 

Heliostat mirrors can track the sun

An ETES revelation 

Up until recently, RayGen's sales pitch revolved around its ability to generate maximum megawatts in minimum space. A RayGen solar farm uses technology known as PV Ultra. PV Ultra generates 1MW of energy with just four square metres of panels, a tower and a field of mirrors. To put it in more concrete terms, the solar modules needed for a RayGen 1GW solar farm (one capable of powering roughly 200,000 suburban homes) could fit comfortably in 12 shipping containers. In contrast, it would take 24,000 shipping containers to house the solar panels required for a conventional solar farm. 

Proprietary PV Ultra technology provided enough of a competitive advantage for RayGen to get two successful pilot projects - one in China and one in Australia - up and running in 2015 and 2018. But RayGen still faced two issues. 

First, intermittency. Second, a whole lot of wasted heat energy as a result of concentrating sunlight. (Remember what inevitably ended up happening when you played around with a magnifying glass on a sunny day as a child?) 

It wasn't immediately apparent that there was a relationship between those two problems. So, like many renewable-energy start-ups before it, RayGen looked into lithium-ion battery storage to address the intermittency issue. 

Despite all the progress that's been made with batteries in recent years, Dr Lasich and his team concluded the cons (i.e. expense and substantial e-waste) still outweighed the pros. Some sort of electro thermal energy storage (ETES) seemed like a better fit. Especially given RayGen's solar had an abundance of hot water as a result of a need to prevent the aforementioned modules overheating. However, there was no off-the-shelf ETES solution suited to RayGen's unique solar-farm set-up. RayGen's engineers had to work out how to mix and match technologies from all around the world. Over the last two years, RayGen has been doing this in order to create what they describe now as a 'Solar Power Plant'.   

RayGen's Newbridge site in operation in Victoria.

Fitting the pieces together       

With backing from heavy hitters such as Australian electricity and gas company AGL and Dutch multinational Photon Energy, funding from the Australian Renewable Energy Agency, and advice and equipment supplied by, among others, a Turkish heat-to-energy engine maker and a Danish engineering consultancy, RayGen is about to start work on a world-first facility. One that will combine a solar farm with a tailored electro-thermal storage cycle that will generate 4MW of solar electricity paired with 3MW/50MWh of reliable, low-cost storage. This facility is being built near Mildura, Victoria - a part of Australia that has lots of sunlight but a small population - and is scheduled to be commissioned sometime in 2021. 

As well as a PV Ultra solar farm, RayGen will also build two agricultural-style dams or 'pits' - one for chilled water and one for hot water. These pits will be about 16m deep with slanted sides and resemble small mine pits. They will be insulated with rubber and essentially be sealed dams (leaving aside some inflow and outflow pipes). It's a closed-loop system, so there is no water loss. 

These pits can store thermal energy for months at a time with minimal energy losses. In Denmark - where pit thermal energy storage was pioneered in the 1990s as a solution for clean, low-cost space-heating - pit stores are heated using solar hot water collectors in summer and discharged into district heating systems six months later, in the depths of winter. Over that half-year between heating and discharging, more than 90 per cent of the thermal energy is retained in the pits.  

The pits work together by forming the heat source and cold sink needed to drive a heat engine. In the hot-water pit, one pipe will draw 70 degree Celsius water from the bottom of the pit and send it up the PV Ultra tower to cool the modules at the top. That water, having been heated to above 90 degrees Celsius as it passes across the modules, will then be returned to the top of the pit. In the cold-water pit, electricity produced by the solar farm will be used to keep the water cooled to near zero degrees Celsius. (This is done with chiller technology commonly used in industrial refrigeration.)

When on-demand electricity is required, the hot and cold pits are discharged. The hot and cold water will be pumped into an Organic Rankine Cycle (ORC) engine via heat exchangers, where the water will evaporate and condense ammonia in a cycle, causing a turbine to spin. Spinning the turbine and generator converts the thermal energy to electricity that can be dispatched into the grid on demand. 

After the water exits the engine cycle, the hot water will have cooled to around 70 degrees Celsius and is returned to the bottom of the hot pit. The cold water will have heated to 15 degrees Celsius and is returned to the top of the cold pit. Diffusers are used in the pits to minimise disturbance to the thermally stratified water.

To summarise, using little more than water, mirrors and sunshine, this A$25 million facility will be able to feed low cost solar electricity into the grid during the day and equally low cost hydroelectricity to it during the night. RayGen estimates that they can create electricity at a price point - approximately A10c/kWh - substantially lower than existing battery projects. 

Schematic to show system layout

Hydro's new frontier? 

When hydro is mentioned people automatically think of pumped hydro. This is perhaps nowhere truer than Australia, where the Snowy Mountains Scheme - a post-war, nation-building initiative involving sixteen major dams, seven power stations and 225km of tunnels, pipelines and aqueducts - is widely celebrated. Hydro is undoubtedly an attractive source of renewable energy. However, it requires a waterway, public support, political will and deep pockets to construct one or more dams.  

RayGen's Solar Power Plant doesn't use traditional hydropower, but it does use water to store potential power for a long duration. RayGen's 'thermal hydro' pits can be dug almost anywhere relatively cheaply. And a temperature difference of 90 degrees Celsius (i.e. the difference in temperature between the hot-water and cold-water pits) in a thermal hydro system generates the same results as a height difference of 1000m does with pumped hydro. 

Confident

At this juncture, I feel duty-bound to point out that while all the different components of RayGen's system have been individually proven, it's yet to be demonstrated that they will work when combined. 

That noted, there's good reason to believe they will and it's not only RayGen that is betting heavily on solar plus thermal hydro being a success. More on that momentarily, but first a little background on the achievements of RayGen and its suppliers. 

RayGen is confident that the marriage of solar and thermal hydro has game-changing potential. Not just in regional Australia, but also in other sparsely populated places with good solar resources, such as Arizona and China's Gansu Province. 

All images copyrighted to RayGen and used with kind permission for the purposes of this article.