On the development of high temperature ammonia–water hybrid absorption–compression heat pumps

Ammonia–water hybrid absorption–compression heat pumps (HACHP) are a promising technology for development of efficient high temperature industrial heat pumps. Using 28 bar components HACHPs up to 100 °C are commercially available. Components developed for 50 bar and 140 bar show that these pressure limits may be possible to exceed if needed for actual applications. Feasible heat supply temperatures using these component limits are investigated. A feasible solution is defined as one that satisfies constraints on the COP, low and high pressure, compressor discharge temperature, vapour water content and volumetric heat capacity. The ammonia mass fraction and the liquid circulation ratio both influence these constraining parameters. The paper investigates feasible combinations of these parameters through the use of a numerical model. 28 bar components allow temperatures up to 111 °C, 50 bar up to 129 °C, and 140 bar up to 147 °C. If the compressor discharge temperature limit is increased to 250 °C and the vapour water content constraint is removed, this becomes: 182 °C, 193 °C and 223 °C.     

A basis for the development of new ammonia–water–sodium hydroxide absorption chillers

Adding NaOH to ammonia/water improves the separation of ammonia in the generator and reduces both chiller driving temperature and rectification losses. In this paper the main disadvantages for the implementation of these new mixtures are addressed: a) few experimental data or reliable correlations of the fluid mixture properties exist; b) selection of an adequate system for separating the hydroxide; and c) evaluation of potential corrosion problems. Our results show that the separation of NaOH from an ammonia/water solution with a maximum approximate mass fraction of 0.05 (5% weight) is viable when reverse osmosis is used with membranes BW30 and SW30HR LE. Cycle simulation using experimental data to calculate the vapour–liquid equilibrium properties shows that the COP is approximately 20% higher than with a conventional ammonia/water chiller working under the same conditions and using a hydroxyl separation efficiency of 99% for NaOH, which is feasible according to the experimental tests.     

Development of microwave assisted zeolite–water adsorption heat pump

A microwave assisted zeolite–water adsorption heat pump system was designed, manufactured and investigated experimentally. The influence of operation time of microwave oven on performance of the adsorption heat pump was studied. The performance criteria: coefficient of performance, specific cooling power and volumetric cooing power, were calculated for the designed and tested adsorption heat pump system. The regeneration of adsorbent bed was achieved very rapidly (35 min) by using microwave heating system. The poor thermal conductivity of adsorbent did not affect the periods of isosteric heating and isobaric desorption processes.     

Diagrams of entropy for ammonia–water mixtures: Applications to absorption refrigeration systems

The objective of this work is to calculate the entropy of ammonia–water mixture as a function of temperature, pressure, concentration, and other thermodynamic properties associated to absorption process, to support energy and exergy analysis of absorption refrigeration systems. This calculation is possible because a novel mathematical modelling was developed for this attempt. This determination will allow simulation and optimisation of absorption refrigeration systems, giving major importance in determining the values of thermodynamic properties of ammonia–water mixtures, such as enthalpy and entropy. A mathematical modelling for thermodynamics properties calculation at liquid and vapour phases of ammonia–water system is developed. The studies were based on the enthalpy vs. concentration diagram obtaining the enthalpy in the liquid phase corresponding at a temperature range from 80 °C to −40 °C. The mixtures enthalpy values were calculated for ammonia (h_1c) and water (h_2c) by using a non-linear regression program. The evaluation of thermodynamic properties in this work was discretised by formulating appropriate equations for each type of substance. However, thermodynamic properties of mixtures can be determined based on data from simple substances and mixing laws, or from an equation of state that considers the mixture concentration. The consistency of experimental data indicates the most suitable method to be used in entropy calculation.     

Experimental analysis of a novel concept of a “thermally driven” solution pump operating a small-capacity ammonia/water absorption heat pumping system

Only a few electrical solution-pumps are available on the market suitable for ammonia/water absorption heat pumping systems with an evaporator capacity lower than 20 kW. In order to improve this, a “thermally-driven” solution-pump is proposed, offering several advantages, as e.g. an oil-free, simple and leak-proof design. “Thermally-driven” means this pump is driven by a power process within the absorption heat pumping-cycle instead of electricity. The experimental analysis of this novel pump operating in a commercially available ammonia/water-absorption chiller shows that it can be easily integrated and works without any relevant operating limits. However, the measured COP of the considered chiller is decreased by at least 0.1 (15%) by this thermal-pump compared to the existing electrical-pump. Nevertheless, thermal energy (e.g. cost and CO₂-free waste heat) can be used to drive this pump and due to lower production cost it can be an interesting alternative from an economical and ecological point of view.     

Heterocyclic methacrylates for clinical applications–further studies of water sorption

The room temperature polymerizing system comprising poly(ethyl methacrylate)-tetra hydrofurfuryl methacrylate (PEM/THFMA) has potential in orthopaedic and dental applications, and earlier work has shown it to have unusual water absorption characteristics. This aspect has been studied in further detail, by studying the water absorption behaviour from some biological solutions, and the effect of the addition of an antibiotic (gentamicin). For comparison purposes, a parallel system whereby tetrahydrofuryl methacrylate was replaced by hydroxyethyl methacrylate (PEM/HEMA), was studied. In the case of PEM/THFMA, water uptake was substantially reduced when absorption was carried out from solutions (from about 30% in water to about 1.5% in solutions of higher concentrations), and the corresponding diffusion coefficient increased (by a factor of several hundred). The addition of gentamicin increased uptake, but the extent of increase also decreased in solutions. It was concluded that uptake was related to the osmolarity of the external solution, and also on the presence of osmotic sites within the polymer; hence the uptake process appears to be governed by chemical potential considerations. At the higher uptakes, there was evidence of water clusters. In marked contrast, the uptake by the PEM/HEMA system was independent of the osmolarity of the external solutions, presumably due to the hydrophilic nature of HEMA.     

Fabrication of Mn–ZnO photoanodes for photoelectrochemical water splitting applications

Journal of Materials Science: Materials in Electronics - A photoelectrochemical (PEC) water splitting ability of pure ZnO and manganese-incorporated ZnO thin films fabricated via a simple...     

Maldistribution in air–water heat pump evaporators. Part 1: Effects on evaporator,heat pump and system level

This paper presents an approach to quantify the effect of evaporator maldistribution on operating costs of air–water heat pumps. In the proposed simulation model maldistribution is induced by two parameters describing refrigerant phase and air flow distribution. Annual operating costs are calculated based on heat pump performance at distinct operating conditions. Results show that percentage increase of operating costs is similar for the three considered climate zones, even though the effect of maldistribution on heat pump performance varies with operating conditions. Differences in terms of absolute cost increase for the climate zones arise mainly due to a varying number of operating hours. Absolute cost increase is considerable in the average and especially colder climate zone and can only partly be reduced by enlarging the evaporator.     

Creep resistance of Fe–Ni–Cr heat resistant alloys for reformer tube applications

Relations between microstructure, phase transformations and creep resistance of austenitic Fe–Ni–Cr alloys are investigated. As-cast alloys with different silicon contents and an ex-service tube are submitted to laboratory agings to trigger specific phase transformations, and subsequently creep-tested at 950°C under stresses of 24–48?MPa. As-cast microstructures contain interdendritic chromium-rich M₇C₃ carbides with niobium-rich MC carbides. After aging at 950°C, primary M₇C₃ carbides transform into chromium-rich M₂₃C₆ carbides, associated to a loss in creep strength. The G phase present in the ex-service alloy is reversed into MC carbides by a heat treatment at 1100°C, associated to a slight decrease in creep resistance. Besides, the addition of silicon is highly detrimental to creep strength. Results can be used for alloy design.     

Maldistribution in air–water heat pump evaporators. Part 2: Economic analysis of counteracting technologies

In this study a methodology is applied to quantify the effect of evaporator maldistribution on operating costs of air–water heat pumps. The approach is used to investigate the cost-effectiveness of two technologies enabling to counteract maldistribution: a flash gas bypass setup and the individual superheat control in parallel evaporator channels. In the total cost of ownership analysis, different scenarios for climatic conditions, severity of maldistribution, and economic framework are considered. Results show that the flash gas bypass system is cost-effective only in a few conditions, namely severe maldistribution, high electricity prices, and colder climate. Investment in the individual superheat control technology, however, can be quickly amortized in many scenarios. For the warmer climate zone with a small number of operating hours counteracting of maldistribution does not pay off under the used economic assumptions.     

Design,construction and operation of a solar powered ammonia–water absorption refrigeration system in Saudi Arabia

This study presents an experimental investigation of a solar thermal powered ammonia–water absorption refrigeration system. The focus of this study lies on the design of the components of the absorption chiller, the ice storages and the solar collector field as well as the integration of the data acquisition and control unit. An ammonia–water (NH₃/H₂O) absorption chiller was developed in the laboratory of the Institute of Thermodynamics & Thermal Engineering (ITW) at the University of Stuttgart (Germany). A demonstration plant was built in the laboratory of the CoRE-RE at King Fahd University of Petroleum & Minerals (KFUPM – Saudi Arabia). The whole system was tested successfully. The results of the experiments indicated a chiller coefficient of performance (COP) of 0.69 and a cooling capacity of 10.1 kW at 114/23/−2 (°C) representing the temperatures of the generator inlet, the condenser/absorber inlet and the evaporator outlet respectively. Even at 140/45/−4 (°C), the chiller was running with a cooling capacity of 4.5 kW and a COP of 0.42.     

Solar cooling with water–ammonia absorption chillers and concentrating solar collector – Operational experience

Concentrating solar collectors provide high efficiency at high driving temperatures favourable for thermally driven chillers. Therefore, they offer applications for hot climates and industrial process integration, especially in combination with NH₃–H₂O chillers that provide refrigeration temperatures below 0 °C. The presented solar cooling installation comprises a linear concentrating Fresnel collector that provides the driving heat for two NH₃–H₂O absorption chillers at temperatures up to 200 °C. Chilled water temperatures are produced in the range between −12 °C and 0 °C. Collector capacities reached up to 70 kW at peak times and the total cooling capacity of both chillers showed peak values up to 25 kW. For good operating conditions, the thermal system EER was 0.8 and an electrical system EER of 12 was easily achieved. The system showed a sound operating behaviour. The performance of different operation and control strategies was analysed, evaluated and enhanced within the two year operation phase.     

Modeling the solar absorption performance of Copper@Carbon core–shell nanoparticles

Journal of Materials Science - The working fluid is a critical component in direct absorption solar collectors. Nanoparticle (NP) suspensions can be used as efficient solar absorption media. In...     

Engineered chemistry of Cu–W composites sintered by field-assisted sintering technology for heat sink applications

High performance microelectronics require superior thermal management systems. Heat sink plates that dissipate the heat generated by the microelectronic components are of critical importance for this purpose. The two primary concerns with heat sink materials are the coefficient of thermal expansion (CTE), which should be low to match that of silicon (4 × 10⁻⁶/°C), and the thermal conductivity, which should be as high as possible to move the heat efficiently. This study created copper–tungsten composite materials using the field assisted sintering technology (FAST) process. The amount of tungsten used in the composites was varied from 0 to 70 wt% to study the effects of tungsten as well as to show the ability of the composite’s properties to be tailored based on the percentage of tungsten in the composite. It is shown that increasing pressure has a positive effect on final density while heating rate has no effect. High densities of >97.5% were achieved in all of the composites. As predicted, the CTE decreases as the percentage of tungsten increases. Thermal conductivities are also reported for each sample.     

Experimental investigation on NH3–H2O compression-assisted absorption heat pump (CAHP) for low temperature heating in colder conditions

The coefficient of performance (COP) and heating capacity of the absorption heat pump (AHP) decreased obviously as the evaporator inlet temperature dropped. Compression-assisted AHP (CAHP) could operate efficiently in colder conditions, and a prototype was constructed for experimental investigation. At a generator inlet of 130 °C, as the evaporator inlet decreases from −5 °C to −25 °C, the COP drops from 1.513 to 1.372, while the heating capacity deteriorates from 77.26 kW to 47.11 kW. Comparisons between CAHP and normal AHP indicated that CAHP can extend the lower limit of evaporator inlet temperature from −10 °C to −25 °C. Besides, CAHP can enhance the heating capacity by approximately 55.5–85.0% even when AHP can operate normally. Moreover, the improvement contributed by CAHP is greater under much colder conditions. The heating COP and capacity of CAHP are improved in all the conditions, while the primary energy efficiency is advantageous under lower evaporator inlet temperatures.     

Experimental evaluation of a variable effect LiBr–water absorption chiller designed for high-efficient solar cooling system

A variable effect LiBr–water absorption chiller is studied in this paper based on a real developed 50 kW prototype. The chiller is designed specifically for the high-efficient utilization of the solar power with variable temperature. It can obtain the optimized COP and cooling power under different heat source temperatures. The construction, circulation and testing system of the chiller were introduced. A typical running condition of the chiller from the starting to the steady operating was given to show the dynamic performance. Several groups of the temperatures and COPs were given to show the steady state performance. These data showed that the COP increased from 0.69 to 1.08 under generation temperature from 95 °C to 120 °C. Besides, the effects of chilled water temperature, cooling water temperature, pump frequency and opening of valve on COP and cooling power were analyzed respectively.     

An experimental–numerical study of moisture absorption in an epoxy

It is well known that moisture absorption impairs the mechanical and physical properties of polymers. Conventionally, the material’s hygric strains are described as the product of a constant coefficient of moisture expansion (CME) and moisture concentration. This hypothesis, however, has not been thoroughly examined experimentally. In this paper, the hygro-mechanical response of a DGBA based epoxy is reported as a function of moisture uptake. Cylindrical specimens are made of epoxy with an axially located optical fiber that contains a 23 mm Bragg grating sensor (FBG). Strain data from the sensor and from a micrometer are combined with experimental absorption curves to determine the resin’s CME. The data indicate that diffusion and CME depend on moisture. Analysis of the experiments is carried out by numerical simulations of heat transfer, moisture diffusion and elastic stress analysis of the single fiber composite. The simulated results correlate well with the experimental data.     

Non-equilibrium sorption performances for composite sorbents of chlorides–ammonia working pairs for refrigeration

The sorption/desorption characteristics under the non-equilibrium conditions are extremely important for the refrigeration. The experimental test unit is set up, and the p–T–x curves of different working pairs are tested under the non-equilibrium working conditions. The results show that non-equilibrium sorption/desorption processes are related with dual variables rather than a single variable. Then we tested the isobaric sorption/desorption processes of chlorides. Results show that the hysteresis phenomena are related with the desorption enthalpy. Sorption/desorption dynamic models are studied. For sorption process the model is in accord with D-A equation, but for the desorption process both D-A equation and Spinner model should be considered. The Arrhenius equation is used to fit the relation between reaction heat and desorption hysteresis. The results show that the non-equilibrium equations fit the refrigeration performance well, whereas the theoretical equilibrium curves have big error.