Optimising design for hybrid CSP-biomass plants

The recent retrofit of the 14MW Falck biomass plant in Italy to incorporate CSP to make it a hybrid biomass-CSP project raises the question. When might it make sense to retrofit a CSP plant to be a hybrid CSP-biomass plant?

Desert regions in the Middle East, and North and South America all have good DNI for CSP, but no spare biomass, so hybridising CSP and biomass makes little sense there. But Italy, Spain, the South of France, Turkey, India and Australia are among areas wit

By Susan Kraemer

To get an idea of the challenges involved and the advantages to be gained in hybridising CSP and biomass in a retrofit, CSP Today talked with researcher Juergen Peterseim, who has 10 years of experience with Germany’s ERK Eckrohrkessel GmbH in optimisation design for cogeneration, waste and hybrid energy systems, ranging from 5MW to 200MW.

Peterseim recently submitted his PhD thesis on CSP-biomass hybrid plants, which included a 35.5MWe CSP-biomass hybrid plant case study in Queensland, Australia.

He received funding for this case study from the recycling firm Thiess Services (now Remondis).

The obvious advantage of hybridising CSP with biomass energy is that such a plant could operate day or night, and even in prolonged periods of cloudiness.

But it turns out there are many other surprising advantages to biomass hybrids.

To increase cycle efficiency

CSP trough plants normally operate at lower temperatures - thus lower efficiencies - than CSP power tower plants. But if hybridised, a trough CSP-plant can be designed to run at higher temperatures so its efficiency rivals a power tower.

“Borges was the first plant to combine biomass and solar at power plant scale; 25 MW of capacity, and it uses in a pretty clever way.” says Peterseim.

“The solar thermal plant produces saturated steam, and the biomass is used to increase the steam temperature to 520° C - a temperature that normally a parabolic trough plant couldn’t get to.”

Only small changes needed

The only difference between a CSP plant and a biomass plant is the boiler. Everything else remains the same.

Normally a CSP plant includes a small natural gas fired boiler anyway; for start-up purposes, and to guarantee a controlled shutdown. So, in itself, a biomass boiler is not a real addition.

Of course a biomass fired boiler does have some added complexity, as it requires a fuel storage facility and must be designed to prevent particulate matter entering the air. And there are customisations needed for each type of biomass.

Optimise boilers for different fuels

To optimise the boiler, there are several caveats. You need to know which fuel you are burning.

”You can design for multiple feedstocks or multiple fuels in one boiler but that usually comes with a bit of a penalty,” he explains.“ In a boiler design where you have to accept 10 fuels, you have to design for the worst one.”

So first you must know which type of biomass is available. Regional availability would determine fuel source. Desert regions in the Middle East, and North and South America all have good DNI for CSP, but no spare biomass, so hybridising CSP and biomass makes little sense there.

But Italy, Spain, the South of France, Turkey, India and Australia are among areas with both good DNI for CSP, and good biomass resources, from horticultural waste like straw for wheat farming, to urban construction and demolition timber.

Overlaying geospatial modelling of DNI with biomass resources reveals the optimum locations for both.

Match biomass fuel to CSP type

The choice of biomass fuel is also determined by CSP technology, because of the differing temperature requirements of Fresnel, dish, parabolic trough or power tower CSP.

For a trough plant - with its mid-range temperatures, a boiler should be designed to deal with biomass sources that yield mid-range temperatures. One fuel to consider might be construction demolition timber.

“Generally, steam temperatures from construction demolition timber are lower compared to clean wood chips because of high temperature corrosion issues inside the boiler,” Peterseim explains. “And so you have to limit the steam temperature to something like 450°C.”

“But to match the higher temperature needed for solar power tower, the biomass needs to generate higher steam temperatures. If you use clean wood chips you can get temperatures of say 540°C,” he relates.

“Some fuels can’t reach power tower steam temperatures. But a lower temperature might be good enough for trough and Fresnel installations.”

Falck CSP-Biomass hybrid plant

The new CSP-biomass hybrid Falck plant just announced in Cambria, Italy is an example of a low temperature hybrid that does use Fresnel CSP technology.

The Rende Integrated Solar Combined Cycle plant operates at 300°C and the biomass fuel is virgin wood, according to spokeswoman Alessandra Ruzzu.

This is a retrofit of an existing 14MWe biomass plant that was originally built in 2000, by converting it into a hybrid biomass-CSP plant.

The plant received a major upgrade in 2010, and has since been converted to a hybrid by attaching a small (1MWe) patented Fresnel-type CSP component, supplied by Falck partner Elianto.

Ruzzu conveys that in addition to using the heat from both the burning of the biomass fuel and the concentrated solar thermal power from the Fresnel CSP system, this project also captures the non-recoverable waste heat originating in the biomass plant.

How best to retrofit trough?

Retrofitting an older renewable project to a hybrid with another renewable like the Falck plant does make sense. But Fresnel plants are relatively few. What would be the optimum way then to retrofit an older parabolic trough plant to make it a CSP-biomass hybrid?

“Older trough plants are designed and built for 380° C steam,” Peterseim cautions. “So you cannot increase temperature. The steam turbine could not cope with higher temperatures.”

“So in a retrofit you wouldn’t use biomass to increase the steam temperature, like the Borges plant; which was designed from the start to handle higher temperatures,” he explains.

“Turbines can’t cope with higher temperatures than what they are designed for. But you could add a biomass boiler for steam at the same temperature.”

That way you can continue to run the parabolic trough plant, even without sunshine, and even when the storage is empty.

One contender for such a retrofit might be the original SEGS project built in the 1980s in a farming area of California, now negotiating its second round of 25-year contracts with utilities on a grid that has become over-saturated with daytime solar - creating a greatly increased demand for coverage of the new peak load in the evening.

“We are still trying to negotiate a deal that would make the most beneficial use of the site and its resources,” says Brad Bergman, General Manager at Cogentrix Energy.

“The amount of existing solar capacity does impact the utilities’ relative need for more solar power, but it’s more of a timing issue, as to when they have a need for additional solar capacity.”

A retrofit can be fairly inconsequential and really make sense, because the power block is the same for both CSP and biomass plants, regardless of biomass fuel type and form of CSP.

The benefit of such a retrofit is being able to operate more hours each year, and since the power block is used for longer hours per year; it has now become a more efficient investment.

In that sense, from an investment point of view, a hybrid is more efficient than a pure CSP plant.

To comment on this article, please contact the author, Susan Kraemer.