Concentrated Solar Power (CSP) technologies use mirrors to concentrate (focus) the sun’s light energy and convert it into heat to create steam to drive a turbine that generates electrical power.
CSP plants generate electric power by using mirrors to concentrate (focus) the sun’s energy and convert it into high-temperature heat.
That heat is then channeled through a conventional generator. These plants consist of two parts:
1) Collect and convert solar energy to heat
2) Convert the heat energy to electricity
Heat storage in molten salts allows solar thermal plants to continue to generate after sunset and adds value to such systems when compared to photovoltaic panels.
Water and Wastewater treatment
CSP plants consume a large amount of water, which is usually drawn from rivers and wells to generate vapor. The vapor is generated by turbines which require ultra-pure water in order for them to generate high quality vapor.
For this reason, the incoming water to the power plant needs to be treated at a water treatment plant (WTP) before use.
As a general rule, such treatment plant applies a reverse osmosis pre-treatment followed by a post-treatment by means of resins or continuous electrodeionization (CEDI) treatment.
Once ultra-pure water is obtained, it is sent, by and large, to the steam generator turbine, a small amount of which is set aside to clean solar panels.
On the other hand, the generated effluents containing water treatment plant rejects (soil and sand, bacteria and different types of salt) will have to be treated at a wastewater treatment plant (WWTP) to make them appropriate for discharge.
Wastewater is treated at WWTP’s by means of different technologies for salt crystallization. The processes used to treat wastewater are mainly chemical pre-treatment, membranes, vacuum evaporation and crystallization.
Air emissions treatment
The heat captured by solar collectors used in solar thermal power plants is pumped to the power block by means of an organic heat transfer fluid, which contains benzene-derived molecules.
This fluid undergoes degradation, which poses a considerable security risk, since some of the degradation by-products are potentially hazardous.
Typical heat transfer fluid degradation mechanisms are contamination from water pipe residues and water from the water-vapor cycle; oxidation caused by the reaction of oil with oxygen in the air; and cracking that occurs inside absorber tubes and the auxiliary boiler, when the heat transfer fluid reaches very high peak temperatures.
Three are the products resulting from degradation:
- Solid wastes, mainly carboxylic acids, carbon and carbon deposits, which are highly flammable and corrosive, due to their acidic properties.
- Short-chain hydrocarbons produced by the breaking apart of by biphenyl and diphenyl ether molecules. This type of hydrocarbons has a very low boiling point and alter the viscosity and flash point.
- Long-chain hydrocarbons produced by several short-chain residues joining together. This type of hydrocarbons has a very low boiling point and alter the viscosity and thermal properties.
In order to remove them, solar thermal power plants are equipped with three technologies:
- Primary filter
- Ullage Systems
It can therefore be noted at this point that vapours vented from the ullage system contain benzene and other contaminants, which may include phenol, toluene and xylene.
Given the danger of these contaminants, especially benzene (it is carcinogenic and must be removed) the law establishes very strict emission limits that enforce the treatment of the vents before their release into the atmosphere.
The ullage system’s function is to recuperate the thermal oil and to do this, it evaporates it, giving rise to vapours that contain the above-mentioned contaminants.
The most appropriate technology for benzene emissions reduction is the use of activated carbon filters, since they contain inert material, which helps to retain volatile organic compounds and return purified air.
A real example of a plant applied by Condorchem Envitech in a solar power plant:
It consists of an adsorption unit with two separate contact and serial chambers.
The air arrives at the adsorption unit and crosses the activated carbon bed where the contaminants are retained, and the clean air goes out, complying with the legal limits for emissions.
The adsorption unit must be installed so that it is easy to extract and reinsert activated carbon. It is therefore important to mention that the adsorption of volatile substances over time involves the saturation of the carbon bed, which is why periodically, and depending on the quantity of Volatile Organic Compounds contained in the processed air, the carbon must be replaced.
To determine when the carbon must be replaced, the total bed mass is measured via 4 load cells per filter. At the outlet of the filtration unit, a gas flow meter is installed to continuously read and record the precise, average and accumulated flow emitted.
Once the bed of the first chamber is saturated, and after replacing it, the adsorption unit has the possibility of sending the contaminated flow initially to the second chamber and immediately afterward to the first, and vice versa. This is made possible thanks to the installation of the 4 manual valves.
It is also important that the adsorption unit has a safety valve, which if necessary, activates to prevent excessive pressure of the system.