By Xavier Lara
‘Bigger is better’ is a good rule of thumb when planning renewable energy projects. On wind farms, bigger turbines produce more energy. With PV, bigger panels can pack in more power per string. And plants tend to benefit from economies of scale, too.
Thus, a 120 MW project might be more cost effective than a dozen 10 MW projects, because you only pay for one permit, one grid connection, one engineering, procurement and construction contract, and so on. In general terms, this holds true for CSP as much as any other renewable technology.
In fact, CSP plants possibly benefit more than most from economies of scale because, in general terms, larger projects can collect more heat and achieve higher efficiencies. But size in CSP is also more nuanced than in many other types of renewable energy.
One important point, for example, is that CSP cannot be deployed as a distributed energy resource in the same way as rooftop PV or small-scale wind. It is not that small-scale modular CSP technologies do not exist—they have just proved difficult to commercialize.
There is a small-scale CSP technology called dish Stirling that showed promise a decade ago, for example. But it didn’t have energy storage and could not compete with PV for low-cost, distributed energy production.
Around the same time, another CSP technology, linear Fresnel, also had potential for relatively small-scale deployments. It was more successful than dish Stirling, but ultimately also failed to gain a foothold in the market. Today it is restricted to a few legacy projects.
That leaves the two CSP technologies commonly used today: power tower and parabolic trough. In parabolic trough plants, solar energy is collected in parabola-shaped reflectors and focused onto a receiver tube that runs the length of each collector.
In theory, this technology could be built on an almost infinitely large scale. All that is needed is increasing rows of parabolic troughs and a big enough steam turbine and storage tank.
In practice, however, it is important that the troughs are not too far distant from the balance of plant because otherwise the heat transfer fluid—typically synthetic oil - could cool down on its way to the turbine and storage.
This would increase so-called parasitic losses, as well as operations and maintenance costs. As a result, recent studies estimated that the optimum size for a parabolic trough plant is probably around 200 MW. Getting the size right is also important for power towers.
Like parabolic trough plants, these projects superficially benefit from scale because bulk purchase and manufacturing of components can help bring down unit costs. But very big power towers suffer from a problem.
These plants use mirrors called heliostats to focus sunlight on a single receiver high up on a tower. If the heliostat is too far from the tower, the light can be attenuated by airborne particles such as dust and soot. That reduces the efficiency of the system.
Also, getting heliostats to focus on very distant receivers requires a high level of precision both in the mirror and the control system, which does not help to keep costs down.
Because of this, some researchers believe the optimum size for power towers could be much less than the gargantuan projects that have taken shape in the United States or North Africa, for example.
One study, led by former European solar thermal association president Dr Luis Crespo, concluded that multiple power tower units of around 30 MW each might be preferable to a single massive plant; but it is even possible that it will be lower, on the next wave of modular high temperature towers, with supercritical CO2 cycles.
In both parabolic trough and power tower projects, however, the size of the solar field is just one factor. Another important feature is the size of the power block. In the existing plants, the steam turbines can range from 10 MW up to 200 MW, but this technology too, benefits from economies of scale.
Thus, a 40 MW turbine has less than half the installed cost (per kW of capacity) of a 3.3 MW system. For CSP, power block considerations could constrain most projects to a minimum size of around 50 MW.
Besides these technical considerations, perhaps one of the biggest determinants of CSP plant size is financing. The bigger the plant, the more it will cost in capital terms. And the higher the capital cost, the more difficult it can be to find finance.
Financing CSP plants is complicated by the fact that, unlike PV projects, they cannot be built out yet a modular basis. Whereas you can start generating electricity from PV with just a single array and inverter in place, for CSP you must commission the whole plant in one go.
That somewhat heightens the risk for investors and underscores the role that concessional finance can have in helping bring new CSP plants to fruition.
Somewhat perversely, however, concessional finance works best with very large projects, because of the costs involved in due diligence and getting other sponsors on board. Perhaps because of this, recent CSP projects in developing economies have tended to be very large.
Witness the Ouarzazate Solar Power Station in Morocco, for example. This was, until DEWA Phase IV (under construction), the world’s largest CSP project, at 510 MW, and combines two parabolic trough plants with a 150 MW power tower and 72 MW of PV.
Other major CSP projects built using concessional finance include the 110 MW Cerro Dominador power tower in Chile and the 100 MW KaXu parabolic trough plant in South Africa.
In such projects, concessional finance helps keep funding costs low, which can justify the development of very large plants even if slightly smaller ones might have been technically more appropriate.
What is clear is that thinking about the size of a CSP plant can have a significant bearing on cost and efficiency. That is why it pays to speak to an experienced developer, such as Pacific Green Solar Technologies, before embarking on a project.
At Pacific Green, we not only have the expertise to resolve your technical issues, but can also provide low-cost, high-quality components thanks to our manufacturing alliances in China. And that’s a big deal no matter what size of project you have in mind.