In this blog, the MinWaterCSP project partners Fraunhofer ISE (Germany) and Waterleau (Belgium) will give some insights into their work in the project to achieve water savings in CSP and answer two questions:

  • How to develop treatment strategies in Belgium and Germany for used water from mirror cleaning which is collected in southern Spain?
  • What is zero blowdown technology (ZBT) for cooling systems and what are the chances of this approach for water savings?

How to develop treatment strategies in Belgium and Germany for used water from mirror cleaning which is collected in southern Spain?
Collecting the water after spraying and brushing the mirrors with sophisticated trucks is a big challenge. The mirrors are huge and water drops down through gaps between the mirror segments. This effort of water collection only pays off if the recovered water can be reused. But which treatment is necessary to restore the original water quality for mirror cleaning? For experimental work with particle filters, ion exchange resins, small settling devices or hydrocyclones, several cubic meters of raw water are needed, especially as the concentration of contaminants is low.

While the project partners Ecilimp in Spain and Soltigua in Italy provide mirror cleaning solutions the partners in the MinWaterCSP consortium who are experts in water analysis and treatment are based in Belgium (Waterleau) and Germany (Fraunhofer ISE).

So do we have to transport several cubic meters of water from Italy and Spain to Belgium and Germany? This would not be an ideal solution, as transportation is cost and time intensive. Besides, the water properties would change on their way to Belgium and Spain due to biological growth, temperature differences, degassing and other effects.

Another possibility would be to analyse the water samples or even the dust on the mirrors in detail and mix similar artificial water samples in the labs of Waterleau and Fraunhofer ISE. Their experiences show that working with such artificial water mixtures results in completely different observations than working with the original water.

Thanks to Waterleau, a better solution could be found. Waterleau can provide a small and compact water treatment unit which is a real all-rounder. It includes lamella settlers, activated carbon filters, microfiltration, ultrafiltration, reverse osmosis and ion exchanger units. This compact treatment unit will be transported to Ecilimp (Spain) in January 2018 and can treat the collected water immediately on-site.

Picture 1: Photo of the all-rounder water treatment unit by Waterleau (photo: Ron Gerards, Waterleau).

Fraunhofer ISE worked out a special testing procedure to evaluate, which of the treatment steps are really necessary to achieve the required water quality.

Figure 1: Hydraulic design of the all-rounder treatment unit by Waterleau, which is used to analyse the need for treatment steps of the mirror cleaning water (figure: Ron Gerards, Waterleau).

What is zero blowdown technology (ZBT) for cooling systems and what are the chances of this approach for water savings?
In the MinWaterCSP approach, the development of a hybrid cooling system is the most important step toward water savings. The substitution of wet cooling towers by the new deluge cooling system makes up more than 85% of water savings compared to wet cooling towers. But there is still potential for improvements. One approach is the operation mode of zero blowdown technology (ZBT).

The high cooling efficiency of wet cooling has its price as about 80% of the incoming heat load is removed by water evaporation. As the cooling water evaporates, the dissolved solids present in makeup water become more concentrated. When the dissolved materials exceed their solubility limit, this results in precipitation and formation of an undesirable scale. Scale inhibitors, and sometimes acid, are added to the cooling water to increase solubility of dissolved solids. At some point, water has to be drained from the cooling system to keep the concentration of the salts below the solubility limit. The water drained from the cooling tower is termed “blowdown”. The so-called “cycles of concentration” (COC) express dissolved solids in the cooling water divided by the dissolved solids in the makeup water.

Figure 2 : Cycles of concentration and blowdown flow for an evaporation rate of 50 m³/hr (figure 2: Joachim Went, Fraunhofer ISE).

There are several approaches to achieve zero blowdown. For example, there are groups that define ZBT for a value of the COC higher than 20. Others try to eliminate the blowdown by increasing the drift losses and the COC, so the complete drain water is carried away with the air flow. In all cases, ion exchange and softening the water plays an important role. Special measures for scale and corrosion inhibition are necessary.

For MinWaterCSP, the amount of make-up water is already drastically reduced. Therefore, a higher effort for the treatment e.g. softening or complete demineralization of the cooling water can be justified. With the overall CSP simulation tool developed by Fraunhofer ISE, the MinWaterCSP consortium will investigate, whether ZBT is profitable and could significantly contribute to water savings.

Author: Joachim Went, Fraunhofer ISE, Germany

About Fraunhofer ISE
Fraunhofer is Europe’s largest solar energy research institution. In the solar thermal field, Fraunhofer ISE is pursuing R&D on materials research and component development for both improved flat-plate and concentrating collectors since about 15 years, but also on process integration and systems.

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