Solar Thermal Energy and Environment

Renewable energies are, without a doubt, the most viable and interesting alternative to the energy needs of the modern world, currently based mostly on non-renewable resources, giving our current system the only opportunity for long-term continuity.

However, the fact that an energy is nourished by a renewable resource is far from equivalent to calling that energy “clean energy”, and we must always be aware that any model of energy generation has an impact on the environment that may be more or less relevant depending on:

1. The type of system chosen. Since it is not only necessary to take into account the generation source used, but also the rest of the resources and materials used directly or indirectly, throughout the entire life cycle of the system, to extract so much desired energy.
2. The maturity of the technology used, associated with the generation system, and the degree of innovation and development achieved by it for the sake of a lower associated environmental impact.
3. The responsibility of the operator and their level of awareness in the operation and maintenance of the plant in their daily lives, as well as of the procedures and means available for operation under normal, abnormal and/or emergency conditions.
4. The sensitivity and resources of the environment in which the chosen technology is located.

And this is just what we find when we talk about Solar Thermal Energy, a renewable energy whose environmental problems associated with its exploitation and maintenance can even be very relevant.

First of all, we must know that there are different systems for the generation of solar thermal energy, each with very different characteristics and with different associated environmental problems: From those known as Stirling Disks, based on the engine of the same name, which work with hydrogen as a working gas (technology still in the maturation phase), to tower technology, consisting of a field of heliostats that concentrate sunlight at a fixed point to superheat a fluid at high temperatures (a technology that is more mature than above), or the most common cylinder-parabolic solar concentrators, which operate by concentrating solar radiation on receiving tubes with transmitting fluid (the latter being the most widespread technology).

These generation systems must be combined with the very diverse technological options available for their design and operation, and among others, the fluids used for heat exchange (synthetic oils, salts, water,...), the possibility of integrating hybridizations, thermal storage systems such as The ones we've already seen, etc.

In this way, environmental problems can be very diverse and complex, and in practice each plant or project will have different environmental impacts with different relevance, and it is common to encounter the following aspects:

The occupied surface.

Concentrating the sun and taking advantage of its thermal power requires a lot of space, since it is necessary for the mirrors or heliostats to have maximum solar radiation throughout the day, avoiding shadows between them and achieving the necessary generation temperatures depending on the cycle in which they are working.

This implies that, for example, a mirror field of a solar thermal plant using cylinder-parabolic concentrators may be requiring, depending on the average solar radiation available, between 3 and 5 hectares per MW of generation.

To the impact exerted during construction on flora and fauna by the elimination of the first and the displacement of the second on large areas, it should be added that during its exploitation it is important to avoid the spontaneous generation of vegetation in the solar field, since this can decrease productivity due to the albedo effect and be a potential cause of the spread of fires. This implies the necessary periodic use of plant protection products aimed at eliminating plant cover and preventing its growth, with the consequent environmental impact that could result from their use.

Water is also a resource.

Generating energy involves generating heat and therefore the need to dissipate it, something that is usually achieved through closed cooling circuits powered by water. Water that is usually lost due to evaporation and purges, which is necessary to maintain adequate salinity in the circuit, and which needs to be replaced with water that, previously, had to be demineralized on many occasions to be able to use the maximum number of cycles in the circuit.

It should also be taken into account in these cases, that solar thermal plants usually require sites with high insolation, in order to be as productive as possible, and that in general these locations tend to have the habit of coinciding with places where the availability of water is not very high.

This implies a strong impact on the environment, both because of the consumption necessary to maintain generation levels, which are around 8000 m3/year per MW installed, and because of the discharges carried out, of high salinity and carried out to an environment that is already usually affected by this same problem.

It is true that this factor, well managed, can have a good solution and even become an environmental advantage through the application of techniques and technologies aimed at environmental compensation or even the implementation of zero waste technologies.

Not everything that glitters is renewable.

Another environmental factor to consider in solar thermal energy is the temporality of a resource that, despite being renewable, is available only in a cyclical way (day/night), with seasonal fluctuations and depending on weather conditions.

In these cases, solar thermal technology cannot simply stop generating, as is the case with other renewable energies, since it has to maintain a minimum temperature in the exchange circuits and the solar field to avoid the generation of structural damage due to the solidification of the exchange fluids used (salts, thermal oils, etc.).

This involves the necessary installation of combustion systems to maintain these temperatures, which can be powered by traditional fossil fuels such as natural gas, thus becoming dependent on non-renewable resources, or by alternative fuels, hybridizing plants with biomass generation systems, thus maintaining their renewable nature even when emissions continue to be generated into the atmosphere.

The dependence on combustion can also be reduced if thermal storage systems are adopted that allow even the generation of energy for a limited time beyond the period of maximum solar radiation, currently being at maximum heat storage that allow generation for more than 8 hours.

Contaminant fluids.

A solar thermal power plant is like a gigantic radiator that operates at high temperatures with exchange fluids designed specifically for their function, fluids whose management must be carried out with special care to avoid as much as possible any environmental impact related to the emission of their vapors, their discharge to soils, or their transfer to other media such as water, through discharges.

In the case, in addition to cylinder-parabolic concentrator power plants, these fluids move through a circuit that occupies hundreds of hectares of surface and has moving parts, which considerably increases the likelihood of suffering incidents that may affect the environment.

The operator's awareness and adequate action during control, maintenance operations or even during incidents or leaks that may occur are vital so that this environmental aspect does not escape the company's control, a factor that our experience also indicates is vital during the start-up operations of these plants.

The maintenance and operation of these solar thermal power plants is perhaps the most complex that can exist in the field of renewable energy, and proof of this is the considerable volume of hazardous waste generated (usually above 10 Tm/year), a generation that is often susceptible to significant optimization.