What is Solar thermal and what are its types?

  

   Solar energy is one of the renewable sources of energy, through which we can harness more power. Generally, solar energy can be harnessed in two different ways, namely solar photovoltaic (operated by light energy) and solar thermal (operated by heat energy). In one of my previous posts, I had explained the construction and operation of the solar photovoltaic in which the solar energy (light energy) is converted into electrical energy, by the solar cells. In this topic, let us discuss the construction and operation of solar thermal technology. 

Difference between solar thermal and solar photovoltaic

   Solar thermal technology generally uses solar collectors to collect heat from the sun and to heat up a liquid to either generate electricity or to provide hot water. Solar collectors should be designed in such a way that they should have high absorption and low emission. The collectors used in solar thermal technology is generally classified into two types, they are,

1.     Non-concentrating type solar collector.

2.     Concentrating (focusing) type solar collector. 

  Here the non-concentrating type collector is a normal collector which is used in homes, as a part of solar water heaters, and in various places to generate electricity. Whereas the concentrating type collector is mainly used in large areas such as deserts to generate electricity. Concentrating type collectors usually have a high amount of heat concentration and produce high temperature due to their parabolic type structure, whereas non-concentrating type collector has a less concentration of sunlight and produce less temperature due to their flat type structure. Let’s look at the function and operation of the non-concentrating and concentrating type solar collectors.

NON-CONCENTRATING TYPE SOLAR COLLECTOR

  A Non-concentrating type collector is a type of collector which does not have much concentration of heat or temperature on its surface. Due to this property, it is mainly used in home appliances such as solar water heaters and solar air heaters. There are mainly two types of Non-concentrating type solar collector, namely

1.      Flat Plate collector

2.      Evacuated Tubular Collector

 

Both types of collectors get the input from the sun as solar energy and use the heat to heat up a liquid.

 

FLAT-PLATE COLLECTOR

    A Flat plate collector is simply a collection of small copper tubes placed on an absorber plate (a sheet of copper or aluminium) painted with black to absorb maximum heat. These tubes and plates are together placed inside a flat panel. The top surface of the panel is made up of glass and the bottom surface is made up of insulating material to ensure maximum capturing of sunlight inside the panel. The fluid to be heated is passed inside the tubes. The absorber plate absorbs the heat energy from the sun and transmits it to the copper tubes, thus heating of the water takes place. Thus the hot water can be used for our purposes.

Credit (Flat plate collector): By SolarCoordinates - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=14917757

Construction of Flat plate Collector

    The flat plate collector is used in the solar water heater in such a way that it directly heats the fluid or indirectly heats the fluid. Based on the heating process, the methods are classified as

1.     Direct hot water system

2.     Indirect Hot water system

DIRECT HOT WATER SYSTEM

  In the direct hot water system (also called an active open-loop system); the cooler water from the main storage tank is fed into a hot water storage tank. From this tank, the cold water is made to run through the copper tubes of the flat plate collector using a pump. After capturing the heat the hot fluid is again made to flow to the hot water storage tank. From there the hot water is used for our needs.

 

Direct hot water system-(Active Open-loop system)

Direct hot water system- (Passive Open-loop system)

    There is also another type of direct hot water system, called a passive open-loop system. In this system, the water does not require any pump to force it to the tank; thereby it uses the natural gravity system. At first, the cold water from the main storage tank is made to flow to the hot water storage tank. Now the water from the hot water storage tank is made to flow to the collector tubes to get heated. Now as the liquid gets hotter, it expands and gets to the top of the storage tank. But as the cold water is dense it remains at the bottom of the tank. The effect of rising hot water and sinking cold water is called the thermosyphon effect. Now the hot water can be used for our purpose and the cold water from the main storage tank can be circulated again to get heated. This is the most common type of solar water heating method used nowadays.

 

Direct hot water system- (Passive open-loop system)

INDIRECT HOT WATER SYSTEM

  The indirect hot water system is also the same as the direct water system but it heats the water indirectly. The fluid used is an anti-freeze liquid, which is made to flow inside the copper tubes. When the liquid gets hot enough, it is made to pass over a heat exchanger. This heat exchanger is filled with cold water and placed with two coils inside the cold water. In one coil, the liquid from the collector tubes is made to run. When the hot liquid comes in contact with the filled in cold water, it makes the cold water hot. In another coil, temporary water is filled up which gets heated up by an additional boiler. In case the heat supplied by the collector tubes fails, the water from the boiler heats the water, filled inside the exchanger. This method is also called a closed-loop system. This system is mainly used in cooler areas, because when normal water is made to run along the copper tubes, the water sometimes gets freezes up, which also makes the tubes corroded. Thus to avoid this situation anti-freeze liquid is used inside the tubes, from which the heat is collected.

Indirect hot water system

EVACUATED TUBULAR COLLECTOR

   In an evacuated tube collector, the flat panel surface is replaced by hollow tubes. These tubes are made of copper tubes and are surrounded by a glass medium with a vacuum present in between to prevent heat loss.  Inside the tubes, a default liquid is filled up. Absorber plates are placed around the copper tubes to trap the heat from the sunlight and to heat up the liquid running inside the tubes. The top surface is covered by glass and the bottom surface is covered by a high-grade insulating material, to capture the maximum amount of sunlight.

Credit(Evacuated Tubular Collector) :By F.MYKIETA - HELIOFRANCE, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=9930112

 

    

Construction of single tube

    The working of the evacuated tubular collector is slightly different than the flat plate collector. Instead of passing the water through the tubes and getting used by us, the liquid used in the evacuated tubes stays in the tube itself. When the sun’s rays start to hit the absorber plate, the liquid inside the tube gets heated up. So the liquid starts to rise up and reaches the top of the evacuated tubes. The top of the tubes is placed inside the hot water storage tank. So when the cold water from the tank comes in contact with the tubes, heat transfer takes place. Thus the heat from the tube gets transferred to the water, by which it becomes hot and can be used by us. Now as the heat transfer has taken place, the liquid inside the tube becomes cooler again and comes down within the tube, by which it gets heated up again by the sun’s radiation. The main advantage of this tubular collector is that its efficiency remains the same even though the sun changes its path. So the heating process occurs over a maximum time per day.

 

Construction of Evacuated tubular Collector

 

   Non-concentrating type collectors are also used for generating electricity. By supplying water to collector panels, heat can be fetched off from the panels. This heat is transferred to the Butane liquid (organic fluid) in the butane boiler where it becomes the butane gas. The purpose of using an organic fluid separately is that the normal water cannot be converted into steam with the heat produced by the flat plate collector, so a liquid must be needed which can be quickly converted into steam with a nominal heat provided. Now this steam is passed to the steam turbine where the heat energy of the steam is converted into rotational energy. This rotational energy is given to a generator where electricity is generated. 

Collectors used for generating Electricity

CONCENTRATING TYPE SOLAR COLLECTORS

   Concentrating type collectors are mainly used for generating electricity. To generate a huge amount of electricity a large number of solar panels are placed at a pattern; so that the entire mirror focuses the heat at one point. At first, a long tower is constructed at the place, and a boiler is placed at the top of the tower. The boiler contains the working fluid. Parabolic-shaped mirrors are constructed around the tower. As the mirrors are parabolic, wherever the sun rays strike in the mirror, it all will be focused at one point. So this point is made at the point at the boiler. Each mirror is placed at a different tilted angle, such that all the points of focus will be at the boiler. Thus a very high amount of heat will be generated. This accumulated heat on the boiler will make the liquid in the boiler get converted into steam. This high-pressure steam is made to flow over the steam turbine by which the steam turbine rotates and also rotates the generator, thus generating electricity. This steam can be condensed by a condenser and can be reused again.

 

Credit (Concentrating type Collectors): By Amble - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=37624786

  Concentrating mirrors can also be arranged in such a way, that at the point of concentration of the mirrors an oil pipeline is made to flow. Now when the oil comes in contact with the heat point, it gets heated up and is passed over to a heat exchanger, where the heat from the oil is transferred to the water. Now the water gets converted into steam, which is passed over to a steam turbine to produce rotational energy. From this rotational energy, electricity can be generated by using a generator. 

 

Credit (Concentrating type collectors) :By Tommaso.sansone91 - Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=76571900

 

 

 

 

 

 

 

 

 

Written by [Jerine Victor]