Energy performance analysis of a forced circulation solar water heating system equipped with a heat pipe evacuated tube collector under the Mediterranean climate conditions

doi:10.1016/j.renene.2019.03.109

Renewable Energy

Volume 140, September 2019, Pages 874-883

https://doi.org/10.1016/j.renene.2019.03.109 Get rights and content

Highlights

•
An experimental study on an active SWHS equipped with a HP-ETC is conducted.
•
The annual energy evaluation and analysis of an active SWHS are performed.
•
Useful heat gains from the solar collector and the storage tank are calculated.
•
Losses in the solar collector, solar circuit and in the storage tank are analysed.
•
The annual efficiency for the HP-ETC was 0.62, while for the system it was 0.516.

Abstract

This work represents the energy performance analysis during the annual time period of a forced circulation solar water heating system equipped with a heat pipe evacuated tube collector having an aperture area of 1.476 m² under the Mediterranean climate conditions. For the purpose, recorded data from a field-trial installation are exploited. The recorded data obtained every minute are used to perform the energy analysis during an annual period. The analysis is performed by using mathematical models and by representing the results for each month. Monthly values of measured parameters and calculated quantities offered a clear view regarding the operation of the considered field-trial installation for this climate region. For an annual irradiation on solar collector plane of 2,212 kWh/year, it was noticed that the annual useful heat gain for the heat pipe evacuated solar tube collector, the useful energy delivered by the solar circuit to the storage tank, and the delivered energy to the thermal consumer were 1,345 kWh/year, 1,311 kWh/year, and 1,009 kWh/year, respectively. The annual efficiency for the heat pipe evacuated solar tube collector was 0.62, while for the forced circulated system was 0.516. Also, the annual energy balance of the system obtained from the calculation is built.

Introduction

The usage of solar energy for thermal purposes varies greatly from a region to another. There are huge differences regarding the type of the system, solar collector, and application. For these reasons the energy evaluation of solar water heating systems and solar collectors under different climate conditions is very important. The same configured and designed system equipped with the same solar collector represents different energy performances in different climate regions.

Around $74 %$ of solar thermal systems installed in Europe by the end of 2016 were for domestic hot water usage. Leaning on the system type, it was noticed that the share of forced circulation systems in Europe was $61 %$ . These systems are more common in Central and North Europe. Referring to the total water collector area in Europe, evacuated tube collectors (ETC) accounted for $13.7 %$ of the total capacity in operation by the end of 2016. While, for Albania their share was $1.33 %$ , or $A_{E T C} = 2,760$ m² [1].

The Evacuated Tube Solar Collector (ETSC) consists of several evacuated tubes. Inside each evacuated tube, a vacuum is created between the absorber and the transparent glass cover. The absorber is connected to a heat pipe. Heat pipes are used to transfer heat by using the phase change of the fluid inside the pipe. It consists of a perfectly insulated (to minimize losses from the fluid to the outside) circular pipe with an annular wick layer. Solar radiation incident at the evaporator end boils the fluid inside the pipe. The vapour migrates to the condensing end, and transfers the heat of vaporization to the fluid loop at the condensing end. The heat created by the fluid loop is used at the end-use point. After thermal energy is released into the heat transfer fluid, the vapour is condensed and is returned to the evaporator end by capillary action through the wicks or by gravitational force [2].

In the field of solar thermal applications, the knowledge about the thermal performance of different solar collectors and solar water heating systems are very important.

The solar collectors are considered a particular kind of heat exchanger, which converts the solar radiant energy into heat. Many researchers have put efforts in evaluating, analysing, and improving the efficiency of different solar collectors utilized in solar thermal applications.

Gill et al. studied a heat pipe evacuated tube collector installed in an active system under Northern Maritime climate conditions. The annual efficiency of the considered collector was $0.82$ . Also, efficiency values were higher in winter months [3].

Ayompe et al. compared the performance between a flat plate collector and a heat pipe evacuated tube collector under temperate climate conditions. The annual average collector efficiency was $0.461$ and $0.607$ for the flat plate collector and the evacuated tube collector, respectively [4].

Sakrieh and Al-Ghandoor investigated the performance of five types of solar collectors experimentally under the climate conditions of Al-Zarqa, Jordan. The average efficiency achieved for the evacuated tube collector was $0.76$ , while for the flat plate was lower [5].

Hayek et al. investigated the overall performance of solar collectors experimentally under weather conditions of the Eastern Mediterranean Sea. They concluded that collector efficiency for heat pipe evacuated tube collectors was (15–20) % higher compared to the water-in-glass evacuated tube ones [6].

Hassanien et al. investigated the performance and the viability of using an evacuated tube solar collector to assist a heat pump for greenhouse heating in Kunming, China. The annual thermal efficiency of the collector was 0.49 [7].

Sabiha et al. enhanced the performance of a heat pipe evacuated tube collector by using single walled carbon nanotubes nanofluids under climate conditions of Kuala Lumpur, Malaysia. In the case of water as heat transfer fluid the highest efficiency was $0.5437$ , while for the new fluid it was $0.9343$ for the same flow rate of $0.025$ kg/s [8].

Depending on whether they require circulation pumps or not to run, solar water heating systems can be grouped into two basic categories: passive circulation systems and active circulation systems (forced circulated systems). The second systems eliminate the restrictions regarding the placement of the collector towards the storage tank. In this system type, a pump is used to circulate the heat transfer fluid through the closed collector loop to a heat exchanger [9].

Mahbubul et al. analysed the effect of single walled carbon nanotube-water nanofluid on the efficiency of an evacuated solar tube collector. They used a system placed in Dhahran, Saudi Arabia. When as heat transfer fluid was used water and 0.2% nanofluid, the obtained collector efficiencies were 0.567 and 0.66, respectively [10].

Sharafeldin and Gróf studied the thermal performance of an evacuated tube solar collector using WO₃/Water nanofluid. The system was with forced circulation and was situated in Budapest, Hungary. They achieved a thermal-optical efficiency of the evacuated tube solar collector of 0.7283 [11].

Sokhansefat et al. simulated through TRNSYS model two active solar water heating systems for the cold region of Iran. One of the systems was equipped with an evacuated tube collector having an aperture area of 2.04 m². They annual useful energy gain from the evacuated tube collector was 30,260 MJ/year [12].

Many researchers have studied the solar water systems of different types and equipped with different solar collectors.

Ayompe and Duffy analysed the energy performance of a forced circulation solar water heating system with a heat pipe evacuated tube collector situated in a temperate climate (Dublin, Ireland). The annual average daily collector efficiency was $0.63$ , while the system efficiency was $0.52$ [13].

Hazami et al. validated the TRNSYS model of a forced circulation solar water heating system equipped with a heat pipe evacuated tube collector in a region with typical North-African climate conditions (Borj Cedria, Tunisia). The study highlighted that the annual system efficiency was $0.76$ [14].

Mazarron et al. analysed the effect of tank water temperature on the delivered energy for an active solar water heating system with evacuated tube collector. When the required temperature was 50 °C, the annual system efficiency was $0.64$ [15].

Chow et al. carried out experimental and numerical evaluation of thermosiphon systems with single-phase and two-phase solar collector. It was noticed that the annual thermal performance of the two-phase solar collector was higher [16].

Redpath investigated the performance of two configurations for thermosiphon heat pipe evacuated tube solar water heaters for the northern maritime climate. The study revealed that the efficiency for the system with internal heat pipe condenser was $0.63$ , while for the other configuration with external heat pipe condenser was $0.475$ [17].

Daghigh and Shafieian evaluated theoretically and experimentally the performance of a solar water heating system with evacuated tube heat pipe collector. They presented the effect of several factors on the thermal performance for the studied solar collector [18].

Kaligorou presented optical, thermal and thermodynamic analysis of collectors. Also, a description of methods used to evaluate the collector performance was offered [19].

Erdenedavaa et al. analysed the performance of an active solar water heating system equipped with heat pipe evacuated tube collectors under the climate conditions of Ulaanbaatar, Mongolia. They performed measurements for a period of 8 months in a region with cold climate and dust presence. The average collector efficiency was 0.498, while the system efficiency was 0.37 [20].

Halawa et al. presented a comparative study of methods for evaluating the thermal performance of solar water heating systems in Australia, Taiwan and Japan. Also, they discussed the merits and weaknesses of each approach [21].

Mathioulakis et al. investigated the suitability and reliability of available assessment methods for energy performance indicators of solar domestic water heating systems in the context of their energy labelling [22].

For the selected region there is a gap regarding papers related to forced circulation solar water heating systems equipped with evacuated tube collectors. A forced circulation solar water heating system equipped with two flat-plate collectors connected in parallel with a total absorber area of 4.41 m² placed side by side with the studied system was evaluated by Maraj et al. [23]. The results showed that during the same time period, the annual averaged value of collector efficiency and system efficiency were respectively 0.494 and 0.411.

In general, the research work was focused mainly on the performance of different solar collectors under different operation and climate conditions. Several studies were performed on thermosiphon and forced circulation systems installed in locations with climate conditions (Northern Maritime, North Africa, Eastern Mediterranean, etc.) different from the Mediterranean climate. Also, the length of measurement period is very important to perform the energy analysis and evaluation of a solar water heating system. The considered long-term system performance was based on a 12-months period.

In this study, the energy performance analysis and evaluation of a forced circulation solar water heating system equipped with a 1.476 m² heat pipe evacuated tube collector was performed for the Mediterranean climate conditions. Their analysis and evaluation were based on parameters obtained from measurements and quantities calculated through the mathematical model. The studied trial system is a single family one and its collector is with heat pipe evacuated tubes. It was above mention that this collector type is very rare in the selected region, because of the massive diffusion of flat-plate solar collectors. Consequently, this study provides detailed information regarding the annual and monthly values of the useful gain from the solar collector, collector efficiency, useful energy delivered by the solar circuit to the storage tank, delivered energy to the thermal consumer, system efficiency, solar fraction, losses, etc.

Section snippets

Experimental setup

Fig. 1 shows the external and the internal part of the studied Solar Water Heating (SWH) system, which is installed in the premises of the Department of Energy, Faculty of Mechanical Engineering, Polytechnic University of Tirana – Albania. Tirana is the capital of Albania and is situated in the central part of the country. The average altitude of the city is 110 m above the sea level and the geographical coordinates are $41.33$ °N and $19.82$ °E. The selected town has a typical Mediterranean

Energy performance analysis

SWH system refers to the complete arrangement of many components that together collect, store, and deliver useful heat to the thermal consumer. It includes the solar collector, pipes, storage tank, circulation pump, controls, etc. Each component has its effect on the energy performance of the SWH system.

The performance and energy evaluation of the considered system are performed leaning on the following mathematical model.

The required rate of addition of sensible heat is given as [29]: $\dot{L} = {\dot{m}}_{w} \cdot c_{p w}$

Results and discussions

In this study, the annual energy evaluation and analysis of a forced circulation SWH system equipped with a heat pipe evacuated tube collector installed in a region with typical Mediterranean climate conditions belonging to the “Cs” group were performed.

For this purpose, the database of measured parameters of the considered SWH system for a 12-months period was utilized. Their description is performed leaning on their placement in the subsystems of collection, circulation, and storage.

The

Conclusions

The annual energy performance of a forced circulation solar water heating system equipped with a heat pipe evacuated solar tube collector installed in a particular region with typical Mediterranean climate conditions of “Cs” group was carried out. Recorded data obtained from this trial installation during a 12-months period were used. The conclusions are obtained as follows:

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In the selected region the annual irradiation on solar collector plane for the considered time period was $H_{T}^{y e a r} = 2,212$

Acknowledgements

The authors acknowledge the financial support from the Agency for Research, Technology, and Innovation- Government of Albania through the project “Water and Energy 2010–2012”, No. 397. The utilized trial solar water heating system was provided through this project, which included also a dissertation thesis.

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Energy performance analysis of a forced circulation solar water heating system equipped with a heat pipe evacuated tube collector under the Mediterranean climate conditions

Highlights

Abstract

Introduction

Section snippets

Experimental setup

Energy performance analysis

Results and discussions

Conclusions

Acknowledgements

Sol. Energy

Energy

Energy Procedia

Energy Build.

Energy Convers. Manag.

Renew. Energy

Renew. Energy

Renew. Energy

Sol. Energy

Sol. Energy

Energy Convers. Manag.

Energy Build.

Sol. Energy

Appl. Therm. Eng.

Prog. Energy Combust. Sci.