Thermal energy storage(TES) air-conditioning system is a electrical load management technology with great potential to shift load from peak to off-peak utility periods. TES is now in widespread use for electric rate structures and energy policies and becomes a great contribution to energy conservation. However, TES is more complicated than conventional air-conditioning system in design and control strategies. According to practical field operation, the control of dynamic characters is especially difficult, and so are storage capacity design , discharging rate and charging capacity selection. This study set an example how to improve the energy performance of a retrofitted thermal energy storage air-conditioning system of a marine museum. Through full-scale experiment, historical air-conditioning operation data and computer simulation, the solution is provided on condition of unchanging major water pipe and equipment. This example can be a good demonstration for upping performance of TES. After testing and recording data for one year, this case indicates the investigation is effective and valuable to electric power management and green technology.

Distributed power generation through renewable sources of energy has the potential of meeting the challenge of providing electricity access to the off-grid population, estimated to be around 1.2 billion residing across the globe with 300 million in India, in a sustainable way. Technological solutions developed around these energy challenges often involve thermal systems that convert heat available from sources like solar, biomass, geothermal or unused industrial processes into electricity. Conventional steam based thermodynamic cycle at distributed scale (< 1 MWe) suffers from low efficiency driving scientific research to develop new, scalable, efficient and economically viable power cycles. This PhD work conducts one such study which provides a database of thermal power blocks optimized for the lowest initial investment cost to developers of distributed power plants. The work is divided in two steps; a) feasibility study of various thermodynamic cycles for distributed power generation covering different operating temperature regimes and b) perform their detailed thermo-economic modelling for the heat sources mentioned above. Thermodynamic cycles are classified into three temperature domains namely, low (< 450 K), medium (< 600 K) and high (< 1000 K) T cycles. Any fluid whose triple point temperature is below the typical ambient temperatures is a potential working fluid in the power cycle. Most of the organic and the inorganic fluids satisfy this criterion and can be perceived as potential power cycle fluids. The general notion is that organic fluids are more suited for low or medium temperature cycles whereas inorganic fluids for high temperature ones. Organic fluids can further be classified into hydrofluorocarbon and hydrocarbon. While the former has high global warming potential (GWP), the latter is flammable in nature. Their mixture in certain compositions is found to obviate both the demerits and perform equally well on thermodynamic scales for low T cycles. On the similar lines, mixture of HCs and inorganic fluids, such as propane+CO2 and isopentane+CO2 are found to be more appropriate for medium T applications if the issues like pinch temperature in the regenerator arising due to temperature glide are taken care of. In the high temperature domain, high efficiency Brayton cycle (supercritical CO2) and transcritical condensing cycles are studied with the latter being 2 % more efficient than the former. However, application of the condensing cycle is limited to low temperature ambient locations owing to low critical temperature of CO2 (304 K). In the same cycle configuration, mixture of CO2 and propane (52 and 48%) with a critical temperature of ~ 320 K is observed to retain the thermodynamic performance with the increased heat rejection temperature matched to the tropical ambient conditions. However, these cycles are plagued by the high operating pressures (~300 bar) calling for high temperature steel making the power block uneconomical. In this regard, the advanced CO2 cycles are developed wherein the optimum operating pressures are limited to 150 bar with an increased cycle efficiency of 6 % over the S-CO2 cycle. Feasibility study carried out on these cycles in the Indian context indicates the low and medium T cycles to be better suited for distributed power generation over the high T cycles. In the second part of work, a comprehensive study is performed to optimize the low and the medium T cycles on a thermo-economic basis for the minimum specific investment cost ($/We). Such a study involves development of component level models which are then integrated to form the system of interest, thus, following a bottom-up approach. A major emphasis is given on the development of scroll expander and low cost pebble bed thermal energy storage system that are the reported in the literature as the areas with high uncertainties while connecting them to the system. Subsequently, the key design parameters influencing the specific cost of power from an air-cooled ORC are identified and used to formulate a 7-dimensional space to search for the minimum costs for applications with a) geothermal/waste or biogas heat sources and b) solar ORCs. Corresponding maps of operating parameters are generated to facilitate distributed power engineers in the design of economic systems within constraints such as available heat source temperatures, maximum expander inlet pressures imposed, etc. Further, the effect of power scaling on these specific costs is evaluated for ORC capacities between 5 and 500 kWe.

Ice based Cool Thermal Energy Storage (CTES) systems have attracted much attention during last few decades. The reasons are mainly of economical and environmental nature. Compared to conventional refrigeration and air-conditioning systems without cool thermal energy storage, implementation of CTES will increase environmental standards and overall efficiency of the energy systems as it contributes to the phase-out of synthetic refrigerants and reduces peak loads in electricity grids. For the application of a cool thermal energy storages in refrigeration installations and HVAC systems in industry and building sector, it is necessary to have appropriate design tools in order to sufficiently accurate predict their performance. In this thesis theoretical and experimental investigations of two ice based cool thermal energy storage systems, namely static, indirect, external melt, ice-on-coil, i.e. ice bank system and dynamic, ice slurry cool thermal energy storage system are carried out. An ice bank storage technology for cooling purposes is known for a long time. The main drawbacks which are hindering its wider use are the system complexity, high first costs, system efficiency which is highly dependant on design, control and monitoring of the system, etc. On the other hand, ice slurry technology was not well studied until recently, while in the current scientific literature there are still differences between results and conclusions reported by different investigators. The aim of the present thesis is to extend the knowledge in the field of ice based CTES systems, thereby contributing in the development and wider utilization of those systems. In the first part of the thesis a computer application, named “BankaLeda” is presented. It enables simulation of an ice bank system performance. In order to verify developed simulation model an experimental evaluation has been performed. Field measurements have been conducted on a two module silo which was installed as a part of the refrigeration system in dairy and cheese factory “Antun Bohnec” in the city of Ludbreg in Croatia. Experimental findings were compared to the simulation model. The software „BankaLeda“ presents a strong optimization tool for designers and engineers in the field by providing a high degree of freedom in defining particular system design and operating parameters. It offers a basis for assessment and testing of a new energy efficient system arrangements and measures. Besides it will give decisionmakers the ability to test potential solutions in the process of CTES system design. In the second part of the thesis ice slurry pressure drop and heat transfer in horizontal straight tubes have been experimentally investigated. In particular a mixture of 10.3 % of ethanol and water with an initial freezing point of -4.4 °C was considered. It was found that the behaviour of ice slurry flow is changing with time and that ice slurry pressure drop is generally higher than for single phase flow. However for ice concentrations of 15 % and higher, for certain velocities ice slurry pressure drop is found to be of a similar value as for single phase fluid. Moreover, if ice slurry is to be used as a energy transport media it is recommended to keep the ice mass fraction at a level of 20 %. With tube geometry and thermophysical properties of a carrier fluid the heat transfer of ice slurry is generally a function of ice mass fraction and velocity. The imposed heat flux has no or has just minor influence on the heat transfer coefficient. Up to ice mass fraction between 10-15 % the mean heat transfer coefficient shows only slight (laminar flow) or no increase (turbulent flow) in comparison to single phase flow. Beyond that ice mass fraction the heat transfer coefficient is increasing significantly. The test data for pressure drop and heat transfer in laminar and turbulent regime was compared to several correlations from the literature. A new correlations for ice slurry pressure drop and heat transfer in the laminar flow regime, for 10.3 % ethanol and water mixture, were derived based on the present experimental data. The correlation for pressure drop predicts 82 % of the experimental data with ±15 % accuracy, while the correlation for heat transfer predicts 75 % of the data with the same accuracy. In order to investigate advantages and disadvantages of a dynamic, ice slurry system over a static, indirect, external melt, ice-on-coil CTES system and to assess their differences from economical aspects, a theoretical simulation model of an ice slurry CTES have been developed. It was found that the ice slurry based CTES systems posses higher economic and energy saving potential than static type systems. In the best case scenario the total energy consumption of dynamic CTES system was found to be approximately 25 % lower than for a static CTES system. / QC 20100715

Geothermal energy is the energy naturally present inside the earth crust. When a large volume of hot water and steam is trapped in subsurface porous and permeable rock structure and a convective circulating current is set up, it forms a geothermal reservoir. A geothermal system can be defined as - convective water in the upper crust of earth, which transfers heat from a heat source (in the reservoir) to a heat sink, usually the free surface. A geothermal system is made up of three main elements: a heat source, a reservoir and a fluid, which is the carrier that transfers the heat. As an alternative source of energy geothermal energy has been under attention of the researchers for quite some time. The reason behind this is the existence of several benefits like clean and renewable source of energy which has considerable environmental advantage, with no chemical pollutants or wastes are generated due to geothermal emissions, and the reliability of the power resource. Hence research has been directed in several directions like exploration of geothermal resources, modeling the characteristics of different types of geothermal reservoirs and technologies to extract energy from them. The target of these models has been the prediction of the production of the hot water and steam and thus the estimation of the electricity generating potential of a geothermal reservoir in future years. In a geothermal power plant reinjection of the heat depleted water extracted from the geothermal reservoir has been a common practice for quite some time. This started for safe wastewater disposal and later on the technology was employed to obtain higher efficiency of heat and energy extraction. In most of the cases a very small fraction of the thermal energy present in the reservoir can be recovered without the reinjection of geothermal fluid. Also maintaining the reservoir pressure is essential which gradually reduces due to continuous extraction of reservoir fluid without reinjection, especially for reservoirs with low permeabilities. Although reinjection of cold-water has several benefits, the possibility of premature breakthrough of the cold-water front, from injection well zone to production well zone, reduces the efficiency of the reservoir operation drastically. Hence for maintaining the reservoir efficiency and longer life of the reservoir, the injectionproduction well scheme is to be properly designed and injection and extraction rates are to be properly fixed. Modeling of flow and heat transport in a geothermal reservoir due to reinjection of coldwater has been attempted by several researchers analytically, numerically and experimentally. The analytical models which exist in this field deal mostly with a single injection well model injecting cold-water into a confined homogeneous porous-fractured geothermal reservoir. Often the thermal conductivity is neglected in the analytical study considering it to be negligible which is not always so, as proved in this study. Moreover heterogeneity in the reservoir is also a major factor which has not been considered in any such analytical study. In the field of numerical modeling there also exists a need of a general coupled three-dimensional thermo-hydrogeological model including all the modes of heat transport (advection and conduction), the heat loss to the confining rocks, the regional groundwater flow and the geothermal gradient. No study existing so far reported such a numerical model including those mentioned above. The present study is concerned about modeling the non-isothermal flow and heat transport in a geothermal reservoir due to reinjection of heat depleted water into a geothermal reservoir. Analytical and numerical models are developed here for the transient temperature distributions and advancement of the thermal front in a geothermal reservoir which is generated due to the cold-water injection. First homogeneous geothermal aquifers are considered and later heterogeneities of different kinds are brought into picture. Threedimensional numerical models are developed using a software code DuMux which solves flow and heat transport problems in porous media and can handle both single and multiphase flows. The results derived by the numerical models have been validated using the results from the analytical models derived in this study. Chapter 1 of the thesis gives a brief introduction about different types of geothermal reservoirs, followed by discussion on the governing differential equations, the conceptual model of a geothermal reservoir system, the efficiency of geothermal reservoirs, the modeling and simulation concepts (models construction, boundary conditions, model calibration etc.). Some problems related with geothermal reservoirs and geothermal power is also discussed. The scenario of India in the context having a huge geothermal power potential is described and different potential geothermal sites have been pointed out. In Chapter 2, the concept of reinjection of the heat depleted (cold) water into the geothermal reservoir is introduced. Starting with a brief history of the geothermal reinjection, the chapter describes the purpose and the need of reinjection of geothermal fluid giving examples of different geothermal fields over the world where reinjection has been in practice and benefitted by that. The chapter further discusses on the problems and obstacles faced by the geothermal projects resulting from the geothermal reinjection, most important of which is the thermal-breakthrough and cooling of production wells. Lastly the problem of this thesis is discussed which is to model the transient temperature distribution and the movement of the cold-water thermal front generated due to the reinjection. The need of this modeling is elaborated which represents the motivation of taking up the problem of the thesis. Chapter 3 describes an analytical model developed for the transient temperature in a porous geothermal reservoir due to injection of cold-water. The reservoir is composed of a confined aquifer, sandwiched between rocks of different thermo-geological properties. The heat transport processes considered are advection, longitudinal conduction in the geothermal aquifer, and the conductive heat transfer to the underlying and overlying rocks of different geological properties. The one-dimensional heat transfer equation has been solved using the Laplace transform with the assumption of constant density and thermal properties of both rock and fluid. Two simple solutions are derived afterwards, first neglecting the longitudinal conductive heat transport and then heat transport to confining rocks. The analytical solutions represent the transient temperature distribution in the geothermal aquifer and the confining rocks and model the movement of the cold-water thermal front in them. The results show that the heat transport to the confining rocks plays an influential role in the transient heat transport here. The influence of some parameters, e.g. the volumetric injection rate, the longitudinal thermal conductivity and the porosity of the porous media, on the transient heat transport phenomenon is judged by observing the variation of the transient temperature distribution with different values of the parameters. The effects of injection rate and thermal conductivity have been found to be high on the results. Chapter 4 represents another analytical model for transient temperature distribution in a heterogeneous geothermal reservoir underlain and overlain by impermeable rocks due to injection of cold-water. The heterogeneity of the porous medium is expressed by the spatial variation of the flow velocity and the longitudinal effective thermal conductivity of the medium. Simpler solutions are also derived afterwards first neglecting the longitudinal conduction, then the heat loss to the confining rocks depending on the situation where the contribution of them to the transient heat transport phenomenon in the porous media is negligible. Solution for a homogeneous aquifer with constant values of the rock and fluid parameters is also derived with an aim to compare the results with that of the heterogeneous one. The effect of heat loss to the confining rocks in this case is also determined and the influence of some of the parameters involved, on the transient heat transport phenomenon is assessed by observing the variation of the results with different magnitudes of those parameters. Results show that the heterogeneity plays a major role in controlling the cold-water thermal front movement. The transient temperature distribution in the geothermal reservoir depends on the type of heterogeneity. The heat loss to the confining rocks of the geothermal aquifer also has influence on the heat transport phenomenon. In Chapter 5 another analytical model is derived for a heterogeneous reservoir where the heterogeneous geothermal aquifer considered is a confined aquifer consisted of homogeneous layers of finite length and overlain and underlain by impermeable rock media. All the different layers in the aquifer and the overlying and underlying rocks are of different thermo-hydrogeological properties. Results show that the advancement of the cold-water thermal front is highly influenced by the layered heterogeneity of the aquifer. As the cold-water thermal front encounters layers of different thermo-hydrogeological properties the movement of it changes accordingly. The analytical solution derived here has been compared with a numerical model developed by the multiphysics software code COMSOL which shows excellent agreement with each other. Lastly it is shown that approximation of the properties of a geothermal aquifer by taking mean of the properties of all the layers present will lead to erroneous estimation of the temperature distribution. Chapter 6 represents a coupled three-dimensional thermo-hydrogeological numerical model for transient temperature distribution in a confined porous geothermal aquifer due to cold-water injection. This 3D numerical model is developed for solving more practical problems which eliminate the assumptions taken into account in analytical models. The numerical modeling is performed using a software code DuMux as mentioned before. Besides modeling the three-dimensional transient temperature distribution in the model domain, the chapter investigates the regional groundwater flow has been found to be a very important parameter to consider. The movement of the thermal front accelerates or decelerates depending on the direction of the flow. Influence of a few parameters involved in the study on the transient heat transport phenomenon in the geothermal reservoir domain, namely the injection rate, the permeability of the confining rocks and the thermal conductivity of the geothermal aquifer is also evaluated in this chapter. The models have been validated using analytical solutions derived in this thesis. The results are in very good agreement with each other. In Chapter 7 the main conclusions drawn from the study have been enlisted and the scope of further research is also pointed out.