14th International Combustion Sysmposium (INCOS2018) 25-27 April 2018 EXERGY AND THERMOECONOMIC ANALYSIS OF NON-CONDENSING AND CONDENSING BOILERS Hamdi S. Celik*, L. Berrin Erbay+ #Vaillant Group Turkey Turk Demirdokum Fabrikalari 4 Eylul Mah. Ismet Inonu Cad. No:263 11300 Bozuyuk Bilecik TURKEY [email protected] +Mechanical Engineering, Eskisehir Osmangazi University Eskisehir, Turkey [email protected]

Abstract— In this paper; re-evaluating waste heat, from flue of it is important to see how system exergy changes with combi-boilers and the effect on energetic and exergetic efficiencies decreased flue gas temperature. of device have been researched. Flue-gas of a hermetic boiler In our study, exergy analyses of both condensing and non- temperature is measured and then, if it is reduced to desired value, condensing domestic type boilers were investigated. Effect of waste-heat energy was calculated and with an assumed recovery heat exchanger for flue gases (recuperator) was recuperator; this energy was used for the boiler, by this way amortization time is found, also system benefits are emphasized to compared. Additionally, differences of costs, flue emissions environment. Consequently, advantages of preferring condensing and environmental effects were discussed for both systems. boiler is discussed with comparisons in the consist of this research. II. MATERIALS AND METHOD Keywords— Exergy Destruction, Effectiveness, Heat Transfer, Domestic type boilers are devices which consumes natural Recuperator gas or liquid petroleum gas to generate heat which is transferred by heat exchanger to circulated water in system. There are two I. INTRODUCTION main group of domestic type boilers based on their condensing Recovery of waste energy by reusing it in arisen system or technology. to power up another cycle has huge importance to utilise diminishing energy resources. Through, both new resource A. Non-condensing Boilers requirement and harm to the environment is decreased. Exergy Flue gases of non-condensing boilers leaves system without analysis has a key factor in energy efficiency. another contact by power provided by fan in system (Fig. 1). Exergy shows performance of a system more conspicuously Due to absence of additional heat transfer apparatus, to energy by describing whole energy transfer and temperature of flue gases reaches about 140-150 °C on exit of irreversibility due to exergy destruction [1]. Exergy analysis chimney and that results in both loss of energy and adversely has been primarily subject that designers and greenhouse effect. thermodynamicists concentrated, especially in last 20 years. Exergy is defined as generated work due to balance between B. Condensing Boilers material and general components of environment by reversible In last decades, researchers and companies focused on processes [2]. overcoming of disadvantages described above and given efforts Exergy on heat generating devices is interesting subject resulted in generation of condensing boilers. There are different which exergy analysis has also been considered significantly. types of condensing boilers which have same main operation. Exergy analysis for flue gas of a boiler was investigated [2]. It Unlike non-condensing boilers, on condensing boilers, flue gas was calculated that if supplemental recovery system with 3,35 is passed over further heat exchange element to decrease their kW exergy loss is implemented, 3.38% of exergy loss can be temperature and steam in flue gas condenses to water. On these restored. systems, temperature of flue gases are about 50-60 °C. On the Erbay, Yılmaz and Yeşilaydın [3] inspected thermodynamic other hand, more powerful fan and pump are used in system in analysis of heat exchangers in point of irreversibilities and order to compensate pressure drop of gas and water due to entropy generating and noted the effect of irreversibilities on further heat exchange element. heat exchanger design and optimisation. They suggested that effect of entropy generation on heat transfer should also be In this study, exergy analyses of commercial non- evaluated. condensing and condensing boilers constructed in our company In another study, Kalina cycle was used for lowering flue gas were studied and influence of exergy to performance was temperature [4]. It was shown that decreasing flue gas shown. Recuperator, which is an extra heat exchanger placed temperature by 44 K increases exergy efficiency by 0,255% and after combustion chamber, was used to enable condensing (Fig.

379 14th International Combustion Sysmposium (INCOS2018) 25-27 April 2018

2). In this system, water in instalment returns to boiler and is  Heat transfer between system and environment was pumped to recuperator. Water passes to main exchanger and neglected, then leaves boiler to instalment. Water flow across whole  In calculations of recovered heat on recuperator, system observed and these data used in exergy analysis of flue thermophysical properties of air used for flue gas. gas. In order to calculate continuous flow open systems mass flow rate becomes important. Mass conservation equation for multi entrance and exit system is shown in Equation 1 [3], [4], [5]. ∑ 푚̇ 𝑖 = ∑ 푚̇ 푓 (1) Also energy conservation is given for given in Equation 2. Potential and kinetic energies were neglected during in this study [5]. 푉2 푉2 ∑ 푄̇ − ∑ 푊̇ = ∑ 푚̇ 𝑖 (ℎ + + 𝑔푧) − ∑ 푚̇ 푓 (ℎ + + 𝑔푧) 2 𝑖 2 푓 (2) where Q is total heat transfer between system and environment and W is work done in control volume. Based on our second assumption Q was zero. Efficiency of first law of thermodynamics is defined as ratio of total enthalpy of material exits control volume to total enthalpy of material enters control volume [5], [6]. Algebraic definition of efficiency is given as Equation 3. ∑ 퐸̇푓 휂퐼 = (3) ∑ 퐸̇푖 Exergy calculation depends of some properties of environment such as temperature, pressure and chemical composition and assumption of reversible process exists. As in Fig. 1 Non-condensing boiler system schematic all other thermodynamic analyses, information about properties at entrance and exit of reversible process are adequate [5], [7]. Exergy input and output of continuous flow open control volume are generally calculated as in Equation 4 and 5. 푉2 퐸̇ 푥 = 푚̇ [(ℎ − ℎ ) − 푇 (푠 − 푠 ) + 푖 + g푧 ] (4) 𝑖 𝑖 𝑖 0 0 𝑖 0 2 𝑖 푉2 퐸̇ 푥 = 푚̇ [(ℎ − ℎ ) − 푇 (푠 − 푠 ) + 푓 + g푧 ] (5) 푓 푓 푓 0 0 푓 0 2 푓 Exergy balance for continuous flow open systems; 퐸̇ 푥𝑖 − 퐸̇ 푥푓 = 퐸̇ 푥퐷퐿 (6) where 퐸̇ 푥퐷퐿 consists both exergy destruction and all exergy losses. Destructed exergy is irreversible but losses can be recovered. Exergy efficiency which describes energy quality is formulated as in Equation 7 [1], [2], [6], [7]. ∑ 퐸̇ 푥퐿 휂퐼퐼 = 1 − (7) ∑ 퐸̇ 푥푓

III. CALCULATIONS

A. Energy and Exergy Analysis of Boilers

1) Non-condensing Boiler (NCB) Analysis Fig. 2 Condensing, including recuperator, boiler system schematic Control volume of system, as shown in Fig. 3, was drawn and all inputs and outputs for the system were determined Theoretical study was made before initiating exergy and before analysis (Table I). These data were used for calculations energy analyses. Some assumptions had to be made afore of energy and exergy according to first and second laws of thermodynamic calculations: thermodynamics.  All control volumes were continuous flow open systems, there is no time dependently change on any point,

380 14th International Combustion Sysmposium (INCOS2018) 25-27 April 2018

퐸̇ 푥1 = 푚̇ 1 [(ℎ1 − ℎ0) − 푇0(푆1 − 푆0)] 푄̇ = 퐸̇ 푥𝑖 ∗ 휂푎𝑖푟

TABLE I

BOILER PARAMETERS Input Parameter NCB CB Water flow rate (l/s) 0.2 Fan power (W) 30 80

Pump power (W) 75 100

Natural gas flow rate (l/s) 12.5 Lower heating value of natural 8250 gas (kcal/m3) Combustion energy of natural 25863,75 gas (W) Input water temperature (°C) 30 Fig. 3 Control volume of non-condensing boiler system including inputs and t1 outputs Output water temperature (°C) 50 51.6 2) Condensing Boiler (CB) Analysis t2

In condensing boiler, there was additional component, Input air temperature (K) t0 298 recuperator, inside the system (Fig. 4). Flue gas temperature (°C) 140 60 hinput (kj/kg) 298.3 Toutput (K) 413 333 P1-2 (bar) 1 h1 (kj/kg) 125.79 h2 (kj/kg) 209.33 216.57 S0 of air at 298 K (kj/K) 1.701 S1 of air (kj/K) 2.016 1.75 S2 of air at 880 K (kj/K) 2.823 hair-input at 880 K (kj/kg) 910.56 hair-output (kj/kg) 412 313.4 Condensation heat of air-Ly 2256 (kj/kg) ηair (kg/s) 0.138

B. Cost Analysis of Systems Cost comparisons were made assuming that both systems were used in a city, like Eskisehir, where winters are cold and Fig. 4 Control volume of condensing boiler system including inputs and outputs 120 days operated. As discussed previously, on condensing As mentioned in assumptions, in absence of real boilers energy consuming equipments are more powerful than thermophysical values for flue gases, properties of air were non-condensing boilers to overcome pressure drops. However, taken as data. Based on assumption of temperature difference recuperator which provides 2 kW heat recovery has key factor on flue gas is 80 °C and efficiency of recuperator is 60%, 2 kW on natural gas consumption. 3 can be recovered. This results about 1.6 °C increase on output Natural gas price supposed as 1.10 TRY/m as average in water temperature. Eskisehir on 2017 and electric price supposed as 0.5 TRY/kWh. Natural gas was supplied to system for heat generation and There is about 300 TRY more investment per product to fan and circulation pump consumed electrical energy. By producing condensing boiler. According to these data, neglecting potential and kinetic energy changes and based on redemption time of condensing boiler system was calculated. equality of mass across the system, energy conservation IV. RESULTS formula was generated as follows; ̇ ̇ 푄 − 푊 = 푚̇(ℎ1 − ℎ2) A. Energy and Exergy Analysis of Boilers Theoretical work output of system was used to calculate All calculations for both boilers were shown in Table II. work efficiency of system. Calculations showed that only 65% of power generation ̇ ( ) 푊푡ℎ = 푚̇[ ℎ1 − ℎ2 − 푇0(푆1 − 푆2) potential was transformed to available power for non- Exergy of system calculated in order to determine heat condensing boiler where this ratio on condensing boiler reached output of system and then to calculate total efficiency.

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70.5%. While energy losses on non-condensing system were REFERENCES including heat transfer between system and environment over [1] L. B. Erbay and B. Cam, “Energy and exergy analysis of Eskişehir control surface and high temperature flue gas, heat recovery Kazım Taşkent sugar plant,” Journal of Engineering and Machinery, vol. over recuperator provided to system again. Moreover, 52, pp. 60-70, 2011. [2] K. Comakli and M. Terhan, “Exergy analysis of waste flue gas coming diminished flue gas temperature resulted that lower CO out a natural gas-fired boiler,” Journal of Engineering and Machinery, emission. In real time tests on condensing boiler, it was seen vol. 56, pp. 58-64, 2015. that emission of CO was decreased in ratio of 50% compared [3] H. Yilmaz, İ. Yesilaydin and L. B. Erbay, “Entropy generatıon and to non-condensing boiler. temperature cross phenomena at heat exchangers,” Journal of Engineering and Machinery, vol. 54, pp. 38-59, 2013. TABLE II [4] O. K. Singh and S. C. Kaushik, “Energy and exergy analysis and optimization of Kalina cycle coupled with a coal fired steam power NON-CONDENSING BOILER CALCULATED VALUES plant,” Applied Thermal Engineering, vol. 51, 2013. [5] N. Sato, Chemical Energy and Exergy: An Introduction to Chemical Thermodynamic Calculations NCB CB Thermodynamics, Elsevier Science & Technology Books, 2004. Work output (W) 24957 27054 [6] Cengel and Boles, Thermodynamics. An Engineering Approach, Literatur Publications, 1996. Theoritical work ouput 25770 27462 [7] J. Szargut, Exergy Method Technical and Ecological Applications, WI Press, 2005. Work efficiency 0,968 0,985 Exergy efficiency 0,65 0,705

B. Cost Analysis of Systems First of all, natural gas consumption requirement was calculated in consideration of recovery on recuperator. 2000 푊 푚3 푚̇ = = 5 푘푐푎푙 4184 푗 1 푑푎푦 (8250 ) ∗ ( ) ∗ ( ) 푑푎푦 푚3 푘푐푎푙 86400 푠 For total working period, total consumption and price of this consumption calculated as follows; 푚3 5 ∗ 120 푑푎푦푠 = 600 푚3 푑푎푦 푇푅푌 600 푚3 ∗ 1.1 = 660 푇푅푌 푚3 Although there was prevention of expenditure via natural gas efficiency, electric consumption and cost difference coming from pump and fan differentiation. (80 + 100) − (30 + 75) = 65 푊 120 푑푎푦푠 푘푊ℎ 65 푊 ∗ ( ∗ 24 ℎ) = 187.2 1 푦푒푎푟 푦푒푎푟 푘푊ℎ 푇푅푌 푇푅푌 187.2 ∗ 0.5 = 93.6 푦푒푎푟 푘푊ℎ 푦푒푎푟 Total saving was about 566 TRY/year and that showed redemption of a recuperator is about half of a year for itself. Assuming lifetime of a boiler is about 10 years, this result can be considered that recovered energy can redeem boiler’s price twice.

V. CONCLUSIONS Two typical domestic heating devices, non-condensing and condensing boilers, in market were compared with regard to exergetic and thermoeconomical manners. It was shown that available power was higher on condensing boiler based on recovered energy from flue gas. This feature also ensures that CO emission is lower and condensing boilers more environment friendly product than non-condensing boilers. Moreover, cost efficiency of condensing boiler was revealed and redemption period was presented.

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