Design, Construction And Performance Evaluation Of A Passive Solar Water Heater

Design, Construction And Performance Evaluation Of A Passive Solar Water Heater

Abstract

A thermosyphon solar water heating system which captures and utilises the abundant solar energy to provide domestic hot water was designed, simulated, constructed and tested. The system was designed to supply a daily hot water capacity of 0.1m3 at a minimum temperature of 70oC for domestic use. The design approach was in three parts; firstly, since solar radiation and weather data which are driving function for solar systems design vary randomly with time, the monthly average daily solar radiation and weather data obtained from the typical meteorological year (TMY) solar data of Zaria were used to determine the design month as the month (August) with the least monthly average daily solar energy ratio. Solar radiation and weather data of the design month were used to design the system. Secondly, the design month solar radiations and weather data were used as input into the design equations coded using MATLAB programming language to determine the system characteristic and components sizes. A parametric study was also carried out to study the effects and sensitivity of varying some design parameters such as number of glass covers , collector tube centre to centre distance W, absorber plate thickness, collector tube internal diameter and collector tilt angle on the design objective function (the heat removal factor ). Thirdly, based on the values of the system characteristics and components sizes obtained from the design calculations and the parametric study, a model for the performance simulation of the system was formulated using the Transient System Simulation (TRNSYS) software. This model was used to predict the annual hourly performance of the system for recommended average day of the months using the TMY solar radiation and weather data of Zaria as input function. The system was then constructed based on the component sizes adopted for the simulation owing to the satisfactory performance of the system as revealed from the simulated results. To validate the simulated system performance, system performance tests were conducted for 3 days and the results were compared with the simulated results. The root mean square error (RMSE) and the Nash-Sutcliffe Coefficient of Efficiency (NSE) statistical tools were used to analyse the experimental and simulated results in order to validate the predictive power of the software. The results of this research led to the conclusion that a thermosyphon solar system with collector area of 2.24 m2 operated under the weather condition of Zaria, would be capable of supplying a daily domestic water of 0.1m3 at temperature ranging from 59oC for the worst month (August) to 81oC for the best month (April).The computed Nash Sutcliffe Coefficient of Efficiency (NSE) values of 0.663, 0.956 and 0.885 and the low RMSE values of 8.09oC, 3.65oC and 5.31oC between the modeled tank inlet temperature and the observed tank inlet temperature for the three days tests conducted indicated that the model formulated using TRNSYS software was valid and closely agreed, capable of predicting the performance of the system with a 66.3 %, 95.6% and 88.5 % degree of accuracy for the 3 days that the experiments were conducted respectively.

Chapter One

Introduction

1.1 Background of the Study

Energy is considered a prime agent in the generation of wealth and a significant factor in economic development. The importance of energy in economic development is recognized universally, and historical data verified that there is a strong relationship between the availability of energy and economic activity (Soteris, 2004). Although in the early seventies, after the oil crises, the concern was on the cost of energy, however during the past two decades, the risk and reality of environmental degradation have become more apparent. The growing evidence of environmental problems is due to a combination of several factors, since the environmental impact of human activities has grown dramatically (Soteris, 2004). This is due to the increase of the world population, energy consumption and industrial activities. Achieving solutions to the environmental problems that humanity faces today requires long term potential actions for sustainable development. Renewable energy resources appear to be one of the most efficient and effective solutions.

Of all the renewable sources of energy available, solar thermal energy is the most abundant one and is available in both direct as well as indirect forms. The Sun emits energy at a rate of 3.8 x 1023 kW, of which, approximately 1.8 x1014 kW is intercepted by the earth, which is located about 150 million km from the sun. About 60% of, this amount reaches the surface of the earth. The rest is reflected back into space and absorbed by the atmosphere. About 0.1% of this energy, when converted at an efficiency of 10% would generate four times the world‟s total generating capacity of about 3000 GW(Mirunalini, et al.,2010). It is also worth noting that the total annual solar radiation falling on the earth ismore than 7500 times the world‟s total annual primary energy consumption of 450 EJ (Mirunalini, et al., 2010). The annual solar radiation reaching the earth‟s surface, approximately 3,400,000 EJ, is an order of magnitude greater than all the estimated (discovered and undiscovered) nonrenewable energy resources, including fossil fuels and nuclear energy ( Mirunaliniet al. , 2010). However, 80% of the present worldwide energy utilisation is based on fossil fuels.

World demand for fossil fuels (starting with oil) is expected to exceed annual production, probably within the next two decades (Mirunaliniet al., 2010). International economic and political crisis and conflicts can also be initiated by shortages of oil or gas. Moreover, burning fossil fuel releases harmful emissions such as carbon dioxide, nitrogen oxides, aerosols, etc. which affect the local, regional and global environment. By means of different mechanisms, solar radiation may be converted into other forms of energy, such as photovoltaic conversion into electrical energy, photochemical conversion into chemically bound energy, and photo thermal conversion into heat. The heat converted from solar radiation, is well suited to provide domestic hot water and space heating. In most parts of the world, the yearly solar radiation received by a single family house is several times greater than the energy needed for domestic hot water and space heating(Mirunaliniet al., 2010).

For many years, solar domestic hot water (DHW) systems have gained great attention due to their considerable energy conservation, environmental protection and relatively good economy. The purpose of using a solar DHW system is to convert the solar radiation into thermal energy, and then to use it for domestic hot water heating, thus reducing the over dependence on and consumption of conventional energy. Recently, environmental issues have led to an even greater interest in solar DHW systems. There are several fundamental conditions that make solar DHW systems very different from conventional fossil-fuel systems. Firstly, the power density of solar radiation is relatively low and the collector has to cover a large area. Thus, the solar DHW systems cannot be as compact as conventional units. Secondly, the solar radiation varies considerably during the day, in the course of a year and between different locations. Therefore, the solar energy received by a collector is an irregular function of time and location, and the power output of the collector cannot be controlled in the same way as conventional heating systems. Consequently, heat storage and auxiliary energy are required to match the supply to the load.

1.2 Statement of the Problem

There is a strong consensus among climate scientists that the environmental problems now observed is caused by human activities targeted to meeting our energy demand, especially the combustion of fossil fuels. When oil, gas, or coal are burned to generate electricity or provide heat, the products of the combustion which include carbon dioxide and nitrous oxide, lead to global warming and acid rain deposition, respectively . The expected impacts of global warming include sea-level rise flooding of coastal areas increased frequency and severity of floods, draughts, storms, and heat waves, reduced agricultural production, massive species extinction, and the spread of vector-borne diseases such as malaria and dengue fever (Christopher and Homola, 2006). Thus, the manners in which we produce and consume energy (conventional way) are to a large extent responsible for this impending environmental problem (Intergovernmental Panel on Climate Change (IPCC), 2001).

According to Christopher and Homola (2006), rising economic losses due to weatherrelated disasters are part of a trend being linked to climate change. The World Health Organization estimates that climate change is already responsible for 150,000 deaths annually (Christopher and Homola, 2006).

Domestic hot water use again represents a large proportion of domestic energy need. This energy need accounts for approximately one third of the total annual energy consumption for domestic purposes and therefore a greater portion of the family income is spent on domestic hot water (Retscreen International, 2004).

1.3 The Present Research

This research involves the design, simulation, construction and performance tests of a solar domestic hot water heating thermosyphon system for Zaria, Nigeria, located on latitude 11.2o N and longitude 7.8oN. The design method employed is the simulation based method, where mathematical models for the determination of the system design parameters and characteristics were coded into a computer programme using the Matrix Laboratory (MATLAB) software in a manner that represents the conceptual design of the system.

The effects and sensitivity of the system design parameters on the collector heat removal factor were studied through programmes codes written in MATLAB in order to determine the size of the various components of the collector that will give better performance.

The system performance was simulated using the Transient Systems Simulation (TRNSYS) software for recommended average days of the months. The system was then constructed based on the adopted system configuration and components‟ sizes obtained from the studies. The performance of the system was then experimentally determined and the results obtained from the test were compared with the simulated results in order to validate the formulated model used for the performance simulation.

1.4 Aim and Objectives

The aim of this research is to design, simulate, construct and test the performance of a solar domestic hot water thermosyphon system for the city of Zaria, Nigeria.

The specific objectives are:

1. To carry out a parametric study on the effect and sensitivity of the tilt angle, , number of glasing , , absorber plate thickness, , collector tube diameter, and collector tube centre to centre distance, , on the objective function which is the percentage expression of the heat removal factor, , using Matrix Laboratory (MATLAB) programming language.
2. To predict through simulation using TRNSYS, the annual performance of the system.
3. To validate the predicted system performance through experiments.
4. To estimate the cost of the system.

1.5 Significance of the Research

Environmental concerns about global warming, local pollution and reduction of over dependence on conventional energy source for domestic hot water need in Zaria, is the primary impetus for this research. This research would provide alternative way to providing hot water for domestic use in various homes. It also has the potential of reducing family utility bills and thereby improving family savings.

Chapter Five

Summary, Conclusions and Recommendations

5.1 Summary

A thermosyphon solar water heating system to supply 100 litres at a minimum temperature of 59oC has been successfully designed, simulated, constructed and tested in the Department of Mechanical Engineering, Ahmadu Bello University (A.B.U) Zaria, Nigeria (latitude 11.2oN and longitude 7.8o E).The design approach was in three parts; firstly the typical meteorological year (TMY) solar data of Zaria was processed to obtain the monthly average daily solar radiation of Zaria for recommended average day of the months. The month of August with the worst amount of the average daily solar radiation was considered as the design month and solar radiation and weather data of this month was used as input data for system design and parametric studies.

Secondly, a parametric studies which studies the effect and sensitivity some selected system components (i.e copper tube diameter, number of glass covers, absorber plate thickness, and centre to centre tube distance) on the design objective function (the heat removal factor. ) were studied through programs written in MATLAB programming language in order to determine the appropriate components size for each component based on solar radiation for the design month.

Thirdly the annual performance (collector and storage tank temperature) of the system was simulated under weather data of Zaria using TRNSYS 16 software. The system was then constructed based on the component sizes and system dimensions used for the simulation of the system. A 3 day test was conducted to experimentally evaluate the performance of the system and the results were compared with the simulated results. The RMSE and NSE statistical tools were employed to compare the experimental results with the simulated results in order to determine the predictive power of the simulation software.

5.2 Conclusions

The results of this research led to the following conclusions:

1. A thermosyphon solar system with collector area of 2.24 m2 and tilted at angle of 12o to the horizontal operated under the weather condition of Zaria, would be capable of producing daily domestic hot water of 0.1m3 at a minimum temperature of 59oC at the end of the day for the worst month and 81oC for the best month.
2. The parametric study reveals that for a solar collector with collector area of 2.24m2 and collector tubes diameter of 0.2m,the heat removal factor is affected in the following ways:
   1. The heat removal factor, ,for a tube diameter of 0.03m increases from a minimum value of 0.446, reaching a maximum value of 0.575 representing an increase of 28.92 % as the tube centre to centre distance, W, was increase from 0 to 0.3m. Further increase in W from 0.3m to 2.0m, decreased the heat removal factor from a value of 0.575 to 0.350 representing a drop of 39.13%.
   2. The heat removal factor increases from a value of 0.6178 to 0.6195 as the plate thickness increases from 0.003m to 0.075m. The increase in represents an insignificant increase of 0.31% in heat removal factor..
3. The computed values of Nash-Sutcliff coefficient of 0.663, 0.956 and 0.885 and the low RMSE values of 8.09oC, 3.64oC and oC between the modeled tank inlet temperature and the observed tank inlet temperature for the three day test confirm that the model formulation using TRNSYS software proposed here for the performance simulation of the system is valid, realistic and is a representative of the real system and can be used confidently to estimate the dynamic behavior of the real system owning to the good quality of fit between experimental results and the simulated results.
4. The collector hourly average efficiency varied with time. The efficiency increased from 16.5% in the morning (8.00 hours) to 66.1% at 10.00 hours and decreased to a value of 19.0 % at the end of the day (17 hours).

5.3 Recommendations

The absorber plate of the solar collector in this study was placed above the copper tubes carrying the circulating water. It would be important here again to recommend that an investigation should be conducted to know if the absorber plate placed below the collector copper tube will give better performance of the system.

It is also recommended that the annual life cycle savings (ALCS) analysis should be conducted so as to compare the difference between the overall cost of a conventional plant that supplies all the desired hot water, and the overall cost of the solar water heating system so as to justify the huge capital investment in the solar system.

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