Progress and latest developments of evacuated tube solar collectors

doi:10.1016/j.rser.2015.07.016

Renewable and Sustainable Energy Reviews

Volume 51, November 2015, Pages 1038-1054

https://doi.org/10.1016/j.rser.2015.07.016 Get rights and content

Abstract

Solar energy is the most available, environmental friendly energy source and renewable to sustain the growing energy demand. Solar energy is captured by solar collectors and an evacuated solar collector is the most efficient and convenient collector among various kinds of solar collectors. In this paper, a comprehensive literature on why evacuated collector is preferable, types of evacuated collectors, their structure, applications and challenges have been reviewed. Latest up to date literature based on journal articles, web materials, reports, conference proceedings and thesis have been compiled and reported. Applications of evacuated solar collectors in water heating, heat engines, air conditioning, swimming pool heating, solar cooker, steam generation and solar drying for residential and industrial sectors have been summarized and presented. Collector efficiency of different types of evacuated collectors and their performance based on different working fluids have been reported as well. Based on the available literature, it has been found that an evacuated tube collector has higher efficiency than the other collector. An evacuated tube collector is also very efficient to be used at higher operating temperature. There are few challenges that have been identified and need to be addressed carefully before installing an evacuated tube solar collector. However, after critically analyzing the available literature, authors have presented some future recommendations to overcome the barriers and for enhanced performance of an evacuated tube solar collector.

Introduction

The most available source of renewable energy on earth is solar energy as the earth receives millions of watts of energy everyday coming from solar radiation. However, only a fraction of it in the form of day lighting and photosynthesis is used by the natural world, one third is reflected back into space and the rest is absorbed by land, oceans and clouds. Thus, it is very reasonable to collect solar energy and utilize it efficiently to generate electric power, heat and also for cooling purposes in a viable way. The effect of using solar energy on the environment for a variety of applications is minimal as it produces no harmful pollutants. Besides environmental consciousness, dwindling of traditional energy sources marks solar energy as the appropriate energy source to meet the increasing demand of energy worldwide. Researchers have investigated and developed technologies on how to harvest solar energy to serve human beings and are still considering new technologies to maximize the collection and utilization of solar energy [1].

There are particular challenges in the effective collection and storage of solar energy though it is free for taking. As solar radiation is only available during daytime, the energy must be collected in an efficient manner to make use of most of the daylight hours and then must be stored. Solar thermal collectors are the existing components to capture solar radiation which is then turned to thermal energy and transferred to a working fluid subsequently. Therefore, solar collectors are the main and most critical components of any solar system [2].

There are basically two types of collectors, stationary and tracking [3] (Fig. 1). Different collector configurations can help to obtain a large range of temperature for example, 20–80 °C is the operating temperature range of a flat plate collector (FPC) [4] and 50–200 °C is for an evacuated tube solar collector (ETSC) [5], [6]. The most productive and mostly used solar collectors are FPCs but these collectors have comparatively low efficiency and outlet temperatures. FPC is popular due to its low maintenance cost and simple design.

However, FPC has two major drawbacks:

i.
convection heat loss through glass cover from collector plate and
ii.
absence of sun tracking.

ETSCs have considerably lower cost and heat loss than the standard FPCs [7], [8]. On the other hand, an ETSC overcomes both these drawbacks due to the presence of vacuum in annular space between two concentric glass tubes, which eliminates sun tracking by its tubular design. Conventional FPCs are mainly designed for sunny and warm climates. Their performance reduces during cold, windy and cloudy days and they are greatly influenced by the weather as moisture and condensation cause early erosion of internal materials which might cause system failure. In contrast, ETSCs have outstanding thermal performance, easy transportability and expedient installation. In addition, ETSCs are suitable for unfavorable climates [9], [10].

This paper presents a review of previous studies on ETSC, their applications, and suitability in solar thermal engineering systems. The former studies on ETSC mainly related to their suitability and performance in various applications. Therefore, this review mainly investigates the performance of ETSC for domestic and industrial applications, factors that influence the collector efficiency, challenges of using this collector as well as economic consideration regarding the usage of this collector. Some suggestions are also made for future research in this field. There is no review on ETSC till now and thus this is the first systematic review paper on recent developments of ETSC and their applications according to the authors׳ opinion. Finally, it is the authors׳ hope that this review will be useful to find more about ETSC, their applications, and challenges and the future recommendations will help in future research work.

Section snippets

Evacuated tube solar collector (ETSC)

A variety of technologies exist to capture solar radiation, but of particular interest of authors is evacuated tube technology. Numerous authors [3], [11], [12] have noted that ETSCs have much greater efficiencies than the common FPC, especially at low temperature and isolation. For instance, Ayompe et al. [13] conducted a field study to compare the performance of an FPC and a heat pipe ETSC for domestic water heating system. With similar environmental conditions, the collector efficiencies

Cost and maintenance

Morisson et al. [38] mentioned that the world market of solar water heater with ETSCs is significantly expanding due to the low cost manufacturing process of tubular solar collectors. According to China industry in 2001, about 65% of 6.5 million m²/year solar water heaters are double glass evacuated tubular solar water heaters.

According to Mangal [17], evacuated tubes are strong and long lasting. In case if any tube is broken, it just requires replacing the broken tube which is cheap whereas for

Economic consideration on the usage of ETSCs

ETSCs have not provided any real competition to FPCs in the past years though ETSCs have been commercially available for more than 20 years. Recently, there has been a major expansion of the evacuated tube solar water heater market in China, Europe, and Japan as a result of globally growing industries of evacuated tube collectors. This review has attempted to identify the economic advantages of ETSC over FPC by comparing the initial and maintenance cost as well as the payback period.

From Table 5

Performance based on working fluids

To improve the efficiency of solar collectors, researchers have mainly focused on several structural changes such as changing the structure of solar collectors or changing the coating to improve absorptivity but from the literature, only few studies focused on changing the working fluid in order to improve the collectors׳ efficiency [57]. From recent studies, it is found that the working fluid can influence the performance of solar collector significantly. Water, oil, and air are the most

Future work

i.
One of the drawbacks of ETSC is that the collector tubes are very fragile and easy to be damaged. To overcome this drawback, research can be carried out on improving the structure of evacuated collector to make their body harder. For example, nanotechnology can be used to build a harder and powerful evacuated collector.
ii.
Evacuated tubes are made of annealed glass which is much more fragile than tempered glass and the material mostly used is borosilicate glass. Experiments can be done on materials

Conclusion

This paper presents an overview of recent studies on ETSCs and revealed that this collector has great potential in industrial, residential and agricultural sectors. ETSCs have been used for water heaters, solar cooker, air conditioning, heating swimming pool, drying agricultural products and for many other purposes. An ETSC is highly recommended for higher temperature applications as they can gain higher temperatures easily and are able to preserve heat even when the outside weather is cold.

Acknowledgments

The authors would like to acknowledge the Ministry of Higher Education Malaysia (MOHE) for financial support. This work was supported by UM-MOHE High Impact Research Grant Scheme (HIRG) (Project no: UM.C/HIR/MOHE/ENG/40).

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Progress and latest developments of evacuated tube solar collectors

Abstract

Introduction

Section snippets

Evacuated tube solar collector (ETSC)

Cost and maintenance

Economic consideration on the usage of ETSCs

Performance based on working fluids

Future work

Conclusion

Acknowledgments

Desalination

Prog Energy Combust Sci

Sol Energy

Renew Sust Energ Rev

Appl Energy

Sol Energy

Energy Convers Manag

Renew Energy

Sol Energy

Energy

Appl Energy

Sol Energy

Sol Energy

Renew Energy

Appl Therm Eng

Appl Therm Eng

Sol Energy

Sol Energy

Int Commun Heat Mass Transfer

Energy Procedia

Energy

Appl Energy

Energy Building

Sol Energy

Sol Energy

Sol Energy

Proc Eng

Energy Convers Manag

Sol Energy

Appl Therm Eng

Renew Sust Energy Rev

Appl Energy

Renew Sust Energy Rev

Sol Energy

Renew Sust Energy Rev

Renew Sust Energ Rev

Appl Therm Eng

Int J Heat Mass Trans

Renew Sust Energ Rev

J Nanomater

Renew Sust Energ Rev

Procedia Comput Sci

Renew Sust Energ Rev