Solar Energy Technologies: Thermal and Photovoltaic Systems

Overview of Solar Thermal Power Plants

• Introduction to Solar Energy Conversion: While photovoltaic (PV) cells are common in devices like calculators and isolated homes, solar thermal power plants represent a significant alternative for large-scale electricity generation. These plants transform solar radiation into heat energy (calorific energy), which is then used to power an electric generator.
• Three Mature Technologies: There are currently three established technological approaches for solar thermal power:     - Tower Central Receiver: Utilizes a field of heliostats to focus light on a central tower.     - Parabolic Trough Collector: Uses curved mirrors to heat a central tube.     - Parabolic Concentrator with Stirling Engine: Combines a parabolic dish with an external combustion engine.

Solar Tower Power Plants (Central Receiver)

• System Components:     - Heliostats: Large, flat mirrors mounted on orientable supports. They are equipped with two electric motors that slowly adjust the mirrors' positions to ensure solar rays are consistently reflected toward the tower receiver.     - Central Tower and Receiver: A tall structure with a receiver at the top where the energy from the entire heliostat field is concentrated.
• Thermodynamic Process:     - Temperature: The concentrated solar energy can reach temperatures up to $1000\,^\circ\text{C}$.     - Heat Transfer: Inside the receiver, a fluid circulates through a boiler, absorbing the heat from the solar rays. This mirrors the function of a coal plant, but uses sunlight instead of burning fuel, making it non-polluting.     - Primary and Secondary Circuits: The fluid in the primary circuit transfers heat to the water in the secondary circuit to generate high-pressure steam.     - Electricity Generation: The steam drives a steam turbine, which in turn powers an electric generator.
• Cooling System:     - Forced Draft Cooling Towers: Exhaust steam from the turbine is cooled using forced-draft towers equipped with powerful fans to create airflow, liquefying the water so it can be reused in the circuit.
• Energy Transformation and Efficiency:     - The process converts radiant energy (sunlight) into calorific energy (heated fluid) and finally into electrical energy.     - These plants typically have an efficiency of approximately $20\,\%$.

Case Study: Gemasolar Power Plant

• Location and Operation: Located in Fuentes de Andalucía (Sevilla), Gemasolar has been operational since 2011. It was the first commercial tower plant capable of producing electricity $24$ hours a day.
• Thermal Storage: It achieves continuous production by storing heat in the form of molten nitrate salts.     - Hot Salt Tank: Stores salts at $565\,^\circ\text{C}$.     - Cold Salt Tank: Salts are returned here at $285\,^\circ\text{C}$ after yielding their heat.     - Autonomy: The system provides up to $15$ hours of electricity production without sunlight.
• Technical Specifications:     - Heliostat Field: $2650$ heliostats covering an area of $195\,\text{Ha}$.     - Tower Height: $140\,\text{m}$.     - Power Output: Maximum electrical power of $20\,\text{MW}$, which is enough to power $2\,\text{million}$ lightbulbs of $10\,\text{W}$ or supply electricity to approximately $27000$ homes.

Parabolic Trough Collector Systems

• Operation Mechanism: These systems use parabolic mirrors to concentrate sunlight onto a central receiver tube.
• Heat Transfer Fluid: Usually, thermal oil circulates through the tube, heating to temperatures exceeding $300\,^\circ\text{C}$, which is sufficient to produce steam for electricity generation. This technology is also utilized in industrial processes for heating fluids.
• Plant Configuration:     - Collectors are connected in series; the receptor tube of one row leads into the start of the next to gradually increase the fluid temperature.     - Large surface areas are required because solar radiation is diffuse; therefore, these plants are often built in deserts or areas with inexpensive land.
• Hybridization: Some plants include an auxiliary natural gas boiler to maintain production during cloudy days or at night.

Parabolic Dish-Stirling Systems

• Disc-Stirling Association: This system couples a parabolic mirror (dish) with a Stirling engine located at the focal point.
• Stirling Engine Principles:     - History: Invented in 1816 by the Scottish engineer Robert Stirling.     - Mechanism: An external combustion thermal engine that operates on the cyclic heating and cooling of a gas (such as air, helium, or hydrogen) enclosed in a cylinder.     - Cycles: Heating causes gas expansion to move a piston; cooling causes contraction. This movement drives an electric generator.     - Modern Applications: Known for being very silent; currently used to power submarines and military boats.
• Performance and Scalability:     - Experimental Results: A system in Odeillo (South of France) with an $8.5\,\text{m}$ diameter mirror produces $9.8\,\text{kW}$ with a $21.6\,\%$ efficiency. Some models reach $30\,\%$.     - Scalability: Units can be used for isolated homes or grouped by the thousands for large-scale plants.     - Economic Viability: While technologically mature, they have not yet proven to be economically viable for commercial large-scale plants.

Global and Local Status of Solar Thermal Energy

• Spanish Leadership: Spain is a global leader in research and commercial use. It hosts the international laboratory Plataforma Solar d'Almeria (PSA).
• Energy Share: As of 2021, approximately $2\,\%$ of the electricity consumed in Spain is produced by solar thermal plants (e.g., Andasol 1, 2, and 3 in Granada).
• Market Competition: Development has slowed due to the drastic cost reductions in solar photovoltaic and wind energy, which have attracted the bulk of renewable investments.
• Pros and Cons Summary:     - Advantages: No emissions or dangerous waste; renewable and free fuel; potential for 24-hour production via thermal storage.     - Disadvantages: Large land requirements; discontinuous production subject to weather; higher costs compared to PV and wind.

Photovoltaic Solar Energy (PV)

• The Photovoltaic Effect: Photovoltaic systems use solar cells to transform the energy of photons (light particles) directly into electrical energy.
• Material: Primarily constructed from semiconductors, most commonly silicon.
• Types of Silicon Cells:     - Monocrystalline Silicon: The entire cell is a single crystal with ordered molecules. High efficiency (about $20\,\%$). Characterized by a uniform dark blue or black color.     - Polycrystalline Silicon: Composed of multiple small fused crystals, giving it a mottled (jaspiat) appearance. Lower efficiency (about $15\,\%$) but cheaper than monocrystalline.     - Amorphous Silicon: Features a thin layer of disordered molecules deposited on glass. Lowest efficiency (about $10\,\%$) and shorter lifespan. Used in low-power devices like calculators and battery chargers.

Mechanism of the Photovoltaic Cell

• Internal Structure:     - n-type Silicon (Negative): Has an excess of electrons.     - p-type Silicon (Positive): Has a deficiency of electrons.     - p-n Junction: The contact between these layers creates an electric field.
• Functioning: When light hits a silicon atom, it can provide enough energy for an electron to break free from the nucleus. The p-n junction induces these free electrons to move toward the n-layer. Connecting a cable between the layers allows the excess electrons to flow, generating an electric current.

Solar Panels and Modules

• Composition: Cells are grouped into panels to protect them from environmental factors (rain, wind) and facilitate installation. They are often sandwiched between protective layers and held by an aluminum frame.
• Output: An individual cell produces about $0.5\,\text{V}$ of direct current (DC). Inside the panel, cells are connected in series to achieve higher voltages (e.g., a standard panel may have $36$ cells).

Types of Photovoltaic Systems

• Isolated Systems (Off-grid):     - Fully autonomous; used where grid connection is impossible or too expensive.     - Surplus energy is stored in batteries.     - Applications: Rural homes, satellites (e.g., International Space Station), solar vehicles (e.g., the "Despertaferro" designed by UPC students), road signage, and parking meters.
• Grid-Connected Systems:     - Energy produced is injected into the electrical grid (sold). During periods of low production, energy is taken from the grid (bought).     - Self-consumption: Users generate their own power on rooftops, reducing bills. Some use batteries for excess production.     - Architectural Integration: Photovoltaic facades (e.g., Pompeu Fabra Library in Mataró) and photovoltaic pergolas to provide shade while generating power.
• Photovoltaic Power Plants: Large-scale installations with thousands of panels on metal structures for mass electricity injection into the grid (e.g., a $20\,\text{MW}$ plant in Llucmajor, Mallorca, and a $110\,\text{MW}$ plant in Guillena, Sevilla).

Global Photovoltaic Statistics

• Growth: Solar PV and wind energy together generated $10\,\%$ of global electricity for the first time in 2021.
• Global Rankings (2021 Installed Capacity):     1. China     2. United States     3. Japan     - Germany leads in Europe with nearly $60\,\text{GW}$.     - Spain ranks 8th globally with $18.5\,\text{GW}$.
• Capacity Expansion (IEA Data):     - 2012: $96.5\,\text{GW}$     - 2015: $227.1\,\text{GW}$     - 2018: $402.5\,\text{GW}$     - 2021: $942\,\text{GW}$