The glass industry is working on ways to reduce the weight of mirror assemblies and increase solar reflectance.

By CSP Today staff writer

A high level of sophistication is required to produce low-iron solar glass for the CSP sector.

Glass manufacturers must control the supply chain in order to achieve repeatability, proper specification and viable cost.

They admit that the main challenge has been developing the ability to produce low-iron solar float in each region of the globe.

In order to accomplish this, a company like Guardian, a manufacturer of float glass and fabricated glass products, says it has had to overcome challenges with raw materials, float design and logistics in order to become a full-service CSP provider.

The challenge for the glass industry is to advance the technology, reduce the weight of the mirror assemblies and increase solar reflectance, said Scott Thomsen, chief technology officer and group vice-president for Guardian Industries Corp’s Science & Technology Centre.

Saint-Gobain Solar, which has been in the CSP business since the 80’s, says till today there is no standardisation at all in the CSP market with regards to mirrors.

“There is a need to educate customers and this can only be done by going through a standardisation process,” said Rafael Sanz de Acedo Hecquet, VP Sales and Marketing, CSP Market, Saint Gobain Solar. The company provides mirror for any technology, ranging from less than 2mm up to 4mm.

Another advance will be the incorporation of first surface mirror technology on glass to further increase solar reflectance.

To date, metal mirrors have been demonstrated on a prototype basis but have yet to be installed in CSP fields, according to Thomsen.

The main improvements that the industry is developing currently are better quality raw glass and low-iron ultra-clear glass.

Jordi Villanueva, Rioglass Solar’s sales manager, international sales division, said the focus was on new bending methods and equipment that allows higher accuracy of shape, repeatability and a fastesr production cycle. He also referred to the emergence of new, top-quality paints with reduced lead and copper content.

Modes of production

Parabolic mirrors made from glass can be ‘single block’ (one single piece of glass) and ‘laminated’ (two pieces of glass ‘glued’ together). Laminated mirrors can initially offer higher reflectivity values but in the long term durability becomes an issue.

Villanueva said that within the single block mirror group, the strength of the glass had three range levels:

 Annealed: Regular raw glass that comes out of the float factory.

Heat strengthened: In between an annealed glass and a tempered glass.

Fully tempered glass: Maximum strength for the glass.

Each of these strength levels has shown exactly the same reflectivity values; therefore the only property affected when tempering is mechanical strength.

As far as glass production costs are concerned, they are affected more by the drive for high solar transmission rather than reduction of stress in the glass substrates.

Tailored offering

The main factors when designing a mirror that will perform as a high precision reflective tool are focal length (distance from the mirror surface to the ‘target’), intercept dispersion (how the light has to be spread over the target) and energy and reflectivity values.

Manufacturers have come up with an offering tailored specifically to CSP technology.

Guardian’s CSP glass products are designed to match the supply channel requirements of each unique CSP technology. For example, for certain applications the company has developed a 0.95mm solar float that has uniform thickness, flatness and ease of fabrication.

Durability and reflectivity

Applications that utilise a monolithic piece of glass require low-iron solar float with a thickness of 3mm to 5mm. Laminated mirror applications require a low-iron solar float with a thickness of 0.95mm to 2mm to achieve the desired performance.

The key requirement in creating a high-reflective mirror is the iron content in the glass, which directly affects solar transmission.

“The glass is so critical today because the industry relies on second surface mirrors to retain solar reflectance,” said Thomsen.

Rafael Sanz de Acedo Hecquet of Saint-Gobain Solar Glass said reflectivity, durability and precision were of paramount importance. For the parabolic trough market, the company supplies tempered mirrors.

“Tempering of the mirrors ensures a high resistance to heavy wind loads but it also leads to safer operation and maintenance,” said Sanz de Acedo , whose company produces its own glass.

Rioglass Solar claims to be the first parabolic mirror manufacturer to offer a fully tempered product.

“That’s our main difference with regard to our direct competitors,” said Villanueva.

This not only reduces the breakage of mirrors in the field, but most importantly allows the plant to stay operational under higher wind loads; which normally would cause the collectors to turn into stow position. This increases the overall efficiency of the plant and therefore reduces the cost of the kWh produced, making the project more viable.

Rioglass Solar offers the following warranties – reflectivity: 92.5 percent or greater; accuracy: 99.5 percent of the mirror surface to intercept on a 60mm absorber tube. Breakage and corrosion are independently negotiated with each customer.

The technology and component standardisation process for such offerings in the CSP sector can be looked upon in two ways.

On the one hand such standardisation allows the component manufacturers to focus on a limited product range and therefore reduce costs. On the other, that same standardisation could become an obstacle to technology and efficiency improvements, since the cost of these ‘prototypes’ would be much higher (at the beginning) than a standard product.

Testing times ahead

High solar reflecting mirrors can be produced using mirrors of 4mm to 5mm thickness or thin glass (1mm). The thickness is dictated more by CSP technology than by glass fabrication or production. The same mirroring processes are typically used whether thick or thin glass mirrors are produced.

First surface mirrors with a protective oxide topcoat do have potential in the CSP sector. However, only significant durability testing and field evaluation will determine whether they will survive 20 to 30 years in the field.

Read more: mirrors, glass, surface, solar reflectance