In recent times, we have increasingly become adept at harnessing the visible light using solar panels. However, we cannot deny that thermal energy is still the dominant component and possibly the oldest energy source. Concentrated Solar Power (CSP) systems harvest the heat energy from the incident infrared radiation using mirrors.
How Concentrated Solar Power Works?
All concentrated solar power (CSP) systems work by using multiple arrays of mirrors to focus a large area of diffused sunlight onto a thermal receiver. Hence, the word “concentrated”! In a nut shell, the process first begins when the sunlight falls on the arrangement of mirrors.
The mirrors then collect the sunlight and reflect (redirect) it to a receiver for longer periods of time. Most modern day mirrors track the sun’s position to collect the maximum amount of sunlight. The receiver is actually a pipe or a tube filled with what experts call working fluid. Consequently, the temperature of the working fluid increases upto 500 degrees (or even higher) depending on the type of mirrors and fluid used.
Finally, the fluid flows to a thermal power generation system where the heat from the fluid creates steam that drive turbines thereby generating electricity. The term “working fluid” refers to the fluid transporting heat mechanically by flowing.
Fig 1 illustrates mirrors focusing sunlight on a central receiver – the power tower. Moreover, CSP plants are expected to produce practically sufficient energy particularly in locations with excess of sunlight. For example, one of the world’s largest CSP plants is in Morocco. It has a capacity of 500 MW and supplies power to 1.1 million Moroccans.
A wide range of CSP systems exist to harness the sun’s thermal energy and common collector technologies are :
- Parabolic Trough Collectors
- Linear Fresnel Collectors
- Solar Power Towers or Power Tower
- Parabolic Dish collectors
I recommend reading How CSP Works: Tower, Trough, Fresnel or Dish by Solar Paces for an elaborate explanation on each of the above technologies.
Thermal Energy Storage (TES) Systems
A major drawback of solar power is its inconsistency during a given time period. For instance, clouds shielding the sunlight inhibits generation of power from solar energy. Therefore, integrating the concentrated solar power plant with thermal energy storage system is a neat trick to counter this problem.
Similar to other most energy systems, the (excess ) heat energy is stored during bright sunshine and released when solar intensity is negligible or not available.
There are three types of thermal energy storage systems:
- Sensible Heat Storage
- Latent Heat Storage
- Thermo-Chemical Heat Storage
Sensible Heat Storage
During energy storage, heat energy is stored by increasing the temperature of the storage material. On the other hand, heat is extracted from the material by lowering the material’s temperature to generate electricity.
However, the material does not undergo any phase change. In other words, the material does not transit between either of the three states of matter – solid, liquid and gas. Since the process does not involve phase change, engineers desire materials with high specific capacity, energy density and thermal conductivity which is also a drawback of this type. More importantly, the entire process without any change in the chemistry of the storage material.
Solid materials used in sensible heat storage provide high thermal conductivity at low cost (0.05 – 5 $/kg). They also provide a wide range of temperature for the heating process (200 – 1200 °C). Concrete and ceramics are popular choices.
While solid have their advantages, liquid storage materials dominate the industry. Molten salts like solar salt and HitecXL are the two most common examples. As the name suggests, the salts though solid at room temperature (25 °C), are melted into liquids when subjected to elevated temperatures. Moreover, molten salts are not toxic and thermally stable.
Not leaving out the third state of matter to fend for itself, some concentrated solar plants use gaseous materials like compressed air or steam. Although the materials are economical and offer a large range of operating temperature, they have a low thermal conductivity and energy density when compared to the liquid or solid materials.
Latent Heat Storage
In latent heat storage, the heat energy is stored/extracted when the storage material undergoes a phase changes at constant temperature. Simply put, when the material melts/solidifies/evaporates/condenses, it either releases or stores the supplied heat energy.
Like sensible heat storage, it is also a pure physical process with no change in the chemistry of the material. These materials are known as phase change materials (PCM). Because the materials store/release during phase change, they enable energy exchange in a narrow range of temperature and exhibit higher energy densities.
However, the major disadvantage is their low thermal conductivity leading to extremely slow rates of transition between phases. To tackle the problem, designers mix additives like graphite to increase the thermal conductivity and vary depending on the amount added.
While better options like materials made of metal alloys are available, they are expensive. As the saying goes, “there is no free lunch!”
Thermo-Chemical Heat Storage
Unlike the previous two thermal energy storage systems, a reversible endothermic chemical reaction consumes the solar energy. Since a chemical reaction occurs, the newly formed products store the solar energy. When these new products convert back into the original reactants, they release he stored solar energy.
Photosynthesis-respiration pair is a great example of thermo-chemical heat storage. On one side, photosynthesis uses solar (although not the infrared range) energy to produce starch (food) and oxygen. On the other side, respiration breaks down the same food in the presence of oxygen to release energy and carbon dioxide. Like I said, sun is the source of all energy forms on earth!
Similar to the photosynthesis-respiration pair, metallic hydrides, carbonate systems, hydroxide systems, etc. convert the sun’s heat energy (infrared range) into chemical energy for further use. Depending on the reaction, the products may cause unwanted problems like unusually slow reaction rate. Moreover, some of the reactions may require a catalyst (an external stimulus) to execute the reaction.
Integrating Thermal Energy Storage with Concentrated Solar Power
Now that we have discussed both the concepts individually, in this section we shall see how they bring out the best of each other. Depending on whether the storage materials can flow (move), the integration process broadly is divided into two categories – active and passive systems. Fig 2 is a flowchart depicting the classification.
Well, they are active because the storage material flows to absorb and release the heat by convection. As you may have already guessed by now, the storage material is typically a liquid. Gases are not a popular choice. The two sub-divisions within active systems are – direct and indirect systems.
- Direct Systems – In direct systems, the storage medium also plays the role of the heat transfer fluid (HTF) or working fluid. During heat absorption, the fluid is directly stored in the hot tank. During heat release and power generation, the fluid passes through a power system which extracts the heat and then flows into a cooling tank for reuse. Fig 3 (a) is a flowchart depicting an active direct system. Although this system does not require a heat exchanger, choice of the right storage material is crucial. For example, molten salts meet the requirements of a good heat transfer fluid as well as that of a good storage material.
- Indirect Systems – Unlike direct systems, the heat transfer fluid and the storage material are not the same in indirect systems. As shown in Fig 3 (b), during heat absorption stage, the storage material from the cold tank flows into the heat exchanger for indirect heating and stored in the hot tank. To release heat and generate power, the flow direction of storage material is reversed.
In contrast to the active systems, the storage material, often a solid, is stationary. A heat transfer fluid releases/absorbs the heat to/from the storage material (refer Fig 4). The choice of the fluid varies with the type of thermal energy system used in concentration solar power plants. However, a fluid with high thermal conductivity is always a requirement.
In conclusion, concentrated solar power plants/systems work on two basic principles – how much of the sun’s heat can the external mirrors capture and how much of the captured heat can the thermal energy storage system deliver for power generation.
Hence, the effective output energy is a product of the two parts. Although the technology has not matured yet, it as the potential to transform arid regions where sun is the dominant source of energy.
Additionally, there are hybrid systems that use both, concentrated solar power and its thermal energy storage as well as the photovoltaic solar panels and its battery technology to tap into the visible and infrared range simultaneously. Exciting times ahead!
Thank you for your time!
Global warming is one of the most exigent problems in today’s world. I aspire to work in the field of energy science by developing sustainable renewable energy systems to contribute towards mitigation of global warming.