Harnessing Solar Energy for Heating Applications by Using Thermal Collectors

Overview

There are two types of solar thermal systems passive systems and active systems. The passive system refers to when the sun shines through the windows and warms up the environment. Some buildings in different parts of the world try to design the buildings so that they use most of the sunlight.

Another type of solar heating is called active systems, which have collectors and heating fluid such as water or air. They use fans or pumps to move the fluid through the collectors to a place where heat is released. They usually have a tank for storing the heated fluid.

 Solar Thermal Collectors

This is a type of active solar heating system. There are two basic types of solar collectors, non-concentrating and concentrating collectors. The non-concentrating collector has the same area for intercepting and for absorbing solar radiation. The most common type is flat-plate collectors and are used for water and space heating in buildings. The concentrating solar collector consists of reflecting surfaces to intercept and focus the sun radiations to a smaller receiver or absorber area, sometimes the radiation area might be hundreds of times greater than the absorber area. 

The concentrating solar thermal collector further has two types as well, imaging concentrators and non-imaging concentrators. Imaging concentrators have a very large aperture area with a small absorber area, effectively reducing thermal losses at high temperatures. They are also used to achieve the highest temperatures that are currently achievable with a solar thermal system. They have been used in Ivanpah Solar Electric Generating System. Non-imaging concentrators do not produce images of the sun on the absorber, instead, radiation is distributed from entire areas of the reflectors onto the entire absorber surface.

 Some types of solar thermal collectors

  1. Flat plate collectors: This is the most common solar thermal technology in Europe. The idea behind this technology is fairly simple. The sun heats up a dark surface and then the absorbed heat is transferred to the fluid.
  2. Evacuated tube collectors: They consist of a set of modular tubes in which the heat loss is reduced because of vacuum. The absorber plate is a metal strip in the center of the tubes. The energy is then transferred to the water by a heat pipe. These are most commonly used for heating applications in China and the rest of the world.
  3. Compound parabolic concentrating/concentrator: Compound parabolic collectors (CPCs) are non-imaging concentrators, that can accept incoming radiation over a relatively wide range of angles. The operating principle is based on edge optics in these collectors. The absorber can be in different configurations like a flat, bifacial, wedge, or cylindrical.

CPCs were proposed in the late 1950s and the basic deThere has been ample amount of research being done on parabolic concentrators. But its impact on the market thus far has been modest. This system has considerable potential for both low-temperature industrial and residential applications. However, the interest in industrial solar low- and intermediate-temperature heat utilization has recently increased which could show that they would be highly relevant in the future. They also have a wide range of applications due to their temperature range. Stationary CPCs are applicable for the medium-temperature range (60 °C–150 °C), and tracking CPCs for the medium- and high-temperature range (100 °C–250 °C).

They can also be combined with photovoltaics. The IR part of the solar spectrum is directly absorbed by the tubular absorber of the CPC by the wavelength-selective film, and the visible part is transmitted to the thin-film PV module. This hybrid PV/T collector can achieve a total energy output conversion efficiency of up to 20% compared to the independent PV or solar collector, as has the advantage of longer wavelengths of solar radiation, resulting in an annual thermal efficiency of 31%.

Conclusion

Solar thermal collectors and in particular compound parabolic concentrators (CPC) have good potential. CPCs impact on the market hasn’t been big so far but the interest in them is slowly rising.  They can also be combined with photovoltaics and can achieve higher efficiency.

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