Effect of groundwater flow on the THM behavior of an energy pile

Bridges are one of the road structures that are most exposed to icing, a problem that is widely dealt with through salting, a technique that reduce the lifespan of the bridge. Seasonal storage of solar heat energy captured directly through the asphalt layer is a promising solution that can be achieved cost-effectively only if a necessary geostructure is used as heat-exchanger. Bridge piles and abutments are founded on piles that can be used as heat exchangers. Seasonal storage and natural thermal reload are two possible solutions for the operation of such a system, depending on local conditions. In particular, the presence of groundwater flow is thought to be a significant factor in such a design and is taken into account. The study aims at determining the geotechnical and energy design parameters through thermo-hydro-mechanical numerical simulations. A single heat-exchanger pile is taken as an example in this study. Given the presence of water flow, a three-dimensional FEM simulation needs to be carried out. The soil is described as a thermo-elastic porous medium, and various groundwater flow scenarios are studied. The capture of energy and the needs for deicing are based on the few existing structures that use other means of energy exchange with the ground. The article proposes an overview of all coupled phenomena, between thermal, hydraulic and mechanical aspects. Results show that only specific sites can be considered for using heat-exchanger piles for deicing bridges, but that the efficiency of the solution in those sites is high. Possible stability problems, in terms of foundation and structure are also considered, such as vertical displacements due to the dual use of the foundation piles.

Theoretical and Experimental Study of a Thermally Driven Heat Pump Based on a Double Organic Rankine Cycle

Jonathan Demierre

Nowadays, one of the main alternatives for a more rational use of energy in heating applications is the heat pumping technologies. The market is dominated by two kinds of heat pump systems: the electrically driven vapor compression heat pumps (EHP), which are the most widely used in residential heating applications and the thermally driven heat pumps (TDHP), that are usually based on a sorption process. In this thesis, theoretical and experimental investigations of a concept of thermally driven heat pump, based on the coupling of a vapor compression heat pump cycle and an organic Rankine cycle, are presented. The system is named here as ORC-ORC. The studied concept uses a single stage centrifugal compressor directly coupled to a single stage radial inflow turbine. The shaft is rotating on gas bearings, which allows the system to be oil-free. Like most of the other TDHP's, this system has the advantage to work with a variety of fuels or heat sources like wood pellets, natural gas, solar heat, geothermal heat or waste heat. The concept studied in this work is a gas red system for space heating and domestic hot water production in small residential buildings. A systematic approach has been used to evaluate, in term of energy efficiency, the potential of ORC-ORC systems and to propose optimal design specifications at different levels of the design process. The method is based on the optimization which allows to identify the best configurations at each design step with respect to the designer choices. This approach is divided in three steps. In the first step, a model of the complete system has been developed based on a process integration approach. This step allows to quickly determine whether the system is potentially attractive or not, for given conditions, before going deeper in the design process. The results show that, in general, it is advantageous for the ORC evaporation to be supercritical. In the second step, based on the process integration results and heuristic rules, a suitable system heat exchanger network has been generated and modeled. The predicted values of the coefficient of performance (COP) at design conditions, for the different situations, are in the range 1.30 and 2.19. In a last step, the off-design characteristics of the compressor-turbine unit are considered, in order to evaluate the COP as well as the heating capacity range that can be covered with a specific compressor-turbine unit design. For this purpose, a model of the compressor-turbine unit has been developed. Experimental investigations have been carried out with R134a as working fluid. A first simple test rig has been developed to test the selected ORC evaporator in supercritical conditions. The measurements have allowed to calibrate and to validate an in-house supercritical evaporator simulation tool. An ORC-ORC prototype has been developed and built. Commercially available equipment has been used, except for the compressor-turbine unit that has been designed especially for this application. The targeted operating point, with an HP evaporation temperature of 0 °C and a condensing temperature of 35 °C, has not been reached, because of experimental difficulties which caused some delay. A maximum temperature difference of 20 °C between the HP evaporation and the condensation has been achieved, which corresponds to a compressor pressure ratio of 1.9. Measurements have been performed with compressor-turbine unit rotational speeds up to 171 krpm. The theoretical investigations shows that the proposed ORC-ORC concept is an interesting alternative of thermally actuated heating device for small residential buildings. Concerning the experimental investigations, although a number of problems have been encountered during the tests, it has been demonstrated that the proposed concept is feasible with today's technology.

EPFL2012

Evaluation of a Combined Cycle Setup for Solar Tower Power Plants

James Spelling

As our awareness of the dangers of global climate change grows, the development of new renewable energy technologies is of primary importance in the effort to reduce emissions of carbon dioxide and other greenhouse gases. Rapid increases in the price of oil and worries about the stability and security of the extraction of fossil fuels have lead to renewed interest in the development of local energy sources, thereby reducing dependence on foreign sources. Amid the plethora of option available, one of the more promising solutions is the increased use of concentrating solar thermal power. Solar thermal power generation shows great potential for supplying the base electricity needs of numerous countries in the sun-belt regions, stretching from the tropics to the Mediterranean. This has lead to a recent renewal in the development of such systems, most notably with the construction of the PS10 and PS20 central receiver power plants in Spain. However, all currently existing solar thermal power plants have been based around the use of Rankine or steam turbine cycles, which are limited in the efficiencies they can achieve. In order to make better use of the Sun’s energy, higher efficiency cycles should be used. Recent developments in the field of high temperature solar receivers [13] have made it possible to imagine the use of solar thermal energy to drive gas turbine units, potentially allowing the use of combined cycle setups in solar thermal power plants. In order to evaluate the potential improvement in efficiency that can be achieved by using combined cycles in solar thermal power plants, a detailed simulation of such a setup will be performed. By taking into account the requirements of thermal energy storage and the design of the necessary heat exchangers, an objective evaluation of the performance of the combined cycle can be obtained. As a key element in the energy conversion process, particular attention will be given to the modelling and simulation of the high temperature receiver. Due to the highly variable nature of the solar flux, the performance of the receiver will be evaluated over a range of operating points. In order to obtain a detailed breakdown of the sources of losses and irreversibilities within the system, a complete exergy balance will be performed on the power plant. Combined with the standard energy balance, this analysis will allow identification of the key areas for improvement in the design of solar thermal power plants. Finally, in order to determine whether such a setup is economically viable, the construction, operation and maintenance costs will be evaluated in order to predict the levelised energy cost associated with electricity production in a combined cycle plant.

2008

Potential of refrigerant based district heating and cooling networks

Samuel Henchoz

Urban areas represent an ever increasing challenge in terms of energy use and environmental impact associated with it. Projections from the United Nation expect this number to reach 66% by 2050. In Europe 40% of the final energy consumption and 36% of the emissions of greenhouse gases are caused by buildings. The building sector is also one of the area where there is a great potential of reduction of the greenhouse gases emissions and of the dependence on fossil fuels. Among the measures that can help realise this potential, efficient energy conversion technologies supplying thermal energy services could play an important role. The present thesis aims at demonstrating the potential of a new type of district network, capable of delivering indistinctly cooling and heating services using a two-pipe network and in which the transfer of energy across the network is done by exploiting the evaporation/condensation of a refrigerant. The main research question is: "Is it possible to build and operate safe, reliable, energy efficient and economically profitable district heating and cooling networks that use a refrigerant as a transfer fluid?" The demonstration followsthree axes, the first being a thermoeconomic analysis. Thisanalysis focuses on a test case area in Genevaâs city centre where 5 variants of refrigerant based district heating and cooling networks, one cold water network and the mix of conversion technologies currently in use are compared on the basis of their energy and exergy performances and on the economic profitability. Considerations on economic uncertainty, safety and technical issues are also included in the analysis. The key findings are: â¢ All the variants of network can potentially reduce the final energy consumption of over 80% as compared to the current situation. â¢ All the variants of network have rather similar exergy efficiencies comprised between 39.5% and 45%. â¢ The most profitable variant uses CO2 as a transfer fluid and an open cycle CO2 heat pump at the central plant. It costs initially between 27 and 35 mio â¬ , reaches break-even in 4 to 6 years and the net present value after 40 years is comprised between 40 and 80 mio â¬. â¢ A cold water network is the second best option, although more expensive initially and thus less profitable, it has several advantages in terms of safety and availability of components. â¢ The CO2 variants exhibit a much better compactness than the cold water network. The second axis is the design, construction and testing of a lab scale refrigerant network. First a description of the design process and of the test facility is provided. It is followed by a presentation of the results of the test campaign. The tests aimed at demonstrating the practical feasibility of the concept, mostly by assessing the controllability of the network. Overall the good behaviour of the test facility and its ability to be smoothly and automatically controlled could be demonstrated, which further improved confidence in the practicality of the concept. The third axis is the development of dynamic models of simulation. These models are described in the present manuscript. They include, heat exchangers, pipes, pumps and valves. A short comparison between experimental and simulation results is also provided. The comparison between experiment and simulation showed that at their current stage of development the models cannot simulate accurately enough a refrigerant based network.

EPFL2016