Exergetic modeling and performance evaluation of solar water heating systems for building applications

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Abstract

Solar water heating (SWH) is a well-proven renewable energy technology and has been used in many countries of the world. The basic technology is straightforward, although there are a variety of various types of SWH systems. In the performance assessment of SWH systems, energy analysis (first law) method has been widely used, while the number of the studies on exergetic assessment is relatively low. The SWH system investigated consists of mainly three parts, namely a flat plate solar collector, a heat exchanger (storage tank) and a circulating pump. The main objectives of the present study are as follows, differing from the previously conducted ones: (i) to model and assess SWH systems using exergy analysis (second law) method as a whole, (ii) to investigate the effect of varying water inlet temperature to the collector on the exergy efficiencies of the SWH system components, (iii) to study some thermodynamic parameters (fuel depletion ratio, relative irreversibility, productivity lack and exergetic factor) and exergetic improvement potential, and (iv) to propose and present an exergy efficiency curve similar to the thermal efficiency curve for solar collectors. The system performance is evaluated based on the experimental data of the Izmir province, Turkey, which is given as an illustrative example. Exergy destructions (or irreversibilities) as well as exergy efficiency relations are determined for each of the SWH system components and the whole system. Exergy efficiency values on a product/fuel basis are found to range between from 2.02 to 3.37%, and 3.27 to 4.39% at a dead (reference) state temperature of 32.77 °C, which is an average of ambient temperatures at eight test runs from 1.10 to 3.35 p.m., for the solar collector and entire SWH system, respectively. An exergy efficiency correlation for the solar collector studied was also presented to determine its exergetic performance. It is expected that the model presented here would be beneficial to the researchers, government administration, and engineers working in the area of SWH systems for residential applications.

Introduction

In developed countries, energy consumption in the building sector represents a major part of the total energy budget. Most of the amount is spent for hot water production and space heating. Hot water is required for taking baths and for washing clothes, utensils and other domestic purpose in urban as well as in rural areas. Water is generally heated by burning non-commercial fuels, namely, firewood as in the rural areas and commercial fuels such as kerosene oil, liquefied petroleum gas (LPG), coal and electricity in urban areas [1]. In this regard, utilization of solar energy through solar water heating (SWH) systems plays a big role in reducing energy amount required.

SWH is a well-proven and readily available technology that directly substitutes renewable energy for conventional water heating. A variety of types of systems are available and suitable for many applications [2]. Small systems are used for domestic hot water applications while larger systems are used in industrial process heat applications. There are two types of water heating systems based on the type of the circulation: natural circulation and forced circulation. Natural circulation solar water heaters are simple in design and low cost. Forced circulation water heaters are used in freezing climates and for commercial and industrial process heat [3].

The main part of a SWH is the solar collector array that absorbs solar radiation and converts it into heat. This heat is then absorbed by a heat transfer fluid (water, non-freezing liquid, or air) that passes through the collector. This heat can then be stored or used directly. Portions of the solar energy system are exposed to the weather conditions, so they must be protected from freezing and from overheating caused by high isolation levels during periods of low energy demand. In solar water heating systems, potable water can either be heated directly in the collector (direct systems) or indirectly by a heat transfer fluid that is heated in the collector, passes through a heat exchanger to transfer its heat to the domestic or service water (indirect systems). Five types of solar energy systems can be used to heat domestic and service hot water: thermosyphon, integrated collector storage systems, direct circulation, indirect, and air [4].

The thermal engineer involved with process simulation is working in a very dynamic environment. Two points for future development are pointed. They are using the exergy concept in modeling a general modular program, and using the new feature of the visual programming in building flexible, and easy to manage, programs with comfortable interface [5].

An exergy analysis (or second law analysis) has proven to be a powerful tool in the simulation thermodynamic analyses of energy systems. In other words, it has been widely used in the design, simulation and performance evaluation of energy systems. Exergy analysis method is employed to detect and to evaluate quantitatively the causes of the thermodynamic imperfection of the process under consideration. It can, therefore, indicate the possibilities of thermodynamic improvement of the process under consideration, but only an economic analysis can decide the expediency of a possible improvement [6], [7].

The concepts of exergy, available energy, and availability are essentially similar. The concepts of exergy destruction, exergy consumption, irreversibility, and lost work are also essentially similar. Exergy is also a measure of the maximum useful work that can be done by a system interacting with an environment, which is at a constant pressure P0 and a temperature T0. The simplest case to consider is that of a reservoir with heat source of infinite capacity and invariable temperature T0. It has been considered that maximum efficiency of heat withdrawal from a reservoir that can be converted into work is the Carnot efficiency [8], [9].

Although numerous studies have been conducted on the performance evaluation of SWH systems by using energy analysis method in the literature, very limited papers have appeared on exergy analysis of these systems. Xiaowu and Ben [10] performed an exergy analysis of a domestic-scale water heater and investigated the effects of collector design parameters on the collector exergy efficiency. They reported that large exergy losses occurred in the storage barrel and to improve the exergy efficiency of domestic-scale water heater, a judicious choice of width of plate and layer number of cover was necessary. Ucar and Inalli [11] studied on the exergoeconomic analysis and optimization of a solar assisted heating system for residential buildings in the Elazig, Turkey. They obtained the optimal sizes of the collector area and storage volume in a seasonal storage solar heating system using the exergoeconomic optimization technique.

Some other studies associated with SWH systems have been performed as follows: (i) the exergetic evaluation of solar collectors [12], [13], [14], [15], [16], which are the main component of a SWH system, (ii) exergy of solar energy [17], [18], [19], which is used in performing exergy analysis of solar collectors, (iii) exergy analysis of indirect thermal storage tanks [20], which use an immersed heat exchanger to charge and/or extract the stored solar energy and (iv) exergetic analysis of a solar process heat system [21].

The present study differs from the previous ones due to the facts that: (i) it covers an exergetic evaluation of each components of the SWH system as well as the whole SWH system, including its collector, circulating pump and water storage tank, (ii) it contains a parametric study on the effect of varying water inlet temperature to the collector on the exergy efficiencies of the SWH system components, (iii) it investigates some thermodynamic parameters [22], such as fuel depletion ratio, relative irreversibility, productivity lack and exergetic factor as well as improvement potential, and (iv) it proposes an exergy efficiency curve similar to the thermal efficiency curve for the solar collector studied.

Section snippets

Theoretical analysis

Energy and exergy analysis is presented for the performance evaluation of the SWH systems. Balance (mass, energy and exergy) equations for steady state, constant-flow control volume systems, and appropriate energy and exergy equations are derived for this system and its components.

System description

Fig. 1 illustrates a schematic diagram of the experimental system tested at Ege University, Izmir, Turkey. This SWH system consists of mainly three parts: (i) the flat plate solar collector (2 m2 aperture area), (ii) the circulating pump and (iii) the heat exchanger with water storage tank. Water is circulated through the closed collector loop to a heat exchanger, where its heat is transferred to the potable water. The collector is oriented facing towards south, inclined at an angle equal to 45°

Conclusions

We have presented exergetic aspects of SWH systems in general and have evaluated the performance of a SWH system along with their essential system components (i.e., solar collector, circulating pump and heat exchanger) through a comprehensive exergy analysis. The experimental and assumed values are utilized in the analysis. The exergy destructions in the overall SWH system are briefly quantified, while some thermodynamic parameters are investigated.

We can extract some concluding remarks from

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    ᴪ in/out = (h in/out – ho) – To (s in/out – so). The exergy efficiency the ratio of useful delivered exergy (Exu) to collector absorbed exergy (Excoll) by the evacuated tube solar collector [43,44]. η exc = Exu / ExcollExu = ṁair [(h – ho) – To (s – so)]Exu = ṁair Cp air [(Tcoa – Tcia) – Tcoa (ln Tcoa/Tcia)]Excoll = AI [ 1 +{ (1/3) (Tamb/Tsr)4} – {(4/3) (Tamb/Tsr)}]

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