To understand the solar thermal basics, we must come to the basic property of heat. Heat results in the molecules of a substance or body to move in a disorderly motion. This result in the development of a form of energy called kinetic energy. Heat is the kinetic energy of the disordered motion. Since heat is a form of energy, it is expressed as Joules (J) and denoted as Q. Heat is also known as thermal energy. Solar thermal energy is a field that takes advantage of the sun’s heat for various thermal applications such as drying or heating. Some of the applications dependent on the temperature range are:
In this article, we will be explaining the various types of solar thermal energy systems, their importance and their applications.
Understanding Solar Thermal Energy
To harness the thermal energy from the sun, it is necessary to consider the available technology that can operate efficiently. This technology is known as solar collectors and these are devices which exchange heat on a low, medium or high-temperature setting depending on the type of application. Their main function is to convert solar radiation into thermal energy to fulfil the energy requirements. The radiation that is absorbed can either be directly used or even stored. The optimum efficiency of the collector depends on the collector design. The cover plate should be able to absorb solar radiation effectively.
Usually, glass plates are used as the glazing/cover plate because of its transparency which helps in absorption and transmission of solar radiation. The selection of the right material also affects the number of heat losses experienced by the collector. Selective coatings are used to improve the thermal conductivity and the thermal receiver must have high absorptivity to effectively absorb the thermal radiation that’s transferred from the glazing material. Apart from these design parameters, depending on the intensity of the application, 3 types of solar collectors have been developed. They are flat-plate collectors, evacuated tube collectors and concentrators.
Types of Collectors
1] Flat plate collectors: These are the most basic types of collectors used in the solar thermal industry. They are designed for low-temperature operations in the temperature range of 60oC to 100oC. Their dimensions generally have an area of around 1.5 to 3 sq. m and are very simple in construction and assembly. Flat plate collectors are used for water heating and space heating.
They consist of a box which has an insulating layer on the bottom and sides of the box. A dark coloured absorber plate with attached tubes is layered on top of the insulation and glazed with a transparent material such as plastic or glass. When solar radiation falls on the absorber plate transmitting through the glass/plastic cover plate, the absorber plate which has tubes attached it heats the liquid inside them. Usually, water is used. The water that is heated is then transferred from the tubes either by the force of gravity, a pump or even due to the pressure of the liquid being used. The efficiency of the heating capacity of the collector heavily depends on the ambient temperature and the availability of solar radiation.
Collectors underperform in cold temperatures but are effective during hot temperatures. Another important design parameter that can affect the performance of the collector is the glazing material. The function of such a material is to effectively transmit radiation and minimize losses associated with heat. Therefore, as mentioned before, materials such as glass are commonly used because they can transmit 90% of short-wavelength radiation. Plastics are also used because they also possess high transmittance rates for values as high as 0.4 but their disadvantage is temperature limitations because, at high temperatures, they are prone to some form of deterioration.
2] Evacuated Tube Collectors: These were developed to overcome some of the limitations of flat plate collectors such as low output during cloudy or cold days and internal regression of system components due to weathering or moisture. Evacuated tube collectors consist of solar collector tubes with heat pipes inside the tubes which is vacuum-sealed. There are many pipes which are connected and laid out in parallel. These tubes have a metal inner tube which acts as an absorber. These metal tubes are attached to a fin which helps in minimizing heat loss due to radiation.
The metal tubes are surrounded by a glass outer tube and the space between the inner and outer tube is vacuum-sealed. The heat transfer fluid that is used in these collectors is liquid-vapour phase change materials to transfer heat. These phase change materials can change their phase from liquid to vapour and back to liquid as it undergoes an evaporating-condensation cycle. An example of this fluid is methanol. The cycle involves evaporating the phase change material and converting it to vapour form when solar radiation absorbed. The vapour passes through the heat sink and condenses during which it releases its latent heat. The latent heat is passed through the heat exchanger where it gets collected by either water or glycol which can then be utilized or stored. Meanwhile, the condensed liquid repeats the process.
3] Concentrators: The third type of collector is known as concentrating collectors or concentrators. They can either be stationary or moving collectors. They’re usually used for high-temperature applications because they make use of a concave reflecting surface to direct the sun’s radiation beam to a focal point which increases the radiation flux. There are 2 types of concentrating collectors:
- Non-imaging collectors such as Compound Parabolic Collectors: In this type of technology, the concentrators are unable to image the sun to one focal point and instead direct the beam to 2 focal points. The concentrators should be employed in such a way that there is a gap or space between the reflector and the receiver to inhibit the absorber from directing the heat away leading to heat losses. The gap shouldn’t be too big as well because that can also affect the performance of the concentrator.
- Imaging collectors such as Parabolic Trough Collectors, Parabolic Dishes, Central Receivers and Fresnel Collectors: These collectors can image the sun’s radiation to one single point which helps in achieving very high temperatures. They help in reducing thermal losses by supporting a small absorber area with a large aperture area. In parabolic concentrators, the geometry of the reflector is used to determine the incident angle and the distribution of the angle across the parabolic shaped reflector. The intensity of the radiation and the width of the sun imaging plays a role in the effectiveness of the concentrator. In Fresnel lens concentrators, a single lens is broken down into concentric annular sets of lens. Fresnel lens helps in reducing the production costs because of the flatness of this type of concentrator. Another type of concentrators that are classified as imaging is heliostats and central receivers. These concentrators have sun-tracking mirrors which are called heliostats. These direct the sun’s radiation to a central point on a tower. They operate at high power and temperature levels in the range of 500MW and around 800oC respectively [1-4].
Types of Thermal Energy Storage Systems
Solar thermal storage systems can be classified as sensible heat storage, latent heat storage and thermochemical storage.
1] Sensible Heat Storage Systems: These systems store thermal energy through the heating and cooling process of a medium such as water, rocks and molten salts. This system is quite popular due to its economic feasibility and non-toxicity. This system depends on the specific heat of the medium, the temperature difference and the amount of storage media being used. Sensible heat storage systems can be classified as:
- Underground storage: Heat that is absorbed through solar radiation is absorbed by underground media such as clay, rocks and sand. Heat is pumped to these media using pipes and boreholes positioned at suitable intervals. The charging and discharging rate depend on the rate of heat transfer and the array area of the pipes used.
- Water tank storage: This is one of the most widely used storage technologies wherein a solar collector heats the cold water coming from the storage tank using solar insolation. The heated water is then transferred back to the stratified tank. The tank is also thermally insulated to prevent heat losses.
- Packed-bed storage: In this storage system, the particulate material is packed in a unit and thermal energy is stored by the bed through fluid circulation. The fluid that’s commonly used is air. When heat is added to the system, the fluid moves downward and when the heat is being removed, fluid flows upward. In such a system, heat addition and removal cannot be performed simultaneously. This system is highly stratified which is an advantage to efficiency. The packed bed of matter is heated as a temperature front is transferred from one side of the bed to the next.
- Molten Salt Storage: When considering solar thermal technology, one of the biggest challenges is storage. For solar thermal collectors and even concentrated solar collectors, one of the major storage options that are commonly employed is molten salt technology. It enables the use of molten salt both as a heat transfer fluid and as a storage medium for solar thermal technologies. It can be used in medium to high-temperature operations for storage and transportation of thermal energy. Their major advantage of water as a heat transfer fluid is its reduced risk of freezing during colder temperatures. Water tends to freeze when it gets too cold which can be an operational hazard by clogging the tubes. Molten salt has an improved anti-freeze property which minimizes this risk. Molten salt also has a wider operational temperature range, increased safety and durability. They also have reduced risk of corrosion and are cost-competitive especially by reducing maintenance costs. Therefore, the selection of the heat transfer fluid can significantly affect system performance.
2] Latent Heat Energy Storage Systems: This type of storage system is classified as phase change system because they absorb and release thermal energy during the medium’s change of state/phase. Therefore, the material used for this storage system is known as phase change materials (PCMs). At first, heat is absorbed sensibly as input temperature increases but soon it will reach a point where the heat is absorbed or released at a constant temperature. A further supply of heat will not increase the temperature of the material but instead will absorb this heat and this is known as latent heat. If a PCM has a large latent heat, then better is the material quality. The point at which heat gets absorbed at constant temperature is usually at the melting point of these materials. Similarly, as the material solidifies, it will release the heat to the surroundings which can be harnessed accordingly. These materials are classified as organic materials such as paraffin, inorganic materials such as salt hydrates and eutectic materials which are a mixture of organic and inorganic substances. These materials have high energy density and take less volume which is why PCMs have been the subject of interest and research in recent years.
3] Thermochemical storage systems: These systems store solar radiation in the form of chemical energy. The heat or thermal energy is used to conduct reversible endothermic reactions by storing energy as a chemical potential. The energy is stored in chemical bonds and when the energy is to be released, reverse chemical reactions take place. There are 2 types of thermochemical storage systems. The first one is a direct system where heat from the solar receiver is directly transferred to the reactor where endothermic reactions take place. The second type is the indirect system where heat is transferred to the reactor from heat transfer fluids. This type of energy storage has higher energy density than sensible heat storage or latent heat storage. These systems also reduce costs by reducing the volume needed to occupy in storage tanks because of the chemical potential [5-8]
Active and Passive Systems
Solar thermal systems can also be classified as active systems or passive systems. Active systems are those systems which use controllers, valves and pumps to conduct heat transfer fluids through the collector. They are more efficient and expensive than passive systems and can operate even during power loss. They’re easier to install as well. Active systems can be classified as open-loop (direct) systems or closed-loop (indirect) systems. Open-loop systems use the same hot water that is pumped through the systems for use in the application. They can be operated either manually or automatically but they are not suitable for temperatures in the sub-zero range where freezing is common. Closed-loop systems are suitable for use in freezing temperature conditions and pump the heat transfer fluid through the water heater.
Passive systems are those systems which do not use external mechanical systems such as pumps or controllers to conduct heat transfer fluid through the collector. They rely on natural mechanisms of conduction, convection and radiation. They use thermal mass mediums such as rocks, water, air, etc. for heat management [9, 10].
Applications of Solar Thermal Energy
1] Water Heating: Now since the basics of solar thermal systems have been introduced, it is time to delve into some of their applications. One of their major applications is water heating. They use thermosiphon and integrated water and storage (ICS) systems to heat the water inside the pipe. Thermosiphons and ICS systems are passive heating systems which use natural convection to aid fluid circulation. They work on the principle that as the water/fluid gets hotter, the better it circulates because of lesser density. The hot fluid flows towards the top and the colder, denser water flows to the bottom to replace the fluid in the collector. If there is no radiation or low radiation, then the water stagnates in the collector and no convection takes place. These passive systems are a good alternative to active systems by helping in reducing costs. They are suitable for milder climates which don’t experience wild fluctuations in the temperature of the surroundings.
2] Solar Distillation and Desalination: Another application for solar thermal systems is for water purification through solar distillation and desalination. Distillation is one of the oldest methods that are commonly used to purify a substance by filtering out the components based on their volatilities. It involves evaporating a solvent in one location and condensing the solvent vapour in another location. This purifies the solvent and when the energy supplied to facilitate this process is provided by solar radiation, it is termed as solar distillation. Conventionally, distillation occurs in constant conditions of temperature, pressure and flow rate but solar distillation is dependent on the solar insolation available with the highest performance shown during maximum irradiance. It also varies throughout the year with it showing better performance during the warmer months as compared to the colder months. The major advantages of opting for solar distillation are because the need for regular operation and maintenance is minimized due to the absence of moving parts. The use of solar radiation also completely avoids burning of fossil fuels and hence has zero greenhouse gas emissions. The versatility in being able to install these systems in remote locations also provides an added advantage. Water desalination using solar thermal energy is a classification of solar distillation which uses passive systems to minimize dependency on construction, operation and maintenance. Using a solar collector significantly reduced thermal losses and increased efficiency because of the small surface area of the absorber and because of the lack of need for extra components.
3]Food Drying: The final application of solar thermal systems is food drying using an indirect passive system. Drying food crops is one of the highly practised methods to remove moisture from food items. Moisture often provides a medium for bacteria and fungi to grow which consequently leads to the food being spoiled. This greatly affects the farmers who harvest these crops and at the same time affect the nation’s economy if not managed effectually. Preventing moisture from seeping into the food helps to retain the flavour and nutritional value of the food. The major factors that affect food drying are airflow, humidity and temperature. The use of passive solar systems uses the process of convection to keep the foods dry by using solar radiation. It is more economical and helps in managing the temperature, humidity and airflow conditions during the different stages of drying [1,11-13]
Overall, it may be concluded that solar thermal energy systems are one of the most promising fields alongside alternating technologies, help to power the society. This article inferred the basic understanding of solar thermal systems, the commercial systems that support solar thermal energy from flat plat collectors to concentrating systems. The article also helped in explaining an important storage technology-molten salt, that’s used in these collector systems which operate considerably better than other heat transfer fluid materials. The article finally explained some of the applications using active and passive solar thermal systems which perform more efficiently than conventional technologies. Therefore, we hope that this article has provided you with added knowledge as to why solar and alternative technologies have to be considered for the future!