Exegetic Efficiency of Passive Solar Air Heater With Phase Change Energy Storage Material
Chapter One
PREAMBLE OF THE STUDY
Furthermore, a comprehensive exergy analysis assessing the magnitude of exergy destruction identifies the location, the magnitude and the source of thermodynamic inefficiencies in a thermal system. This knowledge is useful in directing the attention of process design researchers and practicing engineers to those components of the system being analyzed, that offer the maximum opportunities for improvement.
In addition, exergy analyses are a method that uses the conservation of mass and energy principles together with the second law of thermodynamics for the design and analysis of energy system. It can reveal whether or not, it is possible to design more efficient energy system by reducing inefficiencies in the system. The exergy method gives information on the quality of the energy transferred in latent heat energy storage systems such as PCMs and finally obtain the energetic and exergetic performance efficiency of PCMs.
CHAPTER TWO
LITERATURE REVIEW
Energy and Exergy efficiencies
A numer of existing documents of relevance to the study were reviewed. Key highlights are presented in this chapter.
Kotas(1985) and Bejan.(1988).They defined exergy as the maximum work potential derived from a reversible engine interacting with the environment to reach a dead state, taking temperature and pressure as the reference state. They employed second law of thermodynamic which emphasizes on irreversibility or entropy generation minimization to improve the performance of machine. They showed that exergy analysis can indicate the possibilities of thermodynamic improvement of any system. Their results recorded more meaningful efficiencies than those obtained using energy analysis alone.
Ozturk and Demire (2004) experimentally evaluated the energy and exergy efficiencies of the thermal performance of a solar air heater having its flow channel packed with Raschig rings.
Kurbas and Durmus(2004) experimentally evaluated the energy efficiency, friction factor and dimensionless exergy loss, of a solar air heater, having five solar sub-collections of same length and width arranged in series in a common case, for various values of Reynolds number.
Luminosu and Fara (2005) presented the determination of the optimal operation mode of a flat solar collector by exergetic analysis and numerical simulation. This paper proposes an exergy analysis of a flat plate –solar collector based on the assumption that Tfi =Te = constant (Te – environmental temperature)
The method has proven valuable in the design of solar collectors for the specific climatic insolation conditions of a certain region. And the exergy efficiency of a flat – plate solar collector, ex = f (m, Ac) presents points of local maxima and a point of global maximum.
Gupta and Kaushik (2008) discussed the performance evaluation of solar air heater for various artificial roughness geometries based on energy, and exergy efficiencies. It is found that artificial roughness on absorber surface effectively increases the efficiencies in comparison with smooth surface. The energy efficiency in general increases in the following sequence: smooth surface, circular ribs, v shaped ribs, wedge shaped rib, expanded metal mesh, rib- grooved, and chamfered rib–groove. The effective energy efficiency based criteria also follow same trend of variation among various considered geometries, and trend is reversed at very high Re. It is found that for the higher range of Re, circular ribs and V shape ribs give appreciable exergy efficiency up to high Reynolds number : while for low Re. chamfered rib -groove gives more exergy efficiency.
Gupta and Kaushik (2008) considered the exergetic performance evaluation and parametric studies of solar air heater. It was observed that the exergy output depends on heat gain and entropy created term. If the inlet temperature of air is low, then maximum exergy output is achieved.
Koca. etal.(2008) reported energy and exergy analysis of a latent heat storage system with phase change material for a solar collector. The exergy analysis, which is based on the second law of thermodynamics, and energy analysis, which is based on the first law, were applied for evaluation of the system efficiency for charging period. It was observed that the average net energy and exergy efficiencies are 45% and 2.2% respectively.
Enibe (2002) considered the performance of a natural circulation solar air heating system with phase change material energy storage. He tested the system experimentally under daytime no- load conditions at Nsukka, Nigeria over the ambient temperature range of 19 -410C. The results show that the system can be operated successfully for crop drying applications, with suitable values to control the working chamber temperature. It can also operate as a poultry egg incubator.
Enibe (2003) studied the transient thermal analysis of a natural convections solar air heater. It includes a single-glazed flat –plate solar collector integrated with a paraffin- type PCM energy storage subsystem as an application.
Byornar and Sandness (2003) studied the energy and exergy efficiencies of different types of solar system like photovoltaic, active and passive solar collectors.
Asada and Elisa (2003) investigated the energy of a low temperature radiant heating system. He considered the energy conversion and heat transfer steps in the building where heat is required in the incident solar radiation and in the heating system.
Zohor, Zaeem and Moosavi (2008) investigated the increase in solar thermal energy storage by using a hybrid energy storage system. They studied the energy involved in combining the phase change material energy storage and water with solar air heater. They developed a computer program for an optimization time schedule of changing the storage tanks during each day, according to the solar radiation conditions.
Kreith and Kreider(1978) did thermal analysis of flat plate solar collector and derived fin efficiency and useful energy of the collector.
Duffie and Backman (1991) did an extensive detailed work on collector design to obtain fin efficiency of the collector, flow factor and heat removal factor. They also worked on the phase change material energy storage to calculate the quantity of energy stored per hour, from solid to liquid phase.
Rajput (2007) did analysis of heat influx on three-dimensional heat conduction coordinate axes, using Fourier law equation.
Obi (2008) considered the Performance simulation of a natural circulation solar air heating system with phase change material energy storage for low temperature application.The predicted temperatures of the system is compared with the experimental data under daytime no-load condition over the ambient temperature range of 18.5-36.00C and daily global irradiation of 4.9-20.1 MJ/m2– day. The predicted temperatures agree closely with experimental data to within acceptable limits.
Rosen (1992) studied the energy and exergy efficiencies of close systems for thermal energy storage.
Kreith and Bohn (1993).studied the heat flow into a lump in a cylinder, in three coordinate axes.
Onyegegbu and Morhenne,(1993) studied Transient Multidimensional Second Law analysis of solar collector subjected to time-varying insolation with diffuse components. The instantaneous optimum flow rates were found to follow the insolation pattern. It was also found that the daily optimum exergetic efficiencies and optimum flow rates were 30% and 10% respectively.
Onyegegbu, Morhenne and Norton (1994) analyzed the Second law optimization of integral type natural circulation solar energy crop dryers. It is shown that operating the dryer at conditions of minimum entropy generation yields a useful criterion for choosing dryer dimensions and is compatible with the desire to maintain allowable limits on crop temperature.
Yunus and Michael (2006); considered the entropy generation in a closed system at a steady flows. They analyzed the entropy flows of different systems, both open and closed ones.
Adrian Bejan (1996) studied the entropy generation minimization. He used second law of thermodynamic mainly on different systems to analyze entropy generation of that system.
Rai (2006) looked into how to convert solar energy in different systems from one form to another and other sources of energy conversion.
SOLAR RADIATION
The amount of energy per unit time per unit area received from the sun outside the earth’s atmosphere at the mean earth-sun distance is known as the solar constant, which has a value of (1367+ 23) W/m2 (Duffie and Beckman,1991 ).The extraterrestrial radiation, however, will vary due to the earth’s elliptical orbit around the sun with the consequential variation in the earth-sun distance. The amount of incoming radiation that is not reflected back into space is attenuated by the earth’s atmosphere due to absorption and scattering. Direct or beam radiation reaches the surface with very little directional change. Radiation from the rest of the sky that has been scattered and eventually reaches the surface is known as diffuse radiation. The combination of both direct and diffuse radiation is called global radiation. Solar energy input or collector panels are dependent on the following factors:
- Date & Time
- Latitude
- Climate conditions
- Collector panel orientation
- Geometric properties of the solar collector
The earth’s axis is tilted at an angle of 23.450 relative to the orbital plane. This tilt is the main cause of the seasonal variations as the earth orbits the sun. It is convenient to assume an apparent daily motion of the sun across the sky for all solar geometrical calculations. This motion varies cyclically throughout the year and is defined by the angle of declination, δ, which is the angular position of the sun at solar noon (i.e. when the sun is on the local meridian) with respect to the plane of the equator, (Duffie and Beckman, 1991). This angle varies within + 23.450, affecting the angle of incidence of solar radiation on the surface of the earth and causing seasonal variations in the length of the day.
CHAPTER THREE
ENERGY ANALYSIS OF THE ABSORBER PLATE
Kf– is the flow resistance coefficient, For laminar flow of gases through tubes, Kreith and Bohn (1993) recommend the relation as;
N = 1.86(R P D / L)1 / 3 (m / m
)0.14
(3.3
u e r s
Where D is the flow area or wetted perimeter and L is the Length. The range is
10 < Re Pr D / L < 1500; 0.0044 < (m / ms ) < 9.75
for fully developed laminar flow with L/D > 100
for longer channels (L/D>60) in turbulent flow, the Nusselt no is given by
Nu = 0.023 Re 0.8 Pr n
(3.4)
Where
CHAPTER FOUR
RESULTS AND DISCUSSION.
Energy and exergy analyses were performed for a solar collector with thermal energy system (TES) unit using paraffin wax as a PCM. From the literature, the exergy analysis is a better method to improve system performance efficiency. Analysis of exergy takes into account, the loss of availability of heat in TES. Thus, it reflects the thermodynamic, energy and economic value of the system.
In this study, the experimental data, reported in Ugwu and Ujah (2006) and Obi (2008) were used as an input to the energy and exergy model equations in chapter three. The dada analysis was undertaken using the Engineering Equation Solver (EES), software. The program code is presented in Appendix A. The graphs were plotted from the results in the tables 4.1 to 4.11, which were obtained after running the computer program of the equation model. The graphs and their descriptions are highlighted below:
CHAPTER FIVE
CONCLUSION
A complete analysis of the flat-plate solar collector with phase change material energy storage subjected to time varying insolation was carried out. The analysis of the system which is based on the first and second law of thermodynamics was carried out, such as, the energy of the collector, the exergy of the collector, the entropy generation of the collector, energy of the PCM, the exergy of the PCM and the entropy generation of the PCM. Input parameters reported by Ujah and Ugwu (2006) and Obi (2008) were applied on the equation model, to generate results using computer program. Graphs were plotted for each day, between 9 hours to 16 hours, which reveals the daily amount of energy and exergy storage by the system for ten days.
The energy and exergy efficiencies of PCM are considered with one- dimensional heat conduction in cylindrical pipe for storing periods. Exergy analysis was employed in order to improve system performance efficiency by reducing system irreversibility or internal loss or loss of availability which leads to maximization of energy system performance efficiency. Energy efficiency of the system was improved by minimizing energy losses to the environment. The analysis reveals that solar radiation and temperature are the most effective parameters and the average energy and exergy efficiencies were 48% and 35% respectively.
REFERENCES
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