Enthalpy and exergy analysis in the framework of a district scale energy balance

This report presents a study of an enthalpy and exergy analysis in the framework of a district scale energy balance. Energy demand (heating, cooling and lighting) for buildings represents more than 30 % of the total worlds energy use. Main eﬀorts have been made on increasing performances of new building envelop and on the use of low temperature processes in order to increase the exergy eﬃciency. On a district scale, diﬀerent temperature and energy ﬂow levels are present, meaning a great potential to make use of synergetic eﬀects by rationally managing energy ﬂow interactions between buildings as synergies by enhanced energy management and the usability of waste heat source. An optimisation of exergy ﬂows for the heating systems allows to identify the temperatures levels according to the energy demand, which helps to implement the best choice of available technologies. A number of connection cases will be described and studied to show the potential gain in terms of thermal and economical performances, where the thermal performances are deﬁned by both the energy and the exergy eﬃciency. This study shows an improvement of the thermal performances of connection cases where in all scenarios the heat losses are lower than the waste heat recovery. These gains on thermal performances may increase further by the integration of appropriate technologies such as heat pumps. The use of primary resource like gas can also be decreased.

Key Energy and Technological Aspects of Three Innovative Concepts of District Energy Networks

Patrick Chatelan, Daniel Favrat, Samuel Henchoz, François Maréchal

Energy needs in urban areas are heterogeneous by their nature, spatial distribution and temperature level required. Three concepts of district energy networks that could provide the energy services efficiently to a city centre are compared. The focus is on the energy and technological aspects. These networks are characterized by similar temperature levels; between 9.5 °C and 18 °C, rely on free cooling for most of the cooling services and use a combination of centralized and decentralized heat pumps to provide the heating services. Two of these concepts exploit the latent heat of evaporation/condensation of CO2 and of the refrigerant R1234yf to store and transfer heat. The third concept is more conventional and uses the sensible heat of liquid water, however with a small temperature spread. The proposed networks allow the waste heat emitted by the users requiring cooling to be transferred and valourised by the users requiring heating, thus reducing the load on the central plant. For the area considered, where the annual heating and cooling demand are 53.1 GWh and 49.4 GWh respectively, the annual electricity required to supply the thermal services amounts to 10.87 GWh for CO2, 10.52 GWh for water and 9.60 GWh for R1234yf. © 2016 Elsevier Ltd. All rights reserved.

2016

Preliminary Study of a Solar Trough/Tower Power Plant

Jorge López Moreno

The possibility of combining both the advantages of a solar tower thermal power plant and of a solar parabolic trough power plant could be interesting from an energetic point of view as well from an economic point of view. The preliminary study of this combination feasibility is the goal of this project. The study of the project viability consist first of all on the modeling of the trough system, the energy balance of the parabolic troughs will be established at first. Secondly, and choosing a perfect solar day simulating a day when the solar radiation is rather stable, with no cloudy periods along the day, the power available from the trough plant can be calculated. Once we have estimated the power that we can introduce into the cycle of the tower plant, the main issue consists on how and where it is better to use this energy in the cycle. Several possibilities seem to be interesting a priori, however a more detailed study of each one is to be done in order to get to the most economic and efficient layout. The work related to the tower thermal power plant will be done taking as starting point the master thesis of James Spelling, who did an incredible work in this field and whose work will be a valuable help to continue the research on the proposed concept in this project of combining two plants, tower and trough. The main advantage of the tower design compared to the parabolic trough design is the higher temperature. As it will be explained further, thermal energy at higher temperature can be converted more efficiently, the exergy efficiency is better. In terms of storage, the high temperature results in a cheaper and a more efficient storage system. Nevertheless, the set up of the field of heliostats is more complex than the set up of the parabolic trough lines even though the area for a trough plant must be previously flattened. The lower temperature level that entails an inconvenient itself for a parabolic trough power plant may become an advantage when combining both types of concentrating solar power plants. If the results are positive enough that it seems interesting to combine both types of power plants, another step forward will be done towards the use of the sun energy as a renewable source thus making it a more interesting choice for power generation. Sun potential has already shown as an important technology not only at the present time but also in the future. Commercial solar plants have already achieved levelized energy costs around 12-15 UScts/kWh, and the potential for cost reduction is expected to follow this trend even further, to costs as low as 5 UScts/kWh. In addition the taxes imposed for the non-green power should make even more interesting the use of concentrating solar power plants. The requirements of high beam normal radiation lead to a limited amount of suitable places where the implantation of this technology is possible. However many places exist on Earth where the conditions are perfectly adapted, we talk about regions such as United States (the country with more carbon dioxide emissions in the world), some regions in Mexico, North Africa, India, Australia, northeastern Brazil and some locations around the Mediterranean, especially in the south of Spain. The challenge is therefore set for a change in our energy production scheme.

2010

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.