Dynamic Modeling and Simulation of Two-stage Compression Heat Pumps with Flash Economizer

Multistage heat pumps are one of the most efficient technologies for delivering heat at temperatures below 100°C, Thanks to their fairly high exergy efficiency. In the recent years the development of radial compressors mounted on gas bearings have unlock the possibility of a downsizing of multistage heat pumps to the capacity range of domestic applications. Such small scale multistage heat pumps are expected to have increased efficiency at nominal and part load conditions, to be fully hermetic and to be more compact thanks to their oil free operation and the use of dynamic machines. As a result of an industrial collaboration, an experimental two-stage heat pump with flash economizer was built and tested in a previous work. It used radial oil free compressors arranged on a single shaft rotating on gas bearings. A dynamic simulation model has been developed in order to better understand the behavior of the aforementioned prototype, as well as to preliminary test control strategies. As a result, a whole range of dynamic models, including the evaporator, condenser, flash economizer, and compressors were developed using the modeling and simulation software gPROMS. They rely on simple two and three zone models for the evaporator and condenser respectively. Zero dimensional models were used for the compressors, two different types of which were implemented. One is a volumetric compressor with a fixed built-in volume ratio, connected to an asynchronous electric motor the other one is a variable speed radial type based on a simplified compressor map deduced from experimental data. As much as possible, the aforementioned models have been inspired from components available in the base library of gPROMS. The results showed that a two stage heat pump based on quasi constant speed volumetric compressors poses problem at low temperature lift, when the top part of the cycle is degenerated, making it close to a single stage machine. The version with the dynamic compressors does not exhibit any problem of degeneration of the cycle, while having a better performance and load characteristic curve. Both systems showed an insensitivity of the performances to the charge of refrigerant due to the buffering effect of the flash economizer. As a conclusion, a simple dynamic model was developed for simulating various types of two-stage heat pumps with flash economizer. It confirmed the potential of performance improvement of radial compressors based heat pumps over volumetric ones.

Two-Stage Heat Pump using Oil-Free Turbocompressors – System Design and Simulation

Adeel Javed, Jürg Alexander Schiffmann

The combination of multi-stage heat pump cycles with small-scale oil-free turbocompressor technology running on gas bearings could be a promising way to increase performance in domestic and commercial heat pumps. This paper presents a novel two-stage heat pump system with two heat sources at two different temperature levels using two separate turbocompressors rotating on gas bearings optimized for R134a. The system allows integration of unused heat sources, e.g. solar thermal or waste heat, into heat production with a minimal loss of exergy. The cycle comprises an evaporator for the first heat source, a condenser as heat sink, an open economizer with integrated heat exchanger for the second heat source, and a tube-in-tube suction line heat exchanger (SHX) in the high-pressure for superheating and subcooling. The aim of this study is to evaluate theoretically the performance of this heat pump cycle using a system model programmed in the software EES (Engineering Equations Solver). The simulation assumes steady-state, negligible pressure drops and heat losses, and adiabatic expansion processes. The superheating in the evaporator and the SHX is 5°C, and there is no subcooling in the condenser. The heat exchangers are modeled using effectiveness-NTU models. At the design point, the heating capacity of the condenser is set to 6.5 kW and provides hot water of 55°C. The first heat source is brine of 5°C. The second heat source is water of 30°C and has been designed to provide up to 30% of the total condenser heat capacity. The two turbocompressors are designed specifically to meet the heat pump design point. Presently, one-dimensional (1D) compressor maps are used in the heat pump model. Simulation results show that coefficient of performance (COP) improvements of 20% to 30% are achievable, depending on the source temperature levels of the heat pump cycle and the amount of second heat source added to the system. The COP increases with higher source temperatures, higher second heat source capacity, and lower sink temperature. The pressure ratios are defined by the imposed temperature levels. The mass flow rate of the refrigerant in the first stage is mainly determined by the second heat source capacity, and in the second stage by the heat capacity of the condenser. In future work, this novel heat pump concept will be tested experimentally.

2016

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

Pompe à chaleur bi-étagée à haute performance.

Daniel Favrat, Michele Zehnder

The objectives of the present work is to theoretically and experimentally demonstrate the advantages of a two-stage cycle equipped with an economizer and single variable speed compressor for heat pumps using air as cold source. Furthermore, an efficient deicing device based on the two-stage concept is discussed in this paper. A two-stage cycle numerical model has also been developed allowing fast determination of the heat pump’s steady-state operating range as a function of the principal design variables. The experimental validation has been carried out using two successive heat pump biPAC1 uses a two-stage cycle with an economizer and a variable speed compressor with a mid-stage casing pressure (two scroll compressors, the first one with a 30 to 110 Hz frequency range). The second heat pump biPAC2 uses a two-stage cycle with a two-phase injection (limitation of the distillation effect of the non-azeotropic refrigerant in the economizer) for further optimization. Furthermore, another first stage compressor type has been used (piston compressor with a frequency range of 45 to 65 Hz) as well as a new evaporator with an enhanced tube surface and a new plat condenser. The experiments using the biPAC2 have resulted in a 70% increase of the condenser’s power and a 14% increase of the performance coefficient for an operating regime L-7/W50 with the HCFC 22 refrigerant. The good performances of the biPAC2 have been confirmed through further testing using HFC407C, the de-icing cycle and following the normalized test case EN255. These results have been compared to the three best commercial heat pumps. Two-stage cycle results show an excellent exergetic efficiency for the extreme temperatures (50% at L-10/W60). The simulations based on the experimental results are given for a temperature range between L-8/W50 and L-12/35 for typical household requirements (degrees-day of Zurich). the numerical simulations lead to an average annual performance coefficient (with de-icing) of 3.9 in a monovalent operation.

1997