Optimization based approach to assist the transition of large energy users to low temperature networks

Distric Energy System (DES) represents a promising opportunity to improve energy efficiency and boost the decarbonization of the European energy sector. If most of the studies focus on small consumers typically located within urban areas, large energy users like hospitals, university campuses or airports, constitute complex and unique systems with great potential for energy and cost savings. The aim of this work is to propose a robust method to optimize the preliminary design and operation of local DES characterized by complex non-residential buildings. Decision variables include the optimal set of technologies to be installed (type, size, location), their operating schedule, together with temperature and configuration of the thermal network. Focus is given to heat pumping technologies and in particular to the comparison between different degrees of centralization and respectively, decentralization. The mathematical model is based on a Mixed- Integer Linear Programming (MILP) formulation including a set of heat cascade constraints to ensure the feasibility of the heat exchanges. A case study is generated to represent a building-stock constituted by large non-residential buildings, connected to a central plant through an existing network infrastructure. The users differ for power and temperature demand, both estimated as linear function of the ambient temperature. Results show that the optimum degree of decentralization is highly influenced by the distribution of the heat demand among the users at the different temperatures. Therefore, the use of local heat pumps to upgrade the heat produced by a central unit does not always translate in operating cost savings. For the case study presented in this work it is shown that if the lowest temperature user covers 66% or more of the total demand a decentralized configuration is the most cost effective solution.

Key energy and technological aspects of three innovative concepts of District energy networks

Daniel Favrat, Samuel Henchoz, François Maréchal

In urban areas, the use of thermal energy services is intensive and relatively heterogeneous by their nature, spatial distribution and temperature level required. In this study three concepts of district energy networks that could provide these services very efficiently to a specific area representative of mid Europe (Geneva) are compared. The focus is on the energy and technological aspects related to each of the proposed networks. 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 HFO refrigerant R1234yf to store and transfer heat across the network. The third concept is more conventional as it uses the sensible heat of liquid water, however with a small temperature spread (2 - 8 °C). The three proposed networks allow the waste heat emitted by the users requiring cooling to be collected, transferred and valorised 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 final energy (electricity) consumption required to supply the thermal services amounts to 10.87 GWh for the CO2 variant, 10.52 GWh for the water variant and 9.60 GWh for the R1234yf variant.

2015

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

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

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

Jonathan Demierre

EPFL2012