Is solar thermal energy clean?

Gemasolar Thermosolar Plant 4

Solar thermal, a clean energy?

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Solar thermal power plant in Seville, Spain. Stu.dio / Shutterstock
Luis Moreno Merino, Spanish National Research Council (CSIC)

The production of electric power in solar thermal power plants has been considered, for more than twenty years, an environmentally friendly solution that can make a significant contribution to the supply of clean energy. This energy source is proposed as a substitute for other technologies with a significant environmental impact capable of accelerating global warming.

But more than ten years of experience and the objective analysis of the pros and cons of solar thermal energy has shown that it also has weak points. Solar thermal power plants can have a significant impact both on water quality, especially when supplied from groundwater, and on its availability for other uses.

Unlike photovoltaic power plants, which convert sunlight directly into electrical energy, solar thermal power plants need to concentrate solar radiation by means of mirrors. The energy reflected by these mirrors heats a thermal fluid to a very high temperature (around 450 °C).

The hot fluid is transported to the plant to generate steam at high pressure, which moves turbines that finally produce electricity. In other words, what differentiates a coal, gas or even nuclear power plant from a solar thermal power plant is the way in which the water is heated and steam is produced; from that point on, their operating principle is identical.

Apparently the process is totally clean, no fossil fuels are required and no gases or discharges of any kind are produced. But this is a simplification that hides the environmental problems of solar thermal power plants.

Saline and polluted water discharges

There are three points where a solar thermal power plant can come into conflict with the environment, especially with the quality and quantity of water: the heat transport system, its storage and steam production.

The heat transport from the mirror field to the plant is carried out by means of a thermal fluid: the mixture eutectic biphenyl/biphenyl oxide (HTF). This fluid reaches very high temperatures and over time degrades and produces new, more toxic compounds. A typical power plant such as those built in Spain (50 MW) uses several tons of HTF and accidental spills are far from unknown, endangering soil, surface water and groundwater.

To continue producing power when the sun has set below the horizon, solar thermal power plants must somehow store the excess heat captured during the day. The most common way to store heat is by melting salts, usually a mixture of potassium nitrate and sodium nitrate, which is stored in large tanks of hundreds of tons capacity. These tanks are not free from accidents or leaks. However, this type of problem is relatively easy to solve; the salts solidify on cooling and can be easily removed.

Solar thermal power plant.
Concentrating solar thermal power plant with molten salt thermal storage system in Seville (Spain). Kallerna/Wikimedia Commons, CC BY-SA

Once the thermal fluid reaches the plant, the heat it carries is used to generate steam at very high pressure. The steam drives a turbine and electricity is generated. The steam is then converted back to liquid water in a cooling tower.

The cooling towers of a solar thermal power plant consume enormous quantities of water, generating salinized water as a waste product. Depending on the number of cycles to which the cooling water is subjected, the salinity can increase two, three, four... times.

This water can return to the aquifer or watercourse from which it originates, increasing its salinity, or be eliminated in evaporation ponds, in which case it depletes the resources or even the reserves of the bodies of water.

Significant amounts of water are also used in the washing of the mirrors, up to 5 % of total consumption. Although water concessions of up to 1 hm³/year were approved in many of the Spanish plants, actual consumption can be significantly lower, between 3 and 3.5 m³/kWh..

A third problem, which is almost never mentioned, is the use of additives (antioxidants, biocides, descalers, etc.) that are added to the water in order to be used in the industrial process. The problem is that these substances are still present at the end of the cycle, when the water is discharged back into the environment.

How to reduce your environmental impact

Despite all these drawbacks, solar thermal energy can be clean, safe and can successfully replace other conventional sources that require fossil fuels.

Dry cooling systems (dry cooling) can save up to 90 % of the plant's total water consumption. However, these systems are significantly more expensive and the installations required are larger for the same amount of energy produced.

On the other hand, thermal fluid spills can be controlled if this eventuality is considered in the design phase of the plant and the following measures are taken the appropriate waterproofing and neutralization systems.

In summary, solar thermal technology can only be considered as an environmentally acceptable alternative to other sources of electricity production if it is able to ensure the integrity of soils and groundwater against accidental spills of thermal fluids.

Similarly, the safety of the water discharged after passing through the cooling circuit must be ensured. The necessary technologies exist; it is up to the environmental and energy authorities to ensure their correct use in any newly designed plant and the adaptation of existing ones.The Conversation

Luis Moreno MerinoSenior Researcher, Hydrogeology, Spanish National Research Council (CSIC)

This article was originally published in The Conversation. read the original.

Octavio Alonso