Energetic and Exergetic Evaluation of Biomass Fired Water Heating System

This paper deals with thermal and exergy efficiencies of biomass fired water heating system. Water heating system is extensively suitable to generate hot water in rural areas. The developed water heating system was tested with Desi babul ( Acacia nilotica ) wood. Thermal and exergy efficiencies of the system were estimated at 54.5 percent and 6.79 percent, respectively. Citation:  Panwar, N. L., and Paul, A. S. (2017). Energetic and Exergetic Evaluation of Biomass Fired Water Heating System. Trends in Renewable Energy, 3, 153-159. DOI: 10.17737/tre.2017.3.2.0043


Introduction
It is well known fact that biomass is an indirect source of solar energy, and it is a renewable energy resource available where the climatic conditions are favorable for plant growth and production [1][2][3].Biomass combustion is going to increase worldwide for a provision of heat and electricity.Approximately 60% of total biomass is used for energy purposes are traditional biomass that is fuel wood, while crop residues and the remaining biomass are used for modern bio-energy.Biomass can play vital role in responding to concerns over the protection of the environment as well as the security of an energy supply [4][5].From ancient time, biomass is the main fuel which is used for cooking and water heating.But due to urbanization and industrialization, these biomass are replaced by modern fuel like kerosene, LPG and electricity.However, these conventional sources of energy are exhaustible in nature.It is essential to find an economical, convenient and efficient way to replace these conventional sources by renewable sources like biomass and solar as alternate fuel with improved technology to maximize the energy efficiency.
Comprehensive literature is available on various aspects of biomass combustion devices.However, literature on exergy analysis of biomass fired water heating system is limited.Saidur et al. performed an exhaustive literature survey on the exergy assessment of various biomass that can be used as fuel for cookstove [6].Tyagi et al. presented an experimental and comparative performance evaluation, using energy and exergy analyses, of four metallic types of cook stoves [7].Further, Panwar presented a study on assessment of energy and exergy of improved biomass cookstoves [8].Biomass fired water heating systems are capable to generate hot water whenever it is required, and significantly contribute to reduction of greenhouse gases.Despite these advantages, there is very little literature available on energetic and exergetic analyses of such water heating system.154 Therefore, the present experimental study was conducted to assess the energetic and exergetic performance of the biomass fired water heating system.

System Description
The biomass-fired water heating system consists of two concentric cylinders, i.e. the inner cylinder and the outer cylinder made of stainless steel SS 304.Actual combustion takes place in the inner cylinder, which has a diameter of 20 cm, whereas the outer shell has a diameter of 30 cm.During the experiment, the water was poured between these two cylinders.The outer shell was insulated with glass wool to minimize heat loss.The grate was made of a mild steel round bar.To maintain the proper draft during the combustion of the biomass, a chimney was placed on the top of the combustion chamber, as illustrated in Figure 1.One water tap was placed at on the upper side of the water tank to drain hot water, and another tap was placed at the bottom side to drain the water when the system is not in use.The technical specification of the developed water heating system is presented in Table 1.Thermal Performance Proximate analysis of the fuel -Desi Babul wood (Acacia nilotica), which was purchased from a local market, was carried out prior to the test by using the method suggested by the literature [9].The physical and thermal properties of the fuel wood are presented in Table 2.
Seven trials were undertaken under different conditions to study the thermal efficiency of the developed system.The calorific value of fuel wood was calculated by using a digital bomb calorimeter (Advance Research Instruments Company, Delhi, India).A multi-channel temperature scanner (ADI-Vadodara, Gujarat, India) with calibrated NiCr-Ni thermocouples was used to measure the ambient air temperatures, water inlet and out temperature.

Eqn. 3
Exergy Analysis Energy assessment is the customary method of assessing the way that energy is used in an operation involving the physical or chemical processing of materials and the transfer and/or conversion of energy.Energy analysis is based on the first law of thermodynamics: Net heat supplied is converted in order to work.Thus, energy analysis ignores reductions of energy potential and can provide sound management guidance for those applications in which the usage effectiveness depends exclusively on energy quantities.Such analyses are suitable for sizing and analyzing of systems using only one form of energy [10].
The exergy contents of biomass can be calculated by using their higher heating or lower heating values.Both higher and lower heating values are the function of the weight fraction (wt%, dry) of the chemical composition of biomass such as carbon (C), hydrogen (H), oxygen (O), nitrogen (N), etc.The higher heating value (HVV) of biomass can be calculated by using the correlation proposed by [11][12].
Eqn. 5 where  2   and   state the moisture content of biomass and the weight of hydrogen in biomass.
Correlation can be used for estimating the exergy of biomass suggested by [14].where  is the quality factor and can be calculated as follows: Peer-Reviewed Article Trends in Renewable Energy, 3 Tr Ren Energy, 2017, Vol.

Thermal Performance
Thermal efficiency of the developed system was estimated using equations (1-3).To assess the thermal performance, total 80 liters of water was used to raise the temperature from 30°C to average temperature about 61°C in one hour, and one kg of babool (Acacia nilotica) wood was consumed.The developed biomass fired system illustrated in Figure 2. Thermal efficiency was estimated at about 54.50 %.

Table 1 .
Technical specification of biomass fired water heating system.