- : Ms Word, Ms Word Format
- : 100 Pages
- : ₦5000
- : 1-5 Chapters
- Click to DOWNLOAD Materials
TRANSIENT PERFORMANCE EVALUATION OF DIURNAL HEATING AND NOCTURNAL COOLING OF WATER USING A HYBRID FLAT-PLATE SOLAR COLLECTOR/RADIATOR
ABSTRACT
Present energy dependence on fossil sources has been identified as the primary cause of the twin menace of global warming and climate change, hence the focus of the ensuing investigation on renewable energy resources to address the current environmental concerns. The transient performance evaluation of a hybrid solar collector/nocturnal radiator (SCONOR) for diurnal water heating and nocturnal water cooling is thus presented. The mathematical models of the physical system are based on the heat transfer mechanism in the SCONOR-bond-tube-water arrangement while for accumulative heating and cooling performances the models were based on lumped energy equations in the SCONOR and multi-zone stratification models in the thermal energy storage tanks. The resulting equations were discretized using finite element numerical scheme for ease of implementation in a digital computer. The transformed equations were solved using a scripted code written with FlexPDE finite element model builder and numerical solver version 7.12. The numerical results obtained from the models were compared with experimental data from literature sourced from three different climatic conditions. In the transient diurnal heating mode, the time of occurrence of SCONOR, bond, tube and water peak temperatures were accurately predicted while a mean deviation of 4-8.6oC was observed between the reported data and numerical predictions. For the transient nocturnal performance evaluation of the SCONOR, the model results closely matched experimental results with a mean deviation of less than 0.2oC, given that the initial conditions varied at about 7oC which was maintained throughout the nocturnal period with minimal fluctuations. The model results from a 24-hour performance cycle covering diurnal heating, cool down and nocturnal cooling of the SCONOR closely matched experimental results for the absorber and radiator functions of the hybrid system with mean deviations of 3-5oC in the diurnal heating mode and 0.2-0.4oC in the nocturnal cooling mode. The accumulated performance results during the diurnal thermal storage and nocturnal thermal storage were also compared with available experimental data from literature, and the results show good agreement with mean deviations of 0.2-5oC and 7oC during heating and cooling respectively. The thermal energy accumulated in the hot tank during diurnal heating maintained the modelled room temperature at approximately 5oC above ambient during the cold harmattan night and that accumulated in the cold storage tank maintained the modelled room temperature at 5.7oC below ambient during the hot day. From parametric analyses, the optimal design parameters are 0.26m, 0.003m, 0.1m, and 1 for the bond width, bond thickness, riser tube spacing and number of windscreen respectively. The overall results show that the developed models are useful design tools for the integrated diurnal water heating-and-nocturnal water cooling.
Keywords: Models, Numerical Simulation, FEM, Hybrid SCONOR, Transient, Diurnal Solar Energy, Nocturnal Radiation, Thermal Storage, Comfort Cooling, Comfort heating.
TABLE OF CONTENTS
Title page i
Certification ii
Dedication iii
Acknowledgements iv
Abstract v
Table of contents vi
List of tables x List of figures xi
Nomenclature xv
CHAPTER 1 INTRODUCTION
1.1 Background Information 1
1.2 Problem Statement 6
1.3 Objectives of Study 8
1.4 Justification of study 9
1.5 Scope of Study 10
CHAPTER 2 REVIEW OF THE RELEVANT LITERATURE
2.1 Background Information on Existing Systems 11
2.2 Solar Water Heating Systems 12
2.3 Nocturnal Water Cooling Systems 28
2.4 Hybrid solar water heating and nocturnal water cooling systems 39
2.5 Thermal Stratification in Hot Water and Cold Water Storage Tanks 46
2.6 Summary of findings from literature 51
CHAPTER 3 RESEARCH METHODOLOGY
3.1 The hybrid solar water heating and nocturnal cooling system configuration 54
3.1.1 The Solar Collector/Nocturnal Radiator 55
3.1.2 Mode of Operation 57
3.2 Formulation of system model equations 58
3.2.1 The SCONOR Model 60
3.2.1.1 Simplifying assumptions for model formulation 61
3.2.1.2 Spectral selective surface-coated absorber control volume 62
3.2.1.3 The bond zone 66
3.2.1.4 The Tube Zone Control Volume 68
3.2.1.5 The Fluid Layer 72
3.2.1.6 Heat losses from SCONOR 74
3.2.1.6.1 Top heat losses from the SCONOR surface 74
3.2.1.6.2 The convective heat transfer coefficients 76
3.2.1.6.3 The radiation heat transfer coefficients 78
3.2.1.6.4 Radiopaque-transparent cover radiative and convective heat exchanges 79
3.2.1.6.5 Bottom and edge losses from the SCONOR plate 80
3.2.1.7 The climate model 82
3.2.1.7.1 Models of solar radiation on tilted SCONOR surface 82
3.2.1.7.1.1 Beam radiation on the tilted SCONOR surface 83
3.2.1.7.1.2 Diffuse and reflected solar radiations on the tilted SCONOR surface 85
3.2.1.7.2 The transmittance-absorptance product 88
3.2.1.7.2.1 Formulation of transmittance for beam radiation component 90
3.2.1.7.2.2 Diffuse and ground reflected transmittances due to reflection losses 92
3.2.2 Thermal modeling of the nocturnal cooling process 97
3.2.2.1 The SCONOR model_Nocturnal Phase 98
3.2.2.1.1 The fluid region 100
3.2.2.1.1.1 The tube region 101
3.2.2.1.2 The bond region 102
3.2.2.1.3 The Nocturnal Radiator region 103
3.2.2.1.3.1 Radiator on a horizontal surface 106
3.2.2.1.3.2 Radiator on a tilted surface 107
3.2.3 Thermal stratification in the water storage tanks 108
3.2.3.1 Modelling the hot water storage tank 109
3.2.3.1.1 Differential equation of stratification in the hot water tank 110
3.2.3.2 Modelling the cold water storage tank 117
3.2.3.2.1 Differential equation of stratification in the cold water tank 119
3.2.4 Space conditioning for thermal comfort 122
3.2.4.1 Modelling of comfort cooling 122
3.2.4.1.1 The Room Convector 125
3.2.4.1.2 Heat gain into the room as a result of transmission 127
3.2.4.1.3 Heat gain into the room as a result of infiltration 128
3.2.4.1.4 Heat internally generated in the room 129
3.2.4.1.4.1 Heat generated by people 129
3.2.4.1.4.2 Heat gain in the room by appliances 131
3.2.4.1.4.3 Heat generated from light bulbs 132
3.2.4.2 Modelling of comfort heating 133
3.2.4.2.1 The radiator control volume 136
3.2.4.2.2 Internal heat generation and heat losses by transmission and air renewal 138
3.3 Numerical Implementation 140
3.3.1 Formulation of finite element equations 140
3.3.2 Computational domain 141
3.3.3 Finite element formulation of model equations 142
3.3.3.1 Initial and boundary conditions 145
4.1.1 Thermal distribution on the SCONOR 158
4.1.2.2 SCONOR cool-down phase 170
4.1.2.3 Nocturnal water cooling in the SCONOR 176
3.3.3.3 The tube/fluid region 149
3.3.4 Numerical simulations 151
3.3.4.1 FlexPDE scripting procedure 152
3.3.4.2 Grid Independence 154
3.3.4.3 Numerical solution stability 155
3.3.4.4 Convergence Criteria 155
3.3.4.5 Outline of simulations undertaken 156
CHAPTER 4 RESULTS AND DISCUSSIONS
4.1 Results 158
4.1.3 Performance evaluations during 24-hour period in 5 Nigerian cities 185
4.1.4 Stratification in the storage tanks and space conditioning 187
4.1.4.1 Diurnal stratification and nighttime comfort heating 187
4.1.4.2 Nocturnal stratification and daytime comfort cooling 189
4.1.5 Comparison of model results 190
4.1.6 Parametric analyses 192
4.2 Discussions 197
4.2.1 Thermal energy distribution on the SCONOR 197
4.2.2 Predictions using Owerri climatic data 198
4.2.2.1 Solar water heating in the SCONOR 198
4.2.2.2 The SCONOR cool down phase 206
4.2.2.3 Nocturnal water cooling in the SCONOR 207
4.2.3 Performance evaluation in 5 Nigerian cities 214
4.2.4 Stratification in the storage tanks and comfort conditioning 215
4.2.4.1 Diurnal stratification and nighttime comfort heating 215
4.2.4.2 Nocturnal stratification and daytime comfort cooling 218
4.2.5 Comparison of model results 220
4.2.6 Parametric analyses 222
CHAPTER 5 CONCLUSIONS AND RECOMMENDATIONS
5.1 Conclusions 226
5.2 Recommendations 229 5.3 Contributions to knowledge 230
References 231
Appendix i: Numerical Data for location studies 237
Appendix ii: Numerical Data for surface coatings 242
Appendix iii: Numerical Data for validation studies 243
Appendix iv: Model input parameters 245
CHAPTER 1 INTRODUCTION
1.1 Background Information
Global interest is currently increasing on the exploitation of hybrid active heating/passive cooling, low energy water/air heating, and ventilation/air conditioning (comfort cooling) applications. A hybrid system involves the use of a single unit/machine to perform two different functions that otherwise would have been carried out by two independent systems. Several countries are committing a great deal of resources into the research and implementation of such techniques. Both the Evapcool of the European Union Fifth Framework/Energy sub-programme and Solar Heating and Cooling Programme of the International Energy Agency are some of the current collaborative efforts in this area (Balen et al, 2003). Solar energy utilization has been on the increase in recent years due to the declining fossil fuel resources and environmental concerns about global warming and air pollution. Although solar energy is inexpensive relative to conventional sources of energy like natural gas, the overall cost of its exploitation continues to decrease due to advances in technology.
Energy utilization worldwide depends heavily on fossil resources (oil and gas products). Environmental sustainability is highly threatened owing to the pollutant emissions from heavy dependence on these fossil resources (Asere et. al., 2011). Oil prices and energy consumption demand have been on the increase. The finite nature of fossil resources is also a major concern, since studies have revealed that they are tending towards exhaustion (World Energy Council, 2013; Cappellan-Perez et al., 2014; Khan et al., 2015). These have become contending challenges. Consequently, research and technological investments are gaining serious attention in renewable and sustainable energy (Sinan, et. al., 2012; Jung-Sik, et. al., 2010).
Renewable energy technology generates energy by transforming natural phenomena (or natural resources) into useful energy forms and is strongly believed to be one of the promising solutions to the energy challenge. Renewable energy will contribute a great deal in the new millennium in adding the much needed flexibility to the energy mix and decrease dependence on conventional fossil fuels, thus drastic reduction in global warming and climate change. Solar energy is the most sustainable renewable energy source (Eduardo et. al., 2010). The sun is a continuous fusion reactor, turning hydrogen into helium, and producing an effective average body temperature of about 5762K and total output energy of approximately 3.8 x 1020 MW amounting to 63 MW/m2 of its surface. This energy radiates outwards in all directions and about 1.7 x 1014 kW reaches the earth within 8 minutes 20 seconds after leaving the giant furnace. However, it’s been found that harnessing this minute fraction for about 30 minutes is sufficient to meet the world’s annual energy demand (Kalogirou, 2004).
Solar thermal energy systems have emerged over the recent decades for several applications in domestic and industrial activities like hot water supply, space heating and cooling, desalination, sterilization, pasteurization, drying, refrigeration, distillation, washing, etc, that can conveniently be carried out within the temperature ranges of 60 to 2800C (Parimal and Parishwad, 2013). For domestic water heating and cooling applications, flat plat collectors have been extensively used. Nocturnal cooling concept has many potential applications in the storage of food, seed and medicine, air conditioning of buildings and water desalination (Dobson, et al., 2003). By integration of this concept into the architectural design of building envelopes, thermal comfort can also be provided during hot/dry seasons (summer) with minimum energy utilization, without recourse to the mechanical vapour compression systems extensively used for air conditioning. As global industrialization and standard of living improve, energy consumption related to space cooling increases and hence associated energy cost. This cooling concept is likely to reduce the energy bills associated with conventional mechanical cooling as well as greenhouse gas emissions.
Thermal water heating dates back a long way, at least as far as Archimedes‟ use of a concave mirror to heat water in 214 BC (Renewable Energy World, 2004). Solar water heating (SWH) systems harness irradiation from the sun to heat water either for domestic or industrial use. In 1891, Kemp patented the first solar water heating system in the United States. This was essentially boxes that were painted black and filled with water and exposed to the sun to heat up the water. Early 20th century witnessed several efforts to improve the design of the SWH systems (Iordanou, 2009). In Japan, a simple solar water heater was created consisting of a basin with its top covered by glass and by the 1960s more than 100,000 collectors of this type were in use. Australia witnessed an increase in the number of solar water heaters as a result of oil price increases, but the discovery of natural gas in the late 1980s led to a drop in the growing interest in solar water heaters. Levi Yissar, an Israeli engineer pioneered the introduction of solar water heater in Israel around the mid 50s, and by early 1980 about 60% of Israeli population heated their water with energy from the sun. More so, following the drop in oil price in the mid 1980s, Israeli government made it a law, making well above 90% of Israeli households to install solar water heaters today (Iordanou, 2009; REN21, 2017).
According to Veeraboina and Yesuratnam (2014), solar water heating systems can span a period of up to 15 to 20 years and favour regions that have good water quality and are non-hilly. Such systems are available in multiples of 100 LPD (Litres per day) i.e. 100, 200, 300 LPD, etc. Among the common household appliances, solar water heaters consume the highest amount of power on the annual scale and hence constitute the highest source of energy cost for households, as revealed by the following chart showing the quantity of energy a typical appliance uses annually with a corresponding cost based on national averages, as shown in Fig. 1.1.
Fig. 1.1 Annual appliance energy usage and their costs (Veeraboina and Yesuratnam, 2014).
Globally, many countries are embracing the solar water heating technology. While countries like
Austria, Turkey and Germany have high share of installation of SWH per capita, Israel and
Cyprus have almost attained saturation, standing as leaders in world‟s installed capacity per
capita as reflected in figure 1.2 (Leonardo-energy.org; Iordanou, 2009).
Figure 1.2: Solar water heater installed capacity per thousand inhabitants and share of homes* equipped (source: Enerdata, 2009 as reported in Iordanou, 2009).
China and Brazil are constituents of the emerging markets for solar water heating systems, with
China ranking first in terms of installed capacity of about 11 million m², installed up to 2009 (almost 2/3 of the world‟s capacity). Hence, China has become the leader in solar water heating system additions, surpassing Turkey (380,000 m2), Austria (about 200,000 m2) and Portugal (around 88,000 m2) in that order.
Figure 1.3: Global installed capacity of solar water (source: Enerdata, 2009).
Preliminary estimates indicate that the solar water heater market would have risen to well above 185 GWth in 2010, from 153 GWth estimate of 2009, with China alone contributing about 70% of prospective capacity and only 10% coming from European Union, as shown in Fig. 1.3. The European Union, has included the development of solar water heating (SWH) systems in the National Renewable Energy Action Plans by member states and most of EU countries, therefore, have set targets for solar heat by 2020 (Leonardo-energy.org). According to ECOWAS Centre for Renewable Energy and Energy Efficiency status report (REN21, 2014), solar water heaters are being utilized in West Africa but estimates of the penetration are limited and difficult to assess. Mali has installation of about 1,500 200-litre solar water heaters while mention is made of its use in other countries like Niger, the Gambia, Nigeria, etc. However, Olusola et al (2017) revealed that about 4,000 solar water heating units were installed as at 2015 in Nigeria, with the Nigerian Federal Government setting a target of installations of about 60,000 and 150,000 units in 2020 and 2030 respectively.
1.2 Problem Statement
According to Ogueke et al (2009), hot water demands appear to be highest within the periods of the day when electric energy demand for other purposes is high, usually early in the mornings and late in the evenings. Hot water for domestic activities, industrial process heat, space heating, sterilization in hospitals, bathing at homes, schools and hotels, etc, in Nigeria are currently largely being met by the use of fossil-driven grid electricity and natural gas. These fossil fuel resources, apart from being finite, are reputed to be the sources of the twin menace of global warming and climate change, which have become the two most current contending political issues around the globe today. Global atmospheric temperature is increasing, mostly during dry seasons of the year when ambient temperature is above that needed for human comfort, making comfort cooling inevitable, hence, increased demand on grid electricity to drive the conventional air conditioning systems (Ogueke et al, 2011).
The immediate consequences are progressively increasing cost of energy supply and harmful emissions into the earth environment from conventional energy consumption to meet man‟s
needs. Population is also on the increase, especially in the developing nations, thereby placing a more increased burden on energy demand. Therefore, there is need for nations to develop and provide energy systems commensurate with the growing population but free from adverse environment effects. Another prominent contending issue stifling access to energy in developing nations is the erratic nature of electricity supply, leading to power rationing in most cases. During the periods of highest need for hot water, it is either that the electric power is not available or certain areas are completely shut off due to insufficiency of available grid supply. It is, therefore, foreseeable that removing the energy demand for hot water purposes from electricity is the most seemingly convenient means to eliminate or reduce these problems. Use of the ambient energy is speculated to significantly address the problems.
Solar energy is an immense free energy resource, renewable, free from pollution and capable of meeting the energy needs for hot water during the day, as any object placed on the earth absorbs a great deal of this energy depending on the nature of its surface. Also, the sky during the night, acts as a heat sink as it remains colder that any object placed on the earth‟s surface. This implies that a surface exposed to the night sky dissipates heat to the environment thereby cooling down. Thus, if the roof of a building is coupled to a thermal storage, nocturnal cooling can be used to keep it cool and provide comfort throughout the day. The urgent need to harness nocturnal cooling resources to supplement our cooling needs cannot be overemphasized.
Accordingly, Nwaigwe et al (2010) documented a comprehensive report of the principles and theories of nocturnal sky radiation cooling and its available systems, including the pertinent experiences of its operations, prospects and technical constraints. Unlike the solar water heating system (SWHS), works in nocturnal radiation cooling systems (NRCS) have remained at the laboratory experimental stage. Independently, each of these systems is quite expensive to produce. This problem can be solved by utilizing a hybrid flat-plate solar collector/radiator to harness solar radiation during the day and night sky radiation during the night. This is integrated into a building envelope and coupled to hot water and cold water storage tanks, room convector and radiator for performing the two different functions that otherwise would have been done by two different independent systems. In which case, diurnal solar water heating and night time or nocturnal radiation water cooling would have been successfully undertaken using a single system. Because the thermal energy supplies for running the heating and cooling phases of this hybrid system are ambient energy, its operations correspond to the natural availability of diurnal
(day time) solar energy and nocturnal (nighttime) sky radiation periods respectively. Amraoui and Aliane (2015) reported that solar energy application is closely related to the performance of the collectors which convert it. This is also true for a nocturnal radiator.
Many objects lack the capability of both absorbing the incoming solar radiation during the day and at the same time radiating heat to the cold sky during the night because their surfaces are non-selective to radiation wavelength within the solar radiation and atmospheric window spectra. Therefore, improvement in their operating condition and geometrical configuration would definitely result in saving conventional fuel and cost.
In synopsis, the demand placed on conventional energy utilization for water heating/cooling and space conditioning can be greatly reduced and hence reduction in energy costs by the effective exploitation of the concepts of diurnal and nocturnal radiant flux interchanges.
1.3 Objectives of Study
The main objective of the present investigation is to carry out a transient performance evaluation of diurnal heating and nocturnal cooling of water using a hybrid flat-plate solar
collector/radiator. To achieve the main objective, the specific objectives include, to:
- Develop a 2-D dynamic thermal model that will enable comprehensive overview of the effects of thermo-physical and material properties as well as weather conditions on the performance of the H-SWH-NC system.
- Determine the mean diurnal heating and nocturnal cooling outputs of the collectorradiator surface under some selected locations in Nigeria.
(iii)Study the effect of stratification in the storage tanks on the performance of the hybrid collector/radiator for diurnal and nocturnal thermal energy storage and thus the possibility of comfort heating and cooling.
(iv)Carry out parametric analyses to ascertain the effects of riser tube spacing, optimum configuration of the bond region, insolation intensity and surface spectral properties on the dynamic and thermal behaviour of the hybrid SCONOR, to enable further improvements on the H-SWH-NC system.
1.4 Justification of study
The advent of hardware-in-the-loop technology or virtual prototyping brought a revolution in engineering products/systems development. Engineering today has shifted greatly from the traditional method that dominated the period of early development of the field. Nowadays, prototypes can be modelled and tested in the virtual environment to ascertain its functionality before finally building the physical prototype. Virtual prototyping and high speed digital computers coupled with available and powerful numerical solvers have made this possible. This has led to considerable conservation of time, material and money and other resources that go into product and systems development. Also, there are parameters that are difficult to measure experimentally due to either the complexity of the system or expensive nature of transducers to carry out such measurements. Modelling and simulation will provide comprehensive insight into the nature of the parameter and how it affects the system performance and can therefore help in system optimization studies.
Water heating forms significant part of energy consumption of many countries and has been accomplished greatly using electricity and natural gas. Where consumption data are available, for instance, in the United States, Europe and Japan, statistics show that water heating alone represents about 15% of households‟ energy use in Europe, 20% in the United States and as much as 30% in Japan. Hence, switching from electricity and gas to solar water heating could strongly reduce fossil fuel consumption and CO2 emissions (Leonardo-energy, n.d). Also, available statistical data for building energy consumption show that the heating, ventilation and air conditioning (HVAC) component, primarily provided by the mechanical vapour compression systems accounts for as much as 70% of the total energy consumption (Martin, 2000). Modelling the exploitation of space heating and passive cooling techniques using solar water heating and nocturnal cooling concepts cannot only reduce the costs involved in experimentation of the innovative system but would also enable recommendations to be made about the concepts economic importance in achieving drastic reductions in household energy consumption.
1.5 Scope of Study
This investigation is limited to numerical analyses of hybrid solar water heating and cooling, both for production of hot water for domestic use and cool water for space conditioning. This involves the development of a 2-D dynamic thermal model for comprehensive evaluation of the effects of thermo-physical properties and weather conditions on the performance of the hybrid solar water heating and nocturnal cooling system. It is also intended to examine the performance of the system in some selected Nigerian locations as well as the effects of stratification in the cold and hot water storage tanks on the overall performance. Moreso, it will encompass a parametric analyses to ascertain optimum design parameters that would aid prototype development. It is an integrated sustainable energy system that uses a solar thermal panel for hot and cold water production plus space ventilation system with air-heating/cooling by means of a water coil for the supply of air conditioning, mostly during the periods of adverse climatic conditions during the year, for the Nigerian climate, through sufficient preparation of cold water.
TRANSIENT PERFORMANCE EVALUATION OF DIURNAL HEATING AND NOCTURNAL COOLING OF WATER USING A HYBRID FLAT-PLATE SOLAR COLLECTOR/RADIATOR
