A DRYING HEAT PUMP USING CARBON DIOXIDE AS WORKING FLUID

Carbon dioxide as a working fluid in drying heat pumps

Chapter 7 - Exergy analyses of refrigeration and heat pump systems

A review of transcritical carbon dioxide heat pump and refrigeration cycles

Air dielectric coaxial cables as cryogenic transfer lines

Two-phase flow distribution in parallel flow mini/micro-channel heat exchangers for refrigeration and heat pump systems: A comprehensive review

Novel molecules as working fluids for refrigeration, heat pump and organic Rankine cycle systems

Future military space missions will introduce significant new technological needs for spacecraft energy systems. It is generally accepted that spacecraft heat rejection systems that use heat pumps to boost the radiator temperature will reduce the radiator area. However, these systems must also result In weight savings and high rellabliity. Mainstream Engineering Corporation is involved in the development of advanced, high-temperature, thermallyand electrically-driven heat pumps for spacecraft applicatioits. This paper discusses the development of several heat pump configurations. This presentation also reviews the potential benefits of an innovative, spacecraft heat pump, thermalmanagementlthermal-transport loop for high-power applications. Practical Spacecraft Vapor-Compression Heat Pump Conflguratlons Deslgnlng a heat pump for spacecraft operations would require the use of onboard power to drive the compressor. However, tl high-temperature waste heat Is available (such as waste heat from a nuclear or sdar dynamic power system), then a thermailydriven system, rather than an electrically-driven system, mlght be preferred. Mainstream is pursulng development of both thermallyand 6iectrlcailydrlven heat pump systems. In the area of thermaiiydriven systems, the englneer has the choice of several chemlcal heat pump systems or the use of a heat engine to power a vapor-compresslon heat pump. The chemical heat pumps (absorptlon, metal-hydride, or complex-compound heat pumps) typically have only ilqukl pumps and therefore tend to be falsely represented as less complex than heat-engine vapor-compression (Rankine-Rankine or Stirllng-Rankine) systems. However, the metal-hydride and complexcompound systems are intermment, or batch, systems requiring several systems operating at different intefvals to approximate contlnuous operation. Besides the control and plumbing problems associated with these designs, chemical heat pumps also tend to have an inherently low Coefficlent of Performance (COPc) and are larger and heavler than many systems of equal capacity. The absorption systems are continuous systems but also tend to be both Inherently low in COPC and heavy. A heat engine drivlng a vapor-compression heat pump typically out performs an absorption heat pump system because the heat engine can utilize the high-temperature heat avallabie (from nuclear power source waste heat, for example), and typically the absorptlon system cannot (although Malnstream Is investigating new absorption-refrigerant pairs that could solve that problem). in addltion, the absorption systems tend to be larger and heavler because they require a large iiquld-vapor surface area to achieve the equilibrium concentrations necessary for optimum refrigerant transport. Metal-hydrlde, complexcompound. and other chemlcal heat pump systems also tend to be heavy and large for a given capacity. A lighter, more compact alternative Is t o utilize a heat engine drlvlng a vapor-compression heat pump. This approach typically has a higher COPC. Brayton cycles are sometimes used in place of Rankine cycles for power converslon (and reverse Brayton cycles are sometlmes used In place of reverse Ranklne cycles for heat pumps). Brayton cycles utilize stable, gaseous RuMs over the entlre temperature range of the cycle. In spite of this thermal stablllty. the Brayton cycles have inherent disadvantages when compared to Rankine cycles. The first disadvantage in utilizing j Copyright 0 American Institute of Aeronautics Astronauries. Inc.. 1989. AI1 rights reserved and Brayton cycles is that they typically have lower cycle efficiencies because only the sensible heating of the working fluid Is available to transfer energy in the cycie(t-3) resulting in larger mass flow rates (compared to the Rankine cycle. which utilizes the latent heat of vaporization of the fluid to carry a slgnfflcant amount of the energy) The second disadvantage Is that the weight of the Brayton cycle is typically higher than the weight of a comparable Rankine cycle due to the low heat transfer coefficient of the gaseous working fluid (which results in larger heat exchangers) and higher flow rates (or higher pressures) for the Brayton system. The weight difference between comparable Rankine and Brayton cycles is documented in the iiterature(4) in terms of Stirling-powered heat pumps, Mainstream has investigated the concept in which a free-piston Stirling engine drives a third compression piston within the Stirling enclosure in this design, the Stirling working fluid and the vaporcompression heat pump working fluid would be the same, allowing the system to be hermetically sealed The working fluid is selected to insure superheated conditions in the Stirling system. Although the Stirling-powered vaporcompression heat pump has the potential for higher performance due to the higher theoretical petformance of the Stirling heat engine (compared to the Rankine heat engine), this performance benefit has never been practically demonstrated in any Stirling heat engine in addition, the Stirling heat engine will be heavier than the Rankine heat engine, and long-term reliability of the Stirling heat engine has not been demonstrated. Finally (for the free piston configuration), it Is not clear that the effect of the variable compressor load on the motion and frequency of the Stirling machine dispiacer piston is well

Transcritical carbon dioxide heat pump systems: A review

Refrigerant significantly influences the performance of air-conditioning and refrigeration system as well as it has some environmental issues that need to be considered before selection. These systems can be made eco-friendly, if it is powered by solar energy or low-grade thermal energy and they use environment-friendly refrigerants. In this chapter, low GWP (global warming potential) refrigerants have been explored for the domestic air-conditioning applications. Refrigerants with high GWP are mostly used in environment control applications such as heating, ventilation, air-conditioning (HVAC), and refrigeration systems. Some refrigerants contribute to significant environmental issues and the Montreal Protocol and Kyoto Protocol have been signed to address the threats of ozone layer depletion and global warming potential. To fulfil the commitments of Kyoto Protocol, meanwhile, governments in many countries instituted phase-out plan for the use of environmentally harmful gasses in heat pump systems. For instance, EU MAC Directives, F-gas regulation, and Japan METI directives, which clearly declared their target year to use new refrigerant of GWP below 150 for mobile air conditioner and GWP below 750 for the residential air conditioner. Research interest has been stimulated to find alternative refrigerants with low or ultra-low GWP for energy conservation and environmental sustainability. Hydrofluoroolefins (HFOs) have a very low environmental impact, and thus HFOs are considering as potential candidates to replace the hydrofluorocarbon (HFC) refrigerants such as R410A, a near-azeotropic mixture of difluoromethane (R-32) and pentafluoroethane (R-125) and is commonly used refrigerant in air-conditioning applications. The limited number of pure fluids sometimes cannot meet the excellent heat transfer criteria due to their low volumetric capacity and moderate flammability or toxicity. Mixing of HFOs and HFCs refrigerants, in this case, allows the adjustment of the most desirable properties of the refrigerant by varying the molar fraction of the components. Different combination of mixture presented here cannot be claimed as the best mixture, but it might be a good choice for further study. This chapter focuses on the research trend in finding low GWP refrigerants and its application in different heat pump system considering the system performance, safety, and the overall environmental impact. The conventional vapor compression system, thermally driven adsorption system, and sorption-compression hybrid system have been taken into consideration.

Carbon Dioxide, CO2 (R744) was one of the first commonly applied working fluids in the infancy of refrigeration more than 100 years ago. In contrast to ammonia it mainly disappeared after the first generation of synthetic refrigerants have been introduced to the market after 1930. One reason was that the transition from low-rpm belt driven compressors towards the direct electrical motor driven compressors (50-60 Hz) was not performed for CO2 compressors before the revival introduced by Gustav Lorentzen in the 90is of last century. Since 1988 an enormous R & D effort has been made to further develop CO2 refrigeration technology in spite of the opposition from the chemical industry.Today CO2 refrigeration and heat pumping technologies are accepted as viable and sustainable alternatives for several applications like commercial refrigeration, transport refrigeration, vehicle air conditioning & heat pumping, domestic hot water heat pumps and industrial applications. For some applications, the current threshold to introduce R744 technology can be overcome when the system design takes into account the advantage of the thermo dynamical- and fluid properties of CO2. I.e. the system is designed for transcritical operation with all it pros and cons and takes into consideration how to minimize the losses, and to apply the normally lost expansion work. Shortcut-designs, i.e. drop in solutions, just replacing the H(C)FC refrigeration unit with an CO2 systems adapted for higher system pressures will not result in energy efficient products. CO2 systems do offer the advantage of enabling flooded evaporators supported with adapted ejector technology. These units offer high system performances at low temperature differences and show low temperature air mal-distributions across evaporators.This work gives an overview for the development possibilities for several applications during the next years. Resulting in a further market share increase of CO2 refrigeration and heat pump systems, as energy efficient alternatives to current systems not applying natural working fluids.

Selection of pure and binary working fluids for high-temperature heat pumps: A financial approach

A review on substitution strategy of non-ecological refrigerants from vapour compression-based refrigeration, air-conditioning and heat pump systems

Chapter 4 - Thermophysical and rheological properties of unitary and hybrid nanofluids

In 2014, European F-gas directive plans the prohibition of fluorinated working fluids with GWP of 2500 or more from 2020. Consequently, new working fluids have to be considered in the future such as HydroFluoroOlefin, carbon dioxide or mixture of HFO with hydro-fluoro-carbon or CO2. The knowledge of the thermo-physical properties of working fluid is essential for the evaluation of performance of heat pumps, ORC and refrigeration. Herein, several “French” laboratories proposed to investigate the thermo-physical properties of the R744 + R1234yf binary system. In 2014, Juntarachat et al. measured and correlated vapour liquid equilibria including mixture critical point. New experimental determinations for density and viscosity using vibrating densitometer and capillary viscometer are presented. Also, enthalpies of mixing are determined using BT-215 Calvet calorimeters. In addition, molecular simulation based on empirical force have been realized. Several thermodynamic and transport properties have been obtained using Monte Carlo and Molecular Dynamics simulations. The PPR78 cubic equation of state and a multipolar version of SAFT-Mie are used to predict the new experimental data for thermodynamic properties. The model developed is used as an input to simulate the performance of new heat pump system. An experimental heat pump is used to evaluate the achievable energy efficiency with this mixture in real machines with one test molar composition of 5% R1234yf). The optimal pressure and COP will be compared to those obtained with pure CO2 in similar operating conditions.

Heating application efficiency is a crucial point for saving energy and reducing greenhouse gas emissions. Today, EU legal framework conditions clearly define how heating systems should perform, how buildings should be designed in an energy efficient manner and how renewable energy sources should be used. Using heat pumps (HP) as an alternative “Renewable Energy System” could be one solution for increasing efficiency, using less energy, reducing the energy dependency and reducing greenhouse gas emissions. This scientific article will take a closer look at the different efficiency dependencies of such geothermal HP (GHP) systems for domestic buildings (small/medium HP). Manufacturers of HP appliances must document the efficiency, so called COP (Coefficient of Performance) in the EU under certain standards. In technical datasheets of HP appliances, these COP parameters give a clear indication of the performance quality of a HP device. HP efficiency (COP) and the efficiency of a working HP system can vary significantly. For this reason, an annual efficiency statistic named “Seasonal Performance Factor” (SPF) has been defined to get an overall efficiency for comparing HP Systems. With this indicator, conclusions can be made from an installation, economy, environmental, performance and a risk point of view. A technical and economic HP model shows the dependence of energy efficiency problems in HP systems. To reduce the complexity of the HP model, only the important factors for efficiency dependencies are used. Dynamic and static situations with HP´s and their efficiency are considered. With the latest data from field tests of HP Systems and the practical experience over the last 10 years, this information will be compared with one of the latest simulation programs with the help of two practical geothermal HP system calculations. With the result of the gathered empirical data, it allows for a better estimate of the HP system efficiency, their economic costs and benefits and their environmental impact.