ABSTRACT
In this study, a gas assisted solar powered absorption air – conditioning system was designed, constructed and tested under Zaria weather conditions in Nigeria. A technical and economic viability study of the system was first carried out by developing and simulating a TRNSYS model of the system to cool an office block with an average space cooling demand of 151.8kWh/m2 per annum, under Zaria weather conditions. System components were sized and optimized to give maximum coefficient of performance (COP), solar coefficient of performance (SCOP) and solar fraction (SF). Design of the optimised system components was done through a programming based design, optimisation and performance evaluation method using MATLAB. A reduced size version of the system, which includes a 4 kW absorption chiller prototype was constructed. Experimental tests were conducted on the constructed system. The experiments were carried out on a test room in the months of April and June 2017. The experimental days were classified as: hot clear sky days, fairly clear sky days, cloudy days and rainy days. Results indicated that the system is capable of operating in any of the days as classified. However, the cloudy days recorded the highest gas consumption of 0.95kg, as the system relied more on the gas boiler heating mode. The hot clear sky days recorded the least gas consumption of 0.48 kg as the system operated more on the solar power heating mode. Room temperatures were cooled to between 24oC and 27oC, from initial temperatures ranging from 29oC to 33oC. Maximum cooling power of 2.7 kW was achieved by the absorption chiller. Maximum daily average COP of 0.438 was attained. Solar fraction of 0.55 was achieved in the experimental day of April, while 0.27 was attained for June. Results from life cycle cost savings simulation showed energy savings of N2, 172,558 with 35m2 collector area for a period of 20 years. Life cycle cost savings analysis of the constructed system showed the system is economically viable in the Nigerian climate studied, with a savings of 𝑁39,991.93 achievedwith a collector area of 1.7 m2over a 20 year period. Environmental impact assessment showed CO2 emission reduction in the range of 2.7% to 58.4%. It was concluded that the system operates satisfactorily in the Nigerian climate studied and it is economically viable. Recommendations for further improvements were made.
TABLE OF CONTENTS
In this study, a gas assisted solar powered absorption air – conditioning system was designed, constructed and tested under Zaria weather conditions in Nigeria. A technical and economic viability study of the system was first carried out by developing and simulating a TRNSYS model of the system to cool an office block with an average space cooling demand of 151.8kWh/m2 per annum, under Zaria weather conditions. System components were sized and optimized to give maximum coefficient of performance (COP), solar coefficient of performance (SCOP) and solar fraction (SF). Design of the optimised system components was done through a programming based design, optimisation and performance evaluation method using MATLAB. A reduced size version of the system, which includes a 4 kW absorption chiller prototype was constructed. Experimental tests were conducted on the constructed system. The experiments were carried out on a test room in the months of April and June 2017. The experimental days were classified as: hot clear sky days, fairly clear sky days, cloudy days and rainy days. Results indicated that the system is capable of operating in any of the days as classified. However, the cloudy days recorded the highest gas consumption of 0.95kg, as the system relied more on the gas boiler heating mode. The hot clear sky days recorded the least gas consumption of 0.48 kg as the system operated more on the solar power heating mode. Room temperatures were cooled to between 24oC and 27oC, from initial temperatures ranging from 29oC to 33oC. Maximum cooling power of 2.7 kW was achieved by the absorption chiller. Maximum daily average COP of 0.438 was attained. Solar fraction of 0.55 was achieved in the experimental day of April, while 0.27 was attained for June. Results from life cycle cost savings simulation showed energy savings of N2, 172,558 with 35m2 collector area for a period of 20 years. Life cycle cost savings analysis of the constructed system showed the system is economically viable in the Nigerian climate studied, with a savings of 𝑁39,991.93 achievedwith a collector area of 1.7 m2over a 20 year period. Environmental impact assessment showed CO2 emission reduction in the range of 2.7% to 58.4%. It was concluded that the system operates satisfactorily in the Nigerian climate studied and it is economically viable. Recommendations for further improvements were made.
CHAPTER ONE
INTRODUCTION
1.1 Background to the Study
Energy is an essential need of all human beings in the world. The technological advancement and economic growth of any country rests on the availability of utilisable form of energy in that country (Sarbu and Sebarchievici, 2015). So far, fossil fuels are considered and utilised as the prominent source of generating utilisable form of energy (Choudhury et al. 2013). It is estimated that 80% of the energy currently used on earth is generated from fossil fuel sources (Sarbu and Sebarchievici, 2015). However, concerns are growing on a daily basis over the negative effects on the environment that are caused by burning fossil fuels that include global warming and ozone layer depletion (Said et al., 2015).
The energy demand for air conditioning is increasing continuously, due to growing thermal loads, changing building architectural modes and especially due to increasing occupants indoor comfort demands resulting in higher electricity demand notably during peak periods (Kalkan et al., 2012).Vapour compression based refrigerating systems are generally employed in refrigeration and air conditioning units. These units operate with synthetic refrigerants such as chlorofluorocarbons (CFCs) and hydro – chlorofluorocarbons (HCFCs), which when released into the atmosphere deplete the ozone layer and contribute to the global warming (Sarbu and Sebarchievici, 2015). Also, usual vapour compression based cycles are electrically powered, consuming large amounts of high quality energy. According to the International Institute of Refrigeration in Paris, the amount of electricity consumption from different types of refrigeration and air conditioning processes is approximately 15% of the electricity produced worldwide (Kalkan et al., 2012). Also, electricity consumption for air conditioning systems has been
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estimated around 45% of electricity consumption in residential and commercial buildings (Kalkan et al., 2012).
Renewable energy sources are being sought as alternative sources of energy for refrigeration and air conditioning. Amongst the many renewable energy sources, solar energy stands out as a very attractive alternative. The main reason being in its cleanliness and natural availability (Anand et al., 2015). Also, the cooling load usually reaches its peak value when solar energy is mostly available. This synchronism between solar irradiance and building cooling load gives an advantage (Li and Sumathy, 2000).
Solar electrical and solar thermal powered refrigeration systems can be used to produce cooling. The first is photovoltaic based in which solar energy is converted directly to electricity then utilised for producing cooling, similar to conventional methods. The second utilises solar thermal energy to power the generator of an absorption refrigeration system (Otanicar et al., 2012).Solar thermal cooling systems have the advantage of using absolutely harmless working fluids such as water, or solutions of certain salts as refrigerants. They are energy efficient and environmentally safe. Due to the advantages of solar thermal cooling systems over solar photovoltaic cooling systems, over the years, more research has been carried out in the field of solar thermal cooling (Otanicar et al., 2012).However, despite the increased level of research in the field of solar thermal cooling over the years and the seeming advantage it can potentially have, the absorption air conditioning system still lacks the ability to compete favourably with its vapour compression counterpart in the market.This is due most especially to the initial high cost of the absorption air conditioning systems and the relatively low coefficient of performance (COP).
At present absorption chillers are mainly established for capacities in the range of tens of kilowatts to hundreds of kilowatts. Research works are presently ongoing to establish absorption chillers in the smallest of capacities, improve Coefficient of Performance and also to reduce the initial cost.
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Research works are also being carried out to design Solar Absorption Cooling systems that will yield high Solar Fractions (SF) and be economically viable.
1.2 Statement of the Problem
The following are some of the problems that have necessitated the present research work.
i. Cost of electricity generated from fossil fuels is high.
ii. The vapour compression units consume high power which ultimately increases electricity bills.
iii. Most vapour compression systems commonly use chlorofluorocarbons (CFCs) and hydro – chlorofluorocarbons (HCFCs) refrigerants, which cause depletion in the ozone layer. The replacements, which are hydrofluorocarbons (HFCs) have zero ozone depleting potential (ODP), but high global warming potential (GWP) (Benhadid – Dib, 2012). To safeguard the environment, harmless refrigerants must be used as agreed in the “Kyoto Protocol” (Pachauri and Reisinger, 2007).
iv. In Nigeria, as a developing nation, the cooling load increases daily as a result of better comfort demands. This causes more load on the grid, especially during peak periods, which leads to total collapse in some areas. In addition to this, electricity tariff has been on the increase. As at October 2017, electricity tariff stands at N26.37 per kilowatt hour (residential),N37.88 per kilowatt hour (commercial) and N44.22 per kilowatt hour (industrial) with increase still expected over the years (MYTO 2.1 report, 2015).The need for an alternative source of power cannot be overemphasized.
v. In the department of mechanical engineering, A.B.U. Zaria, the central vapour compression air conditioning unit for the office block has broken down beyond repair. There is need to design and simulate a solar absorption air conditioning system as a possible replacement.
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vi. Solar absorption cooling is a very promising technology, however there is the need for more research work to improve COP to make it even more favourable.
1.3 The Present Research
The present research work is intended to develop a solar absorption air conditioning system. The system will comprise of an absorption chiller which will operate on the lithium bromide/ water pair. An array of solar collector to generate the required thermal energy. A storage tank to store the resulting hot water. An auxiliary heater powered by liquefied natural gas is to be incorporated to complement the solar power during periods of low solar radiation. A cooling tower for heat rejection. An indoor fan coil unit. This will be achieved by carrying out an initial simulation using TRNSYS to optimally size system components required to give monthly maximum COP and SF for an office block under Zaria weather conditions. The design of the system components will be done using program based design and calculation in MATLAB, where each of the system components will have design parameters optimized to give overall optimum COP. The system will be constructed using locally available materials and expertise. The constructed system will be tested on a test room to evaluate the performance. An economic evaluation of the system using life cycle cost savings analysis method will be carried out. An environmental impact assessment of the system to determine the carbon dioxide (CO2) emission reduction will be carried out.
1.4 Aim and Objectives
The aim of this research is to simulate the performance of a TRNSYS model of agas assisted solar absorption air conditioning system under Zaria weather conditions, design, construct and test the system.
Specific objectives are:
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i. To developa model to obtain optimum sizes of absorption chiller, collector area, storage tank volume required to achieve monthly maximum COP and SF for an office block under Zaria weather conditions using TRNSYS.
ii. To carry out the design and optimisation of the system components using MATLAB.
iii. To construct the optimised system design.
iv. To carry out experimental tests on the constructed system.
v. To carry out an economic analysis of the system.
vi. To carry out an environmental impact assessment of the system.
1.5 Justification for the Research
i. Synchronism between building cooling load and solar irradiance can lead to a successful operation of a solar absorption cooling system.
ii. Nigeria is a country with a vast availability of solar energy. A modest estimate of the technical potential of solar energy in Nigeria with 5% device efficiency is put at 15x1014kJ of useful energy annually (Onyebuchi, 1989). This translates to about 4.2×105 GWh of electricity production annually (Oyedepo, 2012). Successful implementation of solar cooling technology in Nigeria could lead to significant reduction in power consumption.
iii. The refrigerant to be used (water) will have no global warming potential on the environment.
1.6 Scope and limitations of the Research
The scope of the present research is limited to:
i. Initial simulation of a solar absorption cooling system under Zaria weather conditions as a feasibility study and to optimally size system components.
ii. Design of the system components using programme based design and optimisation.
iii. Construct the optimised system.
iv. Test the performance of the system.
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Limitations to the research work:
i. Due to the high cost required to construct the system as sized and limited funds available, reduced size version of the system was constructed within the limits of available resources.
ii. Due to the reduced size version constructed, system was tested using a test room rather than the office block.
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